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Theoretical Background of Helium Neon Lasers

Theoretical Background of Helium Neon Lasers. William “Buck” Scougale. [1] W. R. Scougale, ‘Interferometer Operations Manual’, unpublished. Background. Michelson Experiment: measure wavelength of light. Angles of reflection created/exaggerated to illustrate light path more clearly.

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Theoretical Background of Helium Neon Lasers

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  1. Theoretical Background of Helium Neon Lasers William “Buck” Scougale [1] W. R. Scougale, ‘Interferometer Operations Manual’, unpublished

  2. Background • Michelson Experiment: measure wavelength of light. • Angles of reflection created/exaggerated to illustrate light path more clearly. • Path Difference D can be calculated! … Just not as simply as one might think from the diagram. • Need to understand the light from the laser to use properly. [1]

  3. Background: Lasers • What is it? LASER (acronym) • Light • Amplification by • Stimulated • Emission of • Radiation • Anatomy • HeNe discharge tube. (pump) • ~ 90% Helium, 10% Neon • Mirror Cavity • Reflectivity • What Do they Do? • Provide optical amplification • Produces ‘Coherent’ light [2] • Temporal Coherence (time) • Spatial Coherence (space) [2] A.T. Forrester, Am. J. Phys. 24, 192, August 1955, p. (1), [3] Young & Freedman, ‘Sears and Zemansky’s University Physics with Modern physics 13th Ed.’, 2004, p. 1309

  4. Function: Basic Principles 2) 1) • Energy Absorption & Emission • Einstein Coefficients Relate Rate Processes[4] • Spontaneous Absorption • B is Probability of Induced Absorption (fig 1.) • Spontaneous Emission • A is Probability of Spontaneous Emission (fig 2.) • Induced Emission • B’ is Probability of Induced Emission (fig 3.) • B = B’ is crucial for laser function. For a system subjected to Thermal Radiation: B is a B’ = B 3) [4] D. V. Schoeder, ‘Thermal Physics’, 2000, p. 293-294

  5. Function: Basic Principles • 4 State System (in order of occupation). • E0: Electron in Ground State • T01: Pumping Transition (IE discharge tube) • E1: lifetime ~10^(-8) s • T12: Transition to Stable state. • E2: lifetime ~10^(-3) s • T23: Stimulated Lasing transition • E3: lifetime ~10^(-8) s • T34: Transition to maintain Population Inversion. (?) • E0: Electron in Ground State [3]

  6. Function: Basic Principles • 4 State System • T01: Pumping Transition (b) • T12: Transition to meta-stable state (c) [3]

  7. Function: Basic Principles • 4 State System • Einstein Coefficients Require “Population Inversion”. • IE: for lazing to occur: N2 >> N3 • Remember that B = B’ in probability, so the rates of transition are the same. [5] Melissinos & Napolitano, ‘Experiments in Modern Physics: Second Edition’, 2003, p. 160

  8. Function: Beam Properties Cavity Length= L • Beam Properties Determined by the Cavity. • ‘n’ is mode number. n= 1,2,3,… • ; • ; • Only accounts for mirror losses! • R = 99%, • R = 95%,

  9. Function: Beam Properties • Beam Properties Determined by the Gain Medium. • ; h is Planck’s const. • Pumping Produces Signal Noise • Width from Doppler Broadening • Energy Conservation [6] • Doesn’t account for vector nature of velocities. • Phaser Approach [6] • Gaussian profile • From Boltzmann’s Velocity Distribution. ( Speed Dist.) Remember ~90:10 He:Ne mixture. He Ne He Ne Ne Ne He He [6] S. F. Jacobs, Am. J. Phys. 47(7), July 1979

  10. Function: Beam Properties • Beam Properties Determined by the Gain Medium. • ; h is Planck’s const. • Pumping Produces Signal Noise • Width from Doppler Broadening • Energy Conservation [4] • Doesn’t account for vector nature of velocities. • Phaser Approach [4] • Gain: Gaussian profile • From Boltzmann’s Velocity Distribution. ( Speed Dist.) • Beam: Cutoff Gaussian profile

  11. Safety: HeNe Laser • Safety Notes • No glasses, (so we can see the beam) • Block all stray beams. • Spot beams with index cards, not your body. • What we have in the lab. • Uniphase Model 1125p • HeNe 10mW • Class IIIb • Lumentum Model 1508-1 • HeNe <4mW • Class IIIa [5] FDA, ‘Radiation Emitting Products’, updated 10/12/2018: https://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProductsandProcedures/HomeBusinessandEntertainment/LaserProductsandInstruments/default.htm

  12. Conclusion • References: [1] W. R. Scougale, ‘Interferometer Operations Manual’, unpublished [2] A.T. Forrester, Am. J. Phys. 24, 192, August 1955, p. 1 [3] Young & Freedman, ‘Sears and Zemansky’s University Physics with Modern physics 13th Ed.’, 2004, p. 1309 [4] D. V. Schoeder, ‘Thermal Physics’, 2000, p. 293-294 [5] Melissinos & Napolitano, ‘Experiments in Modern Physics: Second Edition’, 2003, p. 160 [6] S. F. Jacobs, Am. J. Phys. 47(7), July 1979 • Special thanks: • Dr. Rudi Michalak for reviewing my presentation. • Questions?

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