Optical parametric oscillator (OPO)

In an optical parametric oscillator a pump beam passes through a crystal with a second-order nonlinearity, and pump photons split into pairs of lower-energy photons, the signal and the idler. Energy conservation fixes their frequencies, $1/\lambda_p = 1/\lambda_s + 1/\lambda_i$, but not how the energy is shared, so the output wavelengths are chosen by phase matching: only the pair whose wavevectors match the pump's builds up. Placing the crystal in a cavity that resonates the signal, the idler, or both turns this parametric gain into an oscillator once the gain exceeds the round-trip loss. It is the reverse of second-harmonic generation and sum-frequency mixing, and uses the same crystals.

An OPO has no energy levels to set its wavelength, which is its main advantage over a laser. Changing the phase-matching condition, by rotating a birefringent crystal, heating it, or moving to a different poling period in a periodically poled crystal, tunes the output over hundreds of nanometers or more. A 1064 nm pump with the signal at 1550 nm produces its idler at 3393 nm, in the mid-infrared where few convenient lasers exist, and that combination, a mature 1 µm pump laser and periodically poled lithium niobate, is a standard route to tunable light between about 1.4 and 4 µm. The quantum defect sets the ceiling on efficiency: at most 1064/1550, or 68.6%, of the pump power can end up in that signal, with the rest in the idler.

The cavity design sets the threshold and stability. A singly resonant OPO, which resonates only the signal, has a high threshold but tunes smoothly; a doubly resonant OPO has a much lower threshold, reachable with continuous-wave pumps, but its output hops between modes because both waves must fit the cavity at once. Pulsed OPOs pumped by Q-switched lasers reach threshold easily from the high peak power, and synchronously pumped OPOs, whose cavity length matches the repetition period of a mode-locked pump, produce tunable femtosecond pulses. Walk-off between the beams in birefringent crystals limits the useful crystal length, one reason quasi-phase-matched crystals with noncritical geometry are preferred.

Below threshold, the same device is the standard source of squeezed light and of correlated photon pairs. On chips, OPOs have been built in thin-film lithium niobate waveguides and resonators, and in silicon nitride microresonators using the third-order process of four-wave mixing in place of the second-order one. Uses include mid-infrared spectroscopy and gas sensing, where many molecules have their strongest absorption lines, and tunable sources for nonlinear microscopy.

References: R. W. Boyd, Nonlinear Optics, 4th ed. (Academic Press, 2020); L. E. Myers et al., J. Opt. Soc. Am. B 12, 2102 (1995).