A flat-top birefringent interleaver based on ring-cavity architecture

We propose and demonstrate a flat-top 25-GHz birefringent interleaver using a novel ring-cavity architecture. The ring-cavity provides the phase shifts needed to achieve a flat-top spectral passband at the output. Fresnel reflectivities (for s- and p-polarization states) at the prism-air interface of the ring cavity are employed so that highly accurate thin-film coatings are not needed for the phase shifts. By choosing the appropriate incident angle near the Brewster angle, the optimum interface reflectivities for flat passbands and low crosstalk can be obtained. The ring cavity based interleaver in a 25-GHz channel spacing application exhibits a 0.5-dB passband larger than 0.14 nm, a 25-dB stopband greater than 0.14 nm, and a channel isolation higher than 28 dB over the entire C-band.