OPO

An optical parametric oscillator (OPO) is a tunable light source that uses a second-order nonlinear crystal inside an optical cavity to convert a pump beam into two lower-frequency waves: the signal and the idler. Energy conservation requires ωp = ωs + ωi, while momentum (phase) matching ensures efficient interaction. Phase matching can be achieved by adjusting crystal temperature or angle, or by using quasi-phase matching with periodically poled crystals (for example, PPLN). Type I or Type II phase matching describes the polarization relation of the generated waves.

OPOs can be singly resonant (one generated wave is resonant in the cavity) or doubly resonant (both are resonant). They may be pumped by continuous-wave or pulsed lasers; in ultrafast setups, synchronously pumped OPOs produce femtosecond pulses. The output wavelength is highly tunable by changing the phase-matching condition, allowing operation from the near-infrared to the mid-infrared, depending on the pump wavelength and nonlinear medium.

Common nonlinear media include periodically poled lithium niobate (PPLN), KTP, BBO, and LBO. OPOs are widely used for spectroscopy, trace-gas sensing, atmospheric science, and ultrafast or quantum optics experiments. They can serve as bright, tunable infrared sources and, in certain configurations, as sources of entangled photons through parametric down-conversion.

Compared with other tunable lasers, OPOs offer broad wavelength coverage and high brightness but require precise alignment, stabilization, and careful management of crystal properties and thermal effects. Practical implementations balance tunability, stability, and output power for the target application.