Study on spectral deviations of high-order optical fiber comb filter based on polarization-diversity loop configuration

In this paper, we investigate optical factors that may cause deviations of experimentally observed output spectra from theoretically predicted ones in a high-order optical fiber comb filter based on a polarization-diversity loop configuration. They include the wavelength dependency of half-wave plates (HWPs) contained in the filter for wavelength switching and the weak birefringence of single-mode fibers (SMFs) that connect optical components comprising the filter. The wavelength dependence of the HWPs is considered using the phase difference curve of a commercial HWP, which is a function of the wavelength of an incident light. In order to examine the effect of the SMF birefringence on the spectral characteristics of the high-order comb filter, the SMF is modeled as a low birefringence fiber whose principal axes are arbitrarily oriented with a certain small birefringence value. It is concluded from the calculation results that the SMF birefringence rather than the wavelength dependency of HWPs strongly affects the spectral characteristics of the filter including an extinction ratio, wavelength location, and spectral shape of the filter channel.