Учебно-методическое пособие по чтению специальной литературы для студентов 1 курса физического факультета. Дроздова И.В - 25 стр.

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     beams are called incoherent. A laser is useful because it produces light that is
     not only of essentially a single frequency but also coherent, with the light
     waves all moving along together in unison. The MASER, using the same
     principle of operation, generates or amplifies electromagnetic radiation in the
     longer-wavelength microwave region of the electromagnetic spectrum.

2.   A laser is made up of several basic components. One is the so-called active
     medium, which may consist of atoms of a gas, molecules in a liquid, ions in a
     crystal, or any of several other possibilities. Another component consists of
     some method of introducing energy into the active medium. Such as a flash
     lamp, for example. The third basic component is a pair of mirrors placed on
     either side of the active medium, one of which transmits part of the radiation
     that strikes it. In the following discussion the active component is taken to be a
     gas.

     Each atom in the active medium of a gas laser is characterized by a set of
     energy states, or energy levels, in which it may exist. These states may be
     pictured as unevenly spaced rungs of a ladder, with higher rungs representing
     states of higher energy. Left undisturbed for a long enough time, an atom will
     fall to its lowest state. This is called the ground state. As a simple example,
     suppose that an atom has only two energy states that differ by a certain amount
     of energy. Then consider how this atom interacts with light. According to
     QUANTUM MECHANICS, the atom will interact with light of only one
     particular frequency (determined by a relationship involving a physical
     constant known as PLANCK’S CONSTANT).

     Three kinds of interactions can take place between the atom of gas in a laser
     and light. Either the light is absorbed, or spontaneous emission occurs, or
     stimulated emission occurs. That is, an atom in its lower energy state can
     absorb light and be excited to its upper state. If the atom is instead in its upper
     energy state, it can fall spontaneously to its lower state and emit light in the
     process. The third possibility is that the atom is stimulated by the presence of
     light to jump down to its lower energy state, emitting additional light while
     doing so.

     Spontaneous emission is unaffected by the presence of light and occurs on a
     time scale characteristic of the states involved. This time is called the
     spontaneous lifetime. In stimulated emission the additional light emitted has
     the same frequency and directional characteristics as the light that stimulates it.
     This is the crucial feature on which the properties of the laser are based. In
     order for the laser to work effectively, stimulated emission must predominate
     over both absorption and spontaneous emission.

3.   The probabilities of occurrence of stimulated emission and absorption are both
     proportional to the intensity of the light. Stimulated emission, however, can