Techniques are presented for obtaining stable, single mode output from an optical parametric oscillator and for aligning such an oscillator with a molecular or atomic transition. A line width of 10-3 cm-1 or less was obtained with a long term frequency stability of 10-3 cm-1 or better. It has been demonstrated that an OPO can stay aligned for several hours with an absorption line of half-width 0.01 cm-1.
Audun Hordvik and Philip B. Sackett, "Characteristics of the Optical Parametric Oscillator and Its Use for Selective Excitation," Appl. Opt. 13, 1060-1064 (1974)