Resonators with Several Active Media

Abstract

In high power lasers, the maximum output power scales proportionally with the length of the active medium. In general, there is an upper bound for the output power that can be attained with a medium of a certain length. For solid state lasers this limit is caused by the pump induced thermal heat P _H which has to stay below the fracture limit to avoid damage to the active material. For a laser rod the maximum output power P _out per length is given by:

$$\frac{{{P_{out,\max }}}}{\ell } = \frac{{{\eta _{extr}}}}{\chi }\;\frac{{{P_{H,\max }}}}{\ell }$$

((13.1))

where η _extr is the extraction efficiency, χ is the thermal load parameter, and P_Hmax is the maximally allowed thermal heat (see Table 13.1 for typical values). In gas lasers the limitation of the output power per tube length is also a result of the heat deposition in the gas. For sealed-off CO₂ lasers in which the gas is not constantly removed, the maximum output power is on the order of 100W/m for tube lasers. Better heat removal can be achieved by flowing the gas through the electrical discharge region. In fast flow CO₂ lasers, maximum output powers per length on the order of 2,000W/m can be realized. Unfortunately, using a longer active medium to increase the output power is often not feasible due to space requirements, limitations on the length (solid state laser rods, for instance, are usually not grown longer than 200mm), or technical difficulties in providing efficient heat removal.