Since the invention of the laser in 1960, a major cornerstone of photonics has been the generation of coherent light in different regions of the optical spectrum. Many efforts have been devoted to deliver laser radiation at new wavelengths, but fundamental barriers including a lack of suitable gain materials have severely hampered the development of lasers in various spectral regions. After more than six decades of research and innovation, vast regions of the optical spectrum from the UV to deep-IR still remain inaccessible to conventional lasers.
The discovery of nonlinear optics in 1961 opened the door to an effective new approach for the generation of laser light in new spectral regions. When a laser beam is focused into a suitable transparent dielectric crystal, under appropriate conditions, the high optical intensity in the material can lead to generation of new wavelengths of coherent radiation through nonlinear dipole oscillations in the medium. A particularly powerful example of nonlinear processes is the optical parametric oscillator (OPO), which enables an input laser wavelength to be efficiently converted into widely tunable output radiation over expansive spectral regions with a single device.
In its simplest form, an OPO consists of a nonlinear crystal enclosed in an optical cavity formed by pair of mirrors. The crystal is irradiated by an input pump laser (ω3), and the microscopic nonlinear dipole oscillations lead to the generation of two optical waves, the so-called signal and idler, at lower frequencies (ω1, ω2), or longer wavelengths (λ1, λ2). The generated fields are subject to energy conservation with respect to the input pump (ω3=ω2+ω1) and must satisfy phase-matching (Δk=k3-k2-k1=0) for amplification to practical macroscopic levels. By controlling the phase-matching condition (Δk=0), using temperature, angle, or grating period of the nonlinear crystal, the generated output frequencies (ω1, ω2) can be differentially adjusted, thus providing tunable radiation over extended regions, from a fixed input pump frequency (ω3=ω1↑+ω2↓). The OPO can be operated in different time-scales (cw, ns, ps, fs), depending on the temporal structure of the input pump laser, and in various resonance configurations, depending on the characteristics of the optical cavity at the signal, idler, and pump wavelengths.