Lasers Flashcards
LASER
Light Amplification by Stimulated Emission of Radiation
what are lasers
Laser is a source of coherent directional (collimated ie parallel) monochromatic
(single wavelength) light
electrons in laser
Electrons can move between different orbits at different energy levels within an
atom
This is accompanied by absorption or emission of a photon
Stimulated emission: a stimulating photon can cause an atom that is in an
excited state to emit another photon which will have the same phase, direction
and wavelength as the stimulating photon
what happens to achieve light amplification
the optical material needs to have more excited
atoms than lower state atoms so that emission occurs more than absorption
This state is achieved by “inversion of population” using an excitation source
(which could be a discharge lamp, pulsed flash lamp, electric discharge,
chemical reaction, other laser or electron beam etc.)
what can the lasing materilal be
a gas, liquid or solid and light energy is “pumped” into
it by a power source.
lasers and cavity
require a cavity (optical resonator) bordered by two mirrors which
circulated the emitted light through the lasing material so that it stimulates the
emission of new photons
laser and mirror
A fraction of the photons escapes via one mirror (semi-transparent) to form the
laser beam
Continuous production of light occurs in a laser in continuous-wave (CW)
mode
q-switched mode
a time-variable absorber can be
incorporated so that laser output is brief and very powerful. An absorber (eg.
rotating mirrors, saturable dye cells and electro-optic switches) blocks one of
the mirrors so that the lasing material is raised to high energy levels but
resonance does not occur. Population inversion is very great in this case. If the
absorber suddenly becomes transparent, a single, powerful pulse is emitted
convex lens in laser
used to focus a beam to spot with fixed diameter
lasers in optical fibres
used to transport lasers via total internal reflection and
deliver them to tissues
define a fundametnal mode
Energy focussed on the smallest spot is known as the fundamental mode
wavelength range of ophthalmic lasers
193nm to 10800nm (including the
visible spectrum).
The shorter the wavelength, the higher the frequency and energy
laser tissue interactions
: depend on the wavelength, pulse duration and irradiance
(power per unit area)
photochemical reaction
eg. photo-transduction in photoreceptors. Used in
photodynamic therapy in ARMD or corneal crosslinking. Very low irradiances
and long exposures
photo-thermal reaction
tissue effects depend on the temperature but range from necrosis
to coagulation and vaporization.
photo-thermal reaction in PRP
pulses of 10-200ms and transient hyperthermia.
Nd:YAG (532nm) are currently most commonly used for this.
The energy is absorbed mainly by the melanin in the RPE and choroid and by
haemoglobin (only about 5% is absorbed by the neural retina)
photo-thermal reaction in PASCAL
semi-automated pattern scanning
photocoagulator that delivers square arrays of up to 5×5 spots ir rings or
arcs for macular photocoagulation. Focal or grid laser is used for macular
oedema: mechanism of effect is debated. It may stimulate RPE and
endothelial cell proliferation to strengthen the inner blood-retinal barrie
photo-thermal reaction in Argon and Nd:YAG
used in laser trabeculoplasty for IOP
control (it is thought that thermal burns in the TM contract the tissue and
open spaces within the TM to increase aqueous flow. Selective Laser Trabeculoplasty uses lower energy laser than Argon and leaves the TM
intact with minimal damage to the endothelial cells or scarring. The IOPlowering effects of LT diminish with time
krypton-red
Krypton red (646nm) is useful to penetrate vitreous haemorrhage for
PRP
photo-mechanical
photoablation and photodisruption occur when laser
absorption results in tissue temperature exceeding the vaporization threshold
(100-305 degrees C). Expanding and collapsing vapor bubble lead to tissue
rupture and ejection of tissue fragments
argon laser
wavelength: blue (488nm) and green (514)
uses: outer retina, iris (thermal, photocoagulation)
notes: goldstandard for ROP, not used for macular as much due to blue light component
krypton
wavelength: yellow (568nm) and red (647nm)
uses: macula
notes: xanthophyll pigment does not absorb this wavelength as much as CP argon
Helium-neon (He-Ne)
wavelength: 630bm
uses: aiming beam
notes: low power
Diode lasers
wavelength: 790-950nm (infrared)
uses: CB destruction
notes: semi-conductor, extremely compact
Neodymium-yttrium-alum-inim-garnet (Nd-YAG)
wavelength: 1065nm (infrared)
uses: ionising, caps, PI
notes: usually used in q-switch mode
excimer
wavelength: argon fluoride 193nm (UV)
uses: corneal cutting
notes: photoablation
carbon dioxide
wavelength: 10600nm (infrared)
uses: vaporising, bloodless incisions
notes: 90% absorbed within a thickness of 200microns
complications of PRP
CNVM
Tractional RD
ERM
Angle-closure glaucoma: secondary to choroidal effusion
Vitreous haemorrhage
Foveal burns
Retinal vascular occlusions
scanning laser poliarimetry
Utilises the birefringent properties of the RNFL: birefringent because the axons are
arranged in a parallel fashion
Polarised light passes through the nerve fibre layer and is reflected back
This induces an alteration in its polarisation: the degree of change is known as
retardation
This can be used for RNFL thickness measurement
