Abstract
We present a compact and low loss 90° optical hybrid on a silicon-on-insulator (SOI) platform for coherent receiving systems. Our 90° optical hybrid uses a novel topology, comprising one Y-junction and three 2x2 multimode interference (MMI) couplers. The geometry of the 90° optical hybrid is fully optimized using particle swarm optimization (PSO). The fabricated 90° optical hybrid has a compact footprint of 21.6 μm x 27.9 μm, with an insertion loss less than 0.5 dB, a common mode rejection ratio (CMRR) larger than 30 dB, and phase error smaller than 3° in the C-band across 22 reticles on one wafer. The measured phase error (< 3°) in a packaged coherent receiver further confirms the excellent performance of the 90° optical hybrid.
Highlights
The rapid growth of data transmission capacity has driven the development of optical transmission systems with high spectral efficiency, high channel data rate, and low cost [1,2]
There are two major types of 90° optical hybrids: Type-I is based on several discrete components such as 1x2 couplers, 2x2 couplers and phase shifters [8,9,10,11]; Type-II is based on a single device such as a 4x4 multimode interference (MMI) coupler [12,13]
There are designs constructed by 2x4 MMI couplers followed by 2x2 MMI couplers [14,15,16], which can be regarded as a derivative of Type-II hybrid
Summary
The rapid growth of data transmission capacity has driven the development of optical transmission systems with high spectral efficiency, high channel data rate, and low cost [1,2]. Type-I hybrids which avert waveguide crossing to mitigate crosstalk have been demonstrated on a silicon-on-insulator (SOI) platform [10,11] These designs require accurate control of phase in the waveguides [19], a feature which is difficult to achieve in high-index-contrast photonics platforms due to waveguide sidewall roughness and thickness variations. Governed by the self-imaging theory [22,23,24], to realize optimum self-imaging, 90° optical hybrids based on 4x4 MMI couplers or 2x4 MMI couplers usually have a length of hundreds of micrometers This requirement hinders the further scaling-down of the footprint, but more importantly it results in low-quality imaging because of the strong phase errors of the higher-order modes [21]. We benchmark our work against previous works and conclude our discussions
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