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Applications of LASERs

Applications of LASERs. University of Surrey School of Physics and Chemistry Guildford, Surrey GU2 7XH, UK. 3MOLS 23/11/01. Jeremy Allam Optoelectronic Devices and Materials Research Group Tel +44 (0)1483 876799 Fax +44 (0)1483 876781. 1. General lasers. • coherent • monochromatic.

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Applications of LASERs

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  1. Applications of LASERs University of Surrey School of Physics and Chemistry Guildford, Surrey GU2 7XH, UK 3MOLS 23/11/01 Jeremy Allam Optoelectronic Devices and Materials Research Group Tel +44 (0)1483 876799 Fax +44 (0)1483 876781

  2. 1. General lasers • coherent • monochromatic • Interferometry • Holography 2. High power lasers • material processing • medical applications • nuclear fusion • high CW power • high pulsed powers 3. ‘Ultrafast’ lasers • short pulses (<5fs) • broadband gain(>300nm) • high peak powers (>TW) • dynamics of physical, chemical, biological processes • spectroscopy, pulse shaping • high energy processes, wavelength conversion Applications of lasers

  3. Longitudinal Coherence of Laser Light phase noise or drift (spontaneous emission, temperature drift, microphonics, etc) leads to finite spectral width phasor at t=0 phasor at t=t1 leads to finite coherence time tc (or length lc) tc (orlc)

  4. Measuring Longitudinal Coherence M1 optical fibre M2 L1 BS M1 BS L1 detector M2 L2 detector use interferometer e.g. Michelson interferometer for long coherence lengths, use optical fibre delay D(path length) = 2L1-2L2 << coherence length lc 2L1-2L2 ~ lc

  5. Applications of interferometers Measurement of length: {see Smith and King ch. 11} LINEAR TRANSLATION: interferometric translation stage FLATNESS/UNIFORMITY: e.g. Twyman-Green interferometer LINEAR VELOCITY OF LIGHT: famous Michelson-Morley experiment c is independent of motion of reference frame DETECTING GRAVITATIONAL WAVES: minute movement of end mirrors ROTATION (e.g. of earth): Sagnac interferometer as an optical gyroscope: For N loops of area A and rotation rate W, phase difference is: Measurement of optical properties: REFRACTIVE INDEX: Rayleigh refractometer LIGHT SCATTERING: heterodyne spectrometry ULTRAFAST DYNAMICS: pump-probe / coherent spectroscopy Numerous other applications...

  6. Holography photograph illuminating beam 2D representation of image (no depth) object photographic plate eye reference beam beam expander illuminating beam reconstruction beam LASER hologram BS object reconstructed image Hologram (photographic plate) diffracted reference beam eye {see Smith and King ch. 19} RECORDING READING / RECONSTRUCTING Photography - record electric field intensity of light scattered by object Holography - record electric field intensity and phase

  7. Laser fabrication of Be components http://www-cms.llnl.gov/wfo/laserfab_folder/index.html • a high-speed, low-cost method of cutting beryllium materials • No dust problem (Be dust is poisonous) • autogenous welding is possible • Achieved using a 400-W pulsed Nd-YAG laser and a 1000-W CW CO2 laser • Narrow cut width yields less Be waste for disposal • No machining damage • Laser cutting is easily and precisely controlled by computer

  8. 1kW Nd:YAG cutting metal sheet

  9. Laser Tissue Welding Photograph of the laser delivery handpiece with a hollow fiber for sensing temperature. The surgeon is repairing a 1 cm-long arteriotomy. http://lasers.llnl.gov/mtp/tissue.html Laser tissue welding uses laser energy to activate photothermal bonds and/or photochemical bonds. Lasers are used because they provide the ability to accurately control the volume of tissue that is exposed to the activating energy.

  10. Nuclear Fusion: National Ignition Facility http://www.llnl.gov/str/Powell.html

  11. Why femtosecond lasers? (Titanium-sapphire properties) • timing physical processes • time-of-flight resolution ultrashort pulses (5fs) THz pulse generation 1 broadband gain (700-1000nm) • pulse shaping • coherent control 2 generate: • UV • X-rays, • relativistic electrons high power (TW) parametric conversion 3

  12. Coherent control of chemical pathways Spectral-domain pulse shaping: Coherently-controlled multi-photon ionisation:

  13. Imaging using femtosecond light pulses Nonlinear imaging for 3D sectioning (e.g. TPA fluorescence) femtosecond pulse detection region of TPA Time-resolved imaging for scattering media scattering medium diffusive photons early photons ‘snake’ photons time ballistic photons

  14. Why femtosecond lasers in biology and medicine? Conventional laser applications Benefits by using femtosecond lasers • more controllable • less damage ablation • wide spectral range • coherent control spectroscopy • nonlinear imaging (e.g. TPA, THG) ->3D optical sectioning -> contrast in transparent samples • time-of-flight resolution: early photons in diffusive media • THz imaging imaging

  15. Ablation with femtosecond lasers Conventional lasers (high average power) Femtosecond lasers (high peak, low av. power) • dominated by thermal processes (burning, coagulation), and acoustic damage • collateral damage (cut cauterised) • absorption within illuminated region • stochastic -> uncontrolled ablation • dominated by non-thermal processes (‘photodisruption’) • little collateral damage (cut bleeds) • strong NL effects only at focus (-> sub-surface surgery) • deterministic -> predictable ablation * due to dynamics of photoionisation (by light field or by multi-photon absorption) and subsequent avalanche ionisation

  16. Femtosecond vs. picosecond laser ablation deterministic -> predictable ablation stochastic -> uncontrolled ablation

  17. Ultra Short Pulse Laser for Medical Applications -1 http://lasers.llnl.gov/mtp/ultra.html Using ultra-short duration bursts of laser energy, surface material is removed without any significant transfer of energy to the surrounding areas. For laser pulses less than about 10 ps (1/100th of a billionth of a second), we can cut without collateral damage to surrounding tissues. Tiny cuts with amazingly small kerf (>100 um) are produced, without thermal or mechanical damage to surrounding areas. Histological section of a pig myocardium drilled by an excimer laser, illustrating extensive thermal damage surrounding the hole. Histological section of a pig myocardium drilled by an USPL showing a smooth-sided hole free of thermal damage to surrounding tissue.

  18. Ultra Short Pulse Laser for Medical Applications -2 http://lasers.llnl.gov/mtp/ultra.html Extensive thermal damage and cracking to tooth enamel caused by 1-ns laser ablation. Smooth hole with no thermal damage after drilling with a USPL.

  19. Femtosecond laser surgery of cornea - 1 Femtosecond LASIK Femtosecond interstroma

  20. Femtosecond laser surgery of cornea - 2 Lenticle removal using Femtosecond LASIK

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