Researchers achieve 8 W output from optical parametric oscillator

Researchers have demonstrated a total output power of more than 8 W from a high-power mid-infrared (mid-IR) cadmium silicon diphosphide (CSP) optical parametric oscillator (OPO) pumped at 2.06 μm. The researchers showed that CSP crystals can out-perform zinc germanium phosphide (ZGP) for frequency down-conversion from 2 microns to the mid-IR band. This high-power mid-IR output could be useful for infrared spectroscopy, laser surgery, remote sensing and defence communications.

Marcin Piotrowski from the French-German Research Institute in France of Saint Louis led this research project and said that laser power scaling in this spectral band is important for infrared countermeasures in defence — protecting aircraft and other platforms from the threat of infrared-seeking missiles. “Efficient sources capable of delivering high power with good beam quality are critical, as the laser beam must travel long distances through the atmosphere,” Piotrowski said.

CSP crystals are a promising alternative to ZGP for frequency conversion in OPOs. However, few studies have examined CSP for parametric conversion with pump wavelengths that are longer than one micron. In this study, the researchers focused the pump laser to a 1-mm diameter spot inside an OPO resonator formed by two flat mirrors. The CSP sample was nine millimetres long with a 4 x 4 mm² aperture. A ZGP sample of the same size was also used for comparison.

Although the conversion threshold for the two materials was different, CSP demonstrated a better conversion efficiency for high pump power. The absorption properties of ZGP also caused thermal effects to become more significant. The researchers found that CSP had a lower thermal load and less beam degradation while ZGP showed reduced beam quality at high powers due to higher residual absorption.

These experimental results suggest that CSP crystals may outperform ZGP for frequency down-conversion from 2 μm to the mid-IR range.

“We look forward to continuing our investigations on nonlinear crystals such as CSP for OPO emission in the mid-IR. In particular, as the quality and maturity of the CSP material allows, we will combine it with the technologies actively investigated in our research group: monolithic non-planar cavities such as the FIRE cavity — particularly well suited for power scaling while maintaining good beam quality. In addition, compact and robust 2 µm fibre lasers are planned as future pump sources for these OPO cavities,” Piotrowski said.

Image credit: iStock.com/Torsten Asmus