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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
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

slide2

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

slide3

1. General lasers

• coherent

• monochromatic

  • Interferometry
  • Holography

Applications of lasers

longitudinal coherence of laser light
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.)

measuring longitudinal coherence
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.

applications of interferometers
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...

holography
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

slide8

2. High power lasers

• material processing

• medical applications

• nuclear fusion

  • high CW power
  • high pulsed powers

Applications of lasers

slide9

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
slide11

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.

slide12

Nuclear Fusion: National Ignition Facility

http://www.llnl.gov/str/Powell.html

slide13

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

what is ultrashort
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

slide15

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!

slide16

–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.

ultrafast laser spectroscopy why
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.

ultrafast spectroscopy of photosynthesis
Ultrafast Spectroscopy of Photosynthesis

The initial events in photosynthesis occur on a ps time scale.

Arizona State University

slide20

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.

selective photochemistry
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.

slide22

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
pulse shaping in time and frequency domains
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!
the fourier synthesis pulse shaper
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

slide27

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
slide28

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

slide29

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
slide30

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...
slide31

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.

slide32

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.

slide33

Femtosecond laser surgery of cornea - 1

Femtosecond

LASIK

Femtosecond

interstroma

slide34

Femtosecond laser surgery of cornea

Lenticle removal using Femtosecond LASIK

biomedical imaging using ultrashort laser pulses
(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
slide36

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

two photon fluorescence imaging
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

imaging by third harmonic generation thg
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)

sectional thg images of spiral algae formation
Sectional THG images of spiral algae formation

Squier et al, Optics Express 3, p. 315 (1998)

more real time thg images
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.

slide41

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 ...
optical coherence ranging and tomography
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)

inside a blood vessel in vitro
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)

thz imaging for biomedical applications
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)
thz imaging of biomedical samples
THz imaging of biomedical samples

Centre of Medical Imaging Research University of Leeds

TeraVision project (EU-IST)

slide48

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
slide49

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
broadband sources for spectroscopy
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