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Re: LASERS



I read through your "ABOUT LASERS" handout. A few minor corrections that have nothing to do with subject matter, which I'll leave to those more informed. The third sentence should have opaque replaced with transparent. Misspellings include: uncorelated, spectorscopists, lavels, loosing, resent, requirment, continous. We're not English teachers, but we should be able to use spell check before handing stuff out to our students.

Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU> 05/01 11:24 AM >>>
This is a LONG handout for my students. Feel free to
use it, and to comment. See safety aspects at the end.

Ludwik Kowalski
-------------------------------------------------------------
ABOUT LASERS

It is well known that light traveling through a material
is usually absorbed, sooner or later. We say that a material
is opaque when a very thin layer is sufficient to absorb
light, as in metals. And we say a medium is opaque
when only a negligible fraction of energy is lost by light
in a thin layer, as in glass. In general, the intensity of
light decreases exponentially with the depth of the
traversed material. In other words: I(x)=I(o)*exp(-a*x).

It turns out that under some "unnatural" conditions a
medium can behave in such a way that I(x) increases
with the traversed depth, x. When this happens light
is actually amplified (rather than attenuated) as it travels
along the x direction. The first two letters in the word
LASER stand for Light Amplification. The last three
letters stand for Stimulated Emission of Radiation.
Laser is a device which can amplify light. A small
amount of light is received as input and a stronger light,
of the same frequency, is produced at the output.

Sometimes the initial light is generated in the same
medium in which the amplification takes place. In such
cases the device is actually a generator but the name
laser is still used. I suppose this can be justified by
saying that a generator is an amplifier with a strong
positive feedback. A common laboratory laser is an
electrical discharged tube filled with a mixture of two
gases, He and Ne. If it had pure He, or pure Ne, then
the tube would produce an ordinary light.

But a mixture of these two gasses, under proper
conditions, is able to generate a very unusual (highly
coherent) light. It is not possible to explain this to those
who are not familiar with the phenomenon of "stimulated
emission of radiation" (by atoms and molecules). This
process, by the way, was recognized by Einstein in
1916; the first laser was constructed in 1960.

It is common knowledge today that a quantum system
is excited from the energy level E1 to E2 when a photon
(whose energy is E=E2-E1) is absorbed. What happens
next? The system may get rid of the received energy by
radiating one or more photons. This process is called
natural (spontaneous) emission. Each system (atom or
molecule) gets rid of the excess energy independently
of other systems. The emitted photons travel in all
directions and moments of emission are totally
uncorelated. Some excited atoms decay more rapidly
than others.

An excited level is said to be metastable if its half-life
is unusually long, for example, 1 ms instead of 1 ns,
or so. Metastable quantum levels play a very important
role in lasers. Suppose that the half-life of an excited
atom is 1 ns. It means that only 1/2 of excited atoms
will remain after 1 nanosecond, 1/4 after two ns, etc.
This is not different from what we know about
emission of nuclear radiation.

Einstein recognized that an excited atom should be able
to decay more rapidly than what is expected from the
half-lives of its excited levels. The condition for this is
presence of nearby photons. In other words, a photon
passing near an excited atom may cause it to decay
suddenly. This process is called stimulated emission.
It turns out that photons produced by stimulated
emission travel in the same direction as those which
trigger the decay. Furthermore, they are perfectly
synchronized with the original photons.

Note that original photons are not lost in the process.
This suggest that a single photon at the input of a tube
may, under suitable conditions, create several identical
photons at the output. That is the main idea behind light
amplification; a chain reaction involving photons, if you
wish. The probability of stimulated emission, however,
is very small, in comparison with probabilities of
processes in which photons are lost. The designer of
a laser must overcome this, in one way or another.

The common way to accomplish "lasing" is to create
an enormous number of excited atoms. Referring to
previously introduced levels E1 and E2 one must have
more atoms at E2 (they amplify) than at E1 (they cause
absorption). This is not natural; under ordinary thermal
equilibrium, the number of atoms exited to E2 is much
smaller than the number of atoms excited to E1. This
is governed by the Boltzmann distribution law.

A condition under which N2 (number of atoms excited
to the energy E2) is higher than N1 (number of atoms
excited to the lower energy E1) is called "population
inversion". Amplification of light is impossible in a
thermally equilibrated system. But it is possible in a
system whose population is inverted. To create the
"population inversion" one must supply energy.This
process is called "optical pumping".

The optical pumping processes, in the very first
two lasers built (ruby and He-Ne), are explained on
pages 635 and 636 of our textbook. Note that in both
of these systems photons traverse the lasing medium
many times before they escape from the cavity
through a semitransparent mirrow. The two mirrors
must be highly parallel to each other.

You are likly to be confused by the unfamiliar
notation used by spectorscopists to describe quantum
levels of Figure 30H. The essential point is that the
two excited lavels of Helium (about 19.6 and 20.5 eV,
as indicated on the right axis) are metastable and that
Neon atoms have several excited levels which are very
close to those of He. Long halflife of excited He atoms
increases their chance of loosing energy through
collisions with He atoms.

Collisions of that kind produce Ne atoms of several
energies near 19.6 and 20.5 eV. These decay by
emitting photons (wavelength of 6328 A from 5s
and infrared of 11177 and 11523 A from 4s). Then
other photons are emitted and Ne atoms return to
the ground state. As far as the visible red light is
concered (6328 A) the level near 20.5 eV should
be called E2 while the level near 16.6 eV should
be called E1 (in terms of our previous notation).
The population inversion occurs when the density
of Ne atoms at E2 is larger than the density at E1.

*****************************************
Working with lasers one must be aware of dangers
associated with highly concetrated, and often invisible,
light. From the point of view of safety laser are
identified as class 1, class 2, class 3A, class 3B and
class 4. I learned about these things from a resent
(April 99) "Laser Safety Awareness Training" course
at Brookhaven National Laboratory. [Passing an exam
was a requirment for participating in an experiment
near a laser. Also the nuclear radiation worker exam.
I even had to sign something as if I were a woman.]

Class 1 --> Can not exceed any known hazard levels.
hazard warning not required.
Class 2 --> May produce retinal domage when stared
at for a long time. Radiation must be
visible and of the continous kind. The
power <1 mW. Warning label and an
area sign are required.
Class 3A --> Continous output in the visible 1 to 5 mW
Class 3B --> Continous output 5 to 500 mW [or pulsed
but not to exceed 10 J/cm^2 (fire hazard)].
Class 4 --> Even light from specular reflection can
be harmful. Also danger from ignited fire,
etc., etc..
*****************************************