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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 narrowband-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 (??) 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.
Date of Publication: Dec.15, 2009