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.
Highlights
Due to simple design, ease of use, and good fiber compatibility, fiber comb filters have been considered as useful wavelength-selective elements that can be employed to route and process optical signals or block unwanted signals causing crosstalks in dense wavelength-division-multiplexed (DWDM) optical networks
The coefficient of determination R2 for the linear regression was estimated as a value of ~0.99948, which is closest to unity, showing considerably linear relationship between applied phase shift φ and λdip
It was experimentally checked that any arbitrary phase shifts of 0°−360° except for the eight discrete ones could be introduced into the filter transmittance by selecting proper waveplate angles
Summary
Power superposition of the two interference spectra. As arbitrarily polarized light is given by the linear superposition of LHP and LVP components, the filter output spectrum is independent of input polarization[23]. The flat-top comb spectrum of the filter is red-shifted by π/4, π/2, 3π/4, π, 5π/4, 3π/2, and 7π/4 at Sets II, III, IV, V, VI, VII, and VIII, respectively, compared with the spectrum obtained by (3−2cosΓ−cos2Γ)/4 at Set I This kind of waveplate angle sets for the wavelength tuning can be obtained for θq2 = 157.5° at all Sets I−VIII like the case of θq2 = 67.5° (see Supplementary Table S1 and part 3 of Supplementary Information). The length L and birefringence B of each PMF were set as 7.2 m and 4.166 × 10–4 to achieve an FSR of ~0.8 nm at 1550 nm, respectively It can be found from the figure that the passband-flattened comb spectrum, compared with the zeroth-order comb spectrum indicated as a black dashed line, moves toward a longer wavelength region while the waveplate angle set changes from Set I to Set VIII. If we restrict this discussion to θh[2] only and the angular resolution of the waveplate is assumed as 1°, the theoretical tuning step becomes ~0.0089 nm, or ~1.11 GHz for a spectral shift of 0.1 nm during the adjustment of θh[2] by 11.25°
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