Silicon-Based Hybrid (de)Multiplexer for Wavelength-/Polarization-Division-Multiplexing

A novel hybrid multiplexer for wavelength-division- multiplexing (WDM) and polarization-division-multiplexing (PDM) is proposed and realized by integrating a polarization-splitter-rotator (PSR) and an optical-filter array based on novel microring resonators (MRRs). With the PSR, the launched TM-polarized light is rotated to be TE-polarized and outputs from the cross port while the launched TE-polarized light outputs from the through port directly. In the proposed novel configuration, the cross- and through-ports of the PSR are connected through the bus waveguide for all the MRR-based optical filters. In this way, each MRR-based optical filter works bidirectionally and has two drop ports, from which the same wavelength channel of TE- and TM-polarizations are dropped separately. As an example, a 16-channel hybrid WDM-PDM multiplexer is designed and realized with eight wavelength channels and dual polarizations by integrating a PSR and eight MRR-based optical filters with box-like responses. The fabricated PSR has an excess loss of ∼0.6 dB and an extinction ratio (ER) of ∼20 dB in the wavelength range from 1550 to 1580 nm. The optical filters are designed by using cascaded MRRs with bent directional couplers, which works for TE polarization only with very high ER (>35 dB) and, thus, reduces the polarization crosstalk greatly. For the MRR-based optical filters of the present hybrid (de)multiplexer, the channel spacing is 400 GHz (3.2 nm), the crosstalk between the adjacent channels is <−25 dB, and the 3 dB bandwidth is as large as 250 GHz (2 nm). For the demonstrated 16-channel hybrid (de)multiplexer, the on-chip loss is ∼3 dB for all the channels and the footprint is 500 
μ
m × 300 
μ
m. The present hybrid (de)multiplexer can be extended for more channels by reducing the channel spacing as well as increasing the free-spectral range of the MRRs.