JP5326601B2 - Optical transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low-cost and low-noise optical transmission device which reduces mode-hopping noise of an external resonator type laser and does not require precise temperature control. <P>SOLUTION: An optical transmission device includes a laser beam generating device having an optical amplifying unit and an optical waveguide unit, including a second core layer provided with a diffraction grating and an optical waveguide that is optically combined with the optical amplifying unit; and an electrical signal source for applying electrical signals to the optical amplifying unit to inject the electric current to the semiconductor layer. The laser beam generating device generates optical signals, by reciprocating between a first state of laser-oscillating in a plurality of resonator modes of the optical resonator, formed by the light reflecting surface and the diffraction grating, and a second state with the luminous intensity smaller than that of the first state in accordance with the electrical signals. Furthermore, the frequency corresponding to the interval of the resonator mode is higher than the frequency that corresponds to the bit rate of the electrical signal. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to an optical transmission apparatus that generates an optical signal and sends it to an optical transmission line.

  At present, as a light source for optical communication, a distributed feedback laser (hereinafter abbreviated as DFB laser) that stably oscillates laser in a single longitudinal mode is widely used.

  However, it is known that the oscillation wavelength of a DFB laser is relatively susceptible to fluctuations with respect to changes in element temperature. This is because the diffraction grating that determines the oscillation wavelength of the DFB laser is formed of a semiconductor (InP, GaAs, etc.) whose refractive index easily changes depending on the temperature. For this reason, in a conventional optical transmission device (device that generates an optical signal and sends it to an optical transmission line), the temperature of the DFB laser used as a light source is precisely controlled by a temperature control device to keep the oscillation wavelength constant. . However, such a temperature control device is expensive and hinders cost reduction of the optical transmission device. This problem is particularly apparent in an optical transmitter for wavelength division multiplexing (hereinafter abbreviated as WDM) that requires many light sources having slightly different oscillation wavelengths.

  In view of this, an external resonator type laser (hereinafter referred to as an external resonator type laser) in which a diffraction grating is formed of a dielectric material having a small refractive index temperature change has been proposed (Non-Patent Document 1).

This external cavity type laser is composed of a SiO ₂ optical waveguide having a diffraction grating and a semiconductor laser in which an antireflection film is applied to the first end face and a high reflection film is applied to the second end face. Is formed.

  In the external resonator type laser, the first end face of the semiconductor laser and the end face of the optical waveguide face each other, and the semiconductor laser and the optical waveguide are optically coupled. Further, an optical resonator (external resonator) is formed by a highly reflective film applied to the second end face of the semiconductor laser and a diffraction grating formed in the optical waveguide.

  In this external resonator type laser, light within the reflection band of the diffraction grating is amplified by the semiconductor laser to cause laser oscillation.

By the way, the temperature change coefficient of the refractive index of a dielectric such as SiO ₂ is about an order of magnitude smaller than the temperature change coefficient ( ^{−10 −4} degree ⁻¹ ) of the semiconductor. For this reason, the temperature dependence of the oscillation wavelength of the external cavity laser is 0.01 nm / degree, which is an order of magnitude smaller than the temperature dependence of the oscillation wavelength of the DFB laser (0.1 nm / degree).

  Therefore, if the optical transmission device is formed by an external resonator type laser, an expensive temperature control device becomes unnecessary.

ELECTRONICS LETTERS, Vol. 32, page1202-1203, 1996. 2007 4th IEEE International Conference on Group IV Photonics, ThA2.

  The external resonator laser oscillates at a wavelength at which the reflectance by the diffraction grating is the highest among the wavelengths of a number of resonator modes (longitudinal modes) formed in the external resonator.

As described above, the oscillation wavelength of the external resonator laser in which the diffraction grating is formed by the dielectric (SiO ₂ or the like) hardly changes with the change of the element temperature.

  However, when the element temperature changes, the longitudinal mode during laser oscillation causes a mode hop that suddenly changes to another mode at a certain temperature. Such a mode hop is caused by a difference in temperature change rate between the refractive index of the dielectric forming the diffraction grating and the refractive index of the semiconductor (semiconductor laser) forming a part of the external resonator.

  When mode hop occurs, noise is generated in the laser beam. Such noise degrades the quality of optical communication. Therefore, it has been difficult to form an optical transmission device that is constantly exposed to changes in environmental temperature using an external resonator laser as a light source.

  Accordingly, an object of the present optical transmitter is to provide a low-cost and low-noise optical transmitter that does not require precise temperature control by reducing the mode hop noise of the external cavity laser.

  In order to achieve the above-described object, the present optical transmission device includes a waveguide having an optical gain (hereinafter referred to as a waveguide having an optical gain) having a semiconductor layer that generates optical gain by an injected current as a first core layer. And an optical amplifying unit in which a light reflecting surface is formed on the first end face of the gain waveguide, and a second core layer provided with a diffraction grating. A laser light generating device comprising: an optical waveguide unit including a third end surface facing the second end surface of the unit, and an optical waveguide optically coupled to the optical amplification unit at the third end surface; An electric signal source that applies an electric signal to the optical amplifying unit and injects the current into the semiconductor layer; and the laser light generating device forms the light reflecting surface and the diffraction grating according to the electric signal. Multiple resonators of optical resonator A light signal is generated by reciprocating between a first state in which laser oscillation occurs in a mode and a second state in which the emission intensity is lower than that in the first state, and further corresponds to the interval between the resonator modes. The frequency is higher than the frequency corresponding to the bit rate of the electrical signal.

  According to the present optical transmission device, since mode hop noise is reduced, it is possible to provide a low-cost and low-noise optical transmission device that does not require precise temperature control.

It is a perspective view explaining an example of an external resonator type laser (related technology). It is a figure explaining how the oscillation wavelength of an external resonator type laser (related technology) is determined. It is a figure explaining the reflectance of a diffraction grating and the resonator mode of an optical resonator in the state where temperature rose from the state demonstrated in FIG. It is a figure explaining the relationship between the reflectance of a diffraction grating, and the several resonator mode which is carrying out laser oscillation (embodiment). It is a perspective view explaining the principal part of the optical transmission apparatus according to Examples 1-4. It is a figure explaining the cross section along the AA line of FIG. 5, and a cross section perpendicular | vertical to the AA line. It is a figure explaining the cross section of the optical waveguide unit of Example 1. FIG. It is a figure explaining the relationship between the reflectance of a diffraction grating, and a wavelength. It is a figure explaining an example of the electric signal which an electric signal source applies to an optical amplification unit. It is a figure explaining the cross section of the optical waveguide unit of Example 2. FIG. It is a figure explaining the cross section of the optical waveguide unit of Example 3. FIG. 6 is a plan view of a second core layer on which the diffraction grating of Example 4 is formed. FIG. FIG. 10 is a diagram for explaining the distribution of the coupling coefficient κ of the diffraction grating of Example 4. It is a figure explaining the relationship between the reflectance of the diffraction grating of Example 4, and a wavelength. FIG. 10 is a perspective view illustrating a main part of an optical transmission apparatus according to a fifth embodiment. It is the figure which looked at the cross section along the AA line of FIG. 15 from the direction of the arrow. It is a figure explaining the relationship between the reflectance of a diffraction grating of Example 5, and a wavelength. FIG. 10 is a perspective view illustrating a main part of an optical transmission apparatus according to a sixth embodiment. It is a top view explaining the structure of the optical multiplexer of Example 6. FIG. Example 7 is a perspective view for explaining a main part of an optical transmission apparatus. It is a figure explaining the cross section of an optical modulator perpendicular | vertical to the advancing direction of signal light. It is a figure explaining the relationship between the reflectance of the diffraction grating of Example 7, and a wavelength. FIG. 10 is a perspective view illustrating a main part of a WDM optical transmitter according to an eighth embodiment. FIG. 10 is a conceptual diagram of an optical LAN constructed using the WDM optical transmission apparatus according to an eighth embodiment. It is a conceptual diagram of an optical transmission / reception module.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the technical scope of the present invention is not limited to these embodiments, but extends to the matters described in the claims and equivalents thereof.

  First, the above-described external cavity laser will be described in some detail with the inventor's unique knowledge.

  FIG. 1 is a perspective view for explaining an example of the external resonator type laser 2. In the following, including the drawings, the same parts are denoted by the same reference numerals, and the description thereof is omitted.

  As shown in FIG. 1, the external cavity laser 2 includes a semiconductor laser 6 having a high reflection film 4 formed at one end and an antireflection film (not shown) formed at the other end. The semiconductor laser 6 is originally an optical element in which one end of a semiconductor optical amplifier that should be covered with an antireflection film at both ends (traveling waveform) is covered with a high reflection film.

  For this reason, the semiconductor laser 6 does not oscillate alone. Therefore, it is not appropriate to call the semiconductor laser 6 a semiconductor laser. However, in order to unify the terminology with Non-Patent Document 1, the term “semiconductor laser” is used here.

The external resonator type laser 2 includes a planar optical circuit 14 in which an optical waveguide 12 is formed on a SiO ₂ layer 10 stacked on a Si substrate 8. A diffraction grating 16 is formed in the optical waveguide 12.

  The semiconductor laser 6 is fixed to a terrace provided on the Si substrate 8 and is optically connected to the optical waveguide 12 at an end face provided with an antireflection film.

  Here, the diffraction grating 16 functions as a distributed-bragg reflector (hereinafter referred to as DBR), and is integrated with the highly reflective film 4 of the semiconductor laser 6 so as to be necessary for laser oscillation. Form. On the other hand, the semiconductor laser 6 functions as an optical gain medium.

  FIG. 2 is a diagram for explaining how the oscillation wavelength of the external cavity laser 2 is determined.

  The horizontal axis in FIG. 2 is the wavelength. The left vertical axis in FIG. 2 represents the reflectance (light reflectance) 18 of the diffraction grating 16. On the other hand, the right vertical axis in FIG. 2 represents the light intensity of the resonator mode 20 in which laser oscillation is performed.

  In FIG. 2, a mode which is a resonator mode (longitudinal mode) 22 generated in the optical resonator 5 formed by the highly reflective film 4 and the diffraction grating 16 and does not oscillate is also indicated by a broken line.

  In the external resonator laser 2, the optical resonator 5 is significantly longer than the DFB laser, so that the resonator mode interval δλ is narrowed. Therefore, as shown in FIG. 2, a plurality of resonator modes coexist in the wavelength band 24 where the diffraction grating 16 (DBR) exhibits high reflectance.

  As shown in FIG. 2, the reflectance of the diffraction grating 16 (DBR) changes sharply within the wavelength band 24. For this reason, the external resonator type laser 2 oscillates in the resonator mode 20 having the highest reflectance among the plurality of resonator modes coexisting in the wavelength band 24.

The laser light whose wavelength is determined in this way is emitted from the end face of the optical waveguide opposite to the semiconductor laser 6 and enters the optical fiber 17. The optical fiber 17 is made of SiO ₂ (the same applies to the following description).

  FIG. 3 is a diagram for explaining the reflectance 18 of the diffraction grating 16 and the resonator mode 22 of the optical resonator 5 in a state where the temperature is higher than the state described in FIG. The horizontal and vertical axes are the same as in FIG.

  The temperature change of the reflectance 18 of the diffraction grating 16 and the resonator mode 22 of the optical resonator 5 depends on the temperature change of the refractive index of each member forming the diffraction grating 16 and the optical resonator 5.

The diffraction grating 16 is formed of a dielectric (SiO ₂ ) having a small refractive index temperature change rate. For this reason, the reflectance 18 of the diffraction grating 16 hardly changes even if the temperature changes.

On the other hand, inside the optical resonator 5, there is a semiconductor laser 6 formed of a semiconductor having a large refractive index temperature change rate. For this reason, when the element temperature rises, the resonator mode 22 is shifted to the long wavelength side as a whole. Δλ _T shown in FIG. 3 indicates such a wavelength shift.

  The wavelength shift of the resonator mode 22 causes a mode hop as will be described below.

  When the temperature of the external resonator type laser 2 rises, the resonator mode 20 that oscillated in FIG. 2 moves to the vicinity of the long wavelength end of the wavelength band 24 as shown in FIG. On the other hand, the resonator mode 26 present in the vicinity of the short wavelength end of the wavelength band 24 in FIG. 2 moves to the center of the reflection band 24 (see FIG. 3).

  For this reason, the resonator mode in which the reflectance is maximum changes from the resonator mode 20 to the resonator mode 26. As a result, laser oscillation in the resonator mode 20 suddenly stops, and laser oscillation in the resonator mode 26 having a higher order starts. That is, a mode hop from the resonator mode 20 to the resonator mode 26 occurs. When the temperature of the external cavity laser decreases, a mode hop occurs through the reverse process.

  The light intensity of the resonator mode 20 oscillating before the mode hop and the light intensity of the resonator mode 26 after the mode hop usually do not match. For this reason, when a mode hop occurs, the output light intensity of the external cavity laser 2 varies. This fluctuation of the output light intensity generates noise in the output light of the external resonator type laser 2.

  Noise caused by mode hop has a large noise component, that is, RIN (relative intensity noise) on the lower frequency side than the bit rate (several hundred MHz to several tens GHz) used for optical communication.

  Similarly, the optical signal used for communication also has many frequency components on the lower frequency side than the bit rate. Moreover, the signal component on the lower frequency side than the bit rate is important for the detection of the optical signal. Therefore, it is difficult to remove only noise caused by mode hopping (hereinafter referred to as mode hopping noise) by signal processing after photoelectric conversion (such as noise removal by a filter circuit). For this reason, when mode hop noise occurs, the error rate (code error rate) of the transmission signal increases.

  Therefore, it is important to suppress mode hop noise in the external cavity laser.

  As a countermeasure for this, it is conceivable that the temperature of the external cavity laser 2 is precisely controlled so that mode hops do not occur. However, such a measure detracts from the advantages of an external cavity laser that does not require precise temperature control.

  In addition, the temperature at which mode hop occurs varies from device to device. Therefore, the temperature at which the mode hop occurs must be specified for each element. Such an operation is extremely complicated and causes high costs.

  Therefore, in the optical transmission apparatus according to the present embodiment, laser oscillation occurs simultaneously in a plurality of resonator modes so that the intensity fluctuation of the entire output light is reduced even if a mode hop occurs.

  FIG. 4 is a diagram schematically illustrating the relationship between the reflectance 27 of the diffraction grating forming the laser beam generation apparatus (external resonator type laser) of this embodiment and the resonator mode 28 during laser oscillation. . The horizontal axis and the vertical axis are the same as those in FIG.

  As shown in FIG. 4, the reflectance 27 of the diffraction grating used in the present embodiment is substantially flat in the vicinity of the maximum value. For this reason, the reflectance is substantially constant for a plurality of resonator modes. Therefore, laser oscillation occurs simultaneously in a plurality of resonator modes.

  In the example shown in FIG. 4, laser oscillation occurs simultaneously in the six resonator modes 28. Moreover, the light intensities of the laser beams oscillating in the respective resonator modes are substantially equal. For example, six resonator modes simultaneously oscillate at a light intensity within 3 dB from the light intensity of the resonator mode oscillating at the maximum emission intensity.

  When the element temperature rises (or falls) in such a state, the resonator mode located on the longest wavelength side (or the shortest wavelength side) among the resonator modes 28 that are oscillating laser is at a certain temperature. The laser oscillation is stopped, and a resonator mode for laser oscillation appears on the short wavelength side (or long wavelength side) instead. That is, a mode hop occurs in which one of a plurality of resonator modes that are lasing is replaced with one of the resonator modes that are not lasing.

  In such a mode hop, even if there is a difference in laser oscillation intensity between the changing resonator modes, the change in the light intensity of the entire laser light is slight. On the other hand, when mode hopping occurs in a conventional external cavity laser that oscillates in a single cavity mode, the laser light intensity varies greatly.

  That is, the mode hop noise generated by the laser light generation apparatus of the present embodiment is smaller than the mode hop noise generated by an external resonator type laser that oscillates in a single resonator mode.

  By the way, as shown in FIG. 4, when laser light (optical signal) including a plurality of resonator modes is input to the photodetector on the receiving side, a frequency corresponding to the resonator mode interval δλ (hereinafter referred to as resonator mode interval δf). The frequency component that is vibrated by the equation (2) described later) is generated in the electrical signal output from the photodetector.

  As is well known, the output of the photodetector is proportional to the square of the electric field strength of the input light. For this reason, when two optical signals having different wavelengths are incident on the photodetector, a frequency difference between the two optical signals (= light speed / λ), that is, beat noise that vibrates at the difference frequency is generated.

  The frequency of beat noise of the DFB laser is usually several hundred GHz. This frequency greatly exceeds the frequency band of a receiver (a circuit formed by a photodetector and an amplifier) that photoelectrically converts an optical signal. Therefore, the beat noise of the DFB laser is not detected.

  However, in the case of an external resonator type laser, the resonator mode interval δf may be less than or equal to the frequency corresponding to the bit rate of the optical signal (hereinafter referred to as the modulation frequency). The frequency band of the receiver is naturally higher than the modulation frequency of the optical signal. Therefore, in such a case, beat noise appears at the output of the receiver.

  The frequency corresponding to the bit rate is a frequency having the same numerical value as the bit rate. For example, the frequency corresponding to a bit rate of 10 Gbit / s is 10 GHz.

  For example, in the example described in Non-Patent Document 1, the resonator mode interval is 8 GHz. Therefore, when optical transmission is performed at a bit rate of 8 Gbit / s or more, the modulation frequency of the optical signal exceeds the resonator mode interval δf.

  In such a case, the beat signal is mixed into the received signal (output of the optical receiver). On the other hand, the received signal includes a frequency component equal to or lower than the modulation frequency. Moreover, such frequency components are important for information transmission. Therefore, it is not possible to remove only beat noise from the received signal by a filter circuit or the like.

  Therefore, when high-speed optical transmission of, for example, 10 Gbit / s or more is executed using the laser light generation apparatus described with reference to FIG. 4 as a light source, beat noise occurs in the received signal, and error rate (code error rate) is It will increase.

  Therefore, in this embodiment, the length of the optical resonator 5 formed by the reflecting mirror 4 and the diffraction grating 16 is shortened so that the resonator mode interval δf is higher than the modulation frequency of the optical signal. In this way, beat noise can be removed from the received signal by a filter circuit or the like, so that an increase in error rate can be prevented.

Incidentally, the resonator mode interval δf is higher than the modulation frequency f _m of the optical signal preferred. For example, the resonator mode interval δf is preferably higher than twice the modulation frequency f _m of the optical signal, further preferably higher than 3-fold, and most preferably greater than four times.

However, if the resonator mode interval δf becomes too wide, a plurality of resonator modes cannot exist in the high reflection band of the diffraction grating. Therefore, the resonator mode interval δf is, for example, the modulation frequency f _m (for example, 10 GHz). 10 times (for example, 100 GHz) or less, more preferably 8 times or less, and most preferably 6 times or less.

  FIG. 5 is a perspective view illustrating a main part of the optical transmission device 30 according to the present embodiment. Fig.6 (a) is the figure which looked at the cross section along the AA of FIG. 5 from the direction of the arrow. FIG. 6B illustrates a cross section perpendicular to the AA line.

  The optical transmission device 30 of the present embodiment includes a laser light generation device 32 and an electric signal source 34.

(1) Laser Light Generation Device (i) Optical Amplification Unit This laser light generation device 32 is a gain waveguide in which a semiconductor layer (active layer 36) that generates optical gain by an injected current is a first core layer 37. , And an optical amplification unit 44 having a light reflection surface 42 formed on the first end face 40 of the gain waveguide 38 (see FIGS. 5 and 6).

  Here, the optical amplifying unit 44 is an optical amplifying element in which one of antireflection films applied to both end faces of a semiconductor optical amplifier (SOA) is replaced with a high light reflecting film.

  For example, as shown in FIG. 6B, the optical amplification unit 44 is a semiconductor laminate in which a lower clad layer 48, an active layer 36, an upper clad layer 50, and an electrode layer 52 are sequentially laminated on a semiconductor substrate 46. It has a structure.

  Here, the semiconductor substrate 46 is made of, for example, n-type InP. The lower cladding layer 48 is made of n-type InP. The active layer 36 is formed of a multiple quantum well made of InGaAsP (or AlGaInAs). The upper cladding layer 50 is made of p-type InP. The electrode layer 52 is made of p-type InGaAsP.

  The optical amplifying unit 44 has a current blocking layer 58 in which p-type InP 54 and n-type InP 56 are stacked so as to surround both sides of the linear active layer 36. This current blocking layer 58 also functions as a cladding layer surrounding both sides of the active layer 36.

  These semiconductor layers, that is, the lower cladding layer 48, the active layer 36, the upper cladding 50, and the current blocking layer 58 form the gain waveguide 38.

  The optical amplification unit 44 includes an n electrode 60 formed on the back surface of the substrate 46 and a p electrode 62 formed on the electrode layer 52.

  A high-reflection (HR) film that forms the light reflecting surface 42 is formed on the first end face 40 of the gain waveguide 38. In addition, an anti-reflection (AR) film 68 is formed on the second end face 66 of the gain waveguide 38. Note that the reflectance of the highly reflective film is, for example, 99% to 100%. The reflectance of the antireflection film is, for example, 0% to 1%.

  With this configuration, the optical amplification unit 44 amplifies the light incident from the second end face 66 by the gain waveguide 38. Next, the light amplification unit 44 reflects substantially all of the amplified incident light by the light reflection surface 42 (high reflection film 64). Next, the light amplification unit 44 amplifies the light reflected by the light reflection surface 42 again by the gain waveguide 38 and emits the light from the second end surface. That is, the optical amplification unit 44 has a function of amplifying the light incident from the second end face 66 and emitting the light from the second end face 66 again.

  In the present embodiment, the optical gain of the optical amplifying unit 44 becomes the highest in the vicinity of 1.3 μm. The length of the light amplification unit 44 along the light guiding direction is 400 μm.

(Ii) Optical Waveguide Unit The laser light generator 32 further has an optical waveguide unit 80 including an optical waveguide 78 (see FIG. 5).

  FIG. 7 is a view for explaining a cross section of the optical waveguide unit 80. FIG. 7A is a view of a cross section along the optical axis of the optical waveguide 78 as viewed in a region 88 provided with the diffraction grating 70. On the other hand, FIG. 7B is a view of a cross section perpendicular to the optical axis of the optical waveguide 78 as seen in a region 82 where the diffraction grating 70 is not provided.

  As shown in FIGS. 5 and 7, the optical waveguide unit 80 includes an optical waveguide 78 including a second core layer 74 provided with a diffraction grating 70. Here, the second core layer 74 forms an optical waveguide 78 together with a clad layer 75 surrounding the second core layer 74. In FIG. 5, a reference numeral (78) representing the optical waveguide is attached to the second core layer forming the center of the optical waveguide so that the drawing is not complicated.

  Further, as shown in FIG. 6A, the optical waveguide 78 is formed with a third end face 76 that faces the second end face 66 of the optical amplification unit 44. The optical waveguide 78 is optically coupled to the optical amplification unit 44 at the third end face 76. Here, the optical axes of the gain waveguide 38 and the optical waveguide 78 of the optical amplification unit 44 are adjusted so as to substantially coincide.

  Further, as shown in FIG. 5, the optical waveguide 78 is optically coupled to the optical fiber 17 at a fourth end face 84 provided on the side opposite to the third end face 76. The optical signal generated by the optical transmitter 30 is output from the fourth end face 84 and sent to the optical fiber 17.

  In the example shown in FIG. 5, no diffraction grating is formed in the optical waveguide 78 in the region 82 between the diffraction grating 70 and the third end face 76. As described above, the diffraction grating 70 and the third end face 76 may be separated from each other, but the diffraction grating 70 may be in contact with the third end face 76.

  Next, a specific configuration and characteristics of the optical waveguide unit 80 will be described in detail.

As shown in FIG. 7, the optical waveguide unit 80 includes a Si substrate 85, a SiO ₂ dielectric layer 86 formed on the substrate, and a SiO ₁ embedded in the periphery of the dielectric layer 86. _-x N _x (x is 0.55 to 0.65) has made of the dielectric layer 72 (see FIG. 7).

Here, the refractive index of the dielectric layer 72 made of SiO _1-x N _x is 1.76, and the refractive index of the dielectric layer 86 made of SiO ₂ embedding the dielectric layer 72 is 1.45. Therefore, the dielectric layer 72 having a high refractive index functions as a core layer, and the dielectric layer 86 having a low refractive index functions as a cladding layer. The dielectric layer 72 and the dielectric layer 86 form an optical waveguide 78 having an equivalent refractive index of 1.54.

  The dielectric layer 72 (second core layer 74) is a flat plate member having a width of 1.2 μm and extends from the third end surface 76 to the fourth end surface 84.

  As shown in FIG. 7B, the dielectric layer 72 has a substantially rectangular cross-sectional structure with a width of 1.2 μm and a height of 0.4 μm. The dielectric layer 72 is divided into a first region 82 in which nothing is formed on the upper surface and a second region 88 in which the diffraction grating 70 is formed on the upper surface.

  The first region 82 extends from the third end surface 76 to the second region 88 by 377 μm.

  The second region 88 extends from the end of the first region 82 to the fourth end face 84 by about 155 μm. In the second region 88, grooves having a depth of 32 nm are periodically formed on the upper surface of the dielectric layer 72 to form the diffraction grating 70. The pitch of the grooves (the pitch of the diffraction grating 70) is about 421 nm. The ratio of the portion where the groove is formed is 50%.

  Here, the coupling coefficient of the diffraction grating 70 is about 125 / cm. The Bragg wavelength of the diffraction grating 70 is 1.30 μm (= 2 × 1.54 × 421 nm).

  In FIG. 5, the width of the optical waveguide 78 is drawn so as to periodically change in the second region 88. However, in practice, the width of the optical waveguide 78 is constant, and its height is periodic. Has changed.

  Next, on the side opposite to the optical fiber 17, the cladding layer (dielectric layer 86) and the core layer (dielectric layer 72) that form the optical waveguide 78 are removed, and a terrace is formed on the Si substrate 85. (See FIGS. 5 and 6). A metal pad 90 is formed on the terrace. The optical amplification unit 44 is placed on the pad 90, and the n-electrode 60 of the optical amplification unit 44 is electrically connected to the pad 90. Here, the optical amplification unit 44 is soldered to the pad 90.

  As described above, in the present laser light generation device 32, the optical amplification unit 44 and the optical waveguide 78 are hybridly mounted on the Si substrate 85.

  A 10 μm gap 96 is provided between the second end face 66 of the optical amplification unit 44 and the optical waveguide 78 (see FIG. 6A). Further, the optical amplifying unit 44 is placed on the pad 90 so that the optical axis of the gain waveguide (active layer) 38 is slightly inclined (several degrees) with respect to the normal of the third end face 76 of the optical waveguide unit 80. It is placed. Accordingly, the light that is emitted from the optical amplification unit 44 and is not incident on the optical waveguide 78 and is reflected by the third end face 76 hardly enters the optical amplification unit 44 again. That is, there is almost no return light to the optical amplification unit 44.

  By the way, a diffraction grating in which the refractive index of the core layer of the optical waveguide is periodically changed like the diffraction grating 70 is called a distributed Bragg reflector (DBR). A distributed Bragg reflector (DBR) can have an effective reflection end defined therein. It is known that the reflected light from the distributed Bragg reflector can be handled as being reflected by this effective reflection end.

  Therefore, the light reflecting surface 42 and the diffraction grating 70 of the optical amplification unit 44 form an optical resonator 94. In the optical resonator 94, a resonator mode corresponding to the optical length between the effective reflection end 92 and the light reflecting surface 42 (of the optical amplification unit 44) is generated. On the other hand, the DBR strongly reflects light within a specific wavelength range (reflection band) centered on the Bragg wavelength. For this reason, the laser light generation device 32 in which one end of the optical resonator 94 is formed by DBR having such wavelength selectivity performs laser oscillation in the resonator mode within the reflection band of the DBR.

  FIG. 8 is a diagram for explaining the relationship between the reflectance of the diffraction grating 70 and the wavelength.

  The vertical axis in FIG. 8 is the reflectance, and the horizontal axis is the wavelength. On the other hand, the vertical axis is a logarithmic display.

  As shown in FIG. 8, the reflectance of the diffraction grating 70 becomes maximum at a Bragg wavelength of 1.3 μm. The reflectance at this wavelength, that is, the maximum value of the reflectance is 90%.

  The width of the wavelength range 98 (referred to as a high reflection band) in which the reflectance exists within 0.1 dB (within a reflectance of 89.8%) from the maximum value is 2 nm. As a result of various studies by the present inventor, the resonator modes included in such a high reflection band simultaneously oscillate and oscillate at an emission intensity within 3 dB of the emission intensity of the resonator mode in which laser oscillation is the strongest.

  By the way, the interval between the resonator modes of the optical resonator can be expressed by the following equation.

ここで、式（１）は、共振器モード間隔δλを波長で表示した式である。一方、式（２）は、共振器モード間隔δｆを周波数で表示した式である。

Here, the expression (1) is an expression in which the resonator mode interval δλ is expressed by a wavelength. On the other hand, Expression (2) is an expression in which the resonator mode interval δf is expressed by frequency.

λ _c is the center wavelength. c is the speed of light. i is a subscript for distinguishing regions having different refractive indexes included in the optical resonator 94. For example, i = 1 represents the gain waveguide 38 and i = 2 represents the gap 96. L _i and N _i are the length and effective refractive index of each region.

Here, the effective refractive index N _i can be represented by the following formula (3).

尚、ｎ_iは夫々の領域の等価屈折率である。また、λは波長である。

Note that n _i is the equivalent refractive index of each region. Λ is a wavelength.

  Here, the equivalent refractive indexes of the gain waveguide 38 (optical amplification unit 44) and the optical waveguide 78 (optical waveguide unit 80) are 3.3 and 1.54, respectively. By substituting these values and the lengths of the above-described regions into the formula (1), a resonator mode interval of 0.393 nm is obtained.

  In this embodiment, the effective reflection end 92 of the diffraction grating 70 is located at the end of the diffraction grating 70 on the optical fiber 17 side. The effective refractive index N is assumed to be equal to the equivalent refractive index. This also applies to the following examples unless otherwise specified.

  Accordingly, there are at least five (= 2 nm / 0.393 nm) resonator modes in the wavelength range. Each resonator mode simultaneously oscillates and oscillates at a light intensity within 3 dB of the light intensity of the resonator mode that emits the strongest light.

  In other words, the laser light generating device 32 oscillates in a plurality of resonator modes of the optical resonator 94 formed by the light reflecting surface 42 and the diffraction grating 70.

  Since the diffraction grating 70 is formed of a dielectric having a small temperature change rate of the refractive index, the center wavelength of the signal light of the optical transmission device 30 hardly changes even if the environmental temperature changes. Therefore, temperature control is not performed in the optical transmission apparatus of this embodiment. That is, this optical transmission device does not require a temperature control device. Alternatively, even if the temperature control is performed by the present optical transmission device, simple temperature control is sufficient.

(2) Electric signal source The electric signal source 34 is electrically connected to the optical amplification unit 44. Specifically, the positive electrode of the electrical signal source 34 is connected to the p-electrode 62 of the optical amplification unit 44. On the other hand, the negative electrode of the electrical signal source 34 is connected to a pad 90 to which the n-electrode 60 of the optical amplification unit 44 is soldered.

  The electrical signal source 34 applies an electrical signal corresponding to information to be transmitted to the optical amplification unit 44 and injects a current corresponding to the information into the semiconductor layer (active layer 36). The injected current generates an optical gain in the active layer.

FIG. 9 is a diagram illustrating an example of the electrical signal 100 that the electrical signal source 34 applies to the optical amplification unit 44. In the upper left of FIG. 9, a diagram for explaining the relationship between the current (injection current) injected into the active layer 36 of the optical amplification unit 44 and the optical output of the laser light generation device 32 is shown. The vertical axis represents light output (light intensity of output light), and the horizontal axis represents injection current. Here, I _th on the vertical axis represents the threshold current of the laser beam generator 32. On the other hand, I _b represents a bias current superimposed on the electric signal 100.

  In the lower half of FIG. 9, a diagram for explaining a time change of the electric signal 100 applied by the electric signal source 34 to the optical amplification unit 44 is shown. In this figure, the time change of the electric signal 100 is illustrated so that time passes from the top to the bottom. The horizontal position of the electrical signal 100 is opposed to the horizontal axis (injection current) in the upper left diagram. The bit string (... 101...) Shown in the figure represents the original signal of the electrical signal 100. The electrical signal 100 is an RZ (retuen to zero) signal.

  In the upper right of FIG. 9, a diagram for explaining the temporal change of the optical output (optical signal 102) of the laser light generation device 32 is shown. In this figure, the time change of the optical output 102 is illustrated so that time passes from the left to the right. The vertical position of the light output 102 corresponds to the vertical axis (light output) in the upper left diagram.

As shown in the lower half of FIG. 9, a bias current I _b slightly larger than the threshold current I _th of the laser light generation device 32 is superimposed on the current injected by the electric signal source 34 into the active layer. In the first half of the period in which the intensity of the electric signal 100 corresponds to the original signal “1”, the electric signal source 34 injects a current I _b + I _p into the active layer 36 of the optical amplification unit 44. On the other hand, in the second half of the period in which the intensity of the electric signal 100 corresponds to the original signal “1”, the electric signal source 34 injects a current I _b into the active layer 36 of the optical amplification unit 44. Further, the electric signal source 34 injects the current I _b into the active layer 36 during the entire period in which the intensity of the electric signal 100 corresponds to the original signal “0”.

  Then, as shown in the upper right diagram of FIG. 9, the laser light generation device 32 has a first state 104 with a large light output (light emission intensity) and a light emission intensity lower than that in the first state in accordance with the electric signal 100. The light signal 102 is generated by reciprocating between the two states 106.

Note that the bias current I _b may be equal to or less than the threshold current. Alternatively, the bias current I _b may not be superimposed on the electric signal 100. In these cases, no laser oscillation occurs in the second state 106.

  The bit rate of the electrical signal 100 generated by the electrical signal source 34 of this embodiment is 10 Gbit / s. On the other hand, the resonator mode interval δf of the laser light generation device 32 is 70 GHz.

  That is, the frequency (70 GHz) corresponding to the resonator mode interval of the laser light generation device 32 is higher than the frequency (10 GHz) corresponding to the bit rate (10 Gbit / s) of the electrical signal 100.

  By the way, when the electrical signal 100 is a non-retuen to zero (NRZ) signal, even if the electrical signal 100 is most frequently turned on and off, the frequency (for example, 5 GHz) of the electrical signal 100 is a bit rate (for example, 10 GHz). ) Is only 1/2. In this case, since the resonator mode interval δf is twice or more the frequency of the electric signal 100, the beat noise becomes smaller.

(3) Operation First, the electric signal source 34 injects a current (I _b + I _p ) into the active layer. When current is injected, the active layer 36 generates optical gain. At this time, the active layer 36 simultaneously generates light (spontaneous emission light). This light is emitted from the second end surface 66 on which the antireflection film 68 having a reflectance of approximately 0% is formed, propagates through the gap 96, and then enters the optical waveguide 78 (see FIG. 6A). .

  The light incident on the optical waveguide 78 propagates through the optical waveguide 78 and enters the diffraction grating 70 (see FIG. 5). The light incident on the diffraction grating 70 is reflected by the diffraction grating 70 in accordance with the reflectance characteristics described with reference to FIG. 8, travels back through the optical waveguide 78, and reenters the active layer 36.

  The light re-entering the active layer 36 is reflected by the light reflecting surface 42 (high reflection film 64) having a reflectance of approximately 100%, and reverses the traveling direction and enters the optical waveguide 78 again (FIG. 6A). reference). During this time, light is amplified by the active layer 36.

  Thereafter, the light reciprocates many times in the optical resonator 94 and grows into laser light. However, light that can grow into laser light is only light (resonator mode) in which the phase change during one round trip of the optical resonator 94 is equal to an integer of 2π. Further, of such light, only light having a sufficiently high reflectivity at the diffraction grating 70 becomes laser light because the gain received from the active layer 36 exceeds the absorption loss or mirror loss of the active layer 36.

  For example, in the laser light generation device 32 of the present embodiment, as described above, five resonator modes oscillate simultaneously within the high reflection band 98 having a central wavelength of 1.3 μm (see FIG. 8). Moreover, since there is almost no difference in reflectance within the high reflection band 98, the light intensity ratio of the laser light emitted in these five resonator modes is within 3 dB (0 dB to −3 dB) (here, “The laser light intensity ratio” is the ratio of the light emission intensity of each resonator mode to the light emission intensity of the resonator mode emitting the strongest light. FIG. 8 shows a wavelength range 152 exhibiting a reflectance within 5 dB from the maximum reflectance. Hereinafter, the term “reflection band” shall mean such a wavelength range.

Next, it is assumed that the electric signal source 34 injects the current _Ib into the active layer. Also in this case, in the same manner as when the current (I _b + I _p ) is injected into the active layer, the laser light generating device 32
Five resonator modes simultaneously oscillate inside the high reflection band 98 (provided that I _b > I _th ). In addition, the light intensity ratio of laser light oscillating in these five resonator modes is also within 3 dB. However, the light emission intensity at this time (second state 106) is lower than that in the first state (the state where I _b + I _p is injected into the active layer; the first state 104) (see FIG. 9). The second state 106 may be a non-lasing state of the current injected into the active layer is less than the threshold current I _b.

  As is well known, a Fabry-Perot laser that oscillates in a plurality of resonator modes oscillates in more resonator modes as the injection current decreases. Similarly to the Fabry-Perot laser, the laser light generation device 32 of the present embodiment also tends to oscillate in more resonator modes as the injection current decreases. Therefore, the second state 106 with a small injection current is a state in which oscillation is more likely in more resonator modes than the first state 104. Therefore, in this embodiment in which laser oscillation is performed in five resonator modes in the first state 104, at least five resonator modes (and light intensity within 3 dB in the second state 106). Ratio).

  As described above, the laser beam generating apparatus 32 oscillates in a plurality of resonator modes of the optical resonator 94 formed by the light reflecting surface 42 and the diffraction grating 70.

  Further, according to the electrical signal 100, the laser light generating device 32 has a first state in which there is another resonator mode that emits light (laser oscillation) with a light intensity within 3 dB of the resonator mode that emits the strongest light. The optical signal 104 is generated by reciprocating between the second states where the emission intensity is lower than that in the first state.

  Therefore, in the present optical transmission device 30, only one of the plurality of resonator modes that are performing laser oscillation causes mode hopping, so that mode hop noise is small. Furthermore, since the change in the light emission intensity of the resonator mode (resonator mode before and after the mode hop) related to the mode hop is small, the mode hop noise becomes smaller.

  Further, as described in “(2) Electric signal source” above, since the resonator mode interval δf (70 GHz) is higher than the modulation frequency (10 GHz) of the electric signal 100, beat noise based on the difference frequency between the resonator modes. However, it does not increase the error rate of the optical signal.

  Furthermore, in the present embodiment, the diffraction grating 70 and the cladding layer 75 surrounding the diffraction grating are formed of a dielectric whose temperature change in refractive index is about one digit smaller than that of a semiconductor. Accordingly, the Bragg wavelength of the diffraction grating 70 hardly changes even if the element temperature changes. Therefore, the oscillation wavelength of the laser light generation device 32 hardly changes even if the temperature of the environment where the optical transmission device 30 is installed (environment temperature) changes. Therefore, the optical transmission device 30 does not require precise temperature control using a thermoelectric conversion element having a sufficient cooling capacity, a complicated temperature control circuit, or the like.

  Note that when the temperature of the laser light generation device 32 changes significantly, the number of resonator modes oscillating within the high reflection band, the light intensity ratio, and the like may change. In this case, it is only necessary that the above condition (a plurality of resonator modes having a light emission intensity ratio of 3 dB or less simultaneously oscillate) within the operating temperature range of the optical transmitter 30 is satisfied.

  Further, in the above-described example, the emission intensity ratio (laser light intensity ratio) of the resonator mode in which laser oscillation is simultaneously performed is limited to 3 dB. The reason is that the smaller the emission intensity ratio, the greater the effect of suppressing mode hop noise. Therefore, the smaller the emission intensity ratio, the better. For example, the emission intensity ratio in the resonator mode is preferably within 3 dB, more preferably within 2 dB, and most preferably within 1 dB. However, even if such a restriction is not provided, the present optical transmission device 30 exhibits the effect of reducing mode hop noise.

  As described above, the number of resonator modes that simultaneously perform laser oscillation may be plural. However, in order to effectively suppress the mode hop noise, the larger the number of resonator modes, the better. For example, the number of resonator modes that simultaneously oscillate is preferably 3 or more, more preferably 5 or more, and most preferably 10 or more.

  However, if the number of resonator modes that cause laser oscillation simultaneously increases, the spectral width of the output light of the laser light generation device 32 becomes too wide. If such an optical transmission device is used for WDM, the wavelength width per wavelength channel becomes too wide, and the number of wavelength channels that can be set decreases. Therefore, the number of resonator modes in which laser oscillation occurs simultaneously is preferably, for example, 35 (= resonator mode interval δf / modulation frequency) = 355 GHz / 10 GHz) or less. The number of resonator modes is more preferably 30 or less, and most preferably 20 or less.

(4) Manufacturing Method First, a 3 μm thick SiO ₂ layer is deposited on the Si substrate 85 by chemical vapor deposition.

Next, a SiO _1-x N _x (x is 0.55 to 0.65) film having a thickness of 0.4 μm is deposited on the SiO ₂ layer by PCVD (Plasma Chemical Vapor Deposition).

Next, the SiO _1-x N _x film is processed into an elongated flat plate having a width of 1.2 μm by photolithography and RIE (Reactive Ion Etching). The elongated flat plate eventually becomes the second core layer 74.

Next, a groove that repeats with a period of 421 nm is formed at a position where the diffraction grating 70 of the SiO _1-x N _x film is to be formed by the electron beam exposure technique and the RIE method. The depth of the groove is 32 nm, and the ratio (duty) of the portion where the groove is not formed is 50%. In this way, grooves are periodically formed in the SiO _1-x N _x film to form the diffraction grating 70.

Next, SiO ₂ having a thickness of 3 μm is further deposited by CVD on the SiO _1-x N _x film and the SiO ₂ layer on which the diffraction grating 70 is formed.

Next, the deposited SiO ₂ layer and SiO _1-x N _x layer are etched at the planned mounting position of the optical amplification unit 44 to expose the surface of the Si substrate 85. The exposed portion of the Si substrate 85 is a terrace.

  Next, a metal pad 90 is formed on the terrace by vapor deposition and lift-off of a metal film.

  Finally, the optical amplification unit 44 is soldered to the pad 90. At this time, the optical axis of the gain waveguide (active layer) 38 of the optical amplification unit 44 is inclined slightly (several degrees) with respect to the normal of the third end face 76 of the optical waveguide unit 80.

  In the optical transmission device 107 of this embodiment, the upper half of the dielectric layer that embeds the second core layer 74 of the optical waveguide unit 108 is formed of an ultraviolet curable epoxy resin (hereinafter referred to as an epoxy resin) ( (See FIG. 5). Other configurations and operations are substantially the same as those of the optical transmission device 30 according to the first embodiment. Therefore, the description regarding a common part is abbreviate | omitted.

  FIG. 10 is a diagram for explaining a cross section of the optical waveguide unit 108 of the present embodiment. FIG. 10A is a view of a cross section along the optical axis of the optical waveguide 111 as seen in a region 88 where the diffraction grating 70 is provided. On the other hand, FIG. 10B is a view of a cross section perpendicular to the optical axis of the optical waveguide 108 as viewed in a region 82 where the diffraction grating 70 is not provided.

In this embodiment, a second core layer 74 having a thickness of 0.4 μm and a width of 1.2 μm is formed on the first SiO ₂ layer 110 having a thickness of 3 μm laminated on the Si substrate 85. Is formed. Furthermore, the second core layer 74 and the first SiO ₂ layer 110 are covered with a _second SiO ₂ layer 112 having a thickness of 0.1 μm.

An epoxy resin 114 having a thickness of 1.5 μm or more is laminated on the second SiO ₂ layer 112. In other words, in this embodiment, the cladding layer surrounding the second core layer 74 is formed of the first SiO ₂ layer 110, the second SiO ₂ layer 112, and the epoxy resin 114.

The refractive index of the epoxy resin 114 used in this embodiment is 1.45, which is the same as that of SiO ₂ . The polymer material such as the epoxy resin 114 is a material whose refractive index decreases (temperature change coefficient of the refractive index becomes negative) with increasing temperature. On the other hand, a dielectric such as SiO ₂ is a material whose refractive index increases with increasing temperature (the temperature change coefficient of the refractive index becomes positive).

  That is, in the present embodiment, a part (upper half) of the cladding layer surrounding the second core layer 74 is formed of a material (epoxy resin) whose refractive index decreases with increasing temperature.

As described above, the refractive indexes of the epoxy resin 114 and SiO ₂ are substantially the same. Further, the structure of the second core layer 74 including the structure of the diffraction grating 70 (pitch, groove depth, and duty) is the same as that of the second core layer of the first embodiment.

  Accordingly, the optical characteristics of the waveguide unit 108 of the present embodiment are the same as the optical characteristics of the waveguide unit 80 of the first embodiment. For example, the wavelength dependency (reflectance characteristic) of the reflectance of the diffraction grating 70 is the same as that of the diffraction grating of Example 1 described with reference to FIG. The resonator mode interval is also 0.39 nm, which is substantially the same as in the first embodiment. Accordingly, the laser beam generator 109 oscillates in the five resonator modes belonging to the high reflection band 98 as in the first embodiment. The light intensity ratio of laser light emitted in these five resonator modes is also within 3 dB as in the first embodiment. Further, the resonator mode interval δf (70 GHz) of the laser light generation device 109 is also higher than the modulation frequency (10 GHz) of the electric signal 100 as in the first embodiment.

  Therefore, the optical transmission device 107 according to the present embodiment exhibits the same effect as the optical transmission device 30 according to the first embodiment. In addition, the temperature change rate of the wavelength of the signal light transmitted from the optical transmission device 107 is one digit greater than the temperature change rate of the wavelength of the signal light transmitted from the optical transmission device 30 of the first embodiment, as will be described below. About small.

The lower half of the second core layer 74 and the cladding layer is formed of a dielectric. The refractive index n of the dielectric increases with increasing temperature. The temperature coefficient dn / dT (degree ⁻¹ ) of the refractive index n of the dielectric is about +10 ⁻⁵ .

On the other hand, the refractive index n of a polymer material (polymer) such as an epoxy resin decreases as the temperature increases. For example, the temperature coefficient dn / dT (degree ⁻¹ ) of the refractive index n of the epoxy resin is about −10 ⁻⁴ .

  By the way, in the optical waveguide 111 of this embodiment, most of the guided light is confined in the second core layer 74, and the ratio of the guided light leaking to the upper half of the epoxy resin clad layer is about 10%.

Therefore, the refractive index change of the dielectric body forming the lower half of the second core layer 74 and the cladding layer (+10 about ^-5), the refractive index change of the epoxy resin forming the upper half of the clad layer (-10 ^- since about ⁴⁾ cancels, the temperature change of the equivalent refractive index of the optical waveguide 111 is approximately one digit than that of example 1 becomes smaller.

  Therefore, the temperature change of the Bragg wavelength of the diffraction grating 70 is also about one order of magnitude smaller than that of the first embodiment. Therefore, in this embodiment, the oscillation wavelength of the laser beam (wavelength of the optical signal) generated by the laser beam generation device 109 changes with temperature from the oscillation wavelength (wavelength of the optical signal) of the laser beam generation device 32 of the first embodiment. The rate is reduced by about an order of magnitude.

  The manufacturing method of the optical waveguide unit 108 of the present embodiment is as follows.

First, similarly to the method of manufacturing the optical waveguide unit 80 of the first embodiment, the second core in which the lower clad layer made of SiO ₂ (the lower half of the clad layer) and the diffraction grating 70 are provided on the Si substrate 85. Layer 74 is formed.

  Next, an ultraviolet curable epoxy resin is applied on the lower cladding layer and the second core layer 74. Next, heat treatment of the epoxy resin for vaporizing the solvent and ultraviolet irradiation to the epoxy resin are performed. The epoxy resin is cured by this ultraviolet irradiation.

  Thereafter, similar to the manufacturing method of the first embodiment, the surface of the Si substrate 85 is exposed and the pad 90 is formed on the exposed surface (terrace). Further, the optical amplification unit 44 is soldered to the pad 90, and the optical waveguide unit 108 is completed.

  In this embodiment, an ultraviolet curable epoxy resin is used as the polymer material (polymer), but other polymer materials such as an ultraviolet curable acrylate, a fluorinated polyimide, a benzocyclobutene, and a deuterated silicone are used. Deuterated PMMA (polymethyl methacrylate) or the like may be used.

  Further, not only the upper half, but the entire cladding layer may be formed of a polymer material (polymer).

  In the optical transmission device 115 of the present embodiment, the upper half of the clad layer of the optical waveguide unit is formed of a polymer (epoxy resin or the like) as in the second embodiment. Furthermore, in this embodiment, the second core layer of the optical waveguide unit 116 is formed of silicon (Si). Other configurations are substantially the same as those of the optical transmission apparatus according to the first embodiment. Therefore, the description regarding the part common to Example 1 is not performed.

  FIG. 11 is a diagram illustrating a cross section of the optical waveguide unit 116 of the present embodiment. FIG. 11A is a view of a cross section along the optical axis of the optical waveguide 122 as viewed in a region 88 where the diffraction grating 118 is provided. On the other hand, FIG. 11B is a view of a cross section perpendicular to the optical axis of the optical waveguide 122 as seen in the region 88 where the diffraction grating 118 is provided.

In the present embodiment, a first SiO ₂ layer 110 having a thickness of 3 μm is laminated on a Si substrate 85, and a second core layer 117 made of Si is further formed thereon. Note that the refractive index of Si is 3.44.

  In the region 88 where the diffraction grating 118 is formed, the width and thickness of the second core layer 117 are 0.28 μm and 0.24 μm, respectively. On the second core layer 117, SiN films are formed with a period (pitch) of 287 nm. The thickness of the SiN film is 15 nm, and the width is 0.28 μm, which is the same as that of the second core layer 117. On the other hand, the length of the SiN film 120 in the optical axis direction is 144 nm, which is half of the formation period of the SiN film.

  On the other hand, in the region 82 where the diffraction grating 118 is not formed, the width of the second core layer 117 is 0.4 to 0.5 μm. On the other hand, the thickness of the second core layer 117 is 0.24 μm, which is the same as the region 88 where the diffraction grating 118 is formed.

  The length of the region 82 where the diffraction grating 118 is not formed is 215 μm. On the other hand, the length of the region 88 where the diffraction grating 118 is formed is 155 μm, which is the same as in the first embodiment.

Then, an epoxy resin 114 having a thickness of 1.5 μm or more is laminated on the first SiO ₂ layer 110 and the second core layer 117 as in the second embodiment.

Thus, the first SiO ₂ layer 110 and the epoxy resin 114 form a clad layer surrounding the second core layer 117.

  In this embodiment, the second core layer 117 is formed of a semiconductor (Si) whose refractive index increases as the temperature rises. Further, the upper half of the clad layer 117 surrounding the second core layer is formed of a material (epoxy resin) whose refractive index decreases as the temperature rises.

  Incidentally, the average value of the equivalent refractive index of the region 88 where the diffraction grating 118 is formed is 2.26. Therefore, the black wavelength of the diffraction grating 118 is 1.3 μm (= 2 × 2.26 × 287 nm), which is the same as that in the first embodiment. Further, the coupling coefficient κ of the diffraction grating 118 is also substantially the same as that of the first embodiment, which is 124 / cm.

As described above, the Bragg wavelength, the coupling coefficient κ, and the length L _B of the diffraction grating 118 of the diffraction grating 118 of this embodiment are substantially the same as those of the first embodiment. For this reason, the characteristic of the reflectance of the diffraction grating 118 is substantially the same as the characteristic of the diffraction grating 70 of Example 1 demonstrated with reference to FIG. The resonator mode interval is also 0.39 nm, which is substantially the same as in the first embodiment.

  Therefore, the laser light generator 123 oscillates in the five resonator modes (the emission intensity ratio is within 3 dB) belonging to the high reflection band 98 as in the first embodiment. Further, the resonator mode interval δf (69 GHz) of the laser light generation device 109 is also higher than the modulation frequency (10 GHz) of the electric signal 100 as in the first embodiment. The light intensity ratio of laser light emitted in these five resonator modes is also within 3 dB as in the first embodiment.

  Therefore, the optical transmission device 115 of the present embodiment exhibits the same effect as the optical transmission device 30 of the first embodiment. In addition, the temperature change rate of the wavelength of the signal light transmitted by the optical transmission device 115 is about an order of magnitude smaller than the temperature change rate of the wavelength of the signal light of the optical transmission device 30 of the first embodiment.

The refractive index n of the semiconductor (Si) forming the second core layer 117 increases as the temperature rises. As described above, the temperature coefficient dn / dT (degree ⁻¹ ) of the refractive index of the semiconductor is about +10 ⁻⁴ .

On the other hand, the refractive index n of a polymer material (polymer) such as an epoxy resin decreases as the temperature increases. For example, the temperature coefficient dn / dT (degree ⁻¹ ) of the refractive index of the epoxy resin is about −10 ⁻⁴ .

  By the way, since the thickness and width of the second core layer 117 are both 1 μm or less, about 50% of the guided light oozes out into the cladding layer surrounding the second core layer 117. For this reason, the temperature change of the refractive index of Si forming the second core layer 117 and the refractive index of the epoxy resin forming the upper half of the cladding layer cancel each other, and the temperature change of the equivalent refraction of the optical waveguide 122 becomes small.

  For this reason, the temperature change rate of the wavelength of the signal light transmitted from the optical transmission device 115 is about one digit smaller than the temperature change rate of the wavelength of the signal light of the optical transmission device 30 of the first embodiment.

  As described above, the thickness and width of the second core layer 117 are both 1 μm or less. When the optical waveguide 122 having such a core layer is bent, the radius of curvature becomes several μm. Therefore, if the optical waveguide of this embodiment is applied to an optical waveguide unit having a bent optical waveguide as described in Example 6 described later, the size of the optical waveguide unit can be reduced.

  In the first to third embodiments, the diffraction grating is formed in the optical waveguide unit and separated from the semiconductor optical amplification unit. In such a structure, the temperature change rate of the Bragg wavelength of the diffraction grating can be reduced by appropriately selecting the material of the optical waveguide on which the diffraction grating is formed. Therefore, even if the environmental temperature of the apparatus changes, the wavelength of the signal light of the optical signal generator can be hardly changed. Therefore, an optical transmitter that does not require precise temperature control can be realized.

  The following description relates to a method for manufacturing the optical waveguide unit 116.

First, an SOI (silicon on insulator) substrate in which an SiO ₂ film and an Si (single crystal) film are sequentially laminated on an Si substrate is prepared.

  Next, a SiN film is deposited on the Si film by a PCVD method. Next, this SiN film is processed by an electron beam exposure technique and RIE to form a SiN film periodically arranged at a position where a diffraction grating is to be formed.

  Further, the Si film is processed by the electron beam exposure technique and RIE to form the second core layer.

Thereafter, an epoxy resin layer serving as the upper half of the cladding layer is formed on the second core layer and the SiO ₂ film.

  The subsequent manufacturing method is the same as the manufacturing method of Example 2.

  The present embodiment relates to an optical transmitter that suppresses noise caused by spontaneous emission light (particularly amplified spontaneous emission light) that does not contribute to laser oscillation by making the reflectance characteristics of the diffraction grating unimodal. In this embodiment, the form of the diffraction grating formed in the second core layer is special. Other configurations and operations are substantially the same as those of the optical transmission device 30 according to the first embodiment. Therefore, the description regarding a common part is not performed.

  FIG. 12 is a plan view of the second core layer 128 on which the diffraction grating 126 is formed. As shown in FIG. 12, in this embodiment, the width of the second core layer 128 repeats increasing and decreasing periodically. Moreover, as shown in FIG. 12, the amplitude of the width of the second core layer 128 gradually decreases from the center of the diffraction grating 126 toward both ends, and becomes zero at both ends. On the other hand, the groove as in the first embodiment is not formed in the second core layer 128.

  A period 131 in which the width of the second core layer 128 increases or decreases is 412 nm. A ratio (duty) of the widened portion 129 to the width increase / decrease period 131 is 50%. The maximum value 130 of the width of the core layer is 1.5 μm. On the other hand, the minimum value 133 of the core layer width is 1.2 μm. The length of the diffraction grating 128 is 257 μm.

The material (SiO ₂ ) and configuration of the cladding layer are the same as those in the first embodiment. Further, the material (SiO _1-x N _x ), thickness (0.4 μm) of the second core layer 128, and the width (1.2 μm) in the region 82 where the diffraction grating 128 is not formed are also described in the example. Same as 1. Therefore, the equivalent refraction of the optical waveguide 134 is 1.54, which is the same as that in the first embodiment.

  FIG. 13 is a diagram for explaining the distribution of the coupling coefficient κ of the diffraction grating 126. The vertical axis represents the coupling coefficient κ. The horizontal axis is the position coordinate along the optical axis.

  As shown in FIG. 13, the coupling coefficient κ is the strongest at the center of the diffraction grating and is 170 / cm. The coupling coefficient κ gradually decreases toward both ends of the diffraction grating and becomes 0 / cm. As shown in FIG. 12, the width of the second core layer 128 is maximized at the center of the diffraction grating. Accordingly, the coupling coefficient κ is also maximized at the center of the diffraction grating. On the other hand, the coupling coefficient κ is minimized at both ends of the diffraction grating where the amplitude of the width 130 is minimized.

  FIG. 14 is a diagram for explaining the relationship between the reflectance of the diffraction grating 126 and the wavelength.

  The vertical axis in FIG. 14 is the reflectance, and the horizontal axis is the wavelength. The vertical axis is logarithmic.

  As shown in FIG. 14, the reflectance of the diffraction grating 126 becomes a maximum value at a wavelength of 1.3 μm, like the diffraction grating 70 of the first embodiment. Further, the maximum value of the reflectance is 90% as in the first embodiment. Further, the width of the high reflection band 98 where the reflectance is within 0.1 dB (within the reflectance of 89.8%) from the maximum reflectance (90%) is also 2 nm as in the first embodiment.

  Further, the resonator mode interval of the optical resonator 136 is also 0.393 nm (70 GHz) as in the first embodiment. Therefore, the resonator mode interval δf (70 GHz) of the laser light generation device 141 is higher than the modulation frequency (10 GHz) of the electric signal source 34.

  Therefore, this optical transmission device 121 exhibits the same effect as the optical transmission device 30 of the first embodiment. In addition, as will be described below, the present optical transmitter 121 can suppress noise due to spontaneously emitted light (particularly amplified spontaneously emitted light) that does not contribute to laser oscillation.

  As shown in FIG. 8, in the relationship between the reflectance and wavelength (reflectance characteristics) of the diffraction grating of Example 1, a side lobe 140 is generated accompanying the main reflection band 138. In such a resonator mode in the side lobe 140, laser oscillation does not occur, but amplified spontaneous emission light, so-called ASE (Amplified Spontaneous Emission) is generated. As is well known, ASE is a noise source of laser light.

  Such a side lobe does not exist in the reflectance characteristics of the diffraction grating of the present embodiment as shown in FIG. Therefore, in the present embodiment, it is possible to prevent noise caused by ASE generated in the side lobe.

  The following description relates to a method for manufacturing the optical waveguide unit 124.

The manufacturing method of the optical waveguide unit 124 is substantially the same as the manufacturing method of the optical waveguide unit 80 of the first embodiment. However, in this embodiment, the SiO _1-x N _x film to be the second core layer 128 is processed so that the width periodically increases and decreases in the region where the diffraction grating is to be formed (see FIG. 12). The processing of the SiO _1-x N _x film is performed using an electron beam exposure technique and RIE. The manufacturing method other than this step is substantially the same as the manufacturing method of Example 1.

  FIG. 15 is a perspective view illustrating a main part of the optical transmission device 142 according to the present embodiment. 16 is a cross-sectional view taken along the line AA in FIG. 15 as seen from the direction of the arrow. The configuration and operation of the optical transmission device 142 have many parts in common with the optical transmission device 30 of the first embodiment. Therefore, description of common parts is omitted.

  In this embodiment, the maximum reflectance of the diffraction grating 144 forming the optical waveguide unit 143 is approximately 100%. On the other hand, as shown in FIG. 16, unlike the first embodiment, a highly reflective film is not formed on the first end surface 40 of the optical amplification unit 146 (the end surface opposite to the optical waveguide unit 143).

  In the present embodiment, the first end face 40 remains cleaved. Therefore, the first end surface 40 functions as a light reflecting surface 42 having a reflectance of about 30%.

  The first core 37 (active layer 36) of the optical amplification unit 146 is optically coupled to the optical fiber 17 at this cleavage plane (see FIG. 16). That is, the optical transmitter 142 outputs an optical signal from the first end face 40 (light reflecting face 42). The length of the optical amplification unit 146 is 300 μm.

The diffraction grating 144 of the present embodiment is an optical member in which grooves are periodically formed in the second core layer 145 made of SiO _1-x N _x (0 ≦ x ≦ 1) as in the first embodiment ( (See FIG. 16). The diffraction grating 144 is surrounded by a cladding layer 75 made of SiO ₂ as in the first embodiment.

  However, unlike the diffraction grating 70 of the first embodiment, the present diffraction grating 144 is directly connected to the third end face 76 on the optical waveguide unit 143 side (end face on the optical amplification unit 143 side) (FIGS. 15 and 15). 16).

  The pitch and duty of the diffraction grating 144 are 412 nm and 50%. These are the same as the pitch and the juty of the diffraction grating 70 of the first embodiment. On the other hand, the depth of the groove of the diffraction grating 144 is 25 nm. This depth is shallower than the depth (32 nm) of the groove of Example 1. The coupling coefficient κ of the diffraction grating 144 is 80 / cm. This coupling coefficient is smaller than the coupling coefficient of Example 1 (125 / cm).

  On the other hand, the length L of the diffraction grating 144 is 725 μm. This length is longer than the length of the diffraction grating of Example 1 (155 μm).

  Therefore, κ · L of the diffraction grating is 5.8. On the other hand, κ · L of the diffraction grating 70 of Example 1 is 1.9. Thus, κ · L of the diffraction grating is much larger than κ · L of the diffraction grating 70 of the first embodiment. For this reason, the maximum value of the reflectance of the diffraction grating 144 is approximately 100%. On the other hand, in this diffraction grating 144, since the coupling coefficient κ is small, the width of the reflection band is narrowed.

  FIG. 17 is a diagram for explaining the relationship between the reflectance of the diffraction grating 144 and the wavelength. The vertical axis (logarithmic display) is the reflectance, and the horizontal axis is the wavelength.

  As shown in FIG. 17, the maximum value of the reflectance of the diffraction grating 144 is approximately 100% (see FIG. 17). Due to this high reflectivity, an effect peculiar to this embodiment described later is exhibited.

  On the other hand, the width of the high reflection band 150 where the reflectance is within 0.1 dB (the reflectance is within 99.8%) from the maximum value (approximately 100%) is 2 nm as in the first embodiment. The resonator mode interval is also approximately 0.40 nm (71 GHz) as in the first embodiment.

  Therefore, as in the first embodiment, in this embodiment, five (= 2 nm / 0.4 nm) resonator modes are formed in the high reflection band 150. Further, the resonator mode interval δf (71 GHz) of the laser light generation device 148 is also higher than the modulation frequency (10 GHz) of the electric signal source 34 as in the first embodiment. Therefore, the optical transmission device 142 of the present embodiment also exhibits the same effect as the optical transmission device 30 of the first embodiment.

  In addition, the present optical transmission device 142 exhibits an effect of stable laser oscillation in a plurality of resonator modes.

  In the diffraction grating 144 of the present embodiment, as shown in FIG. 17, the curve indicating the relationship between the wavelength and the reflectance is substantially flat (flat top) in the vicinity of the maximum reflectance (approximately 100%). In such a case, the laser light generation device 148 stably oscillates in a plurality of resonator modes. Therefore, the optical transmitter 142 stably oscillates in a plurality of resonator modes.

  Further, when the present optical transmission device 142 is applied to a WDM optical transmission device (hereinafter referred to as a WDM optical transmission device), the present optical transmission device 142 exhibits the effect that a large number of wavelength channels can be set. .

  As described above, a plurality of resonator modes reliably oscillate inside the high reflection band 150. However, laser oscillation may occur even outside the high reflection band 150.

  For this reason, when the optical transmitter 142 is applied to WDM, the width of the wavelength channel must be set wider than the width of the high reflection band 150 so that light does not leak into the adjacent wavelength channel. As the width of the wavelength channel, for example, a wavelength range in which the reflectance exists within 5 dB of the maximum reflectance (approximately 100%), that is, the wavelength band 152 is preferable.

  By the way, the width of the wavelength band 152 is proportional to the coupling coefficient κ of the diffraction grating. Therefore, if the coupling coefficient κ is reduced, the reflection band 152 can be narrowed. However, if the reflectance curve indicating the relationship between reflectance and wavelength is not a flat top, the proportion of the high reflection band in the reflection band is small. In such a case, if the reflection band is narrowed, the high reflection band becomes too narrow. As a result, the number of resonator modes that stably oscillate within the high reflection band decreases, and mode hop noise increases.

  On the other hand, in the diffraction grating 144 of the present embodiment, since the reflectance curve is a flat top, the ratio of the high reflection band 150 in the reflection band 152 is large. Therefore, according to the diffraction grating 144 of the present embodiment, the width of the reflection band 152 can be narrowed while ensuring the necessary width of the high reflection band 150.

  For example, in the diffraction grating of Example 1, in order to ensure a high reflection band 98 with a width of 2 nm, a reflection band 152 with a width of 9 nm is provided (see FIG. 8). On the other hand, according to the present embodiment, the high reflection band 150 having the same width (2 nm) as that of the first embodiment can be ensured only by providing the reflection band 152 having a width of 3 nm (see FIG. 17).

  Therefore, when the present optical transmission device 142 is applied to a WDM optical transmission device, the wavelength range per wavelength channel is narrowed, so that a large number of wavelength channels can be set.

  Even if the reflectance curve is not a perfect flat top, the present optical transmission device 142 exhibits such an effect. In order to exert such an effect, the maximum value of the reflectance of the diffraction grating is preferably higher than 90%, the maximum value of the reflectance is more preferably higher than 95%, and the maximum value of the reflectance is Most preferably, it is higher than 98%.

  FIG. 18 is a perspective view illustrating a main part of the optical transmission device 154 according to the present embodiment. The configuration and operation of the optical transmission device 15 4 are the same as the configuration and operation of the optical transmission device 142 of the fifth embodiment, but there are parts. Therefore, the description about a common part is not performed.

  The configuration of the optical amplification unit 156 is substantially the same as that of the optical amplification unit 44 of the first embodiment. However, the element length of the optical amplification unit 156 is 300 μm (the same as in Example 5).

  The configuration of the electrical signal source 34 is the same as that of the electrical signal source 34 of the first embodiment.

  The optical waveguide unit 158 includes an optical multiplexer (coupler) 160.

  FIG. 19 is a plan view illustrating the configuration of the optical multiplexer 160.

  The optical multiplexer (coupler) 160 is provided between the third end face 76 of the optical waveguide unit 158 and the diffraction grating 161, that is, in the middle of the optical waveguide 78 (see FIG. 18). The optical multiplexer (coupler) 160 is optically connected to the first optical input port 162, the second optical input port 164, and the diffraction grating 161 optically connected to the third end face 76. One light output port 166 and a second light output port 168 are provided. As is apparent from FIGS. 18 and 19, the optical multiplexer 160 is a multi-mode interference (MMI) coupler having two inputs and two outputs. Note that an optical coupler such as an MMI functions as an optical coupler even if light is input to the optical output port. That is, the names of the light input port and the light output port are convenient.

  Further, the optical waveguide unit 158 is formed with another diffraction grating 170 having the same structure as that of the diffraction grating 161, and another optical waveguide optically connected to the second optical output port 168 of the optical multiplexer (coupler) 160. 172 (see FIG. 18).

  Here, the configurations of the diffraction grating 161 and the other diffraction gratings 170 are the same as those of the diffraction grating 144 of the fifth embodiment. That is, the maximum reflectance of the diffraction grating 161 and the other diffraction grating 170 is approximately 100%. Therefore, the reflectance curves of the diffraction grating 161 and the other diffraction grating 170 are flat top.

  The optical waveguide unit 158 includes an output waveguide 174 that is optically connected to the second optical input port 164 of the optical multiplexer (coupler) 160 and outputs an optical signal.

  Here, the output waveguide 174 has a bent portion 176. The output waveguide 174 changes the direction of 180 ° at the bent portion 176, then extends to the opposite side of the optical amplification unit 156, and reaches the fifth end surface 184. The output waveguide 174 is optically coupled to the optical fiber 17 at the fifth end face 184.

  In the optical waveguide unit 158, the optical length of the first optical path 180 from the diffraction grating 161 to the first optical output port 166 of the optical coupler (coupler) 160, and the second optical output from the other diffraction grating 170. The optical lengths of the second optical paths 182 reaching the mouth 168 do not match. Further, the difference between both optical lengths is a value obtained by multiplying the wavelength λ of the optical signal output from the optical transmitter 154 by an integer and dividing by 4 (0, 0.25λ, 0.5λ, 0.75λ, 1.0λ, ..) Are formed so as not to coincide with each other.

  In this embodiment, the difference in optical path length is 228 nm. 228 nm is approximately 1/6 of the wavelength (1.3 μm) of the signal light generated by the optical transmitter 154. The difference in the physical length of each optical path is 148 nm (= 228 nm / 1.54).

Here, in addition to the diffraction grating 161 and the other diffraction grating 170, the optical multiplexer 160, the other optical waveguide 172, and the output waveguide 174 are similar to the diffraction grating 144 of the fifth embodiment in the form of SiO _1-x N _x. The film is processed by RIE. The cladding layer surrounding the diffraction grating 161 and the like is also made of SiO ₂ as in the fifth embodiment.

  As is clear from the above description, a reflection type Mach-Zehnder interferometer is formed by the diffraction gratings 161 and 170 and the multiplexer 160 in the optical waveguide unit 158 of the present embodiment. The length along the optical path from the third end face 76 of the optical waveguide unit 158 to the diffraction grating 161 is 200 μm.

  The following description relates to the operation of the laser beam generator 178.

  When a current is injected from the electric signal source 34 into the optical amplification unit 156, the active layer 36 of the optical amplification unit 156 forms an optical gain and generates spontaneous emission light (see FIG. 6). Light (spontaneously emitted light) generated in the active layer 36 is emitted from the second end face 66 and is incident on the optical waveguide 78 of the optical waveguide unit 158.

  The light incident on the optical waveguide 78 is equally divided by the optical multiplexer (MMI coupler) 160 and output to the first optical output port 166 and the second optical input port 168. The lights divided by the optical multiplexer 160 enter the diffraction grating 161 and the other diffraction grating 170, respectively, and are reflected according to the reflection characteristics described with reference to FIG. As shown in FIG. 17, the light in the high reflection band 150 is reflected approximately 100%.

  The light reflected by the respective diffraction gratings 161 and 170 travels back along the optical path that has traveled, and re-enters the first optical output port 166 and the second optical output port 168 of the optical multiplexer (MMI coupler) 160, respectively. Incident. The light re-entering the optical multiplexer (MMI coupler) 160 is combined and interferes, and is emitted from the first optical input port 162 and the second optical input port 164.

  At this time, the optical length of the first optical path 180 from the diffraction grating 161 to the first optical output port 166 of the optical multiplexer 160 and the second optical output port 168 of the optical multiplexer 160 from the other diffraction grating 170. The output of the optical multiplexer (MMI coupler) 160 changes depending on the difference Δ of the optical length of the second optical path 182 to reach. When the optical length difference Δ is zero, all the light re-entering the optical multiplexer 160 is emitted from the second optical input port 164. On the other hand, when the optical length difference Δ is 1 / 4λ, all light is emitted from the first light input port 162.

  In the laser beam generator 178 of this embodiment, the optical length difference Δ is set to approximately λ / 6. In this case, 80% of the combined light is emitted to the first light input port 162 connected to the light amplification unit 156. On the other hand, the remaining 20% of light is emitted from the second light input port 168.

  The light emitted from the first light input port 162 is incident on the active layer 36 of the light amplification unit 156 again. The re-incident light is reflected by the light reflection surface 42 (high reflection film 64) having a reflectivity of approximately 100%, reverses the traveling direction, and enters the optical waveguide 78 again (see FIG. 6). During this time, light is amplified by the active layer 36.

  Thereafter, the light reciprocates between the optical resonators formed by the diffraction gratings 161 and 170 and the light reflection surface 42 (high reflection film 64), and grows into laser light. A part of the generated laser light is branched to the output waveguide 174 and output to the optical fiber 17.

  Here, the Bragg wavelength of the diffraction gratings 161 and 170 is 1.3 μm. The resonator mode interval is 0.35 nm (62 GHz). The width of the high reflectance band 150 is 2 nm.

  Accordingly, similarly to the laser light generation device 148 of the fifth embodiment, the laser light generation device 178 oscillates in five resonator modes with a light intensity ratio of 3 dB or less around 1.3 μm. Further, the resonator mode interval δf (62 GHz) of the laser light generation device 178 is higher than the modulation frequency (10 GHz) of the electric signal source 34 as in the fifth embodiment.

  Further, the maximum reflectance of the diffraction gratings 161 and 170 is approximately 100%.

  Therefore, the optical transmission device 154 of the present embodiment exhibits the same effect as the optical transmission device 142 of the fifth embodiment. In addition, since the optical transmission device 154 is optically coupled to the optical fiber 17 by a dielectric output waveguide 174 as described below, coupling to the optical fiber is facilitated.

Coupling between optical waveguides is easier as the equivalent refractive indexes of both are closer. In the fifth embodiment, the optical amplification unit 146 and the optical fiber 17 (made of SiO ₂ ) are optically coupled (see FIG. 15). Here, the optical amplification unit 146 is made of a semiconductor, and the optical fiber 17 is made of a dielectric. Since the refractive index of the semiconductor and that of the dielectric are greatly different, it is not easy to optically couple the optical amplification unit 146 and the optical fiber 17 with a high coupling rate.

In this embodiment, since both the output waveguide 174 and the optical fiber 17 (made of SiO ₂ ) are made of a dielectric, it is easy to increase the optical coupling ratio between them. Therefore, alignment is easy. That is, the optical coupling between the optical transmitter 154 and the optical fiber 17 is easy.

  By the way, in the WDM optical transmission apparatus, signal light is wavelength-multiplexed and output by an arrayed-waveguide grating (AWG) multiplexer.

  The cross-sectional structure of the output waveguide 174 of the present optical transmitter 154 is the same as this arrayed waveguide diffraction grating. Therefore, it is easy to integrate the optical transmission device 154 and the AWG multiplexer on the same substrate.

  FIG. 20 is a perspective view illustrating a main part of the optical transmission device 186 according to the present embodiment.

  The configuration of the optical transmission device 186 is partially in common with the configuration of the optical transmission device 30 of the first embodiment. Therefore, description of common parts is omitted.

  As shown in FIG. 20, the optical transmission device 186 of this embodiment includes an optical amplification unit 198.

  The structure of the optical amplification unit 198 is substantially the same as that of the optical amplification unit 44 of the first embodiment described with reference to FIG. However, the element length is 1000 μm, which is longer than the optical amplification unit 44 of the first embodiment. The element length of the optical amplification unit 44 of Example 1 is 400 μm.

  Further, the optical transmission device 186 of this embodiment includes an optical waveguide unit 188 having an optical waveguide 204 on which the diffraction grating 202 is formed and an optical modulator 190.

  The configuration of the optical waveguide 204 is substantially the same as the optical waveguide 78 of the first embodiment. However, the coupling coefficient κ of the diffraction grating 202 is 51 / cm, which is smaller than the coupling coefficient κ (125 / cm) of the diffraction grating 70 of the first embodiment. The length of the diffraction grating 202 is 379 μm. The distance between the third end face 76 on the optical amplification unit 198 side and the diffraction grating 202 is 153 μm.

  An optical waveguide 204 is optically coupled to the optical modulator 190 at an end different from the third end surface 76 (opposing the optical amplification unit 198). That is, the input end 192 of the optical modulator 190 is connected to the diffraction grating 202. On the other hand, the output end 194 of the optical modulator 190 is connected to the output waveguide 196. The output waveguide 196 is optically connected to the optical fiber 17.

  As is apparent from FIG. 20, the optical modulator 190 is a Mach-Zehnder type optical modulator. As shown in FIG. 20, the optical waveguides forming the optical modulator 190 are once branched into a pair of parallel optical waveguides, and then merged to become one optical waveguide again. The optical modulator 190 of this embodiment is formed such that a current is injected into one (or both) of a pair of branched optical waveguides, and the refractive index thereof changes. Due to this refractive index change, the phase of the guided light changes, and the incident light is modulated.

  FIG. 21 is a diagram illustrating a cross section of the optical modulator 190 perpendicular to the traveling direction of the signal light (Non-Patent Document 2). FIG. 21 shows one section of the pair of optical waveguides.

In the optical modulator 190, the Si core layer 206 is surrounded by the SiO ₂ cladding layer 208. Si forming the core layer 206 is non-doped. On the other hand, Si layers 210 and 212 thinner than the core layer 206 extend on both sides of the core layer 206. One Si layer 210 is n-type, and the other Si layer 212 is p-type. That is, the n-type Si layer 210 and the p-type Si layer 212 form a pn junction with the core layer 206 interposed therebetween.

  A via hole 214 is formed in the cladding layer 208. The via hole 214 is filled with a metal, and the n-type Si layer 210 and the p-type Si layer 121 are connected to the electrodes 216 formed on the surface of the cladding layer 208 by the metal.

  In addition, the optical transmission device 186 includes an electric signal source 218 that drives the optical modulator 190 by applying an electric signal. When the pn junction is forward-biased by this electrical signal, current is injected into the core layer 206. The injected current reduces the refractive index of the core layer 206. For this reason, a difference occurs in the optical path length between one optical waveguide to which the electrical signal is applied and the other optical waveguide to which the electrical signal is not applied, and the optical signal is modulated.

  In addition, the optical transmission device 186 includes a current supply source 220 that supplies a direct current to the gain waveguide of the optical amplification unit 198.

  In this embodiment, an optical resonator 220 is formed by the optical waveguide 204 on which the diffraction grating 202 is formed and the light reflecting surface 42 of the optical amplification unit 198. Therefore, when a direct current is supplied from the current supply source 220 to the optical amplification unit 198 that forms a part of the optical resonator 220, continuous laser oscillation occurs in the optical resonator 220, and a direct current laser beam is generated.

  This laser light enters the modulator 190 from the input end 192. The modulator 190 modulates the incident laser light in accordance with the electrical signal applied by the electrical signal source 218. The modulated laser light is emitted from the output end 194 and enters the output waveguide 196. The laser is guided to the end of the laser light generation device 200 by the output waveguide 196 and enters the optical fiber 17.

  FIG. 22 is a diagram for explaining the relationship between the reflectance of the diffraction grating 202 and the wavelength. In FIG. 22, the vertical axis represents the reflectance, and the horizontal axis represents the wavelength.

  As shown in FIG. 22, the reflectance of the diffraction grating 202 becomes a maximum value at a wavelength of 1.3 μm. The maximum value of this reflectance is 90%, which is the same as in Example 1. Here, the width of the high reflection band 98 of the diffraction grating 202 (the wavelength range in which the reflectivity is within 0.1 dB of the maximum reflectivity) is 1 nm. The resonator mode interval of the optical resonator 220 is 0.2 nm (36 GHz).

  Therefore, there are five resonator modes in the high reflection band 98. The resonator mode interval (36 GHz) is sufficiently higher than the modulation frequency (10 GHz) of the electric signal source 218.

  Therefore, the optical transmission device 186 of the present embodiment exhibits the same effect as the optical transmission device 30 of the first embodiment. In addition, when each wavelength channel of the WDM optical transmission apparatus is formed by the optical transmission apparatus 186 of the present embodiment, an effect of increasing the number of wavelength channels of the WDM optical transmission apparatus is exhibited.

  When each wavelength channel of the WDM optical transmission apparatus is formed by the present optical transmission apparatus 186, the width of the wavelength channel can be set to 1 nm equal to the reflection bandwidth of the diffraction grating 202 as described below. On the other hand, when the WDM optical transmission apparatus is constructed by the optical transmission apparatus 30 according to the first embodiment, the settable wavelength channel width is 2 nm, which is equal to the reflection bandwidth of the diffraction grating 70. Therefore, if the WDM optical transmission apparatus is constructed by the optical transmission apparatus 186 of the present embodiment, the number of wavelength channels is doubled.

  The reason why the width of one wavelength channel can be set narrow in this way is that the width of the reflection band 152 of the diffraction grating 202 (the wavelength range in which the reflectance is within 5 dB of the maximum reflectance) is 1 nm. This is because it is half of the diffraction grating (see FIGS. 22 and 8).

  As shown in FIG. 22, the reflectance curve of the diffraction grating 202 of the present embodiment is not a flat top like the diffraction grating 70 and the reflectance curve of the first embodiment. For this reason, when the reflection band 152 is narrowed, the width of the high reflection band 98 is also narrowed, and the number of resonator modes existing in the high reflection band 98 is reduced. Such a decrease in resonator mode results in an increase in mode hop noise.

  Therefore, in this embodiment, the optical length (resonator length) of the optical resonator 220 is increased, and the resonator mode interval is decreased.

  However, as the resonator length increases, the mirror loss decreases and the lifetime of photons generated in the optical resonator increases. As a result, the relaxation frequency of the laser light generation apparatus 200 increases. For this reason, in the optical transmitters according to the first to sixth embodiments that directly modulate the laser beam generator (laser oscillator) to generate the signal light, the resonator mode interval cannot be made very narrow. By the way, the reflection band 152 is required to accommodate a plurality of resonator modes in order to reduce mode hop noise. Therefore, the width of the reflection band 152 cannot be made too narrow.

  On the other hand, in the optical transmitter 186 of this embodiment, the continuous laser light (CW light) generated by the optical amplification unit 198 and the optical resonator 220 is modulated by the optical modulator 190 to form signal light. Therefore, the modulation band of the present optical transmitter 186 is not limited by the relaxation frequency of the laser light generator 200.

  Therefore, even if the optical resonator 220 is lengthened and the resonator mode interval is narrowed, the modulation band of the optical transmitter 186 is not narrowed. Therefore, in the present optical transmitter 186, the resonator mode interval can be narrowed, and the required number of resonator modes can be secured in the high reflection band 98.

  Therefore, when this transmission apparatus 186 is applied to a WDM optical transmission apparatus, the width of the wavelength channel can be reduced as much as necessary, so that the number of wavelength channels can be increased.

  Further, an electrical signal for modulating the laser beam is not applied to the optical amplification unit 198 of the optical transmitter 186. For this reason, even if the electric capacity of the optical amplification unit 198 increases, the modulation band of the optical transmission device 186 does not narrow. Therefore, in the optical transmission device 186 of the present embodiment, the optical amplification unit 198 can be lengthened. Prolonging the optical amplification unit 198 has the following advantages.

  In the present embodiment, not the optical waveguide 204 but the optical amplification unit 198 having a large equivalent refractive index is lengthened to narrow the resonator mode interval. Therefore, an increase in the length of the entire laser beam generator 200 is suppressed.

  Furthermore, in this embodiment, the optical amplifier unit 198 that functions as a gain medium is long, so that the maximum output of the optical transmitter 186 is large.

  FIG. 23 is a perspective view for explaining a main part of the WDM optical transmitter 222 according to the present embodiment.

  The WDM optical transmitter 222 includes four optical transmitters 226 integrated on a Si substrate 224. The configuration of the optical transmission device 226 is substantially the same as that of the optical transmission device 30 described in the first embodiment. However, the pitch of the diffraction grating 70 is different from that of the optical transmitter 30 of the first embodiment.

  The pitch of the diffraction grating 70 of the optical transmission device 226 is different for each optical transmission device. The pitch of the diffraction grating 70 of each optical transmission device 226 is 418.07 nm, 420.01 nm, 421.96 nm, and 423.90 nm in order from the shortest. Corresponding to these pitches, the Bragg wavelength of each diffraction grating is also different. The Bragg wavelength of each diffraction grating is 1291 nm, 1297 nm, 1303 nm, and 1309 nm in order from the shortest.

  Further, the WDM optical transmission apparatus 222 includes a wavelength multiplexing unit (λ-MUX) 228 that wavelength-multiplexes the optical signals output from the plurality of optical transmission apparatuses 226. As shown in FIG. 23, the wavelength division multiplexing unit 228 is integrated on the same substrate 224 together with a plurality of optical transmission devices 226. The wavelength multiplexing unit 228 is, for example, an AWG multiplexer.

  The plurality of input units 230 of the wavelength multiplexing unit 228 are connected to output ends (end portions of the diffraction grating 70) of the respective optical transmission devices 226, respectively. On the other hand, the output section of the wavelength multiplexing unit 228 is optically connected to the optical fiber 17.

  Note that the broken lines shown in FIG. 23 indicate boundaries where the WDM optical transmitter 222 is divided by function. This broken line does not indicate the physical boundary of the WDM optical transmitter 222.

  Each optical transmission device 226 generates an optical signal according to the operation described in the first embodiment. Accordingly, each optical transmission device 226 oscillates in a plurality of resonator modes centered on the Bragg wavelength of each diffraction grating 70.

  Therefore, the center wavelengths of the optical signals generated by the respective optical transmission devices 226 are 1291 nm, 1297 nm, 1303 nm, and 1309 nm in order from the shortest. The interval between the wavelengths is 6 nm.

  These optical signals are incident on the input unit 230 of the wavelength multiplexing unit 228, respectively. The wavelength multiplexing unit 228 collects incident optical signals into one and outputs them to the optical fiber 17 from the output unit 232. As is clear from the above description, the number of wavelength channels in the WDM optical transmitter 222 is four, and the wavelength channel width is 6 nm.

  Here, the optical signal generated by the optical transmitter 226 is formed by a plurality of resonator modes that stably oscillate within the high reflection band 98 of the diffraction grating 70 (see FIG. 8). However, as described in the seventh embodiment, laser oscillation may occur in the resonator mode outside this high reflection band.

  On the other hand, there are very few cases where laser oscillation occurs in the resonator mode outside the reflection band 152. Even if laser oscillation occurs in such a resonator mode, the emission intensity is extremely weak.

  Therefore, the optical signal generated by each optical transmission device 226 is substantially formed only by the resonator mode existing inside the reflection band 152.

  In this embodiment, the wavelength channel width of the WDM optical transmission device 222 matches the width of the reflection band 152. Therefore, even if the optical signal generated by each optical transmission device 226 is multiplexed by the wavelength multiplexing unit 228, it can be easily separated on the receiving side of the WDM signal.

  The WDM optical transmission apparatus 222 exhibits the same effect as the optical transmission apparatus according to the first embodiment. In addition, the WDM optical transmission device 222 exhibits an effect that the setting accuracy of the center wavelength of each wavelength channel is higher than that of the WDM optical transmission device using a DFB laser as a light source.

  In general, the control of the equivalent refractive index is easier in the dielectric optical waveguide than in the semiconductor optical waveguide. Therefore, the Bragg wavelength control is also easier with the optical transmission device 226 of this embodiment in which the diffraction grating is formed of a dielectric optical waveguide than the DFB laser. Therefore, the setting accuracy of the center wavelength of each wavelength channel of the WDM optical transmitter 222 is higher than that of the WDM optical transmitter using the DFB laser as a light source.

  The optical signal generated by the optical transmission device described in the first to eighth embodiments is formed by a plurality of resonator modes. Therefore, the optical transmission devices described in the first to eighth embodiments are more suitable for short-distance optical communication than long-distance optical communication. Specifically, it is suitable for short-distance optical communication for transmitting and receiving optical signals at a distance of 1 cm to 1 km, such as optical communication between chips, optical communication between boards, and optical LAN.

  FIG. 24 is a conceptual diagram of an optical LAN 234 (optical local network) constructed using the WDM optical transmission apparatus 222 of the eighth embodiment. This optical LAN 234 is connected to an information processing device 236 such as a server or a PC (personal computer) and these information processing devices 236, receives signals output from one information processing device 236, and transmits them to other information processing devices 236. Formed by the hub 238. The signal transmission / reception is also performed between the hubs 238 and 239.

  Each information processing device 236 and hubs 238 and 239 are equipped with an optical transmission / reception module 240 (optical transceiver) for connecting the devices with optical signals at a short distance (1 m to 100 m) such as a premises.

  As shown in FIG. 24, at least one optical transmission / reception module 240 is mounted on the information processing device 236 and is connected to a plurality of optical transmission / reception modules 240 mounted on the hub 238 by optical fiber pairs 242.

  FIG. 25 is a conceptual diagram of the optical transceiver module 240. The optical transceiver module 240 is formed by the WDM optical transmitter 222 described in the eighth embodiment and the WDM receiver 246 that separates and detects the wavelength-multiplexed optical signal.

  In the optical transmission / reception module 240, the information supplied by the electrical signal 248 is converted into an optical signal by the WDM optical transmission device 222, further wavelength-multiplexed and output to one of the optical fiber pair 242 as a WDM optical signal 250. On the other hand, the WDM receiver 246 receives the WDM optical signal 252 propagating through the other of the optical fiber pair 246, separates the wavelength-multiplexed WDM optical signal into individual optical signals, and then photoelectrically converts them into electrical signals. It outputs as 235.

  In the present optical LAN 234, communication between the hubs 238 and 239 and the information processing apparatus 240 (or another hub 239) is performed by a WDM optical signal generated by the optical transmission / reception module 240. Therefore, high-speed and large-capacity communication is possible.

  The optical transmitter of the optical transceiver module 240 does not require precise temperature control, and is formed by the low noise WDM optical transmitter 222 described in the eighth embodiment. Therefore, according to the present embodiment, a low-cost and low-noise optical LAN can be constructed.

2 ... external cavity laser 4 ... high reflection film 5 ... optical resonator 6 ... semiconductor laser 8 ... Si substrate 10 ... SiO ₂ layer 12 ... optical waveguide 14, Planar optical circuit 16 ... Diffraction grating 17 ... Optical fiber 18 ... Diffraction grating (DBR) reflectivity 20 ... Laser oscillation mode 22 ... Resonator mode 24 ..Waveband 26 (DBR exhibits high reflectivity) ... resonator mode 27 after mode hop ... reflectance of diffraction grating (embodiment)
28... Resonator mode in which laser oscillation occurs 30... Optical transmitter (Example 1)
32 ... Laser light generation device 34 ... Electric signal source 36 ... Active layer 37 ... First core 38 ... Gain waveguide 40 ... First end face 42 ... Light reflection Surface 44 ... Optical amplification unit 46 ... Semiconductor substrate 48 ... Lower cladding layer 50 ... Upper cladding layer 52 ... Electrode layer 54 ... p-type InP
56 ... n-type InP 58 ... current blocking layer 60 ... n-type electrode 62 ... p-electrode 64 ... high reflection film 66 ... second end face 68 ... antireflection film 70 ... Diffraction grating 72 ... Dielectric layer 74 ... Second core layer 75 ... Clad layer 76 ... Third end face 78 ... Optical waveguide 80 ... Optical waveguide unit (implementation) Example 1)
82 ... first region 84 (without diffraction grating) ... fourth end face 85 ... Si substrate 86 ... dielectric layer 88 (formed on the Si substrate) Second region 90 (provided with a diffraction grating) ... Pad 92 ... Effective reflection end 94 ... Optical resonator 96 ... Gap 98 ... High reflection band 100 ... Electrical signal 102 ... Optical signal 104 ... First state 106 ... Second state 107 ... Optical transmitter (Example 2)
108: Optical waveguide unit (Example 2)
109... Laser light generator (Example 2)
110: First SiO ₂ layer 111: Optical waveguide (Example 2)
112 ... Second SiO ₂ layer 114 ... Epoxy resin 115 ... Optical transmitter (Example 3)
116: Optical waveguide unit (Example 3)
117... Second core layer (Example 3)
118: diffraction grating (Example 3) 120 ... SiN film 122 ... optical waveguide (Example 3) 121 ... optical transmitter (Example 4)
123... Laser light generator (Example 3)
124: Optical waveguide unit (Example 4)
126 ... Diffraction grating (Example 4) 128 ... Second core layer (Example 4)
129... Widened portion 130... Maximum value of core layer width (Example 4)
131: Increasing / decreasing period of the core layer width 132 ... A portion 133 where the width is narrowed ... The minimum value of the width of the core layer (Example 4)
134: Optical waveguide (Example 4)
136 ... Optical resonator (Example 4) 138 ... Main reflection band 140 ... Side lobe 141 ... Laser beam generator (Example 4)
142... Optical transmitter (Example 5)
143 ... Optical waveguide unit (Example 5) 144 ... Diffraction grating (Example 5)
145: Second core layer (Example 5) 146: Optical amplification unit (Example 5)
148... Laser light generation device (Example 5) 150... High reflection band (Example 5)
152 ... reflection band 154 ... optical transmitter (Example 6)
156... Optical amplification unit (Example 6)
158 ... Optical waveguide unit (Example 6) 160 ... Optical multiplexer (coupler)
161 ... Diffraction grating (Example 6) 162 ... First light input port 164 ... Second light input port 166 ... First light output port 168 ... Second light output Mouth 170... Other diffraction grating 172... Other optical waveguide 174... Output waveguide 176 .. Bending portion 178.
180: first optical path 182: second optical path 184: fifth end face 186: optical transmitter (Example 7)
188 ... Optical waveguide unit (Example 7)
DESCRIPTION OF SYMBOLS 190 ... Optical modulator 192 ... Input end 194 (of an optical modulator) ... Output end of an optical modulator 196 ... Output waveguide 198 ... Optical amplification unit (Example 7)
200... Laser light generator (Example 7)
202: Diffraction grating (Example 7) 204: Optical waveguide (Example 7)
206 ... Core layer 208 ... Cladding layer 210, 212 ... Si layer 214 ... Via hole 216 ... Electrode 218 ... Electric signal source (Example 7)
220... Optical resonator 222... WDM optical transmitter (Embodiment 8)
224 ... Substrate (Embodiment 8) 226 ... Optical transmitter (Embodiment 8)
228 ... Wavelength division unit 230 ... Wavelength division unit input unit 232 ... Wavelength division unit output unit 234 ... Optical LAN
235, 248 ... Electric signal 236 ... Information processing device 238, 239 ... Hub 240 ... Optical transceiver module 242 ... Optical fiber pair 246 ... WDM receiver 250, 252 ... WDM Optical signal

Claims (4)

An optical amplification unit including a gain waveguide having a semiconductor layer that generates optical gain by injected current as a first core layer, and having a light reflecting surface formed on a first end face of the gain waveguide;
A third end face that includes a second core layer provided with a diffraction grating and is opposite to the second end face of the optical amplification unit, and is optically coupled to the optical amplification unit at the third end face; A laser light generation device having an optical waveguide unit including an optical waveguide;
An electrical signal source that applies an electrical signal to the optical amplification unit and injects the current into the semiconductor layer;
The laser light generation device includes a first state in which laser oscillation is performed in a plurality of resonator modes of an optical resonator formed by the light reflecting surface and the diffraction grating according to the electrical signal, and emission intensity from the first state. Reciprocates between the second states where is small to generate an optical signal,
Furthermore, the frequency corresponding to the interval of the resonator mode is higher than the frequency corresponding to the bit rate of the electrical signal,
Optical transmitter. The optical transmission device according to claim 1,
And this is higher than 90% the maximum value of the reflectance of the diffraction grating,
An optical transmission device. The optical transmission device according to claim 1 or 2,
The optical waveguide unit is
A first light input port provided between the third end face and the diffraction grating and optically connected to the third end face, a second light input opening, and optically connected to the diffraction grating. An optical multiplexer having a first optical output port and a second optical output port;
Another diffraction grating having the same structure as the diffraction grating is formed, and is optically connected to the second light output port; and
An output waveguide that is optically connected to the second optical input port and outputs the optical signal;
A first optical length of a first optical path from the diffraction grating to the first light output port, and a second optical length of a second optical path from the other diffraction grating to the second light output port. And having an optical path difference that is different from the wavelength obtained by multiplying the Bragg wavelength of the diffraction grating by an integer and dividing by 4.
An optical transmission device. An optical amplification unit including a gain waveguide having a semiconductor layer that generates optical gain by injected current as a first core layer, and having a light reflecting surface formed on a first end face of the gain waveguide;
A third end face that includes a second core layer provided with a diffraction grating and is opposite to the second end face of the optical amplification unit, and is optically coupled to the optical amplification unit at the third end face; A laser light generation device having an optical waveguide and an optical waveguide unit including an optical modulator to which the optical waveguide is optically coupled at an end different from the third end surface;
An electric signal source for applying the electric signal to drive the optical modulator;
A current supply source for supplying the current in a DC state to the gain waveguide;
The laser beam generator oscillates in a plurality of resonator modes of an optical resonator formed by the light reflecting surface and the diffraction grating,
Furthermore, the frequency corresponding to the interval of the resonator mode is higher than the frequency corresponding to the bit rate of the electrical signal,
Optical transmitter.

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