Учебно-методическое пособие по чтению специальной литературы для студентов 1 курса физического факультета. Дроздова И.В - 26 стр.

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26
happen only to upper-state atoms, and absorption can happen only to lower-
state atoms. For stimulated emission to dominate absorption, therefore, more
atoms must be in the upper state than in the lower state. This unusual situation
is called population inversion and can be achieved by supplying energy
(pumping the laser) and carefully selecting the active medium. Typical
pumping schemes include the use of light from flash lamps or other lasers,
collisions of the atoms with electrically accelerated electrons in a gas discharge
tube, excitation with energetic particles from nuclear reactions, chemical
reactions, and direct electrical input to semiconductor. Continuous lasing is
harder to achieve than pulsed lasing.
It is necessary to ensure that the stimulating light is sufficiently strong.
Stimulated emission then occurs in a time interval that is short compared to the
spontaneous lifetime of the existed state. This situation is achieved by keeping
a fraction of the laser light trapped between two mirrors enclosing the active
medium. Domination of stimulated emission over spontaneous emission
becomes more difficult to achieve as the spontaneous lifetime becomes
shorter. Because shorter spontaneous lifetimes are associated with states that
emit radiation of higher frequencies, it is difficult to make an ultraviolet-
emitting laser, and an X-ray laser was not successfully demonstrated until
1984. Despite this complexity of construction, however, ultraviolet lasers, or
excimers, have gained widespread use in industry. Emitting ultraviolet light
when a halogen and rare gas atom combine temporarily, they are used in
applications ranging from glass etching and photolithography to sterilization of
wines.
Atoms initially in a lower state are raised to the upper state by energy from a
flash lamp or some other pumping source. Some of these atoms emit light
spontaneously in random directions. Light traveling perpendicular to the
mirrors stays within the active medium long enough to stimulate emission from
other atoms, whereas light traveling in other directions is soon lost. The light
amplified by stimulated emission is now more intense and more likely to
stimulate further emission. Some light reaching the output mirror is transmitted
to form the laser beam; some is reflected back through the medium to continue
the stimulated-emission process.
4. The fundamental principles underlying the operations of the maser and laser
were established long before these devices were successfully demonstrated:
stimulated emission was proposed by Albert Einstein in 1916, and population
inversion was discussed by V.A. Fabrikant in 1940. These fundamental ideas,
followed by two decades of intensive development of microwave technology,
set the stage for the maser, an ammonia maser, constructed in 1954 by J.P.
Gordon, H.J. Zeiger, and Charles H. Townes. Over the next 6 years many
workers, including Nickolai G. Basov, Aleksander M. Prokhorov, Arthur L.
Schawlow, and Townes, made important contributions that helped to extend
                                         26

happen only to upper-state atoms, and absorption can happen only to lower-
state atoms. For stimulated emission to dominate absorption, therefore, more
atoms must be in the upper state than in the lower state. This unusual situation
is called population inversion and can be achieved by supplying energy
(“pumping” the laser) and carefully selecting the active medium. Typical
pumping schemes include the use of light from flash lamps or other lasers,
collisions of the atoms with electrically accelerated electrons in a gas discharge
tube, excitation with energetic particles from nuclear reactions, chemical
reactions, and direct electrical input to semiconductor. Continuous lasing is
harder to achieve than pulsed lasing.

It is necessary to ensure that the stimulating light is sufficiently strong.
Stimulated emission then occurs in a time interval that is short compared to the
spontaneous lifetime of the existed state. This situation is achieved by keeping
a fraction of the laser light trapped between two mirrors enclosing the active
medium. Domination of stimulated emission over spontaneous emission
becomes more difficult to achieve as the spontaneous lifetime becomes
shorter. Because shorter spontaneous lifetimes are associated with states that
emit radiation of higher frequencies, it is difficult to make an ultraviolet-
emitting laser, and an X-ray laser was not successfully demonstrated until
1984. Despite this complexity of construction, however, ultraviolet lasers, or
excimers, have gained widespread use in industry. Emitting ultraviolet light
when a halogen and rare gas atom combine temporarily, they are used in
applications ranging from glass etching and photolithography to sterilization of
wines.

Atoms initially in a lower state are raised to the upper state by energy from a
flash lamp or some other pumping source. Some of these atoms emit light
spontaneously in random directions. Light traveling perpendicular to the
mirrors stays within the active medium long enough to stimulate emission from
other atoms, whereas light traveling in other directions is soon lost. The light
amplified by stimulated emission is now more intense and more likely to
stimulate further emission. Some light reaching the output mirror is transmitted
to form the laser beam; some is reflected back through the medium to continue
the stimulated-emission process.

4.   The fundamental principles underlying the operations of the maser and laser
were established long before these devices were successfully demonstrated:
stimulated emission was proposed by Albert Einstein in 1916, and population
inversion was discussed by V.A. Fabrikant in 1940. These fundamental ideas,
followed by two decades of intensive development of microwave technology,
set the stage for the maser, an ammonia maser, constructed in 1954 by J.P.
Gordon, H.J. Zeiger, and Charles H. Townes. Over the next 6 years many
workers, including Nickolai G. Basov, Aleksander M. Prokhorov, Arthur L.
Schawlow, and Townes, made important contributions that helped to extend