An optical parametric oscillator (OPO) consists of an optical parametric amplifier (OPA) placed in an optical cavity. If the OPA gain is greater than the round-trip optical loss of the cavity, oscillation will occur. OPOs can convert pump light at a fixed wavelength to a pair of tunable redder waves called the signal and idler. OPOs can be built for CW operation, mode locked operation, and nanosecond operation. OPO conversion efficiencies can exceed 90%, high signal/idler beam quality is possible, and broad signal/idler tunability is common.
CW OPOs usually use a stable cavity to keep the beam sizes small in the nonlinear crystal. Pump threshold powers range from microwatts if all three waves are resonated to watts if only the signal is resonated. Threshold powers and conversion efficiencies are determined largely by the unavoidable cavity losses due to imperfect mirrors, crystal ar coatings, and crystal absorption.
Mode locked OPOs are usually synch pumped by a mode locked laser with a matched cavity length. They can produce pulses from 10 fs to 100 ps duration. These OPOs usually use a stable cavity and can have pump thresholds as low as a few nJ. Careful matching of laser and OPO cavity lengths is necessary, and dispersion compensation inside the OPO cavity is necessary for the shortest pulses.
Nanosecond OPOs operate in the transient regime. The pulses are too short to reach a steady state as in CW OPOs. Further, to reach pulse energies of 10 mJ or more requires large beam diameters to avoid optical damage. The transient behavior and large beam diameters make it difficult to analyze their performance except by using detailed numerical models. With careful design nanosecond OPOs can achieve conversion efficiencies of 90% with good beam quality.
The design of an OPO requires the specification of over 50 variables such as crystal type, crystal length, mirror reflectivity at all three wavelengths, mirror curvatures, cavity length, etc. Comprehensive analytical models are not possible but excellent numerical models are available for design of any of the OPO types. They can predict all aspects of OPO performance including efficiency, beam quality, signal/idler spectrum, etc. Several of these models are included in SNLO. They have been extensively compared with laboratory results with excellent agreement.
Extensive modeling of nanosecond OPOs led us to the RISTRA design shown in the RISTRA OPO section. It is a four mirror ring cavity that can contain one or two nonlinear crystals. It resonates only the signal wave, and it rotates the resonated beam by 90 degrees on each round trip. This improves beam quality by averaging over inhomogeneities of the pump beam and also by using the restricted acceptance angle caused by spatial walk off between the signal and idler beams. One RISTRA OPO has achieved 90% conversion efficiency with excellent signal beam quality.
We have extensive experience modeling and building all types of OPOs. We offer custom numerical OPO models optimized to handle your novel design ideas, or we can perform the design and modeling to meet the performance requirements for your application.