For PDF notes Join Telegram Channel link given in description

ENGG. PHYSICS

(BAS-101/201)
UNIT 4
FIBRE OPTICS & LASER
TOPIC- TYPES OF LASER & APPLICATIONS

PREPARED BY: LALIT SIR

For PDF notes Join Telegram Channel link given in description
 Ruby Laser
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 by Maiman 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.

Construction of ruby laser

A ruby laser consists of three important elements: laser medium, the pump
source, and the optical resonator.
Ø Laser medium
Ø Pump source
Ø Optical resonator
Ø Cooling System
Laser medium or gain medium in ruby laser:
In a ruby laser, a single cr ystal of ruby (Al 2 O 3 : Cr 3+ ) in the for m 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.
Pump source or energy source in ruby laser : The pump source is
the element of a ruby laser system that provides energy to the laser medium.
In a ruby laser, population inversion is required to achieve laser emission.
Population inversion is the process of achieving the greater population of
higher ener gy state than the lower ener gy state. In or der to achieve
population inversion, we need to supply energy to the laser medium (ruby).
In a ruby laser, we use flashtube as the energy source or pump
source. The flashtube supplies energy to the laser medium (ruby).
When lower energy state electrons in the laser medium gain
sufficient energy from the flashtube, they jump into the higher
energy state or excited state.
Optical resonator:
The ends of the cylindrical ruby rod are flat and parallel. The cylindrical
ruby rod is placed between two mirrors. The optical coating is applied
to both the mirrors. The process of depositing thin layers of metals on
glass substrates to make mirror surfaces is called silvering. Each
mirror is coated or silvered differently. At one end of the rod, the mirror
is fully silvered whereas, at another end, the mirror is partially silvered.
The fully silvered mirror will completely reflect the light whereas
the partially silvered mirror will reflect most part of the light but
allows a small portion of light through it to produce output laser
light.
Working of ruby laser:
The ruby laser is a three level solid-state laser. In a ruby laser, optical
pumping technique is used to supply energy to the laser medium.
Optical pumping is a technique in which light is used as energy source
to raise electrons from lower energy level to the higher energy level.
Consider a ruby laser medium consisting of three energy levels E1, E2,
E3 with N number of electrons.
We assume that the energy levels will be E1 < E2 < E3. The energy level
E 1 is known as ground state or lower energy state, the energy level
E2 is known as metastable state, and the energy level E3 is known as
pump state. Let us assume that initially most of the electrons are in the
lower energy state (E1) and only a tiny number of electrons are in the
excited states (E 2 and E 3). When light energy is supplied to the laser
medium (ruby), the electrons in the lower energy state or ground state
(E 1 ) gains enough energy and jumps into the pump state (E 3 ). The
lifetime of pump state E3 is very small (10-8 sec) so the electrons in the
pump state do not stay for long period.
After a short period, they fall into the metastable state E2 by releasing
radiation less energy. The lifetime of metastable state E 2 is 10 -3 sec
which is much greater than the lifetime of pump state E 3 . Therefore,
the electrons reach E2 much faster than they leave E2. This results in
an increase in the number of electrons in the metastable state E2 and
hence population inver sion is achieved. After some period, the
electrons in the metastable state E2 falls into the lower energy state E1
by releasing energy in the form of photons. This is called spontaneous
emission of radiation. When the emitted photon interacts with the electron in the
metastable state, it forcefully makes that electron fall into the ground state E1. As a result, two
photons are emitted. This is called stimulated emission of radiation.
When these emitted photons again interacted with the metastable
state electrons, then 4 photons are produced. Because of this
continuous interaction with the electrons, millions of photons are
produced. In an active medium (ruby), a process called spontaneous
emission produces light. The light produced within the laser medium
will bounce back and forth between the two mirrors. This stimulates
other electrons to fall into the ground state by releasing light energy.
This is called stimulated emission. Likewise, millions of electrons are
stimulated to emit light. Thus, the light gain is achieved. The amplified
light escapes through the partially reflecting mirror to produce laser
light.
 Helium-Neon laser
Helium-Neon laser is a type of gas laser in which a mixture of helium
and neon gas is used as a gain medium. Helium-Neon laser is also
known as He-Ne laser.

What is a gas laser?

A gas laser is a type of laser in which a mixture of gas is used as the
active medium or laser medium. Gas lasers are the most widely used
lasers. Gas lasers range from the low power helium-neon lasers to the
very high power carbon dioxide lasers. The helium-neon lasers are
most commonly used in college laboratories whereas the carbon
dioxide lasers are used in industrial applications.
The main advantage of gas lasers (eg.: He-Ne lasers) over solid
state lasers is that they are less prone to damage by overheating
so they can be run continuously.

What is helium-neon laser?

At room temperature, a ruby laser will only emit short bursts of laser
light, each laser pulse occurring after a flash of the pumping light. It
would be better to have a laser that emits light continuously. Such a
laser is called a continuous wave (CW) laser. The helium-neon laser
was the first continuous wave (CW) laser ever constructed. It was built
i n 1 9 6 1 b y A l i Jav a n , B e n n e t t , a n d H e r r i o t t at B e l l Te l e p h o n e
Laboratories.
Helium-neon lasers are the most widely used gas lasers. These lasers
have many industrial and scientific uses and ar e often used in
laboratory demonstrations of optics.
In He-Ne lasers, the optical pumping method is not used instead an
electrical pumping method is used. The excitation of electrons in the
He-Ne gas active medium is achieved by passing an electric current
through the gas. The helium-neon laser operates at a wavelength of
632.8 nanometers (nm), in the red portion of the visible spectrum.
 Helium-neon laser construction
The helium-neon laser consists of three essential components:
Ø Pump source (high voltage power supply)
Ø Gain medium (laser glass tube or discharge glass tube)
Ø Resonating cavity
Advantages of helium-neon laser
Ø Helium-neon laser emits laser light in the visible portion of the
spectrum.
Ø High stability
Ø Low cost
Ø Operates without damage at higher temperatures

Applications of helium-neon lasers

Ø Helium-neon lasers are used in industries.
Ø Helium-neon lasers are used in scientific instruments.
Ø Helium-neon lasers are used in the college laboratories.
 APPLICATIONS OF LASER:

ØEngineering:
1. The lasers have wide industrial applications. They can
blast holes in diamond and hard steels.
2. Laser beam is highly intense. Hence it can be utilized in
applications such as welding cutting and ablation of material.
One advantage of laser is that the beam can be focused into
the fine spot. Small spot size implies that high ener gy
densities are possible so that it is suitable for machining and
drilling holes.
ØMedical:
1. Lasers have wide medical applications. They have been used successfully in the
treatment of detached retinas.
2. Preliminar y success had also been obtained to treat the human and animal
cancers.
3. Microsurgery is also possible because laser beams can be focused on very small
areas and hence one har mful component can be destroyed without seriously
damaging neighboring regions.
4. Lasers are used for treatment of dental decay, the destruction of malignant
tumors and the treatment of skin diseases.
5. In recent years genetic research has wide scope with the help of fine highly
penetrating laser beam.
ØCommunication:
1.Due to nar r ow band width, laser s ar e used in micr owave
communication.
2.By the use of lasers, the storage capacity for infor mation in
computers is generally due to the narrowness of the bandwidth.
3.Low power semiconductor lasers are used in CD players, laser
printers, office automation equipment, optical floppy disc,
optical processing devices, range finders, strain gauges, optical
m i c r o m e t e r s , ve l o c i t y m e t e r s , m e a s u r i n g i n s t r u m e n t s ,
infor mation transmission in optical communication, radar
signaling etc.
ØMilitary:
1. Their study also oriented for military purpose. They can be served as

war weapon.