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Laser basics

Laser basics. Optics, Eugene Hecht, Chpt. 13; Optical resonator tutorial: http://www.dewtronics.com/tutorials/lasers/leot/. Ruby laser example. Laser oscillation. Laser is oscillator Like servo with positive feedback Greater than unity gain. Laser gain and losses.

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Laser basics

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  1. Laser basics Optics, Eugene Hecht, Chpt. 13; Optical resonator tutorial: http://www.dewtronics.com/tutorials/lasers/leot/

  2. Ruby laser example Laser oscillation Laser is oscillator • Like servo with positive feedback • Greater than unity gain Laser gain and losses Laser turn-on and gain saturation • Gain decreases as output power increases • Saturation

  3. Fabry-Perot cavity for feedback • High reflectivity mirrors • Low loss per round trip • Must remember resonance conditions • round trip path is multiple of l

  4. Classical mechanics analog Laser longitudinal modes • High reflectivity Fabry-Perot cavity • Boundary conditions • field is zero on mirrors • Multiple wavelengths possible • agrees with resonance conditions Fabry-Perot boundary conditions Multi-mode laser Multiple resonant frequencies

  5. Single longitudinal mode lasers • Insert etalon into cavity • Use low reflectivity etalon • low loss

  6. Laser transverse modes Transverse laser modes • Wave equation looks like harmonic oscillator • Ex: E = E e -iwt • Separate out z dependence • Solutions for x and y are Hermite polynomials Frequencies of transverse modes

  7. Single transverse mode lasers • Put aperture in laser • Create loss for higher order modes Multi-longitudinal Multi-transverse&long. Single mode

  8. Gaussian beams • Zero order mode is Gaussian • Intensity profile: • beam waist: w0 • confocal parameter: z • far from waist • divergence angle Gaussian propagation

  9. Power distribution in Gaussian • Intensity distribution: • Experimentally to measure full width at half maximum (FWHM) diameter • Relation is dFWHM = w2 ln2 ~ 1.4 w • Define average intensity • Iavg = 4 P / (p d2FWHM) • Overestimates peak: I0 = Iavg/1.4

  10. Resonator options Special cases • Best known -- planar, concentric, confocal • Confocal unique • mirror alignment not critical • position is critical • transverse mode frequencies identical Types of resonators

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