Semiconductor lasers and two-channel laser arrays

Abstract

The semiconductor laser (10) of the present invention comprises a distributed feedback (DFB) region (100) having a uniform diffraction grating (104) and an active layer (105), and two distributed Bragg reflector (DBR) regions (200, 300) having a uniform diffraction grating (104) and a core layer (103), optically coupled to the respective ends of the DFB region (100) in the waveguiding direction, and the lengths of the DFB region (100) and the DBR region (200, 300) in the waveguiding direction are set so that the photon-photon resonance frequency is 40 GHz to 50 GHz when the operating temperature is 25°C to 75°C. The semiconductor laser of the present invention can optimize the PPR effect to maximize the modulation bandwidth.

Description

[Technical Field]

[0001] The present invention relates to a semiconductor laser that transmits data by directly modulating laser light. [Background technology]

[0002] Due to the rapid growth of traffic in data center networks and high-performance computing systems, future standards such as Terabit Ethernet (TbE) envision data rates of 800Gbps or 1.6Tbps using multiple transmission lanes of 100Gbps per lane or more.

[0003] Therefore, low-power photonic integrated circuits (PICs) are expected to play an important role in short-distance communications of less than 10 km. Such links will operate in the O-band communication window, which corresponds to an oscillation wavelength of around 1.3 μm.

[0004] The PIC-based transmitters (Tx) and receivers (Rx) are expected to operate over a wide temperature range of 25°C to 75°C and have a small size and footprint. Each transmit / receive pair can use multiple spatial or wavelength channels using multiplexing methods called space division multiplexing (SDM) or wavelength division multiplexing (WDM). Each transmitter (Tx) and receiver (Rx) can have 4, 8, 16, or 32 channels.

[0005] The above requirements can be supported by fast and energy-efficient Directly Modulated Lasers (DMLs) operating in the O-band. DMLs are based on InP technology and can have a distributed reflector (DR) longitudinal structure consisting of various distributed feedback (DFB) and distributed Bragg reflector (DBR) regions. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6588859 [Patent Document 2] Patent No. 6927153 [Non-patent literature]

[0007] [Non-Patent Document 1] G. Morthier, et al., “Extended Modulation Bandwidth of DBR and External Cavity Lasers by Utilizing a Cavity Resonance for Equalization,” IEEE J. Quantum Electron., vol. 36, no. 12, pp. 1468-1475, Dec. 2000. [Non-patent document 2] M. Radziunas, et al., “Improving the Modulation Bandwidth in Semiconductor Lasers by Passive Feedback,” IEEE J. Sel. Top. Quantum Electron., vol. 13, no. 1, pp. 136-142, Jan.-Feb. 2007. [Non-patent document 3] H. Dalir and F. Koyama, “Bandwidth enhancement of single-mode VCSEL with lateral optical feedback of slow light,” IEICE Electron. Expr., vol. 8, no. 13, pp. 1075-1081, July 2011. Summary of the Invention [Problem to be solved by the invention]

[0008] To realize a low-power DML, it is necessary to realize a high optical confinement structure. Such a structure can be realized by forming a thin layer (thickness less than 350 nm) of III-V materials on a SiO2 / Si substrate. In a Distributed Reflector-Directly Modulated Laser (DR-DML), the DFB region can include an active layer based on a multiple quantum well (MQW) structure.

[0009] For operation in the O-band, the MQW active layer can be based on InGaAlAs compounds, which can achieve very high optical confinement factors.

[0010] Furthermore, the availability of large Si wafers and well-established fabrication methods ensure low manufacturing costs, and more complex silicon photonics PICs can be realized by coupling Si waveguides to the active layer.

[0011] As shown in Patent Document 1, the modulation bandwidth of a membrane DML is usually limited to a 3-dB bandwidth of about 20 GHz. A vertical laser that enables the photon-photon resonance (PPR) effect via optical feedback has been proposed as a method for widening the modulation bandwidth of a membrane DML (see, for example, Patent Document 2 and Non-Patent Documents 1-3). However, the optimized membrane DML structure for maximizing the modulation bandwidth has not been clarified. [Means for solving the problem]

[0012] In order to solve the above-mentioned problems, the semiconductor laser of the present invention is a semiconductor laser comprising: a distributed feedback region having a uniform first diffraction grating and an active layer; a first distributed Bragg reflector region having a uniform second diffraction grating and a core layer, and optically coupled to one end of the distributed feedback region in the waveguide direction; and a second distributed Bragg reflector region having a uniform third diffraction grating and a core layer, and optically coupled to the other end of the distributed feedback region in the waveguide direction, wherein the lengths of the distributed feedback region, the first distributed Bragg reflector region, and the second distributed Bragg reflector region in the waveguide direction are set so that the photon-photon resonance frequency of the semiconductor laser is 40 GHz to 50 GHz when the operating temperature is 25°C to 75°C. [Effects of the Invention]

[0013] According to the present invention, a membrane DR-DML structure with optimized PPR effect can be realized, and the modulation bandwidth can be maximized when the operating temperature is 25° C. to 75° C. Furthermore, by using the membrane DR-DML structure of the present invention, a two-channel laser array can be realized that supports 200 (2×112) Gbps Non-Return to Zero (NRZ) signals and 400 (2×200) Gbps Pulse Amplitude Modulation-4 (PAM-4) signals in the O-band communication window with low power consumption. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a diagram showing the configuration of a semiconductor laser according to an embodiment of the present invention. [Figure 2] FIG. 2 is a vertical cross-sectional view of a semiconductor laser according to an embodiment of the present invention. [Figure 3] FIG. 3 is a lateral cross-sectional view of a distributed feedback (DFB) region of a semiconductor laser according to an embodiment of the present invention. [Figure 4] FIG. 4 is a lateral cross-sectional view of a distributed Bragg reflector (DBR) region of a semiconductor laser according to an embodiment of the present invention. [Figure 5] FIG. 5 is a diagram showing the relationship between damping and wavelength detuning in a semiconductor laser according to an embodiment of the present invention. [Figure 6] FIG. 6 is a diagram showing the relationship between the resonance frequency and the length of the distributed feedback region in the semiconductor laser according to the embodiment of the present invention. [Figure 7] FIG. 7 is a diagram showing the carrier density dependence of the gain in the semiconductor laser according to the embodiment of the present invention. [Figure 8] FIG. 8 is a diagram showing the results of a simulation of the frequency dependence of the EO response in the semiconductor laser according to the embodiment of the present invention. [Figure 9] FIG. 9 is a diagram showing an example of the configuration of a two-channel laser array using a semiconductor laser according to an embodiment of the present invention. [Figure 10A] FIG. 10A is a diagram showing the measurement results of the LIV characteristics of the two-channel laser array according to the embodiment of the present invention. [Figure 10B] FIG. 10B is a diagram showing the measurement results of the LIV characteristics of the two-channel laser array according to the embodiment of the present invention. [Figure 11] FIG. 11 is a diagram showing the results of spectrum measurement in the two-channel laser array according to the embodiment of the present invention. [Figure 12A] FIG. 12A is a diagram showing the measurement results of the frequency dependence of the EO response in the embodiment of the present invention. [Figure 12B] FIG. 12B is a diagram showing the measurement results of the frequency dependence of the EO response in the embodiment of the present invention. [Figure 12C] FIG. 12C is a diagram showing the measurement results of the frequency dependence of the EO response in the embodiment of the present invention. [Figure 13A] FIG. 13A is a diagram showing a measurement result of an eye diagram according to the embodiment of the present invention. [Figure 13B] FIG. 13B is a diagram showing the measurement results of an eye diagram according to the embodiment of the present invention. [Figure 13C]FIG. 13C is a diagram showing the measurement results of an eye diagram according to the embodiment of the present invention. [Figure 14] FIG. 14 is a diagram showing the measurement results of the BER in the embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the accompanying drawings, in which: FIG.

[0016] First Embodiment <Configuration of semiconductor laser> The configuration of a semiconductor laser according to this embodiment is shown in Fig. 1. A semiconductor laser 10 according to this embodiment has a membrane DR-DML structure including a distributed feedback region (DFB region) 100 and two distributed Bragg reflector regions (DBR-f region 200 and DBR-r region 300) optically coupled to the ends of the DFB region 100 in the waveguiding direction.

[0017] The membrane DR-DML structure of this embodiment is composed of a DFB region 100 sandwiched between a long DBR-f region 200 and a short DBR-r region 300. Laser light is mainly emitted from the facet near the DBR-f region 200.

[0018] A cross-sectional view of the semiconductor laser of this embodiment in the waveguiding direction is shown in Figure 2. DFB region 100 of semiconductor laser 10 of this embodiment has active layer 105 formed on a Si substrate and uniform diffraction grating 104 (first diffraction grating) formed in active layer 105.

[0019] The DBR-f region 200 (first distributed Bragg reflector region) optically coupled to one end of the DFB region 100 in the waveguiding direction comprises a core layer 103 formed continuously with the active layer 105, and a uniform diffraction grating 104 (second diffraction grating) formed in the core layer 103.

[0020] The DBR-r region 300 (second distributed Bragg reflector region) optically coupled to the other end of the DFB region 100 in the waveguiding direction comprises a core layer 103 formed continuously with the active layer 105 and a uniform diffraction grating 104 (third diffraction grating) formed in the core layer 103.

[0021] 3 and 4 show cross-sectional views of the semiconductor laser of this embodiment taken along the transverse direction relative to the waveguide direction. The active layer 105 in the DFB region has a quantum well (MQW) structure. The active layer 105 has a six-period MQW structure based on an InGaAlAs compound. The core width of the active layer 105 is, for example, 600 nm, but is not limited to this.

[0022] The active layer 105 is sandwiched between a p-type InP layer 107 and an n-type InP layer 108. Direct modulation of the laser light is performed via electrodes formed on the p-type InP layer 107 and the n-type InP layer 108.

[0023] The active layer 105 is surrounded by an InP layer 103. The total thickness of the III-V layers (105, 103) is, for example, but not limited to, less than 350 nm. The III-V layers (105, 103) are formed on a SiO2 layer 102 on a Si substrate 101.

[0024] The SiO2 layer 102 is used as an undercladding for the active layer 105. The undercladding further comprises SiO x The overcladding 102 may comprise a low refractive index material such as SiO and / or SiO x It can be made of other low refractive index materials such as SiO2 or BCB.

[0025] The coupling coefficient and Bragg wavelength of the DFB region 100 and the DBR regions (DBR-f200, DBR-r300) can be controlled by a uniform grating 104 formed by periodically etching the upper part of the InP layer 103.

[0026] For low-loss edge coupling to optical fiber, e.g., SiO with a 3 μm × 3 μm core. x A waveguide 106 may be formed on an InP layer 103 surrounded by a cladding layer 102 .

[0027] III-V layers (105, 103) to SiO x Efficient coupling into the waveguide 106 can be achieved by having an InP-based tapered waveguide with a maximum width of 1.5 μm without using a surface InP grating 104. Such InP tapers can be placed on either side of the longitudinal length of the vertical laser structure, as shown in FIG.

[0028] The DBR regions (DBR-f200, DBR-r300) can also be constructed with 1.5 μm wide InP waveguides containing uniform periodic surface gratings 104.

[0029] The length of the DFB region 100 (LDFB) can be set to, for example, 60 μm to 120 μm. The length of the DBR-r region 300 (LDBR-r) and the length of the DBR-f region 200 (LDBR-f) can be set to, for example, 80 μm and 200 μm, respectively.

[0030] The coupling coefficient of the DBR region (DBR-f200, DBR-r300) is, for example, 400 cm -1 The coupling coefficient of the DFB region is, for example, 400 cm -1 ~550cm -1 To operate in the O-band communication window, the Bragg wavelength of the DFB region and the DBR region must be about 1.3 μm.

[0031] The Bragg wavelength detuning between the DBR-f region 200 and the DFB region 100 is preferably within ±1 nm, and the Bragg wavelength of the DBR-r region 300 is preferably +4 nm to +7 nm relative to the Bragg wavelength of the DFB region 100 .

[0032] Such a DR-DML structure ensures single longitudinal mode operation with a small hole burning effect, and oscillation occurs at the long wavelength side of the transmittance and reflectance spectra of the DFB region 100 and the DBR-f region 200. The inherent modulation bandwidth is determined by the relaxation oscillation frequency (f R ) and photon-photon resonance frequency (f PPR ) can be further expanded by the detuning effect between

[0033] <Optimization of photon-photon resonance effects> To optimize the PPR effect and widen the modulation bandwidth, the resonance frequency (f PPR ) needs to be optimized.

[0034] In the membrane DR-DML structure, the length of the DFB region (LDFB) can be changed to PPR Figure 5 shows the longitudinal mode analysis of LDFBs from 60 μm to 120 μm under a given operating bias condition.

[0035] One of the key points to maximize the modulation bandwidth is to achieve a uniform EO response due to the PPR effect. R ) with respect to the resonant frequency (f PPR ) value needs to be optimized.

[0036] f R and f PPR If the frequency separation between f is large, the EO response will have a large dip, and conversely, R and f PPR A small frequency separation between the two may result in a bandwidth less than the maximum potential.

[0037] As a result of the investigation by the inventors of the present application, the relaxation oscillation frequency (f R ) is about 10 to 15 GHz, the resonant frequency (f PPR) is optimally set at approximately 50 GHz.

[0038] The resonant frequency (f PPR The relationship between the length of the DFB region and the DBR-f region (LDBR-f) is 200 μm, and the length of the DBR-r region (LDBR-r) is 80 μm. PPR The length of the distributed feedback (DFB) region (LDFB) corresponding to =50 GHz is approximately 80 μm.

[0039] In order to study the characteristics of the structure of this embodiment by simulation, the relationship between the carrier density and gain in the fabricated membrane DR-DML was studied at operating temperatures of 25° C. to 75° C. Fig. 7 is a diagram showing the carrier density dependence of gain in a semiconductor laser according to an embodiment of the present invention.

[0040] Numerical simulations were performed using a traveling-wave laser simulator based on the carrier density dependence of gain in Figure 7. Figure 8 plots the frequency dependence of the EO response in the membrane DR-DML with LDFB = 80 μm at operating temperatures from 25 °C to 75 °C.

[0041] According to the results in Figure 8, the 3 dB bandwidth at 25°C is around 60 GHz. It can be seen that by adjusting the length of the distributed feedback (DFB) region (LDFB), the PPR effect can be optimized and the 3 dB bandwidth can be expanded.

[0042] According to the results in Figure 8, as the operating temperature increases, f PPR and f R Even if the operating temperature rises, f PPR and f R It can be seen that the frequency separation between is kept within a range suitable for maximizing the EO response.

[0043] The simulation results in Figure 8 show that the relaxation oscillation frequency (f R ) is approximately 15 to 20 GHz, and the resonant frequency (f PPR ) is approximately 50 to 55 GHz. PPR and f R The frequency separation between them is kept at approximately 35 GHz.

[0044] It can also be seen that the operating bias current for maximizing the modulation bandwidth is within the range of ±1 to 2 mA at different operating temperatures from 25°C to 75°C.

[0045] This means that there is no need to incorporate heaters or phase shifters into the distributed reflector (DR) structure to adjust the PPR effect at different operating temperatures.

[0046] Instead, the slower, less expensive control electronics used in DML transmitters can be used to monitor temperature and adjust bias currents within the transmitter module.

[0047] <Experimental results using a two-channel laser array> To experimentally verify the performance of the proposed membrane DR-DML structure, a two-channel laser array with an 80 μm long DFB region was fabricated using membrane III-V-on-Si technology.

[0048] An example of the configuration of a two-channel laser array using the semiconductor laser of this embodiment is shown in Fig. 9. In Fig. 9, two membranes DR-DML having pn junctions in the lateral direction are arranged at a laser pitch of 250 μm.

[0049] Figures 10A and 10B show the measurement results of the optical output power-injection current-voltage (LIV) characteristics of a two-channel laser array. The measurements were performed by fusion-splicing a high numerical aperture fiber (HNAF) to a standard single mode fiber (SSMF) pigtail and butt-coupling it to the front face of the chip. Figures 10A and 10B show the measurement results for CH#1 and CH#2 in Figure 9, respectively.

[0050] Similar to previously reported membrane DR-DMLs with PPR effect, superlinear behavior is observed on the LI curve. The kink in the LI curve corresponds to mode hopping between the lasing mode and the first PPR side mode. Laser output powers of over 1 mW are obtained in CH#1 and CH#2.

[0051] Figure 11 shows the spectrum measurement results for the two-channel laser array described above. The solid line is the static measurement, and the dashed line is the dynamic measurement. For the dynamic measurement (dashed line), a 112 Gbps NRZ signal was used. There is some variation in the Bragg wavelength due to manufacturing, which causes the oscillation wavelengths in CH#1 and CH#2 to be slightly different.

[0052] According to Figure 11, PPR side modes appeared next to the oscillation modes of both channels, confirming that the modulated signal was effectively amplified. This confirmed the existence of the PPR effect to widen the modulation bandwidth.

[0053] Figures 12A, 12B, and 12C show the measurement results of the frequency dependence of the EO response in an embodiment of the present invention. Figures 12A and 12B show the measured EO response of a two-channel array using a membrane DR-DML with a DFB region of LDFB = 80 μm. Figure 12C shows the measured EO response of another membrane DR-DML with a DFB region of LDFB = 100 μm fabricated on the same wafer.

[0054] 12A and 12B, at an operating temperature of 25°C, the 3 dB bandwidth of the membrane DR-DML having a DFB region with an LDFB of 80 μm is approximately 60 GHz. On the other hand, according to FIG. 12C, the 3 dB bandwidth of the membrane DR-DML having a DFB region with an LDFB of 100 μm is approximately 50 GHz. It was confirmed that the 3 dB bandwidth can be expanded by adjusting the length (LDFB) of the DFB region 100.

[0055] According to Figures 12A and 12B, at an operating temperature of 25°C, when the LDFB length is 80 μm, the resonant frequency (f PPR On the other hand, according to FIG. 12C, when the LDFB length is 100 μm, the resonant frequency (f PPR ) is 40 GHz. This result is consistent with the theoretical value shown in Figure 6.

[0056] In Figures 12A and 12B, the relaxation oscillation frequency (f R ) decreases, but the resonant frequency (f PPR For example, in FIG. 12A, the relaxation oscillation frequency (f R ) is approximately 10 to 20 GHz, while the resonant frequency (f PPR ) is approximately 40 to 50 GHz.

[0057] As predicted from the simulation results in Figure 8, even when the operating temperature rises, the f PPR and f R It was confirmed that the frequency separation of f is within the range suitable for maximizing the EO response. PPR and f R The frequency separation between them is kept at approximately 30-35 GHz.

[0058] <Data transmission experiment using a two-channel laser array> Data transmission experiments using 112Gbps NRZ signals and 200Gbps PAM-4 signals were carried out in a two-channel laser array under stage control at 25°C.

[0059] In the data transmission experiment, on the transmitting side, a signal was generated using an arbitrary waveform generator and applied to the two-channel array using an RF cable, RF driver, bias tee, and RF probe. On the receiving side, the signal was detected using a photodiode, RF amplifier, and real-time oscilloscope.

[0060] Measurements were performed in both optical back-to-back (BTB) configuration and in a configuration where the signal is detected after 2 km of transmission over SSMF.

[0061] All signal generation and detection components had sufficient bandwidth of at least 60 GHz and were equipped with offline digital equalization to mitigate linear and nonlinear impairments.

[0062] Figures 13A and 13B show eye diagrams for CH#1 and CH#2 when a 112 Gbps NRZ signal is transmitted over 2 km using SSMF. Figure 13C shows an eye diagram for a 200 Gbps PAM-4 signal measured in a BTB configuration. As shown in the eye diagram measurement results of Figures 13A, 13B, and 13C, eye opening was confirmed.

[0063] Figure 14 shows the measurement results of the bit error rate (BER). The threshold value of KP4-FEC (KP4 Forward Error Correction, used in IEEE 200 / 400Gbps Ethernet standard) is 2.4×10 -4 In this study, we achieved a total data rate of 200 (2 × 112) Gbps using NRZ signals after 2 km transmission over SSMF.

[0064] In addition, when using a 200 Gbps PAM-4 signal in a BTB configuration, the KP4-FEC threshold is not reached. However, the HD-FEC (hard-decision forward error correction) threshold of 1.71 × 10 -2 In this study, we achieved 400 (2 x 200) Gbps using PAM-4 signals in a BTB configuration.

[0065] The bias current and voltage were 11.3 mA and 2.347 V for CH#1, and 13.9 mA and 2.517 V for CH#2. The operating power was approximately 26.5 mW for CH#1 and approximately 35.0 mW for CH#2, for a total of less than 0.3 pJ / bit for a 200 Gbps NRZ signal. This invention has made it possible to realize a low-power laser with an operating power of less than 0.3 pJ / bit.

[0066] <Other embodiments> The DFB region 100 of this embodiment may include a coupled Si waveguide below the III-V layer for coupling to a silicon photonic chip.

[0067] The bias form of the DFB region 100 of this embodiment may be either a lateral pn junction or a vertical pn junction.

[0068] Moreover, as the substrate of the semiconductor laser of this embodiment, other substrates such as InP may be used instead of SiO2 / Si.

[0069] The cladding layer 102 of this embodiment may also be made based on low refractive index materials such as SiO2, BCB, SiOx, or combinations thereof.

[0070] Furthermore, the uniform grating 104 of the semiconductor laser of this embodiment may be realized by other methods, such as by depositing a thin film of SiN or the like and etching it. [Industrial Applicability]

[0071] The present invention is applicable to PIC-based data centers, high performance computing interconnects, and short reach optical communication links. [Explanation of symbols]

[0072] 100…DFB area 101...Si substrate 102...SiO2 layer 103…InP layer 104...Diffraction grating 105...Active layer 106...SiO x Waveguide 107...p-type InP layer 108...n-type InP layer 200…DBR area (DBR-f) 300…DBR area (DBR-r)

Claims

1. a distributed feedback region having a uniform first grating and an active layer; a first distributed Bragg reflector region having a uniform second diffraction grating and a core layer, the first distributed Bragg reflector region being optically coupled to one end of the distributed feedback region in a waveguiding direction; a second distributed Bragg reflector region having a uniform third diffraction grating and a core layer, the second distributed Bragg reflector region being optically coupled to the other end of the distributed feedback region in the waveguiding direction; A semiconductor laser comprising: The lengths of the distributed feedback region, the first distributed Bragg reflector region, and the second distributed Bragg reflector region in the waveguiding direction are set so that the photon-photon resonance frequency of the semiconductor laser is 40 GHz to 50 GHz when the operating temperature is 25° C. to 75° C. Semiconductor laser.

2. The frequency separation between the relaxation oscillation frequency of the semiconductor laser and the photon-photon resonance frequency is 30 to 35 GHz.

2. The semiconductor laser according to claim 1.

3. The lengths of the distributed feedback region, the first distributed Bragg reflector region, and the second distributed Bragg reflector region in the waveguiding direction are set so that the photon-photon resonance frequency is 50 GHz when the operating temperature is 25° C.

2. The semiconductor laser according to claim 1.

4. The lengths of the distributed feedback region, the first distributed Bragg reflector region, and the second distributed Bragg reflector region in the waveguiding direction are 80 μm, 80 μm, and 200 μm, respectively.

2. The semiconductor laser according to claim 1.

5. The 3 dB modulation band of the semiconductor laser is 40 GHz to 60 GHz.

2. The semiconductor laser according to claim 1.

6. When the operating temperature is 25° C., the 3 dB modulation bandwidth of the semiconductor laser is 60 GHz.

6. The semiconductor laser according to claim 5.

7. The present invention comprises two semiconductor lasers according to claim 1 arranged at a predetermined laser pitch, each of the two semiconductor lasers being configured to transmit a 112 Gbps NRZ signal or a 200 Gbps PAM-4 signal in the O-band communication window. Two-channel laser array.

8. When the operating temperature is 25° C., the operating power of the two-channel laser array is less than 0.3 pJ / bit.

8. The dual-channel laser array of claim 7.

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