Ultraviolet lasers

UV laser definition: A laser (or other laser-based light source) that produces ultraviolet light

Technical challenges in generating UV lasers

• For short wavelengths, intense spontaneous emissions result in high threshold pump power (unless the gain bandwidth is narrow).
• For wavelengths below ≈200 nm, the choice of transparent and UV-resistant optical materials is rather limited (see article on UV light).
• Even a slight surface roughness or the bubble content of the optics can cause strong wavefront distortion and scattering losses.

Still, there are a variety of lasers that can directly produce UV light.

How UV lasers are generated

• There are laser diodes, usually based on gallium nitride (GaN), that emit in the near-ultraviolet region. However, the available power levels are limited.
• Some solid-state body lasers, such as cerium-doped crystals, such as Ce3+:LiCAF or Ce3+:LiLuF4, can emit ultraviolet light. In most cases, a cerium laser is pumped with nanosecond pulses from a 4-frequency Q-switched laser, so it can emit nanosecond pulses on its own. With Q-switched microchip lasers, even sub-nanosecond pulse durations are possible.
• Very few fiber lasers can produce ultraviolet light. For example, some neodymium-doped fluoride fibers can be used to emit lasers at about 380 nm, but only at low power levels.
• While most dye lasers emit visible light, some laser dyes are suitable for UV emission.
• Excimer lasers are very powerful sources of ultraviolet light that also emit nanosecond pulses, but have an average output power of between a few watts and several hundred watts. Typical wavelengths are between 157 nanometers (F2) and 351 nanometers (XeF).
• Argon-ion lasers can continuously emit wavelengths of 334 and 351 nanometers, even if the power is lower than the usual 514 nanometer wires. Some other ultraviolet rays can be obtained by krypton ion lasers.
• There are also ion lasers emitted in the extreme ultraviolet spectral region. These can be based on, for example, argon, but unlike normal argon-ion lasers, argon-ion lasers use Ar8+ ions, which are produced in a hotter plasma. The emission then occurs at a wavelength of 46.9 nm. This laser can be pumped by capillary discharge or by a strong laser pulse.
• Nitrogen lasers are molecular gas lasers that emit ultraviolet light. The strongest emission line is at 337.1 nm.
• Free electron lasers can emit ultraviolet light of essentially any wavelength and have a high average power. However, they are very expensive and bulky light sources and therefore are not very widely used.

In addition to true UV lasers, there are UV laser sources based on lasers with longer wavelengths (in the visible or near-infrared spectral regions) and one or several nonlinear crystals for nonlinear frequency conversion.

• A wavelength of 355 nm can be produced by tripling the output of a 1064 nm Nd:YAG or Nd:YVO4 laser.
• The light at 266 nanometers is obtained by two subsequent frequency multipliers, which actually quadruple the laser frequency.
• Diode lasers can be equipped with a nonlinear frequency conversion stage to produce ultraviolet light. For example, a continuous-wave near-infrared laser can be used and two resonant frequency doubles can be applied to reach wavelengths of around 300 nanometers. The main attraction of this method is that a wide wavelength range can be obtained without restrictions on certain laser lines.

UV lasers need to be made with special UV optics with high optical quality and low absorption of UV light (especially pulsed lasers). In some cases, the lifetime of a UV laser is limited by the lifetime of the optical components used, such as laser mirrors.

Ultraviolet laser applications

1. High-power pulsed UV lasers can be used to efficiently cut and drill small holes in a variety of materials, including those that are transparent to visible light.
2. High-energy ultraviolet pulses are used in laser-induced breakdown spectroscopy.
3. Use lower pulse energies in precisely focused beams, such as microdissection of biomaterials under a microscope, or photoluminescence analysis (fluorescence lifetime measurement).
4. Microlithography and wafer inspection require a continuous wave UV source, e.g. in the context of semiconductor chip manufacturing. Another application is UV Raman spectroscopy.
5. Some ophthalmic surgical approaches, particularly corneal refractive laser eye surgery in the form of LASIK, require an ultraviolet (and sometimes deep ultraviolet) laser source。

Ultraviolet laser sources involve some special safety hazards, most of which are related to eye damage and the risk of skin cancer.