Classification of LASER�

By:

SALONI SHARMA

DEPTT. OF PHYSICS

Understanding the principle of laser

• The basic processes involved for the production of LASER are:

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• Stimulated absorption
• Spontaneous emission
• Stimulated emission
• Population inversion

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POPULATION INVERSION

Most lasers are based on 3 or 4 level energy level systems, which depends on the lasing medium..

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When the population inversion exists between upper and lower levels among atomic systems, it is possible to realize amplified stimulated emission and the stimulated emission has the same frequency and phase as the incident radiation

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Laser Action

• Laser action is preceded by three processes, namely, absorption, spontaneous emission and stimulated emission - absorption of energy to populate upper levels, spontaneous emission to produce the initial photons for stimulation and finally, stimulated emission for generation of coherent output or laser
• Interaction of electromagnetic radiation with matter produces absorption and spontaneous emission. Absorption and spontaneous emission are natural processes. For the generation of laser, stimulated emission is essential. Stimulated emission has to be induced or stimulated.

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• The atom stays at the higher level for a certain duration and decays to the lower stable ground level spontaneously, emitting a photon, with a wavelength decided by the difference between the upper and the lower energy levels. This is referred to as natural or spontaneous emission and the photon is called spontaneous photon. The spontaneous emission or fluorescence has no preferred direction and the photons emitted have no phase relations with each other, thus generating an incoherent light output.

• But it is not necessary that the atom is always de-excited to ground state. It can go to an intermediate state, called metastable state with a radiation less transition, where it stays for a much longer period than the upper level and comes down to lower level or to the ground state. Since period of stay of atoms in the metastable state is large, it is possible to have a much larger number of atoms in metastable level in comparison to the lower level so that the population of metastable state and the lower or ground state is reversed. i.e. there are more atoms in the upper metastable level than the lower level. This condition is referred to as population inversion.

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• The atom in the metastable state comes down to the ground state emitting a photon. This photon can stimulate an atom in the metastable state to release its photon in phase with it. The photon thus released is called stimulated photon. It moves in the same direction as the initiating photon, has the same wavelength and polarization and is in phase with it, thus producing amplification. Since there are a large number of initiating photons, it forms an initiating electromagnetic radiation field. An avalanche of stimulated photons is generated, as the photons traveling along the length of the active medium stimulates a number of excited atoms in the metastable state to release their photons. This is referred to as the stimulated emission

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• These photons are fully reflected by the rear reflector (100% reflective) and the number and consequently the intensity of stimulated photons increases as they traverse through the active medium, thus increasing the intensity of radiation field of stimulated emission.
• At the output coupler, a part of these photons are reflected and the rest is transmitted as the laser output. This action is repeated and the reflected photons after striking the rear mirror, reach the output coupler in the return path. The intensity of the laser output increases as the pumping continues.
• When the input pumping energy reduces, the available initiating and subsequently the stimulated photons decrease considerably and the gain of the system is not able to overcome the losses, thus laser output ceases. Since the stimulation process was started by the initiating photons, the emitted photons can combine coherently, as all of them are in phase with each other, and coherent laser light is emitted.

• Though the laser action will continue as long as the energy is given to the active medium, it may be stated that pulsed laser is obtained if the population inversion is available in a transient fashion and continuous wave (CW) laser is possible if the population inversion is maintained in a steady-state basis.
• If the input energy is given by say a flash lamp, the output will be a pulsed output and the laser is called a pulsed laser. If equilibrium can be achieved between the number of photons emitted and the number of atoms in the metastable level by pumping with a continuous arc lamp instead of a flash lamp, then it is possible to achieve a continuous laser output, which is called continuous wave laser.

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Basic components of laser system

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• Active medium- An active medium with a suitable set of energy levels to support laser action.
• Pump- A source of pumping energy in order to establish a population inversion.
• Resonant cavity- An optical cavity or resonator to introduce optical feedback and so maintain the gain of the system overcoming all losses.

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1. Active laser medium or gain medium: 

• Laser medium is the heart of the laser system and is responsible for producing gain and subsequent generation of laser.
• It can be a crystal, solid, liquid, semiconductor or gas medium and can be pumped to a higher energy state.
• The material should be of controlled purity, size and shape and should have the suitable energy levels to support population inversion. In other words, it must have a metastable state to support stimulated emission.

2. Excitation or pumping mechanism: 

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• One of the requirements of laser action is population inversion in the levels concerned. i.e. to have larger population in the upper levels than in the lower ones. Otherwise absorption will dominate at the cost of stimulated emission.
• The commonly used excitation or pumping mechanisms ones are optical, electrical, thermal or chemical techniques, which depends on the type of the laser gain medium employed.
• Solid state lasers usually employ optical pumping from high energy xenon flash lamps (e.g., ruby, Nd:YAG) or from a second pump laser or laser diode array (e.g., DPSS frequency doubled green lasers).
• Gas lasers use an AC or DC electrical discharge through the gas medium, or external RF excitation, electron beam bombardment, or a chemical reaction. The DC electrical discharge is most common for 'small' gas lasers (e.g., helium-neon, argon ion, etc.). DC most often pumps semiconductor lasers current.
• Liquid (dye) lasers are usually pumped optically.

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Optical Resonator

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• To sustain laser action, one has to confine the laser medium and the pumping mechanism in a special way that should promote stimulated emission rather than spontaneous emission.
• Photons need to be confined in the system to allow the number of photons created by stimulated emission to exceed all other mechanisms.

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• This is achieved by bounding the laser medium between two mirrors . On one end of the active medium is the high reflectance mirror (100% reflecting) or the rear mirror and on the other end is the partially reflecting or transmittive mirror or the output coupler. The laser emanates from the output coupler, as it is partially transmittive.

Stimulated photons can bounce back and forward along the cavity, creating more stimulated emission as they go. In the process, any photons which are either not of the correct frequency or do not travel along the optical axis are lost.

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• Optical resonator plays a very important role in the generation of the laser output
• in providing high directionality to the laser beam
• producing gain in the active medium to overcome the losses due to, straying away of photons from the laser medium,
• diffraction losses due to definite sizes of the mirrors,
• radiation losses inside the active medium due to absorption and scattering etc.

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Types of Resonant Cavities

Classification of LASER

• LASERS can be classified based on
• Energy Levels involved in LASER action:
• Three level LASER
• Four level LASER

(b) Mode of Operation:

• Continous wave LASER
• Pulsed LASER

(c) Type of Active Medium:

• Solid LASER
• Gas LASER
• Dye LASER

Three level LASER

• In case of a three-level laser, the material is pumped from level 1 to level 3, which decays rapidly to level 2 through spontaneous emission. Level 2 is a metastable level and promotes stimulated emission from level 2 to level 1.

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Four level LASER

•  In a four level laser, the material is pumped to level 4, which is a fast decaying level, and the atoms decay rapidly to level 3, which is a metastable level.
• The stimulated emission takes place from level 3 to level 2 from where the atoms decay back to level 1.
• Four level lasers is an improvement on a system based on three level systems. In this case, the laser transition takes place between the third and second excited states.
• Since lower laser level 2 is a fast decaying level which ensures that it rapidly gets empty and as such always supports the population inversion condition.

MODE OF OPERATION

• Continous LASER:
• If the output is obtained in continous waveform.
• Generally four level LASERs operate in continous mode.
• Pulsed LASER:
• If the output is obtained in the form of pulses.
• Generally three level LASERs operate in pulsed mode.

Based on Type of Active Medium

• This active element is cut into a cylindrical rod. The ends of the cylindrical rod are highly polished and they are made optically flat and parallel. This cylindrical rod (laser rod) and a pumping source (flash tube) are placed inside a highly (reflecting) elliptical reflector cavity.

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• The active medium Nd: YAG rod is optically pumped by Krypton flash tubes. The Neodymium ions (Nd3+) are raised to excited levels. During the transition from meta stable state to ground state, a laser beam of wavelength 1.064μm is emitted.

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• The optical resonator is formed by using two external reflecting mirrors. One mirror (M1) is 100% reflecting while the other mirror (M2) is partially reflecting.

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�Nd: YAG laser�

Energy level diagram

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These energy levels are those of Neodymium (Nd3+) ions.

•  When the krypton flash lamp is switched on, by the absorption of light

radiation of wavelength 0.73μm and 0.8μm, the Neodymium(Nd3+) atoms are raised from ground level E0 to upper levels E3and E4 (Pump bands).

2.     The Neodymium ions atoms make a transition from these energy levels E2 by non-radiative transition. E2 is a metastable state.

3.     The Neodymium ions are collected in the level E2 and the population inversion is achieved between E2 and E1.

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4.     An  ion  makes  a  spontaneous  transition  from  E2  to  E1,  emitting  a photon of energy hγ = E2 - E1. This emitted photon will trigger a chain of stimulated photons between E2 and E1.

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5.     The photons thus generated travel back and forth between two mirrors and grow in strength. After some time, the photon number multiplies more rapidly.

6.     After enough strength is attained (condition for laser being satisfied),

an intense laser light of wavelength 1.06μm is emitted through the partial reflector. It corresponds to the transition from E2to E1.

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�Characteristics�

1.  Type: It is a four level solid state laser.

2.     Active medium: The active medium is Nd: YAG laser.

3.     Pumping method: Optical pumping is employed for pumping action.

4.     Pumping source: Xenon or Krypton flash tube is used as pumping source.

5.     Optical resonator: Two ends of Nd: YAG rod is polished with silver (one end is fully silvered and the other is partially silvered) are used as optical resonator.

6.     Power output: The power output is approximately 70 watt.

7.     Nature of output: The nature of output is pulsed or continuous beam of light.

8.     Wavelength of the output: The wavelength of the output beam is 1.06μm(infra-red)

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Applications

• Nd:YAG laser can be used in manufacturing for engraving and etching various metals and plastics, and for cutting and welding semiconductors, steel and other alloys.
• It is also employed for making subsurface markings in transparent materials such as acrylic glass or glass. It produces continuous laser at room temperature, and can be used as a portable system as the rods are small.
• It is used in ophthalmology to correct posterior capsular opacification, and oncology to treat benign thyroid nodules, primary and secondary malignant liver lesions and skin cancer.
• It can also be used for flow visualization techniques in fluid dynamics
• It is the most common laser used in laser rangefinders and laser designators
• It is used as pumping tunable visible light lasers
• It is used for research applications such as mass spectrometry, remote sensing and Raman spectroscopy.

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Ruby Laser

CONSTRUCTION

• The Ruby laser consist of a single ruby crystal in the form of a cyclindrical rod of length about 5 cm and diameter 0. 5 cm.

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• The Ruby rod is placed inside a Xenon flash lamp. Ruby crystals have high mechanical strength, high thermal conductivity.

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• The flash lamp is connected to a capacitor which discharges a few thousand joules of energy in a few milliseconds. This results in power output of a few megawatt from Xenon lamp.

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• Its end surfaces are accurately plane and parallel. One of the ends is silvered with 100% reflectivity and the other end with 10% transmission. The two ends thus form a resonant cavity.

• Chromium ions gets excited to these levels by absorbing energy from xenon flash lamp (i. e. optical pumping).

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• There are to main pump bands ( called the blue band & green band). 4 F 1 & 4 F 2 centered at 0. 42 μm & 0. 55μm respectively. These bands are about 1000Å wide each and they have a very small lifetime (≤ 10 -9 s)

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• There is a fast non-radiative decay from these bands to 2 E-state, which is a metastable state and has a life time of 3*10 -3 s. The metastable level 2 E is split into two sub levels with a separation of ΔE=29 cm-1.

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• Thus , population inversion takes between 2Ā and Ē levels and the ground state 4 A 2 emitting a sharp doublet R 1( λ=6943Å) and R 2(λ=6928Å) lines.

Applications

• Range finding is one of the first applications of the ruby laser.
• It was initially used to optically pump tunable dye lasers.
• Laser metal working systems for drilling holes in hard materials
• High-power systems for frequency doubling into the UV spectrum
• High-brightness holographic camera systems with long coherent length
• Medical laser systems for tattoo removal and cosmetic dermatology
• High-power Q-switched system

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GAS LASERS

He-Ne LASER

It consists of a long and narrow tube about 25 cm in length and 1cm in diameter filled with He and Ne gases in the ratio of 10:1, at a pressure of about 1mm of mercury for He and 0.1mm of mercury for Ne.

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High potential difference is applied between two electrodes provided near the ends of the tube to excite the gas atoms through electrical pumping.

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The gas mixture is enclosed between two mirrors forming resonant cavity.

A standing wave is produced by the cascading photons if mirrors at an appropriate distance apart are placed at either end of the gain medium in the lasing cavity

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He-Ne LASER

The mechanism producing population inversion and light amplification in a He-Ne laser originates with inelastic collision of energetic electrons with ground-state helium atoms in the gas mixture.

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When energetic electrons move between the electrodes and collide with the helium, the helium atoms become excited from the ground state to higher energy excited states, among them the 2³S₁ and 2¹S₀ are long-lived metastable states.

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• Because of a near-coincidence between the energy levels of the two He metastable states and the 5s₂ and 4s₂ levels of neon, the collisions between these helium metastable atoms and ground-state neon atoms results in a selective and efficient transfer of excitation energy from the helium to neon atoms collide with the neon atoms with enough energy to push the neon atoms into a higher, stable, state which causes a population inversion..

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• When a photon with proper energy collides with one of these neon atoms, the atom emits an additional photon of this same energy and momentum. This collision and emission process causes a cascade of photons from the neon population that all occur at the same phase angle and frequency.

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• The various transitions leads to emission at wavelength 3.39 μm( corresponding to 5s₂ to 4p₄ transition), 1.15 μm (corresponding to the 4s₂ to 3p₄ transition) and in a narrow band at 632.8 nm (corresponding to the 5s₂ to 3p₄ transition).

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•  The first two lie in IR, the 3.39 μm transition has a very high gain, but is prevented from use in an ordinary He-Ne laser, because the cavity and mirrors are lossy at that wavelength.

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• The best-known and most widely used He-Ne laser operates at a wavelength of 632.8 nm, in the red part of the visible spectrum.

• The mirrors used in internal cavity lasers get eroded by gas discharge and have to be replaced by external mirrors.

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• External mirrors cause reflection losses which can be avoided using Brewster’s windows at two ends of the discharge tube which produces a laser beam polarized in plane of paper.

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• Spherical mirrors in confocal arrangement can be used as resonant cavity.

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Carbon di-oxide LASER

Molecular Gas laser

In a molecular gas laser, laser action is achieved by transitions between vibrational and rotational levels of molecules. Its construction is simple and the output of this laser is continuous.

In CO2 molecular gas laser, transition takes place between the vibrational states of Carbon dioxide molecules.

CO2 Molecular gas laser

It was the first molecular gas laser developed by Indian born American scientist Prof.C.K.N.Pillai.

It is a four level laser and it operates at 10.6 μm in the far IR region. It is a very efficient laser.

Energy states of CO2 molecules.�

A carbon dioxide molecule has a carbon atom at the center with two oxygen atoms attached, one at both sides. Such a molecule exhibits three independent modes of vibrations. They are

a)     Symmetric stretching mode.

b)    Bending mode

c)     Asymmetric stretching mode

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Symmetric stretching mode

• In this mode of vibration, carbon atoms are at rest and both oxygen atoms vibrate simultaneously along the axis of the molecule departing or approaching the fixed carbon atoms.

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 Bending mode

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In this mode of vibration, oxygen atoms and carbon atoms vibrate perpendicular to molecular axis.

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Asymmetric stretching mode

• In this mode of vibration, oxygen atoms and carbon atoms vibrate asymmetrically, i.e., oxygen atoms move in one direction while carbon atoms in the other direction.

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Construction and Working

It consists of a quartz tube 5 m long and 2.5 cm in the diameter. This discharge tube is filled with gaseous mixture of CO2(active medium), helium and nitrogen with suitable partial pressures.

The terminals of the discharge tubes are connected to a D.C power supply.

The ends of the discharge tube are fitted with NaCl Brewster windows so that the laser light generated will be polarized.

Two concave mirrors one fully reflecting and the other partially form an optical resonator.

The active medium is a gas mixture of CO2, N2 and He. The laser transition takes place between the vibrational states of CO2molecules.

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When an electric discharge occurs in the gas, the electrons collide with nitrogen molecules and they are raised to excited states. This process is represented by the equation N2 + e* = N2* + e

N2 = Nitrogen molecule in ground state e* = electron with kinetic energy

N2* = nitrogen molecule in excited state e= same electron with lesser energy

Now N2 molecules in the excited state collide with CO2 atoms in ground state and excite to higher electronic, vibrational and rotational levels.

This process is represented by the equation N2* + CO2 = CO2* + N2

CO2 = Carbon dioxide atoms in ground state CO2* = Carbon dioxide atoms in excited state

• Since the excited level of nitrogen is very close to the E5 level of CO2 atom, population in E5 level increases.

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• As soon as population inversion is reached, any of the spontaneously emitted photon will trigger laser action in the tube. There are two types of laser transition possible.

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• 1.Transition E5  to E4 :
• This will produce a laser beam of wavelength 10.6μm.

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• 2.Transition E5  to E3
• This transition will produce a laser beam of wavelength 9.6μm. Normally 10.6μm transition is more intense than 9.6μm transition. The power output from this laser is 10kW.

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Characteristics

1.  Type: It is a molecular gas laser.

2.     Active medium: A mixture of CO2 , N2 and helium or water vapour is used as active medium

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3.     Pumping method: Electrical discharge method is used for Pumping action

4.     Optical resonator: Two concave mirrors form a resonant cavity

5.     Power output: The power output from this laser is about 10kW.

6.     Nature of output: The nature of output may be continuous wave or pulsed wave.

7.     Wavelength of output: The wavelength of output is 0.6μm and 10.6μm.

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