Continuous-wave Optical Parametric Oscillators

Since the publication of an earlier review on optical parametric oscillators (OPOs) in 2000,1 there has been remarkable progress in the technological development and applications of OPO devices. Once considered an impractical approach for the generation of coherent radiation, OPOs have now been finally transformed into truly viable, state-of-the-art light sources capable of accessing difficult spectral regions and addressing real applications beyond the reach of conventional lasers. While the first experimental demonstration of an OPO was reported in 1965,2 for nearly two decades thereafter there was little or no progress in the practical development of OPO devices, owing to the absence of suitable nonlinear materials and laser pump sources. With the advent of a new generation of birefringent nonlinear crystals, most notably b-BaB2O4 (BBO), LiB3O5 (LBO), and KTiOPO4 (KTP), but also KTiOAsO4 (KTA) and RbTiOAsO4 (RTA) in the mid-1980s, and advances in solid-state laser technology, there began a resurgence of interest in OPOs as potential alternatives to conventional lasers for the generation of coherent radiation in new spectral regions. The high optical damage threshold, moderate optical nonlinearity, and favorable phase-matching properties of the newfound materials led to important breakthroughs in OPO technology. In the years to follow, tremendous progress was achieved in the development of OPO devices, particularly in the pulsed regime, and a variety of OPO systems from the nanosecond to the ultrafast picosecond and femtosecond timescales, and operating from the near-ultraviolet (near-UV) to the infrared (IR) were rapidly developed. These developments led to the availability of a wide range of practical OPO devices and their deployment in new applications, with some systems finding their way to the commercial market. A decade later, in the mid-1990s, the emergence of quasi-phase-matched (QPM) ferroelectric nonlinear crystals, particularly periodically poled LiNbO3 (PPLN) stimulated new impetus for the advancement of continuous-wave (cw) OPO devices, traditionally the most challenging regime for OPO operation due to almost negligible nonlinear gains available under cw pumping. The flexibility offered by grating-engineered QPM materials, allowing access to the highest nonlinear tensor coefficients, combined with noncritical phase matching (NCPM) and long interaction lengths (>50 mm in PPLN), enabled the low available nonlinear gains to be overcome, hence permitting the development of practical cw OPOs in a variety of resonance configurations. As such, the advent of QPM