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Since the invention of the laser, extensive efforts have been made to achieve stimulated emission of radiation in different regions of the optical spectrum. In particular, much effort has been directed to the development of tunable laser sources with extended spectral emission.
Yet, after 45 years, important regions in the UV, visible and IR, where many new scientific and technological applications can benefit, still remain inaccessible to conventional lasers.
One of the most effective techniques to overcome the spectral limitations of conventional lasers is to exploit nonlinear optics. Nonlinear optical techniques can take a variety of forms. The most important in the context of frequency conversion are second harmonic generation (SHG), sum- and difference- frequency generation (SFG and DFG), and optical parametric generation and amplification (OPG and OPA).
Frequency conversion sources offer important advantages for the generation of tunable coherent radiation. As such, they represent attractive alternatives to conventional lasers and other technologies for the generation of tunable radiation in new spectral regions in the UV, visible and IR, where conventional laser technology can not reach.
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