As integrated circuit manufacturers continue to push toward higher integration densities, one of the most promising candidate technologies to emerge is SOI (silicon-on-insulator) nanowire. Its ultra-high index contrast permits ultra-sharp bending, while the CMOS compatibility of the fabrication process indicates great potential for use in low-cost photonics integrated circuits (PICs). Significant progress toward the goal of low-loss silicon photonic devices has been made lately, in the form of reductions in the scattering loss of SOI nanowires and the coupling loss to standard single-mode fibers.1
Figure 1. (a) Scanning electron micrograph of an arrayed waveguide grating (AWG) router with microbends. (b) Spectral responses of all 15 channels. FPR: Free propagation region. FSR: Free spectral range.
Recently, we developed and characterized several SOI nanowire-based photonic integrated devices, including arrayed-waveguide grating (AWG) (de)multiplexers and microring resonators. The former is one of the most important components of various wavelength-division multiplexing modules and systems used for optical communications. We have developed several types of ultrasmall AWG (de)multiplexers, using novel layouts that take advantage of the sharp bending available with SOI nanowires. We have also developed SOI nanowire-based optical sensors featuring microring resonators and cascaded rings.2–4
Figure 1(a) shows a scanning electron micrograph of our 15-channel AWG router. In the present design, the many microbends help to minimize the area occupied by the 34 arrayed waveguides, as does the overlap of the two free propagation regions (FPRs). The resulting device's total size is only 163×147μm.5 Figure 1(b) shows the measured spectral response. With 400GHz channel spacing, crosstalk between adjacent channels is around −5∼−8dB, mainly due to phase errors.
Figure 2. (a) Reflective AWG (de)multiplexer. (b) Spectral responses of all channels.
Figure 3. (a) Fabricated Mach-Zehnder interferometer-coupled microring. (b) Transmission response under deionized water cladding. (c) Wavelength shift after application of various liquids. ΔλMRR0: Wavelength shift relative to deionized water.
Placing a photonic crystal (PhC) reflector at the end of each arrayed waveguide permits the waveguides to be freely arranged according to specific requirements.6 For instance, if a high diffraction order (requiring longer waveguides) is desired, the area occupied can be minimized by incorporating several bends, as in the reflective AWG shown in Figure 2(a). By contrast, the very short waveguides needed for lower diffraction orders might require no bends at all. Figure 2(b) shows our reflective AWG's measured spectral response to transverse electric polarization. Crosstalk between adjacent channels is about –12dB, while the excess loss is about 3∼4dB, mainly from coupling between the FPR and the arrayed waveguides and from loss at the PhC reflectors.
In the realm of optical sensing, SOI nanowires can form the basis of a high-precision platform, due to their enhanced evanescent field. To that end, we fabricated the MZI (Mach-Zehnder interferometer)-coupled microring resonator shown in Figure 3(a).2 Figure 3(b) shows the measured spectral response when the device is covered by deionized water (DI-H2O). Over a broad range of wavelengths (1520∼1630nm), limited by the tunable range of the laser source, each resonant wavelength is distinctly depressed, with a major resonant wavelength λMRR0 (extinction ratio ∼25dB) occurring near 1540nm. Figure 3(c) shows the results of varying the ambient refractive index between 1 and 1.538 by applying various liquids. The linearity of the ΔλMRR0/Δn curve confirms that the MZI-coupled microring sensor works well even under significant refractive index change.
Figure 4.(a) Fabricated cascaded-ring sensor with suspended SOI nanowires. (b) Wavelength shift Δλpeakas a function of solution concentration. λpeak: Resonance wavelength.
The cascaded-ring sensor shown in Figure 4(a) features two rings whose unequal radii correspond to different free spectral ranges (FSRs).4 In the present design, ring #2 is exposed to the liquid sample to be measured. To enhance sensitivity, a suspended silicon nanowire is used for this sensing ring. The highest power level seen in the spectral response at the drop port corresponds to the system's resonance wavelength λpeak. Due to the Vernier effect, changes in the sample's refractive index cause λpeak to shift by integral multiples of the FSR of ring #1 (λFSR1). Figure 4(b) shows λpeak over a sample concentration range of 0–0.11%wt, for which a best-fit curve gives a slope of approximately 330nm/%wt. This corresponds to a sensitivity of about S=4.6×105nm per refractive index unit (RIU), which is several orders higher than conventional single-ring sensors. Since the detection limit of the cascaded-ring sensor is given by Δnmin=ΔλFSR1/S, where ΔλFSR1=2.2nm in our design, our detection limit is estimated to be about 4.8×10−6RIU.
The results of characterizing the SOI nanowire-based photonic integrated devices we have developed demonstrate their suitability for optical communications and optical sensing. Our future work will focus on realizing large-scale PICs on silicon.
The authors would like to thank various colleagues for their contributions. This project was supported by research grants R1080193 (Zhejiang Province of China), 61077040 (National Science Foundation of China), and 863 project 2011AA010301 (Ministry of Science and Technology of China).
Daoxin Dai, Sailing He
Daoxin Dai received his BEng from the Department of Optical Engineering of Zhejiang University (2000) and his PhD from the same department (2005). His current research interests include silicon micro/nanophotonics. He is lead author on over 80 papers in refereed international journals.
Sailing He received his licentiate of technology and PhD in electromagnetic theory from the Royal Institute of Technology, Stockholm, Sweden (1991 and 1992, respectively). After obtaining his PhD, he worked at the Royal Institute of Technology as an assistant professor, an associate professor, and a full professor. He has been with Zhejiang University since 1999, when he was appointed as a Chang-jiang project professor by the Ministry of Education of China. A chief scientist for the Joint Research Center of Photonics of the Royal Institute of Technology and Zhejiang University, he is lead author on one monograph (Oxford University Press) and has authored or coauthored about 400 papers in refereed international journals.
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IEEE Photon. Technol. Lett. 23, no. 13, pp. 842-844, 2011.
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