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Applications of LASERs

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

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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 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. 1. General lasers • coherent • monochromatic • Interferometry • Holography Applications of lasers

  4. 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 tcoh. (or length lcoh.) tcoh. (orlcoh.)

  5. 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 lcoh. 2L1-2L2 ~ lcoh.

  6. 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...

  7. 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

  8. 2. High power lasers • material processing • medical applications • nuclear fusion • high CW power • high pulsed powers Applications of lasers

  9. 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

  10. 1kW Nd:YAG cutting metal sheet

  11. 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.

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

  13. 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

  14. What is “ultrashort”? One month Computer clock cycle Human existence Camera flash Age of pyramids 10 fs light pulse Age of universe 1 minute Very short pulses! Very high powers! -14 -9 -4 1 6 11 16 10 10 10 10 10 10 10 Kilo (k) 10+3 Milli (m) 10-3 Mega (M) 10+6 Time (seconds) Micro (µ) 10-6 Giga (G) 10+9 Nano (n) 10-9 Tera (T) 10+12 Pico (p) 10-12 Peta (P) 10+15 Femto (f) 10-15 Atto (a) 10-18

  15. Current record: 4.0 fsec Baltuska, et al. 2001 Active mode locking Passive mode locking Colliding pulse mode locking Intra-cavity pulse compression Extra-cavity pulse compression Mode-locked Ultrafast Lasers A 4.5-fs pulse… 1000 Shortest Pulse Duration (femtoseconds) 100 10 '65 '70 '75 '80 '85 '90 '95 Year Ultrafast Ti:sapphire laser Reports of attosec pulses, too!

  16. –6 10 –9 10 –12 10 –15 10 Ultrafast Optics vs. Electronics Electronics Speed (seconds) Optics 1960 1970 1980 1990 2000 Year No one expects electronics to ever catch up.

  17. Ultrafast Laser Spectroscopy: Why? Most events that occur in atoms and molecules occur on fs and ps time scales. The length scales are very small, so very little time is required for the relevant motion. Fluorescence occurs on a ns time scale, but competing non-radiative processes only speed things up because relaxation rates add: 1/tex = 1/tfl + 1/tnr Biologically important processes utilize excitation energy for purposes other than fluorescence and hence must be very fast. Collisions in room-temperature liquids occur on a few-fs time scale, so nearly all processes in liquids are ultrafast. Semiconductor processes of technological interest are necessarily ultrafast or we wouldn’t be interested.

  18. Ultrafast Spectroscopy of Photosynthesis The initial events in photosynthesis occur on a ps time scale. Arizona State University

  19. The 1999 Nobel Prize in Chemistry went to Professor Ahmed Zewail of Cal Tech for ultrafast spectroscopy. Zewail used ultrafast-laser techniques to study how atoms in a molecule move during chemical reactions.

  20. Selective photochemistry Gustav Gerber • A chemists dream: control of chemical reaction pathway by selective optical excitation of chemical bond The difficulty with using CW light or long pulses is intramolecular vibrational redistribution: excite one bond, and a few fs later, the whole molecule is vibrating and the weakest bond breaks.

  21. Gustav Gerber Coherent control with shaped fs pulses • SOLUTION: • (1) Use fs pulse to break bond before IVR occurs • (2) shape the pulse to optimise the desired yield • Termed “coherent control” of chemical reactions

  22. Pulse shaping in time and frequency domains • Intensity and phase of an optical pulse may be specified in either the time or frequency domain: • Similarly, modulation can be performed in time or frequency domain: • easy! • difficult - modulators too slow!

  23. The Fourier-Synthesis Pulse-shaper Amplitude mask Transmission = T(x)= T(l) Phase mask Phase delay = j(x)= j(l) grating grating f f f f f f Fourier Transform Plane

  24. Micromachining with CW lasers • Laser ablation with CW and long pulse (ns) : • High average power • Dominant process: thermal • material heated and vaporised • expansion and expulsion of target material • Possible problems • crater formation • heat affected zone (HAZ) • surface contamination (dross) • shock wave damage to underlying material • limiting precision / resolution • collateral damage • absorption within illuminated region • poor vertical control

  25. Femtosecond pulses in micromachining • Ultrashort high peak intensity (ps or fs) pulses: • High peak power, low mean power • Dominant process: creation of plasma • direct and rapid generation by multi-photon ionisation • incident energy absorbed in plasma • negligible cratering, HAZ, shock-wave damage or dross • strong NL effects only at focus -> sub-surface machining Extreme conditions* at focus of ultrashort pulse: 1µJ pulse focussed to (1 µm)3 gives: T~1MK p~10Mbar *Eric Mazur, Harvard University

  26. Femtosecond vs. picosecond laser ablation • ablation with fs pulses appears to be more deterministic • due to (?) statistics of photoionisation (by light field or by multi-photon absorption) and subsequent avalanche ionisation

  27. Applications of femtosecond micromachining http://tops.phys.strath.ac.uk/machining.htm • high-precision ablation • encoding information on micron scale • engineering dielectrics for e.g. optical waveguides • surgery...

  28. Surgery with femtosecond laser pulses - 1 http://lasers.llnl.gov/mtp/ultra.html • small, high precision cuts without kerf • no thermal or mechanical damage to surrounding areas • i.e. no burning or coagulation • sub-surface surgery pig myocardium drilled by excimer laser, illustrating extensive thermal damage surrounding the hole. pig myocardium drilled by an USPL showing a smooth-sided hole free of thermal damage to surrounding tissue.

  29. Surgery with femtosecond laser pulses - 2 http://lasers.llnl.gov/mtp/ultra.html thermal damage and cracking to tooth enamel caused by 1-ns laser ablation. smooth hole with no thermal damage after drilling with a USPL.

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

  31. Femtosecond laser surgery of cornea Lenticle removal using Femtosecond LASIK

  32. (Biomedical) imaging using ultrashort laser pulses • Problems with conventional microscopy • transparent objects require staining (toxic, fading) • 3D imaging by sectioning • internal structures (e.g. retina) not always accessible • opaque objects cannot be viewed in transmission • low contrast due to background transmission • Ultrashort pulse imaging methods address some of these problems : • Multi-photon imaging • ballistic photon imaging • optical coherence tomography • T-rays

  33. t t Nonlinear microscopy for 3D imaging filter z femtosecond pulse detection of nonlinear signal region of NL interaction Linear processes do not favour the focus signal~intensity x area~z-2 x z2 ~constant Nonlinear (‘multi-photon’) processes favour the focus signal~(intensity)2 x area~z-4 x z2 ~ z-2 (2-photon) signal~(intensity)3 x area~z-6 x z2 ~ z-4 (3-photon) Two photon fluorescence Three photon fluorescence Third harmonic generation

  34. Two-Photon Fluorescence* Imaging *requires fluorescent dye Pollen grain (Clivia Miniata) Conventional image (using fluorescence) ~14 µm 46 sections separated by 0.5 µm in the axial dimension. 2 seconds/image 1.5 µm axial resolution 200 mW in 16 beamlets

  35. Imaging by Third Harmonic Generation (THG) 125 µm • THG occurs at focus of intense ultrashort pulse • Uniform material: • THG light from either side of focus interferes destructively • Discontinous material: • allows some constructive interference and THG emission. • THG imaging depends on Dc(3) • THG is sensitive to interfaces Demonstration using an optical fiber in index-matching fluid (~100 fs pulses at 1.2 µm, 1 kHz repetition rate.) Barad et al, Appl. Phys. Lett. 70, 922 (1997)

  36. Sectional THG images of spiral algae formation Squier et al, Optics Express 3, p. 315 (1998)

  37. More Real-Time THG Images Artificial blood vessel (two cover slips) with real red blood cells flowing in it. Scanning scheme used a Lissajou pattern.

  38. Time-resolved imaging for opaque media scattering medium diffusive photons (late arrival): large lateral scattering, high intensity) ‘snake’ photons ‘ballistic’ photons (early arrival): small lateral scattering, low intensity • Scattering is a major problem in e.g. mammography • The problem is weak signals: • mean free path for photons = Ls ~ 0.5 mm for breast tissue • sample length = L=25mm • fraction of ballistic photons is exp(–L / Ls) = exp(–50) = 10–22 • but … • for a pulsed laser with 1 Watt average power, there are only • 1019 photons per second ...

  39. Optical Coherence Ranging and Tomography • cross-sectional micron-scale imaging • real-time, in-situ, in-vivo • optical fibre coupling for internal organs • commercial device available for ophthalmologists This work has been pioneered by Jim Fujimoto and coworkers of MIT. Huang, et al., Science, 254 (1991)

  40. OCT can see otherwise invisible micro-tears in the retina Photographs can’t see the tears

  41. Inside a blood vessel (in vitro) OCT IVUS The OCT images have significantly higher resolution than intravascular ultrasound (IVUS). Brezinski, et al., Am. J. Cardiology 77 (1996)

  42. THz imaging for biomedical applications • fills “THz gap” between microwave and optical frequencies • mixed time / frequency domain spectroscopy • chemical fingerprints at THz frequencies • (e.g. rotational transitions) • strong sensitivity to water content … • coherent method (like OCT) • imaging on 100 micron scale • many variation of imaging method: • intensity • time-of-flight • absorption at key frequencies (f1) • relative absorption (f1/f2)

  43. THz imaging of biomedical samples Centre of Medical Imaging Research University of Leeds TeraVision project (EU-IST)

  44. Surrey Femtosecond high-power broadband source Principles: System:

  45. High rep rate near-infrared system (Spectra) • high rep rate (80MHz) for good signal-to-noise • workhorse system for communications wavelengths • <200fs pulses over range 350 - 1600 nm

  46. Dual colour / mid-infrared system (Coherent) • Ti-sapphire oscillator and regenerative amplifier • high pulse energies for THz beam generation, material processing, and upconversion of weak luminesence • dual parametric amplifiers for non-degenerate pump-probe, and difference frequency generator for mid-infrared • wavelength range 550nm to >10m • ultrashort pulse version: < 60fs pulses

  47. Broadband sources for spectroscopy UV visible NIR MIR FIR mmW RF Ti-S THG Ti-S SHG Ti-S laser OPA SFM DFM HG-OPA THz Ultrafast electronics FEL

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