= Optical parametric amplifier
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 $\omega_p=\omega_s+\omega_i$. 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>.<Overview of Nonlinear Processes by Camacho Lab>[Sum and Difference Frequency Generation: pump, signal, idler and difference-frequency amplification]{ref}
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