A diamond brightness converter

Light at wavelengths within the so-called “eye-safe” region of 1.4–1.8 microns is strongly absorbed in the eye before it reaches the retina. That spectral range is thus particularly desirable for laser applications such as range finding, remote sensing, and lidar. But current laser designs for those wavelengths suffer various performance and practicality limitations, including low pulse-repetition rates, low power levels, and poor beam quality. One promising alternative exploits stimulated Raman emission: Photons from an external pump source are inelastically scattered to a lower frequency, with some energy being lost to heat; above a sufficient pump energy threshold, the scattered photons will lase. Richard Mildren and colleagues at Macquarie University in Sydney, Australia, have now shown that diamond is particularly attractive for such Raman lasers. With its high thermal conductivity, diamond readily dissipates the waste heat. Moreover, the Raman process inherently concentrates the diamond’s laser light into a tightly collimated beam. When the researchers placed a 1-mm-thick synthetic diamond crystal (seen here in the foreground) in a resonator and pumped it with 1.064-µm light from a simple, multimode neodymium pulsed laser, they obtained an eye-safe output beam of up to 16 W at 1.485 µm and 35 kHz. Although the conversion efficiency was only 40%, the diamond nevertheless functioned as a brightness converter: The brightness or radiance of the output beam, defined as the beam’s power divided by its area and divergence, was 1.7 times the input beam’s brightness. (A. McKay, O. Kitzler, R. P. Mildren, Laser Photonics Rev. 8, L37, 2014, doi:10.1002/lpor.201400012/abstract .)