Optical Parametric Generators
A parametric generator is an optical parametric amplifier with fairly high gain (many tens of decibels), so that a substantial output power is generated even without any input signal. The physical origin of this emission is parametric fluorescence, amplified to high levels. This phenomenon is similar to amplified spontaneous emission (ASE) in a laser amplifier; in both cases, quantum fluctuations of the vacuum (→ vacuum noise) are amplified to macroscopic power levels.
High enough gain for parametric generation can only be achieved by pumping with intense laser pulses. Typical OPG setups are based on nanosecond pump pulses from a Q-switched laser, but it is also possible to operate OPGs with femtosecond or picosecond (ultrashort) pulses from a mode-locked laser. (Note that the parametric gain builds up and decays as fast as the pump pulse.) In most cases, ultrashort pulses first need to be amplified (e.g. in a regenerative amplifier), but by combining certain high-power mode-locked lasers with highly nonlinear crystals it is possible to use the laser pulses directly .
Compared with an optical parametric oscillator, the setup of a parametric generator is simpler, because it does not need a resonator. The wavelengths of the signal and idler can simply be tuned by influencing the phase-matching conditions, e.g. by varying the crystal temperature or rotating the crystal (for critical phase matching). It is convenient that there is no need to keep a resonator synchronized or well aligned. On the other hand, a parametric generator gives less control over the properties of the pulses than would be possible with a synchronously pumped optical parametric oscillator, which can also have a much lower threshold pump power. Also, the required high optical intensities in OPG setups sometimes enforce operation close to the optical damage threshold of the nonlinear material.
It is possible to lower the threshold and significantly decrease the linewidth by injection seeding a parametric generator with some signal light, e.g. from a laser diode. However, the device should then actually be seen as a parametric amplifier.
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See also: optical parametric amplifiers, parametric amplification, parametric fluorescence, optical parametric oscillators, amplified spontaneous emission
and other articles in the categories nonlinear optics, photonic devices, non-laser light sources