  We experimentally demonstrate spectral broadening and shaping of
exponentially-decaying nanosecond pulses via nonlinear mixing with a
phase-modulated pump in a periodically-poled lithium niobate (PPLN) waveguide.
A strong, 1550~nm pulse is imprinted with a temporal phase and used to
upconvert a weak 980 nm pulse to 600 nm while simultaneously broadening the
spectrum to that of a Lorentzian pulse up to 10 times shorter. While the
current experimental demonstration is for spectral shaping, we also provide a
numerical study showing the feasibility of subsequent spectral phase correction
to achieve temporal compression and re-shaping of a 1~ns mono-exponentially
decaying pulse to a 250 ps Lorentzian, which would constitute a complete
spectro-temporal waveform shaping protocol. This method, which uses quantum
frequency conversion in PPLN with >100:1 signal-to-noise ratio, is compatible
with single photon states of light.
