Polarization Control

Since Nd:YAG exhibits severe birefringence when put under a thermal load (due to thermally induced stress), even a well polarized beam will undergo depolarization as it passes through the rod. A problematic part of this depolarization effect is that the magnitude and direction of it varies across the rod. Rays going through the rod close to the center thus undergo a very different change in polarization state than a beam going through the rod closer to the edge.

An example of these effects can be seen in the mode pictures below. In each recording a well polarized beam is sent through Nd:YAG rod and then only the depolarized component is filtered out and sent to the (CCD) camera. As can be seen the amount of depolarization typically exhibits a “clover leaf” pattern. This is due to the fact that an incoming linearly polarized beam will not undergo depolarization in directions that are either parallel or orthogonal to the polarization vector.

JMAR has developed a way to compensate for the effects of depolarization through careful design and selection of pump diodes and optical components. In the BriteLight laser a suppression of depolarization effect of better than 10⁴ compared to an uncompensated system is achieved.