Slope Efficiency in laser is the efficiency by which the input pump power is converted into signal power. It is one of the most important properties of a laser and is simply the slope of the output power versus input power.

For a medium to be able to amplify incident radiation, the state of population inversion must prevail in the medium. To generate radiation, this amplifying medium is placed in an optical resonator which is usually a pair of mirrors facing each other. Radiations bouncing back and forth between the resonator are amplified by the amplifying medium. But this radiation also suffers losses due to finite reflectivity of the mirrors and diffraction and scattering losses. So, for the oscillations to sustain in the cavity, the losses must be compensated by the gain. The state where the total gain in the system equals the total loss is known as the steady state condition. So if we pump further, the condition of population inversion occurs and the pump photons are converted to the signal photons, which are the photon of the desired wavelength which we obtain as the laser output.

Beyond the threshold, where the gain equals the loss, all the pump photons will escape the cavity, i.e., they are converted into signal photons. This results in the generation of the laser output. So the cavity consumes a certain amount of pump power to reach the threshold, any more power beyond that is getting converted to a signal photon.