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

Abstract

By harnessing the heterogeneous combination of waveplates, we demonstrated a wavelength-tunable passband-squeezed polarization-diversified fiber loop (PDFL) comb filter. The PDFL comb filter is composed of a polarization beam splitter (PBS), two polarizationmaintaining fiber (PMF) segments of the same length, one waveplate combination (WC) of dual quarter-wave plates (QWPs), and another WC of a QWP and a half-wave plate. The former and latter WCs are located before the first and second PMF segments, respectively, and the second PMF is butt-coupled to the PBS so that its slow-axis orientation angle (OA) should be 22.5 deg for the horizontal axis of the PBS. This is the first report on the implementation of the wavelength tunability in the passband-squeezed PDFL comb filter using this heterogeneous WC. Through the Jones calculus, we derived the sinusoidal passband-squeezed transmittance function of the filter. From the derived transmittance, we found the slow-axis OA sets of the four waveplates where the extra phase ϕ of the transmittance function could be continuously changed from 0 deg to 360 deg and graphically represented these OA sets as functions of ϕ along with the relationship between the OA sets. For eight OA sets selected from these OA sets, allowing ϕ to regularly increase from 0 deg to 315 deg by 45 deg, we calculated the passband-squeezed transmission spectra of the filter and confirmed a redshift of the passband-squeezed spectrum by 0.1 nm per set. Then, this theoretical prediction was validated by experimental verification, which corroborates that our filter can offer the wavelength tunability of the passband-squeezed comb spectrum.

Continuously wavelength‐tuneable general transmittance function in first‐order fibre multiwavelength filter based on composite combination of wave retarders

“…To be more specific, O ( λ ini ) shifts to the location of P II , and with the increase of λ from λ ini , O ( λ ) follows the OPS trace starting at P II , in the direction indicated by the blue dotted arrow. A full description of this spectral evolution of O ( λ ) is given in previous studies [16, 25]. With this position shift of O ( λ ), the position of O ( λ ini + S R /8) coincides with P I on the Poincaré sphere.…”

“…In (2), θ Q1 , θ Q2 , and θ Q3 have an angle symmetry of 180 • while θ H1 has an angle symmetry of 90 • . Before we delve into the principle of phase modulation of a GTF, let us revisit the earlier findings about the correlation between changes in the OPSs of individual filter components and the continuous wavelength tuning of a passband-squeezed transmittance function t s of the PDFL multiwavelength filter with the same wave retarder configuration as the filter studied here, which is shown in (3) [25].…”

“…In other words, ψ plays a role of the absolute phase of the transmittance function. In a prior study [25], a total of eight sets of WRAs, labelled Set I to Set VIII, were utilised to present eight passband-squeezed spectra with tuned wavelengths, which were obtained from the transmittance t s using eight different values of ψ from 0 • to 315 • with 45 • increments. As mentioned earlier, since multiwavelength spectra of the same FSR are obtained along the CW and CCW paths in Figure 1b, we will limit our discussion of light propagation along the CW path.…”

“…Before we delve into the principle of phase modulation of a GTF, let us revisit the earlier findings about the correlation between changes in the OPSs of individual filter components and the continuous wavelength tuning of a passband‐squeezed transmittance function t s of the PDFL multiwavelength filter with the same wave retarder configuration as the filter studied here, which is shown in (3) [25]. $$$$\begin{equation}{t}_s = {{\left[ {3 + 4\cos \left( {\Gamma + \psi } \right) + \cos 2\left( {\Gamma + \psi } \right)} \right]} \mathord{\left/ {\vphantom {{\left[ {3 + 4\cos \left( {\Gamma + \psi } \right) + \cos 2\left( {\Gamma + \psi } \right)} \right]} 8}} \right.$$…”