Lasers are devices that
produce intense beams of light which are monochromatic, coherent, and highly
collimated. The wavelength (color) of laser light is extremely pure
(monochromatic) when compared to other sources of light, and all of the photons
(energy) that make up the laser beam have a fixed phase relationship
(coherence) with respect to one another. Light from a laser typically has very
low divergence. It can travel over great distances or can be focused to a very
small spot with a brightness which exceeds that of the sun. Because of these
properties, lasers are used in a wide variety of applications in all walks of
life. The basic operating principles of the laser were put forth by Charles
Townes and Arthur Schalow from the Bell Telephone Laboratories in 1958, and the
first actual laser, based on a pink ruby crystal, was demonstrated in 1960 by
Theodor Maiman at Hughes Research Laboratories. Since that time, literally
thousands of lasers have been invented (including the edible “Jello” laser),
but only a much smaller number have found practical applications in scientific,
industrial, commercial, and military applications. The helium neon laser (the
first continuous-wave laser), the semiconductor diode laser, and air-cooled ion
lasers have found broad OEM application. In recent years the use of
diode-pumped solid-state (DPSS) lasers in OEM applications has been growing
rapidly. The term “laser” is an acronym for (L)ight (A)mplification by
(S)timulated (E)mission of (R)adiation. To understand the laser, one needs to
understand the meaning of these terms. The term “light” is generally accepted
to be electromagnetic radiation ranging from 1 nm to 1000 mm in wavelength. The
visible spectrum (what we see) ranges from approximately 400 to 700 nm. The
wavelength range from 700 nm to 10 mm is considered the near infrared (NIR),
and anything beyond that is the far infrared (FIR). Conversely, 200 to 400 nm
is called ultraviolet (UV); below 200 nm is the deep ultraviolet (DUV). To
understand stimulated emission, we start with the Bohr atom. THE BOHR ATOM In
1915, Neils Bohr proposed a model of the atom that explained a wide variety of
phenomena that were puzzling scientists in the late 19th century. This simple
model became the basis for the field of quantum mechanics and, although not
fully accurate by today’s understanding, still is useful for demonstrating
laser principles. In Bohr’s model, shown in figure 36.1, electrons orbit the
nucleus of an atom. Unlike earlier “planetary” models, the Bohr atom has a
limited number of fixed orbits that are available to the electrons. Under the
right circumstances an electron can go from its ground state (lowest-energy
orbit) to a higher (excited) state, or it can decay from a higher state to a
lower state, but it cannot remain between these states. The allowed energy
states are called “quantum” states and are referred to by the principal
“quantum numbers” 1, 2, 3, etc. The quantum states are represented by an
energy-level diagram. Basic Laser Principles www.mellesgriot.com Introduction
to Laser Technology For an electron to jump to a higher quantum state, the atom
must receive energy from the outside world. This can happen through a variety
of mechanisms such as inelastic or semielastic collisions with other atoms and
absorption of energy in the form of electromagnetic radiation (e.g., light).
Likewise, when an electron drops from a higher state to a lower state, the atom
must give off energy, either as kinetic activity (nonradiative transitions) or
as electromagnetic radiation (radiative transitions). For the remainder of this
discussion we will consider only radiative transitions. PHOTONS AND ENERGY In
the 1600s and 1700s, early in the modern study of light, there was a great
controversy about light’s nature. Some thought that light was made up of
particles, while others thought that it was made up of waves. Both concepts
explained some of the behavior of light, but not all. It was finally determined
that light is made up of particles called “photons” which exhibit both
particle-like and wave-like properties. Each photon has an intrinsic energy
determined by the equation
E h = n
where n is the frequency
of the light and h is Planck’s constant. Since, for a wave, the frequency and
wavelength are related by the equation
ln = c
where l is the
wavelength of the light and c is the speed of light in a vacuum, equation 36.1
can be rewritten as
E hc = l .
It is evident from this
equation that the longer the wavelength of the light, the lower the energy of
the photon; consequently, ultraviolet light is much more “energetic” than
infrared light. Returning to the Bohr atom: for an atom to absorb light (i.e.,
for the light energy to cause an electron to move from a lower energy state En
to a higher energy state Em), the energy of a single photon must equal, almost
exactly, the energy difference between the two states. Too much energy or too
little energy and the photon will not be absorbed. Consequently, the wavelength
of that photon must be
D D = = − hc E
D = = − hc E EE E where
m n.
Likewise, when an
electron decays to a lower energy level in a radiative transition, the photon
of light given off by the atom must also have an energy equal to the energy
difference between the two states.
SPONTANEOUS AND STIMULATED EMISSION
In general, when an
electron is in an excited energy state, it must eventually decay to a lower
level, giving off a photon of radiation. This event is called “spontaneous
emission,” and the photon is emitted in a random direction and a random phase.
The average time it takes for the electron to decay is called the time constant
for spontaneous emission, and is represented by t. On the other hand, if an
electron is in energy state E2, and its decay path is to E1, but, before it has
a chance to spontaneously decay, a photon happens to pass by whose energy is
approximately
E24E1, there is a
probability that the passing photon will cause the electron to decay in such a
manner that a photon is emitted at exactly the same wavelength, in exactly the
same direction, and with exactly the same phase as the passing photon. This
process is called “stimulated emission.” Absorption, spontaneous emission, and
stimulated emission are illustrated in figure 36.2.
Now consider the group
of atoms shown in figure 36.3: all begin in exactly the same excited state, and
most are effectively within the stimulation range of a passing photon. We also
will assume that t is very long, and that the probability for stimulated
emission is 100 percent. The incoming (stimulating) photon interacts with the first
atom, causing stimulated emission of a coherent photon; these two photons then
interact with the next two atoms in line, and the result is four coherent
photons, on down the line. At the end of the process, we will have eleven
coherent photons, all with identical phases and all traveling in the same
direction. In other words, the initial photon has been “amplified” by a factor
of eleven. Note that the energy to put these atoms in excited states is
provided externally by some energy source which is usually referred to as the “pump”
source.
Of course, in any real
population of atoms, the probability for stimulated emission is quite small.
Furthermore, not all of the atoms are usually in an excited state; in fact, the
opposite is true. Boltzmann’s principle, a fundamental law of thermodynamics,
states that, when a collection of atoms is at thermal equilibrium, the relative
population of any two energy levels is given by where N2 and N1 are the
populations of the upper and lower energy states, respectively, T is the
equilibrium temperature, and k is Boltzmann’s constant. Substituting hn for
E24E1 yields For a normal population of atoms, there will always be more atoms
in the lower energy levels than in the upper ones. Since the probability for an
individual atom to absorb a photon is the same as the probability for an
excited atom to emit a photon via stimulated emission, the collection of real
atoms will be a net absorber, not a net emitter, and amplification will not be
possible. Consequently, to make a laser, we have to create a “population
inversion.” POPULATION INVERSION Atomic energy states are much more complex
than indicated by the description above. There are many more energy levels, and
each one has its own time constants for decay. The four-level energy diagram
shown in figure 36.4 is representative of some real lasers. The electron is
pumped (excited) into an upper level E4 by some mechanism (for example, a
collision with another atom or absorption of high-energy radiation). It then
decays to E3, then to E2, and finally to the ground state E1. Let us assume
that the time it takes to decay from E2 to E1 is much longer than the time it
takes to decay from E2 to E1. In a large population of such atoms, at
equilibrium and with a continuous pumping process, a population inversion will
occur between the E3 and E2 energy states, and a photon entering the population
will be amplified coherently.
THE RESONATOR
Although with a
population inversion we have the ability to amplify a signal via stimulated emission,
the overall single-pass gain is quite small, and most of the excited atoms in
the population emit spontaneously and do not contribute to the overall output.
To turn this system into a laser, we need a positive feedback mechanism that
will cause the majority of the atoms in the population to contribute to the
coherent output. This is the resonator, a system of mirrors that reflects
undesirable (off-axis) photons out of the system and reflects the desirable
(on-axis) photons back into the excited population where they can continue to
be amplified.
Now consider the laser
system shown in figure 36.5. The lasing medium is pumped continuously to create
a population inversion at the lasing wavelength. As the excited atoms start to
decay, they emit photons spontaneously in all directions. Some of the photons
travel along the axis of the lasing medium, but most of the photons are
directed out the sides. The photons traveling along the axis have an
opportunity to stimulate atoms they encounter to emit photons, but the ones
radiating out the sides do not. Furthermore, the photons traveling parallel to
the axis will be reflected back into the lasing medium and given the
opportunity to stimulate more excited atoms. As the on-axis photons are
reflected back and forth interacting with more and more atoms, spontaneous
emission decreases, stimulated emission along the axis predominates, and we
have a laser.
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