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Optical parametric amplifier (OPA), is a coherent light source that emits light of variable wavelengths generated by an optical parametric amplification process without optical cavity. The signal beam and the pump beam propagate through the crystal. Photons of the pump wave are then converted into lower-energy signal photons and idler photons; the photon energy of the idler wave is the difference between the photon energies of pump and signal waves, determined by phase matching condition. In an OPA, the input is two light beams, of frequency ωp and ω1. The OPA will make the ωp beam weaker, and amplify the ω1 beam, and also create a new beam at the frequency ω2: ωp=ω1+ω2. As an example of OPA, the incident pump pulse is at 800 nm (12500 cm−1), and the two outputs, signal and idler, are in the near-infrared region, the sum of the wavenumber of which is equal to 12500 cm−1.