LASERs

• 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

Ophthalmic lasers

• Surgical lasers are capable of damaging the eye and so are classed 3b or 4.
• They require training for use and strict regulation

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

• The shorter the wavelength, the higher the frequency and energy

• 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. 

• Krypton red (646nm) is useful to penetrate vitreous haemorrhage for PRP

• 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

Complications of PRP

• CNVM
• Tractional RD
• ERM
• Angle-closure glaucoma: secondary to choroidal effusion
• Vitreous haemorrhage
• Foveal burns
• Retinal vascular occlusions

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

• Full thickness hole created through the iris
• Permits flow of aqueous from the PC to the AC
• Avoid placing in the axis of the marginal tear strip
• Indication
• Angle closure with pupillary block
• Occludable angles (prophylaxis)
• Complications
• Bleeding
• Inflammation
• Raised IOP
• Corneal burns
• Glare
• Optical aberrations
• Benefits of YAG vs argon PI
• Less energy needed
• Less risk of corneal oedema, iritis, and pupillary distortion
• Less risk of late angle closure
• However bleeding is more common

Cyclodiode

• Selective destruction of the ciliary body
• Complications
• Anterior inflammation including hypopyon
• Hypotony
• Scleral thinning/perforation
• Cataract
• Lens subluxation
• Sympathetic endophthalmitis