Channel Spacing-Switchable Comb Spectra for Orthogonal Input Polarization States using Polarization-Diversified-Loop

Abstract

In this paper, we propose an optical comb filter based on polarization-diversified-loop (PDL) configuration that offers switching operation between dual transmission spectra with different channel spacings by controlling the state of polarization (SOP) of input light with orthogonal relation. As shown in Fig. 1(a), the PDL-based filter consists of a four-port polarization beam splitter, combination of three half-wave plates (HWPs), one Faraday rotator (FR), and two bow-tie polarization-maintaining fiber (PMF) segments with different lengths (L and 2L) and equal birefringence (B). By controlling an input polarizer located before port 1 of the PBS, ASE light is linearly polarized such as linear horizontal polarization (LHP) and linear vertical polarization (LVP) components which propagate along clockwise (CW) and counterclockwise (CCW) directions in the PDL loop, respectively [1]. Fig. 1(b) represents principal optical paths (CW and CCW paths) divided by orthogonal SOPs of input light. Generally, when PMF segments are utilized as birefringence elements, channel spacings (Δλ) of output transmission spectra are determined by the product of their effective length (L eff ) and birefringence expressed as the equation Δλ=λ2/B-L eff where λ is wavelength in vacuum. In the proposed filter, the effective length of the two PMF segments can be the sum of or difference between their two lengths according to their angular combination of the orientation angles of the three HWPs for orthogonal input SOPs. That is, by properly adjusting angle sets of the three HWPs, the proposed filter can create two sinusoidal transmittance functions with different channel spacings determined by LB and 3LB for orthogonal input SOPs of LHP and LVP, and vice versa. According to theoretical analysis, the resulting filter transmittance is the algebraic sum of two sub transmittances along the CW and CCW paths calculated by Jones matrix formulation, given by (1-a)t CW + at CCW where a is a splitting ratio of optical power coupled to x- and y-axes at the PBS and ranges from 0 to 1 for LHP and LVP input components, respectively. Fig. 1(c) shows the switching operation between dual transmission spectra with different channel spacings for orthogonal input signals at different angle set of three HWPs, measured in a wavelength range from 1548 nm to 1552 nm. In the upper figure of Fig. 1(c), two measured transmission spectra with channel spacings of ∼0.75 nm and ∼0.25 nm were obtained for LHP and LVP input SOPs, indicated as blue solid and red dotted lines, respectively. In a similar manner, two transmission spectra with channel spacings of ∼0.25 nm and ∼0.75 nm were achieved for LHP and LVP input SOPs, indicated as blue solid and red dotted lines in the lower figure of Fig. 1(c), respectively. Consequently, the fabricated filter could demonstrate the input polarization-controlled switching operation between two transmission spectra with different channel spacings of −0.25 nm and −0.75 nm without modifying the filter structure.