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An integrated laser is a key component in silicon based photonic integrated circuits. Beyond incorporating the gain medium, on-chip cavity design is critical to device performance and yield. Typical recent results involve cavities utilizing distributed Bragg gratings that require ultra-fine feature sizes. We propose to build laser cavity on silicon using a Sagnac loop mirror and a micro-ring wavelength filter for the first time. The Sagnac loop mirror provides broadband reflection, which is simple to fabricate, has an accurately-controlled reflectivity, and negligible excess loss. Single-mode operation is achieved with the intra-cavity micro-ring filter and, using a 248 nm stepper, the laser wavelength can be lithographically controlled within a standard deviation of 3.6 nm. We demonstrate a proof-of-concept device lasing at 1551.7 nm, with 44 dB SMSR, 1.2 MHz linewidth and 4.8 mW on-chip output power.
© 2014 Optical Society of America
1. Introduction
Silicon photonics has attracted extensive attention in both academia and industry in recent years, as an enabling technology to address the exponentially increasing demands for communication bandwidth. It brings state-of-the-art complementary metal-oxide-semiconductor (CMOS) processing technology to the field of photonic integration. The high yield and uniformity of silicon devices make it possible to build complex photonic systems on chip at large production volumes. While the preferred substrate is silicon-on-insulator (SOI), there exist a number of different SOI platforms due to concurrent efforts in academic and industrial research labs, pioneering start-ups, and foundry service providers [1–7]. The main difference among these platforms is the thickness of the top silicon film of the SOI “sandwich”, chosen based on the need to merge into existing CMOS process flows, differing polarization management techniques, ease of packaging, and considerations of device performance. SOI wafers with submicron top silicon, usually 220 nm, gained more and more popularity, and became the choice of multiple parties [3–7]. Cutting-edge device performance has been demonstrated on this platform, including high-speed modulators [4,5,8], photodetectors [5,9,10], and passive devices such as the Y-junction [11], waveguide crossing [12], and arrayed waveguide gratings [13].
As the device library quickly matures, an integrated laser source for a transmitter remains missing from the design kit of many platforms [3–7]. Basically a laser consists of some gain medium for light amplification and a resonant cavity to provide positive feedback. Due to silicon’s indirect bandgap, various approaches to achieve gain in a CMOS material system have been reported, including heavily N-doped Germanium [14], quantum-dot structures [15], direct bonding of III/V material [16–18], and edge coupling to a reflective semiconductor amplifier (SOA) chip [19–22]. With optical gain available from the above approaches, a partial reflective mirror is needed to close the cavity. The mirror should meet the following requirements to be used in practical applications: 1) manufacturable with high yield in typical silicon photonics processes, where 193 nm or 248 nm lithography tools are usually used; 2) wavelength selective and ideally tunable; 3) low excess loss; 4) with well-defined, preferably adjustable, transmittance (or reflectivity).
An intuitive way to build mirrors on-chip is to utilize distributed Bragg reflectors (DBRs), as implemented in [18, 19] on their 1.5 µm and 3 µm SOI platform. However, because of the high index contrast between silicon and its dioxide, narrow-band gratings on submicron waveguides require sub-50 nm feature size, which is beyond the resolution of mainstream 193 nm or 248 nm lithography tools used in typical silicon photonics processes, and is sensitive to fabrication variations. For example, square gratings of size 40 nm × 145 nm used to achieve a 0.43 nm pass band turned into sinusoidal-shape on fabricated devices [23]. Aware of this limitation, wide-band DBRs in combination with ring filters were used in [17,20]. The grating in [20] was patterned by electron beam lithography. Although a shallow etch could be utilized to create a fine grating in the vertical dimension [17], the constraint on fabrication sensitivity was still not fully relaxed. An eye-shaped reflector does not need the ultra-fine features needed for DBRs, but it does require dedicated control of resonance near critical coupling, which is challenging and could introduce additional loss [21].
In this paper, we propose a laser cavity design with a Sagnac loop mirror and a micro-ring wavelength filter. A diagram of the laser cavity configuration is in Fig. 1. The Sagnac loop mirror, i.e. Mirror A in Fig. 1, is made up of a directional coupler (DC) with its branches tied together on one side. It contains no ultra-fine features other than two parallel waveguides, and can be fabricated by a single etch step. Implementation of Mirror B depends on the gain medium integration technique. It could be either the high reflection end of a reflective SOA in the case of edge-coupled integration or another Sagnac loop mirror in the case of the direct bonding approach. Mirror A and Mirror B, together with the gain medium, readily satisfy the requirement for a Fabry-Perot laser. The transmittance (or reflectivity) of the Sagnac loop mirror can be easily and accurately tuned by adjusting the DC length. Although tapping the cavity waveguide using a DC could also achieve accurate control of power extraction [16, 20, 22], such scheme leads to two output beams, which are challenging to combine. To build a single-mode laser a wavelength filter, such as the critically coupled micro-ring resonator shown in the diagram, must be inserted in the cavity. Note that the spectral response of the filter is fully decoupled from mirror reflectivity, allowing each parameter to be chosen and optimized separately to fit different needs. We also show that the fabricated ring resonance can be lithographically controlled to within σ = 3.6 nm. We demonstrate a prototype single mode laser working at 1551.7 nm, with 44 dB side mode suppression ratio (SMSR), 1.2 MHz linewidth and 4.8 mW on-chip output power.
2. Device design and characterization
We prototyped the aforementioned device as part of a multi-project-wafer run offered by the OpSIS foundry. Fabrication occurred at the Institute of Microelectronics in Singapore. Starting substrate was an 8-inch SOI wafer with 220 nm silicon on top of a 2 µm thick buried oxide layer. Devices were patterned using 248 nm lithography and dry etching. The layout uses 500 nm wide strip waveguide, which was measured to have 2 dB/cm propagation loss. For the DC, two such strip waveguides were routed near each other (200 nm edge-to-edge) to achieve evanescent coupling. Thanks to maturity of silicon processing and submicron thickness of SOI device layer, the directional coupler, which is challenging to fabricate in conventional IIIV optoelectronic material, became a flexible yet reliable building block of silicon based photonics integrated circuits (PICs) [1]. Coupling strength can be accurately modeled by solving the effective index difference of even and odd mode of the structure, or by finite difference time domain (FDTD) method. DC is usually a standard element in the process design kit provided by silicon photonics foundries. From calibration structures, it was found that a 38 µm long coupling region with parallel straight waveguides was needed to couple all the optical power from one waveguide to the other waveguide at 1550 nm wavelength. About 1.5% of the optical power would couple with 0 µm straight coupling length, due to the coupling from waveguide bends.
2.1 Sagnac loop mirror
From the diagram in Fig. 1, it is straightforward to see that the Sagnac loop mirror has 100% transmittance for a DC coupling ratio of either 0 or 100%. Since the DC is symmetric, transmittance at an arbitrary coupling length, x, can be predicted by:
where is the 100% coupling length, and represents the contribution of coupling from waveguide bends. Reflectivity equals to 1-T since excess loss of DC is negligible. To characterize the Sagnac loop mirror transmittance or reflectivity, structures shown as Mirror A in Fig. 1 with different coupling lengths, directly connected to two grating couplers, were measured using a tunable laser. Typical measured spectra are shown in Fig. 2(a). Note that the parabolic line shape and ripples are caused by the spectral response of the grating couplers. Reduction in power indicates decrease of transmittance as coupling length varies from 3 µm to 12 µm. Transmittance and reflectivity as a function of coupling length at 1550 nm wavelength is plotted in Fig. 2(b), where the measured data matched well to the analytic prediction above, meaning that the mirror transmittance and reflectivity can be accurately controlled by choosing the corresponding coupling length.
Fig. 2 (a) Sagnac loop mirror transmission spectrum measured using a tunable laser and grating couplers; Normalized transmittance spectrum is shown in the inset; (b) Transmittance and reflectivity of Sagnac loop mirror as a function of DC coupling length at 1550 nm wavelength.
2.2 Micro-ring filter
Waveguide confinement decreases as the working wavelength is red shifted, hence evanescent coupling, and as a result the reflectivity of Sagnac loop mirror, is stronger in longer wavelength, as shown in the inset of Fig. 2(a). But wavelength dependence of DC is monotonic and gradual, which won’t introduce enough gain difference of cavity Fabry-Perot modes and achieve single mode lasing. Micro-ring resonators, enabled by the tight confinement of silicon waveguides and unique to the submicron silicon platform, serve as a compact and effective intra-cavity wavelength filter for single mode operation. The ring could be made tunable by utilizing thermal or free carrier plasma effect. The Lorentz filter shape of a critically coupled micro-ring provides strong side-mode suppression, and reduces laser linewidth [24]. A major drawback of micro-rings is their sensitivity to fabrication variations. For wafers processed in a commercial CMOS fab, it has been reported that the cross-wafer spread in resonant wavelength is as large as its free spectral range (FSR) [25]. If the micro-ring is used as a WDM modulator, the ring resonance can be thermally tuned to the nearest grid channel, thus mitigating the fabrication sensitivity to a certain extent. However, if the micro-ring is used inside a laser cavity, the non-predictability of lasing wavelength greatly impedes the practical application of such device.
The effect of waveguide geometry variation on micro-ring resonance wavelength can be modeled as a perturbation to the waveguide effective index. The FSR depends on the group index of the waveguide, which is immune to fabrication errors and can be accurately controlled among wafers and lots [25]. If the FSR is increased to be significantly larger than the random spread of wavelengths, that spread determines the range of possible lasing wavelengths. The spread depends on ring waveguide design, the SOI wafer, and silicon processing. We chose an adiabatically widened micro-ring (AMR), which has a large FSR [26] and is more robust against fabrication variations [27]. In an AMR, waveguide is narrow near the coupling region to ensure single mode operation, and then gradually widened to support tight bend and possible need to form metal contact. For an AMR of 2 µm radius, the FSR is as large as 54 nm. As shown in Fig. 3(a), there is only one resonance peak in our testing laser’s sweepable range, 1500 nm to 1580 nm. Resonance FWHM is 1.38 nm, corresponding to a finesse of 39 or Q-factor of 1100. We measured the same device design on all 31 complete 2.5 cm × 3.2 cm reticles across an 8-inch wafer. Wafer chuck temperature was set to 30 °C, where it is most stable. The resonant wavelength distribution contours and statistics are shown in Figs. 3(b) and 3(c). The mean is 1528.76 nm and standard deviation is 3.32 nm.
Fig. 3 (a) Spectrum of AMR drop (solid) and through (dashed) ports. Inset is schematic of AMR layout, where w1 = 0.3 µm, w2 = 0.46 µm, w3 = 0.76 µm, and w4 = 0.2 µm; (b) Contour plot for resonant wavelength distribution across an 8-inch wafer; (c) Statistics of the resonant wavelength distribution.
To further validate the predictability of resonant wavelength, AMRs with slightly different radii on the same wafer were also measured, and summarized in Fig. 4 and Table 1. The wavelength range, maximum minus minimum, falls between 12.30 nm and 16.30 nm. Standard deviation is between 3.32 nm and 3.78 nm, with an average of 3.6 nm. Note that the device is patterned using 248 nm lithography on SOI wafers with 20 nm 3σ thickness variations. Significant device uniformity improvement was observed by switching to 193 nm [28], 193 nm immersion lithography, and more uniform wafers [29]. For WDM applications, the target wavelength can be set as the lower bound of the wavelength spread, and then locally and thermally tuned to the grid wavelength and stabilized with active feedback control [30]. Since the tuning range is a very small fraction of the FSR, thermal tuning power is minimal [31].
Fig. 4 AMR resonance increase as ring radius increases, measured on 31 reticles across an 8-inch wafer.
Table 1. Resonant wavelength distribution of AMR with slightly different radius.
2.3 Sagnac loop mirror and micro-ring based single mode laser
A diced silicon chip was first polished to create a flat and smooth sidewall for butt coupling to the SOA. A silicon nitride waveguide of thickness 0.2 µm and width 4.25 µm is used to better match the SOA mode, as shown in Fig. 5. Transition between silicon and silicon nitride waveguide is based on inverse tapers and measured to have 0.3 dB loss. 0.2 µm silicon nitride is not thick enough to provide effective guiding, thus the mode leaks out in the vertical direction and gets larger. The mode size is similar to the ultra-thin silicon waveguide coupler used in our previous work [21], and expected to have around 3.5 dB loss. A half-cavity with 50% reflectivity Sagnac loop mirror and 2.055 µm AMR on the silicon chip was aligned to the SOA using a six-axis stage. The SOA has anti-reflective coating on the facet in contact with the silicon chip, and 90% reflection on the other end. No index matching oil was used. A pair of probes was used to supply pump current. The SOA was kept at 25 °C by a TEC. The silicon chip sat on metal chuck and stayed at room temperature, 15 to 20 °C. A lensed fiber was used to collect light from the high reflection end of the SOA to monitor the intra-cavity power as a feedback signal during alignment. An image of the testing setup and a zoomed-in view of the SOA silicon chip interface are shown in Fig. 5.
When the power at the lensed fiber side was maximized, a fiber array was brought in to probe the output grating coupler. On the silicon chip, the output grating coupler is firstly connected to a y-junction, which has 3 dB intrinsic loss due to power splitting and 0.3 dB excess loss [11]. One branch of the y-junction is connected to another grating coupler 127 µm away, matching the fiber pitch in the fiber array, while the other branch lead to the output waveguide of the hybrid laser. With the hybrid laser turned off, we actively aligned the fiber array to the grating coupler loop using an Agilent laser and power meter. The grating coupler loss was simultaneously characterized to be 8.5 dB, which is higher than we typically see during wafer scale testing, because it was kept further to the chip surface as precaution. Then the Agilent laser was turned off and hybrid laser turned on, a sharp threshold behavior near 60 mA was observed when varying the pump current. At 170 mA, about 3 × threshold current, optical power measured from the power meter is −5 dBm, which corresponds to on-chip power before the y-junction of 6.8 dBm or 4.8 mW.
The fiber array output was connected to an optical spectrum analyzer (OSA) with 0.1 nm resolution. The trace obtained from the OSA is plotted in Fig. 6(a). The lasing peak appears at 1551.7 nm, with 44 dB SMSR. It is blue shifted compared to the AMR cross-wafer characterization data in Fig. 4 due to temperature difference between the two testing conditions. We performed a heterodyne experiment to measure the laser linewidth. The hybrid laser output from the fiber array was combined with the output of a narrow linewidth laser (Agilent 81600B, linewidth 100 kHz) by a 2 × 2 fiber coupler. The combined signal was sent to a photodetector, with its photocurrent fed into a RF spectrum analyzer. The heterodyne spectrum is plotted in Fig. 6(b), together with a Lorentzian fit. The fitted curve has a full width half maximum (FWHM) of 1.28 MHz, indicating the hybrid laser linewidth is about 1.2 MHz.
Fig. 6 (a) Optical spectrum with 0.1 nm resolution; (b) Heterodyne spectrum (blue dot) and a Lorentzian fit curve with 1.28 MHz FWHM.
3. Conclusion
To conclude, we propose Sagnac loop mirror and micro-ring based laser cavity for photonic integration on silicon. The proposed device avoids the use of DBRs, which significantly relaxes the requirements on the process resolution. We also show the micro-ring resonant wavelength, i.e. the lasing wavelength, can be lithographically controlled to σ = 3.6 nm. A proof-of-concept device lasing at 1551.7 nm, with 40 dB SMSR, 1.2 MHz linewidth and 4.8 mW on-chip output power was demonstrated.
Acknowledgments
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 would also like to thank Mentor Graphics and Lumerical for their continuing support of the OpSIS project. We gratefully acknowledge support for this work by Portage Bay Photonics, LLC.
References and links
1. A. Mekis, S. Gloeckner, G. Masini, A. Narasimha, T. Pinguet, S. Sahni, and P. De Dobbelaere, “A grating-coupler-enabled CMOS photonics platform,” IEEE J. Sel. Top. Quantum Electron. 17(3), 597–608 (2011). [CrossRef]
2. D. Feng, S. Jatar, J. Luff, and M. Asghari, “Micron-scale silicon photonic devices and circuits,” in Optical Fiber Communications Conference (OFC) (2014), paper Th4C. [CrossRef]
3. S. Assefa, S. Shank, W. Green, M. Khater, E. Kiewra, C. Reinholm, S. Kamlapurkar, A. Rylyakov, C. Schow, F. Horst, H. Pan, T. Topuria, P. Rice, D. M. Gill, J. Rosenberg, T. Barwicz, M. Yang, J. Proesel, J. Hofrichter, B. Offrein, X. Gu, W. Haensch, J. Ellis-Monaghan, and Y. Vlasov, “A 90nm CMOS integrated nano-photonics technology for 25 Gbps WDM optical communications applications,” in IEEE International Electron Devices Meeting (IEDM) (2012), pp. 33.8.1–33.8.3.
4. P. Dong, X. Liu, S. Chandrasekhar, L. L. Buhl, R. Aroca, and Y.-K. Chen, “Monolithic silicon photonic integrated circuits for compact 100+ Gb/s coherent optical receivers and transmitters,” IEEE J. Sel. Top. Quantum Electron. 20(4), 6100108 (2014). [CrossRef]
5. T.-Y. Liow, J.-F. Song, X. Tu, A. E.-J. Lim, Q. Fang, N. Duan, M. Yu, and G.-Q. Luo, “Silicon optical interconnect device technologies for 40Gb/s and beyond,” IEEE J. Sel. Top. Quantum Electron. 19(2), 8200312 (2013). [CrossRef]
6. S. K. Selvaraja, W. Bogaerts, P. Dumon, D. Van Thourhout, and R. Baets, “Subnanometer linewidth uniformity in silicon nanophotonic waveguide devices using CMOS fabrication technology,” IEEE J. Sel. Top. Quantum Electron. 16(1), 316–324 (2010). [CrossRef]
7. A. Novack, Y. Liu, R. Ding, M. Gould, T. Baehr-Jones, Q. Li, Y. Yang, Y. Ma, Y. Zhang, K. Padmaraju, K. Bergmen, A. E.-J. Lim, G.-Q. Lo, and M. Hochberg, “A 30 GHz silicon photonic platform,” Proc. SPIE 8781, 878107 (2013). [CrossRef]
8. D. J. Thomson, F. Y. Gardes, J.-M. Fedeli, S. Zlatanovic, Y. Hu, B. P.-P. Kuo, E. Myslivets, N. Alic, S. Radic, G. Z. Mashanovich, and G. T. Reed, “50-Gb/s silicon optical modulator,” IEEE Photon. Technol. Lett. 24(4), 234–236 (2012). [CrossRef]
9. Y. Zhang, S. Yang, Y. Yang, M. Gould, N. Ophir, A. E.-J. Lim, G.-Q. Lo, P. Magill, K. Bergman, T. Baehr-Jones, and M. Hochberg, “A high-responsivity photodetector absent metal-germanium direct contact,” Opt. Express 22(9), 11367–11375 (2014). [CrossRef] [PubMed]
10. 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]
11. Y. Zhang, S. Yang, A. E.-J. 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]
12. Y. Zhang, M. Streshinsky, A. Novack, Y. Ma, S. Yang, A. E.-J. Lim, G.-Q. Lo, T. Baehr-Jones, and M. Hochberg, “A compact and low-loss silicon waveguide crossing for O-band optical interconnect,” Proc. SPIE 8990, 899002 (2014). [CrossRef]
13. 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]
14. 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. T. Wang, H. Liu, A. Lee, F. Pozzi, and A. Seeds, “1.3-μm InAs/GaAs quantum-dot lasers monolithically grown on Si substrates,” Opt. Express 19(12), 11381–11386 (2011). [CrossRef] [PubMed]
16. 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(5), 2315–2322 (2007). [CrossRef] [PubMed]
17. 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]
18. T. Creazzo, E. Marchena, S. B. Krasulick, P. K. 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] [PubMed]
19. 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]
20. 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]
21. S. Yang, Y. Zhang, D. W. Grund, G. A. Ejzak, Y. Liu, A. Novack, D. Prather, A. E.-J. Lim, G.-Q. Lo, T. Baehr-Jones, and M. Hochberg, “A single adiabatic microring-based laser in 220 nm silicon-on-insulator,” Opt. Express 22(1), 1172–1180 (2014). [CrossRef] [PubMed]
22. J. H. Lee, I. Shubin, J. Yao, J. Bickford, Y. Luo, S. Lin, S. S. Djordjevic, H. D. Thacker, J. E. Cunningham, K. Raj, X. Zheng, and A. V. Krishnamoorthy, “High power and widely tunable Si hybrid external-cavity laser for power efficient Si photonics WDM links,” Opt. Express 22(7), 7678–7685 (2014). [CrossRef] [PubMed]
23. 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]
24. B. Liu, A. Shakouri, and J. E. Bowers, “Passive microring-resonator-coupled lasers,” Appl. Phys. Lett. 79(22), 3561–3563 (2001). [CrossRef]
25. A. V. Krishnamoorthy, X. Zheng, G. Li, J. Yao, T. Pinguet, A. Mekis, H. Thacker, I. Shubin, Y. Luo, K. Raj, and J. E. Cunningham, “Exploiting CMOS manufacturing to reduce tuning requirements for resonant optical devices,” IEEE Photon. J. 3(3), 567–579 (2011). [CrossRef]
26. M. R. Watts, “Adiabatic microring resonators,” Opt. Lett. 35(19), 3231–3233 (2010). [CrossRef] [PubMed]
27. J. C. Mikkelsen, W. D. Sacher, and J. K.-S. Poon, “Adiabatically widened silicon microrings for improved variation tolerance,” Opt. Express 22(8), 9659–9666 (2014). [CrossRef] [PubMed]
28. S. K. Selvaraja, P. Jaenen, W. Bogaerts, D. Van Thourhout, P. Dumon, and R. Baets, “Fabrication of photonic wire and crystal circuits in silicon-on-insulator using 193-nm optical lithography,” J. Lightwave Technol. 27(18), 4076–4083 (2009). [CrossRef]
29. S.-H. Jeong, D. Shimura, T. Simoyama, M. Seki, N. Yokoyama, M. Ohtsuka, K. Koshino, T. Horikawa, Y. Tanaka, and K. Morito, “Low-loss, flat-topped and spectrally uniform silicon-nanowire-based 5th-order CROW fabricated by ArF-immersion lithography process on a 300-mm SOI wafer,” Opt. Express 21(25), 30163–30174 (2013). [CrossRef] [PubMed]
30. K. Padmaraju, J. Chan, L. Chen, M. Lipson, and K. Bergman, “Thermal stabilization of a microring modulator using feedback control,” Opt. Express 20(27), 27999–28008 (2012). [CrossRef] [PubMed]
31. P. Dong, W. Qian, H. Liang, R. Shafiiha, D. Feng, G. Li, J. E. Cunningham, A. V. Krishnamoorthy, and M. Asghari, “Thermally tunable silicon racetrack resonators with ultralow tuning power,” Opt. Express 18(19), 20298–20304 (2010). [CrossRef] [PubMed]
