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Fundamentals of Laser Operation

Fundamentals of Laser Operation. Laser Fundamentals. Monochromatic intensity Directional coherent These properties of laser light are what can make it more hazardous than ordinary light. Laser light can deposit a lot of energy within a small area. N 12 α N 1 Q N 12 = B 12 N 1 Q.

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Fundamentals of Laser Operation

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  1. Fundamentals of Laser Operation

  2. Laser Fundamentals • Monochromatic • intensity • Directional • coherent • These properties of laser light are what can make it more hazardous than ordinary light. Laser light can deposit a lot of energy within a small area.

  3. N12α N1Q N12 = B12N1Q

  4. N21α N2 N21 = A21N2 N21α N2Q N21 = B21N2Q

  5. under thermal equilibrium The rate of absorption = The rate of emission B12N1Q = A21N2 + B21N2Q Q [B12N1 - B21N2] = A21N2 Divide by N2

  6. From MB distribution N1 = N0 e – E1/kT N2 = N0 e – E2/kT where k – Boltzmann constant T – absolute temperature N0 – number of atoms at absolute zero At equilibrium, we can write the ratio of population levels as follows, N1/N2 = e (E2 – E1) / kT Since E2 – E1 = hν N1/N2 = e (hν) / kT

  7. A and B are called Einstein’s coefficients, which accounts for spontaneous and stimulated emission probabilities.

  8. Conclusions of Einstein’s theory • Population inversion should be achieved to get laser action • In general the PI can be achieved only at – ve temperatures • To get the PI at +ve temperature the pumping process is needed • To get the laser action we need minimum of three levels (the third level is known as metastable state) • In UV range the laser is not possible

  9. N1/N2 = e (E2 – E1) / kT case – i N1 = N2 e (E2 – E1) / kT = N2 e+ve = 5.e+2 = 36.9 i.e. N1>N2 case – ii N1 = N2 e (E2 – E1) / kT = N2 e-ve = 5.e+2 = 0.6766 i.e. N2 > N1 This shows that N2 > N1 is possible at –ve temperatures only, but it is practically not possible. So this is achieved by some artificial process known as pumping process

  10. Methods of pumping • Optical pumping • Direct electron excitation • Inelastic collision • Chemical process • Direct conversion

  11. Common Components of all Lasers • Active Medium • The active medium may be solid crystals such as ruby or Nd:YAG, liquid dyes, gases like CO2 or Helium/Neon, or semiconductors such as GaAs. Active mediums contain atoms whose electrons may be excited to a metastable energy level by an energy source. • Pumping Mechanism • Excitation mechanisms pump energy into the active medium by one or more of three basic methods; optical, electrical or chemical. • High Reflectance Mirror • A mirror which reflects essentially 100% of the laser light. • Partially Transmissive Mirror • A mirror which reflects less than 100% of the laser light and transmits the remainder.

  12. Laser Components

  13. Stimulated Emission of Radiation Lasing Action Diagram Excited State Spontaneous Energy Emission Energy Introduction Metastable State Ground State

  14. WAVELENGTHS OF MOST COMMON LASERS Argon fluoride (Excimer-UV)Krypton chloride (Excimer-UV)Krypton fluoride (Excimer-UV)Xenon chloride (Excimer-UV)Xenon fluoride (Excimer-UV)Helium cadmium (UV)Nitrogen (UV)Helium cadmium (violet)Krypton (blue)Argon (blue)Copper vapor (green)Argon (green)Krypton (green)Frequency doubled      Nd YAG (green)Helium neon (green)Krypton (yellow)Copper vapor (yellow) 0.1930.2220.2480.3080.3510.3250.3370.4410.4760.4880.5100.5140.5280.5320.5430.5680.570 Helium neon (yellow)Helium neon (orange)Gold vapor (red)Helium neon (red)Krypton (red)Rohodamine 6G dye (tunable)Ruby (CrAlO3) (red)Gallium arsenide (diode-NIR)Nd:YAG (NIR)Helium neon (NIR)Erbium (NIR)Helium neon (NIR)Hydrogen fluoride (NIR)Carbon dioxide (FIR)Carbon dioxide (FIR) 0.5940.6100.6270.6330.6470.570-0.6500.6940.8401.0641.15  1.5043.392.709.6   10.6    Key:      UV   =   ultraviolet (0.200-0.400 µm)              VIS   =   visible (0.400-0.700 µm)              NIR   =   near infrared (0.700-1.400 µm) Laser Type Wavelength (mm)

  15. Continuous Output (CW) Pulsed Output (P) Energy (Watts) Energy (Joules) Time Laser Output Time watt (W) - Unit of power or radiant flux (1 watt = 1 joule per second). Joule (J) - A unit of energy Energy (Q) The capacity for doing work. Energy content is commonly used to characterize the output from pulsed lasers and is generally expressed in Joules (J). Irradiance (E) - Power per unit area, expressed in watts per square centimeter.

  16. Laser Class • The following criteria are used to classify lasers: • Wavelength. If the laser is designed to emit multiple wavelengths the classification is based on the most hazardous wavelength. • For continuous wave (CW) or repetitively pulsed lasers the average poweroutput (Watts) and limiting exposure time inherent in the design are considered. • For pulsed lasers the total energy per pulse (Joule), pulse duration, pulse repetitionfrequency and emergent beam radiant exposure are considered.

  17. Laser Classifications • Class 1 denotes laser or laser systems that do not, under normal operating conditions, pose a hazard. • Class 2 denotes low-power visible lasers or laser system which, because of the normal human aversion response (i.e., blinking, eye movement, etc.), do not normally present a hazard, but may present some potential for hazard if viewed directly for extended periods of time (like many conventional light sources).

  18. Class 3 denotes lasers or laser systems that can produce a hazard it viewed directly. This includes intrabeam viewing of specular reflections. Normally, Class 3b lasers will not produce a hazardous diffuse reflection. • Class 4denotes lasers and laser systems that produce a hazard not only from direct or specular reflections, but may also produce significant skin hazards as well as fire hazards.

  19. Vibrational states of CO2 molecule

  20. Animation CO2

  21. Band gap – types Semiconductor laser – animation

  22. Free Electron Laser • FEL’s differ from conventional lasers in that they use the electron beam as the lasing medium rather than a gas or a solid. • FELs are usually based on the combination of a linear accelerator followed by a high-precision insertion device, which may also be placed in an optical cavity formed by mirrors. • The accelerated electrons in the insertion device bunch together. • Over the length of the insertion device, the electrons in the microbunches begin to oscillate in step (coherently), thereby giving rise to light with properties characteristic of conventional lasers. • Because the microbunches are so tiny, the light generated comes in ultrashort pulses that can be used for strobe-like investigations of extremely rapid processes. • Current FEL’s cover wavelengths from millimeter to visible and are nudging into the ultraviolet. • New facilities designed specifically to produce x rays are under construction.

  23. Electrons are released from the source at the lower left, and are accelerated in a linear accelerator (linac). After emerging from this linac, the electrons pass into a laser cavity. In this cavity electrons to oscillate and emit light.

  24. A high voltage is applied to the gas mixture, Argon and Fluorine, to induce an excited state. • The excited argon atom now appears as potassium, an alkali metal, and thus easily reacts with Fluorine, forming the Ar-F excited dimmer (excimer). • The electromagnetic force between them increases with decreasing distance. • The potential energy separating the two, likewise decreases, reaching a minimum at an inter-nuclear distance of about 2.3 Angstroms, in which metastable state occurs. • As an excimer is hit by another photon, it returns to the ground state, releasing UV photons, and it returns to the original elements.

  25. Population inversion is greatly facilitated with Argon and Fluorine gas because as the excimer returns to the ground state, the molecule regains the original property. • The repulsion is so great that it makes the ground state lifetime no more than a few femtoseconds. • Thus, population inversion is greatly facilitated even though the process of pushing the argon atoms requires a lot of energy.

  26. Applications Communication Materials processing Heat treatment Medicine Holography

  27. Binary data is written as dark or light "dots" in two dimensional pages, with the pages stacked one on top of the other within a photosensitive crystal, the stacking of pages creates the third dimension . Crystals of a chemical compound called Strontium-barium-niobate are used most often as the recording media because they combine high sensitivity with high speed. The electronic charge patterns created by the interference of two laser beams is used to create the holograms.

  28. In the most basic systems, light from a laser source passes through a beam splitter that divides the beam into a data beam and an interference beam. • The reference beam will eventually be used to create the interference pattern. • It is directed into a path that includes a polarization rotor and a page-addressing deflection system (Li, 1994). The data beam, on the other hand, passes into an optical system that expands into the surface of what is called a "page composer", which is implemented as a spatial-light-modulator. • Digital data is superimposed on the expanded beam using the spatial-light modulator. • The images appear as dark or light spots depending on the value of the digital data. From the page composer, the data beam is converted using Fourier-transform optics. • From there, it is focused on the crystallite structure that will hold the hologram.

  29. At this point the data beam and reference beam come together again, with the resulting interference pattern on photo refractive material. • This in turn, modifies the optical properties of the crystallite material with an electronic charge pattern. • Multiple holograms are stored in a single crystal by altering the angle at which the beams enter the crystal (Ajluni, 1994). • In the read cycle, instead, the data beam is turned off, allowing only the reference beam to focus on the crystal. • The reference beam's location is determined by the particular pages to read. • The beam illuminates the interference grating or patterns stored at this location, resulting in the reconstruction of original light-and-dark- spots pattern. • The pattern is read by a charge-coupled device that converts the dark and light spots back to digital electronic data (Shandle 1993).

  30. Laser Cooling and Trapping

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