Problem of Heat Sink
The power scaling is limited by the ability to dissipate the heat. Usually, the thermal conductivity of materials designed for efficient laser action, is small compared to that of materials optimal for the heat transfer (metals, diamonds). For the efficient drain of heat from a compact device, the active medium should be a narrow slab; in order to give advantage to the amplification of light at wanted direction over the ASE, the energy and head would be withdrawn in orthogonal directions, as it is shown in figure. At low background loss (typically, at the level of 0.01 or 0.001) the heat and the light can be withdrawn in the opposite directions, allowing active elements of wide aperture. In this case, combining of several active elements is used for the power scaling.
Read more about this topic: Power Scaling
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