Laser Crystals

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

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Definition: transparent crystals with laser-active dopants, used as laser gain media

More general term: laser gain media

More specific terms: composite laser crystals, neodymium-doped / ytterbium-doped / erbium-doped / chromium-doped / titanium-doped / thulium-doped / holmium-doped / praseodymium-doped laser crystals

Categories: optical materials, laser devices and laser physics

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For purchasing laser crystals, use the RP Photonics Buyer's Guide – an expert-curated directory for finding all relevant suppliers, which also offers advanced purchasing assistance.

Laser crystals are optical crystals – typically single crystals (monocrystalline optical materials) – which are used as gain media for solid-state lasers. In most cases, they are doped with either trivalent rare earth ions or transition metal ions. These ions enable the crystal to amplify light at the laser wavelength via stimulated emission, when energy is supplied to the crystal via absorption of pump light (→ optical pumping).

Compared with doped glasses, crystals usually have higher transition cross-sections, a smaller absorption and emission bandwidth, a higher thermal conductivity, and possibly birefringence. (The article on laser glasses discusses the differences in more detail.) In some cases, monocrystalline laser materials may be replaced with ceramic laser gain media, which have a fine polycrystalline structure.

Common Laser-active Dopants

The most common laser-active rare earth ions and host media together with some typical emission wavelengths are shown in the following table:

Ion Common host crystals Important emission wavelengths neodymium (Nd3+) Y3Al5O12 (YAG), YAlO3 (YALO), YVO4 (yttrium vanadate), YLiF4 (YLF), tungstates (KGd(WO4)2, KY(WO4)2) , , , , 946 nm ytterbium (Yb3+) YAG, tungstates (e.g. KGW, KYW, KLuW), YVO4, borates (BOYS, GdCOB), apatites (SYS), sesquioxides (Y2O3, Sc2O3) , – nm erbium (Er3+) YAG, YLF 2.9, 1.6 μm thulium (Tm3+) YAG 1.9–2.1 μm, 1.47 μm, 2.3 μm holmium (Ho3+) YAG 2.1, 2.94 μm cerium (Ce3+) YLF, LiCAF, LiLuF, LiSAF, and similar fluorides 0.28–0.33 μm

The following table lists common transition-metal doped crystals:

Ion Common host crystals Important emission wavelengths titanium (Ti3+) sapphire 650– nm chromium (II) (Cr2+) zinc chalcogenides such as ZnS, ZnSe, and ZnSxSe1-x 2–3.4 μm chromium (III) (Cr3+) Al2O3 (ruby), LiSrAlF6 (LiSAF), LiCaAlF6 (LiCAF), LiSrGaF6 (LiSGAF) 0.8–0.9 μm chromium (IV) (Cr4+) YAG, MgSiO4 (forsterite) 1.35–1.65 μm (YAG), 1.1–1.37 μm (forsterite)

These tables contain only the most common host crystals; many others exist, which are less frequently used.

Important Properties of Host Crystals

The host crystal is much more than just a means to fix the laser-active ions at certain positions in space. A number of properties of the host material are important:

• The medium should have a high transparency (low absorption and scattering) in the wavelength regions of pump and laser radiation, and good optical homogeneity. To some extent, this depends on the quality of the material, determined by details of the fabrication process.
• The host medium influences strongly the wavelength, bandwidth and transition cross-sections of pump and laser transitions and also the upper-state lifetime. For example, Nd:YVO4 has much higher cross-sections, a larger gain bandwidth, and a smaller upper-state lifetime than Nd:YAG. Other neodymium hosts provide other transition wavelengths, e.g.  or  nm from Nd:YLF.
• Non-radiative transitions (e.g. multi-phonon transitions) are also strongly influenced by the host, in particular by its maximum phonon energy. Some of these transitions are very detrimental, leading to quenching of the upper-state population (thus lowering the quantum efficiency). Others are essential for laser operation, e.g. for removing ions from the lower laser level. Energy transfer processes are also dependent on the host material.
• The maximum possible doping concentration can depend strongly on the host material and its fabrication method.
• Different crystalline materials are very different concerning their hardness and other properties, which determine with which methods and how easily they can be cut and polished with good quality.
• Some materials are chemically not stable, e.g. hygroscopic.
• Particularly for high-power lasers (but often enough also for medium and low powers), a high thermal conductivity low thermo-optic coefficients (for weak thermal lensing) and a high resistance to mechanical stress are desirable.
• Optical isotropy can be beneficial, but in other cases birefringence (reducing thermal depolarization) and possibly polarization-dependent gain is preferable (see also: polarization of light).
• A high damage threshold in terms of pulse fluence or peak intensity can be important for high-energy amplifiers.

It is apparent that different applications lead to very different requirements on laser gain media. For this reason, a broad range of different crystals are used, and making the right choice is essential for constructing lasers with optimum performance.

Common Crystalline Laser Host Media

There is a wide range of crystalline media, which can be grouped according to important atomic constituents and crystalline structures. Some important groups of crystals are:

• garnets such as Y3Al5O12 (YAG), Gd3Ga5O12 (GGG), and Gd3Sc2Al3O12 (GSGG): hard and chemically inert materials, optical isotropic, with high thermal conductivity
• sapphire (Al2O3) (e.g. for titanium–sapphire lasers) and aluminates such as YAlO3 (YALO, YAP) for neodymium doping: high hardness and thermal conductivity, anisotropic
• sesquioxides such as Y2O3, Sc2O3: isotropic, high hardness and thermal conductivity
• vanadates such as YVO4 and GdVO4: very high laser cross-sections of Nd3+, anisotropic
• fluorides, e.g. YLiF4 (YLF): good UV transparency, birefringence, large energy storage capability of Nd:YLF; also LiCAF, LiLuF, LiSAF as chromium-doped broadband gain media
• silicates, e.g. MgSiO4 (forsterite): broad gain bandwidth
• monoclinic double tungstates such as KGd(WO4)2 (KGW) and KY(WO4)2 (KYW): combination of relatively high Yb3+ laser cross-sections, large gain bandwidth, and high thermal conductivity
• disordered tetragonal double tungstates such as NaGd(WO4)2 (NGW) and NaY(WO4)2 (NYW): particularly large gain bandwidth of ytterbium
• chalcogenides such as ZnS or ZnSe for mid-infrared lasers

Laser Crystals with Integrated Saturable Absorber

A few laser crystal materials have been demonstrated where some saturable absorber material is incorporated for passive Q switching of a laser. For example, Cr4+ ions can be incorporated into such Nd-doped crystals for emission in the 1-μm spectral region. This has been tried with Cr:Nd:YAG and Cr:Nd:YVO4, for example.

With that concept, one does not need an additional saturable absorber crystal, so that one may make more compact Q-switched laser setups with lower internal parasitic losses. However, unwanted side effects may also occur, such as obtaining unwanted valence states of the involved ions or energy transfers. In addition, some flexibility is lost in experiments if one cannot try out absorbers with different thickness or doping concentration, for example, without exchanging the laser crystal itself.

Geometries of Laser Crystals

Different geometric forms can be used in lasers:

• A common form is that of a cuboid. The crystal can be, e.g., a thin coplanar plate, with transverse dimensions (perpendicular to the laser beam) and a thickness of a few millimeters. It may be oriented for near perpendicular incidence of the laser beam, or at Brewster's angle. It can be fixed in some solid mount which also acts as a heat sink. Larger crystals are usually used for side pumping e.g. with high-power diode bars.
• In some cases, extreme angles between the end faces are required, e.g. if one end face has to be Brewster-angled while the other one is made for perpendicular incidence.
• Slab lasers are based on relatively flat slabs, which may or may not be of cuboid form.
• Many side-pumped lasers use relatively long cylindrical laser rods, e.g. made of Nd:YAG. Particularly for lamp-pumped lasers, the rod length can be several centimeters, whereas the rod diameter is much smaller (a few millimeters).
• Thin-disk lasers require a disk, often with circular cross-section, having a thickness of only e.g. 100–200 μm and a relatively high doping concentration.
• Special geometries are required for monolithic solid-state lasers, such as nonplanar ring oscillators.
• For various reasons, composite crystals are becoming popular. These have a spatially varying chemical composition and can be made with special shapes.
• There are so-called single-crystal fibers, where a monocrystalline material (often containing a laser-active dopant) is pulled into the form of a fiber. Here, a waveguide effect is obtained from the crystal–air interface, possibly also from thermal lensing, doping gradients or other effects.

Bulk Properties

For a given dopant and host medium, the doping concentration is the most important parameter. Other issues of interest are the uniformity of doping (influencing the tendency for quenching), the level of impurities (e.g. unwanted other rare earth ions), and the optical homogeneities. Several of these factors influence the absorption and scattering losses of the material, and/or the strength of thermal lensing.

Of course, it is very desirable that a given crystal quality is produced consistently, although different laser designs can have a different sensitivity to material parameters.

Optimization of Geometry and Parameters

Which geometry, dopant and doping concentration of the gain medium are most advantageous depend on several factors. The available pump source (type of laser diode or lamp) and the envisaged pumping arrangement are important factors, but the material itself also has some influence. For example, titanium–sapphire lasers have to be pumped with high intensities, for which the form of a transversely cooled rod, operated with relatively small pump and laser beam diameter, is more appropriate than e.g. a thin disk. As another example, Q-switched lasers reach a higher population density in the upper laser level and are therefore more sensitive to quenching effects and energy transfer processes; therefore, a lower doping density is often appropriate for these devices. For high-power lasers, lower doping densities are often used to limit the density of heat generation, although thin-disk lasers work best with highly doped crystals. Many laser products do not reach the full performance potential because such details have not been properly worked out.

Besides the material parameters, the used length (thickness) of crystal is an important parameter. In some cases (e.g. often for four-level laser gain media), it is only important to have it long enough to achieve sufficiently efficient pump absorption at the used pump wavelength, although a shorter length may be better in terms of cost. For quasi-three-level laser gain media or even three-level gain media, one often needs to limit the length to a value which is too short for efficient pump absorption, as reabsorption effects would otherwise spoil the performance. For side pumping, additional considerations come into play; besides efficient pump absorption, it is important to obtain a suitable spatial shape of the gain profile.

Laser modeling and simulation can be used to reliably determine the optimum crystal dimensions, doping density and possibly values of additional parameters.

Optical Surfaces

Those surfaces which are passed by the laser beam are normally either oriented at Brewster's angle or have an anti-reflection coating. Even AR-coated crystals are often slightly tilted against the beam so as to prevent back-reflections staying in the laser resonator. This is important for, e.g., mode-locked lasers and tunable single-frequency lasers.

A high surface quality is of course important. Specifications of surface flatness are often better than ($\lambda / 10$). This helps to avoid both scattering losses and wavefront distortions which can degrade the laser's beam quality. In addition, scratch and dig specifications (cosmetic surface quality) limit the density of small-scale surface defects; they may read e.g. “80–50” for medium quality mass production, or “10–5” for particularly demanding laser applications. Proper surface treatment also influences the damage threshold, which is important e.g. for high-energy pulse amplifiers. Finally, a high degree of end face parallelism can be important for avoiding changes in beam direction in a crystal.

Related Articles

Suppliers

The RP Photonics Buyer's Guide contains 92 suppliers for laser crystals. Among them:

GWU-Lasertechnik

GWU offers all common laser crystals (Nd:YVO4, Nd:YAG, Yb:YAG etc.) with a broad variety of specifications. Besides the well-established materials, innovative crystals with outstanding properties like Yb:CALGO can open new horizons for demanding laser applications. No matter whether individual pieces for R & D purposes are required or cost-efficient numbers in small, medium or large batches with in-time delivery for the production line are needed: GWU’s dedicated service helps to find the best core components for your application. GWU-Lasertechnik has more than 30 years of experience in distributing laser crystals. Choose GWU to benefit from our wide knowledge and in-field experience!

Contact us to discuss your requirements of Scintillation Crystal. Our experienced sales team can help you identify the options that best suit your needs.

ALPHALAS

ALPHALAS offers many standard laser crystals like Nd3+:YAG, Yb3+:YAG, Nd3+:YVO4 and Nd3+:GdVO4 with large stock inventory. Customer-specific dimensions, doping levels as well as HR, PR and AR coatings are also available.

Optogama

Optogama supplies high-performance laser crystals engineered for demanding applications in scientific, industrial, and medical systems. Our crystal portfolio includes a wide variety of doped materials optimized for different laser wavelengths and performance needs:

• Yb-doped: Yb:KGW, Yb:KYW, Yb:CaF₂ — excellent for high-power, ultrafast lasers
• Nd-doped: Nd:KGW, Nd:YLF — widely used for nanosecond pulsed systems
• Ti:sapphire — ultrafast laser gain medium with broad tunability
• Er:Yb:phosphate glass & Er:Yb:YAB — efficient gain media for compact eye-safe lasers
• Tm:YLF, Ho:YLF, Pr:YLF — suitable for mid-infrared and eye-safe spectral ranges

Our crystals are precisely cut, polished, and coated to meet tight optical tolerances, excellent beam quality, thermal handling, and reliability under high pump powers.

EKSMA OPTICS

Laser crystals of various materials including neodymium-doped yttrium aluminum garnet (Nd:YAG), Yb-doped potassium gadolinium tungstate (Yb:KGW), Yb-doped potassium yttrium tungstate (Yb:KYW), Nd-doped potassium gadolinium tungstate (Nd:KGW, titanium-doped sapphire (Ti:sapphire) are available from EKSMA Optics.

Shalom EO

Shalom EO offers various kinds of laser crystals, including: Nd:YAG, Nd:Ce:YAG, Nd:YVO4, CTH:YAG, Er:YAG, Yb:YAG, Ti:sapphire crystals and diffusion-bonded crystals. Nd:YAG and Nd:YVO4 are popular crystals for -nm lasers, Nd:Ce:YAG, CTH:YAG, and Er:YAG crystals are often used in the medical and cosmetic lasers, Ti:sapphire is for ultrafast lasers, alexandrite is for long-pulsed and picosecond 755-nm lasers, diffusion bonding crystals are useful for OEM compact lasers.

Megawatt Lasers

MegaWatt Lasers Inc. has a large inventory of Nd:YAG, Er:YAG and CTH:YAG laser rods. We also can assist in the design and modeling for various applications.

Bibliography

[1]A. A. Kaminskii, “Laser crystals and ceramics: recent advances”, Laser Photon. Rev. 1 (2), 93 (); https://doi.org/10./lpor. [2]A. A. Kaminskii, Laser Crystals, Springer, New York () [3]R. C. Powell, Physics of Solid-State Laser Materials, AIP Press, Springer () [4]W. Koechner, Solid-State Laser Engineering, 6th edn., Springer, Berlin () [5]F. Träger (ed.), Handbook of Lasers and Optics, Springer, Berlin ()

(Suggest additional literature!)

This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics AG. How about a tailored training course from this distinguished expert at your location? Contact RP Photonics to find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, training) and software could become very valuable for your business!

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6 Questions to Ask Yourself If You Are Thinking of Buying a Laser Engraver

Laser engravers are a useful tool and it makes sense to purchase one if you use it often enough. If you are considering buying a laser engraver, there are many things that you need to think about before buying it. There are a number of options to choose from and there are good reasons to choose one kind of engraver over another one. It is important to weigh the pros and cons of each of the laser engraver before you decide on the one that you would like to purchase. Here are some questions that you should ask yourself before you decide to purchase a laser engraver.

1. How Often Will I Use It?

If you only plan on using the laser engraver once in a while, it may not be the best option to purchase a laser engraver. It may be more cost-effective for you to simply rely on a professional laser engraving service to manage your project. With a professional service, you can just have the company manage your projects for you and you can trust that it will be done properly. However, if you are planning on using your engraver very often, you have to decide which one is best for the kinds of project that you will be working on.

2. Is a Laser Engraver Difficult to Learn How to Use?

If you are worried about learning how to use a laser engraver, that shouldn’t prevent you from buying a laser engraver. You can use many different kinds of design software with laser engravers, so you may not need to learn how to use a new design software in order to use the laser engraver. Once the system and the laser engraver has been set up it is simple to understand how to use.

3. What Materials Will I Be Engraving?

Laser engravers are extremely useful because of the variety of materials that they can engrave. You should think about the different projects that you will be working on and the materials that you are engraving before you choose which laser engraver you wish to purchase. Both the HTL QT Desktop Laser Engraver and the CO2 Laser Engraver have the ability to engrave many different types of materials. Here are the many different materials that can be engraved with a laser engraver.

Metal – Laser engraving on metal can be used for key chains, jewelry, gifts, name tags, and many other applications. The engraving is exact and can be extremely precise. You can use either the HTL QT Desktop Laser Engraver and the CO2 Laser Engraver to engrave many different types of metals.

Plastic and Rubber – The benefit of using laser engravers instead of a traditional engraver is that you can very precisely engrave plastic without needing to worry about melting the plastic. The CO2 Laser Engraver is the best option for anyone who is planning on engraving on plastic or rubber.

Glass or Crystal – Since glass and crystal are so brittle, they cannot be engraved with a traditional engraver. With a laser engraver, you can get permanent, precise engravings that are perfect for awards, trophies, or even wine glasses. The CO2 Laser Engraver is the best option for engraving on glass.

Paper – Using a laser engraver to cut paper is an excellent use of a laser engraver. It can be used to create invitations, artwork, and other creative uses. A laser engraver can make custom cuts and designs. The HTL QT Desktop Laser Engraver and the CO2 Laser Engraver can both be used to engrave paper.

4. Are There Any Materials that Should Not Be Engraved?

This is something that you definitely need to know before you begin doing engraving. You want to make sure that you are not using the laser engraver on anything that could potentially damage what you are trying to engrave. Also, certain things could react with the laser and they could be harmful to you. For example, when PVC is cut with a CO2 laser, it can create hydrochloric acid and toxic fumes which are dangerous for you, and less importantly, will corrode the laser engraver itself. It is important for you to discuss this with the company that you purchase the laser engraver from to ensure that you only engrave what is safe and best for you and the laser engraver.

5. What Size of a Laser Engraver Do I Need?

While both the HTL QT Desktop Laser Engraver and the CO2 Laser Engraver have a compact design, the size of the laser engraver depends on the projects that you will be working on. You will need an engraver that will easily fit in your office space or workshop but will be large enough to handle the projects that you will be working on. If you are working on extremely large projects, you may want to use a professional service. However, if you are working on smaller projects, then either the HTL QT Desktop Laser Engraver or the CO2 Laser Engraver will be an excellent option.

6. Where Should I Place My Laser Engraver?

Before you purchase your laser engraver, it is a good idea to think about where you will be placing it. You need to make sure that you have enough space in your workshop or office for your laser engraver, and you also need to make sure that it is properly ventilated. It is a good idea to keep your laser engraver about a foot away from any walls or barriers. This ensures that air can circulate through the engraver’s fans and keep the engraver cool throughout the engraving process. You need to also be able to plug it into an outlet that is able to provide the right amount of electricity that the laser engraver needs.

Choosing the Right Laser Engraver for Your Needs

Before deciding which laser engraver you want to purchase, it is important that you walk yourself through these questions, but also discuss any further questions with the company that you purchase the laser engraver from. Talk to the company about what you are planning on using your engraver for and they should be able to recommend helping you choose the best engraver for your needs. This will ensure that you are completely informed before you begin to use your laser engraver.