Ultraviolet Lasers

Although most lasers emit at longer wavelengths, e.g. in the infrared spectral region, there are some laser types emitting ultraviolet light. That technology involves a number of challenges:

• For short emission wavelengths, strong spontaneous emission leads to a high threshold pump power (except when the gain bandwidth is narrow).
• For wavelengths below ≈ 200 nm, the choice of transparent and UV-resistant optical materials (e.g., for laser optics used inside the laser resonators) is fairly limited (see the article on ultraviolet light).
• Even weak surface roughness or bubble content of optical components can lead to strong wavefront distortions and scattering losses.

Ultraviolet lasers need to be made with special ultraviolet optics, having a high optical quality and (particularly for pulsed lasers) a high resistance to UV light. In some cases, the lifetime of a UV laser is limited by the lifetime of the optical elements such as laser mirrors.

The following types of lasers can directly generate ultraviolet light:

• There are laser diodes, normally based on gallium nitride (GaN), emitting in the near-ultraviolet region. The available power levels, however, are limited.
• Some solid-state bulk lasers, e.g. based on cerium-doped crystals such as Ce³⁺:LiCAF or Ce³⁺:LiLuF₄, can emit ultraviolet light. Cerium lasers are in most cases pumped with nanosecond pulses from a frequency-quadrupled Q-switched laser, and thus emit nanosecond pulses themselves. With Q-switched microchip lasers, even sub-nanosecond pulse durations are possible. Mode-locked operation has also been demonstrated [14].
• Few fiber lasers can generate ultraviolet light [10]. For example, some neodymium-doped fluoride fibers can be used for lasers emitting around 380 nm, but only at low power levels.
• Although most dye lasers emit visible light, some laser dyes are suitable for ultraviolet emission.
• Excimer lasers are very powerful UV sources, also emitting nanosecond pulses, but with average output powers between a few watts and hundreds of watts. Typical wavelengths are between 157 nm (F₂) and 351 nm (XeF).
• argon-ion lasers can continuously emit at wavelengths of 334 and 351 nm, even though with lower powers than on the usual 514-nm line. Some other ultraviolet lines are accessible with krypton-ion lasers.
• There are also ion lasers emitting in the extreme ultraviolet spectral region. These can be based on, e.g., argon, but unlike in ordinary argon-ion lasers one operates with Ar⁸⁺ ions, generated in a much hotter plasma. The emission then occurs at 46.9 nm. Such lasers can be pumped either with a capillary discharge or with an intense laser pulse.
• Nitrogen lasers are molecular gas lasers emitting in the ultraviolet. The strongest emission line is at 337.1 nm.
• free-electron lasers can emit ultraviolet light of essentially any wavelength, and with high average powers. However, they are very expensive and bulky sources, and are therefore not very widely used.

Apart from real ultraviolet lasers, there are ultraviolet laser sources based on a laser with a longer wavelength (in the visible or near-infrared spectral region) and one or several nonlinear crystals for nonlinear frequency conversion. Some examples:

• The wavelength of 355 nm can be generated by frequency tripling the output of a 1064-nm Nd:YAG or Nd:YVO₄ laser.
• 266-nm light is obtained with two subsequent frequency doublers, which in effect quadruple the laser frequency.
• 213-nm light corresponds to the 5^th harmonic of 1064 nm, obtained by frequency tripling or quadrupling plus sum frequency generation. Overall, that conversion may not be very efficient, but relatively low output powers are sufficient for some applications.
• Diode lasers can be equipped with nonlinear frequency conversion stages to produce UV light. For example, one may use a continuous-wave near-infrared laser and apply resonant frequency doubling twice, arriving at wavelengths around 300 nm. A main attraction of this approach is that a wide range of wavelengths is accessible, with no limitations to certain laser lines.

Note that nonlinear frequency conversion also involves various special challenges for short output wavelength, related to phase matching (due to strong chromatic dispersion in the UV) and material degradation problems, for example.

For the extreme ultraviolet region, there are sources based on high harmonic generation. Such sources can reach wavelengths down to a few nanometers while still having a table-top format. The average output powers, however, are fairly low.

Ultraviolet lasers find various applications:

• Pulsed high-power ultraviolet lasers can be used for efficient cutting and drilling of small holes in a variety of materials, including materials which are transparent to visible light. They have a substantial market share in the area of laser micromachining, despite the higher cost compared with infrared laser sources.
• High energy UV pulses are used for the technique of laser-induced breakdown spectroscopy.
• With far lower pulse energies in a precisely focused beam, one can, for example, do microdissection of biological materials under a microscope, or perform photoluminescence analysis (fluorescence lifetime measurements).
• Continuous-wave UV sources are required for micro-lithography and for wafer inspection, e.g. in the context of semiconductor chip manufacturing. Another application is UV Raman spectroscopy.
• Both continuous-wave and pulsed UV lasers are used for fabricating fiber Bragg gratings.
• Some methods of eye surgery, in particular refractive laser eye surgery of the cornea in the form of LASIK, requires UV (sometimes even deep-UV) laser sources.

Ultraviolet laser sources involve some special safety hazards, mostly related to the risks of eye damage and causing skin cancer. The article on laser safety gives some details.

The delivery of ultraviolet light in optical fibers is possible even at rather short wavelengths, but involves more serious limitations, compared with sources for the visible or infrared spectral region. For example, silica fibers may exhibit substantial degradation (called solarization) when exposed to short-wavelength light, but that tendency depends strongly on the chemical composition of the fused silica. There are also attempts to use hollow-core fibers and capillaries for UV transmission; the basic idea is to have most of the UV light in the air core, with only little overlap with the silica material which provides the guiding. That principle can be utilized even in wavelength regions where the absorption of fused silica is substantial.

For professional purchasing, our buyer's guide for ultraviolet lasers explains additional buyer-oriented technical background and suggests selection criteria: Emission wavelength, Average output power, Pulse regime, Pulse energy, Beam quality, Power stability, Lifetime and maintenance, Cooling requirements, Environmental requirements. It also lists 92 suppliers of ultraviolet lasers. Some of these show their product descriptions as sponsored content:

⚙ hardware

TOPTICA provides lasers in the UV range from 190–390 nm. Proprietary technology and high-end clean room manufacturing capabilities enable stable long-term operation at all wavelengths.

⚙ hardware

LightMachinery excimer lasers are powerful and reliable sources for ultraviolet light. They now feature exciPure™ technology, introduced in 2016; exciPure represents the greatest improvement in excimer gas lifetime and reduction in operating costs in a generation.

The IPEX-700 series is designed for medium duty cycle operation in industrial and R & D environments. These lasers deliver high power ultraviolet laser machining combined with state-of-the-art performance. They are ideal for applications such as pulsed laser deposition.

The IPEX-800 series is designed for high duty cycle operation in a manufacturing environment. These lasers deliver high power ultraviolet laser machining combined with state-of-the-art performance. They offer long gas lifetimes, superior optical stability and precise control of laser operating parameters. Easy to use, simple to service and economical to operate, they combine the benefits of high precision excimer processing with the lowest total cost of ownership and highest uptime in the market today.

⚙ hardware

Serving North America, RPMC Lasers offers a wide range of UV lasers, from ultra-compact CW modules to high-energy pulsed systems, with single/multimode, free-space, or fiber-coupled options, tailored from components to OEM and turnkey solutions.

High average and peak powers deliver precision with smaller spot sizes than green lasers, ideal for demanding applications, with compact, lightweight, and rugged designs suited for portable integration.

Versatile for scientific and industrial uses, they excel in micromachining, LIBS, Raman, fluorescence lifetime spectroscopy, and more, offering unmatched resolution for fine features and sensitive detection.

Let RPMC help you find the right UV laser today!

⚙ hardware

Sub-nanosecond passively Q-switched microchip lasers are available with emission wavelengths of 355 nm and 266 nm.

For higher peak powers, we offer the 266 nm PNU-M01210-1x0 lasers, part of the Powerchip series, also available with various wavelengths including 355 nm, 266 nm and 213 nm. Peak powers of tens of kilowatts (or even 160 kW at 1064 nm) are generated, while the pulse durations are always well below a nanosecond. These lasers use a specific long life process to extend lifetime between refurbishments.

⚙ hardware

CNI offers various ultraviolet lasers (diode lasers and diode-pumped solid-state lasers) with many wavelengths between 213 nm and 349 nm. The output power is up to 3 W, and the pulse energy is up to 10 mJ. The laser products include 5 series: high energy, high power, high stability, low noise and single longitudinal mode laser.

⚙ hardware

PreciLasers utilizes high-efficiency optical frequency conversion technologies to deliver high-power continuous-wave UV lasers. Our ultraviolet lasers cover typical wavelengths such as 190 nm, 266 nm, 313 nm, and broadly tunable ranges from 305 nm to 383 nm. With powers up to 4 W and excellent beam quality (M² < 1.3), they are perfect for photolithography and wafer metrology.

⚙ hardware

Most of the pulsed lasers offered by ALPHALAS are optionally available with harmonics in the UV range:

• third harmonic (315, 343, 349, 351, 355 nm)
• fourth harmonic (236, 257, 262, 263, 266 nm)

with pulse durations from picoseconds (PICOPOWER series) to sub-nanoseconds and nanosecond (PULSELAS-A/P series). While the passively Q-switched sub-nanosecond microchip UV lasers are the best alternative for low-cost and maintenance-free operation, the harmonically tripled and quadrupled regeneratively amplified picosecond lasers offer very high peak powers for material processing and nonlinear optical applications.

⚙ hardware

GWU-Lasertechnik has more than 30 years of experience in lasers and non-linear optics. We are the pioneer of commercial BBO OPO technology. Our widely tunable laser sources cover especially the UV and deep-UV range down to a wavelength of < 190 nm. We offer pulsed solutions for nano-, pico- and femtosecond pulses with best performance and highest reliability. Our rugged and thoroughly tested all-solid-state Laser technology does not require any consumable supplies and is thus providing most convenient usability, longest lifetime and excellent total costs of ownership. With a vast flexibility and a huge versatility, GWU’s laser products can serve the needs even for the most demanding scientific and industrial applications.

⚙ hardware

HÜBNER Photonics specializes in UV lasers, providing advanced solutions tailored for precise applications. Their offerings include:

• Cobolt 05-01 Series: Single-frequency or Q-switched lasers emitting at 355 nm.
• Cobolt 06-01 Series: Continuous-wave diode lasers emitting at 375 nm.

For more detailed specifications and potential applications, visit the HÜBNER Photonics website.

⚙ hardware

Our H-Model ultraviolet laser modules are versatile and compact UV sources:

• fiber-optic output
• CW power levels up to 4 W at 405 nm or 1 W at 375 nm
• CW or pulse operation mode
• ruggedized for harsh environments

Applications are in chemical and biological detection, water purification, disinfection, skin treatment and fluorescence imaging.

⚙ hardware

Sacher Lasertechnik has developed a frequency-doubled laser system where a resonant cavity including a frequency doubler crystal is pumped via a tunable diode laser. Depending on the required SHG power, the tunable diode laser is either a high power external cavity laser, or a two stage Master Oscillator Power Amplifier System. The covered wavelength regime ranges from 365 nm up to 540 nm.

Sacher Lasertechnik also offers the Jaguar UV laser, a MOPA system with fourth harmonic generation for output wavelengths from 205 nm to 270 nm.

⚙ hardware

Monocrom has the CiOM Q-switched Nd:YLF lasers, emitting nanosecond pulses with up to 2 W average power at 351 nm.

⚙ hardware

VEXLUM offers products starting from the ultraviolet wavelength of 350 nm, suitable for applications in semiconductor technology, quantum technology, and more. For specific requirements, please contact VEXLUM directly.

(Suggest additional literature!)