Lecture 5. Physical Basis of Laser Technology Fedor A. Gubarev
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Lecture 5. Physical Basis of Laser Technology Fedor A. Gubarev 2017 1 How Laser Work The lasing medium will normally emit photons in specific spectral lines when excited by an energy source. The different quantum levels, or energy states, of the material, determine the wavelength. Normally, most atoms in a medium are in the ground state. Some small percentage will exist at higher energies as well. Normally, these higher energy states are unstable and the electrons will release this excess energy as photons almost immediately and return to the ground state. 2 How Laser Work Energy is supplied to the laser medium by the energy pumping system. This energy is stored in the form of electrons trapped in the metastable energy levels. Pumping must produce a population inversion (i.e., more atoms in the metastable state than the ground state) before laser action can take place. When population inversion is achieved, the spontaneous decay of a few electrons from the metastable energy level to a lower energy level starts a chain reaction. The photons emitted spontaneously will hit (without being absorbed)other atoms and stimulate their electrons to make the transition from the metastable energy level to lower energy levels ‐ emitting photons of precisely the same wavelength, phase, and direction. This action occurs in the optical cavity. When the photons that decay in the direction of the mirrors, (most are lost ‐ lasers are not as efficient as one would believe) reach the end of the laser material, they are reflected back into the material where the chain reaction continues and the number of photons increase. When the photons arrive at the partially reflecting mirror, only a portion will be reflected back into the cavity and the rest will emerge as a laser beam. 3 Gas Lasers Carbon dioxide laser Invented in 1964, the CO2 gas laser is still one of the most widely used medical lasers. It contains a mixture of helium (60–80%), nitrogen (∼25%) and CO2 (∼5%). The gas is excited with either an electrical ac or dc discharge or with a radio frequency (RF) field. It is broken down to CO and O2 and is usually replenished by a continuous flow, or the gas products are allowed to recombine in sealed tubes. Carbon dioxide lasers can be made with emission up to several kilowatts, but 10–20 W are sufficient for most surgical procedures. 4 Gas Lasers Carbon dioxide laser The wavelength is 10.6 µm and the beam is of high quality with respect to monochromaticity and collimation. They operate either in the CW or in pulsed mode, depending on the application. The radiation of this laser cannot be transmitted through standard silica optical fibers. However, a system of mirrors, hollow waveguides or fibers made of metal halides, e.g. thallium bromide, can be used. The efficiency is high: 10–15% of the input power is converted to laser emission. Since the infrared radiation at 10.6 µm is invisible, He‐Ne lasers are used to generate aiming beams. The penetration depth in water is about 10 µm, and the penetration depths in tissues are largely dependent on the water content. The laser is used for a number of surgical‐, ophthalmologic‐ and cosmetic applications. 5 Gas Lasers Carbon monoxide laser The carbon monoxide (CO) laser operates in the CW mode and has emission lines between 5 and 6 µm. Light in this wavelength region is strongly absorbed by tissues, and the laser has been used for thermal welding of vessels. Argon‐ and krypton ion lasers Unlike the carbon oxide lasers, the gases in these lasers must be ionized by electrical discharge to operate. They are not very efficient and a large fraction of the input energy is lost as heat, which requires efficient cooling systems. The Ar ion laser has its main output lines at 488 and at 514.5 nm, while the main lines of the krypton laser are at 530.9, 568.2 and 676.4 nm. While the argon ion laser can give outputs larger than 20 W, the output at each of the krypton lines is lower than 10 W. The emission from these lasers is mainly in the visible range, can be transmitted through optical fibers and is absorbed by numerous tissue chromophores. Notably, hemoglobin absorbs argon lines strongly. Therefore, the laser has excellent coagulative properties and can be used for vaporization of pigmented lesions in the skin, endometric lesions and retina. 6 Gas Lasers Carbon monoxide laser The carbon monoxide (CO) laser operates in the CW mode and has emission lines between 5 and 6 µm. Light in this wavelength region is strongly absorbed by tissues, and the laser has been used for thermal welding of vessels. Argon‐ and krypton ion lasers Unlike the carbon oxide lasers, the gases in these lasers must be ionized by electrical discharge to operate. They are not very efficient and a large fraction of the input energy is lost as heat, which requires efficient cooling systems. The Ar ion laser has its main output lines at 488 and at 514.5 nm, while the main lines of the krypton laser are at 530.9, 568.2 and 676.4 nm. While the argon ion laser can give outputs larger than 20 W, the output at each of the krypton lines is lower than 10 W. The emission from these lasers is mainly in the visible range, can be transmitted through optical fibers and is absorbed by numerous tissue chromophores. Notably, hemoglobin absorbs argon lines strongly. Therefore, the laser has excellent coagulative properties and can be used for vaporization of pigmented lesions in the skin, endometric lesions and retina. 7 Gas Lasers Helium–neon laser The basic constituents of this laser are helium and neon. It operates in a continuous mode and has an average output of a few milliwatts. The laser can be made emitting at various wavelengths, e.g. 543 nm (green), 594 nm. (yellow) and 633 nm (red). Its reliability, lightweight and good beam qualities make it suitable for alignment and analytical purposes. Excimer lasers In 1971, the first excimer laser was discovered using a xenon dimer (Xe2)excitedbyan electron beam to produce stimulated emission at 172 nm wavelength. Excimer is a short form of the expression ‘excited dimers’. Some molecules, such as rare gas halides (ArF, KrF, XeCl, XeF), are stable only in their excited states, and not in their ground state. The laser medium consists of such molecules in a buffer gas (helium or neon). They have low efficiency (∼2%), give short pulses (10–100 ns) and emit in the UV region. Typical wavelengths are 193 nm (ArF), 249 nm (KrF), 308 nm (XeCl) and 351 nm (XeF). The penetration of radiation at these wavelengths in tissues is low, which makes the lasers suitable for many surgical applications. UV radiation, notably at 193 and 249 nm, can ionize molecules in tissue, break bonds and lead to abrasive reactions. High energy (average power up to 200 W) and short pulses are excellently suited for this including some applications in the field of ophthalmology. 8 Metal Vapour Lasers Copper vapour and gold vapour lasers The lasing medium of these lasers is a mixture of neon and the vapour of a metal, usually copper or gold. They are excited by electric discharge, have rather low efficiencies (less than 1%), emit a few tens of watts at 511 and 578 nm (copper) and 628 nm (gold). To keep the metals in a vaporized state the temperature must be kept above 1500 ◦C. The gold lasers have been used for photosensitizer‐mediated photodynamic therapy (PDT), since the porphyrin photosensitizers most frequently used in this therapy have an absorption band around 630 nm. The copper vapour laser has been used as a pump source for dye lasers or for surgery and tissue destruction, since its lines overlap absorption peaks of hemoglobin. Both lasers are pulsed with repetition rates around 5– 10 kHz. Helium–cadmium The lasing medium of these laser is mixture of Helium–cadmium HeCd. They are excited by electric discharge in metal in metal vapor mixed with helium buffer gas. Wavelength of Helium–cadmium laser is 441.563 nm to 325 nm. This kind of laser is used in Printing and typesetting applications and some scientific research. 9 Metal Vapour Lasers Strontium and Calcium Vapor Lasers The peculiarity of lasers on the vapors of strontium and calcium, is the fact that in their active media, both ionization and recombination mechanisms of inversion formation are realized with high efficiency. In particular, the self‐limited generation regime is realized on a number of IR transitions in the spectra of both atoms and ions of strontium and calcium, and the recombination regime on ionic transitions of strontium and calcium in the violet and UV ranges. In this case, it is possible to realize simultaneous multiwave generation in a wide range of the spectrum due to both mechanisms of inversion formation. Free electron lasers Free electrons emit radiation when forced to change direction by magnetic fields. When electrons accelerated to relativistic speed (i.e. speed close to that of light) wiggle through a periodic array of magnetic fields of alternating directions, emission occurs. The coherent emission, which is mainly in the forward direction, is of the same kind as the non‐coherent emission from cyclotrons. Radiation wavelengths from the x‐ray to the microwave region can in principle be obtained with pulse durations ranging from almost infinite (CW) to sub picoseconds. A number of medical applications are possible and ablation of tissue in ophthalmologic, otolaryngologic, neurosurgical applications and wound healing applications has been proposed.