ABSTRACT
Nonlinear optical materials play a key role in the development of coherent sources of radiation as they permit the frequency conversion of mature solid-state lasers into spectral ranges where lasers do not exist or perform poorly. The nonlinear processes which come into play are generally of second order; to obtain efficient frequency conversion, one needs to compensate for the refractive index dispersion of materials which induces a phase mismatch between the nonlinear polarization and the radiated waves. Taking advantage of the birefringent properties of nonlinear materials usually solves this phase-matching problem. Although this method is straightforward to implement, it also has a number of drawbacks. It is obviously restricted to spectral ranges where the birefringence is high enough to compensate for the dispersion. The effective nonlinear coefficient is fixed by the crystal orientation and is not necessarily the highest one. Finally, one also faces beam walk-off problems limiting the interaction length.
