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Due to advantages of low dark-count rate, reduced dead-time, and room-temperature operation, single-photon upconversion detectors for the telecom band are gaining strong interest as an alternative to other single-photon counters. In this work, we investigate the spatial and spectral distribution of upconverted spontaneous parametric downconversion (USPDC) noise, which is the typical dominant noise source in short-wavelength-pumped single-photon upconversion detectors for 1.5 µm - 1.6 µm. Our upconversion detector relies on a bulk periodically poled lithium niobate (PPLN) crystal and a 1064 nm intracavity pump system that spectrally translates the signal to the visible (~630 nm) where efficient, uncooled, and low dark-count Si based single-photon detectors operate. Experimental results show that the spectral and spatial distribution of the USPDC noise has a relatively broadband and radially modulated pattern that depends on the PPLN temperature, which is in good agreement with our numerical simulations. We also demonstrate that for narrow-linewidth 1575 nm signal photons, the dark-count rate can be significantly reduced by (1) using a phase-matched signal angle that corresponds to an upconverted output angle where the USPDC noise is at a "local minimum" and (2) applying a spatial filter (instead of an ultra-narrow bandpass filter) at the output. This simple spatial filtering technique resulted in a 14 dB dark-count rate reduction. Due to a corresponding decrease in the interaction length of the signal with the pump, the upconversion efficiency also decreased, but only with a 2.2 dB penalty.

Enhancing the detectivity of an upconversion single-photon detector by spatial filtering of upconverted parametric fluorescence