Resolution of Conundrum

Posted on 2006-09-03 as part of the Photonics Spotlight (available as e-mail newsletter!)

Permanent link: https://www.rp-photonics.com/spotlight_2006_09_03.html

Author: Dr. Rüdiger Paschotta, RP Photonics AG, RP Photonics AG

Abstract: This article presents the resolution to a physics conundrum in the area of nonlinear optics. Essentially the question was 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. A closer inspection shows that time reversal of frequency doubling in fact leads to a an OPO with an subharmonic input wave.

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 anymore, 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 RP Photonics Encyclopedia.

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