Asymmetric waveguides vertical couplers for polarization-independent coupling and polarization-mode splitting

This paper presents a coherent approach to the design of compact vertical coupler (VC) mode-size converters with variable polarization dependence. As a polarization-independent (PI) coupler, the VC is shown to transfer light with more than 90% efficiency for both transverse-electric (TE) and transverse-magnetic (TM) modes over a transfer length of only 150 μm. As a polarization-mode splitter (PMS), the VC is shown to preferentially couple TE or TM modes with a contrast ratio of up to 20 dB. This versatility makes the VC a compact and useful input-stage device that not only maximizes input/output coupling efficiency to small active devices but also provides a degree of polarization control before the actual device. The design principle is based on resonant coupling between the top tapered active waveguide and the underlying uniform large and passive waveguide. For the PI coupler, the taper is designed so that resonant transfer occurs near the critical width where the upper waveguide is nonbirefringent. For the PMS, the resonant transfer for TE and TM are designed to occur at widely separated points far from the critical width. Resonant coupling is sensitive to exact dimensions and the refractive index of the waveguides. The taper design is the key in ensuring that the VC performance is robust as well as compact.