Thermal effects in a gain medium of a high-power laser can cause significant power losses through depolarization, if a gain medium without intrinsic birefringence is used and the laser resonator contains an element with high losses for one of the polarization directions (e.g. a Brewster plate). The reason for this is that the temperature gradients in the gain medium induce mechanical stress and thus some amount of birefringence, with the direction of the local birefringence axis varying over the beam cross-section. (The birefringence axes are those directions of polarization with the maximum and minimum refractive index; they are often oriented in the radial and tangential direction.) As a result, an originally linear polarization state is distorted, so that losses can occur at a polarizing intracavity element.
Thermally induced depolarization is suppressed if the gain medium has a sufficiently strong natural birefringence, so that the birefringent axis can not be significantly rotated by thermal effects. This is the case, e.g., in Nd:YVO4 lasers. For optically isotropic gain media such as Nd:YAG, thermal depolarization loss can be minimized, e.g. by using a Faraday rotator , a λ/4 plate in the laser resonator , or by arranging the laser resonator for a Gouy phase shift of suitable magnitude . The basic idea behind such compensation methods is to create a situation where depolarization from different passes through a gain medium cancel each other at least partially. Depolarization losses can also be reduced by using a YAG crystal with optimized cut direction [6, 8].
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