Diamond Raman Lasers

The wavelength coverage of lasers is limited by the materials nature permits. This constraint is loosened by the engineering that is enabled in epitaxial semiconductors, but gaps remain between materials systems. Thus, there is a continuing requirement to efficiently convert the wavelength of lasers, moving from spectral regions where good sources exist to those where they are scarce. This project targets one such conversion process - the Raman laser - and in particular the novel use of diamond to permit power-scaling. Efficient Raman conversion - the generation of longer wavelengths due to inelastic scatting of light in a medium - is usually considered the preserve of high power pulsed lasers or systems based on long lengths of fibre. Recent work, however, has shown that this need not be so. First in hydrogen gas (Montana State University) and then in crystals (National Academy of Sciences of Belarus; Macquarie University), it has been shown that continuous-wave lasers of modest power can be wavelength-shifted via Raman scattering: the Raman medium is placed inside the laser cavity to exploit the high intensities there in. This approach is important because it expands the wavelength palette available from compact diode-pumped solid-state lasers. Such lasers are typically based on crystals doped with metal ions and the output wavelengths are limited to the finite number of potential laser transitions in such doped-crystal systems. Raman-based approaches allow, for example, the well known 1064 nm transition in Nd:YAG to be shifted into the region around 1200 nm where tissue transmission is high. Furthermore, frequency doubling of this Raman shifted laser gives access to the applications-rich, but currently source-poor, yellow-orange region of the spectrum.So far, the output power from continuous-wave intracavity Raman lasers has been limited to a few Watts. This ceiling arises from thermal problems in the Raman medium. Removal of the heat deposited in the Raman medium due to the inelastic scattering process is seriously inhibited by the low thermal conductivity of the crystals typically used. This leads to excessive thermal lensing effects that complicate scaling to higher powers. Diamond has a higher Raman gain coefficient than most Raman media and much greater thermal conductivity than all of them. However, its use as a Raman medium is usually dismissed: due to the small sample sizes available and the expense of even these small samples. In initial studies at the Institute of Photonics, we have shown that this judgement is too hasty. First, the recent commercial availability of synthetic single crystal diamond will bring down costs and improve quality. Second, our modelling indicates that the high thermal conductivity and damage threshold of diamond means that tight focussing enables the use of short - and therefore available - crystals (<2 mm). This programme will build on this platform, targeting four demonstrations in particular:1. First CW intracavity Raman laser to be based on diamond (target: 12 W at 1240 nm; 5 W in the orange (620 nm) via intracavity second harmonic generation)2. First Raman conversion of a semiconductor disk laser (target: 200 mW at 735 nm and 2 W at 1235 nm)3. First use of adaptive optics for automated beam quality optimisation in a CW Raman laser (target: 10 W, M-squared < 1.1 at 1240 nm)4. First use of diamond micro-optics to Raman convert a compact Q-switched laser (target: 40% efficiency)Achieving these results will establish a strong presence for the UK in this important emerging area of solid-state laser engineering. Furthermore, it will open the way to a range of compact sources in new spectral regions for applications as diverse as subcutaneous photodynamic therapy, underwater vision systems, and multispectral imaging.

The wavelength coverage of lasers is limited by the materials nature permits. Thus, there is a continuing requirement for efficient wavelength conversion of lasers, moving from spectral regions where good sources exist to those where they are scarce. This project targeted one such conversion process - the Raman laser - and in particular the novel use of diamond to permit power-scaling.

Efficient Raman conversion - the generation of longer wavelengths due to inelastic scatting of light in a medium - is usually the preserve of high power pulsed lasers or systems based on long lengths of fibre. However, first in hydrogen gas (Montana State University) and then in crystals (National Academy of Sciences of Belarus; Macquarie University), it has recently been shown that continuous-wave lasers of modest power can be wavelength-shifted if the Raman medium is placed inside the laser cavity to exploit the high intensities there in. This approach expands the wavelength palette available from compact diode-pumped solid-state lasers.

So far, the output power from continuous-wave crystalline Raman lasers has been limited by thermal problems in the Raman medium. Removal of the heat deposited in the Raman medium due to the inelastic scattering process is seriously inhibited by the low thermal conductivity of the crystals typically used. Diamond has a higher Raman gain coefficient than most Raman media and much greater thermal conductivity than all of them. However, its use as a Raman medium has until recently been dismissed: due to the small sample sizes available and the expense of even these small samples. The recent commercial availability of synthetic single crystal diamond, however, has brought down the cost and improved the quality. This programme built on the platform of our earlier work on diamond for laser applications. As a result, we were able to identify early in the programme the remaining issues with the then state of the art specification for synthetic single crystal diamond. Building on our existing relationship, we then worked closely with a world-leading UK-based manufacturer, testing and proving a new generation of material that subsequently enabled significant progress in diamond Raman lasers. Using this material, we made the following significant demonstrations, with particular emphasis on pioneering continuous wave diamond Raman lasers (that is to say lasers that give a continuous output rather than a series of pulses):

1.The first continuous wave diamond Raman laser
2.The highest power from a continuous wave Raman laser (5W at 1.2µm) as of the project end date.
3.The first semiconductor disk laser pumped Raman laser
4.The first tunable diamond Raman laser (>4W over 10nm; >1W over 30nm in the 1.2µm region)
5.Frequency doubling of a tunable diamond Raman laser to the orange.

The success of this programme led directly to significant funding from the European Research Council (Starting Independent Investigator Grant; €1.5M over five years) to extend this work. In addition, close industrial interaction within the programme led to consultancy that assisted with commercialisation of a new grade of diamond material.