Open Access
Add to BibSonomyAbstract
A highly efficient silicon (Si) hybrid external cavity laser with a wavelength tunable ring reflector is fabricated on a complementary metal-oxide semiconductor (CMOS)-compatible Si-on-insulator (SOI) platform and experimental results with high output power are demonstrated. A III-V semiconductor gain chip is edge-coupled into a SOI cavity chip through a SiNx spot size converter and Si grating couplers are incorporated to enable wafer-scale characterization. The laser output power reaches 20 mW and the highest wall-plug efficiency of 7.8% is measured at 17.3 mW in un-cooled condition. The laser wavelength tuning ranges are 8 nm for the single ring reflector cavity and 35 nm for the vernier ring reflector cavity, respectively. The Si hybrid laser is a promising light source for energy-efficient Si CMOS photonic links.
© 2014 Optical Society of America
1. Introduction
Silicon (Si) photonics technology has emerged as a promising solution for high-speed intra-system communication systems that require not only large bandwidth with high density and scalability but also low energy-per-bit [1,2]. Tremendous progress has been made in Si photonics devices and Si photonic link development to meet these requirements. Energy-efficient high-speed Si optical modulators [3,4] and Si/germanium (Ge) photodetectors [5,6], as well as other Si photonics devices have been successfully demonstrated on waveguide integrated Si-on-insulator (SOI) platforms. They have matured into complementary metal-oxide semiconductor (CMOS) blocks capable of being manufactured in high-volume for building highly functional and energy-efficient Si photonic links [7,8].
One of the remaining milestones for Si photonics is a power-efficient laser source compatible with SOI platform. Zheng et al. [9] stipulated that the waveguide-coupled wall-plug efficiency (WC-WPE) needs to be 10% to build ultra-low power Si photonics links for practical implementations with sub-pJ/bit power efficiencies. In order to meet this high WC-WPE requirement for the laser, it is not practical to have an external temperature control unit such as a thermo-electric cooler (TEC). In addition to the energy efficiency requirement, the Si laser source needs to be wavelength tunable with a narrow linewidth for wavelength division multiplexing (WDM) applications. To minimize the footprint of a complete Si photonics integrated chip, it is also desirable to directly build the laser sources into the existing SOI platform. Because of the indirect bandgap nature of Si, one implementation approach is to hybrid integrate III-V material into the SOI platform. One of the hybrid integration approaches is wafer-bonding of the III-V and SOI semiconductor materials, which has been extensively explored recently with significant progress [10–12]. Owing to its high scalability and relaxed alignment tolerances, this hybrid bonding technique is a promising technology for realizing an integrated Si laser. One of the technical challenges for the wafer bonding approach, however, is the considerable thermal mismatch between III-V and Si. Another hybrid integration example is the edge-coupling of a III-V chip to a SOI chip by direct butt-coupling. It is a non-invasive approach for the integration of two heterogeneous materials, Si and III-V, which can be independently optimized for highest performance. Zilkie et al. reported an edge-coupled hybrid laser with 9.5% WC-WPE at 6 mW of output power on a 3-μm SOI platform [13]. On the other hand, building an efficient wavelength tuning filter on such a thick SOI platform may present a challenge. Tanaka et al. demonstrated a flip-chip bonded hybrid Si laser, which exhibits 7.6% WC-WPE with 10 mW of output power in a sub-micron SOI platform with temperature control but no wavelength tuning capability [14]. Due to the progress in modern flip-chip bonding technologies, it is feasible to integrate the edge-coupled hybrid laser with an SOI platform with high precision and at low cost.
In this paper, we present a tunable edge-coupled Si hybrid external-cavity laser with high output power and high efficiency. Si grating couplers (GCs) are incorporated with this hybrid laser for testing and characterization. These GCs can also be readily used for wafer-scale characterization for high density laser integration. A SiNx spot-size converter (SSC) is implemented to provide mode-matching between an SOI waveguide (WG) and a III-V reflective semiconductor optical amplifier (RSOA) chip. The waveguide-coupled laser output power reaches 20 mW with a linewidth of 0.22 pm and a side mode suppression ratio (SMSR) of 40 dB. The maximum WC-WPE is measured to be 7.8% at a laser output power of 17.3 mW. We also demonstrated the laser wavelength tuning capability using an integrated micro-heater. Wavelength tuning ranges of 8 nm and 35 nm, in the C-band, were achieved with a single ring reflector and a vernier ring reflector.
2. Device design and fabrication
Our Si devices were designed and fabricated on a 300-nm SOI platform with an 800-nm-thick buried oxide (BOX) layer. The particular BOX thickness is critical to enhance the GC performance by constructive interference of the reflected light off the Si substrate interface. This BOX thickness also establishes adequate optical isolation from the Si substrate. SOI devices were defined by electron-beam (ebeam) lithography and dry etching. The schematic of a Si hybrid external cavity laser with its features is shown in Fig. 1.A 5-μm radius single ring filter connected by a Y-junction forms a wavelength selective loop mirror with a 20-nm free spectral range (FSR). The ring-bus waveguide gap width was designed to be 150 nm, which corresponds to a power coupling ratio of 0.2 and a 1-dB bandwidth of 0.4 nm. The quality (Q) factor of this ring resonator is estimated to be around 4000, which ensures a single mode lasing condition. Additional details of the micro-ring reflector design can be found in [9]. The scanning electron microscopy (SEM) image of the fabricated single ring resonator is shown in Fig. 1(f). We optically characterized the fabricated single ring resonator and the measured Q factor is approximately 3800 in accordance with our original design. A directional coupler was used to extract laser optical power from the cavity. Its cross coupling ratio determines the effective mirror loss of the laser cavity. In our reported configuration, a 3-dB coupler (48% cross coupling ratio in measurement) is incorporated as the laser output coupler, as shown in Fig. 1(a). Fiber GCs were implemented on the laser output waveguides for laser power output coupling to an external fiber array (Fig. 1(b)). The fiber GCs were designed to convert a sub-micron WG mode into a 10-μm mode-field-diameter (MFD) mode of the optical fiber. Details of the fiber GC design can be found in [18]. In our characterization, a coupling loss of –3.5 dB between the SOI WG and a fiber was measured from the fabricated fiber GCs. A SSC, schematically shown in Fig. 1(a), consists of an inverse Si taper embedded in a dielectric overcladding-type WG. SSCs have been widely adopted as an efficient mode converter for sub-micron SOI platforms [14–17], where silicon oxynitride (SiON) (index of 1.5~1.6) is typically used for the dielectric overcladding material. Based on our numerical simulations in Fig. 2(a), the refractive index of the dielectric layer needs to be higher than 1.8 in order to reduce the optical loss to the bottom Si substrate for our 300-nm SOI/800-nm BOX platform. We, therefore, chose SiNx as the dielectric overcladding material, which exhibits an index above 1.9 and is compatible with standard CMOS process. We then designed the SiNx SSC structure using a 3D semi-vectorial BPM tool. In our design, a Si inverse taper is linearly tapered down from a width of 390 nm to 80 nm along a 200-μm length. The simulation results indicate that, it is possible to achieve –0.5 dB mode conversion loss with optimized design parameters. The resulting mode propagation profile with optimized design parameters is shown in Fig. 2(b). It is noted that the mode conversion loss here does not include losses from surface reflections and mode-mismatch. As schematically shown in Fig. 1(a), the SiNx SSC is angled with respect to the edge-normal axis to minimize back-reflections from the coupling interface. An Si inverse taper with a length of 200 μm and a 80-nm tip width was fabricated using ebeam lithography and dry etching on a 300-nm SOI chip. A low-stress SiNx layer was then deposited using a plasma-enhanced chemical vapor deposition (PECVD) process and a 4 μm(W) × 2.5 μm(H) SiNx WG structure (3.5 μm(W) × 2 μm(H) mode field diameter) was defined by photolithography and a dry-etching process. The final SiNx SSC structure is shown as the SEM images in Fig. 1(d). We optically characterized the coupling loss of the fabricated SiNx SSC using a lensed fiber. The coupling loss between a SiNx SSC and the lensed fiber with a mode size of 2.5 μm was measured at –2.5 dB. Based on this measurement, we estimated the coupling loss between the SiNx SSC and the RSOA with a vertical mode size of 2 μm to be about –2 dB. While the SiNx SSC was formed on a channel-type Si inverse taper waveguide (300-nm-thick WG), other SOI devices including ring resonator, directional coupler, and fiber GC were designed with a rib WG structure with an 80-nm-thick slab. Hence, we implemented a slab taper section for the SiNx SSC to connect to the rest of the SOI circuits. A 80-nm-thick slab was linearly tapered down from each side of the WG to ensure a smooth transition into the rib WG along a 20 μm length (Fig. 1(e)) and a minimal mode mismatch at the interface.
Fig. 1 Edge-coupled Si hybrid external cavity laser: (a) A schematic view of the hybrid laser. Inset shows a photograph of the experimental setup of the edge-coupled RSOA and SOI chip. Optical micrograph of (b) the fiber GC and (c) the single ring reflector with integrated metal micro-heater (c). SEM images of (d) SiNx SSC, (e) the slab taper section, and (f) 5-μm-radius single ring resonator.
Fig. 2 (a) Simulation results for the mode conversion loss of 3 × 3 μm SiNx SSC with the corresponding refractive index of the dielectric overcladding. (b) Mode propagation profile with our optimized SiNx SSC design where 300 nm SOI WG mode is converted to 3 μm SiNx WG mode.
Lasing wavelength tuning capability is a key requirement for WDM applications. The resonant wavelength of a micro-ring reflector can be thermally tuned by an integrated metal heater. A nickel-chromium (NiCr) micro-metal heater was integrated above the ring resonator, as shown in Fig. 1(c). We also designed and processed a vernier ring reflector to achieve a wider wavelength tuning range. The optical microscope image of the vernier ring reflector is shown in Fig. 3(a).Two ring resonators with radii of 7.5 μm and 10 μm are cascaded through a 300-nm-wide common bus waveguide, followed by a broadband distributed Bragg reflector (DBR) mirror at the end of the output waveguide of the second ring resonator. A 3-dB directional coupler was also incorporated as a laser output coupler. Figure 3(b) shows the SEM image of the vernier ring where the ring waveguide width is 480 nm and the gap between the bus and ring waveguide is 180 nm. In theory, the vernier effect of the two rings can achieve a 45 nm tuning range by controlling the two micro-heaters. After device fabrication, the SOI chip was appropriately cleaved in order to form the optical access facets.
Fig. 3 (a) Optical micrograph of the vernier ring reflector with integrated micro-heaters. (b) SEM image of the vernier ring. The width of the ring waveguide is 480 nm and the common bus waveguide (300 nm width) is located between two ring resonators.
3. Laser characterization
For the hybrid laser demonstration, a III-V RSOA chip was mounted on a 6-axis piezo-stage for active alignment with the processed SOI chip. We used a commercial-off-the-shelf RSOA chip with a gain length of 1 mm and a vertical mode field diameter (MFD) of about 2 μm. The RSOA has a high reflection mirror (> 90%) on one side and an anti-reflection (AR) layer coated on the edge-coupling side. To minimize the back-reflection further at the interface, its waveguide is angled by 7° with respect to edge-normal axis. A reference fiber GC loop is placed next to the Si reflector outputs for calibration purposes. The optical outputs from the SOI chip were coupled from on-chip GCs to a fiber array. By powering up the RSOA and monitoring the output power from SOI chip, we actively aligned the two chips. After the best alignment was achieved, we measured the L-I-V curve by sweeping the pumping current to the RSOA chip. Figure 7 shows the measured L-I-V curve of an edge-coupled hybrid external cavity laser. All measurements were done at room temperature with no external temperature control. The I-V curve, red line shown in Fig. 4(a), shows the characteristic of the RSOA chip used for this experiment. It indicates a moderately low serial resistance of 1.8 Ω, which is a key parameter for achieving high WPE. The output laser power of the hybrid laser was measured through a fiber array connected to the power meters. The fiber GC coupling loss was calibrated based on the reference loop measurements, which allows us to obtain the actual laser optical power in the SOI waveguide. We collected the laser output from two fiber GC ports and combined them as a total laser output power. The measured L-I curve (blue line in Fig. 4(a)) shows a lasing threshold of 39 mA and a total maximum optical power of up to 20 mW can be achieved. Such output laser power suggests this hybrid laser is capable of supporting multiple Si photonic links simultaneously. The kinks on the L-I curve are likely attributed to mode-hopping due to the thermal-optic effect in RSOA chip during the current injection. To achieve mode-hopping free laser, a phase tuning section shall be incorporated into the laser cavity to compensate the cavity mode drift. Based on the measured L-I-V data, we calculated the laser WC-WPE, with results shown in Fig. 4(b). The WC-WPE is defined to be the ratio between the total laser output power in Si waveguide and the total electrical pumping power. A maximum WC-WPE of 7.8% can be achieved with a pump current of 160 and 190 mA, where the laser output power is 14 and 17.3 mW, respectively. The WC-WPE drops significantly as the pump current increases beyond 250 mA, which is likely due to thermal roll over in the RSOA chip.
Fig. 4 L-I-V curve of edge-coupled hybrid laser. Blue and red curve are L-I and I-V curves correspondingly (a). Extracted experimental WPE (b).
We also measured the linewidth of this hybrid external cavity laser using a high resolution optical spectrum analyzer (APEX), which has a resolution limit of 0.16 pm (or 20 MHz). Figure 5 shows the spectrum of laser output, which clearly confirms a single wavelength operation. The lasing wavelength is 1566.7 nm and the measured linewidth is 0.22 pm (or 27 MHz) when the pump current is 110 mA. The SMSR is more than 40 dB.
4. Laser wavelength tuning experiment
We built a Ω-shaped NiCr heater directly above the ring resonator for thermal tuning of the laser wavelength. The measured resistance of the metal heater was 190 Ω. First, we observed lasing wavelength shifting by applying currents through the metal heater on the single ring reflector. The measurement results are shown in Fig. 6.As we increase the heater power up to 54 mW, we observed a lasing wavelength red-shift from 1566.3 to 1574.6 nm (or 8.3 nm). Across the 8.3-nm tuning range, single wavelength lasing operation with a SMSR of ~40 dB was achieved for all the stable operation points. When a phase tuner is integrated with the gain chip or in Si, we expect to achieve continuous wavelength tuning with an equally high SMSR. The side-mode fringes in the lasing spectrum correspond to Fabry-Perot resonances in the laser cavity, where the total cavity length is 1.56 mm including the 1-mm RSOA gain length. The output power variation across the tuning range is likely due to the non-flat gain spectrum, directional coupler dispersion and back reflection from the output grating couplers
Fig. 6 Measured lasing wavelength tuning results. (a) Laser spectrum with different tuning power. (b) Peak lasing wavelength as a function of the micro-heater power.
To increase the wavelength tuning range, we edge-coupled the RSOA to a vernier ring reflector and applied electrical power to both micro-heaters in order to align the resonances of both ring filters. We measured the lasing wavelength tuning from 1543 to 1578 nm (35 nm tuning range) by adjusting these two micro-heater power levels. The overlapped measured lasing spectrum for each tuning power is shown in Fig. 7.MH1 and MH2 correspond to the micro-heaters on 7.5 and 10 μm ring resonators, respectively. Single-wavelength lasing operation with a SMSR in excess of 35 dB was maintained across the tuning range of 35 nm.
Fig. 7 Tuning of the lasing wavelength with the vernier ring reflector. The table shows color coded electrical tuning powers on the vernier rings (two micro-heaters: MH1, MH2).
5. Conclusion
We demonstrated an edge-coupled hybrid external cavity laser with a maximum WC-WPE of 7.8% in un-cooled CW operation. At the peak efficiency, an output power of 17.3 mW was obtained and a maximum output power of 20 mW can be achieved while maintaining a WC-WPE above 7%. Lasing wavelength tuning range of 8 nm was demonstrated in the cavity containing a thermally controllable ring reflector. The lasing linewidth was measured to be about 0.22 pm (or 27 MHz) with a SMSR of more than 40 dB. To expand the laser wavelength tuning range, an edge-coupled hybrid laser with the vernier dual-ring reflector was also demonstrated. We observed a 35 nm lasing wavelength tuning range while maintaining the single wavelength lasing operation. The edge-coupling hybrid integration approach enables independent performance optimization of the III-V gain chip and the SOI chip separately. The outputs are available for testing through vertically-access grating couplers. Hence, this integration and testing method is also valuable for edge-couple gain integration at the wafer scale. This demonstration promises an encouraging pathway to ultra-efficient inter/intra-chip Si photonic links using integrated on-chip laser sources.
Acknowledgments
This material is based upon work supported, in part, by the Defense Advanced Research Projects Agency (DARPA) under Agreements HR0011-08-09-0001. The views expressed are those of the author and do not reflect the official policy or position of the Department of Defense or the U.S. Government. The authors thank Dr. Jag Shah of the DARPA Microsystems Technology Office (MTO) for his inspiration and support of this program. Approved for Public Release, Distribution Unlimited.
References and links
1. A. V. Krishnamoorthy, R. Ho, X. Zheng, H. Schwetman, J. Lexau, P. Koka, G. Li, I. Shubin, and J. E. Cunningham, “Computer systems based on silicon photonic interconnects,” Proc. IEEE 97(7), 1337–1361 (2009). [CrossRef]
2. B. Jalali and S. Fathpour, “Silicon photonics,” J. Lightwave Technol. 24(12), 4600–4615 (2006). [CrossRef]
3. J. Liu, M. Beals, A. Pomerence, S. Bernadis, R. Sung, J. Cheng, L. C. Kimerling, and J. Michel, “Waveguide-integrated, ultralow-energy GeSi electro-absorption modulator,” Nat. Photonics 2(7), 433–437 (2008). [CrossRef]
4. G. Li, X. Zheng, J. Yao, H. Thacker, I. Shubin, Y. Luo, K. Raj, J. E. Cunningham, and A. V. Krishnamoorthy, “25Gb/s 1V-driving CMOS ring modulator with integrated thermal tuning,” Opt. Express 19(21), 20435–20443 (2011). [CrossRef] [PubMed]
5. D. Feng, S. Liao, P. Dong, N.-N. Feng, H. Liang, D. Zheng, C.-C. Kung, J. Fong, R. Shafiiha, J. Cunningham, A. V. Krishnamoorthy, and M. Asghari, “High-speed Ge photodetector monolithically integrated with large cross-section silicon-on-insulator waveguide,” Appl. Phys. Lett. 95(26), 261105 (2009). [CrossRef]
6. L. Vivien, J. Osmond, J.-M. Fédéli, D. Marris-Morini, P. Crozat, J.-F. Damlencourt, E. Cassan, Y. Lecunff, and S. Laval, “42 GHz p.i.n Germanium photodetector integrated in a silicon-on-insulator waveguide,” Opt. Express 17(8), 6252–6257 (2009). [CrossRef] [PubMed]
7. X. Zheng, J. Lexau, Y. Luo, H. Thacker, T. Pinguet, A. Mekis, G. Li, J. Shi, P. Amberg, N. Pinckney, K. Raj, R. Ho, J. E. Cunningham, and A. V. Krishnamoorthy, “Ultra-low-energy all-CMOS modulator integrated with driver,” Opt. Express 18(3), 3059–3070 (2010). [CrossRef] [PubMed]
8. X. Zheng, Y. Luo, J. Lexau, F. Liu, G. Li, H. Thacker, I. Shubin, J. Yao, K. Raj, R. Ho, J. E. Cunningham, and A. V. Krishnamoorthy, “A 2 pJ/bit(on-chip) 10 Gbps digital CMOS silicon photonic link,” IEEE Photon. Technol. Lett. 24(14), 1260–1262 (2012). [CrossRef]
9. X. Zheng, S. Lin, Y. Luo, J. Yao, G. Li, S. S. Djordjevic, J.-H. 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]
10. 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]
11. H. Park, M. N. Sysak, H.-W. Chen, A. W. Fang, D. Liang, L. Liao, B. R. Koch, J. Bovington, Y. Tang, K. Wong, M. Jacob-Mitos, R. Jones, and J. E. Bowers, “Device and integration technology for silicon photonic transmitters,” IEEE J. Sel. Top. Quant. 17(3), 671–688 (2011). [CrossRef]
12. 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]
13. 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]
14. 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]
15. T. Tsuchizawa, K. Yamada, H. Fukuda, T. Watanabe, J. Takahashi, M. Takahashi, T. Shoji, E. Tamechika, S. Itabashi, and H. Morita, “Microphotonics devices based on silicon microfabrication technology,” IEEE J. Sel. Top. Quant. 11(1), 232–240 (2005). [CrossRef]
16. N. Fujioka, T. Chu, and M. Ishizaka, “Compact and low power consumption hybrid integrated wavelength tunable laser module using silicon waveguide resonators,” J. Lightwave Technol. 28, 3115–3120 (2010).
17. K. Ohira, K. Kobayashi, N. Iizuka, H. Yoshida, M. Ezaki, H. Uemura, A. Kojima, K. Nakamura, H. Furuyama, and H. Shibata, “On-chip optical interconnection by using integrated III-V laser diode and photodetector with silicon waveguide,” Opt. Express 18(15), 15440–15447 (2010). [CrossRef] [PubMed]
18. J. Yao, I. Shubin, X. Zheng, G. Li, Y. Luo, H. Thacker, J.-H. Lee, J. Bickford, K. Raj, J. E. Cunningham, and A. V. Krishnamoorthy, “Low Loss optical interlayer coupling using reflector-enhanced grating coupler” in Proceedings of IEEE Optical Interconnects Conference (Santa Fe, NM, 2013) pp. 31–32. [CrossRef]
