Conversion Efficiency

In the plane-wave approximation, the conversion efficiency of an SRO for the ideal case of perfect phase-matching and zero losses is given by 
η_par = sin² gl, ..........(1)
where g and l have been defined in (i), (ii). Using this expression, total conversion of the pump can be achieved in theory. As the point of total conversion is exceeded, power starts to couple back into the pump field at the expense of the signal and idler fields and the conversion efficiency decreases.

........(i)

...........(ii)
The conversion efficiency in (1) is defined as the ratio of the sum of signal and idler power to pump power. The energy or power between the signal and idler beams is divided according to the ratio of the photon energies, that is,

..........(2)

From this follows the ratio of the energy of the signal compared to the total energy converted by the OPO

.....(3)

In the so-called degenerate mode, for which λ_s = λ_i = 2λ_p, each pump photon generates two photons at twice the pump wavelength. In this case, two orthogonally polarized beams at the same wavelength are emitted from the OPO.