We demonstrate a laser for the silicon photonics platform by hybrid integration with a III/V reflective semiconductor optical amplifier coupled to a 220 nm silicon-on-insulator half-cavity. We utilize a novel ultra-thin silicon edge coupler. A single adiabatic microring based inline reflector is used to select a lasing mode, as compared to the multiple rings and Bragg gratings used in many previous results. Despite the simplified design, the laser was measured to have on-chip 9.8mW power, less than 220KHz linewidth, over 45dB side mode suppression ratio, less than −135dB/Hz relative intensity noise, and 2.7% wall plug efficiency.
© 2014 Optical Society of America
Silicon photonics has attracted extensive attention in recent years as a promising technology for next-generation high-speed, low power consumption, and low cost communication systems. Near future application is widely seen in Ethernet at 100Gb/s and beyond, likely of use in data centers. A few experiments also indicate long haul capability . The device library has matured over the past decade. High performance and reliable passive components including low-loss waveguide, power splitters, multiplexers [2–4], have been demonstrated. High-speed pn junction modulators, either in ring or Mach-Zehnder form were also reported . Photodetectors based on epitaxial germanium remain CMOS compatible, and approach their III/V counterparts’ performance in key metrics .
An efficient electrically pumped laser source is one of the few bottlenecks to overcome to complete the silicon photonics design kit. Attractive light source solutions have been reported, such as lasers formed by hybrid integration [7, 8] or direct bonding [9–11]. Novel germanium and quantum dot based lasers are also being actively explored [12, 13]. Substantial research efforts are needed for the germanium and quantum dot laser to meet practical application requirements. Direct bonding of III/V materials and hybrid integration are usually regarded as the mainstream solutions. Direct bonding offers the possibility of monolithic integration, but introducing III/V processing is a challenge for CMOS fabs. Hybrid integration, usually based on edge-coupling between the SOI chip and the III/V chip, could leverage advantages of both III/V and silicon material systems, as well as mature packaging processes that are available.
In this paper, we present a laser based on the hybrid integration approach, as shown in Fig. 1. It consists of a commercially available Thorlabs SAF 1126 InP-based reflective semiconductor amplifier (RSOA), and a SOI chip fabricated via the OpSIS multi-project wafer (MPW) foundry service [14, 15] to close the laser cavity. The SOI chip utilized a standard silicon-on-insulator (SOI) wafer with 220 nm thick top silicon film. The choice of a thin silicon layer makes for a higher level of compatibility with many currently utilized silicon photonics device geometries [3,4,6], but introduces a number of challenges. Compared to the hybrid laser in  which uses a SOI wafer with 3 µm top silicon, there is a large mode mismatch between submicron single mode silicon waveguide and the RSOA waveguide mode, introducing a performance-limiting coupling loss. A SiON or polymer spot size converter can be used on the silicon chip to match the RSOA mode, but this approach entails increased process complexity . We used a novel ultra-thin silicon edge coupler to achieve low loss coupling detailed in 2.1 directly from partially-etched silicon, without the requirement of an additional layer.
A key challenge in the design of a hybrid III/V – silicon laser cavity is the need to introduce a reflector that can select a single mode from the high density of fundamental fabry-perot modes created in a relatively large cavity. Previous approaches have included either a distributed Bragg reflector (DBR) , or a combination of a ring resonator and a DBR , or a dual-ring [16, 17] or dual DBR  configuration that makes use of the Vernier effect. Using a single ring-resonator would be desirable, because it would simplify the system design. In [18, 19]. a single ring filter was used, but the laser utilized a directional coupler to extract light from cavity, which resulted in two output ports with same power at the same wavelength. If used in a WDM transmitter, and the two output ports cannot be combined coherently, half of the output power will be wasted. We show that a laser using an adiabatic microring resonator (AMR)  can achieve attractive performance. The large free-spectral range (FSR) of the AMR enables the effective selection of a single lasing mode, without the need for a further frequency selection based on a second ring or a DBR. Figure 1 shows a diagram of the hybrid system. The grating coupler (GC) on the left, GC1 in Fig. 1, is the on-chip laser output port, and where our power measurement is taken. In this case, a grating coupler has been attached to this port to enable us to measure the output power. GC2 and GC3 connect to a 10% directional coupler tap, serving as monitoring ports to help fully characterize the device. This allows the direct measurement of the internal power circulating in the cavity.
2. SOI chip design
2.1 RSOA edge coupler design
Efficient light coupling between a submicron silicon waveguide and an RSOA or laser diode has been challenging due to the large mode mismatch. The mode field diameter (MFD) of a single transverse mode semiconductor laser diode is usually a few microns in the horizontal direction and around 1 um in the vertical direction, while the dimension of typical SOI waveguide is only 500 nm x 220 nm, as shown in Figs. 2(a) and 2(b). Adding a silicon nitride layer can help match the mode at the price of increased fabrication complexity. We adopt an alternative approach, and utilize a partially etched silicon layer. Most silicon photonics processes contain a partial etch step, usually in order to enable electrical contact in modulators. Here, we show that such a partial etch can provide a useful tool for mode-matching.
In the OpSIS-IME MPW platform [14, 15], a 90 nm silicon thickness is available. It is possible to form a waveguide with much looser mode confinement with this geometry, as shown in Fig. 2(c) . The optical mode of the thin waveguide is larger in vertical direction and provides a significantly improved match to the RSOA waveguide mode. Theoretical coupling loss is simulated using finite-difference time-domain (FDTD) method. Coupling loss as a function of RSOA and silicon waveguide facet spacing is summarized in Fig. 3. State-of-the-art motored-controlled optical stages have a resolution better than 0.2 um. At such distance, insertion loss better than 1.9 dB can be achieved.
The thin waveguide width was first tapered from 4.25 µm to 1.2 µm over 150 µm, then a push-pull taper was used for transition between 1.2 µm x 0.09 µm waveguide and 0.5 µm x 0.22 µm routing waveguide . The push-pull taper consists of two 150 µm overlapping tapers in opposing directions. To be consistent with the process design rules and minimize coupling loss, the 0.22 µm thick waveguide tapers from 0.5 µm to 0.18 µm, while the 0.09 µm thick waveguide tapers from 1.2 µm to 0.4 µm. The measured insertion loss of such push-pull taper is 0.8 dB from corresponding characterization structures.
2.2 Inline reflector design based on single AMR
Distributed Bragg reflectors (DBRs) are commonly used for narrow band reflection with large FSR values. Attractive DBRs made on SOI have been reported . However, DBRs usually require high-resolution gratings and are very sensitive to fabrication variations. Inline reflectors appropriate for integrated lasers were proposed and demonstrated by Paloczi et al  and recently implemented in silicon . These consisted of a ring or disk resonator incorporated in a Mach-Zehnder structure. Microring and disk resonators have been widely used for modulation, filtering and sensing in silicon photonics. Particularly as low-loss, sharp bends enabled by submicron silicon waveguides allow extremely small device footprints, thus large FSR. Disk resonators with 2 µm radius and 41.6 nm FSR, and rings with over 50 nm FSR have been reported [20, 24].
In this work, an adiabatic ring-based inline reflector is utilized to achieve narrowband reflection, as shown in Fig. 1. The ring resonator design was adopted from  to achieve a large FSR to ensure single mode lasing. Layout of the AMR is illustrated in Fig. 4. It could be seen as a 3 µm radius micro-disk resonator with the center ellipse carved out. We increased the radius compared to  to make enough space to place metal vias inside the ring for the integrated thermal tuner. The transmission spectrum of the AMR based inline reflector is also plotted in Fig. 4, showing its FSR greater than 30 nm. The Q-factor of the loaded resonator is about 2550.
The intrinsic Q value of the AMR has been degraded via coupling to the two waveguides and losses introduced by doping and metal vias. At the resonance peaks, there is thus preferentially more light reflected back into the laser cavity. The length of RSOA is 1mm, and routing silicon waveguide before the AMR is about 1.35mm, indicating the Fabry-Perot mode spacing is about 0.1nm. Therefore, the overall cavity mode that lines up with this resonance peak will experience lasing.
3. Experiment and results
3.1 Silicon photonics chip fabrication
The SOI chip was fabricated by Institute of Microelectronics in Singapore via the OpSIS foundry service [14, 15]. Starting substrate was a standard 8-inch SOI wafer with 220 nm silicon film on top of 2 µm buried oxide (BOX). Three layers of lithography and etching were utilized to pattern the grating couplers, ultra-thin silicon edge coupler, and strip waveguides. Then dopants and metal interconnects were added to make the AMR tunable, via the creation of an integrated heater inside the AMR. The RSOA (Thorlabs SAF 1126) is commercially available.
3.2 Chip alignment and bonding
The diced SOI chip was first polished to create a smooth waveguide facet. Chip alignment was accomplished with a 5-axis stage from Newport. A lensed fiber was used to probe the high-reflection back facet (90% reflection) of the RSOA. The other end of the lensed fiber was plugged into a photo detector to monitor the laser output power for active alignment. An image of the testing setup and diagram is shown in Fig. 5. When detector showed a maximum reading, the two chips were at optimal relative position, and were bonded together using UV curable epoxy.
3.3 Hybrid laser characterization
3.3.1 Output power and coupling loss
The bonded chip was characterized using a fiber array to probe the output grating couplers, GC1 to GC3 in Fig. 1. The parallel waveguide length in the directional coupler between GC2 and GC3 is 5 um, and the separation between them is 200 nm, corresponding to 10% coupling ratio. GC1 is the main output port. GC2 and GC3 are laid out for monitoring purpose. The GC1 output is plotted in Fig. 6. An SMSR over 45 dB is observed. On-chip output power at the port coupled to GC1 is observed to be 9.8 mW when current is set to 320mA. Stage chuck temperature was set to 20 °C during the measurement. By comparing the power at all three GCs, the transmission and reflection ratio of the inline reflector was found to be 29% and 4%, while 67% of light is lost, most likely because of metal absorption due to contacting the inner part of the ring. Cavity circulating power before the inline reflector is estimated to be 33.8 mW.
The continuous wave (CW) light intensity as a function of pump current, L-I curve, is shown in Fig. 7. The L-I curve shows a threshold at around 60 mA, and slope efficiency of 42 mW/A. Optical probing of grating couplers GC2 and GC3 enables the transmission, reflection and insertion loss of the inline reflector can be characterized. Based on the theory in  and information provided concerning the gain in the RSOA datasheet, coupling loss of external cavity lasers can be derived from its slope efficiency, and it can be calculated that the coupling loss was 3.5 dB. This was higher than the 1.8 dB as we expected, most likely due to a larger gap in our manual alignment.
The true line width cannot be determined from the spectrum in Fig. 6, as the measurement is limited by the optical spectrum analyzer (OSA) resolution to about 0.2 nm. To measure the true linewidth, output of the hybrid laser was combined with output from a test laser (Agilent 81600B) using a fiber 3 dB coupler and the combined signal was fed into a photodetector (PD) to be converted to electrical domain. The PD RF output was monitored by a Tektronix RSA 6114A electrical spectrum analyzer (ESA) with resolution bandwidth (RBW) set at 100 kHz. The wavelength of Agilent laser was set to be very close to the silicon laser, with a difference within the bandwidth of the ESA. A diagram of this experiment and the measured spectrum is shown in Figs. 8 and 9. The Agilent laser is known to have a linewidth of 100 kHz. From the spectrum in Fig. 9, the combined Lorentzian linewidth is 220 kHz. This is a sufficiently narrow linewidth that our hybrid integrated laser can even be used in coherent communication systems, which have some of the narrowest linewidth requirements .
3.3.3 Relative intensity noise
Another metric of interest for the laser when used in data transmission system is the relative intensity noise (RIN). We characterized the RIN of our laser using the same PD and ESA as in the heterodyne measurement. We established an upper bound of the RIN at −135 dB/Hz across the RF spectrum range 20MHz-2.9GHz.
Figure 10 shows the lasing spectra of our laser for different thermal tuning power applied to the ring resonator. Pump current was controlled at 80 mA to get an easier temperature control. Figure 11 shows the center wavelength as a function of thermal tuning power. The lasing wavelength changes linearly as the tuning power increases. 6 nm of tuning wavelength range was obtained by about 6 mW of heating power. As the ring was thermally tuned, its resonance position, the wavelength with most preferential feedback, red shifted. Since the cavity Fabry-Perot mode spacing was around 0.1 nm, about 60 longitudinal modes were progressively selected and deselected as the ring temperature changed, causing mode-hopping to occur during the tuning process. However, the laser wavelength remained stable when the tuning thermal power was fixed. Heater resistance is 29 kΩ at low voltage. Current saturation is observed at voltages higher than 10 V due to the tiny size of the waveguide resistor. At 5.1 mW, the device local temperature is estimated to be about 120 °C, comfortably below the level at which we’d expect damage to the device during the experiment. A number of results [19,27], indicate that heating the ring up to over 300 °C without damaging silicon microrings is possible, indicating that with improved resistors our device could achieve a tuning range of 15 nm or higher.
To conclude, we demonstrate a hybrid-integrated laser for silicon photonics, with CMOS compatible edge coupler and inline reflector. Crucially, laser specifications compatible with even the most stringent communications requirements were achieved despite the use of only a single ring-resonator for lasing mode selection. The laser has a 9.8 mW output power, coupling loss around 3.55 dB, linewidth less than 220 kHz, RIN less than −135 dB/Hz, and over 45 dB side mode suppression ratio.
The authors would like to thank Gernot Pomrenke, of AFOSR, for his support of the OpSIS effort, through both a PECASE award (FA9550-13-1-0027) and ongoing funding for OpSIS (FA9550-10-l-0439). The authors are grateful to Mentor Graphics and Lumerical for their continuing support of the OpSIS effort.
References and links
1. P. Dong, X. Liu, S. Chandrasekhar, L. L. Buhl, R. Aroca, Y. Baeyens, and Y. K. Chen, “Monolithic silicon photonic circuits enable 112-Gb/s PDM- QPSK transmission over 2560-km SSMF,” in European Conference on Optical Communications (2013), paper We.2.B.1.
2. G. Li, J. Yao, H. Thacker, A. Mekis, X. Zheng, I. Shubin, Y. Luo, J. H. Lee, K. Raj, J. E. Cunningham, and A. V. Krishnamoorthy, “Ultralow-loss, high-density SOI optical waveguide routing for macrochip interconnects,” Opt. Express 20(11), 12035–12039 (2012). [CrossRef] [PubMed]
3. Y. Zhang, S. Yang, A. E. Lim, G. Q. Lo, C. Galland, T. Baehr-Jones, and M. Hochberg, “A compact and low loss Y-junction for submicron silicon waveguide,” Opt. Express 21(1), 1310–1316 (2013). [CrossRef] [PubMed]
4. W. Bogaerts, S. K. Selvaraja, P. Dumon, J. Brouckaert, K. De Vos, D. Van Thourhout, and R. Baets, “Silicon-on-insulator spectral filters fabricated with CMOS technology,” IEEE J. Sel. Top. Quantum Electron. 16(1), 33–44 (2010). [CrossRef]
5. G. T. Reed, G. Mashanovish, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4, 548 (2010).
6. L. Vivien, A. Polzer, D. Marris-Morini, J. Osmond, J. M. Hartmann, P. Crozat, E. Cassan, C. Kopp, H. Zimmermann, and J. M. Fédéli, “Zero-bias 40Gbit/s germanium waveguide photodetector on silicon,” Opt. Express 20(2), 1096–1101 (2012). [CrossRef] [PubMed]
7. A. J. Zilkie, P. Seddighian, B. J. Bijlani, W. Qian, D. C. Lee, S. Fathololoumi, J. Fong, R. Shafiiha, D. Feng, B. J. Luff, X. Zheng, J. E. Cunningham, A. V. Krishnamoorthy, and M. Asghari, “Power-efficient III-V/Silicon external cavity DBR lasers,” Opt. Express 20(21), 23456–23462 (2012). [CrossRef] [PubMed]
8. S. Tanaka, S. H. Jeong, S. Sekiguchi, T. Kurahashi, Y. Tanaka, and K. Morito, “High-output-power, single-wavelength silicon hybrid laser using precise flip-chip bonding technology,” Opt. Express 20(27), 28057–28069 (2012). [CrossRef] [PubMed]
9. A. W. Fang, H. Park, O. Cohen, R. Jones, M. J. Paniccia, and J. E. Bowers, “Electrically pumped hybrid AlGaInAs-silicon evanescent laser,” Opt. Express 14(20), 9203–9210 (2006). [CrossRef] [PubMed]
10. S. Keyvaninia, G. Roelkens, D. Van Thourhout, C. Jany, M. Lamponi, A. Le Liepvre, F. Lelarge, D. Make, G. H. Duan, D. Bordel, and J. M. Fedeli, “Demonstration of a heterogeneously integrated III-V/SOI single wavelength tunable laser,” Opt. Express 21(3), 3784–3792 (2013). [CrossRef] [PubMed]
11. T. Creazzo, E. Marchena, S. B. Krasulick, P. Yu, D. Van Orden, J. Y. Spann, C. C. Blivin, L. He, H. Cai, J. M. Dallesasse, R. J. Stone, and A. Mizrahi, “Integrated tunable CMOS laser,” Opt. Express 21(23), 28048–28053 (2013). [CrossRef]
12. R. E. Camacho-Aguilera, Y. Cai, N. Patel, J. T. Bessette, M. Romagnoli, L. C. Kimerling, and J. Michel, “An electrically pumped germanium laser,” Opt. Express 20(10), 11316–11320 (2012). [CrossRef] [PubMed]
15. M. Hochberg and T. Baehr-Jones, “Towards fabless silicon photonics,” Nat. Photonics 4(8), 492–494 (2010). [CrossRef]
16. K. Nemoto, T. Kita, and H. Yamada, “Narrow-spectral-linewidth wavelength-tunable laser diode with Si wire waveguide ring resonators,” Appl. Phys. Express 5(8), 082701 (2012). [CrossRef]
17. R. M. Oldenbeuving, E. J. Klein, H. L. Offerhaus, C. J. Lee, H. Song, and K.-J. Boller, “25 kHz narrow spectral bandwidth of a wavelength tunable diode laser with a short waveguide-based external cavity,” Laser Phys. Lett. 10(1), 015804 (2013). [CrossRef]
18. X. Zheng, S. Lin, Y. Luo, J. Yao, G. Li, S. S. Djordjevic, J. Lee, H. D. Thacker, I. Shubin, K. Raj, J. E. Cunningham, and A. V. Krishnamoorthy, “Efficient WDM Laser Sources towards Terabyte/s Silicon Photonic Interconnects,” J. Lightwave Technol. 31(24), 4142–4154 (2013). [CrossRef]
19. S. Lin, S. S. Djordjevic, J. E. Cunningham, I. Shubin, Y. Luo, J. Yao, G. Li, H. D. Thacker, J. Lee, K. Raj, X. Zheng, and A. V. Krishnamoorthy, “Vertical-coupled high-efficiency tunable III-V-CMOS SOI hybrid external-cavity laser,” Opt. Express 21(26), 32425–32431 (2013). [CrossRef]
21. M. Gould, A. Pomerene, C. Hill, S. Ocheltree, Y. Zhang, T. Baehr-Jones, and M. Hochberg, “Ultra-thin silicon-on-insulator strip waveguides and mode couplers,” Appl. Phys. Lett. 101(22), 221106 (2012). [CrossRef]
22. X. Wang, W. Shi, H. Yun, S. Grist, N. A. F. Jaeger, and L. Chrostowski, “Narrow-band waveguide Bragg gratings on SOI wafers with CMOS-compatible fabrication process,” Opt. Express 20(14), 15547–15558 (2012). [CrossRef] [PubMed]
23. G. T. Paloczi, J. Scheuer, and A. Yariv, “Compact microring-based wavelength-selective inline optical reflector,” IEEE Photonics Technol. Lett. 17(2), 390–392 (2005). [CrossRef]
24. W. Shi, H. Yun, W. Zhang, C. Lin, T. K. Chang, Y. Wang, N. A. F. Jaeger, and L. Chrostowski, “Ultra-compact, high-Q silicon microdisk reflectors,” Opt. Express 20(20), 21840–21846 (2012). [CrossRef] [PubMed]
25. K. Kallimani and M. J. O’Mahony, “Calculation of optical power emitted from a fibre grating laser,” IEE Proc. Optoelectron. 145(6), 319–324 (1998). [CrossRef]
26. L. G. Kazovsky, “Coherent optical receivers: performance analysis and laser linewidth requirements,” Proc. SPIE 0568, 24–31 (1985). [CrossRef]