Applications of LASERs

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Applications of LASERs


Jeremy Allam


Optoelectronic Devices
and

Materials Research Group


Tel +44 (0)1483 876799

Fax +44 (0)1483 876781

University of Surrey



School of Physics and
Chemistry


Guildford, Surrey

GU2 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 Light

phase noise
or

drift

(spontaneous emission,
temperature drift,
microphonics, etc)

leads to

finite spectral
width




phasor at t=0

phasor at t=t
1




L
leads to
finite coherence time
t
c

(
or

length
l
c
)


t
c

1
D
n
L

l
c

c

t
c
t
c

(
or

l
c
)

Measuring Longitudinal Coherence

use

interferometer
e.g.

Michelson interferometer


(path length)
= 2L
1
-
2L
2
<<
coherence length
l
c

M
1

M
2

L
2

L
1

BS

detector



M
1

M
2

L
1

BS

detector

optical
fibre

for long coherence
lengths, use optical
fibre delay

2L
1
-
2L
2
~
l
c

L
1
output
0
L
1
output
0
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:

Applications of interferometers

Measurement of
length:

Measurement of
optical properties:

REFRACTIVE INDEX: Rayleigh refractometer

LIGHT SCATTERING: heterodyne spectrometry

ULTRAFAST DYNAMICS: pump
-
probe / coherent spectroscopy

{see Smith and King ch. 11}

Numerous other applications...


f
S

8
p
W
NA
l
c
For N loops of area A and rotation rate
W,
phase difference is:

Holography

{see Smith and King ch. 19}

eye

reconstructed
image

reconstruction
beam

diffracted
reference beam

hologram

LASER

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 CO
2

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 pulse

generation





• 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)

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)

scattering medium

ballistic photons

‘snake’ photons

diffusive photons

time

early

photons

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 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

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