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Gl *Bandwidth The difference between the frequency limits of a band containing the useful frequency components of a signal. Coherent lightRadiant electromagnetic energy of the same, or almost the same, wavelength, and with definite phase relationships between different points in the field. *Continuous wave laser A laser in which the beam of coherent light is generated continuously, as required for communica- tion and certain other applications. Dispersion In laser telecommunication, the spreading or broadening of light pulses as they pass through an optical fiber. Dispersion limits the rate at which light carrier pulses can be transmitted and decoded without error. *Electro-opticsThe study of the influence of an electric field on optical phenomena. See optoelectronics. *Excimer laser- A laser containing a noble gas, such as helium or neon, which is based on a transition between an excited state in which a metastable bond exists between two gas atoms and a rapidly dissociating ground state. *Gas laser- A laser in which the active medium is a discharge in a gas contained in a glass or quartz tube with a Brewster-angle window at each end; the gas can be excited by a high-frequency oscillator or direct-current flow between electrodes inside the tube; the function of the discharge is to pump the medium, to . . . . O stain popu. .atlon inversion. *SOURCE: McGraw-Hill Dictionary of Scientific & Technical Terms, 3rd ea., 1984. New York: McGraw-Hill. Reprinted with permission. _~ 133

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1 34 GLOSSARY 1 *Hydrogen maser -A maser in which hydrogen gas is the basis for providing an output signal with a high degree of stability and spectral purity. *Klystron An evacuated electron-beam tube in which an initial velocity imparted to electrons in the beam results subse- quently in density modulation of the beam; used as an amplifier or oscillator for microwave radiation. Laser A device that uses the maser principle of amplification of electromagnetic waves by stimulated emission of radiation and operates in the ultraviolet, optical, or infrared region of the spectrum. Derived from light amplification by stimulated . . ~ . . . emlsslon ot raulatlon. *Laser spectroscopyA branch of spectroscopy in which a laser is used as an intense, monochromatic light source. Maser An acronym formed from microwave amplification by stimulated emission of radiation. Molecular beam epitaxy A technique for growing single crys- tals of compound semiconductor thin films through the mol- ecule-by-molecule deposition of material from a molecular beam. The structure of the film is determined by the crystal structure and orientation of the underlying substrate. Neodymium-g~ass laser A laser that produces intense light pulses in the near-infrared portion of the spectrum, specifi- cally with a wavelength of about 1 Em. *Optical fiberA long, thin thread of fused silica, or other transparent material, used to transmit light. Also known as a light guide. *Optoelectronics The branch of electronics that deals with solid- state and other electronic devices for generating, modulating, transmitting, and sensing electromagnetic radiation in the ultra- violet, visible-light, and infrared portions of the spectrum. *Quantum electronics The branch of electronics associated with the various energy states of matter, motions within atoms or groups of atoms, and various phenomena in crystals. Practical applications include the atomic hydrogen maser and the cesium atomic-beam resonator. *Ruby laser An optically pumped solid-state laser using a ruby crystal (A1203) doped with chromium (Cr+3) impurities. Laser emission occurs in the red part of the optical spectrum. Semiconductor laserA laser in which stimulated emission of coherent light occurs in a semiconductor when excited by carrier injection, electron-beam excitation, impact ionization, optical excitation, or other means. The most common form is the diode laser, in which electrons and holes are driven into a pn junction and combine there.