LASER

1. LASER

2. German-born American physicist Birth: March 14, 1879 Death: April 18, 1955 Place of Birth: Ulm, Germany Known for: Proposing the theory of relativity, a physical theory of gravity, space, and time, Explaining the photoelectric effect and Brownian motion. Albert Einstein first proposed stimulated emission, the underlying process for laser action, in 1917. Translating the idea of stimulated emission into a working model, however, required more than four decades. Albert Einstein

3. Charles Townes • American physicist Charles Townes won the 1964 Nobel Prize in physics. He made fundamental contributions in quantum theory and significantly improved radar technology. • In December 1953 Townes and his students constructed a device producing microwaves in a beam. • They dubbed the process “microwave amplification by stimulated emission of radiation,” which led to the more commonly used term maser. • The maser quickly found many applications for its ability to send strong microwaves in any direction. • In 1958 Townes developed the concepts for the visible-light maser, or laser (derived from “light amplification by stimulated emission of radiation”), which delivers infrared or visible light instead of microwaves.

4. Theodore Maiman • Theodore Maiman, born in 1927, American physicist. Theodore Harold Maiman was born in Los Angeles and educated at the University of Colorado and Stanford University. • He was the first to successfully produce a pulse of coherent light from a laser, accomplishing this in May 1960, using ruby as the laser medium. • The first continuously operating laser was achieved a few months later. • Due to his work on the laser, he was twice nominated for a Nobel Prize and was given membership in both the National Academies of Science and Engineering

6. LASERS The laser perhaps is the most important optical device past fifty years. The laser is essentially an optical amplifier. The word Laser is acronym that stands for Light Amplification by Stimulated Emission of Radiation.

8. Lasers • Importance( Introduction & Applications,) • Induced absorption • Spontaneous and stimulated emission • Einstein’s coefficient. • Population inversion • Meta-stable state • Requisites of laser system • Ruby construction and working • He-Ne laser construction and working

9. Medical field a. Eye surgery b. Cosmetic / plastic surgery c. Brain tumor d. Endoscopic e. Dental treatment and extraction Defense field a. Death ray b. Defensive applications c. Strategic defense initiative d. Laser sight e. Illuminator f. Range finder( Measure of distance) g. Target designator ( Measurement of mobile object with high accuracy) Scientific and research field a. Spectroscopy b. Lunar laser ranging c. Photochemistry Importance Excimerlaser used for eye surgery.

10. Surveillance Systems

12. Law Enforcement Target Designators Rangefinding

18. BASICS OF LASERS AND LASER LIGHT L ight A mplification by S timulated E mission of R adiation Laser-Professionals.com

20. CHARACTERISTIC OF THE LASER LIGHT 1. Monochromaticity The light emitted by a laser is almost pure in color, almost of a single wavelength or frequency.

21. 2. Coherence

22. 3. Directionality The astonishing degree of directionality of a laser light is due to the geometrical design of the laser cavity and to the monochromaticity and coherent nature of light generated in the cavity.

23. 4. LIGHT INTENSITY The intensity of laser light is highly intense. For example intensity of light from a 1mW Helium- Neon laser is hundreds of times more intense than the light starting from an equal area on the surface of sun. 5. FOCUSIBILITY Focusing light to a tiny , diffraction limited spot is a challenge. Due to the incoherence and non point source ,it is difficult to focus the ordinary light to tiny spot. But as laser emits intense, coherent light that appears to come from distant point source, it can be focused to a diffraction limited spot.

25. Population of atoms in various energy levels Normal Population & Population Inversion

26. POPULATION OF ENERGY LEVEL – BOLTZMANN FACTOR Hence at thermal equilibrium, the population of higher energy state is always lesser than any of its lower states.

27. INTERACTION OF RADIATION WITH MATTER Stimulated/induced absorption Spontaneous emission Stimulated emission

29. Stimulated absorption: When an electromagnetic radiation of frequency  is incident on a sample of atoms, the electrons in the lower energy state (E1) absorb the energy from the incident radiation & rise to the higher energy state (E2). This process is called stimulated absorption. [Figure (a) above].

30. Spontaneous emission:The atoms excited to higher energy state are unstable there. Their life time  in these states is of the order of 108s. The electrons in these states spontaneously make transition to lower energy states emitting a photon whose energy h is the difference between the two energy states E2 and E1 i.e, E2 – E1. This type of transition of an electron from a higher to a lower energy state without any outside stimulus is called spontaneous emission. The photons so emitted are in random phases and random directions.[Figure (b) previous slide].

31. Stimulated emission: When a photon of energy h = E2E1 is incident on an atom which is already in an excited state E2, the atom being disturbed or stimulated by the incident photon, makes a transition to a lower energy state E1 emitting a photon. The emitted photon has the same frequency, phase & direction as the incident photon. This type of emission is called stimulated emission. The net effect is two identical photons in the place of one thereby increasing the intensity of the incident beam. It is this process of stimulated emission that makes possible the amplification of light in lasers. [Figure (c) previous slide].

33. Emission take place without external agency Independent on incident light intensity Transition take place b/n two states Ordinary light radiation is emitted Emission take place with external agency namely photon of right frequency Dependent on incident light intensity Transition take place b/n three states Laser radiation is emitted Spontaneous and Stimulated emission

34. Einstein A and B coefficients Relation between radiation and matter It was demonstrated by Einstein in 1917 that the rates of the three processes – stimulated absorption, spontaneous emission, and stimulated emission are related mathematically. This is the first step toward understanding lasing action in atomic system. Consider two energy levels E1 and E2 with populations N1 and N2 in an atomic system in thermal equilibrium. E2 is larger than E1 and normally N2 is smaller than N1. There will be a transition between these two energy levels.

35. Radiation of a proper frequency (21) will cause transitions to occur between energy levels E1 and E2. In such a system, the rate of upward transitions must be equal to the rate of downward transitions. The populations of two levels (at thermal equilibrium) are related by Boltzmann statistics, assuming non-degeneracy, as

36. Now consider such an atomic system in presence of radiation. Three possible processes then are, • Stimulated absorption or induced absorption – occur in presence of external radiation • Spontaneous emission – occur even in absence of radiation • Stimulated emission or induced emission – occur in presence of stimulating radiation

37. a)Stimulated absorption or induced absorption – occur in presence of external radiation. The rate of stimulated absorption B12 is a constant, characteristic of the atom called Einstein’s coefficient of stimulated absorption.

38. b) Spontaneous emission – can occur even in absence of radiation The rate of spontaneous emission A21 is a constant, characteristic of the atom called Einstein’s coefficient of spontaneous emission.

39. c) Stimulated emission or induced emission – occur in presence of stimulating radiation The rate of stimulated emission B21 is a constant, characteristic of the atom called Einstein’s coefficient of stimulated emission and .

40. At thermal equilibrium, the number of upward transitions must be equal to the number of downward transitions. Thus, Dividing both numerator & denominator of above equation by N2,

41. Using Eq. (8) in Eq. (7), we have, Dividing both numerator and denominator of R.H.S. by B21, we get

42. Now, the black-body radiation law developed by Planck gives an alternate expression for the energy density per unit frequency range in a photon cloud given by,

43. Comparing Eqs. (10) and (11), with h21 = h12 = h That is, or a simple electronic system with no degeneracy.

44. Thus for a simple electronic system with no degeneracy, the probabilities of stimulated absorption and stimulated emission are the same [Equation (13), B12 = B21] and the ratio of A and B coefficients is given by equation (12)

45. We have At thermal equilibrium, the ratio of the spontaneous to stimulated emission is given by, [using Eqs. (11) and (12)]

46. Special cases – Case 1 We have the ratio of the spontaneous to stimulated emission as • Case – 1: when h << kT, That is, whenh << kT, the number of stimulated emission far exceed the number of spontaneous emission. LASING ACTION CAN OCCUR

47. Special cases – Case 2 • Case – 2: When h >>kT That is, whenh >> kT, the number of spontaneous emission far exceed the number of stimulated emission. LASING ACTION CAN NOT OCCUR. This is the reason why at visible and ultraviolet frequencies lasing action is difficult as compared to microwave frequencies.

48. CONDITIONS FOR LASER ACTION • Population Inversion • Metastable states

49. a) Population Inversion

50.  Depends on population N2  Depends on population N1 If N2> N1 photons will be added to the field  amplification If N1> N2 photons will be added to the field  attenuation Thus for Laser to operate it is necessary to have N2> N1 ,,,that is, population inversion.