Editing Optical parametric amplifier (section)

== Relationship to parametric amplifiers in electronics==
{{main|Parametric amplifier}}
The idea of parametric amplification first arose at much lower frequencies: AC circuits, including radio frequency and microwave frequency (in the earliest investigations, sound waves were also studied). In these applications, typically a strong pump signal (or "[[local oscillator]]") at frequency ''f'' passes through a circuit element whose parameters are modulated by the weak "signal" wave at frequency ''f''<sub>s</sub> (for example, the signal might modulate the capacitance of a [[varactor diode]]<ref name=Das>{{cite book|url=https://books.google.com/books?id=ZU19Uemy83YC&pg=PA397|title=Microwave Engineering|first1=Annapurna|last1=Das|first2=Sisir K.|last2=Das|date=18 February 2019|publisher=Tata McGraw-Hill Education|isbn=9780074635773 |via=Google Books}}</ref>). The result is that some of the energy of the local oscillator gets transferred to the signal frequency ''f''<sub>s</sub>, as well as the difference ("idler") frequency ''f''-''f''<sub>s</sub>. The term ''parametric'' amplifier is used because the ''parameters'' of the circuit are varied.<ref name=Das/>

The optical case uses the same basic principle—transferring energy from a wave at the pump frequency to waves at the signal and idler frequencies—so it took the same name.