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The Characteristics of Laser: Directionality, Monochromaticity, Coherence, and Intensity

This text discusses the characteristics of a laser, including directionality, monochromaticity, coherence, and intensity. It explains the importance of the beam's divergence angle, the spectral width of the emitted light, and the mechanisms that cause spectral broadening. The concept of coherence is also covered, both in terms of temporal and spatial coherence. Additionally, it explores how the intensity of a laser beam is significantly higher than that of ordinary light.

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The Characteristics of Laser: Directionality, Monochromaticity, Coherence, and Intensity

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  1. Laser action summary Step 1 : Choose a proper lasing medium Step 2 : Establish population inversion by suitable pumping Step 3 : Stimulated emission takes place Step 4 : Positive feed back (optical resonator) Step 5 : Amplification of light PH 0101 UNIT 3 LECTURE 2

  2. Characteristics of laser • Directionality • The directionality of a laser beam is expressed in terms of the full angle beam divergence which is twice the angle that the outer edge of the beam makes with the axis of the beam. • The outer edge of the beam is defined as a point at which the strength of the beam has dropped to 1/e times its value at the centre. PH 0101 UNIT 3 LECTURE 2

  3. At d1 and d2 distances from the laser window, if the diameter of the spots are measured to be a1 and a2 respectively, then the angle of divergence (in degrees) can be expressed as • For a typical laser, the beam divergence is about 1 milli radian. PH 0101 UNIT 3 LECTURE 2

  4. (ii) Monochromaticity • The degree of monochromaticity is expressed in terms of line width (spectral width) • The line width is the frequency spread  of a spectral line • The frequency spread  is related to the wavelength spread  as  = -(c/2)   • The three most important mechanisms which give rise to the spectral broadening (frequency spread) are Doppler broadening, Collision broadening and natural broadening . PH 0101 UNIT 3 LECTURE 2

  5. (1) Doppler broadening The atoms which emit radiation are not at rest at the time of emission and depending on their velocities and the direction of motion, the frequency of the emitted radiation changes slightly and this broadening is called Doppler broadening. (2) Collision broadening If the atoms undergo collision at the time of emitting radiation there will be change in the phase of the emitted radiation resulting in frequency shift and is known as collision broadening. PH 0101 UNIT 3 LECTURE 2

  6. (3) Natural broadening In solid materials, an atomic electron emitting energy in the form of a photons leads to an exponential damping of the amplitude of the wave train and the phenomenon is called natural broadening. PH 0101 UNIT 3 LECTURE 2

  7. Coherence The purity of the spectral line is expressed in terms of coherence Coherence is expressed in terms of ordering of light field. (1) Temporal coherence (2) Spatial coherence PH 0101 UNIT 3 LECTURE 2

  8. (i) Temporal coherence Temporal coherence refers to correlation in phase at a given point in a space over a length of time. i.e, if the phase difference between the two light fields E1 (x,y,z,t1) and E2 (x,y,z,t2), is constant, the wave is said to have temporal coherence. The maximum length of the wave train on which any two points can be correlated is called coherent length. Coherent time = The high degree of temporal coherence arises from the lasers monochromaticity. PH 0101 UNIT 3 LECTURE 2

  9. (ii) Spatial coherence • Spatial coherence refers to correlation in phase at different points at the same time. • i.e, if the phase difference between the two light fields • E1( x1,y1,z1,t) and E2 (x2,y2, z2,t) is constant, the wave is said to have spatial coherence. • The high degree of spatial coherence results, since the wave fronts in a laser beam are in effect similar to those emanating from a single point source. PH 0101 UNIT 3 LECTURE 2

  10. (4) Intensity or Brightness • When two photons each of amplitude ‘a’ are in phase with each other, then by young’s principle of superposition the resultant amplitude is ‘2a’ and the intensity is proportional to (2a)2 i.e, 4a2. • In laser, many number of photons (say n) are in phase with each other, the amplitude of the resultant wave becomes ‘na’ and hence the intensity is proportional to n2a2. • Thus due to coherent addition of amplitude and negligible divergence, the intensity increases enormously. • i.e., 1mw He-Ne laser can be shown to be 100 times brighter than the sun. PH 0101 UNIT 3 LECTURE 2

  11. Property Spontaneous emissionb(ordinary light) Stimulated emission (laser light) Stimuli Not required Required Monochromaticity Less High Directionality Less High Intensity Less High Coherence Less High Difference between spontaneous emission and stimulated emission PH 0101 UNIT 3 LECTURE 2

  12. Active Medium Pumping Mechanism Optical resonator Essential components of a laser system Active medium or Gain medium It is the system in which population inversion and hence stimulated emission (laser action) is established. Pumping mechanism It is the mechanism by which population inversion is achieved. i.e., it is the method for raising the atoms from lower energy state to higher energy state to achieve laser transition. PH 0101 UNIT 3 LECTURE 3

  13. Different pumping mechanisms i. Optical pumping Exposure to electromagnetic radiation of frequency  = (E2-E1)/h obtained from discharge flash tube results in pumping Suitable for solid state lasers ii. Electrical discharge By inelastic atom-atom collisions, population inversion is established Suitable for Gas lasers PH 0101 UNIT 3 LECTURE 3

  14. iii. Chemical pumping By suitable chemical reaction in the active medium, population of excited state is made higher compared to that of ground state Suitable for liquid lasers. Optical resonator A pair of mirrors placed on either side of the active medium is known as optical resonator. One mirror is completely silvered and the other is partially silvered. The laser beam comes out through the partially silvered mirror. PH 0101 UNIT 3 LECTURE 3

  15. Types of Lasers • Based on its pumping action • Optically pumped laser • Electrically pumped laser • Basis of the operation mode • Continuous wave Lasers • Pulsed Lasers PH 0101 UNIT 3 LECTURE 3

  16. According to their wavelength • Visible Region • Infrared Region • Ultraviolet Region • Microwave Region • X-Ray Region PH 0101 UNIT 3 LECTURE 3

  17. According to the source • Dye Lasers • Gas Lasers • Chemical Lasers • Metal vapour Lasers • Solid state Lasers • Semi conductor Lasers • other types PH 0101 UNIT 3 LECTURE 3

  18. Gas lasers PH 0101 UNIT 3 LECTURE 3

  19. CO2 Laser • Introduction • 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. PH0101 UNIT 3 LECTURE 4

  20. 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. PH0101 UNIT 3 LECTURE 4

  21. Pumping • Population inversion is created by electric discharge of the mixture. • When a discharge is passed in a tube containing CO2, electron impacts excite the molecules to higher electronic and vibrational-rotational levels. • This level is also populated by radiationless transition from upper excited levels. • The resonant transfer of energy from other molecules, such as, N2, added to the gas, increases the pumping efficiency. • Nitrogen here plays the role that He plays in He-Ne laser. PH0101 UNIT 3 LECTURE 4

  22. A carbon dioxide (CO2) laser can produce a continuous laser beam with a power output of several kilowatts while, at the same time, can maintain high degree of spectral purity and spatial coherence. In comparison with atoms and ions, the energy level structure of molecules is more complicated and originates from three sources: electronic motions, vibrational motions and rotational motions. PH0101 UNIT 3 LECTURE 4

  23. Fundamental Modes of vibration of CO2 • Three fundamental modes of vibration for CO2 • symmetric stretching mode(frequency 1), • bending mode (2) and • asymmetric stretching mode (3). • In the symmetric stretching mode, the oxygen atoms oscillate along the axis of the molecule simultaneously departing or approaching the carbon atom, which is stationary. PH0101 UNIT 3 LECTURE 4

  24. In the bending mode, the molecule ceases to be exactly linear as the atoms move perpendicular to the molecular axis. • In asymmetric stretching, all the three atoms oscillate: but while both oxygen atoms move in one direction, carbon atoms move in the opposite direction. • The internal vibrations of carbon dioxide molecule can be represented approximately by linear combination of these three normal modes. PH0101 UNIT 3 LECTURE 4

  25. CO2 Laser PH0101 UNIT 3 LECTURE 4

  26. Independent modes of vibration of CO2 molecule PH0101 UNIT 3 LECTURE 4

  27. The energy level diagram of vibrational – rotational energy levels with which the main physical processes taking place in this laser. • As the electric discharge is passed through the tube, which contains a mixture of carbon dioxide, nitrogen and helium gases, the electrons striking nitrogen molecules impart sufficient energy to raise them to their first excited vibrational-rotational energy level. • This energy level corresponds to one of the vibrational - rotational level of CO2 molecules, designated as level 4. PH0101 UNIT 3 LECTURE 4

  28. collision with N2 molecules, the CO2 molecules are raised to level 4. • The lifetime of CO2 molecules in level 4 is quiet significant to serve practically as a metastable state. • Hence, population inversion of CO2 molecules is established between levels 4 and 3, and between levels 4 and 2. • The transition of CO2 molecules between levels 4 and 3 produce lasers of wavelength 10.6 microns and that between levels 4 and 2 produce lasers of wavelength 9.6 microns. PH0101 UNIT 3 LECTURE 4

  29. Energy level diagram PH0101 UNIT 3 LECTURE 4

  30. The He molecules increase the population of level 4, and also help in emptying the lower laser levels. • The molecules that arrive at the levels 3 and 2 decay to the ground state through radiative and collision induced transitions to the lower level 1, which in turn decays to the ground state. • The power output of a CO2 laser increases linearly with length. Low power (upto 50W) continuous wave CO2 lasers are available in sealed tube configurations. PH0101 UNIT 3 LECTURE 4

  31. Some are available in sizes like torches for medical use, with 10-30 W power. • All high power systems use fast gas-floe designs. • Typical power per unit length is 200-600 W/m. • Some of these lasers are large room sized metal working lasers with output power 10-20 kW. • Recently CO2 lasers with continuous wave power output exceeding 100 kW. • The wavelength of radiation from these lasers is 10.6m. PH0101 UNIT 3 LECTURE 4

  32. Nd: YAG Laser (Doped insulator laser) • Lasing medium • The host medium for this laser is Yttrium Aluminium Garnet (YAG = Y3 Al5 O12) with 1.5% trivalent neodymium ions (Nd3+) present as impurities. • The (Nd3+) ions occupy the lattice sites of yttrium ions as substitutional impurities and provide the energy levels for both pumping and lasing transitions. PH0101 UNIT 3 LECTURE 4

  33. When an (Nd3+) ion is placed in a host crystal lattice it is subjected to the electrostatic field of the surrounding ions, the so called crystal field. • The crystal field modifies the transition probabilities between the various energy levels of the Nd3+ ion so that some transitions, which are forbidden in the free ion, become allowed. PH0101 UNIT 3 LECTURE 4

  34. Nd: YAG laser PH0101 UNIT 3 LECTURE 4

  35. The length of the Nd: YAG laser rod various from 5cm to 10cm depending on the power of the laser and its diameter is generally 6 to 9mm. • The laser rod and a linear flash lamp are housed in a elliptical reflector cavity • Since the rod and the lamp are located at the foci of the ellipse, the light emitted by the lamp is effectively coupled to the rod. • The ends of the rod are polished and made optically flat and parallel. PH0101 UNIT 3 LECTURE 4

  36. The optical cavity is formed either by silvering the two ends of the rod or by using two external reflecting mirrors. • One mirror is made hundred percent reflecting while the other mirror is left slightly transmitting to draw the output • The system is cooled by either air or water circulation. PH0101 UNIT 3 LECTURE 4

  37. Energy level diagram Simplified energy level diagram for the neodymium ion in YAG showing the principal laser transitions PH0101 UNIT 3 LECTURE 4

  38. This laser system has two absorption bands (0.73 m and 0.8 m) • Optical pumping mechanism is employed. • Laser transition takes place between two laser levels at 1.06mm PH0101 UNIT 3 LECTURE 4

  39. Output characteristics • The laser output is in the form of pulses with higher repetition rate • Xenon flash lamps are used for pulsed output • Nd: YAG laser can be operated in CW mode also using tungsten-halide incandescent lamp for optical pumping. • Continuous output powers of over 1KW are obtained. PH0101 UNIT 3 LECTURE 4

  40. Note: Nd: Glass laser • Glass acts as an excellent host material for neodymium • As in YAG, within the glass also local electric fields modify the Nd3+ ion energy levels • Since the line width is much broader in glass than in YAG for Nd3+ ions, the threshold pump power required for laser action is higher • Nd: Glass lasers are operated in the pulsed mode at wavelength 1.06 m PH0101 UNIT 3 LECTURE 4

  41. Nd:YAG/ Nd: Glass laser applications • These lasers are used in many scientific applications which involve generation of other wavelengths of light. • The important industrial uses of YAG and glass lasers have been in materials processing such as welding, cutting, drilling. • Since 1.06 m wavelength radiation passes through optical fibre without absorption, fibre optic endoscopes with YAG lasers are used to treat gastrointestinal bleeding. PH0101 UNIT 3 LECTURE 4

  42. YAG beams penetrate the lens of the eye to perform intracular procedures. • YAG lasers are used in military as range finders and target designators. PH0101 UNIT 3 LECTURE 4

  43. Semiconductor (Ga-As) lasers Introduction The semiconductor laser is today one of the most important types of lasers with its very important application in fiber optic communication. These lasers use semiconductors as the lasing medium and are characterized by specific advantages such as the capability of direct modulation in the gigahertz region, small size and low cost. PH 0101 UNIT 3 LECTURE 5

  44. 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 electron-hole pair. PH 0101 UNIT 3 LECTURE 5

  45. 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. PH 0101 UNIT 3 LECTURE 5

  46. When a current is passed through a p-n junction under forward bias, the injected electrons and holes will increase the density of electrons in the conduction band. • The stimulated emission rate will exceed the absorption rate and amplification will occur at some value of current due to holes in valence band. • As the current is further increased, at threshold value of the current, the amplification will overcome the losses in the cavity and the laser will begin to emit coherent radiation. PH 0101 UNIT 3 LECTURE 5

  47. Simple structure (Homojunction) • The basic semiconductor laser structure in which the photons generated by the injection current travel to the edge mirrors and are reflected back into the active area. • Photoelectron collisions take place and produce more photons, which continue to bounce back and forth between the two edge mirrors. • This process eventually increases the number of generated photons until lasing takes place. The lasing will take place at particular wavelengths that are related to the length of the cavity. PH 0101 UNIT 3 LECTURE 5

  48. Basic semiconductor laser structure a) Side view b) Projection Hetero structures PH 0101 UNIT 3 LECTURE 5

  49. Heterostructures The hetero structure laser is a laser diode with more than single P and N layers. GaAs/AlGaAs is a Hetero junction laser. The notations P+ and N+ and P- and N- indicate heavy doping and light doping respectively. The P-N structure consists of the two double layers, P+ - P- and N+ - N- . A thin layer of GaAs is placed at the junction, the active region. The substance is selected because the electron-hole recombinations are highly radiative. This increases the radiation efficiency. PH 0101 UNIT 3 LECTURE 5

  50. The P and N regions are lightly doped regions that have an index of refraction n2 less than n1 of the active region. • These three layers, n2-n1-n2, form a light waveguide much like the optical fiber, so that the light generated is confined to the active region. PH 0101 UNIT 3 LECTURE 5

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