Transcript laserapplications - University of Surrey

Applications of LASERs
3MOLS 23/11/01
University of Surrey
Jeremy Allam
School of Physics and
Chemistry
Optoelectronic Devices
and
Materials Research Group
Guildford, Surrey
GU2 7XH, UK
Tel +44 (0)1483 876799
Fax +44 (0)1483 876781
Applications of lasers
1. General lasers
• coherent
• monochromatic
2. High power lasers
• high CW power
• high pulsed powers
• Interferometry
• Holography
• material processing
• medical applications
• nuclear fusion
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
Longitudinal Coherence of Laser Light
leads to
phase noise or drift
(spontaneous emission,
temperature drift,
microphonics, etc)
finite spectral
width
L
phasor at t=0
phasor at t=t1
leads to
finite coherence time tc (or length lc)
1
tc 
lc  c t c
Dn L
tc (or lc)

Measuring Longitudinal Coherence
use interferometer e.g. Michelson interferometer
M1
M1
L1
BS
M2
for long coherence
lengths, use optical
fibre delay
L1
optical
BS fibre
L2
detector
detector
(path length) = 2L1-2L2 << coherence length lc
output
output
2L1-2L2 ~ lc
0
0
L
1
L
1
M2
Applications of interferometers
{see Smith and King ch. 11}
Measurement of length:
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:
fS
8pWNA

lc
Measurement of optical properties:
REFRACTIVE INDEX: Rayleigh refractometer
LIGHT SCATTERING: heterodyne spectrometry
ULTRAFAST DYNAMICS: pump-probe / coherent spectroscopy
Numerous other applications...
Holography
RECORDING
{see Smith and King ch. 19}
READING / RECONSTRUCTING
Photography - record electric field intensity of light scattered by object
illuminating
beam
photograph
object
2D representation of
image (no depth)
eye
photographic
plate
Holography - record electric field intensity and phase
reference beam
beam
expander
illuminating
beam
LASER
BS
Hologram
(photographic
plate)
object
reconstruction
beam
hologram
reconstructed
image
diffracted
reference beam
eye
Laser fabrication of Be components
http://www-cms.llnl.gov/wfo/laserfab_folder/index.html
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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
1kW Nd:YAG cutting metal sheet
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.
Nuclear Fusion: National Ignition Facility
http://www.llnl.gov/str/Powell.html
Why femtosecond lasers?
(Titanium-sapphire properties)
1
2
3
ultrashort
pulses
(5fs)
• timing physical
processes
• time-of-flight
resolution
broadband
gain
(700-1000nm)
high
power
(TW)
THz pulse
generation
• pulse shaping
• coherent control
generate:
• UV
• X-rays,
• relativistic
electrons
parametric
conversion
Coherent control of chemical pathways
Spectral-domain pulse shaping:
Coherently-controlled multi-photon ionisation:
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
‘snake’ photons
ballistic photons
time
diffusive photons
early
photons
scattering medium
Why femtosecond lasers in biology and medicine?
Conventional
laser
applications
Benefits by using
femtosecond
lasers
ablation
• more controllable
• less damage
spectroscopy
• wide spectral range
• coherent control
imaging
• 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 lasers
Conventional lasers
(high average power)
Femtosecond lasers
(high peak, low av. power)
• dominated by thermal
processes (burning,
coagulation), and
acoustic damage
• dominated by non-thermal processes
(‘photodisruption’)
• collateral damage
(cut cauterised)
• little collateral damage
(cut bleeds)
• absorption within
illuminated region
• strong NL effects only at focus
(-> sub-surface surgery)
• stochastic
-> uncontrolled ablation
• deterministic
-> predictable ablation
* due to dynamics of photoionisation (by light field or by multiphoton absorption) and subsequent avalanche ionisation
Femtosecond vs. picosecond laser ablation
deterministic -> predictable ablation
stochastic -> uncontrolled ablation
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.
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.
Femtosecond laser surgery of cornea - 1
Femtosecond
LASIK
Femtosecond
interstroma
Femtosecond laser surgery of cornea - 2
Lenticle removal using Femtosecond LASIK