The basic principles behind the operation of optical parametric oscillators (OPOs) have been known since the early days of nonlinear optics, and many different configurations were explored by the early investigators. However, it is only within the course of the last decade or so that OPOs have been developed as significant practical sources of coherent radiation. This has come about largely through substantial improvements in optically nonlinear materials, combined with an appreciation of the opportunities offered by the different cavity configurations that can be employed. It is now the case that OPOs are capable of generating coherent radiation across all timescales from femtosecond pulses to true continuous-wave (cw). In addition, the spectral coverage of these devices is exceptional, extending all the way from the ultraviolet (200nm) to the mid-infrared (>5#mu#m), in many cases with a single device providing more than a decade of spectral coverage (see Figure 1).
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