Term PaperTOPIC - He Ne Laser

Submitted byParvender Kumar

OE110 – A 4710901667

Submitted ToMr. Vinod Kumar

COURSE CODE : PHY 803

COURSE TITLE : LASER TECHNOLOGY

P a g e | 1

Abstract

The design of the helium-neon laser is not complex by modern standards. They consist of

only three essential components and operate by the process of stimulated emission and light

amplification. Because of their many advantages over other types of lasers, helium-neon

lasers are used for many applications in research and industry.

P a g e | 2

Acknowledgement

I would like to sincerely thank my subject teacher Mr. Vinod sir for allowing me to prepare a term paper. Without his proper teaching and guidance, I would never have been able to complete my term paper successfully. This project work has helped me in learning and gaining knowledge about the subject and the topic.

Further I am thankful to the facilities provided by Lovely Professional University like library

and Wi- Fi.

I hope that my work is going to be appreciated.

P a g e | 3

Table Of Content :

S. No. Topics Page

Abstract 1

Acknowledgement 2

1 Introduction 5

2 Construction And Operation 6

3 Working of He Ne Laser 7 - 8

4 He Ne Energy Level Diagram 9

5 Measurement of Laser Beam Stability 10 – 12

6 Power Supply Characteristics of He Ne Laser 13 - 14

7 Status of He Ne laser Development in India 15

8 Applications 16

References 17

P a g e | 4

List of Figures :

Figures Page

Figure 1 Schematic Diagram of He Ne Laser 6

Figure 2 Population inversion of He Ne Laser 7

Figure 3 Energy Level Diagram of He Ne Laser 9

Figure 4 Optical set-up for measuring of laser power stability. 10

Figure 5 Laser beam stability for 24 mW He-Ne laser with sample

each 100 ms in 10 seconds.

11

Figure 6 Laser beam stability for 24 mW He-Ne laser with sample each second in 30 minutes.

11

Figure 7block diagram of power supply for helium-neon laser

13

Figure 8 Helium–Neon Laser Power Supply 14

P a g e | 5

Introduction

A helium–neon laser or HeNe laser, is a type of gas laser whose gain medium consists of a

mixture of helium and neon inside of a small bore capillary tube, usually excited by a DC

electrical discharge. He-Ne laser is a four-level laser.

The first HeNe laser emitted at 1.15 μm in the infrared and was the first gas laser. However a

laser that operated at visible wavelengths was much more in demand, and a number of other

neon transitions were investigated to identify ones in which a population inversion can be

achieved. The 633 nm line was found to have the highest gain in the visible spectrum, making

this the wavelength of choice for most He Ne lasers. However other visible as well as

infrared lasing wavelengths are possible, and by using mirror coatings with their peak

reflectance at these other wavelengths, He Ne lasers could be engineered to employ those

transitions; this includes visible lasers appearing red, orange, yellow, and green. Lasing

transitions are known from over 100 μm in the far infrared to 540 nm in the visible. Since

visible transitions at wavelengths other than 633 nm have somewhat lower gain, these lasers

generally have lower output powers and are more costly. The 3.39 μm transition has a very

high gain but is prevented from lasing in an ordinary He Ne laser (of a different intended

wavelength) since the cavity and mirrors are lossy at that wavelength. However in high

power HeNe lasers having a particularly long cavity, superluminescence at 3.39 μm can

become a nuisance, robbing power from the lasing medium, often requiring additional

suppression. The best known and most widely used HeNe laser operates at a wavelength of

632.8 nm in the red part of the visible spectrum. It was developed at Bell Telephone

Laboratories in 1962, 18 months after the pioneering demonstration at the same laboratory of

the first continuous infrared HeNe gas laser in December 1960.

P a g e | 6

Construction and Operation

The helium-neon laser is usually constructed to operate in the red at 632.8 nm. It can also be

constructed to produce laser action in the green at 543.5 nm and in the infrared at 1523 nm.

Active medium: The gas mixture of He and Ne forms the active medium. Ne act as active

center.

Pumping Source: Electric discharge method is used for pumping and achieving population

inversion. Normally the population density of atoms/electrons is more in the ground state

than the excited state. But if the process of stimulated emission dominates over the process of

spontaneous emission, then it may be possible that N2 > N1.

Where N1 is the number of atoms in the ground state and

N2 is the number of atoms in the excited state.

The process of achieving greater population density of atoms in the higher energy state as

compared to lower energy state is called population inversion. The atoms from lower energy

states are raised to excited states by external energy.

Optical resonator system: A set of mirrors form the optical resonator system. An optical

resonator is a system or set up, which is used to obtain amplification of stimulated photons by

oscillating them back and forth between systems of two mirrors. Thus, it consists of two

plane or concave mirrors. One of the mirrors is partially reflecting (having reflectivity less

than 100%) and other is totally reflecting (having reflectivity 100%). Laser output is received

from partially reflecting mirror. The space between the two mirrors is called cavity.

Figure 1: Schematic Diagram of He Ne Laser

P a g e | 7

Working of He Ne Laser

Pumping of He atoms: When electric discharge is passed through the gas mixture of He and

Ne, electrons are accelerated down the discharge tube in which mixture of He-Ne is placed.

These accelerated electrons collide with helium atoms and excite them to higher energy

levels (let us say F2 and F3).

Figure 2 : Population Inversion of He Ne Laser

These levels happen to be Metastable and thus the He atoms spend a sufficient amount of

time there before getting de-excited.

Achievement of population inversion of Neon atoms: Some of the excited states of Ne

atoms correspond approximately to the same energy of the excited levels F2 and F3 of He.

Thus, when He atoms in levels F2 and F3 collide with the Ne atoms in the ground state E1,

then energy exchange takes place and this results in the excitation of Ne atoms to the levels

E4 and E6 and de-excitation of the He atoms to the ground level F1. As the helium atoms

have longer life time in excited states F2 and F3, thus this process of energy transfer has high

probability.

Therefore, the electric discharge through the gas mixture continuously populates the Ne

excited levels E4 and E6. This helps to create a state of population inversion between the

levels E4 (or E6) and lower energy levels E5 and E3. Therefore the purpose of He atoms is to

help in achieving a population inversion in the Ne atoms.

Achievement of laser: The following three transitions will occur:

P a g e | 8

E6 to E5 with laser wavelength of 3.39 μm or 33900 Angstroms.

E6 to E3 with laser wavelength of 6328 Angstroms.

E4 to E3 with laser wavelength of 1.15 μm or 11500 Angstroms.

The wavelengths of 3.39 μm and 1.15 μm corresponds to infrared region and wavelength

6328 Angstroms corresponds to red light wavelength (visible region).

Mirrors of the optical resonators are so designed to show low reflectivity for wavelengths

3.39 μm and 1.15 μm. Thus photons of these wavelengths will be eliminated. Therefore, the

photons of wavelengths 6328 Angstroms will move back and forth in optical resonator

system and thus laser of wavelength 6328 Angstroms emerges through the partially reflected

mirror.

The excited Ne atoms drop down from levels E3 to E2 through spontaneous emission and this

process will emit a photon of wavelength 0.6 μm. As the level E2 is also Metastable, there is

a probability of excitation of Ne atoms from E2 to E3 leading to quenching of the population

inversion. To eliminate quenching, the narrow discharge tube is used because Ne atoms de-

excited to level E1 from E2 through collisions with the walls of the tube.

P a g e | 9

He Ne Energy Level Diagram

Figure 3 : Energy Level Diagram of He Ne Laser

The left side of the representation shows the lower levels of the helium atoms.The energy

scale is interrupted and that there is a larger difference in energy in the recombination process

than is evident in the diagram.

A characteristic of helium is that its first states to be excited, 21S1 and 21S0 are metastable,

i.e. optical transitions to the ground state 11S0 are not allowed, because this would violate the

selection rules for optical transitions. As a result of gas discharge, these states are populated

by electron collisions

A collision is called a collision of the second type if one of the colliding bodies transfers

energy to the other so that a transition from the previous energy state to the next higher or

lower takes place. Apart from the electron collision of the second type there is also the atomic

collision of the second type. In the latter, an excited helium atom reaches the initial state

because its energy has been used in the excitation of a Ne atom. Both these processes form

the basis for the production of a population inversion in the Ne system.

P a g e | 10

Measurements of laser beam stability

During the recording process for holographic multi-stereograms, it is important that each of

the 70 part holograms are evenly exposed. If the exposure of the film is varied, there will be

areas of the hologram that are brighter than other and the quality of the hologram will not be

as good as desired. This can also happen if some of the part holograms are under-exposed.

The power stability of the laser beam is not decisive for the visibility of the hologram,

because the ratio between the reference and the object beam will still be constant. For each

part of the hologram the exposure time is constant, and it is then important that the laser's

output power is constant to get the same exposure.

For measuring the power stability of the 24 mW He-Ne laser the following set-up was

arranged on the optical table.

Figure 4 Optical set-up for measuring of laser power stability.

The neutral density filter was used to reduce the laser's output power with 50 %, to a readable

value for the laser power meter. To detect the power of the laser, the laser power meter reads

the data continuously. This data is then logged in the PC  by the data logging software

program PICO ADC-12. The data is logged for two different sampling rates and time lags.

The values from ADC-12 are then converted to LOTUS 1-2-3 to make it possible to present

the data in a suitable way.

The first measurement is a short time logging made with sample pr. 100 ms in 10 seconds.

The other measurement is a long time logging made with 1 sample pr. second in 30 minutes.

The idea behind two different measures is to see how the laser works during holographic

recordings (short time) and how stable the lasers output power is over time.

P a g e | 11

Figure 5 Laser beam stability for 24 mW He-Ne laser with sample each 100 ms in 10 seconds.

Laser output power data from sample rate at 100 ms in 10 seconds (short time).

Average value : 680.6 

Standard deviation : 13.1 

The laser output power stability for this measurement is about 1.9 %.

From Melles Griot product catalog the laser output power stability is given by 2.5 %.

Figure 6 Laser beam stability for 24 mW He-Ne laser with sample each second in 30 minutes.

P a g e | 12

Laser output data from sample rate at 1 second in 30 minutes (long time).

Average value : 673 

Standard deviation : 2.8 

The output power stability for this measurement is about 0.4 % 

During the production of a holographic transmission multi-stereogram, where 70 different

part holograms are exposed onto the film, each exposure is about 10 seconds and the entire

recording process takes about 30 minutes. From figure 4-5 can we see that the laser power

stability for one part exposure of the film is good, and the measurement agrees with the data

from the manufacturer, Melles Griot. In practice, the spikes measured in the short time of

measurement should not reduce the hologram's visibility.

From figure 4-6 can we see that the output power from the laser is quite stable over the whole

recording process of 30 minutes. This means, that each of the part holograms on the multi-

stereogram are evenly exposed on the film. The possibility of getting good results in the

holographic multi-stereogram production with the use of this laser is good.

The laser was turned on at least 3 hours before the measurement was taken. It is very

important that the laser is heated and becomes stable before the recording of holography is

started.

P a g e | 13

Power Supply Characteristics of He-Ne Laser

The excitation of the neon atoms to the upper laser level is derived from

an electrical discharge. In steady operation, the discharge passes a few

milliamperes of current through the gas at a voltage in the range 1-2

kilovolts. Electrons in the discharge collide with the helium and neon

atoms and raise them to excited energy levels. Most of the excitation is

received by the more abundant helium atoms, which easily can transfer

their excitation energy to neon atoms. This produces in the neon a

condition of population inversion The basic blocks of the power supply for

a typical small helium-neon laser are shown in Figure 7. The input voltage

first is increased by a transformer. The high-voltage exciter circuit

supplies a voltage pulse that breaks down the gas and initiates the

electrical discharge through it.

Figure 7: block diagram of power supply for helium-neon laser

Figure 8 shows a schematic diagram of a He-Ne laser power supply. A

detailed description of the operation of this supply is beyond the scope of

this module. The following simplified description illustrates the three

functional components of the power supply.

P a g e | 14

Figure 8: Helium–Neon Laser Power Supply

When the power switch is closed, current flows through the step-up transformer and it

produce an ac output voltage of approximately 750V. remainder of the running supply

consists of a voltage doubler-rectifier circuit that converts this ac voltage to dc voltage of

approximately 1750V. This voltage is not of sufficient value to ionize the tube, but it will

sustain current flow once it has begun. The starter circuit consists of a voltage multiplier

circuit capable of delivering several kilovolts but no current. Once the breakdown voltage is

reached, the tube ionizes, and the diodes in this circuit conduct the current. The voltage

difference across the starter circuit becomes nearly zero, and the voltage of the running

supply is delivered to the tube and ballast resistor. The value of the ballast resistor depends

upon the degree to which the current is to be limited. If this circuit is used with the tube

whose negative resistance region, the ballast resistance value is 300kΩ, which limits tube

current to 4.5mA, resulting in a voltage drop of 800 V across the ballast resistor and a drop of

1750V across the laser tube.

P a g e | 15

Status of He Ne Laser Development in India

Helium-Neon lasers of low power output (2- 5 mW) with lifetimes of a few thousand hours

have been developed at IISc, NPL, and Bharat Electronics Ltd., Bangalore. The technology

has been transferred by NPL to M/s Laser Instruments, New Delhi and by BARC, Bombay to

ECIL, Hyderabad. they started production of these lasers commercially about 20 years back

but stopped production since their performance is far from satisfactory. BEL also made an

attempt about 10 years back and stopped production due to lack of sufficient technology

P a g e | 16

Applications

• The Narrow red beam of He-Ne laser is used in supermarkets to read bar codes.

• The He- Ne Laser is used in Holography in producing the 3D images of objects.

• Free Space optical Communications.

• Laser Tachometer

• Laser burglar alarm

• Laser gyroscope

Laser in Civil Engineering

The negligible divergence of the laser beam stimulated a number of ideas for providing

hitherto impossible accuracy and sensitivity in the alignment of tools. Serving as an optical

axis, the beam guides the machines used for levelling the concrete facing of the airfields,

checking the verticality of the framework of tall buildings, sinking mines, and cutting tunnels

from two ends and joining them without tilt.

In Moscow, a laser centring device was used to control the vertical axis during the

construction of a TV tower with a precision of 6 mm. In the US, a hard rock boring machine

has cut a tunnel, 6.5 m in diameter and 2 1/4 km long, without deviating from its planned

course more than 1.58 cm in any direction. This feat was accomplished with the help of the

laser beam.

Laser direction finders are widely used in coal mines. In view of the high rate of digging in

coal mining, the biggest difficulty is to maintain accurately the given direction, which is

achieved using a highly directional laser beam. Similarly, geodolite is used for the detection

and measurement of the deformation of large dams and bridges.

P a g e | 17

References

http://en.wikipedia.org/wiki/Helium%E2%80%93neon_laser

http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/semiconf.html

http://www.winnerscience.com/science/physics/laser-physics/construction-of-

helium-neon-laser/

http://www.winnerscience.com/science/physics/laser-physics/optical-resonator-

system-in-laser/

http://www.winnerscience.com/science/physics/laser-physics/working-of-helium-

neon-laser/

www.indiastudychannel.com/.../87176-20041- Helium-neon _ laser .ppt ...

http://drdo.gov.in/drdo/data/Laser%20and%20its%20Applications.pdf

http://www.fou.uib.no/fd/1996/h/404001/kap04.htm

http://uotechnology.edu.iq/... laser andoptoelec.../ Laser %20systems.pdf

http://www.worldoflasers.com/lasertypes2.htm