Efficient single-photon counting at 1.55 μm by intracavity frequency upconversion in a unidirectional ring laser

A single-photon signal at 1.55 μm was converted to its visible replica by sum-frequency mixing with a strong intracavity pump of a unidirectional ring laser at 1.06 μm in a periodically poled lithium niobate (PPLN) crystal. In the ring laser configuration, standing-wave photorefractive effects in the PPLN crystal could be suppressed and consequently, the spectral bandwidth of the intracavity pump could be controlled for excellent quasi-phase-matching. A conversion efficiency of 96% was demonstrated, and the corresponding background counts were less than 6.0× 105 s-1. The single-photon upconversion was used for efficient single-photon counting at 1.55 μm with long-term stability.