What is Laser?

LASERS  a device that controls the way that energized

atoms release photons.

Light Amplification by Stimulated Emission of Radiation

First we must understand the difference between incoherent and
coherent radiation
•Ordinary light sources (light bulbs, fluorescent lights, etc)
produce incoherent light.
•lasers produce coherent light  all atoms radiate in the same
manner
• A device produces a coherent beam of optical radiation by
stimulating electronic, ionic, or molecular transitions to higher
energy levels
• When they return to lower energy levels by stimulated emission,
they emit energy.
 Properties of Laser
▪ The light emitted from a laser is monochromatic, that is, it is
of one color/wavelength. In contrast, ordinary white light is a
combination of many colors (or wavelengths) of light.

▪ Lasers emit light that is highly directional, that is, laser light is
emitted as a relatively narrow beam in a specific direction.
Ordinary light, such as from a light bulb, is emitted in many
directions away from the source.

▪ The light from a laser is said to be coherent, which means that

the wavelengths of the laser light are in phase in space and
time. Ordinary light can be a mixture of many wavelengths.

These three properties of laser light are what can make it

more hazardous than ordinary light. Laser light can
deposit a lot of energy within a small area.
3
 Monochromacity

Nearly monochromatic light

Example:
He-Ne Laser Comparison of the wavelengths of red and
λ0 = 632.5 nm blue light
Δλ = 0.2 nm
Diode Laser
λ0 = 900 nm
Δλ = 10 nm
 Coherence

Incoherent light waves Coherent light waves

 Directionality

Conventional light source Divergence angle (θd)

Beam divergence: θd= β λ /D
β ~ 1 = f (type of light amplitude distribution)
λ = wavelength
D = beam diameter
 Population Inversion
• A state in which a substance has been
energized, or excited to specific energy levels.
• More atoms or molecules are in a higher excited
state.
• The process of producing a population inversion
is called pumping.
• Examples:
→by lamps of appropriate intensity
→by electrical discharge
Ruby Laser
 Ruby Laser
Ruby LASER Construction:
(i) Active
medium:
It is a solid-state laser, in which a rod of a
synthetic ruby crystal is used as an active
medium. The ruby crystal is obtained by doping
a small amount (about 0.05% by weight) of
chromium oxide (Cr2O3) in Aluminum oxide
(Al2O3), so that some of the aluminum ions
(Al3+) are replaced by chromium ions (Cr3+).
These chromium ions give the crystal a pink or
red color depending upon the doping
concentration. Al2O3 only acts as the host while
the chromium ions act as active centers in ruby
crystal and responsible for the laser action.
The length of the ruby rod is usually 2 cm to
30 cm and the diameter is 0.5 cm to 2 cm.
To construct the optical resonator cavity,
the ends of the rods are polished such that
they become flat and parallel to each other.
Now one of the ends is coated with silver
completely while the other one is partially
silvered. Thus, the two silver-coated ends of
the rod act as an optical resonator system.
 (iii) Pumping
system:
The ruby rod is placed inside a helically shaped xenon
flash lamp to excite the Cr3+ ions. Thus, in ruby
laser population inversion is achieved by using optical
pumping.
Working of Ruby LASER:
Ruby is a three energy level laser system. After absorbing
light photons of wavelength 5500 Å from xenon flash
lamp, some of the Cr3+ ions at ground energy level E1
get excited to higher energy level E3. At this energy level,
they are unstable and by losing a part of their energy to
the crystal lattice, they fall to the metastable energy level
E2, whose lifetime is much longer (about 10-3 s).
Therefore, the number of Cr3+ ions goes on
increasing in E2 state while the number of these
ions in ground state E2 goes on decreasing due to
pumping by the flash lamp and soon the population
inversion is achieved between states E2 and E1.
Now some of the Cr3+ ions will decay
spontaneously to the ground state E1 by emitting
photons of wavelength 6943 Å. The photons that
are moving parallel to the axis of the rod will
reflect back and forth by the silvered ends of the
rod and stimulate other excited Cr3+ ions to
radiate another photon with the same phase.
Thus, due to successive reflections of these
photons at the ends of the rod, the number of
photons multiplies. After a few microseconds,
a monochromatic, intense, and collimated beam of
red light of wavelength 6943 Å emerges through
the partially silvered end of the rod. The
Ruby laser is a pulsed laser that emits light in the
form of very short pulses.
He-Ne Laser