  As a novel technique for infrared detection, frequency upconversion has been
successfully deployed in many applications. However, investigations into the
noise properties of upconversion detectors (UCDs) have also received
considerable attention. In this letter, we present a new noise source - second
harmonic generation (SHG)-induced spontaneous parametric downconversion (SPDC)
- experimentally and theoretically shown to exist in short-wavelength-pumped
UCDs. We investigate the noise properties of two UCDs based on single-pass 1064
nm-pumped periodically poled LiNbO$_{3}$ bulk crystals. One UCD is designed to
detect signals in the telecom band and the other in the mid-infrared regime.
Our experimental demonstration and theoretical analysis reveal the basic
properties of this newly discovered UCD noise source, including its dependence
on crystal temperature and pump power. Furthermore, the principle behind the
generation of this noise source can also be applied to other UCDs, which
utilize nonlinear crystals either in waveguide form or with different bulk
materials. This study may also aid in developing methods to suppress the newly
identified noise in future UCD designs.
