Editing Optical parametric amplifier (section)

==Optical parametric amplification (OPA)==
[[File:Photon picture of Optical Parametric Amplification (OPA).svg|thumb|Photon picture of optical parametric amplification: A pump photon excites a virtual energy level whose decay is stimulated by a signal photon resulting in the emission of an identical second signal photon and an idler photon under conversion of energy and momentum.]]
The output beams in optical parametric generation are usually relatively weak and have relatively spread-out direction and frequency. This problem is solved by using optical parametric amplification (OPA), also called [[nonlinear optics|difference frequency generation]], as a second stage after the OPG.

In an OPA, the input is ''two'' light beams, of frequency ω<sub>p</sub> and ω<sub>s</sub>. The OPA will make the pump beam (ω<sub>p</sub>) weaker, and ''amplify'' the signal beam (ω<sub>s</sub>), and also create a new, so-called idler beam at the frequency ω<sub>i</sub> with ω<sub>p</sub>=ω<sub>s</sub>+ω<sub>i</sub>.

In the OPA, the pump and idler photons usually travel collinearly through a nonlinear optical crystal. [[Phase matching]] is required for the process to work well.

Because the wavelengths of an OPG+OPA system can be varied (unlike most lasers which have a fixed wavelength), they are used in many [[spectroscopy|spectroscopic methods]].

As an example of OPA, the incident pump pulse is the 800&nbsp;nm (12500&nbsp;cm<sup>−1</sup>) output of a [[Ti-sapphire laser|Ti:sapphire laser]], and the two outputs, signal and idler, are in the near-infrared region, the sum of the [[wavenumber]] of which is equal to 12500&nbsp;cm<sup>−1</sup>.