Optical Device

Abstract

The invention provides an optical device sequentially including a DBR region, an active region having a DFB laser structure, a feedback region, and an amplification region having a semiconductor optical amplifier structure, wherein the DBR region, the active region, the feedback region, and the amplification region include a common core layer, and a semiconductor layer around the core layer, the semiconductor layer is made of a semiconductor having a refractive index lower than a refractive index of a semiconductor that forms the core layer, the DBR region and the active region include a common diffraction grating, in the semiconductor layer of the active region and the amplification region, at least a part on one side surface side of the core layer in a lateral direction is doped with a p-type impurity, and at least a part on the other side surface side is doped with an n-type impurity, in the core layer of the active region, oscillation light is generated by the DFB laser structure and current injection using the semiconductor layer doped with the p-type impurity and the semiconductor layer doped with the n-type impurity, a laser beam is generated using photon-photon resonance that occurs in accordance with a frequency difference between a frequency of the oscillation light and a frequency of a reflection spectrum in the feedback region, the laser beam propagates through the core layer and is reflected in the DBR region and amplified in the amplification region.

Description

Optical Device

[0001] The present invention relates to an optical device having a directly-modulated laser structure.

[0002] Along with the recent increase of data traffic, directly-modulated lasers (DMLs) and externally-modulated lasers (EAMs), which are used in optical transmitters, are necessary. In particular, a DML that uses a photon-photon resonance (PPR) effect expresses a new resonance peak in a frequency region higher than a relaxation oscillation frequency, thereby extending the modulation band. That is, the bandwidth can be increased.

[0003] In these modulated lasers, when a membrane structure (III-V membrane structure) formed by a III-V semiconductor compound on Si is introduced, it is possible to increase the light confinement rate, increase the bandwidth, and reduce power consumption. Based on this configuration, a III-V membrane structure EML having a bandwidth for supporting a data rate more than 100 Gbps per channel and a III-V membrane structure DML having the PPR effect are disclosed (non-patent literatures 1 and 2).

[0004] [NPL 1] N.P. Diamantopoulos, et al., "60 GHz Bandwidth Directly Modulated Membrane III-V Lasers on SiO2 / Si," Journal of Lightwave Technology, vol. 40, no. 10, pp. 3299-3306 (2022), https: / / doi.org / 10.1109 / JLT.2022.3153648. [NPL 2] T. Hiraki, et al., "Over-67-GHz-Bandwidth Membrane InGaAlAs Electro-Absorption Modulator Integrated With DFB Laser on Si Platform," Journal of Lightwave Technology, vol. 41, no. 3, pp. 880-887 (2023), https: / / doi.org / 10.1109 / JLT.2022.3221814

[0005] The above-described III-V membrane structure EML has a problem that the device size, optical loss, power consumption, and cost are increased. On the other hand, the III-V membrane structure DML having the PPR effect can reduce the device size, optical loss, power consumption, and cost.

[0006] In the III-V membrane structure DML having the PPR effect, however, operation bias conditions are optimized to activate the PPR effect and increase the bandwidth. Hence, the problem is that the optical output power decreases.

[0007] In order to solve the problem as described above, according to the present invention, there is provided an optical device sequentially comprising a DBR region, an active region having a DFB laser structure, a feedback region, and an amplification region having a semiconductor optical amplifier structure, wherein the DBR region, the active region, the feedback region, and the amplification region comprise a common core layer, and a semiconductor layer around the core layer, the semiconductor layer is made of a semiconductor having a refractive index lower than a refractive index of a semiconductor that forms the core layer, the DBR region and the active region comprise a common diffraction grating, in the semiconductor layer of the active region and the amplification region, at least a part on one side surface side of the core layer in a lateral direction is doped with a p-type impurity, and at least a part on the other side surface side is doped with an n-type impurity, and in the core layer of the active region, oscillation light is generated by the DFB laser structure and current injection using the semiconductor layer doped with the p-type impurity and the semiconductor layer doped with the n-type impurity, a laser beam is generated using photon-photon resonance that occurs in accordance with a frequency difference between a frequency of the oscillation light and a frequency of a reflection spectrum in the feedback region, the laser beam propagates through the core layer and is reflected in the DBR region and amplified in the amplification region.

[0008] According to the present invention, it is possible to provide an optical device that has a high bandwidth and operates at a high speed and high output.

[0009] Fig.1A is a schematic plan view showing the configuration of an optical device according to the first embodiment of the present invention;Fig.1B is a schematic plan view showing an example of the configuration of the optical device according to the first embodiment of the present invention;Fig.2A is a IIA - IIA' sectional view showing the configuration of the optical device according to the first embodiment of the present invention;Fig.2B is a IIB - IIB' sectional view showing the configuration of the optical device according to the first embodiment of the present invention;Fig.2C is a IIC - IIC' sectional view showing the configuration of the optical device according to the first embodiment of the present invention;Fig.2D is a IID - IID' sectional view showing the configuration of the optical device according to the first embodiment of the present invention;Fig.3 is a III - III' sectional view showing the configuration of the optical device according to the first embodiment of the present invention;Fig.4 is a view for explaining the operation of the optical device according to the first embodiment of the present invention;Fig.5 is a view for explaining the operation of the optical device according to the first embodiment of the present invention;Fig.6 is a view for explaining the operation of the optical device according to the first embodiment of the present invention; andFig.7 is a view for explaining the operation of the optical device according to the first embodiment of the present invention.

[0010] <First Embodiment> An optical device according to the first embodiment of the present invention will be described with reference to Figs. 1A to 7.

[0011] <Configuration of Optical Device> An optical device 10 according to this embodiment includes a membrane structure (III-V membrane structure) formed by a III-V semiconductor compound. The optical device 10 sequentially includes a DBR region 11, an active region 12, a feedback region 13, and an amplification region 14, as shown in Fig. 1A. Also, tapered regions 15 may be provided at two ends of the optical device 10.

[0012] The active region 12 is the active region 12 including a distributed feedback (DFB) diffraction grating 108, and has the DBR region 11 at one end and the feedback region 13 at the other end. The length of the active region 12 is, for example, 50 to 100 μm. The active region 12 outputs a directly-modulated laser beam from the other end.

[0013] The DBR region 11 is a rear DBR (DBR-r) mirror passive region including a distributed Bragg reflector (DBR) diffraction grating 108. The length of the DBR region 11 is, for example, 80 to 100 μm. The DBR region 11 ensures that a laser beam in single mode is output from the other end of the active region 12.

[0014] The feedback region 13 is a front DBR (DBR-f) mirror passive region including DBR diffraction grating 108. The length of the feedback region 13 is, for example, 150 to 300 μm. The feedback region 13 enables the PPR effect.

[0015] As shown in Fig. 1B, a passive waveguide (WG) having a front reflector with a reflectivity 109 lower than 10% at the other end (the end face on the side of the amplification region 14) may be used as the feedback region 13. In this configuration, to control the effective refractive index difference to reflection, the reflection coefficient is controlled, between the feedback region (WG) 13 and the amplification region 14, by adjusting the width of the III-V region or using a method such as surface etching.

[0016] The amplification region 14 is formed by a semiconductor optical amplifier (SOA). The length of the amplification region 14 is, for example, 200 to 1,000 μm. By the amplification region 14, the optical output power can be adjusted and increased.

[0017] When the active region (DFB region) 12 has no phase shift, to implement a single mode operation, the Bragg wavelength of the DBR-r region may be detuned by 4 to 8 nm with respect to the Bragg wavelength of the DFB region.

[0018] In the optical device 10, to implement the PPR effect for maximizing the bandwidth, a DC bias current is injected to the active region (DFB region) 12. In addition, to control and maximize the optical output power, the DC bias current is injected into the SOA region.

[0019] In the optical device 10, the RF signal of a modulation current for direct modulation is supplied to the DFB region.

[0020] The tapered region 15 has an adiabatic tapered structure and condenses light via coupling with another optical waveguide or an optical fiber.

[0021] Figs. 2A to 2D show a IIA - IIA' sectional view, a IIB - IIB' sectional view, a IIC - IIC' sectional view, and a IID - IID' sectional view of the optical device 10, respectively. Fig. 3 shows a III - III' sectional view of the optical device 10.

[0022] As shown in Figs. 2A to 2C, the optical device 10 sequentially includes SiO2102, a III-V membrane structure, and an overclad 105 on an Si substrate 101.

[0023] In the active region 12 and the amplification region 14, as shown in Figs. 2B and 2C, the III-V membrane structure includes an active layer (core layer) 103, and a semiconductor layer arranged around the active layer 103. The refractive index of a semiconductor forming the semiconductor layer is lower than the refractive index of a semiconductor forming the active layer 103.

[0024] In the semiconductor layer, at least a part on one side surface side of the core layer in the lateral direction is doped with a p-type impurity to form a p-type semiconductor 1042, and at least a part on the other side surface side is doped with an n-type impurity to form a n-type semiconductor 1041. In addition, an undoped region, for example, at least semiconductor layers on the upper surface side and the lower surface side of the active layer 103 are i (intrinsic)-type semiconductor layers 1043. A p-type electrode and an n-type electrode are provided on the surfaces of the p-type semiconductor 1042 and the n-type semiconductor 1041, respectively. Here, "lateral direction" is a direction parallel to the surface of the substrate 101 and vertical to the light guide direction. Also, "side surface of the active layer" is an end face parallel to the light propagation direction in the active layer 103 and vertical to the surface of the substrate 101.

[0025] As an example of a layer structure, the active layer 103 is an InGaAlAs-based multi quantum well (MQW) that adjusts gain supply in the O-band. Alternatively, the active layer may be an InGaAsP-based MQW that adjusts gain supply in the C-band. The i-type semiconductor layers 1043 on the upper and lower sides of the MQW are each made of undoped InP having a thickness of 50 nm.

[0026] The p-type semiconductor 1042 is, for example, Zn-doped (1 × 1018cm-3) p-type InP. The n-type semiconductor 1041 is, for example, Si-doped (2 × 1018cm-3) n-type InP.

[0027] Thus, in the optical device 10, a current is injected to the active layer 103 in the lateral direction.

[0028] The thickness of the III-V membrane structure may be 200 μm to 350 μm.

[0029] Also, in the active region 12, as shown in Fig. 2B, the diffraction grating 108 is arranged on the upper surface of the i-type semiconductor layer 1043. The diffraction grating 108 may be arranged on the lower surface of the i-type semiconductor layer 1043. The diffraction grating 108 may be arranged in a region including the upper surfaces or lower surfaces of the p-type semiconductor 1042 and the n-type semiconductor 1041.

[0030] In this embodiment, an example in which the diffraction grating is formed in the semiconductor layer has been described, but the present invention is not limited to this. For example, a diffraction grating made of SiO2and SiN may be arranged on the surface of the semiconductor layer 1043.

[0031] In this embodiment, an example in which the InGaAlAs-based MQW corresponding to the O-band or the InGaAsP-based MQW corresponding to the C-band is used as the active layer has been described, but the present invention is not limited to this. An MQW corresponding to another wavelength band may be used. Also, the MQW may be made to correspond to another wavelength using a GaAs-based material or a GaN-based material other than the InP-based material.

[0032] In the optical device 10, as shown in Fig. 3, the DBR region 11, the active region 12, the feedback region 13, and the amplification region 14 include the active layer (core layer) 103 of the same structure, that is, the common active layer (core layer) 103. The light confinement rate of this structure is about 14%.

[0033] In the DBR region 11 and the feedback region 13, as shown in Fig. 2A, to reduce the optical loss, the n-type semiconductor layer (for example, InP) 1041 is arranged around the active layer (core layer) 103. In the DBR region 11, the diffraction grating 108 is formed, like the active region 12. The rest of the configuration is the same as the active region 12.

[0034] The diffraction gratings 108 in the DBR region 11 and the feedback region (DBR-f) 13 are formed on the InP surface by shallow etching. No diffraction grating is formed in the amplification region 14. If the feedback region 13 is a passive waveguide (WG) with a reflector, no diffraction grating is formed in the feedback region 13.

[0035] Thus, in the optical device 10, as shown in Fig. 3, the common diffraction grating 108 is arranged in the DBR region 11 and the active region 12. Also, the common diffraction grating 108 may be arranged in the feedback region 13 as well.

[0036] The overclad 105 is made of SiO2. Also, the overclad 105 may be made of an oxide or polymer with a low refractive index.

[0037] Optical coupling with an external optical fiber or chip is enabled by one of an SiOxwaveguide 107 in the overclad 105 and a waveguide 106 of Si or SiN in the SiO2on the Si substrate 101.

[0038] In Fig. 2D, in the tapered region 15, for evanescent coupling with an external optical fiber or chip, a tapered shape in which the width of the waveguide changes from 1.5 μm to 0.1 μm is formed by the i-type semiconductor layer (for example, InP) 1043.

[0039] The interval between the waveguide formed by the III-V membrane structure and the i-type semiconductor layer (for example, InP) 1043 and the SiOxwaveguide 107 or the waveguide 106 of Si or SiN is about 100 nm or less.

[0040] <Operation of Optical Device> The PPR effect in the optical device 10 is generated in accordance with the frequency difference between the peak wavelength of oscillation light (oscillation spectrum) of the DFB laser in the active region 12 and the peak wavelength of the reflection spectrum in the feedback region (DBR-f) 13.

[0041] Thus, the side mode on the long wavelength side of the oscillation wavelength is fed back, and the light intensity increases. As a result, a laser beam for which the light intensity of the side mode is increased by the PPR effect is output from the other end of the feedback region 13.

[0042] The laser beam output from the feedback region 13 is amplified by SOA in the amplification region 14.

[0043] As described above, in the optical device 10, oscillation light is generated, in the core layer 103 of the active region 12, by the DFB laser structure and current injection using the semiconductor layer 1042 doped with a p-type impurity and the semiconductor layer 1041 doped with an n-type impurity, and a laser beam is generated using photon-photon resonance that occurs in accordance with the frequency difference between the frequency of the oscillation light and the frequency of the reflection spectrum in the feedback region 13.

[0044] Fig. 4 shows the gain (material gain, unit: 1 / cm) of the material used for the SOA region 14 in the optical device 10. The material gain is obtained by data fitting based on experiment results concerning a membrane structure SOA (T. Tsurugaya, et al., "Cross-gain modulation-based photonic reservoir computing using low-power-consumption membrane SOA on Si," Optics Express, vol. 30, no. 13, pp. 22871-22884 (2022), https: / / doi.org / 10.1364 / OE.458264). Here, the DC bias current applied to the SOA region 14 was set to 15 mA (open circles and a broken line in Fig. 4) and 20 mA (open rectangles and a broken line in Fig. 4).

[0045] The active region 12 is InGaAsP-MQW with six wells. Also, the light confinement rate of the MQW active region 12 is 9.4% in a III-V membrane structure having a thickness of 230 nm. The fiber input optical power is -8 dBm to -2 dBm, and a fiber coupling loss is 2 dB per facet.

[0046] When the DC bias current is 15 mA, the material gain is 300 to 620 cm-1. When the DC bias current is 20 mA, the material gain is 370 to 700 cm-1. Thus, the material gain is 300 to 700 cm-1, and is 500 cm-1on average.

[0047] Figs. 5 and 6 show an on-chip gain with respect to the length of the SOA of the optical device 10 and the device-fiber coupling output power. In the optical device 10, the thickness of the III-V membrane structure is 350 nm. At this time, the light confinement rate is 14%. These were obtained based on a material gain of 500 ± 200 cm-1in consideration of a gain saturation effect and an internal loss (20 cm-1).

[0048] In Fig. 5, on-chip gains acquired from a minimum value of 300 cm-1of the material gain and a maximum value of 700 cm-1are indicated by bars in the ordinate direction. A change of the on-chip gain acquired from an average value of 500 cm-1of the material gain with respect to the length of the SOA is indicated by a solid line.

[0049] When the length of the SOA increases up to 500 μm, the on-chip gain increases up to about 11 dB on average.

[0050] Fig. 6 shows, for the sake of comparison, the fiber coupling output power (-2 dBm) of the III-V membrane structure EML obtained based on experiment results described in non-patent literature 2 (a dotted line with “EA-DFB Ref.” in Fig. 6). A target output power of +5 dBm is also shown (a dashed line with “Target +5 dBm” in Fig. 6).

[0051] In the optical device 10, the input power to the SOA region 14 changes because the input power to the SOA region 14 depends on selection of an optimization bias current in the DFB region for PPR optimization. Hence, Fig. 6 shows the plot of the device output power with respect to the change of the input power ("DML Pout on-chip" in Fig. 6) to the SOA from -8 to +1 dBm.

[0052] As can be seen from Figs. 5 and 6, when the on-chip gain (Fig. 5) is larger than 5 dB and the input power ("DML Pout on-chip") to the SOA is at least -5 dBm, the optical device 10 including the SOA region 14 having a length of 250 μm or more can implement an output power larger than that of the membrane structure EML (-2 dBm). At this time, the length of the SOA region 14 may be 500 μm or less.

[0053] Furthermore, when the on-chip gain (Fig. 5) is larger than 6 dB and the input power ("DML Pout on-chip") to the SOA is +1 dBm, the optical device 10 including the SOA region 14 having a length of 300 μm or more can implement an output power larger than +5 dBm. At this time, the length of the SOA region 14 may be 500 μm or less.

[0054] Fig. 7 shows comparison of operation power consumption between the optical device 10 according to this embodiment and a membrane structure EML described in non-patent literature 2. Both devices use DC and AC / RF biases in equal numbers and are equal in the length and whole device size.

[0055] The optical device 10 according to this embodiment can increase the output optical power and reduces total power consumption (about 50 mW) by about 62% as compared to the membrane structure EML device.

[0056] The optical device according to this embodiment can operate with power consumption of 50 mW or less or energy consumption of 1 pJ / bit or less and an optical output power of +5 dBm or more in a bandwidth of 50 GHz.

[0057] <Effects> In the optical device 10, since the active layers in all regions have the same MQW structure, the device can easily be manufactured, as compared to a configuration formed by butt-joint bonding, and an optical loss in a butt-joint bonding portion can be reduced.

[0058] According to the optical device of this embodiment, it is possible to provide an optical device that operates at a high speed and high output in a high bandwidth.

[0059] In the embodiment of the present invention, examples of the structures, dimensions, materials, and the like of constituent parts have been described concerning the configuration of the optical device and a manufacturing method thereof. However, the present invention is not limited to this. Obtaining the functions of the optical device and producing the effects suffice.

[0060] Note that the present invention is not limited the above-described embodiments, and it is obvious that many modifications and combinations can be done by any person ordinarily skilled in the art without departing from the technical scope of the present invention.

[0061] Some or all of the above-described embodiments can also be described as in the following supplementary notes but are not limited to the followings.

[0062] (Supplementary Note 1) There is provided an optical device sequentially comprising a DBR region, an active region having a DFB laser structure, a feedback region, and an amplification region having a semiconductor optical amplifier structure, wherein the DBR region, the active region, the feedback region, and the amplification region comprise a common core layer, and a semiconductor layer around the core layer, the semiconductor layer is made of a semiconductor having a refractive index lower than a refractive index of a semiconductor that forms the core layer, the DBR region and the active region comprise a common diffraction grating, in the semiconductor layer of the active region and the amplification region, at least a part on one side surface side of the core layer in a lateral direction is doped with a p-type impurity, and at least a part on the other side surface side is doped with an n-type impurity, and in the core layer of the active region, oscillation light is generated by the DFB laser structure and current injection using the semiconductor layer doped with the p-type impurity and the semiconductor layer doped with the n-type impurity, a laser beam is generated using photon-photon resonance that occurs in accordance with a frequency difference between a frequency of the oscillation light and a frequency of a reflection spectrum in the feedback region, the laser beam propagates through the core layer and is reflected in the DBR region and amplified in the amplification region.

[0063] (Supplementary Note 2) In the optical device according to Supplementary Note 1, a length of the amplification region is not less than 250 μm and not more than 1,000 μm.

[0064] (Supplementary Note 3) In the optical device according to Supplementary Note 1 or 2, a thickness of the semiconductor layer is not less than 200 μm and not more than 350 μm.

[0065] (Supplementary Note 4) In the optical device according to any one of Supplementary Notes 1 to 3, the feedback region comprises the common diffraction grating.

[0066] (Supplementary Note 5) In the optical device according to any one of Supplementary Notes 1 to 3, the feedback region comprises a reflector at an end on a side of the amplification region.

[0067] (Supplementary Note 6) In the optical device according to any one of Supplementary Notes 1 to 5, the DBR region, the active region, the feedback region, and the amplification region comprise a common Si substrate.

[0068] (Supplementary Note 7) In the optical device according to any one of Supplementary Notes 1 to 6, the core layer is formed by a multi quantum well of a III-V semiconductor compound.

[0069] (Supplementary Note 8) In the optical device according to any one of Supplementary Notes 1 to 7, laser oscillation is caused using photon-photon resonance that occurs in accordance with a frequency difference between a frequency of light generated in the active region and a frequency of a reflection spectrum in the feedback region.

[0070] (Supplementary Note 9) In the optical device according to any one of Supplementary Notes 1 to 8, in the core layer of the active region, light generated by current injection using the semiconductor layer doped with the p-type impurity and the semiconductor layer doped with the n-type impurity oscillates a laser by the DFB laser structure, and a laser is oscillated using photon-photon resonance via the feedback region, propagates through the core layer, and is reflected in the DBR region and amplified in the amplification region.

[0071] The present invention is related to a modulated laser and can be applied to an optical transmitter, an optical communication system, and the like.

[0072] 10...optical device 11...DBR region 12...active region 13...feedback region 14...amplification region 103...core layer (active layer) 1041, 1042, 1043...semiconductor layer 108...diffraction grating

Claims

1. An optical device sequentially comprising: 　　　　a DBR region; 　　　　an active region having a DFB laser structure; 　　　　a feedback region; and 　　　　an amplification region having a semiconductor optical amplifier structure, 　　　　wherein the DBR region, the active region, the feedback region, and the amplification region comprise a common core layer, and a semiconductor layer around the core layer, 　　　　the semiconductor layer is made of a semiconductor having a refractive index lower than a refractive index of a semiconductor that forms the core layer, 　　　　the DBR region and the active region comprise a common diffraction grating, 　　　　in the semiconductor layer of the active region and the amplification region, at least a part on one side surface side of the core layer in a lateral direction is doped with a p-type impurity, and at least a part on the other side surface side is doped with an n-type impurity, and 　　　　in the core layer of the active region, oscillation light is generated by the DFB laser structure and current injection using the semiconductor layer doped with the p-type impurity and the semiconductor layer doped with the n-type impurity, a laser beam is generated using photon-photon resonance that occurs in accordance with a frequency difference between a frequency of the oscillation light and a frequency of a reflection spectrum in the feedback region, the laser beam propagates through the core layer and is reflected in the DBR region and amplified in the amplification region.

2. The optical device according to claim 1, wherein a length of the amplification region is not less than 250 μm and not more than 1,000 μm.

3. The optical device according to claim 1 or 2, wherein a thickness of the semiconductor layer is not less than 200 μm and not more than 350 μm.

4. The optical device according to claim 1 or 2, wherein the feedback region comprises the common diffraction grating.

5. The optical device according to claim 1 or 2, wherein the feedback region comprises a reflector at an end on a side of the amplification region.

6. The optical device according to claim 1 or 2, wherein the DBR region, the active region, the feedback region, and the amplification region comprise a common Si substrate.

7. The optical device according to claim 1 or 2, wherein the core layer is formed by a multi quantum well of a III-V semiconductor compound.

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