FAQ 2
General lasers: Solid State Lasers
Based on the suggestions of large number of students and laser scientists, this section is being renamed as FAQ. The questions being addressed are of very general nature and are being forwarded to us by readers from different parts of India. This section provides an opportunity for interaction where one can ask their clarification either about the articles on web or any other question related to lasers. The reply will be sent to them after discussing with either the authors of a particular article or the experts in that field. Any comments / suggestions to improve the site welcome.
  1. What is M2?
  2. What does TEMpq denote?Show the patterns related to TEM01 and TEM10.
  3. Calculate the number of possible longitudinal modes in a He-Ne laser having a cavity of length of one meter.What is the separation between two modes?
  4. In the above example, if the laser gain profile bandwidth is 500 MHz, how many longitudinal modes are possible?
  5. What are the common techniques of pumping the gain medium?
  6. What are the basic criteria for selecting flash lamps for solid state lasers?
  7. What are the drawbacks of flash lamps?
  8. What are the advantages of diode pumping in solid state lasers?
  9. What is the difference between S- and P- polarization?
  10. Define Slope Efficiency of a Laser?
  11. Define Threshold Pump Power?
  12. Define Wall Plug Efficiency?
  13. Define Luminescence, Fluorescence, and Phosphorescence?
  14. What is a flash lamp?
  15. Why xenon is preferred as the filling gas for the flash lamps?
  16. What is the advantage of using fused silica as the flash lamp envelope?
  17. What is meant by negative resistance in flash lamps?
  18. What is an arc lamp?
  19. What is triggering?
  20. Explain over damped, under damped and critically damped circuits in flash lamp operation.
  21. Explain over voltage, external, parallel and series triggering circuits.
  22. What are the different types of electrode-lamp seals?
  23. What are the basic requirements of an electrical power supply for gas discharge devices?
  24. What are the three basic parts of a flash lamp power supply?
  25. What is a PFN?
  26. What are the factors associated with electrical shock?
  27. Discuss briefly the effect of current in the human body.
  28. What does simmering mean?
  29. What is the star/sun shaped symbol, we see at various places?
  30. Can lasers be dangerous for human beings?
  31. How can lasers prove to be dangerous for eyes?
  32. Why is viewing even the diffused reflection of the pulsed solid-state laser from a roughened surface is usually considered hazardous?
  33. Taking eye as an example, discuss why only particular parts of eye are more susceptible to damage by particular laser radiation and not others.
  34. Which is the most powerful laser in the world?
  35. Why rare earth ions are generally doped as active ions in laser materials?
  36. What are the main lasers being used for medical applications?
  37. Mention the wavelengths of few important Lasers?
  38. What are Eye - safe Lasers?
  39. What are different classes of Lasers?


  1. What is M2?
  2. It is a measure of beam quality. In simple terms it is the ratio of the divergence of the real beam to that of a theoretical diffraction-limited beam of the same waist size with a Gaussian beam profile.


  3. What does TEMpq denote? Show the patterns related to TEM01 and TEM10.
  4. TEM stands for Transverse Electromagnetic modes. The subscripts p and q represent the number of zero illumination points (between illuminated regions) along x-axis and y-axis respectively.

     
    TE01   TE10


  5. Calculate the number of possible longitudinal modes in a He-Ne laser having a cavity of length of one meter.What is the separation between two modes?
  6. The number of possible modes can be estimated using the following equation:
    2nL=Nλ
    where L is the length of the cavity, N is number of all possible modes, n is the refractive index of the lasing medium and λ is the wavelength. Assuming refractive index of the medium as one, the number of modes can be calculated as
      
    However, all these modes will not be supported. These are limited by the fluorescence curve and only the modes for which the gain of laser of the laser medium G (λ) > 1 would be supported.
    The separation between two modes can be estimated using simple relation:
    Δ f = c / 2L, where Δ f is the frequency separation
    In the above example, this separation is
      
    OR one can make use of the following relation and find out the separation in terms of wavelength:
      
    This gives:
      


  7. In the above example, if the laser gain profile bandwidth is 500 MHz, how many longitudinal modes are possible?
  8. As mentioned above that though the number of possible modes exceed three million but the number is restricted by fluorescence curve and only the modes for which the gain of laser of the laser medium G (λ) > 1 would be supported. In the present case, this bandwidth is 500 MHz, and the frequency spacing between two modes is 150 MHz. Thus maximum number of modes can be = (bandwidth / spacing) = 500 / 150 = 3.33. In a practical case only three modes will be excited.


  9. What are the basic criteria for selecting flash lamps for solid state lasers?
  10. Spectral emission characteristics of the flash lamp should match the absorption band of the lasing medium.


  11. What are the common techniques of pumping the gain medium?
  12. Pumping of the gain media is usually performed in one of the following forms:
    • Optical pumping
    • Electrical pumping
    • Chemical pumping
    So far as solid-state lasers are concerned, it is mainly the optical pumping, which is being used. Optical pumping uses either CW or pulsed light emitted by a powerful lamp or a laser beam. Gas lasers such as He - Ne, carbon dioxide are pumped by electrical means. Chemical lasers such as Hydrogen Fluoride (HF), Deuterium Fluoride (DF), Chemical oxygen Iodine Laser (COIL) are pumped through chemical means.


  13. What are the drawbacks of flash lamps?
    • The lifetime of lamps is very limited - normally up to a few thousand hours.
    • Flash lamps have a broad emission spectra (see adjoining figure) whereas the absorption spectra of lasing media have more or less discreet absorption peeks. As a result, most of the optical energy being emitted by the flash lamp goes waste.
    • The wall plug efficiency of the laser (electrical to optical efficiency) is low - typically ~ few percent. This results in a higher heat load, making necessary a more powerful cooling system, and the strong thermal lensing and hence a poor beam quality.
    • Electric power supplies for lamp-pumped lasers involve high electrical voltages, which raise additional safety issues.
    • The low pump brightness (compared with that achievable with diode lasers) and the broad emission wavelength range exclude many solid-state gain media.


  14. What are the advantages of diode pumping in solid state lasers?
  15. The main advantages of diode pumping can be summarized as follows:
    • The compactness of the pump source, the power supply and the cooling arrangement makes the whole laser system much smaller and easier to use.
    • A high electrical-to-optical efficiency of the pump source (order of 50%) leads to a high overall power efficiency i.e. wall plug efficiency of the laser. As a consequence, small power supplies are needed, and both the electricity consumption and the cooling demands are drastically reduced, comparing with those for lamp-pumped lasers.
    • The narrow optical bandwidth of diode lasers makes it possible to directly pump certain transitions of laser-active ions without losing power in other spectral regions. It thus also contributes to a high efficiency.
    • Although the beam quality of high power diode lasers is poor, however, end pumping of lasers provide very good overlap of laser mode and pump region, leading to high beam quality and power efficiency.
    • Diode-pumped low-power lasers can be pumped with diffraction-limited laser diodes. This allows the construction of very low power lasers with reasonable power efficiency.
    • The lifetime of laser diodes is long compared with that of discharge lamps: Further it is much easier to replace laser diodes as compared to discharge lamps.
    • Diode pumping makes it possible to use a very wide range of solid-state gain media for different wavelength regions.


  16. What is the difference between S- and P- polarization?
  17. S-polarization (the s stems from the German word Senkrecht meaning perpendicular) and P-polarization (the p means parallel) are the two main ways to describe two types of linearly polarized light. This perpendicular and parallel are with respect to the surface onto which the light is incident or being reflected.
    So, P-polarization refers to light that is polarized parallel to the plane of incidence and S-polarization refers to light that is polarized perpendicularly to the plane of incidence.


  18. Define Slope Efficiency of a Laser?
  19. Slope efficiency is defined as the slope of the curve obtained by plotting the laser output versus the pump power.
    It is worth mentioning that for a given pump power, the laser (gain medium) having higher pump absorption efficiency will have higher slope efficiency as well.


  20. Define Threshold Pump Power?
  21. The threshold pump power of an optically pumped laser is the value of the incident pump power for just initiation of Laser action. At this point, the small signal gain equals the losses inside a laser resonator. A low threshold power requires low cavity losses and high pump absorption efficiency. In all practical situations, the pump power used in normal operation is several times higher than the pump threshold power.


  22. Define Wall Plug Efficiency?
  23. The wall-plug efficiency of a laser system is its total electrical-to-optical power efficiency. Though the electrical power consumed should include all the electrical instruments like the chillers and re-circulation systems required for a cooling system, however, the convention is to take into account only the electrical power required for running flash lamps or laser diodes, which are used for pumping the gain medium. It is understandable that diode pumped solid state lasers have much higher wall-plug efficiency as compared to flash lamp pumped lasers.


  24. Define Luminescence, Fluorescence, and Phosphorescence?
  25. Luminescence is a collective term for different phenomena where a substance emits light without being strongly heated, i.e., the emission is not simply thermal radiation. This definition is also reflected by the term "cold light".
    Fluorescence and Phosphorescence fall under this category. Fluorescence refers to the light emission caused by irradiation with visible or UV light. The light emission occurs typically within nanoseconds to milliseconds after irradiation. It involves the excitation of electrons into states with a higher energy, from where radiative decay is possible. Typically, the emitted wavelengths are longer than the excitation wavelengths. Phosphorescence, on the other hand, refers to a light emission, which can occur over much longer times (sometimes hours) after irradiation. It involves storage of energy in metastable states and its release through relatively slow processes. This release of energy is usually enhanced when thermally activated.


  26. What is a flash lamp?
  27. A flash lamp or flash tube is a gaseous discharge device filled with Nobel gas (usually Xenon or Krypton) that is designed to produce pulsed radiation.


  28. Why xenon is preferred as the filling gas for the flash lamps?
  29. Total radiation output is maximum for xenon compared to other noble gases (except radon, which is radio-active) for the same electrical input.


  30. What is the advantage of using fused silica as the flash lamp envelope?
  31. It has a spectral transmission from 200 to 4000nm, high thermal conductivity and low thermal expansion.


  32. What is meant by negative resistance in flash lamps?
  33. In flash lamps, as the current increases the resistance decreases and this is referred to as negative resistance


  34. What is an arc lamp?
  35. Arc lamps are gas discharge devices designed for continuous radiation. Krypton filled lamps offer high Nd:YAG pumping efficiency because the emission spectra of the lamp is close to the absorption spectrum of the lasing medium.


  36. What is triggering?
  37. Triggering is the initiation of a discharge through a strobe lamp or arc lamp. There are four methods of triggering a flash tube. They are over voltage triggering, series triggering, external triggering and parallel triggering. The most flexible and commonly used design is external triggering.


  38. Explain over damped, under damped and critically damped circuits in flash lamp operation.
  39. Over damped circuit produces current pulse with low peak value. Under damped circuit generates current pulse with oscillatory nature, resulting in current reversing and swinging negative current. Critically damped circuit produces ideal current pulse of high peak value with no current reversal.


  40. Explain over voltage, external, parallel and series triggering circuits.
  41. In the case of over voltage triggering, the bias voltage across the lamp is high enough to produce the break down of the gas in the flash lamp to begin the discharge. But the high voltage trigger switch (like hydrogen thyratron) is an expensive device.
    In external triggering, a very high voltage of short duration is applied to the trigger wire directly, which is wound outside the envelope of the flash lamp to initiate the discharge. The advantage is that this requires an inexpensive and lightweight transformer
    In series triggering, the secondary of the trigger transformer is in series with the flash lamp and the energy storage capacitor. It produces a safe, highly reliable and reproducible triggering, resulting in a stable and steady light output.
    In parallel triggering, the secondary winding of the trigger transformer is connected in parallel to the lamp, with a diode isolating the secondary winding of the transformer from the energy storage capacitor.


  42. What are the different types of electrode-lamp seals?
  43. There are two types of sealing of the electrodes to the quartz tubes, namely, tungsten rod seals and end cap seals.
    The tungsten rod seal is made with glasses having thermal expansion intermediate between tungsten and the quartz envelope to reduce the stress arising from thermal expansion at material interface. Since this structure (graded seal) is processed at high temperature, flash lamp operation at high current densities with long life is possible.
    In end cap seals, a circular band of invar, constituting the end cap is incorporated with the quartz tube. It is also called solder seal as lead-indium solder with relatively low melting point, is used to make the seal due to its low manufacturing cost. Since the solder material has a low melting point, it cannot withstand high temperature and high current density, in comparison to rod seals. This limits its operation to moderate current applications.


  44. What are the basic requirements of an electrical power supply for gas discharge devices?
  45. It should provide high voltage sufficient to ionize the gas, with adequate voltage and current for a continuous discharge at a pre-selected current level. i.e. Peak voltage exceeding the ionization potential of the gas and a ballast resistor to limit the current passing through gas. Basic reason for controlling the current is that population inversion is dependent on current and not on voltage.


  46. What are the three basic parts of a flash lamp power supply?
  47. Three basic elements are 1) high voltage DC power supply for charging the energy storage capacitor bank 2) pulse forming net work (PFN) comprising of inductance (L), capacitor (C) and resistance (R) being the resistance at the time of gas discharge for transfer of energy from capacitor to flash lamp.3) trigger circuit to provide a low current & high voltage trigger pulse to initiate ionization of the gas and subsequent gas discharge and flash lamp output.


  48. What is a PFN?
  49. A Pulse Forming Network is comprised of an inductor, capacitor, and power supply that generate an electrical pulse to a flash lamp. The values of the capacitor and the inductor decide the electrical pulse to the lamp.


  50. What are the factors associated with electrical shock?
  51. Damage to human body is basically due to the magnitude of the current and not the voltage. But it may be understood that the voltage and the resistance of the body determines the amount of the current flow. The basic factors are magnitude of the current, the flow route and the duration of the current flow. If it is an a.c. circuit, the frequency is also important as it could affect the function of the heart, for example. Damages can be due to thermal and non-thermal effects. Thermal effect produces burns and non-thermal effect produce electrical breakdown of muscles and nerve cells.


  52. Discuss briefly the effect of current in the human body.
  53. The dry skin of the person has a very high resistance compared to the wet skin and as such the amount of current flow in the latter case will be very high. Further the internal parts of the body have low resistance due to the salty and moist nature of the tissues. The high current thus developed cause damage. The function of the heart is affected by the frequency, unabling it to pump the blood properly.


  54. What does simmering mean?
  55. Simmering is the process of maintaining a steady state of partial ionization in the xenon flash lamp during operation between flashes. Simmering avoids the electrode stress associated with continually discharging across a lamp that is not ionized.


  56. What is the star/sun shaped symbol, we see at various places?
  57. The symbol is a simplified version of the international "sunburst" symbol, which stands for lasers. The standard international symbol is shown on the left in the International "Lasers In Use" warning sign which should be posted in areas where unshielded lasers are in use. You may even find a similar sign or logo inside your CD player or CD-ROM drive as they use low power laser diodes to read the data from the disk.



  58. Can lasers be dangerous for human beings?
  59. Yes. Lasers can be dangerous for the part of the body that is most sensitive to light, the eye and laser beams can also cause skin and clothing burns. This is why at laser shows, high power laser beams are usually well above our heads.


  60. How can lasers prove to be dangerous for eyes?
  61. If a high power laser beam strikes in the eye it can cause a burn on the retina. Just as the magnifying glass can focus the sun and burn a piece of paper, the lens in the human eye focuses the laser beam down to a very small point on the retina which can cause a burn. The focusing effect can concentrate a laser beam up to 100,000 times thus the power density of one watt / cm2 beam entering the eye can be focused to a point with power density increasing to 100,000 watts / cm2. This power density is more than enough to cause a severe retinal burn and loss of vision.


  62. Why is viewing even the diffused reflection of the pulsed solid-state laser from a roughened surface is usually considered hazardous?
  63. Though diffused reflection is not focused on the retina, it forms an image and it is likely that the damage threshold may exceed the limit of accepted value due to the high peak power associated with the solid-state lasers.


  64. Taking eye as an example, discuss why only particular parts of eye are more susceptible to damage by particular laser radiation and not others.
  65. Only those parts of the eye that absorb radiation of a particular spectral region are susceptible to damage. It will not be damaged by radiation of a particular wavelength, if it is not absorbed by it.. For example, lasers in the visible and infrared region are transmitted by the lens and therefore the lens is not damaged. As these radiations reach the retina, it absorbs the same and consequently gets damaged. Similarly, cornea absorbs far infrared and short wavelength lasers and thus gets affected only by lasers emitting these radiations


  66. Which is the most powerful laser in the world?
  67. The Nova facility in the USA, now called the NIF (National Ignition Facility) is certainly the most powerful, among the pulsed lasers. However, in case of CW lasers, the military lasers such as HF and DF lasers, which are in the megawatt range, are most powerful.


  68. Why rare earth ions are generally doped as active ions in laser materials?
  69. In most cases, the rare-earth ions replace other ions of similar size and same valence (charge state) in the host medium; for example, a Nd3+ ion in Nd:YAG (yttrium aluminum garnet) substitutes an yttrium (Y3+) ion. The concentration of laser-active rare-earth dopants in the host medium is in most cases only a small molar percentage. A characteristic property of the trivalent rare-earth ions is that their electronic transitions usually occur within the 4f shell, which is somewhat shielded from the host lattice by the optically passive outer electronic shells. This reduces the influence of the host lattice on the wavelengths, bandwidths and cross sections of the relevant optical transitions.


  70. What are the main lasers being used for medical applications?
  71. LASIK: Laser Assisted in situ Keratomileusis
    • UV 193 nm Excimer laser
    PRK: Photorefractive Keratectomy
    • UV 193 nm Excimer laser
    Laser Surgery: Revisualization: For end stage coronary bypass surgery and angioplasty
    • Holmium YAG laser 2.1 micron
    Hair Removal
    • 700 - 1000 nm
    Skin resurfacing
    • 3 - 10 micron
    Vascular and Pigment conditions
    • 532 - 600 nm


  72. Mention the wavelengths of few important Lasers?
  73. Laser Wavelength
    Excimer - Argon Fluoride (ArF) 0.193 micron
    Excimer - Xenon Chloride (XeCl) 0.308 micron
    Excimer - Xenon Fluoride (XeF) 0.351 micron
    Helium Neon (He-Ne) 0. 6328 micron
    Chromium: Aluminum oxide (Cr: Al2O3) - (Ruby) 0.6943 micron
    Neodymium: Yttrium-Aluminum-Garnet (Nd: YAG) 1.064 micron
    Neodymium: Glass (Nd: Glass) 1.054 micron
    Erbium: Glass (Er: Glass) 1.54 micron
    Erbium Yttrium-Aluminum-Garnet (Er:YAG) 2.94 micron
    Holmium: Yttrium-Aluminum-Garnet (Ho:YAG) 2.1 micron
    Chromium: chrysoberyl (Cr+3:BeAl2O4) - Alexandrite 0.700 to 0.820 micron
    Titanium: sapphire (Ti3+:Al2O3) Ti: Sapphire 0.660 to 1.1 micron
    Carbon dioxide (CO2) 10.6 micron
    Gallium Arsenide (GaAs) 0. 840 micron
    Gallium Aluminum Arsenide (GaAlAs) 0.670 - 0.830 micron



  74. What are Eye - safe Lasers?
  75. For a given power levels, Lasers emitting in a wavelength region with relatively low hazards for the human eye are known as eye-safe lasers. Lasers with emission wavelengths longer than 1.4 μm usually fall in this category of "eye-safe", because light in that wavelength range is strongly absorbed in the eye's lens and thus cannot reach the significantly more sensitive retina. Wavelengths between 400 nm and 1400 nm are focused by the curved cornea and lens on to the retina; the optical gain is about 100,000-200,000 times. Viewing a laser beam or Point Source will focus all the light on a very small area of the retina, resulting in a greatly increased power density and an increased chance of damage.
    Threshold energy density for retinal damage at 1064 nm (Nd:YAG) is 10-6 J/Cm2 and at 694.3 nm (Ruby) it is 10-7 J/Cm2. For laser wavelengths above 1400 nm, damage to the retina occurs at very high energy density, since the transmission of the eye is negligible as shown in the adjoining figure. For example, Er:Glass laser emits radiation at 1540 nm and threshold density of retinal damage is 1 J/Cm2. Such high energy density is not normally encountered at work place and these types of lasers are eye safe.


  76. What are different classes of Lasers?
  77. Class I Lasers
    These are lasers that are not hazardous for continuous viewing or are designed in such a way that prevent human access to laser radiation. These consist of low power lasers or higher power embedded lasers. (i.e. laser printers)
    Class 2 Visible Lasers (400 to 700 nm)
    Lasers emitting visible light of low power normally do not present a hazard because of normal human aversion responses. However may be hazardous, if viewed directly for extended periods of time like many conventional light sources.
    Class 2A Visible Lasers (400 to 700 nm)
    Lasers emitting visible light not intended for viewing, and under normal operating conditions would not produce an injury to the eye if viewed directly for less than 1000 seconds. (i.e. bar code scanners)
    Class 3a
    Lasers in this category are normally those, which would not cause injury to the eye if viewed momentarily but would present a hazard if viewed using collecting optics like telescope.
    Class 3b
    Lasers in this category are those, which present an eye and skin hazard if viewed directly. Class 3b lasers do not produce a hazardous diffuse reflection except when viewed at close proximity.
    Class 4 Lasers
    Lasers in this category present an eye hazard from direct, specular and diffuse reflections. In addition such lasers may be fire hazards and produce skin burns.


Google
 


Updated: 6 April, 2015
Visitors: