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# Output Coupling Efficiency

Author: the photonics expert

Definition: a factor influencing the power efficiency of a laser, taking into account intracavity losses

Units: %, dimensionless number

Formula symbol: <$\eta_{oc}$>

The power conversion efficiency of a laser is often reduced by unavoidable parasitic intracavity losses in the laser resonator. More specifically, such losses 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:

$${\eta _{{\rm{oc}}}} = \frac{{{T_{{\rm{oc}}}}}}{{{T_{{\rm{oc}}}} + {l_{{\rm{par}}}}}}$$

Here, the useful losses are given by the output coupler transmission <$T_\rm{oc}$>, and the parasitic losses are <$l_\rm{par}$>. (Both are the losses per round-trip, taking into account a possible double pass in a linear resonator.) A larger output coupler transmission always increases the output coupling efficiency; however, it also increases the laser's threshold pump power, and should therefore not be chosen too high.

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, and a more complicated equation is needed.

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.

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