Output Coupling Efficiency
Unavoidable parasitic intracavity losses in the resonator of a laser tend to reduce the laser's power efficiency. More specifically, they can increase the threshold pump power and reduce the slope efficiency. The sensitivity of the slope efficiency to intracavity losses depends on the transmission of the output coupler mirror: the larger this transmission, the lower is the circulating intracavity power, and the lower is the effect of intracavity losses. The output coupling efficiency is defined as the ratio of the useful losses to the total losses:
Here, the useful losses are given by the output coupler transmission Toc (or twice that value, if the output coupler is a folding mirror in a linear resonator), and the total losses contain both the useful losses and the parasitic losses lpar. This means that a larger output coupler transmission increases the output coupling efficiency; however, it also increases the laser's threshold pump power.
An underlying assumption of the above equation is that all optical losses involved are small, i.e. not more than a few percent. Otherwise, the order of the optical components would matter.
In simple cases, the slope efficiency can be calculated as the product of several efficiency factors, one of them being the output coupling efficiency. Depending on various quantities, a certain value of the output coupler transmission will optimize the overall power efficiency of a laser.