Ultracompact bandwidth-tunable filter based on subwavelength grating-assisted contra-directional couplers
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RESEARCH ARTICLE
Ultracompact bandwidth-tunable filter based on subwavelength grating-assisted contra-directional couplers Kangnian WANG, Yuan WANG, Xuhan GUO (✉), Yong ZHANG, An HE, Yikai SU State Key Laboratory of Advanced Optical Communication Systems and Networks, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
© Higher Education Press 2020
Abstract An ultracompact, bandwidth-tunable filter has been demonstrated using a silicon-on-insulator (SOI) wafer. The device is based on cascaded grating-assisted contra-directional couplers (GACDCs). It also involves the use of a subwavelength grating (SWG) structure. By heating one of the heaters on GACDCs, a bandwidth tunability of ~6 nm is achieved. Owing to the benefit of having a large coupling coefficient between SWG and strip waveguides, the length of the coupling region is only 100 μm. Moreover, the combination of the curved SWG and the tapered strip waveguides effectively suppresses the sidelobes. The filter possesses features of simultaneous wavelength tuning with no free spectral range (FSR) limitation. A maximum bandwidth of 10 nm was experimentally measured with a high out-of-band contrast of 25 dB. Similarly, the minimum bandwidth recorded is 4 nm with an out-of-band contrast of 15 dB. Keywords silicon-based devices, tunable filter, subwavelength, grating waveguide, grating-assisted contra-directional coupler (GACDC)
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Introduction
Optical filters are fundamental components of wavelengthdivision multiplexing (WDM) systems for routing signals [1,2]. WDM systems, initially designed with fixed channel spacing that varies between 20 nm in coarse WDM to 0.8 and 0.4 nm in dense WDM, are now evolving to support flexible grids that meet the increasing demands for network capacity and efficiency [3]. Hence, optical filters with flexible tunable bandwidth and wavelength are becoming Received June 1, 2020; accepted September 2, 2020 E-mail: [email protected]
increasingly important. The widely adopted technologies for tunable filters are mostly based on free-space optics [4] or liquid crystal modulation [5]; however, such filters are often expensive and difficult to use in high dense integration. Conversely, silicon-on-insulator (SOI) platform [6,7] promises a low cost, better stability, and highdensity chip-scale integration; thus, attracting lots of attention in the research community. At the moment, several tunable filters on SOI have been demonstrated, including structures, that are based on microring resonators (MRRs) [6] and Mach Zehnder interferometers (MZIs) [7]. However, these devices provide limited bandwidth (less than 10 nm) and small free spectral range (FSR), thus cannot adequately meet the demands of high-capacity transmission applications [8]. To solve these problems, tunable filters, which are based on cascaded gratingassisted contra-directional couplers (GACDCs), have been proposed [9,10]. These filters realize large tunable bandwidths without exhibiting the FSR limitation; however, the sidewall-etched Bragg gratings
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