Introduction

Although mostlasers emit at longer wavelengths, e.g. in theinfrared spectral region, there are some laser types emittingultraviolet light. That technology involves a number of challenges:

• For short emissionwavelengths, strongspontaneous emission leads to a highthreshold pump power (except when thegain bandwidth is narrow).
• For wavelengths below ≈ 200 nm, the choice oftransparent and UV-resistantoptical materials (e.g., forlaser optics used inside the laser resonators) is fairly limited (see the article onultraviolet light).
• Even weak surface roughness or bubble content of optical components can lead to strong wavefront distortions andscattering losses.

Ultraviolet lasers need to be made with specialultraviolet 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 theoptical elements such aslaser mirrors.

Lifetime Issues

Compared to infrared and visible laser sources, ultraviolet laser sources tend to have more problems with limited device lifetimes. This is essentially because various optical materials (e.g. laser crystals, nonlinear crystals and optical elements) exhibit degradation effects initiated by absorption of ultraviolet light. Another sometimes encountered problem is that hydrocarbons, resulting e.g. from outgassing of lubricants of mirror mounts, are decomposed by ultraviolet light, which can lead to the deposition of black soot on optical elements. Such issues need to be carefully treated in the product development to realize the basic potential for long lifetimes of a particular laser type.

Types of Directly UV-emitting Lasers

The following types of lasers can directly generate ultraviolet light:

• There arelaser diodes, normally based on gallium nitride (GaN), emitting in the near-ultraviolet region. The available power levels, however, are limited.
• Somesolid-statebulk 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 nanosecondpulses from afrequency-quadrupledQ-switched laser, and thus emit nanosecond pulses themselves. With Q-switchedmicrochip lasers, even sub-nanosecond pulse durations are possible.Mode-locked operation has also been demonstrated[14].
• Fewfiber lasers can generate ultraviolet light[10]. For example, someneodymium-dopedfluoride fibers can be used for lasers emitting around 380 nm, but only at low power levels.
• Although mostdye 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 moleculargas 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.

UV Laser Sources Based on Nonlinear Frequency Conversion

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

• The wavelength of 355 nm can be generated byfrequency tripling the output of a 1064-nmNd:YAG orNd:YVO₄ laser.
• 266-nm light is obtained with two subsequentfrequency doublers, which in effectquadruple 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 applyresonant 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 certainlaser lines.

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

For the extreme ultraviolet region, there are sources based onhigh 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.

Applications of UV Lasers

Ultraviolet lasers find variousapplications:

• Pulsedhigh-power ultraviolet lasers can be used for efficientcutting anddrilling 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 oflaser micromachining, despite the higher cost compared with infrared laser sources.
• High energy UV pulses are used for the technique oflaser-induced breakdownspectroscopy.
• With far lower pulse energies in a preciselyfocused beam, one can, for example, do microdissection of biological materials under a microscope, or performphotoluminescence 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 UVRaman spectroscopy.
• Both continuous-wave and pulsed UV lasers are used for fabricatingfiber 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 onlaser safety gives some details.

Fiber Delivery

The delivery of ultraviolet light inoptical 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 thefused silica. There are also attempts to usehollow-core fibers 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.

Frequently Asked Questions

Suppliers

Sponsored content: TheRP Photonics Buyer's Guide contains100 suppliers for ultraviolet lasers. Among them:

⚙ hardware

OurH-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

LightMachineryexcimer 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

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 at355 nm.
• Cobolt 06-01 Series: Continuous-wave diode lasers emitting at375 nm.

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

⚙ hardware

GWU-Lasertechnik has more than 30 years of experience in lasers and non-linear optics. We are the pioneer of commercialBBO 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

Most of thepulsed 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

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

⚙ hardware

CNI offers variousultraviolet 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

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

⚙ hardware

Sub-nanosecond passively Q-switched microchip lasers are available with emission wavelengths of355 nm and266 nm.

For higher peak powers, we offer the266 nm PNU-M01210-1x0 lasers, part of the Powerchip series, also available with various wavelengths including355 nm,266 nm and213 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

Serving North America, RPMC Lasers offers a wide range ofUV 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

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

⚙ hardware

Sacher Lasertechnik has developed afrequency-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 theJaguar UV laser, a MOPA system with fourth harmonic generation for output wavelengths from 205 nm to 270 nm.

Bibliography

(Suggest additional literature!)

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