Wavelength-Tunable Flat-Top Polarization-Diversified Fiber Loop Filter Using Variable Wave Plates

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Abstract
Polarization-diversified loop-based fiber comb filters have required at least two wave plates per birefringence element for polarization interference to achieve continuous wavelength tunability in high-order comb spectra, including flat-top or narrowband comb spectra. By incorporating variable wave plates (VWPs) whose orientation and retardance can be arbitrarily modified, we propose a continuously wavelength-tunable flat-top polarization-diversified fiber loop (PDFL) filter that utilizes one wave plate per polarization-maintaining fiber (PMF) segment used as a birefringence element to generate polarization interference. The proposed PDFL filter consists of a fiber-pigtailed polarizing beam splitter (PBS) to form the PDFL, two PMF segments, and two VWPs, each of which is located in front of each PMF segment. The slow-axis orientation of the PMF segment directly connected to the PBS is set at 22.5° with respect to the horizontal axis of the PBS to ensure optimal visibility of the comb spectrum. Based on the Jones calculus, we theoretically derived the filter transmittance, and the orientation angle (OA) and retardance sets of the two VWPs, which can introduce continuous extra phase shifts from 0° to 360° in the flat-top transmittance function, were found from the filter transmittance. The theoretical flat-top comb spectra calculated from these OA and retardance sets could corroborate the continuous wavelength tunability of the proposed filter. In particular, the wavelength tunability was also confirmed by experimental verification.
Funding Information
  • Regional Innovation System & Education
  • Institute for Regional Innovation System & Education in Busan Metropolitan City
  • Ministry of Education and the Busan Metropolitan City, Republic of Korea (2025-RISE-02-001-010)
  • Pukyong National University (202505780001)

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