Silicon-nanowire-based optical hybrid with insensitive operation for TE/TM states of polarization

An optical 90° hybrid consisting of a polarization splitter–rotator (PSR) and a multimode interferometer (MMI) is proposed to realize insensitive optical frequency mixing for TE/TM states of polarization. The PSR is based on the mode hybridization and mode coupling between waveguides; input light at a fundamental transverse electric (TE) mode and a transverse magnetic (TM) mode are output at separate ports, but both output signals are in the fundamental TE mode. These two output fields are then guided to the MMI, where the optical frequency mixing with the light of a local oscillator takes place. This mixing is then independent of the state of polarization of the hybrid input light. We designed such an optical hybrid based on silicon-nanowire waveguides. At a wavelength of 1540 nm, simulation results for the hybrid show a transmission efficiency of 98.6% or 95.8% for TE- or TM-polarized input light, respectively, while the transmission efficiencies for both polarizations are above 90% in a 30 nm wavelength range, namely [1525 nm, 1555 nm]. Further, in the wavelength range [1510 nm, 1565 nm], the transmission imbalance between the output fields is below 0.85 dB for TE- or TM-polarized input light, respectively, while the phase errors in both cases are less than 4°.