A Compact Silicon-on-Insulator Optical Hybrid for Low Loss Integration with Balanced Photodetectors

An optical hybrid design based on paired multimode interference couplers in silicon-on-insulator process is investigated. The device exhibits greater than 20 dB CMRR and low phase deviation (<10) over 30 nm in the C-band. The design eliminates the use of optical cross waveguides for integration with balanced photodetectors. Introduction Coherent detection scheme provides high spectral efficiency and high sensitivity for dense wavelength division multiplexing (WDM) systems. One of the reasons that coherent detection became popular in the last few years is the possibility of low-cost post-processing electronic compensation on the detected impaired signal. However to get those benefits and reduce the component operation complexity, the key building blocks of a coherent receiver such as polarization beam splitter, 90 optical hybrid, and balanced photodetectors, should be integrated on the same substrate. Recently, InP and silicon-on-insulator (SOI) based integrated coherent receiver have been demonstrated. SOI-based coherent receivers are a cost-effective approach taking advantage of CMOS compatible fabrication processes. Optical hybrid designed using 2×2 and 2×4 general interference (GI) multimode interference (MMI) has been demonstrated in SOI. A four 2×2 GI MMI coupler based optical hybrid with integrated Ge-SOI photodetectors (PD) has been demonstrated in whereas in the PDs were flip-chip onto a passive optical hybrid device. Conventionally, balanced detection requires optical waveguide crossings because the in-phase output (ports 1-3) of the MMI is interleaved with the quadrature-phase output (ports 2-4). This situation increases the optical loss and footprint leading to a more complex solution for integrating the PDs. MMI-based optical hybrids making use of paired interference (PI) do not require optical crossings as the in-phase and quadrature-phase components of the output signal are available at output ports 1-2 and 3-4, respectively. Additionally for the same width, PI-based MMIs are three times shorter in length compared to the GI-based MMIs. With PI-based MMIs, however, a 45 phase shifter and a 2×2 MMI coupler are required at output ports 3 and4 to maintain the same common mode rejection ratio (CMRR) for both the in-phase and quadrature components. While low-loss InP MMI couplers have already been demonstrated, a more compact low-loss PI-based MMI couplers has not been investigated yet in an SOI substrate. An analysis in is presented for the design of a low-loss 1×2 MMI coupler with small footprint (192 μm core MMI waveguide area) in a 220 nm SOI process. In this work, a compact optical hybrid using PI-based MMI couplers is demonstrated for the first time in a SOI process. The compact device (0.39 mm) integrates a 2×4 PI-based MMI, a 45 phase shifter and a 2×2 GI-based MMI coupler (fig. 1). Experimental result shows that the device maintains greater than 20 dB CMRR and minimum phase error between the output ports over the C-band. The footprint of the fabricated optical hybrid is three times smaller compared to recent demonstration of SOI-based optical hybrids. Furthermore, lower cost coherent receiver solutions become possible by eliminating the optical waveguide crossings leading to more practical integration with on-chip balanced PD. Fig. 1: Schematic of the SOI 90 optical hybrid. Design and simulation results First, an analytical equation is used to determine the required length for PI-based and GI-based MMI couplers in SOI. A beam propagation method (BPM) simulation is then performed to determine more precisely the length of the device and resulting phase relation at the optical hybrid output ports for an assumed MMI width. However due to the high refractive index contrast of the structure, the accurate MMI and phase shifter lengths need to be found We.2.B.2.pdf