Alexandrite permits deeper penetration into skin than the ruby, and is used for Laser Hair Reduction. Before you can understand how a laser works, you need to know how an atom can give off light. In laser ranging, a fast laser pulse is sent to a corner reflector at the location to be mapped, and the delay in return of the pulse is precisely measured to obtain the distance from the laser location. A second type of light exists, however, and occurs when an atom or molecule retains its excess energy until stimulated to emit the energy in the form of light. Figure 4(b) illustrates a four-level scenario. The amplification of light by stimulated emission is a fundamental concept in the basic understanding of laser action. Resonance is possible at each integral wavelength increment (for example 200,000, 200,001, 200,002, etc. For example, a gain of 1.5 per centimeter means that a photon generates 1.5 additional photons for every centimeter it travels. The important point is that the amount of amplification increases sharply with the distance traveled through the laser medium. Many of the primary concepts for producing a laser were developed at about the same time, later in the 1950s, by Townes and Arthur Schawlow (of Bell Laboratories), and by Gordon Gould at Columbia University. When the cavity is not an exact multiple of the lasing wavelength, destructive interference will occur, destroying laser action. Several types of transition occur and affect the amount of energy involved in a transition. Although lasers emitting visible light are the most common, the basic principles are applicable through much of the electromagnetic spectrum. The tutorial shows the slow build-up of light energy within the tube prior to establishing a steady state of laser discharge. Although the three-level laser system works for all practical purposes, as exemplified by Maiman's first laser, a number of problems limit the effectiveness of this approach. Interestingly, the photographer commissioned by Hughes to publicize the discovery thought that the actual laser was too small and had Maiman pose with a larger laser that was not operational until later. The ratio of the number of atoms at two energy levels (1 and 2) under thermodynamic equilibrium is given by the following equation: where N(1) and N(2) are the number of atoms in level 1 and level 2, respectively, E(1) and E(2) are the energies of the two levels, k is the Boltzmann constant, and T is the temperature in kelvins. In some systems, with a high-energy laser transition, a great deal of energy must be expended just to raise the laser species to the appropriate level, far above the ground state. 0000038713 00000 n How lasers work. The average lifetime before spontaneous emission occurs for a metastable state is on the order of a microsecond to a millisecond, quite a lengthy period of time on the atomic timescale. Maiman's ruby laser emitted intense pulses of coherent red light at 694 nanometers in a tightly concentrated narrow beam, quite typical of the characteristics displayed by many current lasers. 4 1 Lasers: Fundamentals, Types, and Operations N N E 0 E E 1 E 3 E 0 E E 1 2 Fast decay Fast decay Fast decay Pumping Pumping Lasing Lasing (a) (b) Figure 1.1 Energy level diagram for (a) three- and (b) four level laser systems. Wavelengths of light are extremely small compared to the length of a typical laser cavity, and in general, a complete roundtrip path through the cavity will be equivalent to several hundred thousand wavelengths of the light being amplified. The reason that stimulated emission is difficult to achieve becomes apparent when considering the likely events surrounding the decay of an electron from an exited state with the subsequent and spontaneous emission of light. In reality, the output mirror always transmits a constant fraction of the light as the beam, reflecting the rest back into the cavity. This results in an amplification factor that increases with the path length of the laser cavity. The ground-state molecules were discarded, and the separated excited molecules constituted the required population inversion. 0000002934 00000 n Figure 6(a) shows the beginning of stimulated emission, which is amplified in Figure 6(b) through Figure 6(g) as the light is reflected from the mirrors positioned at the cavity ends. Lasers are designed to produce and amplify this stimulated form of light into intense and focused beams. Thomas J. Fellers and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310. Diffraction is usually described as a beam-spreading effect that results from the formation of diffraction rings (termed Airy rings) that surround a beam when the light waves have passed through a small opening.
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