Optical Parametric Oscillator

An optical parametric oscillator (OPO) is a nonlinear optical device in which a pump photon is converted into two lower-energy photons — signal and idler — inside a birefringent crystal with a second-order nonlinear susceptibility. The process is parametric: it preserves phase coherence between the three fields, enabling the generation of quantum-correlated light states that are essential for quantum optics experiments, spectroscopy, and precision measurement. When driven below threshold, the OPO generates squeezed vacuum; when driven above threshold, it produces coherent tunable radiation across wavelengths that are difficult to reach with conventional lasers.

In gravitational wave detection, the OPO is the core technology behind the squeezed light sources deployed in A+ LIGO and planned for future detectors. The crystal — typically periodically poled potassium titanyl phosphate or lithium niobate — is placed inside an optical cavity resonant with the signal and idler modes, which enhances the nonlinear interaction and increases the degree of squeezing. The engineering challenge is not merely generating squeezed light but stabilizing the OPO against thermal fluctuations, acoustic noise, and pump laser instability over observation runs lasting months. The OPO thus sits at the intersection of quantum optics and systems engineering: a tabletop nonlinear device that must operate with spacecraft-level reliability inside a tonne-scale interferometer.