When to Use Laser Crystal？

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

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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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DOI: 10./pw6   Cite the article: BibTex plain textHTML   Link to this page!   LinkedIn

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

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!)

Questions and Comments from Users

Optical crystals – the heart of medical lasers–transform light into precise tools for advanced treatments and diagnostics that elevate individuals' quality of life.

EBO supply professional and honest service.

Crystals are the backbone of modern medical laser technology. These precisely engineered materials are indispensable for generating and controlling the laser light utilized in multiple medical applications, from noninvasive cosmetic procedures to life-saving surgical interventions.

Crystals convert electrical energy into highly focused—and intense—laser beams, which can be precisely tailored to target specific tissues, structures, or even individual cells within the body.

Without crystals' level of precision and control, medical lasers would lack the power, stability, and versatility needed to perform the delicate procedures transforming the face of healthcare today.

Medical lasers, however, use a range of optical crystals to generate laser light and modify the laser’s wavelength -- and this blog is focused on discussing the details and benefits of the various types of crystals.

Gain and Nonlinear Crystals

Gain crystals—the most commonly used—act as the heart of the laser, converting energy from light sources (like lamps or diode lasers) into focused laser beams. These crystals intensify or amplify the laser light, making it powerful enough for a wide range of Life Science applications.

Neodymium-doped crystals and fibers are used in many lasers where they generate a near-infrared fundamental wavelength around nm. The two most common crystals are Nd:YAG where the host crystal is yttrium aluminum garnet, and Nd:YVO4 where the host crystal is yttrium vanadate.  Medical laser technology relies on YAG and YVO4 as critical components. YAG's inherent properties enable the generation of high-energy pulses, while YVO4 assists by facilitating elevated repetition rates and high average power output.

So where are these lasers used in medicine? In dermatology and aesthetics, Nd:YAG lasers are used for tattoo removal, hair removal, treatment of pigmented lesions, and skin rejuvenation. In ophthalmology, Nd:YAG lasers are employed in certain eye surgeries to treat various eye conditions, including posterior capsular opacification after cataract surgery and glaucoma. And in dentistry, these lasers are used for procedures such as removing tooth decay, gum contouring, and dental surgeries.

SHG Crystals – KTP, LBO, BBO

The fundamental nm from Nd:YAG lasers is sometimes converted to second harmonic green (532nm) output using SHG crystals. Several different nonlinear optical crystals can be used for SHG of Nd:YAG lasers. They are typically referred to using 3-letter acronyms instead of their somewhat cumbersome full names.  The three most common SHG crystals for Nd:YAG—Potassium Titanyl Phosphate (KTP), Lithium Triborate (LBO), and Beta-Barium Borate (BBO)—are categorized based on their nonlinear optical properties that facilitate efficient frequency doubling of Nd:YAG lasers.

These nonlinear crystals all exhibit a property known as birefringence, which allows the phase-matching of the fundamental and the second harmonic waves to occur efficiently. This ensures that the fundamental and second harmonic waves propagate in phase and the conversion efficiency is maximized.

The choice of nonlinear crystal depends on various factors such as the desired wavelength, efficiency of conversion, optical properties, damage threshold, and other specific requirements of the laser system.

Green (532 nm) Nd:YAG applications

The adaptable nature of green (532 nm) Nd:YAG lasers has led to their widespread utilization across a diverse spectrum of medical applications. In dermatology, they are employed in procedures like vascular lesion treatment, where the green wavelength is absorbed by hemoglobin in blood vessels, effectively targeting and treating vascular lesions, port-wine stains, and certain types of birthmarks.

They are also useful in ophthalmology. This includes treatment of certain retinal conditions and ocular surgeries. In cosmetic medicine, the green light is sometimes used in laser hair removal for people with lighter skin tones and darker hair as it's absorbed by melanin in the hair follicles. It is also a useful laser wavelength for breaking down the ink particles in tattoos, especially for green, blue, and black pigments. And we should also mention that green Nd:YAG lasers are sometimes used in photoacoustic imaging techniques that combine optical and ultrasound imaging, enabling visualization of tissues for diagnostic purposes.

Er:YAG Laser Crystals

Lasers based on erbium-doped gain crystals are also widely used in medicine, particularly Er:YAG (Erbium-doped Yttrium Aluminum Garnet). These lasers emit at nm, which is in the mid-infrared and is strongly absorbed by water and hydroxyapatite (a component of teeth and bones). These lasers are therefore known for their precision and minimal damage to surrounding tissues.

Medical applications of erbium lasers include several dental procedures. In dental caries removal, the selective absorption by hydroxyapatite allows for minimal removal of healthy tooth structure, making it suitable for cavity preparation. Soft tissue applications include gum contouring, periodontal treatments, and gingival sculpting. Erbium lasers also get used in root canal treatments – to clean and disinfect the root canals.

In dermatology, Er:YAG lasers are effective in treating wrinkles, scars, and skin irregularities by ablating the superficial layers of the skin to promote collagen remodeling and rejuvenation. They are sometimes used to target and remove pigmented lesions, such as age spots and freckles.

Holmium-Based Lasers

Lasers based on holmium-doped crystals – particularly Chromium, Thulium, Holmium, doped YAG (CTH:YAG) – are another important type of medical laser that emits around nm. These lasers are used mainly in urology, particularly for the treatment of urinary stones (kidney stone, ureteral stones, and bladder stones) by a fragmentation process called lithotripsy. This breaks these stones into smaller pieces that can be naturally passed or extracted.

Lasers based on holmium crystals are often also used in treatment of Benign Prostatic Hyperplasia (BPH). A common procedure here is Holmium Laser Enucleation of the Prostate (HoLEP) for the treatment of BPH, which enables precise tissue removal while minimizing bleeding.

And lastly, holmium lasers are sometimes used for endoscopic procedures in the gastrointestinal tract, primarily for treatments involving strictures, tumors, or other abnormalities.

Nd, Er, and Ho-Doped Fibers

Material compatibility is imperative with the industry's shift to fiber-based laser architectures. Manufacturers now focus on producing gain fibers using the same rare earth elements—Nd, Er, Ho, and others—traditionally found in laser crystals. Using compatible materials ensures optimal alignment of optical properties between the crystals and fibers, maximizing the efficiency, output stability, and performance of the entire laser system.

The Coherent Advantage

It’s not hard to argue that crystals are one of the most critical components of life sciences diagnostics and medical lasers since they create laser light and transform the wavelength when necessary. And in areas like precision cancer surgery, these lasers can literally represent the difference between life and death. That’s why it’s so important to choose the right crystals and the right crystal manufacturer.

Coherent is a market leader in supplying all classes of laser crystals in custom shapes and sizes—like cylindrical rods and rhomboid slabs—used in medical lasers, as well as many other types of optical crystals. With decades of experience and over 400 growth stations across sites globally, we bring an important combination of advantages that underpins this success.

Coherent is the world’s most vertically integrated photonic company, making everything from raw materials like these laser crystals, to complete lasers and specialty laser-based machines. This gives us a unique perspective as both a manufacturer and user of optical components – from emphasis on quality and process control to on-time shipping schedules.

We also have incredible technical breadth. In the specific area of laser crystals, this means we offer an unparalleled selection of laser crystals. We fully understand the relative merits of all these materials and if you’re building medical lasers, we can guide you to the very best solution for your application without any technically limited bias.

With decades of experience growing crystals by every type of commercial process, we understand the importance of QA and metrology. Growing crystals is rather easy. Growing laser-quality crystals with long lifetimes is not.  With our proprietary knowledge, our emphasis on quality, and our comprehensive application of metrology tools, Coherent crystals deliver better performance and extended lifetimes for life sciences instrumentation.

In this era of global supply chain uncertainties, we also have the practical advantage of producing all these crystals at several facilities in the US.

Learn more about Coherent crystals and Coherent solutions for medical applications.