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Lasers in Ophthalmology
Optics & Refraction
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31. Overview
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1Light Amplification by Stimulated Emission of Radiation (LASER)
Properties of Laser Light
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1- Monochromaticity: Single wavelength - precision targeting of specific tissues
- Coherence: Waves in phase (temporal + spatial) - permits interference and holography
- Collimation: Minimal divergence - tight beam over long distances
- High intensity: Enormous power density at focus
Stimulated Emission Mechanism
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1- An incident photon interacts with an excited atom, triggering release of a second identical photon.
- This is the amplification step.
- In a laser cavity, mirrors at both ends reflect photons back and forth through the gain medium, building up an avalanche of coherent photons.
- One mirror is partially transmissive - this is the output beam.
Components of a Laser System
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1| Component | Function | Example |
|---|---|---|
| • Active medium | • Amplifies light via stimulated emission | • neodymium-doped yttrium aluminium garnet (Nd:YAG) crystal • carbon dioxide (CO2) gas • Argon gas • excimer gas |
| • Energy pump | • Creates population inversion | • Flash lamp • electrical discharge • diode laser |
| • Optical resonator | • Two mirrors (one partially transmitting) provide optical feedback and help produce a directional beam. | • Provides amplification and directionality |
| • Delivery system | • Guides beam to target | • Slit lamp • endoprobe • fibre optic |
| • Focusing lens | • Concentrates beam to small spot | • Influences spot size and power density |
| Component | Function | Example |
|---|---|---|
| • Active medium | • Amplifies light via stimulated emission | • neodymium-doped yttrium aluminium garnet (Nd:YAG) crystal • carbon dioxide (CO2) gas • Argon gas • excimer gas |
| • Energy pump | • Creates population inversion | • Flash lamp • electrical discharge • diode laser |
| • Optical resonator | • Two mirrors (one partially transmitting) provide optical feedback and help produce a directional beam. | • Provides amplification and directionality |
| • Delivery system | • Guides beam to target | • Slit lamp • endoprobe • fibre optic |
| • Focusing lens | • Concentrates beam to small spot | • Influences spot size and power density |
Laser Parameters
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1| Parameter | Definition | Clinical Significance |
|---|---|---|
| Wavelength (nm) | Determines tissue absorption | • Retina absorbs 514nm • cornea absorbs 193nm |
| Power (W) | Energy delivered per second | • Higher power → more thermal effect |
| Spot size (µm) | Diameter of focused beam | • Smaller spot → higher irradiance |
| Pulse duration (s) | Duration of single pulse | • Determines thermal spread |
| Energy (J) | Power × time | • Governs total tissue effect |
| Irradiance (W/cm²) | Power / spot area | • Key determinant of tissue damage |
| Fluence (J/cm²) | Energy / spot area | • Total dose per unit area |
| Parameter | Definition | Clinical Significance |
|---|---|---|
| Wavelength (nm) | Determines tissue absorption | • Retina absorbs 514nm • cornea absorbs 193nm |
| Power (W) | Energy delivered per second | • Higher power → more thermal effect |
| Spot size (µm) | Diameter of focused beam | • Smaller spot → higher irradiance |
| Pulse duration (s) | Duration of single pulse | • Determines thermal spread |
| Energy (J) | Power × time | • Governs total tissue effect |
| Irradiance (W/cm²) | Power / spot area | • Key determinant of tissue damage |
| Fluence (J/cm²) | Energy / spot area | • Total dose per unit area |
Laser Modes
- Continuous wave (CW): Constant beam - Argon, diode (photocoagulation)
- Pulsed: Discrete pulses - allows tissue cooling between pulses
- Q-switched: Very short (nanoseconds (ns)), very high energy pulses - neodymium:yttrium-aluminum-garnet (Nd:YAG) (photodisruption)
- Mode-locked: Ultra-short (femtoseconds (fs)) pulses - femtosecond lasers (laser-assisted in situ keratomileusis (LASIK) flap, femtosecond laser-assisted cataract surgery (FLACS))
2. Laser–Tissue Interactions
Four Fundamental Interactions
| Interaction | Mechanism | Energy/Pulse | Example |
|---|---|---|---|
| Photocoagulation | • Absorption → heat → protein denaturation (predominantly 60–70°C) | Continuous-wave, moderate | • Panretinal photocoagulation (PRP) • Laser trabeculoplasty • Retinopexy |
| Photovaporisation | • Temperature 100–400°C → boiling & ablation | Continuous-wave or pulsed, high | • Dacryocystorhinostomy (DCR) • Trachoma • Carbon dioxide (CO2) laser |
| Photodisruption | • Plasma formation → shock wave → mechanical cutting | Pulsed nanosecond (Q-switched) | • Neodymium:yttrium-aluminium-garnet (Nd:YAG) capsulotomy • Neodymium:yttrium-aluminium-garnet (Nd:YAG) iridotomy • Neodymium:yttrium-aluminium-garnet (Nd:YAG) vitreolysis |
| Photoablation | • Ultraviolet photons break molecular bonds directly (photodecomposition) | Pulsed 193 nm | • Excimer photorefractive keratectomy (PRK) • Laser-assisted in situ keratomileusis (LASIK) stromal ablation |
| Interaction | Mechanism | Energy/Pulse | Example |
|---|---|---|---|
| Photocoagulation | • Absorption → heat → protein denaturation (predominantly 60–70°C) | Continuous-wave, moderate | • Panretinal photocoagulation (PRP) • Laser trabeculoplasty • Retinopexy |
| Photovaporisation | • Temperature 100–400°C → boiling & ablation | Continuous-wave or pulsed, high | • Dacryocystorhinostomy (DCR) • Trachoma • Carbon dioxide (CO2) laser |
| Photodisruption | • Plasma formation → shock wave → mechanical cutting | Pulsed nanosecond (Q-switched) | • Neodymium:yttrium-aluminium-garnet (Nd:YAG) capsulotomy • Neodymium:yttrium-aluminium-garnet (Nd:YAG) iridotomy • Neodymium:yttrium-aluminium-garnet (Nd:YAG) vitreolysis |
| Photoablation | • Ultraviolet photons break molecular bonds directly (photodecomposition) | Pulsed 193 nm | • Excimer photorefractive keratectomy (PRK) • Laser-assisted in situ keratomileusis (LASIK) stromal ablation |
Chromophores in the Eye
| Chromophore | Absorption Range | Tissue | Lasers targeting it |
|---|---|---|---|
| • Melanin | • 400–700 nanometres (nm) | • Retinal pigment epithelium (RPE) • iris • choroid | • Argon (514 nanometres (nm)) • Diode (810 nanometres (nm)) |
| • Oxyhaemoglobin | • 418 nanometres (nm) • 542 nanometres (nm) • 577 nanometres (nm) | • Retinal vessels | • Argon (514 nanometres (nm)) • Krypton (568 nanometres (nm)) • Dye (577 nanometres (nm)) |
| • Water | • >1000 nanometres (nm) | • Vitreous • cornea stroma | • Carbon dioxide (CO₂) (10,600 nanometres (nm)) • Erbium:yttrium-aluminium-garnet (Er:YAG) (2,940 nanometres (nm)) |
| • Protein | • Ultraviolet (UV) range | • Lens • cornea collagen | • Excimer (193 nanometres (nm)) |
| • Xanthophyll (macular pigment) | • 460 nanometres (nm) | • Foveal Henle layer | • Absorbed at 450–500 nanometres (nm) → use lasers above 500 nanometres (nm) at the fovea |
| Chromophore | Absorption Range | Tissue | Lasers targeting it |
|---|---|---|---|
| • Melanin | • 400–700 nanometres (nm) | • Retinal pigment epithelium (RPE) • iris • choroid | • Argon (514 nanometres (nm)) • Diode (810 nanometres (nm)) |
| • Oxyhaemoglobin | • 418 nanometres (nm) • 542 nanometres (nm) • 577 nanometres (nm) | • Retinal vessels | • Argon (514 nanometres (nm)) • Krypton (568 nanometres (nm)) • Dye (577 nanometres (nm)) |
| • Water | • >1000 nanometres (nm) | • Vitreous • cornea stroma | • Carbon dioxide (CO₂) (10,600 nanometres (nm)) • Erbium:yttrium-aluminium-garnet (Er:YAG) (2,940 nanometres (nm)) |
| • Protein | • Ultraviolet (UV) range | • Lens • cornea collagen | • Excimer (193 nanometres (nm)) |
| • Xanthophyll (macular pigment) | • 460 nanometres (nm) | • Foveal Henle layer | • Absorbed at 450–500 nanometres (nm) → use lasers above 500 nanometres (nm) at the fovea |
Thermal Spread & Selective Photothermolysis
- Selective photothermolysis (Anderson & Parrish 1983): Laser wavelength absorbed selectively by target chromophore, and pulse duration is ≤ thermal relaxation time (TRT) of target → thermal damage confined to target, sparing surrounding tissue.
- Pattern Scan Laser (PASCAL) and subthreshold micropulse laser use short-pulse photothermal laser delivery; photodynamic therapy (PDT) uses photochemical activation of a photosensitizer.
Exam pearl
- Clinical Analogy:
- Think of a laser as an army of photons marching perfectly in step - same direction, same wavelength, same phase. Unlike ordinary white light (incoherent, divergent, polychromatic), a laser is disciplined, coherent light that can be focused to micron precision.
- This “discipline” - especially coherence and the ability to focus light to a very small spot - is the basis of laser eye surgery’s precision.
Exam pearl
- Clinical Analogy: Imagine sunlight (photons) hitting a dark road vs. a mirror. Absorption generates heat (photocoagulation on the road), reflection does not enter the target tissue, although reflected beams can cause accidental exposure, and scattering spreads the energy. A laser chooses its interaction by selecting the right wavelength for the target chromophore - haemoglobin, melanin, or water.
Laser Physics - Exam Pearls
- Population inversion is the prerequisite for lasing - more atoms in excited than ground state.
- Q-switching stores energy then releases it in a single giant pulse; nanosecond neodymium:yttrium-aluminum-garnet pulses produce photodisruption.
- Femtosecond lasers (10⁻¹⁵ s pulses) produce photodisruption without thermal spread - basis of bladeless laser-assisted in situ keratomileusis (LASIK).
- Irradiance = Power ÷ (π × r²); doubling spot size quarters irradiance.
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Keep reading Lasers in Ophthalmology
9 more sections are included with the Clinical plan.
- Individual Lasers - Viva / Exam Pearls
- Retinal and Glaucoma Lasers
- Lasers in Glaucoma
- Refractive and Cataract Lasers
- Laser Safety
- High-Yield Comparison Tables