We propose and demonstrate a novel Sagnac interferometer based flat-top birefringent optical interleaver employing a ring-cavity as a phase-shift element. The Sagnac interferometer with birefringent crystals provides the optical path difference for interference between the two orthogonal polarization components and the ring-cavity provides the phase shifts needed to achieve a flat-top spectral passband at the output. Fresnel reflections at the prism-air interface of the ring cavity are employed to obtain the desired phase shifts so that highly accurate thin-film coatings are not needed. The Sagnac interferometer based interleaver in a 25-GHz channel spacing (0.2 nm) application exhibits a 0.5-dB passband larger than 0.145 nm, a 25-dB stop band greater than 0.145 nm, and a channel isolation higher than 36 dB over the entire C-band. The superior performance is accompanied with a group velocity dispersion and ripples that can be compensated by using dispersion compensators. (c)2006 Optical Society of America.
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