CHAPTER LASER

TODAYS CHAPTER: : 

11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE 1ii TODAYS CHAPTER: LASER

Slide 2: 

2 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

LASER : 

LASER A laser is an amplifier of light. When the laser is suitably excited by optical or electrical energy, the light of the proper frequency entering the laser cavity is amplified in such a manner that laser output wave is in phase with input. Practical utility of a laser is as an OSCILLATOR –-- a generator of light. Thus laser is also known as GENERATOR of light. 3 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

LASER ACTION : 

4 LASER ACTION Laser action is based on amplification of EM waves by means of forced or induced atoms or molecules. A laser radiation uses three fundamental phenomena when EM waves interacts with the matter namely 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Excited atoms emit photons spontaneously. : 

Excited atoms emit photons spontaneously. When an atom in an excited state falls to a lower energy level, it emits a photon of light. Molecules typically remain excited for no longer than a few nanoseconds. This is often also called fluorescence or, when it takes longer, phosphorescence. Energy Ground level Excited level 11jan 2009 5 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Atoms and molecules can also absorb photons, making a transition from a lower level to a more excited one. : 

Atoms and molecules can also absorb photons, making a transition from a lower level to a more excited one. This is, of course, absorption. Energy Ground level Excited level Absorption lines in an otherwise continuous light spectrum due to a cold atomic gas in front of a hot source. 11jan 2009 6 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Spontaneous absorption : 

7 Spontaneous absorption Let us consider two energy level having energy E1 & E2 resp. The atom will remain in ground state unless some external stimulant is applied to it. When an EM wave i.e photon of particular freq fall on it , there is finite probability that atom will jump form energy state E1 to E2. photon E1 E2 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Spontaneous emission : 

8 Spontaneous emission Consider an atom in higher state (E2). It can decay to lower energy level by emitting photon. Emitted photon have energy hv=E2-E1. Life time of excited state is 10-9sec. Photon hv=E2-E1 E2 E1 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Stimulated emission : 

9 Stimulated emission There are metastable state i.e. transition from this state is not allowed acc to selection rule. There life time is 10-3 sec. Atom in this state can’t jump to lower state at there own. When an photon of suitable freq arrive it make the atom in metastable unstable. The emitted photon is in coherence with incident photon. Incident photon Emitted Photon coherent Metastable state(10-3sec) 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Slide 10: 

10 Stimulated Emission The stimulated photons have unique properties: In phase with the incident photon Same wavelength as the incident photon Travel in same direction as incident photon 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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Stimulated vs Spontaneous Emission Stimulated emission requires the presence of a photon. An “incoming” photon stimulates a molecule in an excited state to decay to the ground state by emitting a photon. The stimulated photons travel in the same direction as the incoming photon. Spontaneous emission does not require the presence of a photon. Instead a molecule in the excited state can relax to the ground state by spontaneously emitting a photon. Spontaneously emitted photons are emitted in all directions. 11jan 2009 11 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

In 1916, Einstein showed that another process, stimulated emission, can occur. : 

In 1916, Einstein showed that another process, stimulated emission, can occur. Before After Absorption Stimulated emission Spontaneous emission 11jan 2009 12 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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13 The processes that account for absorption and emission of radiation and the attainment of thermal equilibrium. The excited state can return to the lower state spontaneously as well as by a process stimulated by radiation already present at the transition frequency. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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Before Absorption Stimulated emission Spontaneous emission After In 1916, Einstein showed that another process, stimulated emission, can occur. 11jan 2009 14 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

EINSTEIN’S THEORY OF RADIATIONS : 

15 EINSTEIN’S THEORY OF RADIATIONS Incident photon Stimulatedemission Spontaneous emission E2 E1 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

EINSTEIN’S THEORY OF RADIATIONS : 

16 EINSTEIN’S THEORY OF RADIATIONS Ra=rate of absorption per unit volume It depends upon: 1.N1: no. of atom in ground state. 2.ρ(v): energy density per unit freq of incident wave. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

EINSTEIN’S THEORY OF RADIATIONS : 

17 EINSTEIN’S THEORY OF RADIATIONS Rsp=rate of emission per unit volume. It depends upon: 1.N2: no. of atom in exicited state. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

EINSTEIN’S THEORY OF RADIATIONS : 

18 EINSTEIN’S THEORY OF RADIATIONS Rst= rate of stimulated emission per unit volume It depends upon: 1.N2: no. of atom in exicited state. 2.ρ(v): energy density per unit freq of incident wave. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Slide 19: 

19 Properties of Laser Monochromatic The light emitted from a laser is monochromatic, that is, it is of one wavelength (color).  In contrast, ordinary white light is a combination of many different wavelengths (colors). 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Properties of Laser : 

Properties of Laser Directional: Lasers emit light that is highly directional.  Laser light is emitted as a relatively narrow beam in a specific direction.  Ordinary light, such as coming from the sun, a light bulb, or a candle, is emitted in many directions away from the source. 20 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Properties of Laser : 

Properties of Laser Coherent The light from a laser is said to be coherent, which means the wavelengths of the laser light are in phase in space and time. 21 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Population Inversion : 

22 Population Inversion A state in which a substance has been energized, or excited to specific energy levels. More atoms or molecules are in a higher excited state. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Population Inversion : 

Population Inversion The process of producing a population inversion is called pumping. Examples: →by lamps of appropriate intensity →by electrical discharge 23 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Achieving inversion: Pumping the laser medium : 

Achieving inversion: Pumping the laser medium Now let I be the intensity of (flash lamp) light used to pump energy into the laser medium: Will this intensity be sufficient to achieve inversion, N2 > N1? It’ll depend on the laser medium’s energy level system. 11jan 2009 24 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

In what energy levels do molecules reside? Boltzmann population factors : 

In what energy levels do molecules reside? Boltzmann population factors Ni is the number density of molecules in state i (i.e., the number of molecules per cm3). T is the temperature, and kB is Boltzmann’s constant. Energy Population density N1 N3 N2 E3 E1 E2 11jan 2009 25 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Boltzmann Population Factors : 

Boltzmann Population Factors In equilibrium, the ratio of the populations of two states is: N2 / N1 = exp(–DE/kBT ), where DE = E2 – E1 = hn As a result, higher-energy states are always less populated than the ground state, and absorption is stronger than stimulated emission. In the absence of collisions, molecules tend to remain in the lowest energy state available. Collisions can knock a mole- cule into a higher-energy state. The higher the temperature, the more this happens. Low T High T Energy Molecules Energy Molecules 3 2 1 2 1 3 11jan 2009 26 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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27 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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28 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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29 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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30 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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31 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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32 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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33 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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34 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Components of LASER : 

Components of LASER 35 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Pump Source : 

36 Pump Source A pump is basic energy source for a laser. It gives energy to various atoms of laser medium & excites them . So that population inversion can take place & it is maintained with time. The excitation of atom occur directly or through atom or atom collision. There is various type of pump depending upon nature of medium .Examples: electric discharges, flashlamps, arc lamps and chemical reactions. The type of pump source used depends on the gain medium. →A helium-neon (HeNe) laser uses an electrical discharge in the helium-neon gas mixture. →Excimer lasers use a chemical reaction. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Gain Medium : 

37 Gain Medium When energy is given to laser medium a small fraction of medium shows lasing action. This part of laser medium is called Active centers. For examples in ruby laser Cr+++ is active center, in He-Ne laser Ne are active centers. It is the Major determining factor of the wavelength of operation of the laser. Excited by the pump source to produce a population inversion. Where spontaneous and stimulated emission of photons takes place. Example: solid, liquid, gas and semiconductor. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Optical Resonator : 

Optical Resonator It is an set up used to obtain amplification of stimulated photons, by oscillating them back & forth between two extreme limits. Consist of: Two plane or concave mirrors placed co-axially. One mirror is reflecting & other is partially reflecting. 38 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Optical Resonator : 

39 Optical Resonator Two parallel mirrors placed around the gain medium. Light is reflected by the mirrors back into the medium and is amplified . The design and alignment of the mirrors with respect to the medium is crucial. Spinning mirrors, modulators, filters and absorbers may be added to produce a variety of effects on the laser output. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Stimulated emission can lead to a chain reaction and laser emission. : 

Stimulated emission can lead to a chain reaction and laser emission. Excited medium If a medium has many excited molecules, one photon can become many. This is the essence of the laser. The factor by which an input beam is amplified by a medium is called the gain and is represented by G. 11jan 2009 40 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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41 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Slide 42: 

Requirements for Laser Action 11jan 2009 42 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Four-level Laser System : 

43 Four-level Laser System Laser transition takes place between the third and second excited states. Rapid depopulation of the lower laser level. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

FOUR LEVEL LASER: : 

44 FOUR LEVEL LASER: STEP 1- PUMPING: atoms are excited to higher energy level by providing energy from ext. source. STEP 2- POPULATION INVERSION: atom via radiation less decay, decays to metastable state and hence population inversion take place. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

FOUR LEVEL LASER: : 

45 FOUR LEVEL LASER: STEP 3- LASER ACTION: atom from metastable state decays to lower state by stimulated emission and hence laser action take place. STEP 4- BACK TO GROUND STATE: atom from excited state decays to lower state by spontaneous emission. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

FOUR LEVEL LASER: : 

46 FOUR LEVEL LASER: 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Three-level Laser System : 

47 Three-level Laser System Initially excited to a short-lived high-energy state . Then quickly decay to the intermediate metastable level. Population inversion is created between lower ground state and a higher-energy metastable state. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Three-level Laser System : 

48 Three-level Laser System 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Two-level Laser System : 

49 Two-level Laser System Unimaginable as absorption and stimulated processes neutralize one another. The material becomes transparent. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Slide 50: 

Two-Level System Even with very a intense pump source, the best one can achieve with a two-level system is excited state population = ground state population 11jan 2009 50 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

The Laser : 

Usually, additional losses in intensity occur, such as absorption, scat-tering, and reflections. In general, the laser will lase if, in a round trip: Gain > Loss This called achieving Threshold. The Laser A laser is a medium that stores energy, surrounded by two mirrors. A partially reflecting output mirror lets some light out. 11jan 2009 51 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Laser Types : 

52 Laser Types According to the active material: solid-state, liquid, gas, excimer or semiconductor lasers. According to the wavelength: infra-red, visible, ultra-violet (UV) or x-ray lasers. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Laser Types : 

53 Laser Types According to the nature of pumping. flash type, chemical pumping & electric discharge lasers According to the nature of output: pulsed & continuous wave lasers. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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54 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Slide 55: 

Continuous vs Pulsed Lasers Pump Source : Excitation of the lasing atoms or molecules by an external source of light (such as a lamp) or another laser The output of the laser light can be a continuous wave (cw) if the pumping is continuous or pulsed if the pumping is pulsed. Pulsed lasers have very high peak intensities because the laser intensity is concentrated in a very short time duration. 11jan 2009 55 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Solid-state Laser : 

56 Solid-state Laser Example: Ruby Laser Operation wavelength: 694.3 nm (IR) 3 level system: absorbs green/blue Gain Medium: crystal of aluminum oxide (Al2O3) with small part of atoms of aluminum is replaced with Cr3+ ions. Pump source: flash lamp The ends of ruby rod serve as laser mirrors. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

How a laser works? : 

57 How a laser works? 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

RUBY LASER : 

58 RUBY LASER 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Slide 59: 

59 1. High-voltage electricity causes the quartz flash tube to emit an intense burst of light, exciting some of Cr3+ in the ruby crystal to higher energy levels. 2. At a specific energy level, some Cr3+ emit photons. At first the photons are emitted in all directions. Photons from one Cr3+ stimulate emission of photons from other Cr3+ and the light intensity is rapidly amplified. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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60 3. Mirrors at each end reflect the photons back and forth, continuing this process of stimulated emission and amplification. 4. The photons leave through the partially silvered mirror at one end. This is laser light. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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61 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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62 As the flash lamp stop operting, the population of the upper level decreases very rapidally & lasing action stops till the further operation of next flash. As the production of laser beam depends upon the operation of flash lamp the ruby laser is pulsed type laser. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Slide 63: 

63 During the period of operation of two flash laser output is oscillating & output is highly irregular function of time, shows random fluctuations in the amplitude. This type of output is called as laser SPIKING. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

He-NE LASER : 

He-NE LASER A helium-neon laser, usually called a HeNe laser, is a type of small gas laser. HeNe lasers have many industrial and scientific uses, and are often used in laboratory demonstrations of optics. Its usual operation wavelength is 632.8 nm, in the red portion of the visible spectrum 64 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

He-Ne laser : 

He-Ne laser He-Ne lasers are normally small, with cavity lengths of around 15 cm up to 0.5 m. The optical cavity of the laser typically consists of a plane, high-reflecting mirror at one end of the laser tube, and a concave output coupler mirror of approximately 1% transmission at the other end. Electric discharge pumping is used. Optical output powers ranging from 1 mW to 100 mW. 65 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Slide 66: 

66 Electron impact Spontaneous emission Radiation less decay 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

CARBON DIOXIDE : 

CARBON DIOXIDE Carbon dioxide lasers are the highest-power continuous wave lasers that are currently available. They are also quite efficient: the ratio of output power to pump power can be as large as 20%. The CO2 laser produces a beam of infrared light with the principal wavelength bands centering around 9.4 and 10.6 micrometers. 67 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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68 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Semiconductor laser : 

Semiconductor laser Lasers which use semiconductor as active medium. The majority of semiconductor materials are based on a combination of elements in the third group of the Periodic Table (such as Al, Ga, In) and the fifth group (such as N, P, As, Sb) hence referred to as the III-V compounds. 69 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

P- and N-type Semiconductors : 

P- and N-type Semiconductors In the compound GaAs, each gallium atom has three electrons in its outermost shell of electrons and each arsenic atom has five. When a trace of an impurity element with two outer electrons, such as zinc, is added to the crystal. The result is the shortage of one electron from one of the pairs, causing an imbalance in which there is a “hole” for an electron but there is no electron available. This forms a p-type semiconductor. When a trace of an impurity element with six outer electrons, such as selenium, is added to a crystal of GaAs, it provides on additional electron which is not needed for the bonding. This electron can be free to move through the crystal. Thus, it provides a mechanism for electrical conductivity. This type is called an n-type semiconductor. 11jan 2009 70 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Slide 71: 

Under forward bias (the p-type side is made positive) the majority carriers, electrons in the n-side, holes in the p-side, are injected across the depletion region in both directions to create a population inversion in a narrow active region.The light produced by radioactive recombination across the band gap is confined in this active region 11jan 2009 71 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

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72 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE

Components of LASER : 

73 Components of LASER PUMP. ACTIVE MEDIUM. OPTICAL RESONATOR. A pump is basic energy source for a laser. It gives energy to various atoms of laser medium & excites them . So that population inversion can take place & it is maintained with time. The excitation of atomoccur directly or through atom or atom collision. There is various type of pump depending upon nature of medium When energy is given to laser medium a small fraction of medium shows lasing action. This part of laser medium is called Active centers. For examples in ruby laser Cr+++ is active center, in He-Ne laser Ne are active centers. It is an set up used to obtain amplification of stimulated photons, by oscillating them back & forth between two extreme limits. Consist of: Two plane or concave mirrors placed co-axially. One mirror is reflecting & other is partially reflecting. 11jan 2009 Made by Mrs MANDEEP KAUR, NWIET, DHUDIKE