Pump depletion is the phenomenon that the pump power for some process occurring in a device can be depleted e.g. due to conversion of the pump power into some other optical power. The term is most frequently used in the context of devices for nonlinear frequency conversion, such as frequency doubling, sum and difference frequency generation, and for optical parametric oscillators.
For example, a pump depletion of 60% in an optical parametric oscillator means that 60% of the injected pump power is converted into signal and idler power, assuming that losses through other processes are negligible. A strong pump depletion is the inevitable consequence of efficient power conversion, and the measurement of pump depletion can be useful for assessing and understanding the performance of nonlinear frequency conversion devices.
The degree of pump depletion (and thus efficiency) in some cases (e.g. for single-pass frequency doublers) rises monotonically with increasing pump power, whereas in other cases (often in OPOs, or in resonant frequency doublers) it is highest for a certain pump power. The maximum pump depletion can be limited by many effects, such as by mode matching and impedance matching of optical resonators, or transverse effects (e.g. back-conversion on the beam axis).