  Multi-longitudinal-mode lasers have been believed to be good candidates as
pump sources for optical frequency conversion. However, we present a
semi-classical model for frequency conversion of optical signals with a
multimode pump laser, which shows that fluctuations of the instantaneous pump
power limit the conversion efficiency. In an experiment, we upconverted a 1550
nm optical signal in a periodically poled lithium niobate waveguide using with
a multi-longitudinal-mode laser, an observed a maximum conversion efficiency of
70%, in good agreement with our theoretical model. Compared to single-mode
pumping, multimode pumping is not a suitable technique for attaining stable
near-unity-efficiency frequency conversion. However, the results obtained here
could find application in characterization of the spectral or temporal
structure of multi-longitudinal-mode lasers.
