Backconversion in a pulsed optical parametric oscillator: evidence from injection-seeded sidebands

A pulsed optical parametric oscillator (OPO) is pumped at 355 nm and injection seeded at two distinct idler wavelengths at ∼855 nm; these are separated by a frequency interval Δ falling within the 350-GHz free-running OPO bandwidth. Above-threshold operation of this dual-frequency seeded OPO causes sidebands to be observed at multiples of Δ in the spectrum of the 607-nm signal radiation. The sideband spacing varies smoothly as Δ is tuned (with the injection-seeded OPO cavity misaligned to reduce its effective finesse), and corresponding sidebands are observed on the transmitted 355-nm pump radiation. This is interpreted as direct evidence of backconversion of signal and idler waves in a pulsed OPO, consistent with previous temporal observations and other aspects of OPO performance. Signal-wave sidebands are observed well beyond the regular free-running OPO gain profile, which is understood in terms of the different phase-matching conditions for the various OPO pump and idler (seed) frequencies that are involved.