M1_Laser_CSE.pptx laser vtu 2nd sem engineering notes

physics notes module 1

1. Module – 1 Lasers & Optical Fiber Lasers : Basic properties of a LASER beam, Interaction of Radiation with Matter, Einstein’s A and B Coefficients, Laser Action, Population Inversion, Metastable State, Requisites of a laser system, Semiconductor Diode Laser, Applications: Bar code scanner, Laser Printer, Laser Cooling, Numerical Problems Optical Fiber : Principle and structure, Acceptance angle and Numerical Aperture (NA) and derivation of Expression for NA, Classification of Optical Fibers, Attenuation and Fiber Losses, Applications: Fiber Optic networking, Fiber Optic Communication. .

2. : MASER :Ruby Laser The Nobel Prize in Physics 1964 was divided, one half awarded to Charles Hard Townes , the other half jointly to Nicolay G B and A M Prokhorov "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle"

3. Ground state energy : -13.6 eV Ist excited state energy : - 3.4 eV E = hγ   Induced Absorption

4.   How radiation is produced   Spontaneous Emission

5. In Spontaneous Emission: Electrons get transferred to lower state with time delay Emitted rays will not be in phase They travel in different directions Hence they will not get amplified The light radiations produced by ordinary sources are due to Spontaneous Emission

8. 4. High energy

9. Properties of Laser Monochromatic Concentrate in a narrow range of wavelengths (one specific colour). Directional A very tight beam which is very strong and concentrated. Coherent All the emitted photons bear a constant phase relationship with each other in both time and phase

10. Basic concepts for a laser Absorption Spontaneous Emission Stimulated Emission Population inversion

11. Stimulated Absorption Energy is absorbed by an atom, the electrons are excited into vacant energy shells.

12. Atom initially in level 1 interacts with an electromagnetic wave of suitable frequency γ. The atom may now undergo a transition to level 2, absorbing the energy from the incident radiation. This is the phenomenon of induced absorption.   Atom + Photon → Atom*

13. Spontaneous Emission The atom decays from level 2 to level 1 through the emission of a photon with the energy hv . It is a completely random process.

14. Atom* → Atom + Photon

15. 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

16. Each electron is triggered into emission by the presence of electromagnetic radiation of the proper frequency. This is known as stimulated emission and it is a key to the operation of laser. Atom* + Photon → Atom + (Photon + Photon)

17. It is an excited state of an atom or other system with a longer lifetime than the other excited states. However, it has a shorter lifetime than the stable ground state. Atoms in the metastable state remain excited for a considerable time in the order of 10 -6  to 10 -3 . A large number of excited atoms are accumulated in the metastable state. Metastable state

18. Population inversion is the state in which the number of atoms in higher energy state is more than those in lower energy state.

19. Expression for Energy Density in terms of Einstein’s Coefficients : energy states E1 and E2 of a system of atoms with E2 > E1. N1 atoms with energy E1 and N2 atoms with energy E2, per unit volume of the system. N1 and N2 are called the number densities of atoms in the states 1 and 2 respectively. radiation of frequency 𝜈 = Hz, and the energy density of radiations U 𝜈       Induced absorption   Spontaneous Emission Non radiative Transfer M Stimulated Emission   N2 = N3 +N4

21.   ………….(4)   Rate of induced absorption = Rate of spontaneous emission + Rate of stimulated emission N 2 = N 3 + N 4 substituting eqns (1), (2) and (3) we get

22.       Energy density,

23. Terminology: Ground State Excited State Metastable state Active Medium Induced Absorption Highest energy level occupied by electrons at absolute zero Energy states above the ground state An excited state with a longer life time Medium which contains ground, excited & metastable states Absorption of energy by a medium and transfer of electrons from lower to higher level LASER

24. Spontaneous Emission Non Radiative Transfer Population Inversion Stimulated Emission Optical cavity Optical Feedback Emission of energy when electron jump from higher to lower level without external help Transfer of electrons from excited to metastable state with out the emission of energy The stage at which the number of electrons in the excited state is more than that in the ground state Emission of energy when electrons jump from higher to lower state by the influence of an external medium The region of the active medium in between the feedback mirrors where lasing take place The process of sending a part of the output radiation back to the input

25. Optical Cavity A pair of plane parallel (concave)mirrors kept on both sides of the active medium for reflecting the rays back and forth several times . One mirror is fully silvered and the other partially silvered Distance between the mirrors are adjusted to have the rays in phase giving a standing wave pattern

26. L = n (λ/2)

27. Requisites of a Laser system.. Medium suitable for the production of laser which contains Ground state, Excited state and at least one Metastable state. Source which can provide energy E2 – E1 with high spectral density   Active Medium Source Optical Cavity

28. Conditions for laser action Metastable state: An energy level above the ground state with higher life time Population Inversion : The condition in which the number of atoms in an excited state is more than the number in the ground state. Optical Feedback : The process by which a part of the out put light radiation is fed back to the input

29. 1. Distinguish between spontaneous emission and stimulated emission. 2. What are the requisites for the laser and explain the conditions to produce laser.

30. Energy of radiation depends on the number of photons present in the beam Energy of a photon ΔE = hγ Energy of the beam E = nhγ , n is the number of photons in unit cross section

34. Semiconductor Laser Principle:   When a p-n junction diode is forward biased, the electrons from n region and the holes from the p region cross the junction and recombine with each other.  During the recombination process, the light radiation (photons) is released from a certain specified direct band gap semiconductors like Ga-As. This light radiation is known as recombination radiation .  The photon emitted during recombination, stimulates other electrons and holes to recombine. As a result, stimulated emission takes place which produces laser.

35. Construction The active medium is a p-n junction diode made from the single crystal. 10 17 to 10 19 dopant atoms/cm 3 This crystal is cut in the form of a platter having thickness of 0.5μm to 1mm . The photon emission is stimulated in a very thin layer of PN junction (in order of few microns 1μm to 100μm). The electrical voltage is applied to the crystal through the electrodes fixed on the surfaces.  The end faces of the junction diode are well polished and parallel to each other. They act as an optical resonator through which the emitted light comes out.

36. Working The region around the junction contains a large amount of electrons in the conduction band and a large amount of holes in the valence band.   The condition of population inversion is achieved by heavy doping. When forward biased the electrons and holes recombine with each other and produce radiation in the form of light.   When the forward – biased voltage is increased, more and more light photons are emitted and the light production instantly becomes stronger. These photons will trigger a chain of stimulated recombination resulting in the release of photons in phase.

38. Disadvantages Due to low power production it is not suited for typical applications. The temperature affects the output Beam divergence is much more Advantages of SC Laser : Light weight and portable Battery supported, easily replaceable Small size low cost Capability of monolithic integrated circuit (IC) with electronic circuitry Compatibility with optical fibers

39. Applications Fibre optical communication Barcode readers Laser pointers Disc reader Laser printing & scanning Lighting sources

40. barcode reader A barcode reader is an optical scanner that can read printed barcodes, decode the data contained in the barcode to a computer.

41.   it consists of : a light source, a lens and a light sensor for translating optical impulses into electrical signals. Additionally, nearly all barcode readers contain   decoder  circuitry that can analyse the barcode's image data provided by the sensor and send the barcode's content to the scanner's output port.

42. So for the code shown here ("black black black white black white black black"), the cell would be "off off off on off on off off." 00010100

43. Pen-type readers Pen-type readers consist of a light source and  photodiode  that are placed next to each other in the tip of a pen. To read a barcode, the person holding the pen must move the tip of it across the bars at a relatively uniform speed. The photodiode measures the intensity of the light reflected back from the light source as the tip crosses each bar and space in the printed code. The photodiode generates a waveform that is used to measure the widths of the bars and spaces in the barcode. Dark bars in the barcode absorb light and white spaces reflect light so that the voltage waveform generated by the photodiode is a representation of the bar and space pattern in the barcode. This waveform is decoded by the scanner in a manner similar to the way  Morse code  dots and dashes are decoded.

44. laser printer They work by using a heated wire to positively charge a drum, which is then passed over by a laser that reverses the charge in the areas that it hits. The now-negatively charged areas of the drum represents the image or text that is to be printed. A toner roller is passed over the drum, and toner particles stick to the negatively charged areas. A sheet of paper is then fed underneath the toner-coated drum and the toner is passed onto its surface, creating a printer copy of a digital document or image.

45. The laser printing process The laser printing process utilises precision streams of ink to print onto the paper. The laser printing process involves different steps. The data required to be printed is transferred from the computer to the laser printer. This is usually via an ethernet cable or wirelessly if the printer has wireless capabilities. The printer then has to reach the required temperature via the heating of the corona wire. This wire once heated, passes an electrical static charge to the drum unit. The drum unit, now positively charged, is ready to receive the laser beam and begin the data transfer process directly onto the drum. Once the laser is activated, the beam reflects off a moving mirror unit which directs the beam directly onto the drum unit.

46. In the areas where the beam hits the drum, the charge is changed from positive to negative. The negatively charged areas now represent where toner particles will adhere to the drum and be directly transferred onto the paper. The ink roller now begins to coat the drum with toner. Toner is comprised of microscopic ink particles which, now positively charged, adhere to the negatively charged areas on the drum unit. A positively charged sheet of paper is now passed close to the drum, attracting the negatively charged toner particles onto the page. The paper, now containing the inked content, is passed into the fuser unit where the rollers fuse the toner particles to the paper. The page is then passed through to the other side of the copier, and you now have one successful printout!

47. Laser printer components A laser printer comprises of different components, each essential in producing the superb print quality. The printer includes: Power supply Requiring a high voltage in order to charge the drum, the power supply works to convert AC current into higher voltages needed for the transfer process . Photosensitive drum The role of the drum unit is to attract positively charged toner particles onto its surface. In order to achieve this, laser printers usually use a corona wire which carries a high voltage. Once the drum is adequately charged, the laser beam is guided onto the surface of the drum via mirrors which precisely scans the images onto the surface of the drum. Toner cartridges Laser printers use microscopic ink particles in a powdered form known as toner. Once heated by the fuser unit, the toner melts allowing it to be fused to the paper fibres under pressure .

48. Corona wires The primary corona wire is responsible for positively charging the drum unit in order for the drum unit to be able to draw the toner particles onto its surface. The transfer corona wire is given a negative charge in order to negatively charge the paper and draw the toner particles from the surface of the drum onto the sheet. Both of these wires require high voltages in order to create adequate charge, this is supplied by the high voltage power supply. Fuser unit The fuser unit comprises of two heated rollers used to physically fuse the toner particles onto the paper. Using high levels of heat and pressure, the powdered toner particles are melted and form a strong bond with the fabric of the paper. Given the high speed that the fuser unit operates, the sheet has as little contact with the paper as possible which avoids the risk of fire. Waste Toner Bottle The waste toner bottle is used to collect excess toner from the photosensitive drum that is unused during the printing process. Excess toner is deposited in a reservoir and once full, the waste toner bottle must be replaced.

49. laser cooling In laser cooling, the basic principle used to cool down atoms is to slow them down. Each atomic species has thermal energy associated with it. So, slowing down an atom would lower its kinetic energy and thermal energy, which will eventually lead to a decrease in its temperature, hence cooling it down. There are many techniques under laser cooling to initiate the cooling. However, one of the most famous and in-demand techniques in this regime is that of Doppler cooling. 

50. Doppler cooling: Whenever a light beam is incident on an object, it exerts a force on it. However, if the incoming laser beam is such that its frequency is somewhat below the atom’s resonant frequency, then the laser beam can be used to slow down the atom. So what leads to this slowdown is the direction-dependent Doppler effect.

51. In technical terms, Doppler’s effect can be defined as an observed change in frequency of a wave when an observer and source have relative motion between them. So, if the radiation source is moving towards the observer, then the frequency of those radiations as perceived by the observer is blue-shifted , which means the observer will observe a higher frequency of radiation traveling towards it than the actual frequency emitted by the source. However, the reverse process of redshift occurs if the source of radiation is moving away from the observer.  A common example of the Doppler effect is the change of pitch heard when a vehicle sounding a horn approaches and recedes away from an observer.

52. So now, owing to the Doppler effect, the laser light, which actually has a frequency lower than the resonant frequency of atom, is brought to resonance for the moving atom “sees” the incoming photon with a higher frequency than what it actually has and hence, that atom can easily absorb it. The photons that will subsequently get radiated by the atom will have a somewhat higher frequency than what was absorbed. To compensate for the gap between the energy of absorbed and emitted photons, the atom suffers a toll on its kinetic energy .

53. As the lack of energy is taken from the atoms’ kinetic energy, they suffer a decrease in their momentum and velocity and hence, are eventually cooled down. Since the atoms return from the optically excited state to the ground state after some 10 ns, the absorption-emission process repeats very rapidly. So mostly, an arrangement of six laser beams is employed so that the atoms’ motion can be slowed down effectively in all directions.

54. Sisyphus cooling In ultra-low-temperature physics, Sisyphus cooling , the Sisyphus effect , or polarization gradient cooling involves the use of specially selected laser light, hitting atoms from various angles to both cool and trap them in a potential well, effectively rolling the atom down a hill of potential energy until it has lost its kinetic energy. It is a type of laser cooling of atoms used to reach temperatures below the Doppler cooling limit.