It is recognized that in the subpicosecond regime of frequency conversion the group-velocity mismatch (GYM) severely limits the interaction length and permits an efficient energy transfer only in a very limited spectral range for a given type of collinear interaction and nonlinear crystal. Solutions for GYM reduction in travelling-wave optical parametric generators and amplifiers have been based on the use of noncollinear phase-matching or of tilted pulse fronts with respect to the phase fronts in a collinear geometry. Here we analyse a third possibility for group-matched interaction of all three waves based on off-plane collinear propagation in biaxial crystals, which avoids complex tilting or angle readjustment procedures, an important advantage in the mid-IR.
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