Light Amplification by Stimulated Emission of Radiation
Laser is a source of coherent directional (collimated ie parallel) monochromatic (single wavelength) light

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
The stimulating photon does not lose energy
For light amplification, the optical material needs to have more excited atoms than lower state atoms so that emission occurs more than absorption
This is achieved by “inversion of population” using an excitation source (various options)

The upper energy level of the stimulated atoms must have a relatively long lifetime so that this population accumulates relative to those at a lower energy level
Once atoms emit a photon and descend to a lower energy level, this level must have a relatively short lifespan so that atoms decay and reenter the cycle.

Lasing material can be a gas, liquid or solid and light energy is “pumped” into it by a power source.
Lasers require a cavity (optical resonator) bordered by two mirrors which circulate the emitted light through the lasing material to stimulate the emission of new photons

The distance between the mirrors is a multiple of the emitted light’s wavelength to achieve “resonance”: upon reflection the light traversing the tube remains in phase (coherent) and reinforces itself.

A fraction of the photons escapes via one mirror (semi-transparent) to form the laser beam

## Summary Table: laser classes

Laser classes
1	Safe for exposure
2m	Safe (without optical instruments)
3R	Not safe but low risk
3B	Hazardous but diffuse reflection can be viewed safely
4	Hazardous including diffuse reflection. Fire risk

## Ophthalmic lasers

The wavelength range of ophthalmic lasers is 193nm to 10800nm (including the visible spectrum).

Laser-tissue interactions: depend on wavelength, pulse duration and irradiance (power per unit area)

Photochemical: eg. photo-transduction in photoreceptors. Used in photodynamic therapy and corneal crosslinking
Photo-thermal: tissue effects depend on the temperature but range from necrosis to coagulation and vaporization.

Retinal PRP: pulses of 10-200ms and transient high temperatures. The energy is absorbed mainly by the melanin in the RPE and choroid and by haemoglobin.
Argon and Nd:YAG lasers are also used in laser trabeculoplasty for IOP control (thermal burns in the TM supposedly contract the tissue and open spaces to increase aqueous flow. Selective Laser Trabeculoplasty uses a lower energy laser than Argon and leaves the TM intact with minimal damage to the endothelial cells or scarring.

The IOP-lowering effects of laser trabeculoplasty diminish with time and can be sudden

Photo-mechanical: photoablation and photodisruption when laser absorption results in tissue temperature exceeding the vaporization threshold (100-305 degrees Centigrade). Expanding vapor bubble leads to tissue rupture and ejection of tissue fragments

LASIK and other corneal refractive surgery with the ArF excimer laser.
Dielectric breakdown refers to tissue rupture in transparent tissues eg in PCO using the nanosecond Nd:YAG
A combination of Argon and ND:YAG can be used in peripheral iridotomy (argon to photocoagulate and prevent bleeding, then Nd:YAG to complete).
Holmium lasers are used to create sclerostomies to increase aqueous outflow

## Summary Table: laser properties and uses

Laser	Wavelength	Uses	Notes
Argon	Blue (488nm) and green (514nm)	Outer retina, iris (thermal, photocoagulation)	Gold-standard for ROP. Not used for macula as much due to blue light component
Krypton	Yellow (568nm) and red (647nm)	Macula	Xanthophyll pigment does not absorb this wavelength as much cp argon
Helium-neon (He-Ne)	630nm (red)	Aiming beam	Low power
Diode lasers	790-950nm range (infrared)	Cyclodiode	Semi-conductor technology, operating in CW mode. Extremely compact
Neodymium-yttrium-aluminium-garnet (Nd-YAG)	1065nm (infrared)	Ionizing: capsulotomy, breaking posterior synechiae, peripheral iridotomy	Neodymium ions which produce the laser are contained within an optically pure YAG crystal therefore achieving higher concentration than as a gasUsually used in Q-switch mode
Excimer (“excited dimer”)	Argon fluoride: 193nm (UV)	Corneal cutting device (photoablation)
Carbon dioxide	10600nm (infrared)	Vaporising: bloodless incisions	90% absorbed within a thickness of 200 microns (ie. limited to surface tissue)

## Complications of PRP

## Scanning laser polarimetry

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 can be used for RNFL thickness measurements

## YAG peripheral iridotomy

## Cyclodiode