Passband-Flattened Frequency-Tunable Optical Fiber Multiwavelength Filter with Composite Combination of Waveplates

We propose a passband-flattened frequency-tunable optical multiwavelength filter with a composite combination of waveplates, which is realized by harnessing a polarization-diversified loop structure. The proposed filter comprises a polarization beam splitter (PBS), two polarization-maintaining fiber (PMF) segments of equal length, an ordered waveplate combination (OWC) of a half-wave plate (HWP) and a quarter-wave plate (QWP) before the first PMF segment, and an OWC of a QWP and an HWP before the second PMF segment. The second PMF segment is butt-coupled to one port of the PBS so that its slow axis is oriented at 22.5° for the horizontal axis of the PBS. Based on the filter transmittance derived through the Jones calculus, we found the orientation angle (OA) sets of the four waveplates, which could induce an extra phase shift Φ from 0° to 360° in the passband-flattened transmittance function. From the transmission spectra calculated at the eight selected OA sets, which caused Φ to increase from 0° to 315° by steps of 45°, it was confirmed that the passband-flattened multiwavelength spectrum can be continuously tuned by properly controlling the OAs. This indicates continuous wavelength tunability based on composite OWCs. Then, this theoretical prediction was verified by experimental demonstration.