  We outline an approach for improving the temporal contrast of a
high-intensity laser system by $>$8 orders of magnitude using non-collinear
sum-frequency generation with the signal and idler of an optical parametric
amplifier. We demonstrate the effectiveness of this technique by cleaning
pulses from a millijoule-level chirped-pulse amplification system to provide
$>$10$^{12}$ intensity contrast relative to all pre-pulses and amplified
spontaneous emission $>$5~ps prior to the main pulse. The output maintains
percent-level energy stability on the time scales of a typical user experiment
at our facility, highlighting the method's reliability and operational
efficiency. After temporal cleansing, the pulses are stretched in time before
seeding two multi-pass, Ti:sapphire-based amplifiers. After re-compression, the
1~J, 40~fs (25~TW) laser pulses maintain a $>$10$^{10}$ intensity contrast
$>$30~ps prior to the main pulse. This technique is both energy-scalable and
appropriate for preparing seed pulses for a TW- or PW-level chirped-pulse
amplification laser system.
