Novel narrow-band spectral interference filter with very high transmittance

We report a novel scheme to improve the effective transmission of a standard interference filter, and demonstrate over 97% passband transmission. Such high efficiency is critical for quantum information applications, e.g. high-efficiency single-photon generation utilizing parametric down-conversion. The scheme can also be modified to function with a tilted filter, thereby allowing tuning of the passband frequency. In addition, the tilted configuration creates an infinite number of consecutive reflections from and transmissions through the filter, further improving the net filter transmission. Because spectral interference filters are a key element in optical quantum information experiments (both on the source and detection side, e.g. to exclude background photons as well as to determine the spectra of the desired photons), our scheme of enhanced interference filter transmission should lead to significant performance improvements in such experiments.