Internal reflection prisms were used to produce a narrow angular range of high reflectivity within a laser cavity. The selectivity was obtained from the sharp angular variation of internal reflectivity near the critical angle. Angular mode selection of a Nd: glass normal laser was achieved by this critical internal reflection (CIR) technique. Two-dimensional angular mode selection was observed with two orthogonal Lummer-Gehrcke plates within the laser cavity.
L. G. DeShazer and E. A. Maunders, "Laser Mode Selection by Internal Reflection Prisms," Appl. Opt. 6, 431-435 (1967)