Integrated Microwave Photonic Filter on a Hybrid Silicon ... - CiteSeerX

IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 58, NO. 11, NOVEMBER 2010

3213

Integrated Microwave Photonic Filter on a Hybrid Silicon Platform Hui-Wen Chen, Student Member, IEEE, Alexander W. Fang, Member, IEEE, Jonathan D. Peters, Zhi Wang, Jock Bovington, Student Member, IEEE, Di Liang, Member, IEEE, and John E. Bowers, Fellow, IEEE

Abstract—A hybrid silicon photonic integrated filter is proposed and demonstrated with a novel structure. This filter incorporates a ring resonator in one arm of a Mach–Zehnder interferometer making it possible to obtain a programmable filter response. The optical filter consists of a 5-mm-long delay loop made of low-loss silicon waveguides with integrated thermal modulators resulting in a 0.164-nm free spectral range with absolute phase tunability and gain elements that allow for the tuning of the filter factor. The microwave response of this integrated filter is measured and display tunability of 20 GHz. Index Terms—Hybrid integrated circuits, microwave filters, optical waveguide components, photonic integrated circuits (PICs).

I. INTRODUCTION ICROWAVE finite impulse response (FIR) and infinite impulse response (IIR) filters implemented using optical delay lines have been proposed and demonstrated over two decades ago [1]. Incoherent microwave filter responses were obtained based on the length of the recirculating loop and the modulation frequency. However, such filters, being discrete in nature, have suffered from various issues such as controllability and complexity. Recent advances in photonic integrated circuit (PIC) technology have made it possible to overcome some of these issues, and consequently, realize compact and environmentally insensitive devices [2]. The demonstrated FIR/IIR filters based on passive ring resonators are of particular interest in wavelength division multiplexing (WDM) systems where a precise filter characteristic can be realized by using a combination of the resonators [3]–[5]. In contrast, the application of such photonic filters in the analog and RF domain

M

Manuscript received December 30, 2009; revised July 02, 2010 and July 24, 2010; accepted July 29, 2010. Date of publication October 18, 2010; date of current version November 12, 2010. This work was supported by the Defense Advanced Research Projects Agency (DARPA) PhASER Project under Contract HR0011-08-1-0006. H.-W. Chen, J. D. Peters, Z. Wang, J. Bovington, and J. E. Bowers are with the Department of Electrical and Computer Engineering, University of California at Santa Barbara, Santa Barbara, CA 93106 USA (e-mail: [email protected]). A. W. Fang was with the Department of Electrical and Computer Engineering, University of California at Santa Barbara, Santa Barbara, CA 93106 USA. He is now with Aurrion LLC, Santa Barbara, CA 93111 USA (e-mail: alexander. [email protected]). D. Liang was with the Department of Electrical and Computer Engineering, University of California Santa Barbara, Santa Barbara, CA 93106 USA. He is now with Hewlett-Packard Laboratories, Palo Alto, CA 94304 USA (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMTT.2010.2074870

have been less explored due to coherent interference on the chip scale, although discrete photonic microwave filters have been widely investigated [6]–[8]. The microwave response of a filter on a PIC is usually interference between modulation frequencies and carrier frequencies because the chip size is smaller than the coherence length of the laser source. Structures such as Mach–Zehnder interferometers (MZIs) combined with ring resonators can be utilized to engineer these filters in the gigahertz response range [9]. This filter has shown tunability from 2 to 15 GHz and has a 3-dB bandwidth of 0.635 GHz. However, this particular scheme, where the ring resonators are cascaded on one arm of the MZI, only a pole or zero, can be achieved at a particular port and the filter quality is fairly sensitive to manufacturing errors, e.g., the coupling coefficient between the ring and one arm of the MZI. In this paper, we propose and demonstrate a microwave filter on the hybrid silicon platform (HSP) [10] incorporating an MZI and a ring resonator where the optical paths of the MZI and ring resonator are partially overlapped. A complete filter response, including both poles and zeros at one port, can be generated by utilizing low-loss silicon-on-insulator (SOI) waveguides and gain sections (amplifiers [11]) on the HSP. The low-loss SOI waveguides give access to longer ring lengths unavailable in InP PIC platforms and enables the realization of free spectral ranges (FSRs) around 1 GHz, while the amplifiers are used to control and achieve specific responses. The filter architecture proposed in this paper is ideal for applications such as band filtering for narrowly spaced signals or arbitrary narrow bandpass filtering. II. DEVICE DESIGN AND FABRCATION The schematic drawing of a hybrid silicon tunable filter based on the MZI and ring resonator is shown in Fig. 1(a) along with a scanning electron microscope (SEM) image of the device in Fig. 1(b). Both SOAs and thermal modulators are control elements used to obtain the desired output function, as well as tune the optical wavelength. The transfer function of this device is , , , and represent the described in (1) where , lengths of different paths of the cell illustrated in Fig. 1(a). The definition of other parameters is listed in Table I

0018-9480/$26.00 © 2010 IEEE

(1)

3214


Fig. 2. (a) Schematic figure of three different configurations realizable with the hybrid silicon filter. (b) Simulated response of the filter operated as a: ring, MZI, and a unit cell.

Fig. 1. (a) Schematic of a hybrid silicon tunable filter. (b) SEM photograph of the fabricated device (c) Close-up schematic of the transition section between SOA and passive WG.

TABLE I DEFINITION OF TERMS IN (1)

The response of such a device can be easily understood by considering this formula as an interferometer with a ring on one arm. Without the feedback from the recirculating loop, i.e., , the equation simply expresses the transfer function of an MZI with a modulator on one arm and an SOA on the other arm, as shown in Fig. 2(a): ring, whereas the second term indicates the transfer function of a ring with an SOA and

a phase modulator inside the loop. The overall transfer function of this filter can be decomposed into three distinct responses, as illustrated in Fig. 2(a): ring, MZI, and cell response. The first two responses can be realized by using one of the SOAs as the optical absorbers to block the light passing through certain paths. For example, in order to obtain ring response, the ) is reverse biased as an abSOA on the forward path ( sorber and the SOA inside the loop ( ) is forward biased as an amplifier to compensate for propagation and splitter loss. is forIn contrast, the MZI response is obtained when is reverse biased. Finally, ward biased as an amplifier and the cell response is obtained when both and are used as amplifiers. This cell response can be further altered by and changing the injection current of both modulators ( ). An identical cell response should be able to be realized at any desire wavelength as long as and are adjusted properly. The simulated transfer functions of this filter based on a 5-mm delay loop are displayed in Fig. 2(b) where is set to 0.7. As can be seen, the difference in FSRs between the ring and MZI response can be used to engineer the desired filter shape wherein certain nearby wavelengths need to be eliminated around the operating wavelength. In this work, the path difference between two arms is four times smaller than the length of the ring resonator such that multiple resonant peaks are present in one MZI cycle. The combination can be manipulated by changing the loop length, as well as the path difference between two arms of the MZI. Consequently, poles and zeros can be achieved in a single cell and the resonance peaks and dips are tunable by applying the appropriate index change using the thermal (phase) modulator. This architecture makes selective filtering possible and the small FSR from the large delay loop provides the ability to filter out adjacent wavelengths on a sub-nanometer scale, which corresponds to an FSR of around several gigahertz in the frequency domain.

CHEN et al.: INTEGRATED MICROWAVE PHOTONIC FILTER ON HSP

3215

Fig. 3. Experimental setup to measure the filter response. An oscilloscope is used instead of an OSA to resolve the spectrum.

III. EXPERIMENT SETUP AND RESULTS A. Fundamental Filter Responses in Optical Domain The experimental setup used to collect the filter responses is illustrated in Fig. 3. A temporally swept tunable laser and a 1-GHz oscilloscope were used instead of an optical spectrum analyzer (OSA) because the FSR of interest was smaller than the resolution of a conventional OSA. An Agilent 8163A tunable laser was used as a laser source, followed by an erbium-doped fiber amplifier (EDFA) and a polarization controller are used to generate the signal. The polarization is maintained at TE because the quantum wells of the SOA are compressively strained and have larger interaction with TE polarized signal. A lensed fiber was then utilized to couple the light into the hybrid silicon filter. The output signal was collected by a lensed fiber and coupled to a photodetector (PD) attached to the oscilloscope. By adjusting the swept speed of the wavelength, a time-varying signal corresponding to the wavelength can be measured where the resolution is much higher than a conventional OSA. Hence, device/ring structures with large loop lengths can be resolved. The experimentally obtained and simulated responses of the ring and MZI are shown in Fig. 4(a) and (b), respectively. First, reverse biased and the ring response was measured with the bias current of varied from 0 to 140 mA (before lasing occurs). As can be seen in Fig. 4(a), the response is almost independent of wavelength at low bias current because the loss inside the loop is very high. The ring resonance with nm then becomes stronger as the bias current increases. The maximum extinction ratio (ER) of 6 dB occurs at 140 mA, where the ring is just below the lasing threshold. The variation over different wavelengths of peak transmission is due to the resolution of the instrument, not due to the inherent characteristics of the filter. In addition, the MZI response as an optical absorber. The was measured by using the experimental curve with 0.654-nm FSR is shown in Fig. 4(b). The largest ER of 8 dB occurs when the amplitudes from two arms of the interferometer are equal, while is biased at 20 mA. The amplitude becomes larger as the current increases to 30 mA, yet the ER is reduced due to extra ASE noise. Next, the cell response was measured by fixing the bias current of at 140 mA so that the ring response is maximized. The cell response resembles more closely the ring response, shown does not have enough gain to compenin Fig. 5(a), when sate the loss from the SOA itself. As the amplitudes of both the

Fig. 4. (a) Measured ring response of a 5-mm-long delay loop at different current levels. (b) Experimental MZI response.

ring and the MZI response are almost identical [see Fig. 5(b)], the cell response has both poles and zeros over the measured wavelength range, which is exactly what is expected from simulation. As the bias current of increases, the amplitude of the MZI response is larger than that of the ring response, and consequently, dominates the cell response, which is also degraded by the higher noise floor from increasing ASE. As can be seen in Figs. 4 and 5, the ERs of the ring, MZI, and cell responses are limited by ASE generated from the on-chip SOAs and the -band EDFA in the measurement system. To improve the signal quality of the filter, there are two approaches that can be applied in the future. The first approach is to replace the -band EDFA with an -band EDFA since the on-chip SOAs have gain characteristics in the range around 1575 nm. An -band EDFA can significantly increase the signal-to-noise ratio of the optical input, and consequently improve the filter ER. The second approach is a structure change, where the input multimode interference (MMI) in Fig. 1(a) is replaced by a tunable coupler. Currently the MZI is not optimized because of power imbalance between two arms. If a tunable coupler is used to adjust the splitting ratio of the incoming signal, the current level of the on-chip SOAs can be reduced. The ASE, therefore, can be decreased as well and introduce better filter response. The tunable coupler can be implemented utilizing the technique

3216


Fig. 5. Experimental cell responses with SOA at 140 mA. The injected current of SOA is adjusted from: (a) 20 mA to (b) 30 mA, to (c) 40 mA to show the difference on cell responses. Fig. 7. (a) Cell response without any thermal modulation. (b) Cell response with I = 0 mA and I = 13 mA. (c) Cell response with I = 0 mA and I = 19:5 mA.

Fig. 8. Schematic of the setup to measure the filter function in the frequency domain.

Fig. 6. (a) Wavelength shift of ring and MZI responses at different current level of MOD , where the ring FSR is 0.164 nm. (b) Wavelength shift of MZI response at different current level of MOD .

developed on this platform, which consists of phase modulators on two arms of an MZI. One of the key components in this filter is the phase modulator. It is used to introduce the necessary index shift in the filter response, which, in turn, allows for different filter shapes to be realized. A resistor in the form of a thin layer of NiCr was deposited next to the waveguide. Passing current through the resistor generates the heat necessary to thermally tune the index. Fig. 6(a) shows the relative wavelength shift in terms of the ring of the thermal modulator inFSR by adjusting the current side the ring ( ). The modulator is 637.5 m long and 10 m wide. As can be seen, a shift over one FSR, which correphase change, can be achieved when equals sponds to the 15 and 11 mA for ring and MZI responses, respectively. The thermally induced index shift is positive, which corresponds to

a shift towards longer wavelengths (red shift). In contrast, the ) introduces a blue shift forward path thermal modulator ( (towards shorter wavelength). This is shown in Fig. 6(b), where it can also be seen that the phase shift introduced by this 950- m can achieve a single FSR at 13 mA. A blue shift is obis on the shorter served rather than red shift because the arm of the MZI such that any positive index change will reduce the phase difference between two arms. The relation between wavelength change and injected current of both MODs can be described in (2), where and are coefficients extracted from the fitted curve in Fig. 6

(2) To verify the tunability of the filter response using thermal modulation, experimental results are presented in Fig. 7. All three figures have the same amplifier bias condition; namely, and are biased at 100 and 24 mA, respectively. and are zero. By inFig. 7(a) is the cell response when creasing to 13 mA, the ring response does not change and the


3217

Fig. 9. (a) Measured filter responses at various wavelength in the frequency domain. SOA is biased at 140 mA and SOA is biased at 32 mA. (b) Simulated filter responses corresponding to the experiment results.

MZI response shifts one FSR down to a lower wavelength [see Fig. 6(b)]. The combined cell response is shown in Fig. 7(b). It is clear that the cell response maintains a similar shape, but is shifted to the left by one FSR. The response can be translated further by increasing to 19.5 mA, as shown in Fig. 7(c). The ER of the filter degrades by 2 dB due to increased thermal effects at higher injection currents into the thermal phase modulator. However, the overall filter response is similar to Fig. 7(a). B. Filter Responses in Frequency Domain To explore the microwave characteristics of the silicon hybrid filter, responses of interest in the frequency domain were measured using the experimental setup illustrated in Fig. 8. An Agilent E8703A lightwave component analyzer (LCA) with an external tunable laser source is used to provide a modulated optical signal. This signal is amplified, filtered to eliminate ASE noise of the EDFA, and then coupled into the hybrid silicon filter. The LCA has a bandwidth of 20 GHz, which is similar to the FSR of the ring resonator shown in Fig. 5(a). Since the coherent microwave response is a combination of the interference between modulation frequencies and the interference between carrier frequencies, arbitrary filter shapes can be generated by adjusting the wavelength of the external laser. A microwave filter with either a pole or a zero can be achieved if the carrier wavelength is at the center of a pole or a zero in the optical domain. When the carrier wavelength differs from the peak of a pole or the dip of a zero, a combined response of the pole and zero will be observed. Fig. 9(a) shows the microwave transfer function of the cell response by changing the carrier frequency from 1575.00 to 1575.22 nm. is kept at 32 mA, while is biased at

140 mA so that the ring response is maximized and the amplitudes of the ring and MZI response are at similar levels. As can be seen, the frequency spacing between a pole and zero changes when the carrier frequency is adjusted. For example, the carrier frequency of 1575.10 nm is in the middle between a pole and zero; thus the frequency difference is about 10 GHz, which is exactly half of the FSR of the filter. It is clear that tunability over 20 GHz of this filter can be realized by varying the carrier frequency by 0.22 nm. The best ER of the pole and zero are 4 and 8 dB, respectively. Again, the ER is limited by the ASE noise. Similar responses can also be achieved by adjusting the thermal modulator to change the phase of the carrier instead of shifting the carrier wavelength. As a comparison, the simulation results of the device are depicted in Fig. 9(b) where the loop gain is set to 0.7. It can be seen that the measured filter responses agree well with the theoretical calculation within the frequency range of interest. Multiple poles and zeros within 20 GHz could also be further demonstrated if the loop delay were longer. IV. CONCLUSION A hybrid silicon microwave filter based on an MZI and a ring resonator was demonstrated. A filter with complete functionality in the optical domain, including ring, MZI, and cell responses, was measured by utilizing the SOAs and modulators as control elements. By taking advantage of the low-loss waveguides and gain elements on the HSP, filter responses in the gigahertz frequency range were also demonstrated. The microwave filter responses have large tunability over a 20-GHz range by slightly changing the carrier frequency. The experimental results in both the optical and frequency domains closely match

3218


the simulated responses. In the future, SOAs as feedback elements can be added to the cell so that accurate and stable control of the cell response can be realized. Cascaded cells with a similar function would be another interesting approach to increase the complexity and ER of the filter response. Additionally, more complicated cell functions are feasible by modifying the length of the delay loop, as well as the coupler splitting ratio. ACKNOWLEDGMENT

Alexander W. Fang (S’05–M’08) received the B.S. degree in electrical engineering (with minors in physics and mathematics) from San Jose State University, San Jose, CA, in 2003, and the M.S. and Ph.D. degrees in electrical engineering from the University of California at Santa Barbara, in 2005 and 2008, respectively. He is the cofounder of and currently with Aurrion LLC, Santa Barbara, CA. His current research interests include the heterogeneous integration of III–V materials with silicon for in-plane lasers. He has authored or coauthored over 60 journal and conference papers. Dr. Fang is a member of the Optical Society of America (OSA).

The authors would like to thank E. Norberg, R. Guzzon, and L. A. Coldren, University of California at Santa Barbara, for useful discussions and suggestions. REFERENCES [1] J. E. Bowers, S. A. Newton, W. V. Sorin, and H. J. Shaw, “Filter response of single-mode fibre recirculating delay lines,” Electron. Lett., vol. 18, no. 3, pp. 110–111, Feb. 1982. [2] B. E. Little, J. S. Foresi, G. Steinmeyer, E. R. Thoen, S. T. Chu, H. A. Haus, E. P. Ippen, L. C. Kimerling, and W. Greene, “Ultra-compact Si/SiO2 microring resonator optical channel dropping filters,” IEEE Photon. Technol. Lett., vol. 10, no. 4, pp. 549–551, Apr. 1998. [3] S. T. Chu, B. E. Little, W. Pan, T. Kaneko, and Y. Kokubun, “Secondorder filter response from parallel coupled glass microring resonators,” IEEE Photon. Technol. Lett., vol. 11, no. 11, pp. 1426–1428, Nov. 1999. [4] J. V. Hryniewicz, P. P. Absil, B. E. Little, R. A. Wilson, and P.-T. Ho, “Higher order filter response in coupled microring resonators,” IEEE Photon. Technol. Lett., vol. 12, no. 3, pp. 320–322, Mar. 2000. [5] F. Xia, M. Rooks, L. Sekaric, and Y. Vlasov, “Ultra-compact high order ring resonator filters using submicron silicon photonic wires for onchip optical interconnects,” Opt. Exp., vol. 15, no. 19, Sep. 2007, Art. ID 11934. [6] S. Sales, J. Capmany, J. Marti, and D. Pastor, “Experimental demonstration of fibre-optic delay line filters with negative coefficients,” Electron. Lett., vol. 22, no. 3, pp. 1095–1096, Feb. 1995. [7] N. You and R. A. Minasian, “A novel high- optical microwave processor using hybrid delay-line filters,” IEEE Trans. Microw. Theory Tech., vol. 47, no. 7, pp. 1304–1308, Jul. 1999. [8] J. Capmany, D. Pastor, and B. Ortega, “New and flexible fiber-optic delay-line filters using chirped bragg gratings and laser arrays,” IEEE Trans. Microw. Theory Tech., vol. 47, no. 7, pp. 1321–1326, Jul. 1999. [9] M. S. Rasras, K.-Y. Tu, D. M. Gill, Y.-K. Chen, A. E. White, S. S. Patel, A. Pomerene, D. Carothers, J. Beattie, M. Beals, J. Michel, and L. C. Kimerling, “Demonstration of a tunable microwave-photonic notch filter using low-loss silicon ring resonators,” J. Lightw. Technol., vol. 27, no. 12, pp. 2105–2110, Jun. 2009. [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. Exp., vol. 14, no. 20, 2006, Art. ID 9203. [11] H. Park, Y.-H. Kuo, A. W. Fang, R. Jones, O. Cohen, M. J. Paniccia, and J. E. Bowers, “A hybrid AlGaInAs-silicon evanescent preamplifier and photodetector,” Opt. Exp., vol. 15, 2007, Art. ID 13539. [12] D. Liang and J. E. Bowers, “Highly efficient vertical outgassing channels for low-temperature InP-to-silicon direct wafer bonding on the silicon-on-insulator substrate,” J. Vac. Sci. Technol., vol. 26, no. 4, pp. 1560–1568, Jul. 2008.

Q

Hui-Wen Chen (S’07) received the B.S. degree in electrical engineering from National Tsing Hua University, HsinChu, Taiwan, in 2003, the M.S. degree in communication engineering from National Chiao Tung University, HsinChu, Taiwan, in 2005, and is currently working toward the Ph.D. degree at the University of California at Santa Barbara. Her interests include design, fabrication, and characterization of high-speed modulators, switches, and filters on the HSP.

Jonathan D. Peters possesses over 13 years of process experience in the microelectromechanical systems (MEMS) and semiconductor field. He was with the Applied Magnetics Corporation, Santa Barbara, CA, as an Operator, Process Inspector, and Plate and Etch Technician for five years before joining Solus Micro Technologies/NP Photonics, Westlake Village, CA, as a Process Engineer for four years. He then joined Innovative Micro Technology, Santa Barbara, CA, as a Process Engineer and Research and Development Engineer for four years. In 2008, he joined the Department of Electrical and Computer Engineering, University of California at Santa Barbara (UCSB).

Zhi Wang was born in Hubei Province, China, in 1971. He received the B.Sc. degree in physics from Beijing Normal University, Beijing, China, in 1993, and the Ms.Eng. and Ph.D. degrees from Beijing Jiaotong University, Baijing, China, in 1996, and 2000, respectively. He is a Professor with the Institute of Optical Information, Beijing Jiaotong University. From July 2004 to August 2005, he was a Research Associate with the Department of Electronics, The Hong Kong Polytechnic University. He is currently a Visiting Scholar with the Department of Electrical and Computer Engineering, University of California at Santa Barbara. His research interests include microstructured optical fibers, optical fiber sensing, all-optical networks, and all-optical signal processing. Dr. Wang is a member of the Optical Society of America (OSA) and The International Society for Optical Engineers (SPIE).

Jock Bovington (S’07) received the Bachelor’s degrees in physics and electrical engineering (with a specialty in computer engineering) from Seattle University (SU), Seattle, WA, in 2006, the M.S. degree in electrical engineering from the University of California at Santa Barbara (UCSB), in 2009, and is currently working toward the Ph.D. degree at UCSB. In 2009, he had an internship with the Photonic Technology Laboratory, Intel Laboratories, where he focused on Si hybrid laser integrated PIC testing. His current research interest is photonics with an emphasis on novel hybrid laser designs enable by integration on low-loss Si and novel ultra-low loss platforms. Mr. Bovington is a National Science Foundation (NSF) Fellow. He is the former president of both SU and UCSB Engineers Without Borders Chapters, with whom he still works to provide sustainable development solutions to a rapidly changing world.


Di Liang (S’02–M’07) received the B.S. degree in optical engineering from Zhejiang University, Hangzhou, China in 2002, and the M.S. and Ph.D. degrees in electrical engineering from the University of Notre Dame, Notre Dame, IN, in 2004 and 2006, respectively. He is currently with Hewlett-Packard Laboratories, Palo Alto, CA. He has authored or coauthored over 50 journal and conference papers. His research interests include Si photonics, diode lasers and photodiodes, hybrid integration techniques, and microelectronic fabrication.

3219

John E. Bowers (F’93) received the M.S. and Ph.D. degrees from Stanford University, Stanford, CA. He is currently a Professor with the Department of Electrical Engineering and the Technology Management Program, University of California at Santa Barbara (UCSB). He is also CTO and cofounder of Calient Networks. He is cofounder of the Center for Entrepreneurship and Engineering Management and founder of Terabit Technology. He was with AT&T Bell Laboratories and Honeywell prior to joining UCSB. He has authored or coauthored eight book chapters, 400 journal papers, and 600 conference papers. He holds 49 patents. His research interests are primarily concerned with silicon photonics, opto-electronic devices, optical switching, and transparent optical networks. Dr. Bowers is a Fellow of the Optical Society of America (OSA) and the American Physical Society. He is a member of the National Academy of Engineering. He was an elected member of the IEEE LEOS Board of Governors, a LEOS Distinguished Lecturer, and vice president for Conferences for LEOS. He was a recipient of the IEEE Lasers and Electro-Optics Soceity (LEOS) William Streifer Award and the South Coast Business and Technology Entrepreneur of the Year Award. He was also the recipient of the ACE Award for Most Promising Technology for the hybrid silicon laser in 2007.