The control of laser transverse mode structure by means of phase-shifting masks

A method for separating the purely transverse modes of a laser beam using a phase-shifting mask is presented. The method is based on the use of a thin band of an optical coating deposited on the laser mirror along nodal lines of the selected mode which acts to shift the phase of reflected radiation by pi radians, causing it to interfere destructively with radiation reflected from noncoated areas of the mirror. Numerical analysis and experimental results confirm the usefulness of the method in the selection for higher-order transverse modes, and demonstrate its advantages in discrimination or selected mode loss reduction over the conventional absorbing mask method.