Unit 6


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Unit 6

  1. 1. UNIT-6Lasers 1
  2. 2. UNIT INDEXS. No. Module Lectur PPT e No. Slide No. 1 Introduction, characteristics L 1 4--5 of lasers. 2 Spontaneous & Stimulated L 2-3 6-14 emission of Radiation Population Inversion. 3. Types of Lasers L 4-5 15-34 4. Applications of lasers L6 35-40 2
  4. 4. Lecture-1 INTRODUCTION The word “LASER” is an acronym for Light Amplification by Stimulated Emission of Radiation. 4
  5. 5. Characteristics Monochromacity High Intensity Coherence Directionality 5
  6. 6. Lecture-2 Types of coherence Temporal coherence Spatial coherence Temporal coherence measures the continuity of a wave along its length. Spatial coherence measures the maximum seperation between any two points on the cross section of the wavefront which maintain correlation between them. 6
  7. 7. Stimulated Absorption Excitation of atoms from lower energy state to higher energy state due to interaction of radiation with matter is known as Stimulated absorption. 7
  8. 8. Spontaneous emission When an electron in the excited level E2 falls spontaneously to lower energy level E1 after its lifetime a photon is emitted. The energy of the emitted photon is given by E2-E1=h 8
  9. 9. Stimulated emission When an electron in the excited level E2 is induced (stimulated) by a photon of energy (E2-E1), the electron moves to lower energy level E1 emitting another photon of energy E2-E1. This process is called stimulated emission. 9
  10. 10.  Both stimulated and stimulating photons are in phase with each other. Stimulated emission of radiation (light) results in amplification of light 10
  11. 11. Lecture-3 Population inversion  For light amplification by stimulated emission of radiation the population of excited state must be greater than the population of lower energy state. This condition is called population inversion. 11
  12. 12. Pumping mechanisms The process of sending atoms from lower energy state to higher energy state is called Pumping. Optical pumping Electric discharge Chemical reaction Injection current through p-n junction 12
  13. 13. Optical Feed back To direct the amplified light to travel back and forth through the active medium many times two end mirrors are kept at both the ends of the laser. These mirrors provide necessary optical feed back. 13
  14. 14. Threshold inversion density Only if the population inversion density is sufficiently large so that the loss is compensated by the gain, lasing action starts. The inversion density for which the gain is just sufficient to compensate for the loss is called threshold inversion density. 14
  15. 15. Lecture-4 Conditions for LasingFor laser action to take place, the three requisites are Suitable active medium Creation of population inversion Proper optical feed back 15
  16. 16. LLe e c RUBY LASERc ttu A ruby laser is a solid-state laser u rr ee --2 It uses a synthetic ruby crystal as its 2 gain medium. It was the first type of laser invented, and was first operated by Theodore H. "Ted" Maiman at Hughes Research Laboratories on 1960. 16
  17. 17.  The ruby laser produces pulses of visible light at a wavelength of 694.3 nm, which appears as deep red to human eyes. Typical ruby laser pulse lengths are on the order of a millisecond. These short pulses of red light are visible to the human eye, if the viewer carefully watches the target area where the pulse will fire. 17
  18. 18. 18
  19. 19. Applications Ruby lasers have declined in use with the discovery of better lasing media. They are still used in a number of applications where short pulses of red light are required. Holographers around the world produce holographic portraits with ruby lasers, in sizes up to a metre squared. The red 694 nm laser light is preferred to the 532 nm green light of frequency-doubled Nd:YAG. 19
  20. 20.  Many non-destructive testing labs use ruby lasers to create holograms of large objects such as aircraft tires to look for weaknesses in the lining. Ruby lasers were used extensively in tattoo and hair removal, but are being replaced by alexandrite lasers and Nd:YAG lasers in this application. 20
  21. 21. Helium-neon laser 21
  22. 22. 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 spect The gain medium of the laser, as suggested by its name, is a mixture of helium and neon gases, in a 5:1 to 20:1 ratio, contained at low pressure (an average 50 Pa per cm of cavity length ) in a glass envelope. 22
  23. 23. He-Ne Laser The energy or pump source of the laser is provided by an electrical discharge of around 1000 volts through an anode and cathode at each end of the glass tube. A current of 5 to 100 mA is typical for CW operation. 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. 23
  24. 24. He-Ne Laser HeNe lasers are typically small, with cavity lengths of around 15 cm up to 0.5 m, and optical output powers ranging from 1 mW to 100 mW. The red HeNe laser wavelength is usually reported as 632nm. However 24
  25. 25.  The true wavelength in air is 632.816 nm, so 633nm is actually closer to the true value. For the purposes of calculating the photon energy, the vacuum wavelength of 632.991 nm should be used. The precise operating wavelength lies within about 0.002 nm of this value, and fluctuates within this range due to thermal expansion of the cavithy. 25
  26. 26.  The laser process in a HeNe laser starts with collision of electrons from the electrical discharge with the helium atoms in the gas. This excites helium from the ground state to the 23S1 and 21S0 long-lived, metastable excited states. Collision of the excited helium atoms with the ground-state neon atoms results in transfer of energy to the neon atoms, exciting neon electrons into the 3s2 level. This is due to a coincidence of energy levels between the helium and neon atoms. 26
  27. 27.  This process is given by the reaction equation:  He(21S)* + Ne + ΔE → He(11S) + Ne3s2* ΔE is the small energy difference between the energy states of the two atoms, of the order of 0.05 eV or 387 cm-1, which is supplied by kinetic energy. 27
  28. 28. . The number of neon atoms entering the excited states builds up as further collisions between helium and neon atoms occur, causing a population inversion. Spontaneous and stimulated emission between the 3s2 and 2p4 states results in emission of 632.82 nm wavelength light, the typical operating wavelength of a HeNe laser. 28
  29. 29. . After this, fast radiative decay occurs from the 2p to the 1s ground state. Because the neon upper level saturates with higher current and the lower level varies linearly with current, the HeNe laser is restricted to low power operation to maintain population inversion. Spectrum of a helium neon laser showing With the correct selection of cavity mirrors, other wavelengths of laser emission of the HeNe laser are possible. There are infrared transitions at 3.39 μm and 1.15 μm wavelengths, and a variety of visible transitions, including a green (543.5 nm, the so-called GreeNe laser), a yellow (594 nm) and an orange (612 nm) transition. 29
  30. 30. SEMICONDUCTOR LASERLecture-5 L e c A semiconductor laser converts electrical energy  t u into light. This is made possible by using a r semiconductor material, whose ability to conduct e - electricity is between that of conductors and 2 insulators. insulators.  By doping a semiconductor with specific amounts of impurities, the number of negatively charged electrons or positively charged holes can be changed.  Compared to other laser types, semiconductor lasers are compact, reliable and last a long time. 30
  31. 31. SEMICONDUCTOR LASER Such lasers consist of two basic components, an optical amplifier and a resonator. The amplifier is made from a direct-bandgap semiconductor material based on either gallium arsenide (GaAs) or InP substrates. These are compounds based on the Group III and Group V elements in the periodic table. Alloys of these materials are formed onto the substrates as layered structures containing precise amounts of other materials. 31
  32. 32. Le Semiconductor lasercture-2 32
  33. 33. HEAVILY DOPED p-n JUNCTION DIODE (a)In equilibrium. (b)With forward bias. 33
  35. 35. L Raw Materials e c t u The conventional semiconductor laser consists of r e a- compound semiconductor, gallium arsenide. This material comes in the form of ingots that are 2 then further processed into substrates to which layers of other materials are added. The materials used to form these layers are precisely weighed according to a specific formula. Other materials that are used to make this type of laser include certain metals (zinc, gold, and copper) as additives (dopants) or electrodes, and silicon dioxide as an insulator. 35
  36. 36. Lecture-6 APPLICATIONS OF LASERS Lasers are uused in local area network to transfer the data from the memory storage of one computer to other computer. These are used to store large amount of data in CD-ROM 36
  37. 37.  Lasers can be used to blast holes in diamonds and hard steel. They are used as a source of intense heat. They are used to cut, drill, weld, and to remove metal from surfaces. 37
  38. 38.  These are used in spacecrafts and submarines. They are also used in high speed photocopiers and printers. They are used in the field of 3-d photography. 38
  39. 39.  Lasers can serve as a war weapon. High energy lasers are used to destroy enemy aircrafts and missiles. These are used to produce certain chemical reactions. 39
  40. 40.  Lasers are used in controlling haemorrhage. Lasers are used for elimination of moles and tumors. Lasers are used in the treatment of glaucoma. 40
  41. 41.  Co2 laser is used in spinal and brain tumor and kidney stone extrusion. Lasers are used to correct a condition called retina detachments by eye specialist. Argon and Co lasers are used in the 2 treatment of liver and lungs. 41