Wavelength-tunability of arbitrary transmittance functions in first-order fiber comb filters based on heterogeneous waveplate combination

We investigated the wavelength tunability of arbitrarily chosen transmittance functions (ACTFs) in first-order fiber comb filters using heterogeneous waveplate combinations: specifically, a combination of dual quarter-wave plates (QWPs) and a set of a half-wave plate and a QWP in a polarization-diversified fiber loop (PDFL). PDFL comb filters studied here create polarization interference using two polarization-maintaining fiber (PMF) segments of equal length and a polarization beam splitter. We considered two comb filters with different waveplate combinations to scrutinize the effect of waveplate combination order. By analyzing the effect of the waveplates and PMF on the output polarization state (OPS) of the filter, we identified the orientation angles of the waveplates that resulted in a redshift of the ACTF by one free spectral range (λS) by making all polarization states on the OPS trace of the second PMF move in the direction of wavelength decrease. For each filter, we theoretically derived 360 sets of waveplate angles that enabled the tuning of its ACTF by λS. Among these sets, we picked eight sets inducing a wavelength shift of λS/8 for each increase in set order and displayed wavelength-shifted comb spectra at these eight sets, which corroborate the wavelength tunability of the ACTFs of the filters.