Diamond is highly attractive as a laser material as it promises capabilities well beyond that possible from other materials in accordance with its extreme properties. Almost all fields of science and technology now benefit from laser technology in some way and there is continued demand for performing devices that meet emerging needs particularly in areas such as medicine and counter terrorism.
Lasers are thus being developed with ever increasing performance and reduced size, and finding new alternatives to the optical gain material fundamental to laser performance is crucial to the process. Although diamond is not a new material, opportunities for its exploitation in lasers have substantially increased recently following advances in synthetic growth of high optical quality material.
Our recent research at the MQ Photonics Research Centre is aimed at exploring undoped diamond as a Raman laser material. This is in contrast to other prior work that has concentrated on doped diamond for color center lasers1, semiconductor diode lasers2 and rare earth doped lasers3. Though the large bandgap and high thermal conductivity of diamond are very attractive properties for a laser host, none of these technologies have been particularly successful due to the difficulty in incorporating suitable concentrations of centers or active laser ions into the tightly bonded lattice.
The most promising to date has been diamond color center lasers that rely on optical excitation of nitrogen vacancies and which have seen optical-to-optical conversion efficiencies only as high as approximately 15%1. Raman lasers on the other hand rely on stimulated scattering from fundamental crystal lattice vibrations. Though the principle of optical amplification is distinct from lasers that rely on a population inversion, in many ways Raman lasers have similar basic properties to other laser-pumped lasers. Raman lasers can be thought of as laser converters that bring about a frequency downshift and improved beam quality. The field has been driven mainly by the need for laser wavelengths that are not fulfilled by conventional laser media and find use in a diverse range of fields such as in telecommunications, medicine, bio-diagnostics, defence and remote sensing.
Of all diamond's outstanding properties there are several that are immediately attractive to the Raman laser designer (Figure 1). Diamond has the highest Raman gain coefficient of all known materials (more than 1.5 times higher than most other Raman crystals) and outstanding thermal conductivity (more than two orders of magnitude higher than most other Raman crystals) and optical transmission range (from 0.23 µm and extending to beyond 100 µm, with the exception of 3-6 µm).
Figure 1. Summary of diamond's outstanding properties.
These properties herald promise for substantially raising average output power of a small device and extending the spectral reach beyond the visible and near infrared. The high Raman gain coefficient relaxes the constraints on pump intensity, performance of optical coatings, parasitic losses and crystal length (the latter being fortunate currently since only relatively small crystals of high quality are available).
Since SRS deposits heat into the Raman material, the high thermal conductivity is crucial for mitigating heat induced lensing and stress forces that introduce birefringence or lead to catastrophic damage at high energies or average powers. The wide transmission range of diamond makes it of interest for generating wavelengths that fall outside the range of other materials at both the ultraviolet and infrared ends of the spectrum.
We have recently demonstrated highly efficient diamond Raman lasers operating in a pulsed regime in the visible4,5,7. This work has benefited from the recent advances in chemical vapour deposition of diamond, which have enabled large single crystals diamond to be reproducibly grown with size and quality well suited for use in Raman lasers.
Raman lasers based on a CVD diamond single crystal were designed to convert a Q-switched (8 ns pulse duration) 532 nm frequency doubled Nd:YAG laser to the first Stokes yellow wavelength at 573 nm. The best performance we obtained was for a Brewster cut crystal grown using special techniques for low birefringence6. For a 5 kHz pump laser at input pulse energies up to 0.4 mJ, the Raman laser threshold was approximately 0.1 mJ and the output increased linearly with slope efficiency 74.9% up to the maximum output pulse energy of 0.24 mJ and maximum average power 1.2 W.
The conversion efficiency at maximum energy is 63.5% and slope efficiency approximately 75%, which are values at the very high end of that achievable in any other material. We have also demonstrated a synchronously pumped diamond Raman laser which efficiently converted picosecond pulses at 80 MHz repetition rate with average power up to 2.2 W7.
The diamond Raman lasers we have demonstrated to date have very similar performance to that seen in other materials. However, development is very much in its infancy. Diamond's starkly different optical and thermal properties compared to "conventional" materials are promising for substantially extending Raman laser capabilities. New devices leveraging these properties are likely to emerge quickly given the high efficiency we have already obtained for external cavity diamond Raman lasers and aided by the large existing knowledge base in Raman lasers. The most notable opportunities lie in high power and high brightness Raman lasers and for wavelength ranges not well serviced by other materials. Yet also diamond's the highly unusual properties offers a vast playing field for major innovation in Raman lasers and nonlinear optical devices.
- S.C. Rand and L.G. DeShazer, Opt. Lett. 10, 481 (1985).
- P. John, Science 292, 1847-1848 (2001).
- Patent: K. Jamison and H. Schmidt, "Doped diamond laser," US Patent No. 5,504,767 (1996).
- R.P. Mildren, J.E. Butler and J.R. Rabeau, "CVD-diamond external cavity Raman laser at 573 nm," Opt. Express 16, pp. 18950-18955, (2008).
- R.P. Mildren and A. Sabella, "Highly efficient diamond Raman laser", Opt. Lett. 34, p2811-3, (2009).
- I. Friel, S.L. Clewes, H.K. Dhillon, N. Perkins, D.J. Twitchen, G.A. Scarsbrook, Diamond and Related Materials 18, 808-815, (2009).
- D.J. Spence, E. Granados and R.P. Mildren, Opt. Lett. 35, Doc ID: 120582, posted 14 Jan 2010.
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