Continuously wavelength‐tunable passband‐flattened fibre multiwavelength filter based on heterogeneous combination of wave retarders

By incorporating the heterogeneous combination of wave retarders, authors demonstrated a continuously wavelength-tunable passband-flattened fibre multiwavelength filter comprised four-port polarization beam splitter (PBS), two equal-length polarization-maintaining (PMF) segments, one retarder (WRC) dual quarter-wave retarders (QWRs), and another WRC half-wave (HWR) QWR. The former lies before first PMF segment, latter is located between segments. From theoretically derived transmittance, found orientation angle (OA) sets four where additional phase ϕ transmittance function could be modified from 0° to 360°. For eight OA allowing increase 315° by 45°, calculated transmission spectrum confirmed its redshift 0.1 nm per set. Then, this theoretical prediction was verified experimental implementation filter, which clearly indicates continuous wavelength tuning capability proposed filter.