Continuous wavelength tuning of the first-order general comb spectrum

Here we show the continuous wavelength tunability of a general transmittance function (GTF), besides flat-top and narrow band transmittance functions, which is mostly obtainable in the first-order fiber comb filter composed of a polarization beam splitter to form a polarization-diversified loop, four waveplates, and two birefringent fiber segments. On the basis of polarization conditions, which should be satisfied to continuously tune the GTF, waveplate orientation angles (WOAs), which gave an extra phase shift of 1-360° to a specifically chosen GTF, were found using a newly proposed WOA search scheme. Wavelength-tuned comb spectra were calculated at eight WOA sets (i.e., Sets I-VIII) selected from the above WOA sets. In the fabricated filter whose free spectral range was ~0.8 nm, its comb spectrum redshifted step by step by ~0.1 nm, resulting in a total displacement of ~0.7 nm, when the WOA set changed from Set I to VIII. The calculated and measured spectra clearly show that an arbitrary GTF can be continuously wavelength-tuned by properly choosing WOAs.