  Quantum frequency conversion (QFC) will be an indispensable ingredient in
future quantum technologies. For example, large-scale fibre-based quantum
networks will require QFC to interconnect heterogeneous building blocks like
emitters, channels, memories and detectors. The performance of existing QFC
devices - typically realised in periodically-poled nonlinear crystals - is
often severely limited by parasitic noise that arises when the pump wavelength
lies between the wavelengths which are inter-converted. Here we investigate the
noise spectrum of a frequency converter pumped by a CW 1064 nm laser. The
converter was realised as a monolithic bulk pump enhancement cavity made from a
periodically-poled potassium titanyl phosphate (ppKTP) crystal -
quasi-phase-matched for the conversion of 637 nm to 1587 nm. In the range from
1140 nm to 1330 nm (up to 60 THz from the pump) Stokes-Raman resonances can be
identified as the dominant noise source while the noise in the range from 1330
nm to 1650 nm can be attributed mainly to parasitic spontaneous parametric
down-conversion (SPDC). Further, a succession of narrow-band peaks is observed
in the spectrum originating from higher-order counter-propagating SPDC. Both
types of counter-propagation, where either the lower-energy idler photon or the
higher-energy signal photon counter-propagate relative to the pump beam, are
observed, with narrow-band peaks corresponding to 10th to 44th order
quasi-phase-matching.
