The problem of the low efficiency of large quantum defect optical parametric
oscillators is solved by recycling unused photons in a process involving pumping
additional parametric processes having a common resonant wavelength. In order to
improve the efficiency, a series of optical parametric oscillators are pumped with
unused signal photons which exist at the output of a previous optical parametric
oscillator that generates both signal and idler outputs, with the idler constituting
the desired output.