JP7524973B2 - Optical Devices - Google Patents

Description

The present invention relates to optical devices using semiconductor lasers.

In recent years, data traffic in data centers has been increasing year by year with the spread of smartphones and cloud services. In order to support this data traffic, it is important to make optical transmitters more compact, less power-consuming, and less costly. Semiconductor lasers, a typical optical transmitter, are capable of transmitting large-capacity signals over long distances. Semiconductor lasers used in information and communication systems include externally modulated lasers, which modulate signals using a modulator installed outside the semiconductor laser. Semiconductor lasers used in information and communication systems also include directly modulated lasers, which directly modulate the output light by modulating the current injected into the active region.

Directly modulated lasers have advantages over externally modulated lasers, such as small size, low power consumption, and low cost. For this reason, directly modulated lasers are widely used as transmitters in information and communication systems for relatively short distances within and between data centers. However, directly modulated lasers have a narrower modulation band than externally modulated lasers, which means that their modulation speed is slow. This is because when the current injection is increased for high-speed operation, the heat generated in the semiconductor laser active layer reduces the light emission efficiency and limits the increase in the modulation band. In general lasers, the intrinsic band is limited by the relaxation oscillation frequency.

To solve this problem of band limitation, a laser structure using photon-photon resonance (PPR), a phenomenon of photon-photon resonance, has been proposed. In a directly modulated laser using PPR, a new resonance peak is generated in a frequency region higher than the relaxation oscillation frequency, where the response has traditionally been reduced, making it possible to expand the modulation band (Non-Patent Document 1, Non-Patent Document 2, Non-Patent Document 3).

In a directly modulated laser using PPR, a distributed feedback (DFB) laser active region and a passive waveguide that serves as the optical feedback mechanism are connected adjacent to each other. The laser active region is optically connected to one end of the passive waveguide. Both ends of the passive waveguide are reflection points (Non-Patent Documents 2 and 3). The laser light generated in the laser active region interacts with the Fabry-Perot type resonance mode formed in the optical feedback region of the passive waveguide, and PPR occurs when the phase matching condition is satisfied. In the passive waveguide, the state of PPR is controlled by adjusting the phase, for example, by changing the refractive index due to the injected current. The PPR frequency is roughly defined as the frequency difference between the laser oscillation mode and the Fabry-Perot type resonance peak.

Therefore, the frequency at which the response enhancement due to PPR occurs is defined within the free spectral range (FSR) defined by the length of the optical feedback region of the passive waveguide. For example, in Non-Patent Document 1, the length of the optical feedback region is limited to 300 μm to generate PPR at approximately 43 GHz. Phase control of this relatively long optical feedback region requires a large amount of power consumption.

Here, for example, in Non-Patent Document 2, the waveguide length is 135 μm, and in Non-Patent Document 3, the waveguide length is 120 μm, and a larger FSR is defined compared to Non-Patent Document 1. By using such a configuration, the PPR frequency determined by the frequency difference between the laser oscillation mode and the Fabry-Perot type resonance peak can be defined larger. Furthermore, in the above-mentioned configuration, since the FSR is large, the phase change with respect to the wavelength change is gentle, and therefore, relatively stable operation can be realized with respect to the change in injection current and the change in temperature.

M. Radziunas et al., "Improving the Modulation Bandwidth in Semiconductor Lasers by Passive Feedback", IEEE Journal of Selected Topics in Quantum Electronics, vol. 13, no. 1, pp. 136-142, 2007. S. Yamaoka, et al., “239.3-Gbit/s Net Rate PAM-4 Transmission Using Directly Modulated Membrane Lasers on High-Thermal-Conductivity SiC”, in Proc. ECOC’19, paper PD.2.1. ,2020. Y. Matsui, et al., “Isolator-free> 67-GHz bandwidth DFB+ R laser with suppressed chirp”, in Proc. OFC’20, paper Th4C. 4. , 2020.

However, although the above technology allows the PPR frequency to be controlled by phase control, it is difficult to simultaneously control the PPR frequency and the laser's electro-optic response (laser S21) response at the PPR frequency. Also, in a laser using PPR that is configured with a relatively short resonator (Non-Patent Document 2), it is considered necessary to compensate for the optical output intensity by using, for example, a semiconductor optical amplifier (SOA) that uses a gain medium.

As described above, the conventional technology had the problem that it was not possible to independently control the PPR frequency and the intensity of the fed-back reflected light and the intensity of the output light, which determine this response.

The present invention has been made to solve the above problems, and aims to make it possible to independently control the PPR frequency and the intensity of the fed-back reflected light and output light that determine this response.

The optical device according to the present invention comprises: a distributed feedback type laser active region formed on a substrate; a passive waveguide formed on the substrate and optically connected to one end of the laser active region in a waveguiding direction, with reflection points formed at both ends in the waveguiding direction, as a Fabry-Perot type resonator structure using an optical waveguide structure; a reflection region formed on the substrate and optically connected to one end of the passive waveguide; an amplification region formed on the substrate and optically connected to one end of the reflection region ; and a DBR region formed on the substrate and optically connected to the other end in the waveguiding direction of the laser active region , wherein the width of the passive waveguide on the laser active region side is different from the width of the reflection point on the passive waveguide opposite to the laser active region, and laser oscillation is caused by photon-photon resonance occurring in response to the frequency difference between the frequency of light generated in the laser active region and the frequency of the Fabry-Perot mode of the passive waveguide.

As described above, according to the present invention, a reflection region and an amplification region are provided following the laser active region and passive waveguide, so that the PPR frequency and the intensity of the fed-back reflected light and the intensity of the output light, which determine this response, can be controlled independently.

FIG. 1 is a diagram showing the configuration of an optical device according to an embodiment of the present invention. FIG. 2 is a cross-sectional view showing the configuration of an optical device according to an embodiment of the present invention. FIG. 3 is an explanatory diagram for explaining the occurrence of PPR. FIG. 4 is a characteristic diagram showing the relationship between the length of the reflective area 103 and the reflectance. FIG. 5 is a characteristic diagram showing the optical spectrum in each region when the reflectance of the reflective region 103 is changed. FIG. 6 is an explanatory diagram for explaining the change in the optical spectrum after amplification in the amplification region 104. In FIG. FIG. 7 is an explanatory diagram illustrating a state in which the S21 response related to PPR increases due to amplification by the amplification region 104. FIG. 8 is a cross-sectional view showing a partial configuration of another optical device according to an embodiment of the present invention. FIG. 9A is a perspective view showing a partial configuration of another optical device according to an embodiment of the present invention. FIG. 9B is a perspective view showing a partial configuration of another optical device according to an embodiment of the present invention.

Hereinafter, an optical device according to an embodiment of the present invention will be described with reference to Fig. 1. This optical device includes a distributed feedback laser active region 101, a passive waveguide 102 optically connected to one end of the laser active region 101 in the waveguiding direction, a reflection region 103 optically connected to one end of the passive waveguide 102 in the waveguiding direction, and an amplification region 104 optically connected to one end of the reflection region 103 in the waveguiding direction.

The passive waveguide 102 has reflection points 105 and 106 at both ends in the waveguiding direction, where reflection occurs. The passive waveguide 102 has an optical waveguide structure and a Fabry-Perot type resonator structure, and is capable of forming a Fabry-Perot (FP) mode. The passive waveguide 102 is also capable of forming a composite mode with the laser active region 101.

This optical device includes, for example, a substrate 111 and a lower cladding layer 112 formed on the substrate 111, as shown in Fig. 2. Note that Fig. 2 shows a cross section parallel to the waveguiding direction and perpendicular to the surface of the substrate 111. The substrate 111 can be made of, for example, InP that is made n-type by doping with Si. The substrate 111 can also be made of, for example, GaAs, _SiO2 , Si, or SiC. The lower cladding layer 112 can be made of, for example, InP that is made n-type. The lower cladding layer 112 can also be made of _SiO2 .

In the laser active region 101, an active layer 113 is formed on the lower cladding layer 112, and a diffraction grating 114 is formed on the active layer 113. The active layer 113 can be a multiple quantum well structure made of, for example, InGaAsP or InGaAlAs. The active layer 113 can also be a bulk structure made of the above materials. The diffraction grating 114 is composed of recesses and protrusions adjacent to the recesses, which are arranged in the waveguiding direction. In addition, the diffraction grating 114 can be formed with a portion (1/4 shift portion) in which the phase is inverted by π in a part (center portion) of the waveguiding direction. The phase shift of this 1/4 shift portion enables single mode emission at the Bragg wavelength.

In addition, a core 115 is formed in the passive waveguide 102. The core 115 can be made of, for example, InGaAlAs, whose lattice constant in the planar direction of the substrate 111 is lattice-matched to InP.

In addition, in the reflection region 103, a core 116 is formed, and a diffraction grating 117 can be formed on the core 116. The core 115 can be made of, for example, InGaAlAs, whose lattice constant in the planar direction of the substrate 111 is lattice-matched to InP. The diffraction grating 117 is made of recesses and protrusions adjacent to the recesses, which are arranged in the waveguiding direction. In this way, the reflection region 103 can have a DBR mirror structure, but is not limited to this, and the reflection region 103 can also be made of a simple air gap, a ring resonator, etc.

In addition, in the amplification region 104, an active layer 118 is formed on the lower cladding layer 112. The active layer 118 may be, for example, a multiple quantum well structure made of InGaAsP or InGaAlAs.

An upper cladding layer 119 is formed on the active layer 113, the core 115, the core 116, and the active layer 118. For example, the active layer 113 extends in the waveguiding direction, and the cross-sectional shape perpendicular to the waveguiding direction is the same as that of the core 115. Similarly, the core 115 and the core 116 also extend in the waveguiding direction. The active layer 118 also extends in the waveguiding direction, and the cross-sectional shape perpendicular to the waveguiding direction is the same as that of the core 116. The upper cladding layer 119 is formed on the lower cladding layer 112, covering the active layer 113, the core 115, the core 116, and the active layer 118. The upper cladding layer 119 can be made of, for example, InP. Note that a part of the upper cladding layer 119 above the active layer 113 is, for example, p-type. Note that the upper cladding layer 119 in other regions, including the upper part of the core 115, is i-type (non-doped).

In the laser active region 101, the n-type lower cladding layer 112, the i-type active layer 113, and the p-type region of the upper cladding layer 119 are stacked in the thickness direction (normal direction of the plane of the substrate 111), forming a so-called vertical n-i-p structure. In this case, the p-type region of the lower cladding layer 112 and the upper cladding layer 119 can form a so-called vertical current injection type current injection structure. In addition, the laser active region 101 can be configured to include an n-type layer and a p-type layer arranged on either side of the active layer 113 as a current injection mechanism. For example, an n-electrode can be formed on the n-type layer, and a p-electrode can be formed on the p-type layer. This is a so-called lateral current injection type current injection structure.

Here, for example, the laser active region 101 having an optical waveguide structure with the active layer 113 as the core and the passive waveguide 102 having an optical waveguide structure with the core 115 can be formed by directly bonding. With this configuration, a reflection point 105 is effectively formed for the light traveling from the passive waveguide 102 to the laser active region 101 due to reflection at the reflection portion of the diffraction grating 114 of the laser active region 101. The position of the reflection point 105 configured in this manner is shifted from the boundary between the laser active region 101 and the passive waveguide 102 by the penetration length of the light.

In addition, the passive waveguide 102 of the optical waveguide structure using the core 115 and the reflection region 103 of the optical waveguide structure using the core 116 can be formed by directly bonding them together. With this configuration, reflection point 106 is effectively formed for the light traveling from the passive waveguide 102 to the reflection region 103 due to reflection at the reflection portion of the diffraction grating 117 of the reflection region 103. Note that reflection points 105 and 106 can also be formed by other structures.

Each layer structure using the compound semiconductors described above can be formed, for example, by epitaxial growth using known metalorganic vapor phase epitaxy. Also, each core, each diffraction grating, etc. can be formed by processing (patterning) using known lithography and etching techniques.

A spot size conversion structure can be provided at the light output section of the optical device to reduce optical coupling loss with an optical fiber or an external optical waveguide. The spot size conversion structure can adiabatically transfer the mode from a conversion core that tapers away from the connection point to a spot size conversion core region.

This optical device also oscillates using photon-photon resonance (PPR) that occurs in response to the frequency difference between the frequency of light generated (oscillated) in the laser active region 101 and the frequency of the FP mode of the passive waveguide 102. As shown in Figure 3, PPR occurs in response to the frequency difference between the peak wavelength of the transmission spectrum 201 in the laser active region 101 (peak wavelength at the oscillation wavelength) and the peak wavelength of the transmission spectrum 202 in the passive waveguide 102 (peak wavelength of the FP mode).

Therefore, the PPR frequency is defined within the FSR determined by the length of the passive waveguide 102. The operating range of the desired PPR frequency can be obtained by adjusting the FSR by changing the length of the passive waveguide 102 according to the application. The S21 response intensity at the PPR frequency increases as the area of the overlapping region 203 of each spectrum shown in FIG. 3 increases. In this optical device, PPR can be expressed regardless of the length of the passive waveguide 102 in the waveguiding direction. According to the optical device of the embodiment, a wide modulation band by PPR that enables high-speed direct modulation can be realized with a short element length, and the effect of PPR can be stably expressed, making it possible to realize a high-speed optical device with high controllability.

Here, the reflective region 103 optically connected to the rear stage of the passive waveguide 102 makes it possible to adjust the optical feedback light intensity to the laser active region 101 in the front stage. For example, when the reflective region 103 is a DBR mirror, the reflectance can be adjusted by changing the length (length in the waveguiding direction) of the reflective region 103 as shown in Figure 4.

Figure 5 shows the optical spectrum when the reflectance of the reflective region 103 is changed. When the reflectance of the reflective region 103 is increased, the transmission spectrum 202 expands to the transmission spectrum 202a, the optical intensity of the feedback side mode increases, and the overlap area of the spectra increases by the overlap region 204 shown in Figure 5.

In addition, an amplification region 104 consisting of an active layer 118, which is a gain medium, is provided downstream of the reflection region 103, and the light intensity can be adjusted. Figure 6 shows the light spectrum after being amplified in the amplification region 104. As is clear from Figure 6, when light is amplified, the transmission spectrum 201 expands to the transmission spectrum 201a, and the transmission spectrum 202 expands to the transmission spectrum 202b, and the overlap region 205 of the effective light spectra increases.

Figure 7 shows the S21 small signal response of a laser using PPR. Increasing the reflectivity of the reflective region 103 increases the effective optical spectral overlap region 205, resulting in an increase in the S21 response at the PPR frequency.

By the way, this optical device can adjust the frequency of the Fabry-Perot mode of the passive waveguide 102, for example, by using a frequency adjustment mechanism. The frequency adjustment mechanism can be configured to inject a current into the passive waveguide 102, control the temperature, or apply an electric field, and adjust the frequency of the Fabry-Perot mode. For example, frequency control can be achieved by providing a resistance heating type heater made of a metal such as tantalum as a temperature control mechanism. Furthermore, the adjustment of the feedback light intensity changes the reflectivity of the reflection region 103. For example, the reflectivity can be controlled to a desired value by changing the length of the reflection region 103 in the waveguiding direction, or by providing a heater on the reflection region 103 to adjust the Bragg wavelength.

The reflection region 103 can also be a simple air gap or a ring resonator. If the optical output of such an optical device is insufficient, the optical output can be amplified, for example, by injecting a current into the amplification region 104. These configurations are effective, for example, when the optical output is insufficient due to a short element length.

As a result, this optical device makes it possible to achieve a wide modulation bandwidth and high optical output power through PPR with a variety of element lengths, and also makes it possible to stably express the PPR effect, enabling the realization of a highly controllable, high-speed optical device.

As shown in Fig. 8, the laser active region 101 may further include a DBR region 121 optically connected to the other end of the laser active region 101 in the waveguiding direction. Fig. 8 shows a cross section parallel to the waveguiding direction and perpendicular to the surface of the substrate 111. Fig. 8 also omits the reflection region 103 and the amplification region 104. In the DBR region 121, a core 122 is formed on the lower cladding layer 112, and a diffraction grating 123 is formed on the core 122. The core 122 may be made of, for example, InGaAlAs.

By providing the DBR region 121 in this way, single mode can be achieved without using phase shift, as shown below. In the DBR region 121, for example, a transmission peak on the short wavelength side of the laser active region 101 can be selected, and the band can be increased by laser operation and PPR. In this case, fringe peaks and FP mode peaks on the longer wavelength side than the peak wavelength of the reflection spectrum of the DBR region 121 are concentrated within the stop band of the transmission spectrum in the laser active region 101. As a result, many of the modes in the region slightly longer wavelength than the transmission spectrum peak, which is important for PPR expression, are attenuated, enabling stable single mode operation and PPR expression.

By making the width of the passive waveguide 102 (core 115) on the laser active region 101 side (the width of the core 115 at the connection with the active layer 113) and the width of the reflection point 106 on the opposite side of the passive waveguide 102 (core 115) from the laser active region 101 different dimensions, the reflectivity of the reflection points 105 and 106 can be adjusted, and it is possible to adjust the S21 response at the PPR frequency and the PPR frequency using the same principle as the enhancement of the PPR application shown in Figure 5.

Next, other configurations of the reflection points 105 and 106 will be described. For example, the core 115 of the passive waveguide 102 and the active layer 113 (core) of the laser active region 101 are butt-coupled with each other by having a structure in which at least one of the thickness and width is different, and the connection point between them can be used as the reflection point 105.

In addition, the connection point between the laser active region 101 and the passive waveguide 102 can be made into a reflection point 105 by constructing the active layer 113 and the core 115 from materials with different refractive indices. For example, the reflection point 105 can be formed by making the active layer 113 a multiple quantum structure using InGaAlAs and constructing the core 115 from InGaAlAs or InGaAsP.

In addition, the reflection point 105 can also be formed by forming a groove extending in a direction intersecting the waveguiding direction in the upper cladding layer 119 at the connection point between the laser active region 101 and the passive waveguide 102. By forming such a groove, an inflection point of the refractive index is formed at this point, and this can be used as the reflection point 105.

In the above-mentioned lateral current injection type laser active region 101, it is possible to increase the refractive index difference between the lower cladding layer 112, the upper cladding layer 119a, and the active layer 113 (core 115), and it becomes possible to confine light more strongly in the active layer 113. This stronger light confinement can increase the interaction between the light returned from the passive waveguide 102 and the light in the laser active region 101. As a result, it is possible to increase the bandwidth by PPR without increasing the reflected back component from the laser active region 101.

As described above, the strong optical confinement eliminates the need for a large reflectance of the passive waveguide 102, making it unnecessary to provide a high reflectance (HR) coating on the end face of the passive waveguide 102, and facilitating the formation of the structure. As described above, when the optical confinement is large and the interaction between the laser oscillation mode and the side mode is large, PPR occurs even when the frequency difference between the transmission spectrum 201 in the laser active region 101 and the transmission spectrum 202 in the passive waveguide 102 is large, making it possible to design a band that increases in the high frequency range.

Furthermore, in a structure in which light confinement in the direction perpendicular to the substrate (thickness) is achieved by the refractive index difference between a III-V group semiconductor/insulator (air, _SiO2 , etc.) or a semiconductor with a low refractive index (SiC, AlN, etc.), a diffraction grating 114 with a large degree of refractive index modulation can be formed in the laser active region 101, so that a laser active region 101 with a diffraction grating 114 having a large coupling coefficient can be achieved.

When the coupling coefficient of the diffraction grating 114 is large, the width of the stop band of the laser active region 101 becomes large, so that many of the maximum peaks of the transmission spectrum of the passive waveguide 102 fall within the stop band. As a result, the laser operation is less likely to become unstable due to interference between the peak of the oscillation light of the laser active region 101 and the peak of the FP mode in the passive waveguide 102.

In typical DFB lasers with a small coupling coefficient for the diffraction grating, the FP mode hardly falls within the stop band of the DFB laser, making it prone to operational instability due to interference between the FP mode and the DFB mode.

Operation can also be affected by adjusting the gain spectrum of the material constituting the active layer 113, but since only a smaller number of DFB and FP mode peaks can be selected compared to when the coupling coefficient is small, single-mode operation or stable operation (ease of occurrence of mode hopping and PPR) becomes possible.

When the refractive index difference between the active layer 113 and the layers sandwiching the active layer 113 above and below is large and the coupling coefficient of the diffraction grating 114 is large, the reflectance of the reflection point 105 formed by reflection at the reflection part of the diffraction grating 114 becomes high. Therefore, in this configuration, it is possible to increase the strength of the optical feedback by the passive waveguide 102. As a result, even when the frequency difference between the transmission spectrum of the laser active region 101 and the transmission spectrum of the passive waveguide 102 is large (the frequency that enhances the response is high), PPR is generated and the bandwidth can be increased.

By the way, in controlling the coupling between the laser light oscillated in the laser active region 101 and the return light from the passive waveguide 102, there are a configuration in which the return light intensity (end face reflectance) from the passive waveguide 102 is specified by the structure, and a configuration in which the return light is appropriately amplified or attenuated during operation. As a configuration in which the end face reflectance is specified by the structure, as described above, there is a configuration in which the shape (cross-sectional shape) of the core 115 of the passive waveguide 102 is changed. For example, the cross-sectional shape of the core 115 may be narrowed or widened, thickened or thinned, etc., relative to the cross-sectional shape of the active layer 113. In addition, the diameter of the core 115 may be made smaller or larger as it moves away from the active layer 113.

As shown in Fig. 9A, the shape of the cross section perpendicular to the waveguiding direction may be multi-staged in the thickness direction of core 115a. As shown in Fig. 9B, the shape of the cross section perpendicular to the waveguiding direction may be multi-staged in the thickness direction of core 115b, with the upper and lower stages being made of different materials.

As described above, according to the present invention, a reflection region and an amplification region are provided following the laser active region and passive waveguide, so that the PPR frequency and the intensity of the fed-back reflected light and the intensity of the output light, which determine this response, can be controlled independently.

It is to be understood that the present invention is not limited to the embodiments described above, and that many modifications and combinations can be implemented by a person having ordinary knowledge in the art within the technical concept of the present invention.

101...laser active region, 102...passive waveguide, 103...reflection region, 104...amplification region, 105...reflection point, 106...reflection point.

Claims (6)

a distributed feedback laser active region formed on a substrate;
a passive waveguide formed on the substrate, optically connected to one end of the laser active region in a waveguiding direction, and having a Fabry-Perot type resonator structure with an optical waveguide structure in which reflection points are formed at both ends in the waveguiding direction;
a reflective region formed on the substrate and optically connected to one end of the passive waveguide;
an amplification region formed on the substrate and optically connected to one end of the reflection region ;
a DBR region formed on the substrate and optically connected to the other end of the laser active region in the waveguiding direction;
Equipped with
a width of the passive waveguide on the side of the laser active region is different from a width of a reflection point on the opposite side of the passive waveguide from the laser active region;
An optical device, characterized in that laser oscillation is achieved by utilizing photon-photon resonance that occurs in response to a frequency difference between the frequency of light generated in the laser active region and the frequency of a Fabry-Perot mode of the passive waveguide. 2. The optical device according to claim 1,
1. An optical device comprising: a first insulating layer that electrically connects a first conductive layer to a first insulating film; a second insulating layer that electrically connects a first conductive layer to a first insulating film; 3. The optical device according to claim 1,
The laser active region comprises:
An optical device comprising: a current injection mechanism for injecting a current in a planar direction of the substrate. The optical device according to any one of claims 1 to 3 ,
An optical device, wherein the reflection region has a DBR mirror structure. The optical device according to any one of claims 1 to 4 ,
An optical device comprising: a core of said passive waveguide that is different from a core of said laser active region in at least one of thickness and width. The optical device according to any one of claims 1 to 4 ,
An optical device, wherein a core of the passive waveguide has a cross-sectional shape perpendicular to a waveguiding direction, the cross-sectional shape having multiple steps in a thickness direction.

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