A spectral interleaver providing flat top spectral transmission passbands and athermal operation is disclosed. The spectral interleaver may comprise a pentagon-shaped birefringent crystal, a polarization beam splitter, and a dielectric mirror at one facet of the crystal. Prisms and polarizing beam splitters can be employed for operation with an unpolarized input beam. The cavity formed by the mirror and the crystal serves as a spectrally dispersive mirror. Front mirror reflectivity is provided by the air-crystal (or other medium-crystal) interface. Proper mirror reflectivities can be achieved by selecting the angle θ of the pentagon-shape crystal. By selecting the proper air space between the mirror and the crystal surface and the crystal length, the interleaver can have a flat top transmission function. A combination of two different crystals which exhibit different thermal-optic effects may be employed such that the total phase retardation is independent of the temperature, leading to athermal operation.