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