Design of a Tunable Single-Polarization Photonic Crystal Fiber Filter With Silver-Coated and Liquid-Filled Air Holes

A tunable single-polarization filter is designed by filling high index liquid into the silver-coated air holes of photonic crystal fiber and the polarization characteristics are analyzed utilizing the finite element method. Results show that the filter can be used to realize, to our best knowledge, the filtering of an incoming signal at 1310 and 1550 nm bands simultaneously in two orthogonal polarization states with narrowband of full width half maximum only 9 nm (at 1550 nm bands). The confinement losses of unwanted polarized mode can reach up to 53.1 and 305.1 dB/cm at the two aforementioned communication windows, with other polarized mode losses as low as 0.8 and 2.4 dB/cm, respectively. For a propagation distance of 1 mm, the crosstalk of x- or y-polarized (x- or y-pol) mode can reach a value of 45.4 and -262.9 dB at 1310 and 1550 nm bands, and the 20 dB bandwidth at 62 and 133 nm. By adjusting the silver-coated air hole diameter, the silver layer thickness, or the filling liquid, the filtering wavelength can be precisely tuned in a broad band range, from 1351 to 1931 nm of x-pol and from 1208 to 1392 nm of y-pol, covering almost all the communication wavelength.