Lasers have been used for several years in the medical and industrial markets. The latest technological advancements mean a range of applications for light-based and laser systems continues to expand across a number of specialist areas such as cosmetic treatments and ophthalmology.
The past five years have seen a considerable rise in demand for cosmetic surgeries such as laser vision correction and hair removal, since these procedures are easily accessible and readily accepted by customers. This has increased demand for improved efficiency from high-intensity pulsed lamps and lasers resulting in the development of new gentle, compact, quick and cost-effective systems.
The pumping cavity design is crucial to the durability and performance of laser equipment with design optimization as the key to increasing efficiency. The limitations of traditional materials imply that manufacturers are now looking at ceramic-based materials to produce the pumping cavity.
For instance, the gold-plated aluminum optical cavities found normally in solid-state lasers can suffer from uniform reflectivity loss due to delamination and flaking of the gold, causing reduced laser efficiency. Glazed alumina cavities, in these applications will last five times longer and require no maintenance because of their innovative material characteristic. Furthermore, there is no mismatch of the coefficient of thermal expansion between the ceramic and the glaze.
Figure 1. Braze assemblies
Figure 2. Ceramic laser reflectors
Solid-State Lasers and High Intensity Lamps
In solid state lasers, diodes and flash lamps optically pump a solid gain medium and need increased reflectivity for increasing radiation transfer from the source to the laser rod.
For such applications a special alumina-based material offers a suitable solution. This is normally of a very high purity, around 99.7% alumina and having a porous structure. This material is especially suitable for lasers using ruby (wavelength: 694nm) and Nd:YAG (wavelength: 1064nm) crystals.
Independent tests done on this alumina-based material shows reflectance figures more than 96% over the 500 to 2000nm wavelengths. The material integrates a strong microstructure with uniform porosity to offer very good laser light reflectance. The reflectance is highly diffuse, with the material acting as a bulk reflector of the source of radiation by both refracting and reflecting light back into the cavity. It boasts high electrical and dimensional stability and strong thermal conductivity at all operating temperatures.
Pump radiation with a longer wavelength when compared to the stimulated emission, has no contribution to the laser output but heats up the laser crystal, causing optical distortions and thus impacting the laser output quality. This can be avoided using liquid or water-cooled reflectors, which must withstand the fluid’s erosive action, absorb generated heat and maintain dimensional stability. It is possible to glaze alumina to improve reflectivity and seal porosity, rendering the ceramic laser cavity impermeable to the cooling fluid.
A specialist samarium oxide glaze is used in yellow-glazed reflectors in which the visible yellow color complements the spectral colors of indigo and violet, effectively absorbing wavelengths upto 450nm. This offers around 98% reflectance in the specific wavelength range and outperforms yellow (GSY) and clear (GSO) glazes between 980nm and 1,064nm, implying that the right reflectance capability across the range of three glazes covers a wavelength range of 500- 2,000nm.
Reflectors made of Polytetrafluoroethylene (PTFE) are used in low-powered solid-state lasers. PTFE is less expensive than alumina, but considerably softer and vulnerable to scratches that diminish reflectivity, particularly during routine maintenance.
Carbon Dioxide (CO2) Lasers
The CO2 laser was the earliest gas laser to be developed and is the most efficient and powerful. It is still used in cosmetic surgical procedures, such as skin resurfacing, since the water in biological tissue readily absorbs the wavelength up to 10,600nm and in the fashion industry for printing patterns onto non-metal substrates such as textile and leather.
In these lasers, it is important that the waveguide channels are properly aligned and straight. Folded waveguide designs in shape of the letter Z evolved due to the need for compact equipment. Alumina ceramics also suit this application appropriately as they have unprecedented optical properties at 10µm wavelength and better mechanical strength, and can survive operating temperatures over 1000°C.
A specialist ceramic material with a high dielectric constant and low loss called Deranox 970 offers high stability to CO2 lasers at low frequencies associated with ionization or trigger probes. Along with ceramics, developments in metallization inks and braze alloys enable material manufacturers to work with customers from the outset to develop fully integrated, optimized solutions, with materials chosen carefully and combined into a design offering reliable long-term performance in the most demanding conditions.
Excimer lasers are used extensively in eye surgery and dermalogical treatments since light is focused and the system can provide very precise and delicate control. Biological matter also absorbs the light. The short wavelength UV light does not penetrate beyond the first nanometer of the target surface and the very short pulse duration of about 10s disintegrates the surface material through ablation rather than burning. As a result, it causes almost no change or heating to the underlying and surrounding material.
Eye correction treatment led to the development of laser in situ keratomileusis (LASIK). Since alumina is thermally stable, it could cope with high temperature shock and high pulse rates.
An excimer laser’s pumping cavity pumps combined gases such as chlorine or fluorine with an inert gas such as argon, krypton or xenon. Hence the insulator must be tough and resistant to corrosion from the highly aggressive gaseous halogens. Alumina has a dielectric strength of 20kV/mm, making it an excellent insulator. Alumina also shows inertness to corrosive halogens and is free from organics that would contaminate the laser gas and shorten the service life of the insulator.
The versatility and strength of ceramic-based laser components and their high purity levels make them suitable for use in a myriad of medical laser applications. Their novel qualities and manufacturability to obtain specific performance attributes enable designers and specifiers to achieve the maximum possible performance and regardless of the application.
This information has been sourced, reviewed and adapted from materials provided by Morgan Advanced Materials.
For more information on this source please visit Morgan Advanced Materials.