Requirements for laser action and Ruby laser

1. LASERS
Dr. SURABHI SINGHAL
Assistant Professor
Government Girls Degree
College, Kharkhoda,
Meerut

2. LASER
A laser is a device that emits light through a process of optical amplification
based on the stimulated emission of electromagnetic radiation. The term
"laser" originated as an acronym for "light amplification by stimulated
emission of radiation".

3. SPONTANEOUS EMISSION
 Spontaneous
emission is the process
in which a quantum
mechanical system (such
as a molecule, an atom or
a subatomic particle)
transits from an excited
energy state to a lower
energy state (e.g., its
ground state) and emits a
quantized amount of
energy in the form of a
photon.

4. STIMULATED EMISSION
Stimulated
emission is the
process by which an
incoming photon of
a specific frequency
can interact with an
excited
atomic electron (or
other excited
molecular state),
causing it to drop to
a lower energy level.

5. Spontaneous v/s Stimulated emission
In stimulated emission the liberated
energy transfers to the electromagnetic
field, creating a new photon with
same phase, frequency, polarization,
and direction of travel that are all identical
to the photons of the incident wave. This
is in contrast to spontaneous emission,
which occurs at random intervals without
regard to the ambient electromagnetic
field.

6. Necessary conditions for laser action
 Population inversion: This is the situation in which the
number of atoms in higher energy state is greater than that in
lower energy state is called population inversion.
 Optical Pumping : This is the process by which the
population inversion is achieved. In optical pumping the
population inversion is achieved by three energy levels
scheme.
 Active system : A medium in which population inversion to
be achieved is called active system.

7. RUBY LASER

8. Details:
 A ruby laser is a solid-state laser that uses the
synthetic ruby crystal as its laser medium. Ruby
laser is the first successful laser developed in
1960.
 Ruby laser is one of the few solid-state lasers
that produce visible light. It emits deep red light
of wavelength 694.3 nm.

9. In a ruby laser, a single crystal of ruby (Al2O3 : Cr3+) in the form
of cylinder acts as a laser medium or active medium. The laser
medium (ruby) in the ruby laser is made of the host of sapphire
(Al2O3) which is doped with small amounts of chromium ions
(Cr3+). The ruby has good thermal properties.
The ruby laser has following main parts:
1. The working substance (active medium)- is in the form of a rod
of ruby crystal (10 cm in length, 0.8 cm in diameter) in which
Cr3+ are active canters while Al and O2 are inert.
2. The resonance cavity- is made by silvering and polishing the
ends of ruby rod. Fully reflecting plates at the left and a partially
reflecting plate at the right, both optically plane and accurately
parallel.
3. The optical pumping system -consists of a helical xenon
discharge tube. It produces flash of few milliseconds.

10. ENERGY LEVEL DIAGRAM

11. The simplified energy level diagram of chromium ions in a ruby laser, indicating
appropriate excitation an decay is shown in Fig. In normal state, most of the chromium
ions are in the ground state E1. When the ruby rod is irradiated by a flash of light,
the 5500A˚ radiation photons are absorbed by the chromium ions which are pumped to
the excited state E3. The excited ion gives up part of its energy to the crystal lattice and
decay without giving any radiation to the meta stable state E2. Since, the state E2 has a
much longer lifetime , The number of ions in this state goes on increasing. Thus
population inversion is achieved between the states E2 and E1. When the excited ion from
the metastable state E2 drops down spontaneously to the ground state E1​, it emits a
photon of wavelength 6943A˚.
This photon travels through the ruby rod and is reflected back and forth by the silvered
ends until it stimulates other excited ion and causes it to emit a fresh photon in phase with
stimulating photon. Thus the reflections will amount to the additional stimulated emission
- the so called amplification by stimulated emission. This stimulated emission is the laser
transition. Finally, a pulse of red light of wave-length 6943A˚ emerges through the
partially silvered end of the crystal.

12. THANK YOU