An optical parametric amplifier transfers energy from a pump beam into a signal and an idler through nonlinear optics. For the common three-wave mixing process, their angular frequencies obey . A strong pump and a weaker signal enter a nonlinear medium; the signal grows and an idler is generated.
Efficient growth requires phase matching so that contributions generated along the medium reinforce. Changing crystal orientation or temperature can tune the matched frequencies. Measuring the signal with and without the pump tests amplification, while observing the corresponding idler checks the frequency-conversion mechanism. The pump supplies the extra energy; the process does not require a stored population inversion.[ref]
Overview of Nonlinear Processes by Camacho Lab. Sum and Difference Frequency Generation: pump, signal, idler and difference-frequency amplification
An optical parametric oscillator places an optical parametric amplifier inside an optical resonator that feeds back at least one generated wave. Pump energy is converted into signal and idler waves satisfying the frequency relation of the underlying three-wave mixing process.
Oscillation begins when round-trip gain overcomes loss at an allowed resonator mode. Below that threshold, weak generated emission can still occur, but sustained oscillation is absent. Tuning the phase matching and resonator together gives a tunable source at wavelengths difficult to obtain directly from a particular laser. Pump power alone does not select the output: cavity resonances, conversion gain and optical losses also matter.
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An Optical Parametric Amplifier (OPA) is a device that amplifies light by utilizing the nonlinear optical process known as parametric amplification. OPAs are key components in the field of nonlinear optics and are widely used in applications such as laser systems, frequency conversion, and pulse compression.