Diamonds sparkle in Raman application

Jörg Schwartz, j.schwartz@europhotonics.com

Researchers from Macquarie University and from the Defence Science and Technology Organisation (DSTO) in Edinburgh have developed a diamond Raman laser with 63.5 percent efficiency, taking diamond into the same conversion league as other Raman laser materials, but with a host of advantages.

Diamond’s hardness makes it an attractive material for high-power lasers because the damage threshold is very high. Equally important, the thermal conductivity of diamond is a magnitude higher than similar materials. This reduces the effect of thermal lensing, which haunts high-power solid-state laser developers. What is more, diamond has exciting light-conversion properties, providing access to new wavelengths and related applications.

Raman lasers typically use silicon, barium nitrate or metal tungstate crystals to amplify light created by a pump laser. The pump light makes some of the photons exchange energy with them, leading to loss or gain of energy, with the scattered light having a different wavelength. A Raman laser takes this secondary light – called Stokes or anti-Stokes – and amplifies it by reflecting it into a cavity while pumping energy into the system to emit a coherent laser beam at the shifted wavelength.

Richard P. Mildren of Macquarie and Alexander Sabella of the DSTO built an efficient 532-nm pumped external-cavity diamond Raman laser generating output chiefly at the 573-nm Stokes line. They used a 6.7-mm-long artificial diamond as the Raman material, generating 1.2 W of output at a pulse repetition rate of 5 kHz with a conversion efficiency of 63.5 percent and a slope efficiency of 75 percent. The maximum output energy of the pulses was 0.67 mJ.

Their results, published in the Sept. 10 issue of Optics Letters, conclude that such efficiency is commensurate with the highest previously reported for other Raman materials pumped by Q-switched lasers.

This is a significant improvement over the first diamond Raman laser that Mildren and co-workers presented in 2008, a device that reached only 22 percent slope efficiency. In the past year, chemical vapor deposition has greatly improved, enabling the synthesis of larger diamond crystals with a lower birefringence, an effect that was a key limitation last year.

Diamond crystals also can be made to generate a wider variety of wavelengths of light, each of which has its own applications – from ultraviolet light at 225 nm to the far-infrared at 100 µm, with only a gap between 3- and 6-µm wavelengths.

The new device is being used to produce yellow laser light for medical applications including eye surgery. Additional applications range from trace gas detectors to defense technologies to satellite mapping of greenhouse gases.