Tunable Narrowband Fiber Multiwavelength Filter Based on Polarization-Diversified Loop Structure

In this paper, we propose a wavelength-tunable narrowband fiber multiwavelength filter based on polarization-diversified loop structure. The proposed filter consists of a polarization beam splitter, three half-wave plates (HWPs), two quarter-wave plates (QWPs) and two polarization-maintaining fiber (PMF) segments. The lengths of the two PMF segments are equal with each other. Among the five waveplates within the filter, a pair of an HWP and a QWP and the other pair of an HWP and a QWP are located in front of each PMF segment. The last HWP is placed after the second PMF and used to adjust the effective azimuthal angle of the second PMF. Azimuthal angle sets of the five waveplates, which can give additional phase shifts from 0 to 360° to the narrowband transmittance function derived from the Jones matrix formula, were theoretically found. Narrowband transmission spectra were calculated at eight waveplate angle sets selected among the angle sets derived above, which were designated as sets I, II, III, IV, V, VI, VII, and VIII that induced additional phase shifts from 0 to 315° (step: 45°) in the transmittance function. The calculated multiwavelength spectra clearly show that the narrowband multiwavelength spectrum can be wavelength-tuned by 0.1 nm as the waveplate angle set switches from set I to set VIII. This theoretical prediction was experimentally verified by appropriately controlling the azimuthal angle of each waveplate.