Continuously wavelength-tunable passband-flattened fiber comb filter based on polarization-diversified loop structure

Abstract

Continuous wavelength tuning of optical comb filters, which is an essential functionality for flexible signal processing in reconfigurable optical systems, has been challenging in high order filter structures with two birefringent elements (BEs) or more due to cumbersomeness in finding a combination of waveplates and BEs and complexity in determining their individual azimuthal orientations. Here, we propose a continuously tunable polarization-independent passband-flattened fiber comb filter with two BEs using a polarization-diversified loop structure for the first time. The proposed filter consists of a polarization beam splitter and two groups of a half-wave plate, quarter-wave plate, and polarization-maintaining fiber (PMF). The azimuthal orientation of PMF in the second group is fixed as 22.5°. Orientation angle sets of the four waveplates, which can induce an arbitrary phase shift from 0 to 2π in the passband-flattened transmittance function, are found from the filter transmittance derived using Jones matrix formulation. From theoretical spectral analysis, it is confirmed that passband-flattened comb spectra can be continuously tuned. Theoretical prediction is verified by experimental demonstration. Moreover, the wavelength-dependent evolution of the output state of polarization (SOP) of each PMF is investigated on the Poincare sphere, and the relationship between wavelength tuning and SOP evolution is also discussed.

Polarization-Independent Wavelength-Tunable First-Order Fiber Comb Filter

“…This elliptical locus of (θ H 1 , θ Q 1 ) is governed by the following Eqs. ( 6) and ( 7), whose detailed derivation can be found in our previous study [24].…”

“…On top of that, wavelength interleaving, that is, half-period frequency switching of an interference comb spectrum, was also achieved in a flat-top or narrow band mode of the PDLC-based first-order comb filter by varying the orientation angles of WRs within the filter [21]- [23]. Recently, continuous wavelength tuning of a PDLC-based passband-flattened comb filter was realized by controlling the orientation angles of four WRs, two of which were located in front of each HBF [24]. Azimuth angle adjustment of the four WRs could provide synchronous modulation of an additional phase difference of 0 to 2π between two orthogonal modes of each HBF for two HBF segments.…”

Wavelength-tunable passband-squeezed polarization-diversified fiber loop comb filter using heterogeneous waveplate combination

“…22 In terms of a first-order PDFL comb filter with two PMF segments, the continuous wavelength tuning of passband-flattened or -squeezed comb spectra was implemented by employing various combinations of half-wave plates (HWPs) and quarter-wave plates (QWPs). [23][24][25][26][27] These WCs adopted in the first-order comb filter also control the effective phase difference between two principal states of polarization of two PMF segments and adjust an extra phase shift ϕ in the filter transmittance function. This phase shift ϕ could be modified from 0 deg to 360 deg by changing the orientation angles (OAs) of the waveplates contained in the filter.…”

“…In the first-order PDFL comb filter, the WC between two PMF segments is required to play more roles than the WC between the PBS and the first PMF segment, e.g., to modify the effective phase difference of the second PMF segment and the effective OA difference between two PMF segments. 23 The convenient SOP control of the WC of a QWP and an HWP will help fulfill these roles of the WC between two PMF segments. Thus, it is worthwhile to investigate the feasibility of the heterogeneous WC in continuously tuning the wavelength location of the passband-squeezed PDFL comb filter.…”

Continuously wavelength‐tunable passband‐flattened fibre multiwavelength filter based on heterogeneous combination of wave retarders

“…Continuous wavelength tuning was realized in zeroth-order PDLS-based multiwavelength filters using some wave retarder combinations (WRCs) [15]. Moreover, continuous frequency tunability was also achieved in the first-order PDLS-based multiwavelength filter, which offers passband-flattened multiwavelength spectra, by employing WRCs of a half-wave retarder (HWR) and a quarter-wave retarder (QWR) [16,17]. However, there was no demonstration of continuous wavelength tuning in the passband-flattened PDLS-based multiwavelength filter by using heterogeneous WRCs, for example, an ordered arrangement of a WRC of dual QWRs and a WRC of an HWR and a QWR.…”