Birefringent interleaver with a ring cavity as a phase-dispersion element

We propose and demonstrate a novel ring-cavity-based flat-top birefringent interleaver. A single ring cavity provides the two phase shifts needed, each for a polarization state, to achieve a flat-top spectral passband at the output. Fresnel reflectivities (for s- and p-polarization states) at the prism 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, we can obtain the optimum interface reflectivities for flat passbands and extremely low cross talk. We also present the results of our experimental investigations of a flat-top 25-GHz optical interleaver that uses a novel ring cavity architecture. In a 25-GHz channel spacing application the interleaver exhibits a 0.5-dB passband larger than 0.14 nm (17.5 GHz), a 25-dB stopband greater than 0.14 nm (17.5 GHz), and channel isolation better than 28 dB over the entire C band.