1. ESSENTIAL COMPONENTS OF A LASER; TYPES OF LASER, CO 2 LASER,
UNIT-3 LEC-3
ESSENTIAL COMPONENTS OF A LASER;
TYPES OF LASER,
CO 2 LASER,
Nd – YAG LASER (Doped Insulator laser),
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2. Essential components of a laser system : Active medium or Gain medium : It is the system in which population inversion and hence stimulated emission (laser action) is established. Pumping mechanism : It is the mechanism by which population inversion is achieved. i.e., it is the method for raising the atoms from lower energy state to higher energy state to achieve laser transition.
Active Medium
Pumping Mechanism
Optical resonator
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3. DIFFERENT PUMPING MECHANISMS :
i. Optical pumping : Exposure to electromagnetic radiation of frequency  = (E2-E1)/h obtained from discharge flash tube results in pumping Suitable for solid state lasers.
ii. Electrical discharge : By inelastic atom-atom collisions, population inversion is established.
Suitable for Gas lasers
Chemical pumping : By suitable chemical reaction in the active medium, population of excited state is made higher compared to that of ground state Suitable for liquid lasers.
Optical resonator : A pair of mirrors placed on either side of the active medium is known as optical resonator. One mirror is completely silvered and the other is partially silvered. The laser beam comes out through the partially silvered mirror.
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4. Types of Lasers(Based on its pumping action) :
Optically pumped laser
Electrically pumped laser
Basis of the operation mode
Continuous wave Lasers
Pulsed Lasers
According to their wavelength :
Visible Region, Infrared Region, Ultraviolet Region, Microwave Region, X-Ray Region and etc.,
According to the source :
Dye Lasers, Gas Lasers, Chemical Lasers, Metal vapour Lasers, Solid state Lasers, Semi conductor Lasers and other types.
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5. Laser gain medium and type Operation wavelength(s)
DYE LASERS
Laser gain medium and type
Operation wavelength(s)
Pump source
Applications
Dye lasers
nm (stilbene), nm (coumarin 102), nm (rhodamine 6G), many others
Other laser, flash lamp
Research, spectroscopy, birthmark removal, isotope separation. The tuning range of the laser depends on which dye is used.
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6. GAS LASERS LASER GAIN MEDIUM AND TYPE OPERATION WAVELENGTH(S)
PUMP SOURCE
APPLICATIONS AND NOTES
Helium-neon laser
632.8 nm (543.5 nm, nm, nm, μm, 1.52 μm, μm)
Electrical discharge
Interferometry, holography, spectroscopy, barcode scanning, alignment, optical demonstrations.
Argon laser
454.6 nm, nm, nm (351 nm,457.9 nm, nm, nm, nm, nm)
Retinal phototherapy (for diabetes), lithography, confocal microscopy, pumping other lasers.
Krypton laser
416 nm, nm, nm, nm, nm, nm, nm
Scientific research, mixed with argon to create "white-light" lasers, light shows.
Xenon ion laser
Many lines throughout visible spectrum extending into the UV and IR.
Scientific research.
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7. UNIT III Lecture 3 LASER GAIN MEDIUM AND TYPE OPERATION WAVELENGTH(S)
PUMP SOURCE
APPLICATIONS AND NOTES
Nitrogen laser
337.1 nm
Electrical discharge
Pumping of dye lasers, measuring air pollution, scientific research. Nitrogen lasers can operate superradiantly (without a resonator cavity).
Carbon dioxide laser
10.6 μm, (9.4 μm)
Transverse (high power) or longitudinal (low power) electrical discharge
Material processing (cutting, welding, etc.), surgery.
Carbon monoxide laser
2.6 to 4 μm, 4.8 to 8.3 μm
Material processing (engraving, welding, etc.), photoacoustic spectroscopy.
Excimer laser
193 nm (ArF), 248 nm (KrF), 308 nm (XeCl), 353 nm (XeF)
Excimer recombination via electrical discharge
Ultraviolet lithography for semiconductor manufacturing, laser surgery
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8. LASER GAIN MEDIUM AND TYPE OPERATION WAVELENGTH(S)
CHEMICAL LASERS
LASER GAIN MEDIUM AND TYPE
OPERATION WAVELENGTH(S)
PUMP SOURCE
APPLICATIONS
Hydrogen fluoride laser
2.7 to 2.9 μm for Hydrogen fluoride (<80% Atmospheric transmittance)
Chemical reaction in a burning jet of ethylene and nitrogen trifluoride (NF3)
Used in research for laser weaponry by the U.S. DOD, operated in continuous wave mode, can have power in the megawatt range.
Deuterium fluoride laser
~3800 nm (3.6 to 4.2 μm) (~90% Atm. transmittance)
chemical reaction
MIRACL, Pulsed Energy Projectile & Tactical High Energy Laser
COIL (Chemical oxygen-iodine laser)
1.315 μm (<70% Atmospheric transmittance)
Chemical reaction in a jet of singlet delta oxygen and iodine
Laser weaponry, scientific and materials research, laser used in the U.S. military's Airborne laser, operated in continuous wave mode, can have power in the megawatt range.
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9. LASER GAIN MEDIUM AND TYPE OPERATION WAVELENGTH(S)
METAL-VAPOR LASERS
LASER GAIN MEDIUM AND TYPE
OPERATION WAVELENGTH(S)
PUMP SOURCE
APPLICATIONS
Helium-cadmium (HeCd) metal-vapor laser
nm, 325 nm
Electrical discharge in metal vapor mixed with helium buffer gas.
Printing and typesetting applications, fluorescence excitation examination (ie. in U.S. paper currency printing), scientific research.
Helium-mercury (HeHg) metal-vapor laser
567 nm, 615 nm
Rare, scientific research, amateur laser construction.
Helium-selenium (HeSe) metal-vapor laser
up to 24 wavelengths between red and UV
Copper vapor laser
510.6 nm, nm
Electrical discharge
Dermatological uses, high speed photography, pump for dye lasers.
Gold vapor laser
627 nm
Rare, dermatological and photodynamic therapy uses.
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10. OPERATION WAVELENGTH(S)
SOLID STATE LASERS
LASER GAIN
MEDIUM AND
TYPE
OPERATION WAVELENGTH(S)
PUMP
SOURCE
APPLICATIONS
Ruby laser
694.3 nm
Flashlamp
Holography, tattoo removal. The first type of visible light laser invented; May 1960.
Nd:YAG laser
1.064 μm, (1.32 μm)
Flashlamp, laser diode
Material processing, rangefinding, laser target
designation, surgery, research, pumping other lasers
(combined with frequency doubling to produce a
green 532 nm beam). One of the most common high
power lasers. Usually pulsed (down to fractions of
a nanosecond)
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11. OPERATION WAVELENGTH(S)
LASER GAIN
MEDIUM AND
TYPE
OPERATION WAVELENGTH(S)
PUMP
SOURCE
APPLICATIONS
Er:YAG laser
2.94 μm
Flashlamp, laser diode
Periodontal scaling, Dentistry
Neodymium doped Yttrium orthovanadate (Nd:YVO4) laser
1.064 μm
laser diode
Mostly used for continuous pumping of mode-locked Ti:sapphire or dye lasers, in combination with frequency doubling. Also used pulsed for marking and micromachining.
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12. LASER GAIN MEDIUM AND TYPE OPERATION WAVELENGTH(S)
PUMP SOURCE
APPLICATIONS
Neodymium doped yttrium calcium oxoborate Nd:YCa4O(BO3)3 or simply Nd:YCOB
~1.060 μm (~530 nm at second harmonic)
laser diode
Nd:YCOB is a so called "self-frequency doubling" or SFD laser material which is both capable of lasing and which has nonlinear characteristics suitable for second harmonic generation. Such materials have the potential to simplify the design of high brightness green lasers.
Neodymium glass (Nd:Glass) laser
~1.062 μm (Silicate glasses), ~1.054 μm (Phosphate glasses)
Flashlamp, laser diode
Used in extremely high power (terawatt scale), high energy (megajoules) multiple beam systems for inertial confinement fusion. Nd:Glass lasers are usually frequency tripled to the third harmonic at 351 nm in laser fusion devices.
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13. LASER GAIN MEDIUM AND TYPE OPERATION WAVELENGTH(S)
PUMP SOURCE
APPLICATIONS
Titanium sapphire (Ti:sapphire) laser nm
Other laser
Spectroscopy, LIDAR, research. This material is often used in highly-tunable mode-locked infrared lasers to produce ultrashort pulses and in amplifier lasers to produce ultrashort and ultra-intense pulses.
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14. Remote atmospheric sensing, LIDAR, optics research.
Cerium doped lithium strontium(or calcium) aluminum fluoride (Ce:LiSAF, Ce:LiCAF)
~280 to 316 nm
Frequency quadrupled Nd:YAG laser pumped, excimer laser pumped, copper vapor laser pumped.
Remote atmospheric sensing, LIDAR, optics research.
Chromium doped chrysoberyl (alexandrite) laser
Typically tuned in the range of 700 to 820 nm
Flashlamp, laser diode, mercury arc (for CW mode operation)
Dermatological uses, LIDAR, laser machining.
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15. SEMICONDUCTOR LASERS :
Laser gain medium and type
Operation wavelength(s)
Pump source
Applications
Semiconductor laser diode (general information)
0.4-20 μm, depending on active region material.
Electrical current
Telecommunications, holography, printing, weapons, machining, welding, pump sources for other lasers.
GaN
0.4 μm
Optical discs.
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16. Telecommunications, solid-state laser pumping, machining, medical..
AlGaAs
 μm
Electrical current
Optical discs, laser pointers, data communications. 780 nm Compact Disc player laser is the most common laser type in the world. Solid-state laser pumping, machining, medical.
InGaAsP
 μm
Telecommunications, solid-state laser pumping, machining, medical..
Vertical cavity surface emitting laser (VCSEL)
nm, depending on material
Telecommunications
Hybrid silicon laser
Mid-infrared
Research
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17. Laser gain medium and type Operation wavelength(s)
OTHER TYPES OF LASERS :
Laser gain medium and type
Operation wavelength(s)
Pump source
Applications
Free electron laser
A broad wavelength range (about 100 nm - several mm); one free electron laser may be tunable over a wavelength range
relativistic electron beam
atmospheric research, material science, medical applications.
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18. "Nickel-like" Samarium laser X-rays at 7.3 nm wavelength
Lasing in ultra-hot samarium plasma formed by double pulse terawatt scale irradiation fluences created by Rutherford Appleton Laboratory's Nd:glass Vulcan laser.
First demonstration of efficient "saturated" operation of a sub–10 nm X-ray laser, possible applications in high resolution microscopy and holography, operation is close to the "water window" at 2.2 to 4.4 nm where observation of DNA structure and the action of viruses and drugs on cells can be examined.
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19. Other laser, mostly Yb-glass fiber lasers
Raman laser, uses inelastic stimulated Raman scattering in a nonlinear media, mostly fiber, for amplification
1-2 μm for fiber version
Other laser, mostly Yb-glass fiber lasers
Complete 1-2 μm wavelength coverage; distributed optical signal amplification for telecommunications; optical solitons generation and amplification
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20. CO2 LASER Introduction :
CO2 lasers belong to the class of molecular gas lasers.
In the case of atoms, electrons in molecules can be excited to higher energy levels, and the distribution of electrons in the levels define the electronic state of the molecule.
Besides, these electronic levels, the molecules have other energy levels.
C.K.N. Patel designed CO2 laser in the year 1964.
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21. Active medium :
It consists of a mixture of CO2, N2 and helium or water vapour. The active centres are CO2 molecules lasing on the transition between the rotational levels of vibrational bands of the electronic ground state..
Optical resonators :
A pair of concave mirrors placed on either side of the discharge tube, one completely polished and the other partially polished.
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22. Population inversion is created by electric discharge of the mixture.
Pumping :
Population inversion is created by electric discharge of the mixture.
When a discharge is passed in a tube containing CO2, electron impacts excite the molecules to higher electronic and vibrational-rotational levels.
This level is also populated by radiationless transition from upper excited levels.
The resonant transfer of energy from other molecules, such as, N2, added to the gas, increases the pumping efficiency.
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23. Contd.
Nitrogen here plays the role that He plays in He-Ne laser.
A carbon dioxide (CO2) laser can produce a continuous laser beam with a power output of several kilowatts while, at the same time, can maintain high degree of spectral purity and spatial coherence.
In comparison with atoms and ions, the energy level structure of molecules is more complicated and originates from three sources: electronic motions, vibrational motions and rotational motions.
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24. Fundamental Modes of vibration of CO2 :
Three fundamental modes of vibration for CO2
Symmetric stretching mode (frequency 1),
Bending mode (2) and
Asymmetric stretching mode (3).
In the symmetric stretching mode, the oxygen atoms oscillate along the axis of the molecule simultaneously departing or approaching the carbon atom, which is stationary.
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25. Contd.
In the ‘bending mode’, the molecule ceases to be exactly linear as the atoms move perpendicular to the molecular axis.
In ‘asymmetric stretching’, all the three atoms oscillate: but while both oxygen atoms move in one direction, carbon atoms move in the opposite direction.
The ‘internal vibrations’ of carbon dioxide molecule can be represented approximately by linear combination of these three normal modes.
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26. CO2 LASER
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27. INDEPENDENT MODES OF VIBRATION OF CO2 MOLECULE
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28. The energy level diagram of vibrational – rotational energy levels with which the main physical processes taking place in this laser.
As the electric discharge is passed through the tube, which contains a mixture of carbon dioxide, nitrogen and helium gases, the electrons striking nitrogen molecules impart sufficient energy to raise them to their first excited vibrational-rotational energy level.
This energy level corresponds to one of the vibrational - rotational level of CO2 molecules, designated as level 4.
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29. collision with N2 molecules, the CO2 molecules are raised to level 4.
Contd.
collision with N2 molecules, the CO2 molecules are raised to level 4.
The lifetime of CO2 molecules in level 4 is quiet significant to serve practically as a metastable state.
Hence, population inversion of CO2 molecules is established between levels 4 and 3, and between levels 4 and 2.
The transition of CO2 molecules between levels 4 and 3 produce lasers of wavelength 10.6 microns and that between levels 4 and 2 produce lasers of wavelength 9.6 microns.
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30. ENERGY LEVEL DIAGRAM
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31. The He molecules increase the population of level 4, and also help in emptying the lower laser levels.
The molecules that arrive at the levels 3 and 2 decay to the ground state through radiative and collision induced transitions to the lower level 1, which in turn decays to the ground state.
The power output of a CO2 laser increases linearly with length. Low power (upto 50W) continuous wave CO2 lasers are available in sealed tube configurations.
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32. All high power systems use fast gas-floe designs.
Contd.
Some are available in sizes like torches for medical use, with W power.
All high power systems use fast gas-floe designs.
Typical power per unit length is W/m.
Some of these lasers are large room sized metal working lasers with output power kW.
Recently CO2 lasers with continuous wave power output exceeding 100 kW.
The wavelength of radiation from these lasers is 10.6m.
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33. Nd: YAG Laser (Doped insulator laser) :
Lasing medium :
The host medium for this laser is Yttrium Aluminium Garnet (YAG = Y3 Al5 O12) with 1.5% trivalent neodymium ions (Nd3+) present as impurities.
The (Nd3+) ions occupy the lattice sites of yttrium ions as substitutional impurities and provide the energy levels for both pumping and lasing transitions.
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34. Contd.
When an (Nd3+) ion is placed in a host crystal lattice it is subjected to the electrostatic field of the surrounding ions, the so called crystal field.
The crystal field modifies the transition probabilities between the various energy levels of the Nd3+ ion so that some transitions, which are forbidden in the free ion, become allowed.
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35. Nd: YAG laser
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36. The ends of the rod are polished and made optically flat and parallel.
The length of the Nd: YAG laser rod various from 5cm to 10cm depending on the power of the laser and its diameter is generally 6 to 9 mm.
The laser rod and a linear flash lamp are housed in a elliptical reflector cavity
Since the rod and the lamp are located at the foci of the ellipse, the light emitted by the lamp is effectively coupled to the rod.
The ends of the rod are polished and made optically flat and parallel.
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37. The system is cooled by either air or water circulation.
Contd.
The optical cavity is formed either by silvering the two ends of the rod or by using two external reflecting mirrors.
One mirror is made hundred percent reflecting while the other mirror is left slightly transmitting to draw the output
The system is cooled by either air or water circulation.
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38. ENERGY LEVEL DIAGRAM
Simplified energy level diagram for the Nd-ion in YAG showing the principal laser transitions
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39. This laser system has two absorption bands (0.73 m and 0.8 m)
Optical pumping mechanism is employed.
Laser transition takes place between two laser levels at 1.06 mm.
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40. OUTPUT CHARACTERISTICS :
The laser output is in the form of pulses with higher repetition rate
Xenon flash lamps are used for pulsed output.
Nd: YAG laser can be operated in CW mode also using tungsten-halide incandescent lamp for optical pumping.
Continuous output powers of over 1KW are obtained.
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41. Glass acts as an excellent host material for neodymium.
Note: Nd: Glass laser :
Glass acts as an excellent host material for neodymium.
As in YAG, within the glass also local electric fields modify the Nd3+ ion energy levels.
Since the line width is much broader in glass than in YAG for Nd3+ ions, the threshold pump power required for laser action is higher.
Nd: Glass lasers are operated in the pulsed mode at wavelength 1.06 m
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42. Nd:YAG/ Nd: Glass laser applications :
These lasers are used in many scientific applications which involve generation of other wavelengths of light.
The important industrial uses of YAG and glass lasers have been in materials processing such as welding, cutting, drilling.
Since 1.06 m wavelength radiation passes through optical fibre without absorption, fibre optic endoscopes with YAG lasers are used to treat gastrointestinal bleeding.
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43. Contd.
YAG beams penetrate the lens of the eye to perform intracular procedures.
YAG lasers are used in military as range finders and target designators.
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