LASERS BASICS

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Information about LASERS BASICS
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Published on March 14, 2014

Author: merryjerry90

Source: slideshare.net

LASERS

>>LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. >>The most important optical device to be developed in the past 50 years. >>Invented in 1958 by Charles Townes and Arthur Schawlow of Bell Laboratories >>Was based on Einstein’s idea of the “particle-wave duality”of light.

Particle-Wave Duality of Light

Incandescent vs laser light  Many wavelengths  Multidirectional  Incoherent  Monochromatic  Directional  Coherent

Principle of lasers The principle of a laser is based on three separate features:  a) stimulated emission within an amplifying medium,  b) population inversion  c) an optical resonator.

Einstein’s quantum theory of radiation According to Einstein, the interaction of radiation with matter could be explained in terms of three basic processes:  absorption  spontaneous emission  stimulated emission.

 Absorption

 An incoming photon excites the atomic system from a lower energy state into a higher energy state. This is called absorption or sometimes stimulated absorption.  It is called stimulated absorption because of the fact that the atoms absorb the incident energy at certain frequencies only.

Spontaneous emission

 Consider an atom (or molecule) of the material is existed initially in an excited state 2.. Since 2> 1, the atom will tend to spontaneously decay to the ground state 1, releasing a photon of energy h = 2- 1 in a random direction . This process is called “spontaneous emission

Stimulated emission

stimulated emission  When an incident photon of energy h = 2- 1 passes by an atom in an excited state 2, it stimulates the atom to drop or decay to the lower state 1. In this process, the atom releases a photon of the same energy, direction & phase as that of the photon passing by.  The net effect is two identical photons (2h ) in the place of one, or an increase in the intensity of the incident beam.  It is precisely this processes of stimulated emission that makes possible the amplification of light in lasers

Population Inversion and Laser Operation  If the higher energy state has a greater population than the lower energy state, then the light in the system undergoes a net increase in intensity. N2 > N1….. Population Inversion  Can be created by introducing a so called metastable centre where electrons can piled up to achieve a situation where more N2 than N1

 The process of attaining a population inversion is called pumping

 To create population inversion, a 3-state system is required.  The system is pumped with radiation of energy E31 then atoms in state 3 relax to state 2 non radiatively.  The electrons from E2 will now jump to E1 to give out radiation.

Optical cavity or resonator  As there are certain losses of the emitted photons within the material itself one has to think about the geometry that can overcome these losses and there is overall gain. This requires an optical cavity or resonator.  A pair of mirrors placed on either side of the active medium .  One mirror is completely silvered and the other is partially silvered.  The laser beam comes out through the partially silvered mirror.

Construction & working of laser A representative laser system is shown in Figure

It consists of three basic parts:-  An active medium with a suitable set of energy levels to support laser action.  A source of pumping energy in order to establish a population inversion.  An optical cavity or resonator to introduce optical feedback and so maintain the gain of the system overcoming all losses.

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.  it must have a metastable state to support stimulated emission

Excitation or pumping mechanism: >>There are various types of excitation or pumping mechanisms available, the most commonly used ones are optical, electrical, thermal or chemical techniques, which depends on the type of the laser gain medium employed. >>eg:Solid state lasers usually employ optical pumping from high energy xenon flash lamps Gas lasers use an AC or DC electrical discharge through the gas medium

Optical resonator:  In practice, photons need to be confined in the system to allow the number of photons created by stimulated emission to exceed all other mechanisms. This is achieved by bounding the laser medium between two mirrors.  Then Stimulated photons can bounce back and forward along the cavity, creating more stimulated emission as they go.  Plays a very important role in the generation of the laser output, in providing high directionality to the laser beam as well as producing gain in the active medium to overcome the losses.

COHERENCE  means that the wavelengths of the laser light are in phase in space and time.  Factors that compromise coherence: 1. thermal fluctuations 2. vibrational fluctuations 3. emission of multiple wavelengths

 Temporal Coherence – How long do the light waves remain in phase as they travel? Coherence Length = 2/n

 Spatial Coherence – Over what area does the light remain in phase?

GAS LASERS  electric current is discharged through a gas to produce coherent light.  The first continuous-light laser & the first laser to operate on the principle of converting electrical energy to a laser light output  He-Ne Atomic gas lasers  Ar+  CO2 molecular gas lasers  Excimer

PROPERTIES OF GAS LAERS  Fixed-frequency  Good beam quality  High frequency stability  Low energy consumption

He-Ne  He-Ne laser is a four-level laser.  Its usual operation wavelength is 632.8 nm, in the red portion of the visible spectrum.  It operates in Continuous Working (CW) mode.  The Helium-Neon laser was the first continuous laser.

simplified energy level diagram of He Ne laser

Advantages of He-Ne Laser  He-Ne laser has very good coherence property  He-Ne laser tube has very small length approximately from 10 to 100cm and best life time of 20.000 hours.  Cost of He-Ne laser is less from most of other lasers.  Construction of He-Ne laser is also not very complex.  He-Ne laser provide inherent safety due to low power output.

Disadvantages of He-Ne Laser  It is relatively low power device means its output power is low.  He-Ne laser is low gain system/ device.  To obtain single wavelength laser light, the other two wavelengths of laser need suppression, which is done by many techniques and devices. So it requires extra technical skill and increases the cast also.  High voltage requirement  Escaping of gas from laser plasma tube.

Applications of He-Ne laser  The Narrow red beam of He-Ne laser is used in supermarkets to read bar codes.  Measuring distances  Red HeNe lasers have many industrial and scientific uses. They are widely used in laboratory demonstrations of optics in view of their relatively low cost and ease of operation .

Argon ion laser  as the name implies it uses high purity argon gas as the lasing medium.  Based on light amplification in ionized argon in a gas discharge  powerful gas lasers, which typically generate multiple watts of optical power in a green or blue output beam with high beam quality.

Construction & Working

ENERGY LEVEL DIAGRAM

Advantages of Argon Laser  Production of multiple wave lengths is the main advantage plus characteristic of argon as well as other ion lasers.  Argon lasers produce high power output as compared to He-Ne laser.  Argon laser is a higher gain system.  Argon laser like He-Ne has very less divergence, typically about 1 milli radian.

Disadvantages of Argon laser  The overall efficiency of argon laser is very less usually lies between 0.01% and 0.1%.  Large amount of power requirement is also its disadvantage.  Construction is very difficult.  Cost of argon laser is not as low as He-Ne laser.  Power supply of high voltages required, because due to solenoid there is extra burden on it.

Applications of argon ion laser  Raman Spectroscopy  Holography  Entertainment  Forensics  Ophthalmic Surgery  Argon ion lasers are also used extensively in scientific, research and educational applications

EXCIMER LASER(EXCIPLEX LASER) :  Excimer lasers produce intense pulsed output in the ultraviolet.  Here the lasing molecule is one consisting of a halogen and an inert gas.  The term excimer is short for 'excited dimer', while exciplex is short for 'excited complex'.

 Under the appropriate conditions of electrical stimulation, a pseudo- molecule called a dimer is created, which can give rise to laser light in the ultraviolet range  Laser action in an excimer molecule occurs because it has a bound (associative) excited state, but a repulsive (dissociative) ground state.

Population inversion  In an excited state they can form temporarily-bound molecules with themselves (dimers) or with halides (complexes) . The excited compound can give up its excess energy by undergoing spontaneous or stimulated emission, resulting in a strongly-repulsive ground state molecule which very quickly (on the order of a picosecond) disassociates back into two unbound atoms. This forms a population inversion between the two states.

CONSTRUCTION AND WORKING

 The heart of the laser is the discharge tube. This is first filled with a low-pressure mixture of an inert gas (e.g.,krypton, argon, xenon) and a halogen or halide gas(fluorine or hydrogen chloride), and then pressurized with an inert buffer gas of either neon or helium. The laser tube has two parallel electrodes running almost the entire length of the tube. The laser is pulsed by discharging across these electrodesThis stripe-shaped discharge lasts from 20 to 50 ns depending on laser gas, laser parameter and discharge pulser design. The resultant plasma contains a high concentration of an excited transient complex (e.g.,ArF, KrF, XeCl, or F2), which emits ultraviolet laser light.Among the most commonly used wavelengths are 248 nm and 308 nm..

ENERGY LEVEL DIAGRAM OF EXCIMER LASER

ADVANTAGES OF EXCIMER LASERS  Excimer lasers are powerful.  Directly generate intense, short ultraviolet pulses.  High optical resolution: less than 1m.  Shallow absorption depth:0.1 to 0.5µm.  Small interaction volume.  Energy highly absorbed by materials.  Uniform power density over relatively large area.  High peak power: approximately 107 watts.

DISADVANTAGES  High performance discharge circuit is required to generate laser light.  Laser gas mixture is toxic and corrosive.  Reactivity of lasant mixtures result in impurities formation during laser operation.  A computer control system is required to maintain stable laser light output.  Changes in gas chemistry affect beam shape and quality.  Advanced optical materials are required to efficiently transmit the beam.  Optic transmissivity degrades over long-term exposure to high- power UV beams.  Optical surfaces and coatings are damaged quickly by laser light if not kept clean.

Uses of excimer laser  Excimer lasers have the useful property that they can remove exceptionally fine layers of surface material with almost no heating or change to the remainder of the material which is left intact. These properties make them useful for surgery (particularly eye surgery) for lithography for semiconductor manufacturing, and for dermatological treatment.

CO2 LASER  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.

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. Pumping : Population inversion is created by electric discharge of the mixture

CONSTRUCTION AND WORKING OF CO2 LASER

ENERGY LEVEL DIAGRAM OF CO2 LASER

ADVANTAGES  A carbon dioxide (CO2) laser can produce a continuous laser beam with a power output of several kilowatts.  Can maintain high degree of spectral purity.  High spatial coherence.  The CO2 laser is the most efficient laser, capable of operating at more than 30% efficiency

APPLICATIONS  Because of the high power levels available (combined with reasonable cost for the laser), CO2 lasers are frequently used in industrial applications for cutting and welding, while lower power level lasers are used for engraving.  In surgical procedures because water (which makes up most biological tissue) absorbs this frequency of light very well.  Other medical uses are  laser surgery,  skin resurfacing  dermabrasion.

Solid-state lasers  "solid-state" refers to a crystal or glass.  Is usually optically pumped.  Common types:-  Ruby laser  Nd:YAG (Yttrium Aluminum Garnet which is Y3Al5O12)  Nd:YVO4  Nd:Glass

Ruby laser  A ruby laser is a solid-state laser that uses a synthetic ruby crystal as its gain medium.  The first working laser was a ruby laser made by Theodore H. Maiman at Hughes Research Laboratories on May 16, 1960.  Ruby lasers produce pulses of visible light at a wavelength of 694.3 nm, which is a deep red color. Typical ruby laser pulse lengths are on the order of a millisecond.

Construction of Ruby Laser  The ruby laser consists of a ruby rod . which is made of chromium doped ruby material. At the opposite ends of this rod there are two silver polished mirrors. Whose one is fully polished and other is partially polished. A spring is attached to the rod with fully polished end for adjustment of wave length of the laser light. Around the ruby rod a flash light is kept for the pump input. The whole assembly is kept in the glass tube. Around the neck of the glass tube the R.F source and switching control is designed in order to switch on and off the flash light for desired intervals.

Operation of Ruby Laser:  When we switch on the circuit the R.F operates. As a result the flash of light is obtained around the ruby rod. this flash causes the electrons within ruby rod to move from lower energy band towards higher energy band. The population inversion take place at high energy band and electrons starts back to travel towards the lower energy band. During this movement the electron emits the laser light . This emitted light travels between the two mirrors where cross reflection takes place of this light. The stimulated lazer light now escapes from partially polished mirror in shape of laser beam.  The spring attached with the fully polished mirror is used to adjust the wave length equal to λ/2 of lazer light for optimum lazer beam. The switching control of the R.F source is used to switch on and off the flash light so that excessive heat should not be generated due to very high frequency of the movement of the electron.

Energy Level Diagram for Ruby Laser

 The above three level energy diagram show that in ruby lasers the absorption occurs in a rather broad range in the green part of the spectrum. This makes raise the electrons from ground state E1 to the band of level E3 higher than E1. At E3 these excited levels are highly unstable and so the electrons decays rapidly to the level of E2. This transition occurs with energy difference (E1 – E2) given up as heat (radiation less transmission). The level E2 is very important for stimulated emission process and is known as Meta stable state. Electrons in this level have an average life time of about 5m.s before they fall to ground state. After this the population inversion can be established between E2 and E1. The population inversion is obtained by optical pumping of the ruby rod with a flash lamp. A common type of the flash lamp is a glass tube wrapped around the ruby rod and filled with xenon gas. When the flash lamp intensity becomes large enough to create population inversion, then stimulated emission from the Meta stable level to the ground level occurs which result in the laser output. Once the population inversion begins, the Meta stable level is depopulated very quickly. Thus the laser output consists of an intense spike lasting from a few Nano sec to µsec. after stimulated emission spike, population inversion builds up again and a 2nd spike results. This process continues as long as the flash lamp intensity is enough to create the population inversion.

Advantages of Ruby laser  From cost point of view, the ruby lasers are economical.  Beam diameter of the ruby laser is comparatively less than CO2 gas lasers.  Output power of Ruby laser is not as less as in He-Ne gas lasers.  Since the ruby is in solid form therefore there is no chance of wasting material of active medium.  Construction and function of ruby laser is self explanatory.

Disadvantages of Ruby Laser  In ruby lasers no significant stimulated emission occurs, until at least half of the ground state electrons have been excited to the Meta stable state.  Efficiency of ruby laser is comparatively low.  Optical cavity of ruby laser is short as compared to other lasers, which may be considered a disadvantage.

Applications of ruby laser  Used in retina surgery and in dermatology. It has the ability to concentrate the energy of optical radiation into a small area and the possibility of cutting and vapourising tissue.  It is used in performing a non contact sharp contour tissue incision and removal of even tiny structuires without any damage to the surrounding tissues and any possible infection to the cut.  In medical diagnostics.  Used in cataract surgey,retina detachment surgery and glaucoma removal surgery.

SEMICONDUCTOR (Ga-As) LASERS  The semiconductor laser is today one of the most important types of lasers with its very important application in fiber optic communication.  Unlike other lasers, semiconductor laser does not need mirrors to obtain the reflectivity needed to produce feedback mechanism

Construction and working

Basic Mechanism :  The basic mechanism responsible for light emission from a semiconductor is the recombination of electrons and holes at a p-n junction when a current is passed through a diode.There can be three interaction processes. 1) An electron in the valence band can absorb the incident radiation and be excited to the conduction band leading to the generation of eletron-hole pair. 2) An electron can make a spontaneous transition in which it combines with a hole and in the process it emits radiation 3) A stimulated emission may occur in which the incident radiation stimulates an electron in the conduction band to make a transition to the valence band and in the process emit radiation.  To convert the amplifying medium into a laser  Optical feedback should be provided  Done by cleaving or polishing the ends of the p-n junction diode at right angles to the junction.

 The heterostructure laser is a laser diode with more than single P and N layers. GaAs/AlGaAs is a heterojunction laser. This increases the radiation efficiency

Advantages of Semiconductor Lasers  Smaller size and appearance make them good choice for many applications.  From cost point of view the semiconductor lasers are economical.  Semiconductor lasers construction is very simple.  No need of mirrors is in semiconductor lasers.  Semiconductor lasers have high efficiency.  The low power consumption is also its great advantage.

Disadvantages of Semiconductor Lasers  Due to relatively low power production, these lasers are not suited to many typical laser applications.  Semiconductor laser is greatly dependent on temperature. The temperature affects greatly the output of the laser.  The lasing medium of semiconductor lasers is too short and rectangular so the output beam profile has an unusual shape.  Beam divergence is much greater from 125 to 400 milli radians as compared to all other lasers.  The cooling system requirement in some cases may be considered its disadvantage.

Application / Uses of Semiconductor Lasers  The semiconductor laser can be pulsed at varying rate and pulse widths. Therefore this laser is a natural transmitter of digital data.  Semiconductor laser is well suited for interface with fiber optic cables used in communication.

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