  The evolution versus pump power of the spectrum of a singly resonant optical
parametric oscillator based on an MgO-doped periodically poled stoichiometric
lithium tantalate crystal is observed. The onset of cascade Raman lasing due to
stimulated Raman scattering in the nonlinear crystal is analyzed. Spurious
frequency doubling and sum-frequency generation phenomena are observed and
understood. A strong reduction of the intracavity Raman scattering is obtained
by a careful adjustment of the cavity losses.
