UK researchers hope to produce a novel solid-state Raman laser using a new low birefringence synthetic chemical vapour deposition (CVD) diamond.

The material, developed by Element Six Ltd, in Ascot, UK, will enable scientists at the University of Strathclyde to produce laser devices that output over a broader optical spectrum, opening up applications in underwater, medical and multi-spectral imaging, as well as in cancer therapy and ophthalmology.

The team at Element Six explains that its material (with a birefringence lower than 10-5, and in some cases less than 10-6) enables end-users to exploit the strong optical and thermal properties of synthetic CVD diamond but without the traditional limitations of depolarisation and absorption losses caused by birefringence and optical scattering.

In addition to its use in Raman lasers, the material could have application in intra-cavity heat spreaders for semiconductor vertical external cavity surface emitting lasers or doped dielectric disk lasers.

Research Scientist at Element Six, Dr Ian Friel, explains, ‘Efficient thermal management within diode-pumped solid-state lasers is of key importance, especially where medium to high power compact laser systems are required. Diamond has the highest thermal conductivity of all known materials, making it a prime candidate. In addition, diamond’s wide optical transparency means it can be exploited in many different laser systems operating over a wide range of wavelengths’.

For Raman lasers, conventionally made using silicon, the ability to shift the output wavelength ‘gives access to the applications rich, but source poor, yellow-orange region of the spectrum’, says Dr Alan Kemp of the University of Strathclyde.

Most commercial lasers operate in the near infrared region of the spectrum, between 0.8 and 1.1µm. ‘Perhaps the most important challenge in modern solid-state laser engineering is to find ways to generate new wavelengths, but in doing so to retain as much as possible of the convenience and performance of current lasers,’ adds Kemp.

Growth potential

An R&D programme at Element Six that focused on diamond material growth, surface processing and achieving consistency of the final product led to the manufacture of the new low birefringence material.

Friel says, ‘Birefringence in an optically isotropic material like diamond is due to strain. Reducing the crystal defects responsible for the strain was key’.

This involved limiting dislocation densities during CVD synthesis by optimising growth conditions and growing on single-crystal diamond substrates that were carefully processed to reduce surface damage.

The material then requires processing to the correct size, shape, flatness and parallelism for the application, and each step has been optimised to avoid unwanted strain.

Friel adds, ‘We also took advantage of the fact that the birefringence in single-crystal CVD diamond is highly anisotropic – it being lowest along a direction perpendicular to the growth direction. The final product [was processed] in such a way that the direction of the laser beam is along the direction of lowest birefringence’.

Further information: Element six