Novel Silica-Based Waveguide-Type Polarization Beam Splitters Using Trenches Filled With Low-Refractive-Index Material

We have proposed and fabricated two kinds of planar-lightwave-circuits-type polarization beam splitters (PBSs) using trenches filled with low-refractive-index material: a nar-rowband-type and a wideband-type. Both PBSs consisted of two 3-dB couplers and two waveguide arms with trenches of different lengths. A narrowband-type PBS used silica-based waveguides on silica substrates. On the other hand, a wideband-type PBS used silica-based waveguides on silicon. The local lateral refractive index difference (Δ) was increased by introducing a pair of trenches. Due to the local lateral enhanced optical confinement structure, the propagation constants of waveguides constructed using trenches filled with a low-refractive-index material depend strongly on the polarization. A narrowband-type PBS, which had the same arm lengths can have a high polarization extinction ratio, although the working bandwidth is restricted. However, a wideband-type PBS, which had the different arm lengths can function over a broad wavelength range, provided that the maximum polarization extinction ratio is acceptable. We optimized the structures of both types of PBS by carrying out simulations. In these simulations, the narrowband-type PBS exhibited a maximum polarization extinction ratio of −34.5 dB for the through-path at 1555 nm, −42.7 dB for the cross-path at 1545 nm, whereas the wideband-type PBS exhibited a −10 dB polarization extinction ratio bandwidth of 105 nm. Our fabricated narrowband-type PBS exhibited a −10 dB polarization extinction ratio bandwidth of 45 nm for both the through-path and the cross-path. At this bandwidth, the insertion loss was less than 9.0 dB. The maximum polarization extinction ratio was −28.9 dB for the through-path at 1570 nm, and −27.5 dB for the cross-path at 1535 nm. The wideband-type PBS exhibited a −10 dB polarization extinction ratio bandwidth of more than 105 nm for both paths. It exhibited an insertion loss of less than 4.4 dB and a maximum polarization extinction ratio of −22.8 dB for the through-path at 1605 nm, −20.0 dB for the cross-path at 1535 nm at this bandwidth.