JP5681082B2 - Tunable semiconductor laser - Google Patents

Description

  The present invention relates to a wavelength tunable semiconductor laser used as a light source for optical fiber communication and a light source for optical measurement, and in particular, characteristics of a light source for optical wavelength (frequency) multiplexing system in optical communication and a light source for optical measurement that covers a wide wavelength band. Regarding improvement.

  In the wavelength division multiplexing communication system in optical fiber communication, a plurality of laser beams having different frequencies (wavelengths) are transmitted using a single optical fiber at intervals determined by the standard. Each frequency is called a channel, and a wavelength tunable laser capable of switching the oscillation frequency at high speed is required for high-speed channel switching.

In a wavelength tunable semiconductor laser for communication, a laser that oscillates at a single wavelength called a single mode laser is used. In order to obtain a single mode, for example, a diffraction grating having periodic irregularities in a waveguide is used. It is used. The semiconductor optical waveguide in which the diffraction grating is formed serves as a distributed reflector (DBR) that selectively reflects at the Bragg wavelength λ _B obtained from the diffraction grating period Λ and the equivalent refractive index n of the optical waveguide. Here, the relational expression between the Bragg wavelength λ _B , the diffraction grating period Λ, and the equivalent refractive index n of the optical waveguide is
λ _B = 2nΛ (1)
It becomes. A tunable semiconductor laser produced by giving a gain to a distributed reflector is called a distributed feedback (DFB) laser.

From formula (1), it can be seen that the Bragg wavelength λ _B can be changed by changing the equivalent refractive index n of the distributed reflector. In other words, a wavelength tunable semiconductor laser capable of changing the oscillation wavelength by changing the equivalent refractive index n can be configured by configuring a resonator using a distributed reflector that can selectively change the wavelength of reflection. Is possible. As a wavelength tunable semiconductor laser using a diffraction grating, it has a plurality of reflection peaks by a distributed reflection type laser (DBR laser) using a uniform diffraction grating distributed reflector, or by periodically providing a diffraction grating. SG (Sampled Grating) -DBR laser, SSG (Super Structure Grating) -DBR laser, etc. using a distributed reflector are known.

As a wavelength tunable semiconductor laser capable of continuously changing the wavelength, there is a distributed active DFB laser (TDA-DFB laser). FIG. 8 is a sectional view showing the basic structure of a conventional distributed active DFB laser. This distribution activity DFB laser, as shown in FIG. 8, on the lower cladding layer 1, an active waveguide layer 2 of a length L _a, inactive length L _t of different composition from the active waveguide layer 2 Waveguide layers (wavelength control layers) 3 are alternately connected periodically. A diffraction grating 5 in which periodic irregularities are formed between the active waveguide layer 2 and the inactive waveguide layer 3 and between the upper cladding layer 4 and the equivalent refractive index of the waveguide is periodically modulated is formed. Has been. Electrodes 7 and 8 are formed on the upper cladding 4 so as to correspond to the active waveguide layer 2 and the inactive waveguide layer 3, respectively. While the common electrode 9 is formed at the lower part of the substrate, the electrodes 7 and 8 formed at the upper part of the substrate are separated by the region of the active waveguide layer 2 and the region of the inactive waveguide layer 3. The electrodes 7 of the active waveguide layer 2 and the electrodes 8 of the inactive waveguide layer 3 are short-circuited on the element.

As described above, the distributed active DFB laser has a structure in which the active waveguide layer 2 and the inactive waveguide layer 3 are alternately and periodically connected in the light propagation direction. Although light is emitted and gain is generated by injecting the current I _a into the active waveguide layer 2, a diffraction grating 5 is formed in each of the waveguide layers 2 and 3, and only a wavelength corresponding to the period of the diffraction grating 5 is formed. Selective reflection causes laser oscillation. On the other hand, by injection of current I _t to the inactive waveguide layer 3 the refractive index by the plasma effect in accordance with the carrier density changes, the optical period of the diffraction grating 5 of the non-active waveguide layer 3 changes. The equivalent refractive index of the inactive waveguide layer 3 changes, and the resonant longitudinal mode wavelength is shifted to the short wavelength side by the ratio of the length of the wavelength control region to the length of one period. If the active waveguide layer 2 of the optical active region length along the propagation direction L _a of the wavelength control region length in the direction of propagation of light in the non-active waveguide layer 3 and L _t, 1 cycle of repeating structural is the length L L _t + L _a, and the ratio [Delta] [lambda] _r / lambda _r of change in the resonance longitudinal mode wavelength lambda _r,
Δλ _r / λ _r = (L _t / (L _t + L _a )) · (Δn / n) (2)
(For example, see Non-Patent Document 1).

On the other hand, each wavelength of the plurality of reflection peaks is also shifted to the short wavelength side as a result of a change in equivalent refractive index due to current injection. The reflection peak wavelength is proportional to the average equivalent refractive index change of the repeating structural one cycle, the ratio [Delta] [lambda] _s / lambda _s of the change in the reflection peak wavelength lambda _s is
Δλ _s / λ _s = (L _t / (L _t + L _a )) · (Δn / n) (3)
It becomes. From equations (2) and (3), the reflection peak wavelength λ _s and the resonance longitudinal mode wavelength λ _r are shifted by the same amount. Therefore, in this laser, the wavelength continuously changes while maintaining the first oscillation mode. However, in the case of the structure shown in FIG. 8, since the diffraction grating 5 is continuously formed at the same period, the wavelength that originally satisfies the oscillation phase condition deviates from the reflection peak wavelength, and the single mode property is poor. In order to improve the single mode characteristic, it is necessary to insert a phase shift (λ / 4) for satisfying the phase condition of the resonator near the center of the resonator.

  In the distributed active DFB laser disclosed in Patent Document 1, active waveguide layers and inactive waveguide layers are alternately and periodically connected on the lower clad, and the upper clad is formed thereon. In this structure, electrodes corresponding to the active waveguide layer and the inactive waveguide layer are formed on the upper clad, and a common electrode is formed below the lower clad. In this distributed active DFB laser, the diffraction grating is formed only in a part, but the wavelength continuously changes in the same manner as the distributed active DFB laser shown in FIG. However, since the diffraction grating is periodically formed in the resonator (referred to as a sample diffraction grating), a plurality of reflection peaks can be formed, and thus it is necessary to improve the single mode property.

  Therefore, Patent Document 1 discloses a structure of a distributed active DFB laser in which two lasers having different repetition periods are integrated in series on the same substrate, and a diffraction grating is formed in each active waveguide layer. ing. The interval between the multiple reflection peaks depends on the sample period of the diffraction grating. By changing this period on the left and right sides of the resonator, the reflection peaks other than the reflection main peak do not overlap on the left and right sides of the resonator. Has improved.

  Furthermore, in Patent Document 2, a structure in which two lasers having different repetition periods of an active waveguide layer and an inactive waveguide layer are connected in cascade and a diffraction grating is formed over the entire resonator, or the active waveguide layer and the inactive waveguide layer are inactive. A structure in which a plurality of laser parts having different repetition periods of the waveguide layer are connected in cascade, and a plurality of laser parts having different repetition periods of the active waveguide layer and the inactive waveguide layer in order to suppress spatial hole burning. A structure in which a phase shift is inserted between the connected laser units is disclosed.

Japanese Patent No. 3237733 JP 2008-103466 A

Ishii et al, "A Tunable Distributed Amplification DFB Laser Diode (TDA-DFB-LD)," IEEE Photonics Technology Letters, vol. 10, no. 1, Jan 1998.

FIG. 12 is a graph showing the difference between the refractive index of the inactive waveguide layer and the refractive index of the active waveguide layer for the distributed active DFB laser having the first laser unit and the second laser unit. Normalized threshold gain difference Δg _th / g _th1 obtained by dividing the difference Δg _th between the threshold gain of the first mode and the threshold gain of the second mode by the threshold gain g _th1 of the first mode (values indicated by black circles and solid lines in the figure). (Corresponding to the left axis) and threshold gain g _th1 of the first mode (values indicated by white circles and wavy lines in the figure, corresponding to the right axis).

In the conventional structure, when a current is passed through the inactive waveguide layer, that is, when carriers are injected, the refractive index decreases. When the refractive index difference between the inactive waveguide layer and the active waveguide layer is 0, the threshold gain g _th1 is the lowest and the normalized threshold gain difference Δg _th / g _th1 is increased. The lower the threshold gain g _th1, the lower the threshold current and the easier the laser oscillation. The normalized threshold gain difference Δg _th / g _th1 is a parameter related to single mode characteristics, and a larger value indicates better single mode characteristics. Therefore, the best characteristics as a semiconductor laser can be obtained when the difference in refractive index is zero.

  For example, assuming that the maximum change amount of the refractive index of the inactive waveguide layer is 0.02, and the refractive index difference in the initial state before current injection into the inactive waveguide layer is 0.01, the refractive index difference is It will vary between 0.01 and -0.01. In this way, by assigning positive and negative with the refractive index difference 0 as the center, it is possible to use a region where good characteristics can be obtained on average.

Furthermore, in reality, when carriers are injected into the inactive waveguide layer, losses such as free electron absorption increase, and thus the threshold gain g _th1 increases. Considering this, if the refractive index difference starts in advance from the point where the refractive index difference is large and the refractive index difference becomes close to 0 after the refractive index change of the inactive waveguide layer, the threshold gain g _th1 shown in FIG. By the decrease, it is possible to cancel the increase in the threshold gain g _th1 due to the increase in loss after the refractive index change.

However, if the initial refractive index difference is too large, the threshold gain g _th1 is high and the normalized threshold gain difference Δg _th / g _th1 is small, so that the refractive index difference is finally near zero as much as possible. You should do it. That is, in the above-described example, it is preferable that the initial refractive index difference is set to about 0.015 to about 0.02 so that the refractive index difference finally becomes around zero.

  However, in order to use the above-described refractive index setting method, either the active waveguide layer or the inactive waveguide layer, that is, once the refractive index is determined, the other waveguide layer is set. The refractive index that can be determined is determined, and the layer structure is limited.

  In order to increase the maximum refractive index difference of the inactive waveguide layer, it is necessary to make the composition of the waveguide core layer close to the oscillation wavelength so that the band gap wavelength is shorter than the oscillation wavelength and no gain is generated. . That is, the wavelength is increased, but in this case, the refractive index is increased.

  In addition, the refractive index of the inactive waveguide layer changes depending on the carriers accumulated in the core layer by current injection, but the optical confinement increases by increasing the thickness of the core layer of the inactive waveguide layer. Becomes larger. However, the thicker the core layer, the higher the refractive index.

  On the other hand, the refractive index of the active waveguide layer can be increased by increasing the number of quantum wells, but if it is increased too much, it adversely affects laser characteristics such as non-uniform carrier injection. .

  As described above, the design for increasing the refractive index change of the inactive waveguide layer is in the direction of increasing the refractive index, but the refractive index of the active waveguide layer must be determined to optimize the laser characteristics. It is desirable to be able to design individually.

  For this reason, the present invention can avoid the restriction of the refractive index difference between the active waveguide layer and the inactive waveguide layer in the conventional structure, and the degree of freedom in design is improved. An object of the present invention is to provide a wavelength tunable semiconductor laser that can have a layer structure suitable for each of the active waveguide layers.

A wavelength tunable semiconductor laser according to a first aspect of the present invention for solving the above-described problems is obtained by alternately and periodically forming an active waveguide layer having gain and an inactive waveguide layer for controlling the wavelength on a semiconductor substrate. A first laser portion in which the active waveguide layer and the inactive waveguide layer are connected at a first period, the active waveguide layer and the inactive structure. A distributed active DFB laser including a second laser unit connected to a waveguide layer at a second period, wherein a diffraction grating is formed over the entire length of the active waveguide layer and the inactive waveguide layer A correction phase shift of a phase shift amount Ω _BJ that satisfies the following expression (4) is inserted at the position of the diffraction grating corresponding to the joint surface between the active waveguide layer and the inactive waveguide layer, Connection between the first laser unit and the second laser unit When the refractive index of the active waveguide layer and the non-active waveguide layer is uniform along the resonance direction in addition to the correction phase shift, the phase condition of the resonator is A phase shift set so as to satisfy is inserted.
-Λ / 4 <Ω _BJ <0 (4)

A wavelength tunable semiconductor laser according to a second aspect of the present invention for solving the above-mentioned problem is that an active waveguide layer having a gain and an inactive waveguide layer for controlling the wavelength are alternately and periodically formed on a semiconductor substrate. A first laser portion in which the active waveguide layer and the inactive waveguide layer are connected at a first period, the active waveguide layer and the inactive structure. A distributed active DFB laser including a second laser unit connected to a waveguide layer at a second period, wherein a diffraction grating is provided only in one of the active waveguide layer and the inactive waveguide layer. The phase relationship between the adjacent diffraction gratings formed is shifted by a phase shift amount 2Ω _BJ that satisfies the following expression (5 −1 ), and the first laser unit and the second laser unit The phase relationship between the diffraction gratings adjacent to each other with the connection part between When the refractive index of the active waveguide layer and the inactive waveguide layer is uniform along the resonance direction, the phase shift set so as to satisfy the phase condition of the resonator is Ω _C0 , and Ω _C0 + 2Ω _BJ It is characterized by only shifting.
-Λ / 4 <Ω _BJ <0 (5 -1 )
A wavelength tunable semiconductor laser according to a third aspect of the present invention for solving the above-described problem is provided by alternately and periodically forming an active waveguide layer having gain and an inactive waveguide layer for controlling the wavelength on a semiconductor substrate. A first laser portion in which the active waveguide layer and the inactive waveguide layer are connected at a first period, the active waveguide layer and the inactive structure. A distributed active DFB laser including a second laser unit formed by connecting a waveguide layer with a second period, wherein diffraction gratings are formed in both the active waveguide layer and the inactive waveguide layer The phase shift amount 2Ω _BJ that satisfies the following expression (5-2) is inserted into only one of the adjacent boundaries of each of the waveguide layers, and the first laser unit and the first laser unit Position correlation of the diffraction gratings adjacent to each other across the connecting part with the two laser parts But the phase shift as Omega _C0 refractive index of the active waveguide layer and the inactive waveguide layer is set in the cavity of the phase condition is satisfied if it is uniform along the resonance direction, Omega _C0 It is characterized by a shift of + 2Ω _BJ .
-Λ / 4 <Ω _BJ <0 (5-2)

A wavelength tunable semiconductor laser according to a fourth aspect of the present invention for solving the above-described problems is the wavelength tunable semiconductor laser according to any one of the first to third aspects, wherein the first laser section and the second laser section are the same. The repetition period of the active waveguide layer and the inactive waveguide layer is different in each of the laser parts.

  According to the present invention, it becomes possible to avoid the limitation of the refractive index difference between the active waveguide layer and the inactive waveguide layer in the conventional structure, and the degree of freedom in design is improved, and the active waveguide layer and the inactive waveguide layer are improved. The layers can have a layer structure suitable for each.

It is explanatory drawing which shows the cross-section of the wavelength tunable semiconductor laser which concerns on 1st embodiment of this invention. 1 is a top view of a wavelength tunable semiconductor laser according to a first embodiment of the present invention. It is a figure explaining the effect of 1st embodiment of this invention. It is a figure explaining the effect of 1st embodiment of this invention. It is a figure explaining the effect of 1st embodiment of this invention. It is a figure explaining the effect of 1st embodiment of this invention. It is a figure explaining a phase shift. It is explanatory drawing which shows the cross-section of the wavelength tunable semiconductor laser which concerns on 2nd embodiment of this invention. It is a schematic diagram explaining the phase relationship of the wavelength tunable semiconductor laser which concerns on 2nd embodiment of this invention. It is explanatory drawing which shows the cross-section of the wavelength tunable semiconductor laser which concerns on 3rd embodiment of this invention. It is explanatory drawing which shows the basic structure of a general distributed active DFB laser. It is a figure explaining the characteristic of a distributed active DFB laser.

  The details of the wavelength tunable semiconductor laser according to the present invention will be described below with reference to the drawings.

(First embodiment)
A first embodiment of a wavelength tunable semiconductor laser according to the present invention will be described below with reference to FIGS.
FIG. 1 is a schematic diagram showing a cross section along the waveguide direction of the wavelength tunable semiconductor laser according to the first embodiment of the present invention.

As shown in FIG. 1, in the wavelength tunable semiconductor laser according to the present embodiments, n-type on the InP lower cladding layer 11, GaInAsP active waveguide layer 12 _a1 of the first length in the laser unit A ₁ L _a1 The active waveguide layer 12 _a1 is alternately and periodically connected to a GaInAsP inactive waveguide layer (wavelength control layer) 13 _t1 having a length L _{t1 having} a different composition. Further, in the second laser portion A ₂ , the GaInAsP active waveguide layer 12 _a2 having a length L _{a2, and} a GaInAsP inactive waveguide layer 13 _t2 having a length L _{t2 having} a different composition from the active waveguide layer 12 _a2 , Are alternately connected periodically. Hereinafter, the GaInAsP active waveguide layers 12 _a1 and 12 _a2 are collectively referred to as GaInAsP active waveguide layers 12 and the GaInAsP inactive waveguide layers 13 _t1 and 13 _t2 are collectively referred to as GaInAsP inactive waveguide layers 13. .

  Periodic irregularities were formed between the GaInAsP active waveguide layer 12 and the GaInAsP inactive waveguide layer 13 and the p-type InP upper cladding layer 14 to periodically modulate the equivalent refractive index of the waveguide. A diffraction grating 15 is formed. A highly doped p-type InGaAs contact layer 16 is provided on the InP upper clad 14 for ohmic contact, and electrodes 17, 17 corresponding to the active waveguide layer 12 and the inactive waveguide layer 13, respectively, are provided thereon. 18 is formed. The electrode 19 formed in the lower part of the substrate is common, but the electrode formed in the upper part of the substrate is separated into the active waveguide layer 12 region and the inactive waveguide layer 13 region. Specifically, the contact layer 16 and the electrodes 17 and 18 are separated by the region of the active waveguide layer 12 and the region of the inactive waveguide layer 13, and further, as shown in FIG. The electrodes 17 and the electrodes 18 of the inactive waveguide layer 13 are short-circuited on the element.

Further, in the wavelength tunable semiconductor laser according to the present embodiment, the repetition period of the active waveguide layer 12 and the inactive waveguide layer 13 is set to L ₁ , in the first laser part A ₁ and the second laser part A ₂ . in place of the L ₂ has a structure in which are connected in series. Further, a phase shift of a phase shift amount Ω _BJ (to be described later) (hereinafter referred to as a corrected phase shift) is put in the diffraction grating near the boundary between all the active waveguide layers 12 and the inactive waveguide layer 13, and the first laser part A _In addition to the correction phase shift, a phase shift for satisfying the phase condition of the resonator having the phase shift amount λ / 4 is inserted between the first laser part A ₂ and the second laser part A ₂ . As a result, the phases of the reflected wave at the first laser part A ₁ and the reflected wave at the second laser part A ₂ are matched so as to satisfy the oscillation condition. Hereinafter, the phase shift inserted between the first laser part A ₁ and the second laser part A _{2 is called} the inter-laser part phase shift 21, and the phase shift inserted in the other part is called the waveguide interlayer phase shift 20. To do.

  Here, when GaInAsP having a band gap wavelength of 1.55 μm is used for the active waveguide layer 12, the non-active waveguide 13 uses a shorter band gap wavelength, for example, GaInAsP having a band gap wavelength of 1.4 μm. Thus, the carrier density does not become constant because it does not contribute to the laser oscillation gain. Thereby, the refractive index can be changed greatly by current injection.

  Further, the active waveguide layer 12 and the inactive waveguide layer 13 do not have to be a bulk material. For example, a quantum well structure, a multilayer quantum well structure in which quantum wells are stacked with a barrier layer interposed therebetween, a quantum dot, It may have a low-dimensional quantum well structure such as a quantum wire. Further, in order to enhance light confinement and carrier confinement in the active layer, a separate confinement heterostructure in which a layer having an intermediate refractive index is introduced between the active layer and the cladding layer may be introduced.

  Since it is important that the refractive index of the diffraction grating 15 fluctuates periodically, the position where the diffraction grating 15 is formed is between the active waveguide layer 12 or the inactive waveguide layer 13 and the upper cladding 14. For example, it may be formed between the waveguide layers 12 and 13 and the lower cladding 11 or at a position away from each layer.

  The semiconductor used in this element is not limited to the combination of InP and GaInAsP, and other semiconductors such as GaAs, GaInNAs, and AlGaInAs may be used. The band gap between the active waveguide layer 12 and the inactive waveguide layer 13 The combination of wavelengths is not limited to the above.

  In the tunable semiconductor laser according to the present embodiment, although not shown, Fe, which is a semi-insulating material, is doped on both sides of the waveguide so that the current Ia is efficiently injected into the active waveguide layer 12. A buried heterostructure (BH) in which InP is buried and regrown is formed. Instead of Fe-doped InP, p-type and n-type semiconductors may be alternately stacked to form a current blocking layer. Moreover, it is good also as an InP layer made into high resistance by doping Ru instead of Fe.

  The waveguide structure employs a buried heterostructure in this embodiment, but the principle of the present invention can also be used in a general ridge structure or high mesa structure.

In the first laser part A ₁ and the second laser part A ₂ , the repetition period L of the active waveguide layer 12 and the inactive waveguide layer 13 is different from L ₁ and L ₂ , respectively, but the active waveguide layer 12 And the ratio of the inactive waveguide layer 13 (L _a1 / L _t1 and L _a2 / L _t2 ) are the same. In this embodiment, this ratio is 1/2. An inter-laser phase shift 21 is inserted between the first laser part A ₁ and the second laser part A ₂ to change the phase of the diffraction grating 15 by ¼ wavelength + Ω _BJ . As a result, the phases of the reflected wave at the first laser part A ₁ and the reflected wave at the second laser part A ₂ are matched so as to satisfy the oscillation condition.

  As described above, the electrodes 17 and 18 provided on the active waveguide layer 12 and the wavelength control non-active waveguide layer 13 are separated from each other. As shown in FIG. The electrodes 17 and the electrodes 18 on the inactive waveguide layer 13 are connected on the element. In this way, by short-circuiting the electrodes 17 and 18 in each region on the element, the current Ia or the wavelength control current can be applied to each region only by bonding a metal bonding wire one by one. It can be injected.

  Next, a method for manufacturing a wavelength tunable semiconductor laser according to this embodiment will be briefly described. First, the active waveguide layer 12 and the inactive waveguide layer 13 are formed on the n-type InP lower clad layer 11 by using a metal organic vapor phase epitaxial growth (MOCVD) method and a selective growth method based thereon. The active waveguide layer and the inactive waveguide layer have a butt joint (BJ) structure.

Thereafter, the pattern of the diffraction grating 15 is transferred to the applied resist using an electron beam exposure method, and etching is performed using the transfer pattern as a mask to form the diffraction grating 15. As shown in FIG. 1, the diffraction grating 15 includes a waveguide layer phase shift 20 of Ω _BJ between the waveguide layers 12 and 13, and a λ between the first laser part A ₁ and the second laser part A _2. A phase shift 21 between the laser parts of / 4 + Ω _BJ is inserted.

  In the present embodiment, the diffraction grating 15 has the same period for both the active waveguide layer 12 and the inactive waveguide layer 13.

  After the p-type InP upper cladding layer 14 and the p-type InGaAs contact layer 16 are grown, in order to control the transverse mode, the waveguide is processed into a stripe shape having a width of 1.2 μm and FeP is doped on both sides of the waveguide. Growing block layer. After the electrodes 17 and 18 are formed, the p-type InGaAs contact layer 16 between the electrodes 17 and 18 is removed in order to electrically isolate the active layer drive electrode 17 and the wavelength control electrode 18. Further, when the waveguide layers 12 and 13 are separated, a separation groove may be formed.

  The semiconductor growth method is not limited to the metal organic vapor phase epitaxial growth method, and a molecular beam epitaxial growth method or other means may be used. The method of forming the diffraction grating 15 is not limited to the electron beam exposure method, and a two-bundle interference exposure method or other means may be used.

In this embodiment, the number of repetitions of the active waveguide layer 12 and the inactive waveguide layer 13 in the first laser part A ₁ and the second laser part A ₂ is set to 6, respectively. Since the first laser part A ₁ and the second laser part A ₂ use the diffraction grating 15 having the same coupling coefficient, the second laser having a long repetition period of the active waveguide layer 12 and the inactive waveguide layer 13 is used. reflectivity for better parts a ₂ becomes larger the coupling coefficient and the product of length increases. Therefore, when the number of repetitions is the same, the output is naturally asymmetric, and the output from the first laser unit A ₁ having a low reflectivity is larger than the output from the second laser unit A ₂ having a high reflectivity. Therefore, the output can be efficiently extracted from the first laser part A ₁ side. Note that the number of repetitions of the active waveguide layer 12 and the inactive waveguide layer 13 is not limited to 6, and the number of repetitions does not have to be the same in the first laser part A ₁ and the second laser part A _2. Therefore, the repetition period and the number of repetitions may be designed according to the required reflectance.

In this embodiment, the repetition period of the first laser part A ₁ is 57 μm, and the repetition period of the second laser part A ₂ is 72 μm. The coupling coefficient of the diffraction grating 15 is 25 cm ⁻¹ .

In the present embodiment, a phase shift of ¼ wavelength is inserted between the diffraction gratings 15 of the first laser part A ₁ and the second laser part A ₂ . The quarter wavelength shift is usually in the ideal state, i.e., even when the refractive indices of the active waveguide layer 12 and the inactive waveguide layer 13 are uniform along the resonance direction, Inserted to satisfy the phase condition. The phase shift of ¼ wavelength is not necessarily between the first laser part A ₁ and the second laser part A ₂ , and is within a range of about ３ of the total resonator length at the center of the resonator. If there is a phase shift of 1/4 wavelength, the phase condition can be satisfied. When the phase shift for satisfying the phase condition of the resonator is not in the vicinity of the waveguide junction surface, this phase shift and the correction phase shift for compensating for the equivalent phase shift caused by the fluctuation of the junction surface are separately Insert it.

In the present embodiment, the ratio of the active waveguide layer 12 and the inactive waveguide layer 13 of the first laser part A ₁ and the second laser part A ₂ is 1: 2. Since the average equivalent refractive index change can be increased by increasing the proportion of the inactive waveguide 13, the amount of wavelength change can be increased. However, if the ratio of the inactive waveguide layer 13 is increased, the ratio of the active waveguide layer 12 is inevitably decreased, and it becomes difficult to obtain a gain necessary for laser oscillation. Therefore, it is necessary to adjust the ratio according to the design such as the number of active layers and the loss of the waveguide. However, the principle of the present invention is that the first laser part A ₁ and the second laser part A _{2 are used.} Since the ratio of the active waveguide layer 12 and the inactive waveguide layer 13 is the same, the ratio can be changed as required.

As described above, in the wavelength tunable semiconductor laser according to the present embodiment, a normal wavelength tunable semiconductor laser is manufactured only in that the active waveguide layer 12 and the inactive waveguide layer 13 are alternately and periodically arranged. It can be easily manufactured using a method. Inserting the waveguide interlayer phase shift 20 of Ω _BJ can be realized only by changing the pattern of electron beam drawing. The diffraction grating 15 can be realized by periodically repeating line and space by electron beam drawing. The waveguide interlayer phase shifts 20 and 21 are the positions of lines or spaces where the waveguide interlayer phase shifts 20 and 21 are to be inserted. Can be realized by shifting a desired amount.

  Further, since the layer structure of a normal pn diode type can be easily integrated with a semiconductor amplifier, a modulator, etc., it can be used as a light source element that is an element of a high-performance multifunctional element.

Subsequently, the phase shift will be described. 3 shows the phase shift amount of the waveguide layer phase shift 20 inserted between the active waveguide layer 12 and the inactive waveguide layer 13 as Ω _BJ , and the first laser part A ₁ and the second laser in FIG. Refractive index difference between the active waveguide layer 12 and the inactive waveguide layer 13 when the phase shift amount of the inter-laser phase shift 21 inserted between the active portion A ₂ and the portion A ₂ is Ω _C (= λ / 4 + Ω _BJ ). Is a diagram in which the normalized threshold gain difference Δg _th / g _th1 is calculated with the horizontal axis. FIG. 4 similarly shows the result of calculating the threshold gain g _th1 . Here, the values when the phase shift amount Ω _BJ is 0 and the phase shift amount Ω _C is 0.25λ are the same as the calculation results in the conventional distributed active DFB laser shown in FIG.

Since the value of the phase shift amount Ω _C is originally a value obtained by adding the phase shift amount Ω _BJ to the λ / 4 shift (0.25λ shift) that is inserted to obtain phase matching, the phase shift amount Ω _BJ is −λ. When / 8 shift (0.125λ shift), the phase shift amount Ω _C is λ / 8, and when the phase shift amount Ω _BJ is −λ / 6 shift (0.167λ shift), the phase shift amount Ω _C is λ / 12 shift (0.083λ shift), and when the phase shift amount Ω _BJ is −λ / 5 shift (0.2λ shift), the phase shift amount Ω _C is λ / 20 shift (0. 05λ shift).

As can be seen from FIG. 3, as the phase shift amount Ω _BJ increases, the peak position (the difference in refractive index that improves single-mode characteristics most) shifts to the positive side of the difference in refractive index. However, the magnitude of the normalized threshold gain difference Δg _th / g _th1 at the peak position becomes small. In addition, it can be seen from FIG. 4 that the minimum value of the threshold gain g _th1 (the refractive index difference at which the threshold current decreases most) is shifted to the positive side of the refractive index difference. However, the threshold gain g _th1 becomes large at the minimum value.

However, for example, when the phase shift amount Ω _BJ is 0 (conventional structure) and when the phase shift amount Ω _BJ is −λ / 8, the characteristics are reversed when the refractive index difference is around 0.015. When the difference in refractive index exceeds 0.015, the characteristics are improved when the phase shift amount Ω _BJ is −λ / 8. Further, when the phase shift amount Ω _BJ is −λ / 6, the characteristics when the phase shift amount Ω _BJ is 0 are reversed at a position where the refractive index difference slightly exceeds 0.02. When the phase shift amount Ω _BJ is −λ / 5, the characteristics when the refractive index difference is between 0.025 and 0.03 and the phase shift amount Ω _BJ is 0 are reversed.

As described above, the Ω _BJ shift shifts the maximum value of the normalized threshold gain difference Δg _th / g _{th1 and} the minimum value of the threshold gain difference Δg _th to a positive region of the refractive index difference, and thus has a large refractive index difference. The characteristics are better than the conventional structure (when there is no Ω _BJ shift).

In FIG. 5, the phase shift amount Ω _BJ is fixed to −λ / 8, the value of the phase shift amount Ω _C is changed, and the normalized threshold gain difference Δg _th / g _th1 with the horizontal axis as the refractive index difference as in FIG. Is the result of calculating. It can be seen that when the value of the phase shift amount Ω _C is made smaller than λ / 8 (0.125), the peak position is further shifted to the side where the refractive index difference is larger. FIG. 6 shows the result of calculating the threshold gain g _th1 with the horizontal axis as the refractive index difference in the same manner as in FIG. 4 with the phase shift amount Ω _BJ fixed at −λ / 8 and the value of the phase shift amount Ω _C changed. is there. In a region where the refractive index difference is larger than 0.01, the threshold gain g _th1 becomes smaller as the value of the phase shift amount Ω _C becomes smaller.

However, if the phase shift amount Ω _C is made too small, as can be seen from the results of Ω _C = 0.025λ in FIGS. 5 and 6, no improvement in characteristics can be seen in a region where the refractive index difference is large. Although not shown in the figure, since no characteristic improvement is observed when Ω _C = 0, it is desirable that the phase shift amount Ω _C is larger than zero.

As described above, when the negative phase shift amount Ω _BJ is inserted, the phase shift amount Ω _C is not a value obtained by adding the phase shift amount Ω _BJ to the conventional λ / 4 shift, but a smaller value than that. It can be seen that the characteristics are improved in the region where the refractive index difference is positive. That is, in the conventional structure, due to the limitation of the refractive index difference between the active waveguide layer and the non-active waveguide layer, it is necessary to design the other layer structure by either layer structure. Therefore, for example, the active waveguide layer 12 is designed such that the threshold current is lowered and the output is increased, and the inactive waveguide layer 13 is refracted. The layer structure can be designed independently, such as a design that greatly changes the rate.

From the results of FIGS. 3 to 6, the ranges of the phase shift amount Ω _BJ and the phase shift amount Ω _C can be expressed by the following equations (6) and (7), respectively.

In this embodiment, since the phase shift amount Ω _C is inserted into the waveguide connection portion, the phase shift amount Ω _C includes the phase shift amount Ω _BJ . However, the phase shift amount Ω _BJ is independently set. It may be arranged. In this case, the phase shift amount Ω _C0 inserted separately from the phase shift amount Ω _BJ may be set to λ / 4 shift or less as shown in the following equation (8).

Here, it is clear that Ω _C , Ω _C0 and Ω _BJ satisfy the relationship of the following equation (9).
Ω _C = Ω _C0 + Ω _BJ (9)

In the present embodiment, the repetition period of the active waveguide layer 12 and the inactive waveguide layer 13 is changed between the first laser part A ₁ and the second laser part A _2. The invention can be applied. However, by changing the period as in the present embodiment, a good range of single mode characteristics with respect to the refractive index difference is widened. Therefore, in the present invention that actively uses a large range of the refractive index difference, as described in the present embodiment, it is better to make the repetition period different between the _first laser part A ₁ and the second laser part A _2. It ’s better.

  The phase shift has a relationship defined in FIG. FIG. 7 is a schematic diagram of the refractive index fluctuation of the diffraction grating when the horizontal axis is the position and the vertical axis is the refractive index. FIG. 7A is a phase shift is zero, and FIG. FIG. 7C shows a case where the phase shift is λ / 4. A negative phase shift will cause the position to shift in the opposite direction.

(Second embodiment)
Hereinafter, a third embodiment of the wavelength tunable semiconductor laser according to the present invention will be described with reference to FIGS. FIG. 8 is a diagram for explaining the third embodiment.

In the first embodiment, the distributed active DFB laser in which the diffraction grating 15 is formed over the entire resonator has been described as an example. In the present embodiment, the positions where the diffraction grating is formed are limited partially and periodically. A distributed active DFB laser as a sample diffraction grating will be described as an example.
Hereinafter, members similar to those shown in FIGS. 1 to 7 and described in the first embodiment are denoted by the same reference numerals, and redundant description is omitted, and different points will be mainly described.

  As shown in FIG. 8, in this embodiment, the active waveguide layer 22 has a diffraction grating 24, and the inactive waveguide layer 23 has no diffraction grating. Since the non-active waveguide layer 23 has no diffraction grating, the phase relationship between the active waveguide layer 22 and the non-active waveguide layer 23 cannot be determined as in the first embodiment. The phase relationship of the diffraction grating 15 between the waveguide layers 22 is important.

That is, for example, light propagating through the active waveguide layer 22 _a11 followed by non-active waveguide layer 23 _t1 is, to enter the next active waveguide layer 22 _a12, the optical length of the non-active waveguide layer 23 _t1, This means that the phase relationship between the diffraction gratings 24 _a11 and 24 _a12 between the active waveguide layers 22 is determined. This may be considered that the inactive waveguide layer 23 is a phase shift of the diffraction grating 24 formed in the active waveguide layer 22.

  FIG. 9 is a diagram for explaining that the inactive waveguide layer 23 becomes a phase shift of the diffraction grating 15 formed in the active waveguide layer 22. For the sake of simplicity, the description here assumes that the average refractive indexes of the active waveguide layer 22 and the inactive waveguide layer 23 are equal.

The active waveguide layer 22 has a diffraction grating 24, and the inactive waveguide layer 23 has no diffraction grating. In FIG. 9, as an example, the diffraction grating when it is assumed that the diffraction grating 24 _a11 of the left active waveguide layer 22 _a11 continues to the inactive waveguide layer 23 is written with a dotted line. In the wavelength tunable semiconductor laser according to the above, it can be seen that the phase of the dotted line and the diffraction grating 24 _{a12 of the} right active waveguide layer 22 _a12 do not match. In other words, by adopting a configuration in which the phases of the diffraction gratings 24 of the adjacent active waveguide layers 22 are not continuous with each other, the same effect as when a phase shift is present between the diffraction gratings 24 of the adjacent active waveguide layers 22 is obtained. An effect can be obtained.

In the present invention, as described in the first embodiment, a phase shift of the phase shift amount Ω _BJ is inserted between the active waveguide layer 22 and the inactive waveguide layer 23. In the second embodiment, since the non-active waveguide layer 23 has a structure without a diffraction grating, the phase relationship between the active waveguide layers 22 is the second phase relationship of the phase shift amount Ω _BJ . The phase shift range defined in one embodiment can be applied. Similarly, the phase relationship between the active waveguide layers 22 sandwiching the position of the phase shift amount Ω _C may be considered as λ / 4 + 2 × Ω _BJ .

  In the present embodiment, a diffraction grating 24 is formed in the active waveguide layer 22 and there is no diffraction grating in the non-active waveguide layer 23. Conversely, a diffraction grating is formed in the non-active waveguide layer 23 to activate the active waveguide. The present invention can be applied even when the waveguide layer 22 has no diffraction grating. In that case, if the active waveguide 26 and the inactive waveguide 27 are replaced, it can be easily analogized.

(Third embodiment)
A third embodiment of the wavelength tunable semiconductor laser according to the present invention will be described with reference to FIG.
FIG. 10 is a diagram for explaining the third embodiment. As can be inferred from the first embodiment and the second embodiment, the phase relationship between the diffraction gratings in the same region is important in order to obtain the effects of the present invention. That is, active waveguides and non-active waveguides.

  Therefore, in this embodiment, as shown in FIG. 10, the phase shift 24 is inserted into only one of the adjacent boundaries of the waveguide of each waveguide layer. In this case, as the phase shift 24, a value twice the waveguide interlayer phase shift 20 inserted in the first embodiment may be inserted as in the second embodiment. Thereby, the range of the phase shift 24 can be considered as the range described in the first embodiment.

  The position where the phase shift 24 is inserted is not limited to the position shown in FIG. 10, and may be inserted at any one of the adjacent boundaries of each waveguide layer.

  Other configurations are the same as the configurations illustrated in FIGS. 1 to 7 and described in the first embodiment, and the same members are denoted by the same reference numerals, and redundant descriptions are omitted.

  The present invention is suitable for application to a wavelength tunable semiconductor laser.

11 n-type InP lower cladding layer 12, 22 GaInAsP active waveguide layer 13, 23 GaInAsP inactive waveguide layer (wavelength control layer)
14 p-type InP upper cladding layer 15, 24 diffraction grating 16 p-type InGaAs contact layer 17 active layer electrode 18 wavelength control electrode 19 electrode 20, 25 waveguide phase shift 21 phase shift between laser parts

Claims (4)

A structure in which an active waveguide layer having gain and an inactive waveguide layer for controlling a wavelength are alternately and periodically formed on a semiconductor substrate, the active waveguide layer and the inactive A first laser part in which a waveguide layer is connected in a first period; and a second laser part in which the active waveguide layer and the inactive waveguide layer are connected in a second period; A distributed active DFB laser comprising:
A diffraction grating is formed over the entire length of the active waveguide layer and the inactive waveguide layer,
A correction phase shift of a phase shift amount Ω _BJ that satisfies the following equation (1) is inserted at the position of the diffraction grating corresponding to the joint surface between the active waveguide layer and the inactive waveguide layer,
In addition to the correction phase shift, the refractive index of the active waveguide layer and the inactive waveguide layer is added to the position of the diffraction grating corresponding to the connection portion between the first laser portion and the second laser portion. A wavelength tunable semiconductor laser, wherein a phase shift is set so as to satisfy the phase condition of the resonator when uniform along the resonance direction.
-Λ / 4 <Ω _BJ <0 (1) A structure in which an active waveguide layer having gain and an inactive waveguide layer for controlling a wavelength are alternately and periodically formed on a semiconductor substrate, the active waveguide layer and the inactive A first laser part in which a waveguide layer is connected in a first period; and a second laser part in which the active waveguide layer and the inactive waveguide layer are connected in a second period; A distributed active DFB laser comprising:
A diffraction grating is formed only in one of the active waveguide layer or the inactive waveguide layer, and the phase relationship between adjacent diffraction gratings is shifted by a phase shift amount 2Ω _BJ that satisfies the following equation (2). And
The phase relationship between the diffraction gratings adjacent to each other across the connecting portion between the first laser portion and the second laser portion is such that the refractive indexes of the active waveguide layer and the inactive waveguide layer are along the resonance direction. Te as the phase shift Omega _C0 is set to the phase condition is satisfied of the resonator when it is uniform, the wavelength tunable semiconductor laser, characterized in that shifted by Ω _C0 + 2Ω _BJ.
-Λ / 4 <Ω _BJ <0 (2) A structure in which an active waveguide layer having gain and an inactive waveguide layer for controlling a wavelength are alternately and periodically formed on a semiconductor substrate, the active waveguide layer and the inactive A first laser part in which a waveguide layer is connected in a first period; and a second laser part in which the active waveguide layer and the inactive waveguide layer are connected in a second period; A distributed active DFB laser comprising:
Diffraction gratings are formed in both the active waveguide layer and the inactive waveguide layer, and a phase shift that satisfies the following expression (3) is provided only at one of the adjacent boundaries of each of the waveguide layers. Quantity 2Ω _BJ And insert
The phase relationship between the diffraction gratings adjacent to each other across the connecting portion between the first laser portion and the second laser portion is such that the refractive indexes of the active waveguide layer and the inactive waveguide layer are along the resonance direction. Phase shift set to satisfy the phase condition of the resonator when _C0 As Ω _C0 + 2Ω _BJ Just shifting
A tunable semiconductor laser characterized by the above.
-Λ / 4 <Ω _BJ <0 (3) In said first laser part and said second laser unit, any one of claims 1 to 3, repetition period of the said active waveguide layer inactive waveguide layer are different from each other wavelength-tunable semiconductor laser according to.

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