Applications of LASERs
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Transcript of Applications of LASERs
Applications of LASERs
Jeremy AllamOptoelectronic Devices and Materials Research Group
Tel +44 (0)1483 876799Fax +44 (0)1483 876781
University of Surrey
School of Physics and Chemistry
Guildford, SurreyGU2 7XH, UK
3MOLS 23/11/01
Applications of lasers1. General lasers
• Interferometry• Holography
• coherent• monochromatic
• dynamics of physical, chemical, biological processes• spectroscopy, pulse shaping• high energy processes, wavelength conversion
• short pulses (<5fs)• broadband gain(>300nm)• high peak powers (>TW)
3. ‘Ultrafast’ lasers
• material processing• medical applications• nuclear fusion
2. High power lasers
• high CW power• high pulsed powers
Longitudinal Coherence of Laser Lightphase noise or drift(spontaneous emission, temperature drift, microphonics, etc)
leads to finite spectral width
phasor at t=0
phasor at t=t1
L
leads to finite coherence time c (or length lc)
t c
1DnL
lc c t c
c (or lc)
Measuring Longitudinal Coherenceuse interferometer e.g. Michelson interferometer
(path length) = 2L1-2L2 << coherence length lc
M1
M2
L2
L1 BS
detector
M1
M2L1 BS
detector
optical fibre
for long coherence lengths, use optical fibre delay
2L1-2L2 ~ lc
L1
outp
ut
0L
1
outp
ut0
LINEAR TRANSLATION: interferometric translation stageFLATNESS/UNIFORMITY: e.g. Twyman-Green interferometerLINEAR VELOCITY OF LIGHT: famous Michelson-Morley experiment
c is independent of motion of reference frameDETECTING GRAVITATIONAL WAVES: minute movement of end mirrorsROTATION (e.g. of earth): Sagnac interferometer as an optical gyroscope:
Applications of interferometersMeasurement of length:
Measurement of optical properties:REFRACTIVE INDEX: Rayleigh refractometerLIGHT SCATTERING: heterodyne spectrometryULTRAFAST DYNAMICS: pump-probe / coherent spectroscopy
{see Smith and King ch. 11}
Numerous other applications...
f S
8pWNAl c
For N loops of area A and rotation rate phase difference is:
Holography {see Smith and King ch. 19}
eye
reconstructed image
reconstruction beam
diffracted reference beam
hologramLASER
Hologram (photographic
plate)
reference beam
beam expander
BS object
illuminating beam
photographic plate
object
illuminating beam
eye
2D representation of image (no depth)
photographPhotography - record electric field intensity of light scattered by object
Holography - record electric field intensity and phase
RECORDING READING / RECONSTRUCTING
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
Laser fabrication of Be components
1kW Nd:YAG cutting metal sheet
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
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.
http://www.llnl.gov/str/Powell.html
Nuclear Fusion: National Ignition Facility
ultrashort pulses (5fs)
broadband gain(700-1000nm)
high power(TW)
THz pulsegeneration
• pulse shaping• coherent control
parametric conversion
Why femtosecond lasers?
• timing physical processes
• time-of-flight resolution
generate: • UV• X-rays,• relativistic
electrons
1
2
3
(Titanium-sapphire properties)
amplitude & phase LCD mask
in out
Coherent control of chemical pathwaysSpectral-domain pulse shaping:
ener
gy
distance
Coherently-controlled multi-photon ionisation:
Imaging using femtosecond light pulsesNonlinear imaging for 3D sectioning(e.g. TPA fluorescence)
scattering medium
ballistic photons‘snake’ photons
diffusive photons
time
early
ph
oton
s
Time-resolved imaging for scattering media
femtosecond pulse
detection
region of TPA
Why femtosecond lasers in biology and medicine?
Conventional laser applications
imaging
Benefits by using femtosecond lasers
• wide spectral range• coherent control
ablation • more controllable• less damage
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
Ablation with femtosecond lasersConventional lasers(high average power)
Femtosecond lasers(high peak, low av. power)
• dominated by thermal processes (burning, coagulation), andacoustic 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
Femtosecond vs. picosecond laser ablation
deterministic -> predictable ablation
stochastic -> uncontrolled ablation
Histological section of a pig myocardium drilled by an USPL showing a smooth-sided hole free of thermal damage to surrounding tissue.
Histological section of a pig myocardium drilled by an excimer laser, illustrating extensive thermal damage surrounding the hole.
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.
http://lasers.llnl.gov/mtp/ultra.htmlUltra Short Pulse Laser for Medical Applications -1
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.
http://lasers.llnl.gov/mtp/ultra.htmlUltra Short Pulse Laser for Medical Applications -2
Femtosecond interstroma
Femtosecond LASIK
Femtosecond laser surgery of cornea - 1
Femtosecond laser surgery of cornea - 2
Lenticle removal using Femtosecond LASIK