الأحد، تشرين الثاني ٣٠، ٢٠٠٨

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 emission Rabi 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 upconversion high-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.

Property	Value
chemical formula	Nd³⁺:Y₃Al₅O₁₂
crystal structure	cubic
mass density	4.56 g/cm³
Moh hardness	8–8.5
Young's modulus	280 GPa
tensile strength	200 MPa
melting point	1970 °C
thermal conductivity	10–14 W / (m K)
thermal expansion coefficient	7–8 × 10⁻⁶/K
thermal shock resistance parameter	790 W/m
birefringence	none (only thermally induced)
refractive index at 1064 nm	1.82
temperature dependence of refractive index	7–10 × 10⁻⁶/K
Nd density for 1 at. % doping	1.36 × 10²⁰ cm⁻³
fluorescence lifetime	230 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 bandwidth	0.6 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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