Laser is acronym for light amplification by the stimulated emission ofradiation. Laser is a device that generates a beam of light that is collimated, monochromatic, and coherent. Laser radiation is characterized by its wavelength, power, and pulse – or continuous wavemode of generation. Any CW lasers can work in the pulse mode by a switch-on and switch – off pumping power but many pulse lasers cannot work in CW. Normally, lasers are characterized by the output wavelength (nm or pm), spectral bandwidth (nm), energy characteristics such as power (mW, W, kW) for CW laser, and energy per pulse (J), pulsewidth (ns, ps, ms, s), repetition rate (Hz), and average power (mW, W, kW) for pulse lasers. An important practical characteristic of a laser is its efficiency, which is the ratio of the output laser power to the input electrical power of laser pumping and expressed in percentage. Lasers of higher efficiency normally have the smallest size and lower cost.
To provide a high precision of laser beam focusing and to transport its radiation through the single-mode fibers, single-mode lasers are used. Such lasers produce a light beam with a Gaussian shape of the transverse intensity profile without any spatial oscillations, the so – called single transverse mode lasers. In general, such lasers generate many optical frequencies (so-called longitudinal modes), which have the same transverse Gaussian shape.
A pulse laser is a laser that generates a single pulse or a set of pulses. Laser pulses can be produced by simple switching the pumping power on and off. However, two technologies are typically used to produce a special laser pulsing, they are: Q-switching and mode-locking. The Q-switching, sometimes known as giant pulse formation, is a technique by which a laser can be made to produce a pulsed output beam with a very high power. The technique allows for the production of light pulses with extremely high (gigawatt) peak power, much higher than would be produced by the same laser if it is operating in a CW mode.
A mode-locked laser is a multimode laser with synchronously irradiating modes, and the regime is obtained by applying an intracavity high-frequency modulator, with typical pulse duration of up to a subpicosecond range, and a repetition frequency of dozens of megahertz.
A cavity-dumped mode-locked laser is a laser that uses a specific technology for producing high-energy ultrashort laser pulses by decreasing the pulse repetition rate. The laser output mirror is replaced by an optical selector consisting of a couple of spherical mirrors and an acousto – or electrooptical deflector, which extracts a pulse from the cavity after it has passed over a few dozen cavity lengths. The pulse energy is accumulated between two sequential extractions: the pulse repetition rate can be tuned in to the range from dozens of hertz to a few megahertz.
Compared to mode-locking, Q-switching leads to much lower pulse repetition rates, much higher pulse energies, and much longer pulse durations; both techniques are sometimes applied simultaneously.
Most lasers emit at a particular wavelength, in tunable lasers, one can vary the wavelength over some limited spectral range.
There are a huge variety of lasers and laser systems available in the market. Lasers can be classified in accordance with the active media used, such as gas, solid state, liquid, and semiconductor (diode) lasers. For example, a gas laser is a laser whose active medium is a gas or mixture of gases. We briefly present the most popular lasers used in medicine.
CO2 (carbon dioxide) laser—a laser in which the lasing medium is CO2 gas with an IR emission from 9.2 to 11.1 pm with the maximal efficiency at 10.6 pm and a power from a few watts to a few kilowatts. Both CW and pulsed regimes are available. Lasers are tunable in the limits of CO2 molecules spectral range (9.2-11.1 pm). CO2 laser has a very high efficiency, up to 40%. Because of a high absorption of tissues in this wavelength range, CO2 laser is mostly used for tissue ablation.
Excimer laser—a laser whose lasing medium is an excited molecular complex, an exci – mer (molecule-dimer). The emission is in the UV range. Examples are: ArF laser, 193 nm; KrF laser, 248 nm; XeCl laser, 308 nm; and XeF laser, 351 nm. These lasers are tunable in some limits (10-20 nm). Because of a high absorption by tissues in the UV range, excimer lasers are widely used for tissue ablation with a high precision in both directions: in tissue depth and transversely. Eye refractive surgery technologies are based on these lasers.
Dye laser—a laser in which the laser medium is a liquid dye. Dye lasers emit in a broad spectral range (e. g., in the visible), and are tunable. Wavelengths range is from 340 to 960 nm, at optical frequency doubling—from 217 to 380 nm, and at parametric conversion—from 1060 to 3100 nm. Its emitted energy is from 1 mJ to 50 J in periodic pulse mode. The mean power is from 0.06 to 20 W. Pulse duration is from several nanoseconds to several microseconds and pulse frequency from a single pulse to 1 kHz. Train of microsecond pulses can be used to generate millisecond pulses. It is used in spectroscopy and photochemistry of biological molecules and is one of the best lasers for blood vessels coagulation.
A solid-state laser has an active medium as a matrix of crystal, glass or ceramic doped by active ions. Different crystal matrices, such as sapphire, yttrium aluminum garnet (YAG), alexandrite, yttrium scandium gallium garnet (YSGG) and others are used in lasers. Active ions can be Nd (neodymium), Cr (chromium), Er (erbium), Ho (holmium), Tm (thulium) and others. Active ions in different matrices have different laser wavelengths. For example Cr3+ doping sapphire (ruby laser) provides laser wavelength of 694 nm, but the same ions doping alexandrite crystal (alexandrite laser) give laser wavelength of 755 nm. Solid-state lasers are pumped by optical radiation from a flash (arc) lamp or from other laser, for example, a diode laser. Efficiency of flash lamp pumped laser is about 0.1-5%. Diode laser pumped solid-state lasers have an efficiency in the range of 10-50%.
Nd:YAG (neodymium:yttrium aluminum garnet) laser is one of the most efficient solid – state lasers whose lasing medium is the crystal Nd:YAG with emission in the NIR at 1064 nm; other less intensive lines at 946, 1319, 1335, 1338, 1356, and 1833 nm are also available. This laser is often used at optical frequency doubling—532 nm, the third harmonic of the radiation (355 nm) is also widely applicable in photomedicine. Both CW and pulsed regimes are available. Typical power of the main harmonic (1064 nm) is from a few watts to a few hundred watts in CW mode. Pulsed lasers are characterized by a high repetition rate, up to 300 Hz; their pulse duration varies from a few nanoseconds to hundred milliseconds, and the pulse energy is 0.05-100 J; the pulse power amounts up to several megawatts, the average power up to 1000 W. Several others lasers are based on active Nd ions, for example, neodymium:yttrium aluminum perovskite laser (Nd:YAP). This laser emits at wavelengths l = 1054, 1341 nm, and other wavelengths. Nd laser is one of most popular lasers in photomedicine.
Er:YAG (erbium:yttrium aluminum garnet) laser—a solid-state laser whose lasing medium is the crystal Er:YAG with emission in mid-IR at 2.79-2.94 ^m—is one of the most effective lasers for ablation of different tissues, including skin and hard tissues, because of its unique wavelength that coincides with the strongest absorption band of water (normal oscillatory modes of water molecules, l = 2.94 ^m). Typical power range is from a few watts to a few tenths of watts. For miniature systems (a crystal 4 mm in diameter and 75 mm long), the pulse duration in the free-run regime is in the microsecond range with the pulse-repetition rate of 25 Hz, pulse energy of a few joules and average power of a few watts. In the Q-switching regime the pulse duration is in the nanosecond range with a pulse energy of ~ 100 mJ.
A diode laser is a semiconductor injection laser. This laser is pumped by electrical current through a multilayered semiconductor structure (heterostructure), including a so-called quantum well heterostructure that maximizes a laser’s optical mode overlap and injected carriers. The optical mode overlap is optimized with the gain to produce lasers with lower threshold currents. One of the widely used diode lasers is GaAs (gallium arsenide) laser—a laser based on the semiconductor material GaAs; the emission is in the NIR, at about 830 nm. More complex compositions allowing one to have the desired wavelength and output power are also designed: GaP^As1-x lasers [emit light from 640 nm (x = 0.4) to 830 nm (x = 0)]. GaxIn1-xAs^P1_y lasers, at y = 2.2xand for different values of x, emit in the range 920-1500 nm. These lasers emit light in range up to 2000 nm. PbxS1-x, SnxPb1-xTe and SnxPb1-xSe lasers, for different values of x, emit in the range 2.5-49 ^m. GaN (gallium nitride) laser emits in the short wavelength range from 360 to 450 nm.
A single diode laser emitter has a typical size of laser aperture of 1.5-100 ^m and cavity length of 0.5-3 mm. The maximum output power of a single diode laser emitter is in the range from 0.5 to 10 W. High-power diode laser is usually a plate planar array of laser bar with laser aperture up to 10 mm. One laser bar comprises 10-90 single laser emitters. Maximum output power of a diode laser bar is in the range from twenty to several hundred watts. Diode lasers are the most efficient lasers with efficiency up to 70%. Diode lasers have a very high beam divergence: 50°-90° in a fast axis and 5°-20° in a slow axis. Special micro optics is necessary to form a low divergence beam or for coupling diode laser power into the fiber. Diode laser can work in the CW mode or in pulse mode by pulsing pumping electrical current.
Diode lasers are used for pumping of other lasers, that allows one to produce very robust and compact totally solid-state systems, such as a diode-pumped Nd:YAG, which is an integrated solid-state laser with a Nd:YAG crystal as a lasing medium and optical pumping provided by a single-diode lasers or by a diode bars. Another example of such a system is a so-called fiber laser. In fiber laser active medium is glass or crystal fiber with core doped by active ions such as Nd, Yt, Er, Tm. Diode laser power is injected in the cladding of such fiber for pumping of active ions. Fiber lasers have a very high efficiency and the best quality of laser beam.