Laser Efficiency Increased Using Ceramic in Optical Pumping Cavities

Lasers have been used in medicine for many years, mostly as minimally invasive surgical cutting tools. However, in recent years there have been many advances in the market, and the range of applications for lasers and light-based systems continues to expand across several specialities, especially in ophthalmology and cosmetic treatments. This has created a demand for increased efficiency of lasers and high intensity pulsed lamps, while creating new systems that are more compact, gentler, quicker and more cost-effective.

The construction of the pumping cavity itself is critical to the performance and durability of laser equipment and engineers can achieve higher efficiency by looking at its design. Traditionally, optical pumping cavities are made of gold-plated aluminium, however they tend to suffer from delamination problems and a loss of uniform reflectivity. Manufacturers are now turning to ceramic as an alternative material from which to manufacture the pumping cavity.

Each type of laser has a different requirement from the pumping cavity, necessitating specialized components with specific material composition and manufacturing techniques. Morgan Technical Ceramics, world-leading innovators in materials science and production, has made significant advances in this area working with leading laser manufacturers in Europe, US and Asia to increase the life of lasers.

Gold-plated aluminium optical cavities that are typically found in solid-state lasers suffer from a loss of uniform reflectivity because of flaking and delamination of the gold and as a result, reduced efficiency of the laser. Alumina cavities, due to the material characteristics will typically last five times longer and require no maintenance.

Solid-state lasers, flash lamps and laser diodes, are commonly found in laser eye surgery procedures. They optically pump a solid gain medium and require a high reflectivity in order to maximize the transfer of radiation from the source to the laser rod.

Morgan Technical Ceramics has developed a special Alumina material, Sintox AL, as a highly cost-effective alternative to metal reflectors. Sintox AL is a high purity porous material (99.7% Alumina) that works particularly well for lasers using Ruby (wavelength of 694nm) and Nd:YAG (wavelength of 1064nm) crystals. Independent tests on Sintox AL alumina have show reflectance figures in excess of 96% over the 500nm to 2000nm wavelengths.

The Alumina material is chosen for its excellent microstructure and its control of defined porosity, which provides excellent reflectance of the laser light. The material provides a highly diffuse reflectance, behaving as a bulk reflector of the source of radiation by both reflecting and refracting light back into the cavity. It has a good thermal conductivity and excellent dimensional and electrical stability at all operating temperatures.

Pump radiation that has a longer wavelength than the stimulated emission does not contribute to the laser output but does heat up the laser crystal, which causes optical distortions affecting the quality of the laser output. Reflectors are often water or liquid cooled to prevent this happening and therefore need to withstand the erosive action of the fluid, absorb the generated heat and remain dimensionally stable. Alumina can be glazed to further increase reflectivity and to seal the porosity, making the ceramic laser cavity impervious to the cooling fluid. It has a high strength to cope with stresses experienced in regular servicing of the laser and is chemically resistant to the cooling solution.

Morgan Technical Ceramics offers yellow glazed reflectors whereby the visible yellow colour is complementary to the spectral colours violet and indigo and effectively absorbs these wavelengths up to about 450nm.

Low powered solid-state lasers can use reflectors made of Polytetrafluoroethylene (PTFE). While less expensive than alumina, PTFE is relatively soft and vulnerable to scratches that diminish reflectivity, especially during routine maintenance.

The CO2 laser was one of the earliest gas lasers to be developed and it is still one of the most powerful and efficient, widely used in cosmetic surgical procedures, such as skin resurfacing, because the water in biological tissue readily absorbs the wavelength.

In CO2 lasers the pumping cavity, or wave guide as it's commonly known, channels the photons into a coherent beam and must therefore be very straight and properly aligned. The demand for more compact equipment has led to the development of folded wave guide designs, in the shape of the letter Z, which have an improved performance/size ratio. The wave guide design occupies only one-third of the space of a conventional straight-line channel design. The overall size of the entire laser engine, including the cooling system is also reduced. This design offers highly-efficient heat extraction, allowing the laser to be air-cooled or liquid-cooled.

Gold plated aluminium has worked well in the past, but now designers are realising that alumina ceramics are well suited to this application. Maintaining precise dimensional tolerances is critical to ensure that the bores properly guide the photon beam and to guarantee a tight seal of the gas medium. Morgan Technical Ceramics has the ability to fabricate intricate internal structures and create highly precise grooves in the wave guides.

Excimer lasers are widely used in eye surgery and dermological treatments because the light is exceptionally well focussed and capable of very precise and delicate control that is required. It is also well absorbed by biological matter. The very short wavelength UV light does not penetrate beyond the first nanometer of the target surface and the very short pulse duration of about 10 seconds disintegrate surface material through ablation rather than burning. As a result, it causes almost no heating or change to the underlying and surrounding material.

The demand for eye correction treatment has led to the development of Laser in situ keratomileusis (LASIK). The requirements from the lasers are to produce a stream of high-repetition, short-duration light pulses with an optical beam. Because of its thermal stability, Alumina is able to cope with high pulse rates and a repeated high temperature shock.

The pumping cavity of an excimer laser typically pumps a combination of gases such as fluorine or chlorine with an inert gas such as argon, krypton or xenon. The insulator therefore needs to be tough and resistant to corrosion from the highly aggressive gaseous halogens. Alumina has an exceptional dielectric strength of 20kV/mm, which makes it an excellent insulator. Insulators can be manufactured from 96.7% - 99.5% pure Alumina and can be accurately machined to include geometries that increase tracking distances and precisely placed and sized bores for electrical feedthrus. It is inert to corrosive halogens and the material has no organics which would pollute the laser gas and reduce the insulator's working life.

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