Threshold for an OPO
The threshold for an OPO occurs when the gain equals the loss in the cavity
For a singly resonant device we resonate only the signal. This condition can be derived as
where as is the fractional loss at the signal around the cavity
If the loss at the signal is dominated by the transmission of the mirror we have
For a doubly resonant oscillator the threshold condition is
The threshold for a doubly resonant oscillator is 2/ai times less than a singly resonant device
Assume that the loss at the idler is due to transmission of the mirrors giving us