Optical parallelism measurement of ruby laser rods

A simple method to examine the optical parallelism between planar end faces of a ruby rod is suggested. When the ruby rod is illuminated by an expanded He-Ne laser beam with linear polarization, the interference pattern due to the recombination of the E and O rays at a Polaroid analyzer shows its optical parallelism. This optical parallelism, which depends not entirely on the mechanical wedge angle but also on the equivalent wedge angle produced by lineage, can be employed as a criterion to show whether or not the ruby rod is suitable in an E-ray oscillation laser system.