Resolution of Conundrum: Threshold Power for Parametric Nonlinear Interactions

Posted on 2006-09-03 as a part of the Photonics Spotlight.

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

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

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.