Applications of LASERs Jeremy Allam Optoelectronic Devices and Materials Research Group Tel +44 (0) Fax +44 (0) 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 leads to finite coherence time  c (or length l c )  c (or l c )

Measuring Longitudinal Coherence use interferometer e.g. Michelson interferometer  (path length) = 2L 1 -2L 2 << coherence length l c M1M1 M2M2 L2L2 L1L1 BS detector M1M1 M2M2 L1L1 BS detector optical fibre for long coherence lengths, use optical fibre delay 2L 1 -2L 2 ~ l c

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... 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 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

 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. 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.

Nuclear Fusion: National Ignition Facility

ultrashort pulses (5fs) broadband gain ( nm) 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 (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. 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. 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