Control of the structure of transverse laser modes by phase-shifting masks

A description is given of a method of selecting pure transverse laser modes by a phase-shifting mask positioned along nodal lines of the selected mode on one of the optical resonator mirrors. This makes it possible to increase the discrimination between the modes compared with the familiar “absorbing” mask method or to reduce the losses of the selected mode with the same degree of discrimination. Results of numerical calculations and an experiment at $\lambda=0.63\mu m$ are reported and these confirm the above conclusion.