Spectrally Flat and Polarization-Diversified Silicon-Nanowire Coarse WDM Demultiplexers Based on Distributed Multimode-Interference Phase Compensations

Spectrally flat-topped and polarization-insensitive coarse wavelength division multiplexing (CWDM)-targeted optical demultiplexers based on distributed multimode-interference (MMI) phase compensations are analytically calculated and experimentally demonstrated. The proposed device for use in a CWDM optical receiver consists of a polarization splitter-rotator (PSR) and two identical silicon-nanowire multiple delayed interferometric (MDI) demultiplexers. The broadband operating nature of MMI couplers is highly suitable for applying the proposed device to >60 nm wide CWDM applications. Moreover, by properly adjusting the relative output phase relations of MMI couplers according to their optical splitting ratios, we experimentally validated a flat-topped CWDM spectral response within the O-band spectral range. Concurrently, stable optical demultiplexing operations were maintained regardless of the input signal polarization states via the monolithically integrated PSR. Fabricated using a silicon photonics foundry process based on ArF-dry lithography technology, the devices exhibited a 1 dB flat bandwidth of >12 nm, a polarization-dependent loss of <1.0 dB, an adjacent-channel isolation of >10 dB, and a polarization crosstalk of <−20 dB across the measured output channels. Although the current spectral crosstalk is ~−10 dB, it can be further suppressed to the −20 dB level through fabrication process optimization to minimize random phase fluctuations, or by incorporating a double-filtering scheme. This distributed MMI phase compensation strategy can be broadly applied to scalable WDM architectures in datacom applications.