Parametric processes (cont.)
Phase-matching determines which pairs of frequencies w1 and w2 will be generated
We can think of the parametric process splitting the higher energy photon into two parts
For historical reasons we refer to w3 as the pump, w1 as the signal and w2 as the idler ( normally choose w1>w2)
When w1= w2 we say that the process is degenerate
In a parametric process gain is produced at the signal and idler frequencies
The signal and idler grow from spontaneous emission noise
In a similar way to the laser we can combine gain and feedback to give us an optical parametric oscillator (OPO)