Polarisation-independent optical filters based on co-directional phase-shifted grating-assisted couplers: theory and designs

Theoretical analysis and design principles for a polarisation-independent optical wavelength filter based on co-directional phase-shifted grating-assisted couplers are developed and presented. Analytical expressions for the period of fine grating and the optimum length of the phase-shifted region are obtained. A design guideline based on the theoretical formulations is explained. It is shown that identical spectral responses for the two polarisations can be despite the polarisation-dependent strengths. Sidelobe suppression is demonstrated by chirping the ratio of the length of phase-shifted section to that of the grating section. Tunability of such devices is also studied