Highly efficient broadband silicon nitride polarization beam splitter incorporating serially cascaded asymmetric directional couplers

The implementation of a polarization beam splitter (PBS) on a silicon nitride platform remains challenging owing to its relatively low index. We therefore propose a silicon nitride PBS that exploits serially cascaded asymmetric directional couplers (ADCs), leading to a high polarization extinction ratio (PER) over a broad bandwidth. The ADC spatially routes incident light through polarization-selective mode coupling under a small footprint of 112 µm. The proposed PBS does not require an active phase control. It is thus effectively realized via a single-step lithography process. The measured transverse-electric and transverse-magnetic PERs were determined to be above 23 dB and 10 dB over an 80-nm bandwidth, respectively, spanning λ=1520−1600nm. The proposed device is thus anticipated to play a key role in providing polarization diversity in photonic-integrated circuits.