A distributed feedback silicon evanescent laser

We report an electrically pumped distributed feedback silicon evanescent laser. The laser operates continuous wave with a single mode output at 1600 nm. The laser threshold is 25 mA with a maximum output power of 5.4 mW at 10 °C. The maximum operating temperature and minimum line width of the laser are 50 °C, and 3.6 MHz, respectively. ©2008 Optical Society of America OCIS codes: (140.5960) Semiconductor lasers; (250.5300) Photonic integrated circuits. References and links 1. 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, 9203-9210 (2006). 2. J. Van Campenhout, P. Rojo Romeo, P. Regreny, C. Seassal, D. Van Thourhout, S. Verstuyft, L. Di Cioccio, J. -M. Fedeli, C. Lagahe, and R. Baets, "Electrically pumped InP-based microdisk lasers integrated with a nanophotonic silicon-on-insulator waveguide circuit," Opt. Express 15, 6744-6749 (2007). 3. T. Maruyama, T. Okumura, S. Sakamoto, K. Miura, Y. Nishimoto, and S. Arai, "GaInAsP/InP membrane BH-DFB lasers directly bonded on SOI substrate," Opt. Express 14, 8184-8188 (2006) http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-18-8184. 4. G. Morthier, P. Vankwikelberge, Handbook of Distributed Feedback Laser Diodes (Arctech House, Norwood, MA, 1997). 5. A. Liu, L. Liao, D. Rubin, J. Basak, H. Nguyen, Y. Chetrit, R. Cohen, N. Izhaky, and M. Paniccia, " HighSpeed Silicon Modulator for Future VLSI Interconnect," in Integrated Photonics and Nanophotonics Research and Applications, OSA Technical Digest (CD) (Optical Society of America, 2007), paper IMD3. 6. H. Park, Y.-H. Kuo, A. W. Fang, R. Jones, O. Cohen, M. J. Pannicia, J. E. Bowers, "A hybrid AlGaInAssilicon evanescent preamplifier and photodetector," Opt. Express 15, No. 21, (2007). 7. H. Park, A. W. Fang, R. Jones, O. Cohen, M. J. Paniccia, and J. E. Bowers, "40 C Continuous-Wave Electrically Pumped Hybrid Silicon Evanescent Laser," International Semconductor Laser Conference 2006 (ISLC 2006), post deadline paper, (2006) 8. H. Park, A. W. Fang, R. Jones, O. Cohen, O. Raday, M. N. Sysak, M. J. Paniccia, and J. E. Bowers, "A hybrid AlGaInAs-silicon evanescent waveguide photodetector," Opt. Express 15, 6044-6052 (2007). 9. A. W. Fang, R. Jones, H. Park, O. Cohen, O. Raday, M. J. Paniccia, and J. E. Bowers, "Integrated AlGaInAs-silicon evanescent race track laser and photodetector," Opt. Express 15, 2315-2322 (2007). 10. M. N. Sysak, H. Park, A. W. Fang, J. E. Bowers, R. Jones, O. Cohen, O. Raday, and M. J. Paniccia, "Experimental and theoretical thermal analysis of a Hybrid Silicon Evanescent Laser," Opt. Express 15, 15041-15046 (2007). 11. D. Derrickson, Fiber optic test and measurement (Prentice Hall,1998), page185.


Introduction
A new type of hybrid integration on silicon has developed in recent years, allowing for alignment free transfer of III-V materials to silicon [1,2].This method allows for the realization of electrically driven lasers on silicon, a key hurdle in creating silicon photonic circuits, while still maintaining a process that is scalable to high volumes.A key element is developing a compact laser that doesn't require polished facets.A second key element is demonstrating a single frequency laser on silicon that can be integrated with an array waveguide grating (AWG) for DWDM or CWDM integrated transmitters.Figure 1 shows a concept for a silicon terabit transmitter utilizing hybrid integration.25 single wavelength lasers are externally modulated at 40 Gb/s and then multiplexed together into a single waveguide to form a wavelength division multiplexed 1 Tb/s data stream.Although micro-disk lasers yield single wavelength output, their wavelength selection is determined by the round trip cavity length; a parameter strongly dependent on fabrication variations, making wavelength targeting a challenge.The inherent mode mismatch between the optical mode in the disk and the bus waveguide also make precise control of coupling extremely challenging.In addition, their small size leads to high thermal impedance limiting the current maximum demonstrated continuous wave operating temperature on silicon to 20 °C [2].Grating based lasers, on the other hand, are more dependent on grating periodicity rather than duty cycle for wavelength selection, giving them an increased wavelength targeting precision.Optically pumped III-V distributed feedback (DFB) membrane lasers on silicon-on-insulator (SOI) have been demonstrated with maximum output powers of 125 nW [3].We report here an electrically pumped DFB silicon evanescent laser (DFB-SEL) lasing CW up to 50 °C.Single wavelength lasing is observed at 1600 nm with a linewidth of 3.6 MHz and maximum output power of 5.4 mW at 10 °C.

Device design
The device is fabricated on the silicon evanescent device platform [4] with an AlGaInAs quantum well based active layer structure wafer bonded to silicon waveguides.Gratings are formed by depositing a 50 nm PECVD SiO 2 hard mask onto the un-patterned silicon on insulator wafer.The hard mask is patterned with electron-beam lithography and inductively coupled plasma dry etching to form a ~25 nm surface corrugated grating with a 238 nm pitch and 71 % duty cycle.The grating stop-band is designed at around 1600 nm, in order to account for the spectral shift seen in previous devices due to device heating [5].Next, silicon waveguides are formed by depositing a 200 nm SiO 2 hard mask.Waveguide patterning is conducted with projection lithography and a second ICP dry etch.The silicon waveguide has a width, height, and rib etch depth of 1.5 µm, 0.7 µm, and 0.5 µm, respectively.This yields a quantum well confinement factor of 5.2% and a silicon confinement factor of 59.2%.Next, the III-V structure is transferred to the silicon wafer with a low temperature wafer bonding technique and processed for electrical current flow control, the definition of passive regions through the selective removal of III-V materials, and the formation of tapers as described by H. Park et al. [4].Figure 2 shows a longitudinal cross section of the laser structure.The grating is 340 microns long with a ¼ wavelength shift in the center of the grating in order to break the modal degeneracy inside the distributed feedback Bragg gratings.The top of Figure 3 shows the device layout.The DFB-SEL consists of a 200 µm long gain region.80 micron long tapers are formed by linearly narrowing the III-V mesa region above the silicon waveguide.This adiabatically transforms the mode from the hybrid waveguide to the passive silicon waveguide allowing for losses on the order of 1.2 dB per taper and reflections on the order of 6 x 10 -4 [4].Two tapers are placed on both ends of this gain region and are also electrically pumped, giving way for a small amount of optical gain.Silicon evanescent photo-detectors are placed on both sides of the laser in order to enable on chip testing of the DFB-SEL performance.The photo-detectors are 240 µm long including the two 80 µm long tapers.The detector to the right is placed 400 microns away in order to allow room for dicing and polishing for off chip spectral tests.

Laser performance
The light-current (L-I) characteristics of the DFB-SEL is measured on chip by collecting light out of both sides of the laser with integrated silicon evanescent photo-detectors.To determine the laser power output, we assume 100% internal quantum efficiency of the photodetectors in order to conservatively assess the laser performance.It can be seen from Figure 3 that at 10 °C, the lasing threshold is 25 mA with a maximum output power of 5.4 mW.The maximum lasing temperature is 50 °C.The secondary y axis of figure 4 shows voltage-current curve.The laser turn on is ~1.8V and the laser has a 13 ohm device series resistance.
The lasing spectrum is taken by dicing off the right photo-detector, polishing, and anti-reflection coating the silicon waveguide output facet.Light is from the collected with a lensed fiber into an HP spectrum analyzer with a 0.08 nm resolution bandwidth.Figure 5 shows the spectrum with a 10 nm span with the laser being driven at 90 mA.The laser has a lasing peak of 1599.3 nm and a side-mode suppression ratio of 50 dB.It can be seen from the inset that the laser operates single mode over a 100 nm span.
The laser linewidth is measured by using the delayed-self heterodyne method [6].The laser light is collected into a lensed fiber and amplified with an L-band amplifier.The ASE from the L-Band amplifier is filtered out with a 5 nm wide tunable band pass filter.The light is then modulated with a lithium niobate modulator at 5 GHz to generate Lorentzian sidebands 5 GHz away from the laser signal.The signal is place through a fiber interferometer with a 3.5 microsecond delay and then collected into a photodetector in an HP lightwave analyzer.The down converted, lineshape is measured at 5 GHz on the spectrum analyzer with a 50 KHz resolution bandwidth.Figure 6 shows the linewidth at 1.8 mW laser output power with a down converted Lorentzian linewidth of 7.16 MHz corresponding to a 3.6 MHz linewidth, a typical value for commercial DFB lasers.

Conclusion
We have demonstrated a single wavelength electrically pumped distributed feedback silicon evanescent laser operating at 1600 nm.The laser threshold is 25 mA, and has a maximum laser output of 5.4 mW at 10 °C with a maximum lasing temperature of 50 °C.The 50 dB of side mode suppression and 3.6 MHz linewidth, are comparable to commercial III-V DFB lasers, and can be used in conjunction with high speed silicon modulators and low loss multiplexors to create wavelength division multiplexed transmitters on silicon.

Figure 4 -
Figure 4 -L-I-V curve for stage temperatures of 10 °C to 50 °C.

Figure 6 )
Figure 6) Delayed-self heterodyned linewidth trace at an output power of 1.8 mW.