Applications of LASERs

Jeremy Allam

Optoelectronic 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 lasers

1. 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

1

DnL 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

8pWNA

l 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)

photograph

Photography - 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 pathways

Spectral-domain pulse shaping:

ener

gy

distance

Coherently-controlled multi-photon ionisation:

Imaging using femtosecond light pulses

Nonlinear imaging for 3D sectioning(e.g. TPA fluorescence)

scattering medium

ballistic photons‘snake’ photons

diffusive photons

time

ear

ly

pho

tons

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.html

Ultra 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.html

Ultra 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