WO2022130622A1

WO2022130622A1 - Optical device

Info

Publication number: WO2022130622A1
Application number: PCT/JP2020/047451
Authority: WO
Inventors: 優山岡; 慎治松尾
Original assignee: 日本電信電話株式会社
Priority date: 2020-12-18
Filing date: 2020-12-18
Publication date: 2022-06-23
Also published as: JPWO2022130622A1

Abstract

This optical device is equipped with a laser-active region (101) of the distributed feedback type, a passive waveguide (102) which is optically connected to one end of the laser-active region (101) in the guided wave direction, a reflection region (103) which is optically connected to the one end of the passive waveguide (102) in the guided wave direction, and an amplification region (104) which is optically connected to the one end of the reflection region (103) in the guided wave direction. A reflection point (105) and a reflection point (106), which are locations where a reflection is produced, are respectively formed on both ends of the passive waveguide (102) in the guided wave direction. In addition, the passive waveguide (102) is equipped with an optical waveguide structure, is configured as a resonator structure of the Fabry-Perot type, and is capable of Fabry-Perot mode formation.

Description

Optical device

The present invention relates to an optical device using a semiconductor laser.

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 reduce the size, power consumption, and cost of the optical transmitter. A semiconductor laser, which is a typical optical transmitter, can transmit a large-capacity signal over a long distance. As a semiconductor laser used in an information communication system, first, there is an external modulation laser that modulates a signal by a modulator provided outside the semiconductor laser. Further, the semiconductor laser used in the information communication system includes a directly modulated laser that directly modulates the output light by modulating the current injected into the active region.

The direct modulation laser has features such as small size, low power consumption, and low cost compared to the external modulation laser. For this reason, the direct modulation laser is widely used as a transmitter of an information communication system for a relatively short distance in or between data centers. However, the direct modulation laser has a problem that the modulation speed is slow because the modulation band is narrower than that of the external modulation laser. This is because when the current injection is increased for high-speed operation, the heat generation of the semiconductor laser active layer lowers the luminous efficiency and limits the increase in the modulation band. In a typical laser, the intrinsic band is regulated by the relaxation vibration frequency.

In order to solve such a problem of band limitation, a laser structure using photon-photon resonance (PPR), which is a resonance phenomenon between photons and photons, has been proposed. In the direct modulation laser using PPR, it is possible to expand the modulation band by expressing a new resonance peak in the frequency region higher than the relaxation vibration frequency, which has been conventionally degraded (non-patented). Document 1, Non-Patent Document 2, Non-Patent Document 3).

The direct modulation laser using PPR has a structure in which a distributed feedback type (DFB) laser active region and a passive waveguide responsible for a light feedback mechanism are connected adjacent to each other. A laser active region is optically connected to one end of the passive waveguide. Further, both ends of the passive waveguide serve as reflection points (Non-Patent Document 2 and Non-Patent Document 3). The laser beam generated in the laser active region interacts with the Fabry-Perot type resonance mode formed in the optical feedback region by the passive waveguide, and PPR is generated when the phase matching condition is satisfied. In a passive waveguide, for example, the state of PPR is controlled by adjusting the phase by changing the refractive index due to the injected current. The PPR frequency is approximately defined by the frequency difference between the laser oscillation mode and the Fabry-Pérot type resonant 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 due to the passive waveguide. For example, in Non-Patent Document 1, the length of the optical feedback region is limited to 300 μm in order to generate PPR at about 43 GHz. Phase control in 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 as compared with Non-Patent Document 1. .. With 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 to be larger. Further, in the above-described configuration, since the FSR is large, the phase change with respect to the wavelength change becomes gentle, so that relatively stable operation can be realized with respect to the injection current change and the temperature change.

However, with the above technique, although it is possible to control the PPR frequency by phase control, it is difficult to control the PPR frequency and the electric-optical response (laser S21) response of the laser at the same time at the PPR frequency. be. Further, in a laser using a PPR composed of a relatively short resonator (Non-Patent Document 2), in order to supplement the optical output intensity, for example, a semiconductor optical amplifier (SOA) using a gain medium is used. It is considered necessary to supplement the output light intensity by using.

As described above, in the prior art, there is a problem that the PPR frequency and the intensity of the reflected reflected light and the intensity of the output light that determine this response cannot be controlled independently.

The present invention has been made to solve the above problems, and the PPR frequency and the intensity of the reflected reflected light and the intensity of the output light that determine this response can be independently controlled. The purpose is to do so.

The optical device according to the present invention has a distributed feedback type laser active region formed on a substrate and an optical device formed on a substrate and optically connected to one end of the laser active region in the waveguide direction. A passive waveguide having a Fabry-Pérot type resonator structure with an optical waveguide structure in which reflection points are formed at both ends of the substrate, and a reflection region formed on a substrate and optically connected to one end of the passive waveguide. And an amplification region formed on the substrate and optically connected to one end of the reflection region, the frequency of the light generated in the laser active region and the frequency of the Fabry-Pérot mode of the passive waveguide. Laser oscillation is performed using photon-photon resonance generated according to the difference.

As described above, according to the present invention, the laser active region, the passive waveguide, the reflection region and the amplification region are provided, so that the PPR frequency and the intensity and output of the feedback reflected light that determines this response are provided. The intensity of light can be controlled independently.

FIG. 1 is a configuration diagram showing a 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 illustrating the generation of PPR. FIG. 4 is a characteristic diagram showing the relationship between the length of the reflection region 103 and the reflectance. FIG. 5 is a characteristic diagram showing an optical spectrum in each region when the reflectance of the reflection region 103 is changed. FIG. 6 is an explanatory diagram illustrating a change in the optical spectrum after being amplified in the amplification region 104. FIG. 7 is an explanatory diagram illustrating a state in which the S21 response regarding 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 the embodiment of the present invention. FIG. 9A is a perspective view showing a partial configuration of another optical device according to the embodiment of the present invention. FIG. 9B is a perspective view showing a partial configuration of another optical device according to the embodiment of the present invention.

Hereinafter, the optical device according to the embodiment of the present invention will be described with reference to FIG. This optical device has a distributed feedback type laser active region 101, a passive waveguide 102 optically connected to one end of the laser active region 101 in the waveguide direction, and optical to one end of the passive waveguide 102 in the waveguide direction. The reflection region 103 is provided with the reflection region 103, and the amplification region 104 optically connected to one end of the reflection region 103 in the waveguide direction.

In the passive waveguide 102,

reflection points

105 and 106, which are locations where reflection occurs, are formed at both ends in the waveguide direction. Further, the passive waveguide 102 has an optical waveguide structure, has a Fabry-Perot type resonator structure, and can form a Fabry-Perot (FP) mode. Further, the passive waveguide 102 can form a composite mode with the laser active region 101.

The optical device includes, for example, a substrate 111 and a lower clad layer 112 formed on the substrate 111, as shown in FIG. Note that FIG. 2 shows a cross section parallel to the waveguide direction and perpendicular to the plane of the substrate 111. The substrate 111 can be composed of, for example, InP which is made n-type by doping with Si. Further, the substrate 111 may be composed of, for example, GaAs, SiO ₂ , Si, or SiC. The lower clad layer 112 can be composed of, for example, an n-shaped InP. Further, the lower clad layer 112 may be composed of SiO ₂ .

In the laser active region 101, the active layer 113 is formed on the lower clad layer 112, and the diffraction grating 114 is formed on the active layer 113. The active layer 113 can have, for example, a multiple quantum well structure made of InGaAsP or InGaAlAs. Further, the active layer 113 may have a bulk structure made of the above-mentioned material. The diffraction grating 114 is composed of a concave portion and a convex portion adjacent to the concave portion, and these are arranged in the waveguide direction. In the diffraction grating 114, a portion (1/4 shift portion) whose phase is π inverted can be formed in a part (central portion) in the waveguide direction. The phase shift of this partial 1/4 shift section enables single-mode light emission at the Bragg wavelength.

Further, in the passive waveguide 102, the core 115 is formed. The core 115 can be composed of, for example, InGaAlAs whose lattice constant in the plane direction of the substrate 111 is lattice-matched to InP.

Further, in the reflection region 103, the core 116 is formed, and the diffraction grating 117 can be formed on the core 116. The core 115 can be composed of, for example, InGaAlAs whose lattice constant in the plane direction of the substrate 111 is lattice-matched to InP. The diffraction grating 117 is composed of a concave portion and a convex portion adjacent to the concave portion, and these are arranged in the waveguide direction. As described above, the reflection region 103 can have a DBR mirror structure, but the present invention is not limited to this, and the reflection region 103 can also be composed of a mere air gap, a ring resonator, or the like.

Further, in the amplification region 104, the active layer 118 is formed on the lower clad layer 112. The active layer 118 can have, for example, a multiple quantum well structure made of InGaAsP or InGaAlAs.

An upper clad 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 waveguide direction, and the shape of the cross section perpendicular to the waveguide direction is the same as that of the core 115. Similarly, the core 115 and the core 116 also extend in the waveguide direction. Further, the active layer 118 also extends in the waveguide direction, and the shape of the cross section perpendicular to the waveguide direction is the same as that of the core 116. Further, the upper clad layer 119 covers the active layer 113, the core 115, the core 116, and the active layer 118, and is formed on the lower clad layer 112. The upper clad layer 119 can be composed of, for example, InP. A part of the upper clad layer 119 above the active layer 113 is, for example, p-shaped. Further, the upper clad layer 119 in other regions including the upper part of the core 115 is i-type (non-doped).

In the laser active region 101, in the thickness direction (normal direction of the plane of the substrate 111), the n-type lower clad layer 112, the i-type active layer 113, and the upper clad layer 119 are p-shaped regions. It can be laminated to form a so-called vertical n-ip structure. In this case, a so-called longitudinal current injection type current injection structure can be configured by the p-type region of the lower clad layer 112 and the upper clad layer 119. Further, in the laser active region 101, the current injection mechanism may be configured to include an n-type layer and a p-type layer arranged with the active layer 113 interposed therebetween. For example, an n-electrode is formed on the n-type layer, and a p-electrode is 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 of the optical waveguide structure having the active layer 113 as the core and the passive waveguide 102 of the optical waveguide structure by the core 115 can be formed by directly joining each other. With this configuration, the reflection point 105 is effectively formed for the light traveling from the passive waveguide 102 to the laser active region 101 by the reflection at the reflection portion by the diffraction grating 114 of the laser active region 101. The position of the reflection point 105 configured in this way deviates from the boundary between the laser active region 101 and the passive waveguide 102 by the penetration depth of light.

Further, the passive waveguide 102 of the optical waveguide structure formed by the core 115 and the reflection region 103 of the optical waveguide structure formed by the core 116 can be formed by directly joining each other. With this configuration, the reflection point 106 is effectively formed for the light traveling from the passive waveguide 102 to the reflection region 103 due to the reflection at the reflection portion by the diffraction grating 117 of the reflection region 103. It should be noted that the reflection point 105 and the reflection point 106 can be formed by another structure.

Each layer structure made of the above-mentioned compound semiconductor can be formed by, for example, epitaxial growth by a known organic metal vapor phase growth method or the like. Further, each core, each diffraction grating, and the like can be formed by processing (patterning) by a known lithography technique and etching technique.

It is also possible to provide a spot size conversion structure in the light emitting portion of the optical device to reduce the optical coupling loss with the optical fiber or the external optical waveguide. In the spot size conversion structure, it is possible to adiabatically shift the mode from the conversion core tapered as the distance from the connecting point to the spot size conversion core region.

Further, this optical device uses photon-photon resonance (PPR) generated according to the frequency difference between the frequency of the light generated (oscillated) in the laser active region 101 and the frequency of the FP mode of the passive waveguide 102. Laser oscillates. As shown in FIG. 3, the PPR has a peak wavelength of the transmission spectrum 201 in the laser active region 101 (peak wavelength in the oscillation wavelength) and a peak wavelength of the transmission spectrum 202 in the passive waveguide 102 (peak wavelength in the FP mode). It occurs according to the frequency difference.

Therefore, the PPR frequency is defined within the FSR, which is 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 waveguide direction. According to the optical device according to the embodiment, a wide modulation band by PPR capable of high-speed direct modulation can be realized with a short element length, the effect of PPR can be stably exhibited, and the controllability is high. High-speed optical devices are feasible.

Here, the reflection 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 of the front stage. For example, when the reflection region 103 is a DBR mirror, the reflectance can be adjusted by changing the length (length in the waveguide direction) of the reflection region 103 as shown in FIG.

FIG. 5 shows an optical spectrum when the reflectance of the reflection region 103 is changed. When the reflectance of the reflection region 103 is increased, the transmission spectrum 202 expands to the transmission spectrum 202a, the light intensity of the feedback side mode increases, and the overlap area of the spectra increases only in the overlap region 204 shown in FIG. do.

Further, in the subsequent stage of the reflection region 103, an amplification region 104 composed of an active layer 118 which is a gain medium is provided, and the light intensity can be adjusted. FIG. 6 shows the optical spectrum after being amplified in the amplification region 104. As is clear from FIG. 6, when light is amplified, the transmission spectrum 201 expands to the transmission spectrum 201a, the transmission spectrum 202 expands to the transmission spectrum 202b, and the effective optical spectrum overlapping region 205 increases.

FIG. 7 shows the S21 small signal response of the laser using PPR. As a result of increasing the reflectance of the reflection region 103 and increasing the overlap region 205 of the effective optical spectrum, the S21 response at the PPR frequency increases.

By the way, in this optical device, for example, the frequency of the Fabry-Perot mode of the passive waveguide 102 can be adjusted by using a frequency adjustment mechanism. The frequency adjustment mechanism can be composed of either injecting a current into the passive waveguide 102, controlling the temperature, or applying an electric field, and adjusts the frequency of the Fabry-Perot mode. For example, frequency control can be realized by providing a resistance heating type heater made of a metal such as tantalum as a temperature control mechanism. Further, the adjustment of the feedback light intensity changes the reflectance of the reflection region 103. For example, the desired reflectance can be controlled by changing the length of the reflection region 103 in the waveguide direction or by providing a heater on the reflection region 103 and adjusting the Bragg wavelength.

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

As a result, according to this optical device, a wide modulation band and high optical output by PPR can be realized with various element lengths, and the effect of PPR can be stably exhibited, and controllability can be achieved. High-speed optical devices with high speed can be realized.

By the way, as shown in FIG. 8, a DBR region 121 optically connected to the other end of the laser active region 101 in the waveguide direction may be further provided. Note that FIG. 8 shows a cross section parallel to the waveguide direction and perpendicular to the plane of the substrate 111. Further, in FIG. 8, the reflection region 103 and the amplification region 104 are omitted. In the DBR region 121, the core 122 is formed on the lower clad layer 112, and the diffraction grating 123 is formed on the core 122. The core 122 can be composed of, for example, InGaAlAs.

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

By the way, what is the width of the passive waveguide 102 (core 115) on the side of the laser active region 101 (the width of the core 115 at the connection portion with the active layer 113) and the laser active region 101 of the passive waveguide 102 (core 115)? The reflectance of the reflection points 105 and 106 can be adjusted by making the width of the reflection point 106 on the opposite side different, and the S21 response at the PPR frequency and the S21 response at the PPR frequency can be adjusted by the same principle as the enhancement of the PPR application shown in FIG. , PPR frequency can be adjusted.

Next, other configurations of the reflection point 105 and the reflection point 106 will be described. For example, the core 115 of the passive waveguide 102 is butt-coupled to the active layer 113 (core) of the laser active region 101 so that at least one of the thickness and the width is different from the active layer 113 (core), thereby reflecting these connection points. It can be point 105.

Further, by forming the active layer 113 and the core 115 from materials having different refractive indexes, the connection point between the laser active region 101 and the passive waveguide 102 can be set as the reflection point 105. For example, the reflection point 105 can be formed by forming the active layer 113 with a multiple quantum structure made of InGaAlAs and the core 115 made of InGaAlAs or InGaAsP.

Further, the reflection point 105 can also be formed by forming a groove extending in the direction intersecting the waveguide direction in the upper clad 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 the reflection point 105 can be obtained.

In the above-mentioned lateral current injection type laser active region 101, the difference in refractive index between the lower clad layer 112 and the upper clad layer 119a and the active layer 113 (core 115) can be increased, and the active layer can be increased. It will be possible to trap light more strongly in 113. Due to this stronger light confinement, the interaction between the light returned from the passive waveguide 102 and the light in the laser active region 101 can be further increased. As a result, the band can be increased by PPR without increasing the reflection return component from the laser active region 101.

Further, as described above, since it is not necessary to take a large reflectance of the passive waveguide 102 due to strong light confinement, a high reflectance (HR) coating is not required on the end face of the passive waveguide 102, and the structure can be easily formed. .. Further, as described above, when the light confinement is large and the interaction between the laser oscillation mode and the side mode is large, 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. Even in this case, PPR is generated, so that it is possible to design to increase the band in the high frequency region.

Furthermore, in a structure that realizes light confinement in the vertical (thickness) direction of the substrate by the difference in refractive index between group III-V semiconductors / insulators (air, SiO ₂ etc.) and semiconductors with low refractive index (SiC, AlN, etc.). Since the diffraction grating 114 having a large degree of refractive index modulation can be formed in the laser active region 101, the laser active region 101 by the diffraction grating 114 having a large coupling coefficient can be realized.

When the coupling coefficient of the diffraction grating 114 is large, the width of the stopband of the laser active region 101 becomes large, so that most of the maximum peaks of the transmission spectrum of the passive waveguide 102 are contained in the stopband. As a result, the instability of the laser operation due to the interference between the peak of the oscillated light in the laser active region 101 and the peak of the FP mode in the passive waveguide 102 is unlikely to occur.

In a DFB laser with a small coupling coefficient of a general diffraction grating, the FP mode hardly enters the stop band of the DFB laser, so that operation instability is likely to occur due to interference between the FP mode and the DFB mode.

The operation can also be affected by adjusting the gain spectrum of the material constituting the active layer 113, but it is possible to select only the DFB and FP mode peaks, which are smaller than when the coupling coefficient is small. Therefore, single mode operation and stable operation (easiness of expression of mode hop and PPR) are possible.

By the way, when the difference in refractive index between the active layer 113 and the layer sandwiching the active layer 113 above and below is large and the coupling coefficient of the diffraction grating 114 is large, the reflection point formed by the reflection by the diffraction grating 114 at the reflection portion. The reflectance of 105 becomes high. Therefore, in this configuration, it is possible to increase the intensity of optical feedback by the passive waveguide 102. As a result, PPR is expressed even in a state where 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 for enhancing the response is high), and the band 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, the intensity of the return light (end face reflectance) from the passive waveguide 102 is defined by the structure. There is a configuration in which the return light is appropriately amplified or attenuated during operation. As a configuration in which the end face reflectance is defined 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 with respect to the cross-sectional shape of the active layer 113. Further, the diameter of the core 115 may be reduced or increased as the distance from the active layer 113 increases.

Further, as shown in FIG. 9A, the core 115a can be formed in which the shape of the cross section perpendicular to the waveguide direction is multi-staged in the thickness direction. Further, as shown in FIG. 9B, the shape of the cross section perpendicular to the waveguide direction may be multi-staged in the thickness direction, and the upper stage and the lower stage may be core 115b made of different materials.

As described above, according to the present invention, the laser active region, the passive waveguide, the reflection region and the amplification region are provided, so that the PPR frequency and the intensity of the feedback reflected light that determines this response and the intensity of the reflected light are determined. The intensity of the output light can be controlled independently.

It should be noted that the present invention is not limited to the embodiments described above, and many modifications and combinations can be carried out by a person having ordinary knowledge in the art within the technical idea of the present invention. That is clear.

101 ... Laser active region, 102 ... Passive waveguide, 103 ... Reflection region, 104 ... Amplification region, 105 ... Reflection point, 106 ... Reflection point.

Claims

A distributed feedback type laser active region formed on the substrate,
A Fabry-Perot type cavity structure with an optical waveguide structure formed on the substrate, optically connected to one end in the waveguide direction of the laser active region, and reflection points formed at both ends in the waveguide direction. Passive waveguide and
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 is provided.
An optical device characterized in that laser oscillation is performed using photon-photon resonance generated according to a frequency difference between the frequency of light generated in the laser active region and the frequency of the fabric perow mode of the passive waveguide.
In the optical device according to claim 1,
A frequency adjustment mechanism that adjusts the frequency of the fabric perlow mode of the passive waveguide by either injecting a current into the passive waveguide, controlling the temperature of the passive waveguide, or applying an electric field to the passive waveguide. An optical device characterized by further provision.
In the optical device according to claim 1 or 2.
The laser active region is
An optical device including a current injection mechanism that injects a current in the plane direction of the substrate.
The optical device according to any one of claims 1 to 3.
An optical device further comprising a DBR region formed on the substrate and optically connected to the other end of the laser active region in the waveguide direction.
The optical device according to any one of claims 1 to 4.
An optical device characterized in that the reflection region has a DBR mirror structure.
The optical device according to any one of claims 1 to 5.
An optical device characterized in that the core of the passive waveguide differs from the core of the laser active region in at least one thickness and width.
The optical device according to any one of claims 1 to 5.
The core of the passive waveguide is an optical device characterized in that the shape of the cross section perpendicular to the waveguide direction is multi-staged in the thickness direction.
In the optical device according to claim 4,
An optical device characterized in that the width of the passive waveguide on the side of the laser active region and the width of the reflection point on the side of the passive waveguide opposite to the laser active region are different.