Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

There are three levels in which population inversion takes place, those are

1. Two level pumping Scheme 2. Three level pumping Scheme 3. Three level pumping Scheme

Two level pumping Scheme: This scheme contains only two energy levels i.e., ground state and

excited state. The atom absorbs the photon energy and jumps to excited state from ground state. In

this case it is difficult to achieve stimulated emission.

This is because, the electron spontaneously returns to ground state (time lag 10-8 sec) by the

emission of radiation and passing photon have practically no time to stimulated excited atom.

There fore only spontaneous emission takes place in two level pumping scheme. Also due to

same reason the density of atoms in excited state (N2) is always less than density of atoms in ground

excited state (N1). As the density atoms in ground state larger than in excited state the photons have

maximum chance undergo induced absorption than the stimulated emission. For this reason it is

difficult to achieve population inversion.

Hence it required create a situation that the electron should stay some more time in the excited

state to get effective stimulation. Introducing another one energy state in between the ground state and

excited may solve this problem. This intermediate state is called as metastable state.

Metastable state: Metastable state is the energy state which lies between ground state and

excited state, transition takes place to this state from excited state without emission of radiation. This

state is more stable than the excited state and electron stay in this state for about 10-3 to 10-2 sec. and

this time is sufficient undergo stimulation emission.

During this time the atom is stimulated by passing photon and atom returns to ground state by

the emission two coherent radiations i.e. laser. The three level pumping schemes contains metastable

state

Three level pumping scheme: This scheme contains three energy levels i.e., ground state,

excited state and a metastable state in between ground state and excited state (As shown in below fig.)

In this scheme the atom in ground state (E1) absorbs photon energy and jumps to excited state (E3).

Within short interval (Spontaneously) atom return to meta-stable state (E2) in which it remain

comparatively more time (10-3 to 10-2). This time is sufficient to achieve population inversion and to

construct the amplification of laser radiation.

Ground state (E1)

Excited state (E2)

Incident photon

In coherent beam

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

The time during which atom was in metal stable state, some more atoms undergo induced

absorption and transit to excited state i.e. to meta-stable state, so that the population of atoms in ground

state decreases and in turn population of atoms in excited or meta-detestable increases or number of

atoms in meta-stable state exceeds the number atoms in ground state this is the condition of population

inversion. The atom in meta-stable state returns to ground state by the emission two coherent laser

rays. The two coherent rays stimulate other two excited atoms and become four and four rays will

become eight by stimulating four this process is continues and construct amplified coherent radiation

i.e. laser.

Example of laser produced in this scheme is Ruby laser and Nd: YAG (Neodymium doped

Yttrium Aluminum Garnet), which is a Solid state laser. The population inversion is achieved in this

laser is by Optical Pumping.

Limitation of this scheme is that, it is also not gets rid of from the spontaneous emission.

Although the stimulated emission is achieved effectively, it suffers from number of spontaneous

emission. Due to this reason, number of stimulated rays cause induced absorption instead of

stimulating other excited atoms, hence laser action will decay and finally ceases. Hence the laser is in

the form of discontinuous pulse. Large amount of incident energy is needed to create effective

population inversion.

Four Level Pumping Scheme: This scheme contains four energy levels i.e., ground state,

excited state, metastable state and intermediate state in between ground state and metastable state (As

shown in below fig.)

In this scheme the atom in ground state (E1) absorbs the photon energy and jumps to excited state

(E4). Within short interval (Spontaneously) atom return to meta-stable state in which it remain

Incident photon

Sponts. emission

Passing photon (E1)

Excited state (E3)

Non Radiative transition

Ground state (E1)

Metastable state (E2)

Coherent beam

Intermediate state (E3)

Sponts. emission

Passing photon (E1)

Excited state (E4)

Non Radiative transition

Ground state (E1)

Metastable state (E2)

Coherent beam (Laser)

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

comparatively more time (10-3 to 10-2). In this time population inversion is achieved not between meta-

stable state and Ground state but it is between metastable state (E3) and intermediate state. The

intermediate energy state (E2) practically empty and metastable state (E3) is completely filled, hence

population inversion achieved effectively and transition takes place from E3 to E2 by the emission of

coherent radiation i.e. laser. Finally the atom goes to ground state from state E2 by the emission of in

coherent radiation i.e. spontaneous emission.

The example of four level pumping scheme is Helium-Neon laser and Carbon di-oxide laser or

Gas lasers. The population inversion is achieved in this laser is by Electrical discharge method.

The advantage of this method is that as the electrons passing three levels to ground state

effective population inversion is achieved. Also number of spontaneous emissions is minimum; the

energy required for pumping also less compared that in three level scheme. This laser gives the

continuous pulses.

Laser Cavity: The medium of a material in which the population inversion is achieved. This is a long

tube with perfectly parallel two opposite faces and is silvered, this sort of material medium is known as

laser cavity, The laser radiation produced in such cavity are made to resonate or amplified.

The Resonance cavity: It is laser cavity with silvered faces; the laser produced in such medium travels

back and forth repeatedly along the axis of the cavity due to reflection at the silvered faces and

contributes for stimulated emission, and produce amplified huge pulse of laser. The highly amplified

pulse escaped from the semi silvered face.

Types of LASER:

There are four type of lasers those are

1) Solid State Laser2) Gas Laser3) Liquid-dye Laser4) Semiconductor laser.

1. Solid State Laser:

Generally, Solid-state laser consists of a glass or crystalline host material which is doped with

impurities such as neodymium, chromium, erbium, or other ions acts as active medium. Many of the

common dopants are rare earth elements, because the excited states of such ions are not strongly

coupled with thermal vibrations of the crystalline lattice (phonons), and the lasing threshold can be

reached at relatively low brightness of pump. These lasers have three level pumping scheme. The

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

population inversion s achieved by the method of optical pumping, so that a flash of either Xe-flash,

Kr-flash or carbon arc lamps itc. is focused on the laser cavity.

There are many hundreds of solid-state media in which laser action has been achieved, but relatively

few types are in widespread use. Of these, probably the most common types are Chromium (Cr) doped

Ruby (Al2O3 crystal) neodymium (Nd)-doped YAG. Neodymium-doped glass (Nd:glass) and

ytterbium-doped glasses and ceramics are used in solid-state lasers at extremely high power (terawatt

scale), high energy (mega joules) multiple beam systems for inertial confinement fusion. Titanium-

doped sapphire is also widely used for its broad tunability.

The first material used for lasing was ruby. Ruby lasers are still used for some applications, but are not

common due to their low efficiency.

SS lasers are used in all sorts of applications including materials processing (cutting, drilling, welding,

marking, heat treating, etc.), semiconductor fabrication (wafer cutting, IC trimming), the graphic arts

(high-end printing and copying), medical and surgical, rangefinders and other types of measurement,

scientific research, entertainment, and many others where high peak power and/or high continuous

power are required. A high energy pulsed YAG laser has even been used in rocket propulsion

experiments (well, at least to send an ounce or so aluminum projectile a few feet into the air using just

the pressure of photons!). The largest lasers (with the highest peak power) in the World are solid state

lasers. Many of the laser projectors for light shows and for other laser displays use solid state rather

than gas lasers like argon or krypton ion. And, that green laser pointer is a Diode Pumped Solid State

(DPSS) laser.

Ruby Laser: Ruby laser is first ever laser invented in 1960 by Maiman. The ruby LASER

consists of Al2O3 crystal. This crystal is doped with 0.05% of Chromium (Cr+++) ions which acts as the

active medium and undergo lasing action. The color of Laser is depends upon percentage of doping.

It consists of Ruby in the form of rod of dimension 4cm in length

and 1cm in diameter. The rod is inserted in the helical Xe-flash

tube. Coherent beam

Passing photon (E1 )

Excited states (E3)

Non Radiative transition

Ground state (E1)

Metastable state (E2)

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

The two ends faces of rod are exactly parallel and highly polished, one of them is completely

silvered and other one is partially silvered and is acts as resonance cavity

When flash of light incident on Ruby rod the Green component of light is absorbed by the Cr3+

ions raising them to excited states E3and E3’. This is short lived state hence, the Cr3+ ions rapidly under

go a non radiative transition to metastable state (E2) and accumulate there.

Due to faster pumping rate, population inversion is established between metastable state (E2)

and Ground state (E1).

The stimulated emission is initiated by a passing photon, the coherent laser radiation is emitted.

The laser rays travel along the axis Ruby rod to and fro, due reflection at two silvered faces and

amplified. The amplified pulse of wave length 694.3nm emerges from the partially silvered mirror.

Dis-advantage of this laser is that it required high pumping power as considerable spontaneous

emission also takes place along with stimulated emission. This is not giving continuous pulse as the

flash last for few milli second.

GAS LASER:

A gas laser is a laser in which an electric current is discharged through a gas to produce light. The

gases like mixture of inert gases, nitrogen and carbon dioxides are the active mediums in gas lasers.

The gas lasers employ mixture two gases; the optical pumping is not suitable for gas lasers.

They are usually excited through electrical pumping. One of them excited by impact

accelerated electron and they in turn transfer their energy to other gas which has the actual active

centers. The laser is obtained by four level pumping scheme in gas lasers.

The helium-neon laser (He-Ne) emits at a variety of wavelengths and units operating at 633 nm

are very common in education because of its low cost.

Carbon dioxide lasers can emit hundreds of kilowatts at 9.6 µm and 10.6 µm, and are often

used in industry for cutting and welding. The efficiency of a CO2 laser is over 10%.

Argon-ion lasers emit light in the range 351-528.7 nm. Depending on the optics and the laser

tube a different number of lines is usable but the most commonly used lines are 458 nm, 488 nm and

514.5 nm.

The nitrogen gas laser due to transverse e lectrical discharge in gas at a tmospheric pressure ,

laser is an inexpensive, gas laser producing UV Light at 337.1 nm.[14]

Metal ion lasers are gas lasers that generate deep ultraviolet wavelengths. Helium-silver (He-Ag) 224

nm and neon-copper (Ne-Cu) 248 nm are two examples. These lasers have particularly narrow

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

oscillation line widths of less than 3 GHz (0.5 picometers),[15] making them candidates for use in

fluorescence suppressed Raman spectroscopy.

Helium-neon laser:

The first gas laser, Helium-neon laser was co-invented by American physicist William R.

Bennett, Jr. and Iranian physicist Ali Javan in 1960.

It consists of a long glass tube of length about 20cm and diameter about 1cm, two end faces of the tube

perfectly parallel and are silvered. The mixture of He and Ne gas is filled in the ratio 10:1 is filled in

the tube at low pressure. A strong electric field is applied across the ends of the tube.

When the power is switched on, high voltage across the ends create pumping action, the gas

molecules inside the tube are ionized. The electrons accelerate towards anode and collide with the He

atom in their path. The K.E. of electrons is transferred to He atoms and raise them to metastable state

F2 of energy 19.81eV and F3 of energy 20.61 eV from ground state.

The He atoms returns to ground state by transferring their energy to the energy states E 4 and E6 of Ne

atoms of energy 18.7eV and 20.66eV nearly equal to that of He (as shown in figure). This is

population inversion in case of Ne atoms.

The atoms of Ne in metastable state E6 transfer its energy to E5 and E3 which are practically empty by

the emission of coherent radiation of 339nm and 632.8nm respectively, optical output powers ranging

from 1 mW to 100 mW.

Similarly, the atoms in metastable state E4 transfer its energy to E3 by the emission of coherent

radiation of 115nm. The atoms return to state E2 by spontaneous emission and finally return to ground

state by non radiative transformation, in this transition, the atoms suffer energy loss by collision with

the fellow atoms and walls of the tube.

F3 = 20.66eV E

6 = 20.61eV

F

2 = 19.81eV E

5

E4

E3

E2

F1

Ground state E1

Passing photon

Non Radiative transition

Metastable state

Coherent beam (Laser)

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

He-Ne laser is a useful source for holography and experiments of spectroscopy. This is cheap laser

that is used for pointing the objects.

Semiconductor Laser:

Commonly known semiconductors Silicon (Si) and Germanium (Ge) indirect band gap conductors, in

which maxima of valance band and minima of conduction band are not at same ‘k’ value. In such

conductors de-excitation of electron takes place with the emission heat.

There are another class of semiconductors called Direct band gap semiconductors, usually these are

compound semiconductors, example are GaAs, AlGaAs, CdSe etc., Here de-excitation of electron

takes place by the emission of electromagnetic radiation and these are used to produce Light emitting

diodes (LED) and Lasers.

Hence the semiconductor like GaAs and compounds of GaAs are the better materials to produce the

semiconductor lasers.

Construction:

The basic type of Semiconductor laser is constructed by the GaAs semiconductor This just a common

p-n junction diode but heavily doped when used as laser. The schematic diagram of a homo junction

semiconductor laser (the p and n type material are made of same type of materials) is shown in figure.

The diode is extremely small in size with sides of the order of few hundreds of microns.

The junction lies in a horizontal plane through the centre. The

top and bottom faces are coated with metal layers and ohmic

contact are provided to pass the current through diode. The

front and rear faces are parallel to each other, perpendicular to

plane of junction and are polished. The junction or depletion

layer is active reason and is about 1μm thick.

Under normal condition of p-n junction the Fermi energy level

is lies in between the conduction band and valancy band (as shown in fig. a). At p side fermi level lies

nearer to the valany band and at n side it lies nearer the conduction band

LASER

Passing photon

Heavily doped GaAs diode with saturation current

Heavily doped GaAs diode Ordinary junction diode

p-type

n-type

p-type

n-type

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

When the p and n semiconductors are doped heavily the thickness of depletion layer decreases

and hence Fermi level crosses the conduction band in n-type material and valency band in p-type

material (in fig. b).

On the application of forward voltage across the diode to heavily doped semiconductor diode,

slowly recombination starts at low current (slightly above barrier potential) and diode emits incoherent

photons of energy lies in infrared or visible reason and the device is called as light emitting diode.

When junction voltage of heavily doped semiconductor is go on increased slowly above the

barrier potential, the current reaches its threshold (maximum) value, due to maximum energy the

electrons surges into depletion region and occupied into conduction band and the valancy band is

occupied with the holes or vacant levels, this create the situation of population inversion. A small

pulse of photon starts the stimulation emission and hence the atoms return to ground state by the

emission of coherent radiation of wavelength range from infrared to visible red depends upon the

percentage of impurities present in the semi conducting material. For Ex. At room temperature

GaAs laser emits light of wave length of 9000Ao in IR reason and GaAsP laser emits light of wave

length 6500Ao

Laser diodes find wide use in telecommunication as easily modulated and easily coupled light

sources for fiber optics communication. They are used in various measuring instruments, eg.

Rangefinders.

Visible lasers, typically red but later also green, are common as laser pointers. Both low and

high-power diodes are used extensively in the printing industry both as light sources for scanning

(input) of images and for very high-speed and high-resolution printing plate (output) manufacturing.

Infrared and red laser diodes are common in CD players, CD-ROMs and DVD technology. Violet

lasers are used in HD-DVD and Blue-Ray technology.

Applications which may today or in the future make use of the "coherent" properties of diode-laser-

generated light include interferometric distance measurement, holography, coherent communications,

and coherent control of chemical reactions.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Applications where the ability to "generate ultra-short pulses of light" by the technique known as

"mode-locking" include clock distribution for high-performance integrated circuits, high-peak-power

sources for laser-induced breakdown spectroscopy sensing, arbitrary waveform generation for radio-

frequency waves, photonic sampling for analog-to-digital conversion, and optical code-division-

multiple-access systems for secure communication.

Characteristics of Lasers: a.

Coherence: The light radiations emitted by laser source will be in phase and are of same frequency.

Therefore light generated by laser is highly coherent. The coherence length lcoh can be determined by

λµλ∆

=2

cohl or dtclcoh .=

where μ – Refractive index of medium for wave length – λ, c- velocity of light, dt – time of pulsed

generated.

b. Directionality: The laser emit light only in one direction as the photons traveling along the optical

axis of the systed are selected and augmented with the help of the optical resonator.

c. Divergence: The divergence or angular spread of the laser beam is extremely small. The little

divergence that exists in it arises out of the wave properties of light. When the light issues out from

the front mirror, it undergoes diffraction because the semitransparent mirror acts as a circular aperture.

Accordingly, it spreads out and the angular spread is given by

δθ = 1.22λ/d

Where d is diameter of the front mirror. In case of gas lasers is as small as 10-5 to 10-6 radians.

d. Intensity: The laser emits light in form of a narrow beam which propagates in the form of plane

waves. As the energy is concentrated in a very narrow region, its intensity would be tremendously

high. It is estimated that light from a typical 1mW laser is 10,000 times brighter than the light from the

sun at earth surface. The intensity of the laser beam stays nearly constant with distance as the light

travels in the form of plane wave.

e. Monochromaticity: The laser light is highly monochromatic. The spread is of the order of a few

angstroms (less than 10Ao) only where as normal monochromatic source has spread 1000 Ao. Such

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

vast difference arises because of conventional sources emit wave trains of the short duration and

length, whereas lasers emit continuous waves of very long duration.

Applications of lasers: The laser has number of applications in the field of communication,

engineering, medical, defense, Entertainment etc.

Holography:

Holography is a technique by which the 3-dimensional image of an object is recorded by using the

laser light.

Holography is invented in 1947 by Hungarian physicist Dennis Gabor (1900–1979), work for which he

received the Nobel Prize in physics in 1971.

The difference between holography and photography is best understood by considering what a black

and white photograph actually is: it is a point-to-point recording of the intensity of light rays that make

up an image. Each point on the photograph records just one thing, the intensity (i.e. the square of the

amplitude of the electric field) of the light wave that illuminates that particular point. In the case of a

colour photograph, slightly more information is recorded (in effect the image is recorded three times

viewed through three different colour filters), which allows a limited reconstruction of the wavelength

of the light, and thus its colour. Although colour holograms are possible, in most cases the holograms

are recorded monochromatically.

But the holographic technique records the information about the variation phase of the light

when illuminated on the object along with the intensity, Such that if the path covered by two different

rays illuminated at two different points then there is variation in phase, hence the difference in the path

hence phase also retained in the image. This variation gives impression of 3-Dimensional image

Holographic recording process: This process contains two parts.

1. Preparation of Hologram 2. Reconstruction of image using Hologram.

1. Preparation of Hologram: The holograms are prepared by the procedure given below.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

AS illustrated in above figure , A weak but broad beam of laser light is split into two beams (in fig. it

is done by using a partial reflector, the beam which reflect is reference beam and that transmits and

focused on object is object beam) namely a reference beam and object beam. The reference beam is

allowed to reach the photographic plate (which later called hologram) directly and while object beam

illuminates the object and the beam scattered from the object also made to reach photographic plate.

Both object beam and reference beams are same source and coherent, are superposed at the

photographic plate and produce a complex interference pattern. The interference pattern of the

photographic plate that carries the complete information about object is called as HOLOGRAM.

2. Reconstruction of image: The hologram will not contain any picture of the object but it contains

interference pattern. One can not get the image directly from the Hologram. To obtain 3-dimantional

image, the Hologram is illuminated with the laser beam that is of same type by which hologram is

prepared. For this beam the interference pattern acts as a diffraction grating. When laser passed

through the hologram two 3-d images are obtain one in front of hologram is called real image and other

is behind the hologram is called as virtual image. The distance of image from the hologram is same as

that of the distance of object from the photographic plate while preparing the hologram.

LASER

Hologram

Virtual image Real image

Reflector

Hologram

Object beam

Reference beam

LASER source

Object

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Rather, a small portion of a hologram's surface contains enough information to reconstruct the entire

original scene, but only what can be seen from that small portion as viewed from that point's

perspective. This is possible because during holographic recording, each point on the hologram's

surface is affected by light waves reflected from all points in the scene, rather than from just one point.

It's as if, during recording, each point on the hologram's surface were an eye that could record

everything it sees in any direction. After the hologram has been recorded, looking at a point in that

hologram is like looking "through" one of those eyes.

Advantages of Holography over the traditional photograph:

1. The traditional photograph is two dimensional image of three dimensional object, where as hologram gives three dimensional image of three dimensional object.

2. In traditional photograph is obtained only one image from one film. The information storage capacity of hologram is extremely high. A hologram of size 9x6 mm2 can store about three hundred photographs.

3. One can not get the complete information about the image even a small part of the film is damaged in traditional photograph, as each region of the film contains a separate and individual part of original object, but a small portion of a hologram also contains complete information of image. Hence hologram is reliable medium for data storage.

4. The information of hologram is used store and send the secret information in the form of interference patterns or codes because, the laser light used to reconstruct the image should have the wave front identical to the wave front of the reference beam by which hologram is constructed and otherwise the interference pattern can not be decoded. It is not possible in traditional photograph.

5. The magnified image of the object can be obtained from the hologram if the laser light have larger wavelength than the wavelength of reference beam, but wave front should be matched.

6. As it is easy to produce coherent sound waves and sound readily passes through the solids and liquids, the three dimensional acoustical hologram of an opaque object is formed. By viewing the hologram in visible light the internal structure of the object can be observed. Such technique is useful in the field of medicine and technology.

Laser Cutting: It is a technology that uses a laser to cut materials, and is usually used in industrial

manufacturing. Laser cutting works by directing the output of a high power laser, by computer, at the

material to be cut. A high power laser about few kW power is focused and moved on the on the cutting

material. The material then either melts, burns, vaporizes away, or is blown away by a jet of gas,[1]

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

leaving an edge with a high quality surface finish. Industrial laser cutters are used to cut flat-sheet

material as well as structural and piping materials.

Advantages of laser cutting over mechanical cutting vary according to the situation, but two important

factors are the lack of physical contact (since there is no cutting edge which can become contaminated

by the material or contaminate the material), and to some extent precision (since there is no wear on

the laser). There is also a reduced chance of warping the material that is being cut as laser systems

have a small heat affected zone. Some materials are also very difficult or impossible to cut by more

traditional means. One of the disadvantages of laser cutting may include the high energy required.

Both gaseous CO2 and solid-state Nd: YAG lasers are used for cutting, in addition to welding, drilling, surface treatment, and marking applications. Common variants of CO2 lasers include fast axial flow, slow axial flow, transverse flow, and slab.

Laser beam welding (LBW): It is a welding technique used to join multiple pieces of metal through the use of a laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. The process is frequently used in high volume applications, such as in the automotive industry.

Laser beam welding has high power density (on the order of 1 Megawatt/cm²(MW)) resulting in small heat-affected zones and high heating and cooling rates. The spot size of the laser can vary between 0.2 mm and 13 mm, though only smaller sizes are used for welding. The depth of penetration is proportional to the amount of power supplied, but is also dependent on the location of the focal point: penetration is maximized when the focal point is slightly below the surface of the workpiece.

LBW is a versatile process, capable of welding carbon steels, HSLA steels, stainless steel, aluminum, and titanium. Due to high cooling rates, cracking is a concern when welding high-carbon steels. The weld quality is high, similar to that of electron beam welding. The speed of welding is proportional to the amount of power supplied but also depends on the type and thickness of the workpieces. The high power capability of gas lasers make them especially suitable for high volume applications. LBW is particularly dominant in the automotive industry.

• The two types of lasers commonly used in are solid-state lasers and gas lasers (especially carbon dioxide lasers and Nd:YAG lasers).

• The first type uses one of several solid media, including synthetic ruby and chromium in aluminum oxide, neodymium in glass (Nd:glass), and the most common type, crystal composed of yttrium aluminum garnet doped with neodymium (Nd:YAG).

• Gas lasers use mixtures of gases like helium, nitrogen, and carbon dioxide (CO2 laser) as a medium.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

PN-junction Laser: A semiconductor laser is a specially fabricated pn junction device (both the p and n regions are highly

doped) which emits coherent light when it is forward biased. It is made from Gallium Arsenide (GaAs) which operated at

low temperature and emits light in near IR region. Now the semiconductor lasers are also made to emit light almost in the

spectrum from UV to IR using different semiconductor materials. They are of very small size (0.1 mm long), efficient,

portable and operate at low power. These are widely used in Optical fibre communications, in CD players, CD-ROM Drives,

optical reading, laser printing etc.

p and n regions are made from same semiconductor material (GaAs). A p type region is formed on the n type by doping

zinc atoms. The diode chip is about 500 micrometer long and 100 micrometer wide and thick. the top and bottom faces

has metal contacts to pass the current. the front and rare faces are polished to constitute the resonator (fig 1).

When high doped p and n regions are joined at the atomic level to form

pn-junction, the equilibrium is attained only when the equalization of

fermi level takes place in this case the fermi level is pushed inside the

conduction band in n type and the level pushed inside the valence band

in the p type (Fig 2).

When the

junction is

forward biased,

at low voltage the electron and hole recombine and cause

spontaneous emission. But when the forward voltage reaches a

threshold value the carrier concentration rises to very high value.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

As a result the region "d" contains large number of electrons in the conduction band and at the same time large number of

holes in the valence band. Thus the upper energy level has large number of electrons and the lower energy level has large

number of vacancy, thus population inversion is achieved. The recombination of electron and hole leads to spontaneous

emission and it stimulate the others to emit radiation. Ga As produces laser light of 9000 Å in IR region.

Energy Level Diagram of Semiconductor Laser

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.

Different Pumping schemes in LASER – by Praveen Vaidya, SDM College of Engg. And Tech. DWR.