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The Photonics Spotlight

Resolution of Conundrum: Threshold Power for Parametric Nonlinear Interactions

Dr. Rüdiger Paschotta

Ref.: encyclopedia articles on parametric amplification, optical parametric oscillators, frequency doubling, etc.

On 2006-07-30, the Photonics Spotlight presented a physics conundrum: the question was essentially why parametric oscillation involves a threshold pump power, while frequency doubling does not, even though both processes appear to be time-reversed versions of each other.

The resolution of this issue is found through a closer inspection of what time reversal really leads to. Consider a resonant frequency doubler operated at a rather low pump power level. It will generate some second-harmonic light, but leaves much of the pump light unconverted. Now imagine the time-reserved situation. This looks like a degenerate optical parametric oscillator (OPO), converting some higher-frequency pump light into lower-frequency light. It is essential not to overlook that the unconverted pump light leaving the frequency doubler now appears as subharmonic light which forms a second input of the OPO. Only with this additional subharmonic input, the OPO can convert light power at arbitrarily low power levels. If you omit this input, you don't have a time-reversed situation of a frequency doubler any more, and therefore it is not surprising that there is then a threshold power involved.

This article is a posting of the Photonics Spotlight, authored by Dr. Rüdiger Paschotta. You may link to this page and cite it, because its location is permanent. See also the Encyclopedia of Laser Physics and Technology.

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