An optical parametric amplifier emits some light even if there is no signal and idler input. This phenomenon is called parametric fluorescence. It is a quantum effect. In a semiclassical model, it can be attributed to vacuum fluctuations (vacuum noise) of the optical field which enters the open input port(s) of the device. In the case of a non-degenerate parametric amplifier, vacuum noise in the signal and idler ports have similar effects.
Parametric fluorescence is similar to the spontaneous emission in a laser amplifier, but occurs essentially only in directions where phase matching occurs. Therefore, it normally carries only a small optical power. There may, however, also be some off-axis parametric fluorescence which is non-collinearly phase-matched. The center wavelength will then depend on the propagation direction.
Parametric fluorescence can be useful for the alignment of a non-collinear parametric amplifier, since it shows the direction where phase matching and thus the maximum parametric gain occurs.
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See also: superfluorescence, optical parametric amplifiers, optical parametric oscillators, optical parametric generators, quantum noise
and other articles in the categories nonlinear optics, quantum optics