الأحد، تشرين الثاني ٣٠، ٢٠٠٨

Brewster Windows

Definition: transparent plates which are oriented at Brewster's angle

There are situations where an optical beam must be sent through some transparent window, whereas the optical losses occurring at this window must be very small. A typical example is a helium–neon laser with a sealed glass tube and external resonator mirrors as shown in Figure 1, where glass windows separate the laser gas mixture from ambient air. Given the small gain and the small output coupler transmission, the losses at these interfaces need to be far below 1% per pass. This is achieved by using Brewster windows, where the angle of incidence is close to Brewster's angle. In that situation, the reflectivity at the air–glass interfaces becomes very small for p-polarized light, i.e., when the polarization direction is in the plane of incidence.

helium–neon laser

Figure 1: Setup of a helium–neon laser. Although two glass windows are within the laser resonator, these induce very little optical loss for p-polarized light, as they are oriented for Brewster-angled incidence.

An uncoated glass plate at normal incidence would normally have a reflectivity of several percent on each side. With an anti-reflection coating, this could be reduced to e.g. 0.2%. Brewster windows can have at least 10 times lower losses. In addition, any residual reflection will leave the resonator, rather than lead to interference effects (as can occur for windows with normal incidence). Of course, both Brewster windows in a setup as shown above must have exactly the same orientation.

Due to the significant loss difference between p and s polarization, the polarization of laser emission is usually forced to be in the p direction. In many lasers, this is the only effect determining the polarization direction.

A potential disadvantage of Brewster windows (or other polarizing optical elements) in a laser resonator is that large depolarization loss can arise if, e.g., thermal effects within a laser crystal affect th

Thin-disk Lasers


Definition: solid-state bulk lasers having a very thin disk of laser-active material as the gain medium

The thin-disk laser (sometimes called thin-disc laser or active-mirror laser) is a special kind of diode-pumpedhigh-power solid-state laser, which was introduced in the 1990s by the group of Adolf Giesen at the University of Stuttgart, Germany. The main difference from conventional rod lasers or slab lasers is the geometry of the gain medium: the laser crystal is a thin disk, where the thickness is considerably smaller than the laser beam diameter, so that the heat generated is extracted dominantly through one end face, i.e., in the longitudinal rather than in the transverse direction. The cooled end face has a dielectric coating which reflects both the laser radiation and the pump radiation.




thin disk laser

Figure 1: Setup of an 8-kW laser based on four thin-disk laser heads (seen on the left side). The photograph was kindly provided by TRUMPF.

The thin disk is also often called an active mirror [1], because it acts as a mirror with laser gain. Within the laser resonator, it can act as an end mirror or as a folding mirror. In the latter case, there are two double passes of the laser radiation per resonator round trip, so that the gain per round trip is doubled and the threshold pump power is reduced.

thin-disk laser head

Figure 2: Schematic setup of a thin disk laser head. The pump optics (not shown) are arranged for multiple passes of the pump radiation. The heat is extracted in the longitudinal direction, which minimizes thermal lensing effects.

The thin-disk laser should not be confused with the rotary disk laser, where the gain medium is a quickly rotating disk, which is usually a few millimeters thick.

Reduced Thermal Issues at High Output Powers; Power Scalability

Due to the small thickness of the disk (e.g. 100–200 μm for Yb:YAG), the temperature rise associated with the dissipated power is small. In addition, the temperature gradients are dominantly in a direction perpendicular to the disk surface and thus cause only weak thermal lensing and depolarization loss. This allows for operation with very high beam quality due to the weak thermal beam distortions, and stable operation can be achieved over a wide range of pump powers.

A very important property arising from the thin-disk geometry is power scalability in a strict and meaningful sense. The scaling procedure is simple: for example, the output power can be doubled by applying twice the pump power to twice the area on the disk, while keeping the disk thickness and doping level constant. The laser resonator has to be modified so as to double the mode area in the disk. With this scaling procedure applied, the new design with twice the output power has unchanged peak optical intensities and a nearly unchanged maximum temperature in the disk (the latter basically because the cooling area has also been doubled). As far as thermal lensing results from the temperature dependence of the refractive index, the dioptric power (inverse focal length) of the thermal lens is reduced to half of the original value, which just compensates the doubled sensitivity of the larger mode to focal length changes. The power has thus been scaled without increasing optical intensities, the magnitude of temperature rises, or thermal lens problems. Strictly, thermal lensing effects can be increased by the power scaling as far as they result from mechanical stress in the disk, but this problem can be kept small by keeping the disk thickness small. Another limitation arises from amplified spontaneous emission (ASE) in the transverse direction, which ultimately limits the gain achievable in the longitudinal direction, but becomes severe only at very high power levels with many kilowatts from a single disk. The use of a composite disk with an undoped part on top of the doped part can also strongly suppress ASE; at least for continuous-wave lasers, the ASE limit may be pushed to the order of 1 MW from a single disk.

Power scalability in a wide range is achieved even for passively mode-locked thin-disk lasers (see below). Here, the doubling of the output power also involves doubling of the mode area on the SESAM, so that optical intensities in that device and also cooling issues are – contrary to naïve expectations – not limiting factors. Limitations rather arise from the problem of doing dispersion compensation at high power levels.

So far, hundreds of watts of output power in a diffraction-limited continuous-wave beam have been obtained. By using multiple thin-disk laser heads within one laser resonator, several kilowatts of power can be generated; thin-disk lasers with 8 kW output power in a multimode beam are commercially available, and similar or even higher power levels in nearly diffraction-limited beam will be realized in the near future. With mode-locked thin-disk lasers, ∼ 80 W of average output power has been achieved.

Multipass Pumping

The small thickness of the disk typically leads to inefficient pump absorption when only a single or double pass is used. This problem is normally solved by using a multi-pass pump arrangement, which can be made fairly compact when using a well-designed optical setup, typically containing a parabolic mirror and prism retroreflectors. Such arrangements easily allow one to arrange for e.g. 8 or 16 double passes of the pump radiation through the disk without excessively stringent requirements on the pump beam quality.

The pump source of a thin-disk laser is usually based on high-power diode bars, either in fiber-coupled form or with free-space power delivery. A typical pump wavelength is 940 nm for Yb:YAG, whereas ytterbium-doped tungstate crystals can be more efficiently pumped near 981 nm.

Thin-Disk Gain Media

The most often used gain medium for thin-disk lasers is Yb:YAG. Compared with Nd:YAG, it has a shorter emission wavelength (typically 1030 nm), a smaller quantum defect (reducing the dissipated power), a longer upper-state lifetime (improving energy storage for Q switching), and a larger gain bandwidth (e.g. for shorter pulses with mode locking). On the other hand, it is a quasi-three-level gain medium with significant reabsorption at the laser wavelength, and thus requires higher pump intensities. The thin-disk principle is well adapted to these parameters.

For broad wavelength tuning and for ultrashort pulse generation, other ytterbium-doped gain media offer a still wider gain bandwidth. Examples are tungstate crystals (Yb:KGW, Yb:KYW, Yb:KLuW), Yb:LaSc3(BO3)44 (Yb:CALGO) and Yb:YVO4. (Yb:LSB), Yb:CaGdAlO

Nd:YAG or Nd:YVO4 may also be used in thin-disk lasers, e.g. when a wavelength of 1064 nm is required, or when the much smaller saturation energy of Nd:YVO4 is relevant.

Spatial Hole Burning

An interesting consequence of the small disk thickness is that spatial hole burning usually cannot be avoided, even if a thin-disk laser is built with a ring resonator. (Note that due to the small ratio of thickness and beam radius, counterpropagating waves in the disk always have a strong overlap, so that an interference pattern is generated even in a ring resonator.) Nevertheless, single-frequency operation is possible by using an appropriate wavelength filter (etalon) in the resonator. For passive mode locking (see below), spatial hole burning in the thin disk distorts the shape of the gain spectrum, causing a variety of instabilities, but also allowing for significantly shorter pulses in the optimum range of parameters [5].

Q-switched Pulse Generation

Thin-disk lasers are well suited for generating high-energy nanosecond pulses with high beam quality, as required for, e.g., some kinds of laser material processing. The typically used gain medium Yb:YAG offers a significantly better energy storage (longer upper-state lifetime) compared with, e.g., Nd:YAG. A somewhat limiting factor, however, is the moderate gain (compared with that of an end-pumped bulk laser), which makes it difficult to achieve very short (few-nanosecond) pulses.

Mode-locked High-power Thin Disk Lasers

Thin-disk lasers are particularly attractive for the generation of ultrashort pulses at very high power levels. In addition to the high-power capability, the main advantages in this context are

  • the ease of achieving diffraction-limited operation (which is a prerequisite for mode locking)
  • the broad gain bandwidth of Yb:YAG (the so far most suitable gain medium for thin-disk lasers)
  • the small nonlinearity of a thin disk, which helps to avoid excessive nonlinear phase shifts despite the high intracavity peak intensities

One of the initial challenges was to find a suitable mode-locking mechanism. Even though originally it was widely believed that passive mode locking with semiconductor saturable absorber mirrors (SESAMs) would not be possible at very high power levels – at least not without first developing special high-power SESAMs, possibly based on improved semiconductor materials –, the author's research group at ETH Zürich demonstrated around 2000 that both thermal and non-thermal issues can be relatively easily managed even at very high power levels, if only the design parameters of the overall laser system (and not only of the SESAM) are properly chosen. In other words, SESAM damage then does not constitute a limiting factor for the power scaling of mode-locked thin-disk lasers; in fact, it was found that the mode-locked thin-disk laser is the first truly power-scalable femtosecond laser. However, the design of such lasers involves a number of subtle issues, related to e.g. spatial hole burningdispersion compensation, and a trial-and-error approach not based on a solid understanding of various details is prone to fail, e.g. by not managing to suppress certain types of instabilities. and to

So far, thin-disk lasers have lead to the highest average output power of 80 W from a mode-locked laser [8, 10], and pulse energies of > 10 μJ combined with sub-picosecond pulse durations are possible [19, 20]. While typical pulse durations with Yb:YAG are around 700–800 fs, significantly shorter pulses are possible, e.g. with ytterbium-doped tungstate crystals such as Yb:KGW or Yb:KYW [7].

Amplifiers for High Pulse Energies

Thin-disk laser heads can also be used for regenerative amplifiers [11]. The relatively small gain of the thin disk can be compensated with a larger number of resonator round trips, even though this makes the amplifier more sensitive to optical losses and nonlinearities. Therefore, it can be advantageous to arrange for multiple passes of the signal radiation through the disk in each resonator round trip.

It is also possible to construct a purely multipass amplifier (without a resonator and optical switch), but this approach limits the overall gain and requires a carefully optimized setup in order to preserve a high beam quality.

Nonlinear Frequency Conversion

High-power continuous-wave green lasers can be easily realized in the form of intracavity frequency-doubledoptical parametric oscillators, amplifiers and generators [6]. A high-power RGB source based on a mode-locked thin-disk laser has also been demonstrated [10]. The high peak power of mode-locked thin-disk lasers allows for efficient nonlinear frequency conversion with critical phase matching in LBO crystals, thus not requiring crystal ovens for most or all conversion stages. thin-disk lasers. For Q-switched or mode-locked lasers, extracavity doubling is often more practical. Thin-disk lasers are also very interesting pump sources for

Further Development of the Thin-Disk Concept

The thin-disk laser concept allows for further variations. For example, side pumping of the disk may allow for even higher output powers while reducing the requirements on the pump beam quality. This approach, developed at the Lawrence Livermore National Laboratory, is based on a composite laser crystal [22]. An undoped YAG disk, which is bonded to a Yb:YAG disk, brings various benefits: It reduces the beam quality requirements for the pump source, reduces the tendency for transverse amplified spontaneous emission (ASE) and parasitic lasing in large disks, increases the mechanical strength and may improve the cooling. The side-pumped concept may thus allow scaling to much higher powers, even though not necessarily with diffraction-limited beam quality. An interesting option is to use a composite ceramic gain medium which is ytterbium doped only in the center region and not in the outside regions, which are used only to deliver the pump power.

It is also interesting to develop cryogenic thin-disk lasers, as cryogenic cooling greatly reduces thermal effects at high power levels.

Competition with Fiber Lasers

Thin-disk lasers are currently facing fierce competition from high-power fiber lasers and amplifiers. In continuous-wave operation, these can currently deliver even higher powers in close to diffraction-limited beams. However, they are generally considered to be still less mature for industrial applications. Within the next few years, both thin-disk lasers and fiber lasers are expected to show significant further progress, and it is currently not clear which technology will acquire the larger market share. See also the article on fiber lasers versus bulk lasers.

In the domain of ultrashort pulse generation, fiber amplifier systems allow one to reach even higher average powers and shorter pulse durations than thin-disk lasers can generate without amplification. However, thin-disk lasers will probably maintain superiority for the generation of pulses with high energies, particularly when high pulse quality (concerning temporal shape, low chirp, and stable linear polarization) is required. A key issue in this context is that both the small disk thickness and the larger beam diameter on the disk lead to a nonlinearity which is much smaller than that in a fiber laser resonator.

A detailed comparison of thin-disk versus fiber lasers is complex and has to take into account many aspects which depend on the particular application. For example, issues such as emission bandwidth, pulse quality and stability of polarization state can be essential in some cases but insignificant in others.

Semiconductor Disk Lasers

The thin-disk geometry is also used in vertical external cavity surface-emitting lasers (VECSELs), a kind of semiconductor lasers. In that case, multiple passes of the pump are usually not required due to the strong absorption of semiconductor materials. However, the concept with multiple pump passes has recently also been applied to such semiconductor lasers, where it allows for a reduced quantum defect and thus for reduced heating and potentially higher powers [12]. To date it is not clear whether this will lead to more efficient and practical lasers; tens of watts of output power have so far been reached with the original concept, not requiring multiple pump passes. Such lasers are actually very interesting, partly because they can be developed at different wavelengths, e.g. for blue light generation with intracavity frequency doubling at very high output power levels.

Gain Media



Definition: media for laser amplification

Within the context of laser physics, a laser gain medium is a medium which can amplify the power of light (typically in the form of a light beam). Such a gain medium is required in a laser to compensate for the resonatoractive laser medium. It can also be used for application in an optical amplifier. The term gain refers to the amount of amplification. losses, and is also called an

As the gain medium adds energy to the amplified light, it must itself receive some energy through a process called pumping, which may typically involve electrical currents (electrical pumping) or some light inputs (→ optical pumping), typically at a wavelength which is shorter than the signal wavelength.

Types of Laser Gain Media

There are a variety of very different gain media; the most common of them are:

Compared with most crystalline materials, ion-doped glasses usually exhibit much broader amplification bandwidths, allowing for large wavelength tuning ranges and the generation of ultrashort pulses. Drawbacks are inferior thermal properties (limiting the achievable output powers) and lower laser cross sections, leading to a higher threshold pump power and (for passively mode-locked lasers) to a stronger tendency for Q-switching instabilities. See the article on laser crystals versus glasses for more details.

The doping concentration of crystals, ceramics and glasses often has to be carefully optimized. A high doping density may be desirable for good pump absorption in a short length, but may lead to energy losses related to quenching processes, e.g. caused by upconversion via clustering of laser-active ions and energy transport to defects.

Important Physical Effects

In most cases, the physical origin of the amplification process is stimulated emission, where photons of the incoming beam trigger the emission of additional photons in a process where e.g. initially excited laser ions enter a state with lower energy. Here, there is a distinction between four-level and three-level gain media.

A less frequently used amplification process is stimulated Raman scattering, involving the conversion of some higher-energy pump photons into lower-energy laser photons and phonons (related to vibrations e.g. of the crystal lattice).

For high levels of input light powers, the gain of a gain medium saturates, i.e., is reduced. This naturally follows from the fact that for a finite pump power an amplifier cannot add arbitrary amounts of power to an input beam. In laser amplifiers, saturation is related to a decrease in population in the upper laser level, caused by stimulated emission.

Thermal effects can occur in gain media, because part of the pump power is converted into heat. The resulting temperature gradients and also subsequent mechanical stress can cause lensing effects, distorting the amplified beam. Such effects can spoil the beam quality of a laser, reduce its efficiency, and sometimes even destroy the gain medium (thermal fracture).

Relevant Physical Properties of Laser Gain Media

A great variety of physical properties of a gain medium can be relevant for use in a laser. The desirable properties include:

Note that in many situations there are partially conflicting requirements. For example, a very low quantum defectgain bandwidth typically means that laser cross sections are smaller than ideal, and that the quantum defect cannot be very small. Disorder in solid-state gain media increases the gain bandwidth, but also reduces the thermal conductivity. A short pump absorption length can be advantageous, but also tends to exacerbate thermal effects. is not compatible with four-level behavior. A large

It is apparent that different situations lead to very different requirements on gain media. For this reason, a very broad range of gain media will continue to remain important for applications, and making the right choice is essential for constructing lasers with optimum performance.

Single-atom Lasers


Definition: lasers with only a single atom as the gain medium

Usually, the gain medium of a laser contains a huge number of laser-active atoms or ions. However, for various reasons it is of interest to study the behavior of a single-atom laser, where a single atom constitutes the whole gain medium. In 2003, Kimble's research group at the California Institute of Technology demonstrated the first realization of a single-atom laser (or one-atom laser) [3]. Cesium atoms were laser cooled and trapped in a magneto-optical trap (MOT) and then released in order to fall downwards. A single cesium atom was then loaded into a far-off-resonance optical trap (FORT), realized between two supermirrors which formed a high-finesse resonator serving as the laser resonator. The resonator length was actively stabilized using an auxiliary laser. The “inversion” of the cesium system (as far as this term makes sense in this regime) was also achieved with optical pumping. Laser emission occurred in the form of two Gaussian beams exiting the laser resonator at the end mirrors.

Such a one-atom laser is not just a miniaturized version of an ordinary laser. Due to the high cavity finesse and the small mode volume, this device operates in the unusual regime of strong coupling between the photons of the light field and the atomic transition. Specifically, the Rabi frequency is well above both the spontaneous emissionRabi cycles before the excitation decays. In this regime, the theoretical predictions from a full quantum description of the dynamics differ strongly from those of a semiclassical model (with classical treatment of the light field) as usually used to describe laser operation. rate and the photon decay rate of the cavity, so that the coupled system can undergo several

The investigation of such devices is of fundamental interest as it makes it possible to test certain predictions of quantum optics. Indeed, the experiments confirmed the prediction that a single-atom laser should have no laser threshold (→ thresholdless lasers), i.e. the laser emission occurs even for the smallest pump powers. Further, the laser output is not in a coherent state as for most other lasers, but consists of nonclassical light. In particular, significant photon antibunching and sub-Poissonian photon statistics could be observed with coincidence measurements, particularly at low pump rates. Much can be learned by comparing various experimental observations with predictions from laser models involving the quantized light–matter interaction. The opportunity to do detailed studies of all these effects makes such efforts worthwhile, even though it is hardly conceivable that a single-atom laser will find any practical application.

Note that there have been single-atom micromasers, where single atoms interacted with a microwave cavity. A significant difference, however, is that in this case one is usually dealing with an atomic beam where different atoms subsequently interact with the light field, even though at each time there is at most one atom in the cavity. In contrast, the single-atom laser really works with just one atom over longer times, as compared with, e.g., the inverse Rabi frequency.

Single-atom lasers should not be confused with atom lasers, which emit coherent matter waves rather than light.

YAG Lasers


Definition: lasers based on YAG (yttrium aluminum garnet) crystals, usually Nd:YAG

The term YAG laser is usually used for solid-state lasers based on neodymium-doped YAG (Nd:YAG, more precisely Nd3+:YAG), although there are other rare-earth-doped YAG crystals, e.g. with ytterbium, erbium, thulium or holmium doping (see below). YAG is the acronym for yttrium aluminum garnet (Y3Al5O12), a synthetic crystal material which became popular in the form of laser crystals in the 1960s. Yttrium ions in YAG can be replaced with laser-active rare earth ions without strongly affecting the lattice structure, because these ions have a similar size.

YAG is a host medium with favorable properties, particularly for high-power lasers and Q-switched lasers emitting at 1064 nm.

YAG lasers are in many cases bulk lasers made from discrete optical elements. However, there are also monolithic YAG lasers, e.g. microchip lasers and nonplanar ring oscillators.

The most popular alternatives to Nd:YAG among the neodymium-doped gain media are Nd:YVO4 and Nd:YLF. Nd:YAG lasers nowadays also have to compete with Yb:YAG lasers (see below).

Properties of Nd:YAG

Nd:YAG is a four-level gain medium (except for the 946-nm transition as discussed below), offering substantial laser gain even for moderate excitation levels and pump intensities. The gain bandwidth is relatively small, but this allows for a high gain efficiency and thus low threshold pump power.

Nd:YAG lasers can be diode pumped or lamp pumped. Lamp pumping is possible due to the broadband pump absorption mainly in the 800-nm region and the four-level characteristics.

energy level structure of the trivalent neodymium ion in Nd:YAG

Figure 1: Energy level structure and common pump and laser transitions of the trivalent neodymium ion in Nd3+:YAG.

The most common Nd:YAG emission wavelength is 1064 nm. Starting with that wavelength, outputs at 532, 355 and 266 nm can be generated by frequency doubling, frequency tripling and frequency quadrupling, respectively. Other emission lines are at 946, 1123, 1319, 1338 and 1444 nm. When used at the 946-nm transition, Nd:YAG is a quasi-three-level gain medium, requiring significantly higher pump intensities.

Nd:YAG is usually used in monocrystalline form, fabricated with the Czochralski growth method, but there is also ceramic (polycrystalline) Nd:YAG available in high quality and in large sizes. For both monocrystalline and ceramic Nd:YAG, absorption and scattering losses within the length of a laser crystal are normally negligible, even for relatively long crystals.

Typical neodymium doping concentrations are of the order of 1 at. %. High doping concentrations can be advantageous e.g. because they reduce the pump absorption length, but too high concentrations lead to quenching of the upper-state lifetime e.g. via upconversionhigh-power lasers. Note that the neodymium doping density does not necessarily have to be the same in all parts; there are composite laser crystals with doped and undoped parts, or with parts having different doping densities. processes. Also, the density of dissipated power can become too high in

Table 1: Some properties of Nd:YAG = neodymium-doped yttrium aluminum garnet.

PropertyValue
chemical formula Nd3+:Y3Al5O12
crystal structurecubic
mass density 4.56 g/cm3
Moh hardness8–8.5
Young's modulus280 GPa
tensile strength200 MPa
melting point1970 °C
thermal conductivity10–14 W / (m K)
thermal expansion coefficient 7–8 × 10−6/K
thermal shock resistance parameter790 W/m
birefringencenone (only thermally induced)
refractive index at 1064 nm1.82
temperature dependence of refractive index 7–10 × 10−6/K
Nd density for 1 at. % doping 1.36 × 1020 cm−3
fluorescence lifetime230 μs
absorption cross section at 808 nm 7.7 × 10−20 cm2
emission cross section at 946 nm 5 × 10−20 cm2
emission cross section at 1064 nm 28 × 10−20 cm2
emission cross section at 1319 nm 9.5 × 10−20 cm2
emission cross section at 1338 nm 10 × 10−20 cm2
gain bandwidth0.6 nm

Other Laser-active Dopants in YAG

In addition to Nd:YAG, there are several YAG gain media with other laser-active dopants:

  • Ytterbium – Yb:YAG emits typically at either 1030 nm (strongest line) or 1050 nm (→ ytterbium-doped gain media). It is often used in, e.g., thin-disk lasers.
  • Erbium – Pulsed Er:YAG lasers, often lamp-pumped can emit at 2.94 μm and are used in, e.g., dentistry and for skin resurfacing. Er:YAG can also emit at 1645 nm [2] and 1617 nm.
  • Thulium – Tm:YAG lasers emit at wavelengths around 2 μm, with wavelength tunability in a range of ∼ 100 nm width.
  • Holmium – Ho:YAG emits at still longer wavelengths around 2.1 μm. Q-switched Ho:YAG lasers are used e.g. to pump mid-infrared OPOs. There are also holmium-doped laser crystals with codopants, e.g. Ho:Cr:Tm:YAG.
  • Chromium – Cr4+:YAG lasers emit around 1.35–1.55 μm and are often pumped with Nd:YAG lasers at 1064 nm. Their broad emission bandwidth makes them suitable for generating ultrashort pulses. Note that Cr4+:YAG is also widely used as a saturable absorber material for Q-switched lasers in the 1-μm region.

Neodymium- or ytterbium-doped YAG lasers in the 1-μm region in conjunction with frequency doublers are often the basis of green lasers, particularly when high powers are required.

السبت، تشرين الثاني ٢٩، ٢٠٠٨

The Argon Ion Laser

Spectra-Physics Model 2020 Argon Ion Laser



Argon Ion Lasers


Definition: gas lasers based on light amplification in ionized argon in a gas discharge

Argon ion lasers are powerful gas lasers, which typically generate multiple watts of optical power in a green or blue output beam with high beam quality.

The core component of an argon ion laser is an argon-filled tube, made e.g. of beryllium oxide ceramics, in which an intense electrical discharge between two hollow electrodes generates a plasma with a high density of argon (Ar+) ions. A solenoid around the tube (not shown in Figure 1) can be used for generating a magnetic field, which increases the output power by better confining the plasma.

argon-ion laser

Figure 1: Setup of a 20-W argon ion laser. The gas discharge with high current density occurs between the hollow anode and cathode. The intracavity prism can be rotated to select the operation wavelength.

A typical device, containing a tube with a length of the order of 1 m, can generate 10 W or 20 W of output power in the green spectral region at 514.5 nm, using several tens of kilowatts of electric power. The dissipated heat must be removed with a water flow around the tube; a closed-circle cooling system often contains a chiller, which further adds to the power consumption. The total wall-plug efficiency is thus very low, usually below 0.1%. There are smaller air-cooled argon ion lasers, generating some tens of milliwatts of output power from several hundred watts of electric power.

The laser can be switched to other wavelengths such as 457.9 nm (blue), 488.0 nm (blue–green), or 351 nm (ultraviolet) by rotating the intracavity prism (on the right-hand side). The highest output power is achieved on the standard 514.5-nm line. Without an intracavity prism, argon ion lasers have a tendency for multi-line operation with simultaneous output at various wavelengths.

There are similar noble gas ion lasers based on krypton instead of argon. Krypton ion lasers typically emit at 647.1 nm, 413.1 nm, or 530.9 nm, but various other lines in the visible, ultraviolet and infrared spectral region are accessible.

Other types of ion lasers are mentioned in the article on gas lasers.

Applications

Multi-watt argon ion lasers can be used e.g. for pumping titanium–sapphire lasers and dye lasers, or for laser light shows. They are rivaled by frequency-doubled diode-pumped solid-state lasers. The latter are far more power efficient and have longer lifetimes, but are more expensive. Argon tubes have a limited lifetime of the order of a few thousand hours. An argon laser may thus be preferable if it is used only during a limited number of hours, whereas a diode-pumped solid-state laser is the better solution for reliable and efficient long-term operation.

Laser safety issues arise both from the high output power of typical ion lasers and from the high voltage applied to the tube.

Krypton/Argon Laser Powers

Argon and Krypton laser single line outputs
Argon Krypton Power
Laser line
(nm)
Laser Line
(nm)
% of Argon
all lines (1)
as % of strongest
line (2)
275.4)
300.3) 6.4 16
302.4)
305.5)
334.0)
337.5)
350.7) 8 57
351.1) 28 70
351.4)
356.4)
363.8)
406.7 3.6 26
413.1 7 51
415.4 1 8
454.6 3 8
457.9 6 15
465.8 3 8
468 2 14
472.7 5 13
476.2 1.6 11
476.5 12 30
482.5 1.6 11
488 32 80
496.5 12 30
501.7 7 18
514.5 40 100
520.8 2.8 20
528.7 7 18
530.9 6 43
568.2 4.4 31
647.1 14 100
676.4 3.6 26
752.5 4.8 34
793-799 1.2 9
Notes: UV lines are combined powers (indicated by parentheses)
(1) Line power expressed as percentage of argon
(2) Line power expressed as percentage of the strongest

What are lasers?

The word "laser" is an acronym for light amplification by stimulated emission of radiation. In most lasers used in ophthalmology, an electric current is passed through a tube that contains an amplifying medium, usually a gas or solid material, which serves to intensify the energy. This energy is emitted as a narrow light beam which, when focused through a microscope, will either cut, burn, or dissolve various tissues.

Different types of lasers emit specific colors of light and are used to treat various eye problems. The lasers are usually named for the amplification materials used. For instance, the carbon dioxide laser is called a CO2 laser, while the YAG laser contains a solid material made up of yttrium, aluminum, and garnet.

Ophthalmic lasers allow precise treatment of a variety of eye problems without risk of infection. Most laser procedures are also relatively painless and can be done on an outpatient basis. This combination of safety, precision, convenience, and reduced cost make lasers one of the most successful medical tools available to physicians.

What are the types of lasers and their uses?

Excimer laser
The Excimer laser is perhaps the best known of all lasers because of its use in laser vision correction surgery such as laser in-situ keratomileusis (LASIK) and photorefractive keratectomy (PRK). The Excimer, or pulsed gas laser, emits an ultraviolet light beam, vaporizing tissue by breaking down molecular tissue bonds in a minute targeted area. It is called a cold laser because it doesn't produce heat that could have harmful effects to the surrounding tissue.

The Excimer laser is precise. Each pulse of the laser removes about 1/500 of the thickness of a human hair. Its precise depth and area control are significant in surgical applications such as refractive vision correction.

YAG laser
An acronym for yttrium-aluminum-garnet, the YAG laser produces short-pulsed, high-energy light beams to cut, perforate, or fragment tissue. This laser may also be called a neodymium-YAG or ND-YAG laser.

Cataract patients often have the misconception that a YAG laser is used to remove their cataracts, but no lasers are used in cataract surgery. This misconception occurs because up to 75 percent of cataract patients develop a condition known as posterior capsular opacification, a clouding of the residual lens capsule left in place after cataract surgery. This gradual loss of vision resembles the symptoms of cataract development, making some people believe that their cataracts have grown back.

The YAG laser is commonly used to vaporize a portion of the capsule, allowing light to pass through to the retina. The procedure is completely painless, takes only a few minutes in the office, and is effective in eliminating the cloudy condition.

Holmium laser
Also known as the infrared holmium YAG laser, this laser is used in a refractive surgery procedure called laser thermal keratoplasty (LTK) to correct mild to moderate cases of farsightedness and some cases of astigmatism. Unlike the Excimer laser, which reshapes the cornea by removing or ablating tissue, the Holmium laser produces infrared light that reshapes the cornea by causing tissue to constrict. The pulsations from the Holmium laser are computer-controlled to produce a pattern of 8 to 16 tiny beams in concentric rings around the periphery of the cornea. The heated fluid in the spots where these beams hit the cornea creates a series of tiny craters. The subsequent shrinkage pulls in the periphery of the cornea, causing the center to bulge, much like tightening a belt, and thus correcting farsightedness.

CO2 laser
The CO2 laser is a specialized laser that is filled with carbon dioxide gas and uses an infrared emission for cutting tissue through heat absorption. It is one of the most common lasers used in surgery and is good for precise cutting and vaporization of tissue, such as that needed in the treatment of superficial lesions or removing small volumes of tissue.

The CO2 laser is used by ophthalmic plastic surgeons to remove fine wrinkles from around the eyes. This laser precisely removes the outermost layer of skin and the underlying dermis, allowing the regrowth of wrinkle-free new skin.

Erbium laser
The Erbium laser, or erbium-YAG laser, is also used in skin resurfacing and is considered to be more precise and accurate than the CO2 laser. It is able to remove finer wrinkles with less damage to the skin. The depth of penetration is about 5 microns compared with the 20 microns typical of the CO2 laser. The Erbium laser also causes less irregular skin pigmentation in darker skinned individuals, because it produces a thinner laser area and less heat. Because the Erbium laser produces minimal thermal scatter, the healing time is less than the healing time with the CO2 laser.

The Erbium laser is also being used in a promising new clinical procedure to emulsify the eye's natural lens during cataract surgery. Most cataract surgeons currently use a piece of equipment called a phacoemulsifier to break up and remove the cloudy lens. The Erbium laser was chosen for the new technique because of its high absorption rate in water, a primary component of the eye's natural crystalline lens.

Argon laser
The argon laser is filled with argon gas that produces blue/green wavelengths. These particular wavelengths are absorbed by the cells that lie under the retina and by the red hemoglobin in blood, but the blue-green wavelengths can pass through the fluid inside the eye without damage. For this reason, the argon laser is used extensively in the treatment of diabetic retinopathy, a severe disorder of the retina that causes blood vessels to leak. The argon laser can burn and seal these blood vessels.

Retinal detachment is another serious eye problem that can be treated by the argon laser. The laser is used to weld the detached retina to the underlying choroid layer of the eye.

Several forms of glaucoma, which is a leading cause of blindness, are also treated with argon lasers. The very serious angle closure glaucoma, for instance, is sometimes treated by using the laser to create a tiny opening in the iris, allowing excess fluid inside the eye to drain to reduce pressure.

Macular degeneration, a severe condition that affects central vision in older adults, is sometimes treated with an argon or krypton laser. In this treatment, the laser is used to destroy abnormal blood vessels so that hemorrhage or scarring will not damage central vision.

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