LASER: Properties, Production, Types, Physiological Effects, and Precautions

1. What is a LASER?

2. Properties of Laser Light

• Fluence (radiant exposure) - energy per unit area (J/cm²)
• Irradiance (power density) - power per unit area (W/cm²)
• Pulse duration - continuous wave (CW) vs. pulsed (milliseconds, microseconds, nanoseconds, picoseconds)
• Spot size - diameter of the beam at the tissue surface

3. Production of Laser (How a Laser Works)

1. Pumping - The energy source (electrical current, flashlamp, or another laser) excites electrons in the lasing medium to a higher energy state ("population inversion")
2. Spontaneous emission - Some excited atoms spontaneously emit photons and return to ground state
3. Stimulated emission - A passing photon stimulates an already-excited atom to release an identical photon (same wavelength, phase, direction) - this is the amplification step
4. Optical resonator / cavity - The medium is enclosed in a tube with two parallel mirrors: one is fully opaque (reflective), the other is partially transmissible. Photons bounce back and forth between the mirrors, amplifying the beam each time
5. Output - Photons exit through the partially transmissible mirror as a coherent, monochromatic, collimated beam

• A solid (Nd:YAG crystal, ruby, alexandrite)
• A gas (CO₂, argon, excimer)
• A liquid (dye solutions - pulsed dye laser)
• A semiconductor (diode lasers)
• Fiber (glass tubes doped with rare-earth elements like erbium or thulium)

• Articulated arms with mirrors - for CO₂ lasers
• Fiber optic cables - for Nd:YAG, diode, and Ho:YAG
• Micromanipulators coupled to microscopes - used in ENT and gynecology

4. Types of Lasers

Classified by Lasing Medium:

Classified by Clinical Use:

• Vascular lasers - PDL, KTP, Nd:YAG (target oxyhemoglobin)
• Pigment/tattoo lasers - Q-switched Ruby, Alexandrite, Nd:YAG (target melanin/ink)
• Hair removal lasers - Alexandrite, Diode, Nd:YAG (target follicular melanin)
• Ablative resurfacing - CO₂, Er:YAG (vaporize tissue - target water)
• Non-ablative resurfacing - 1320 nm Nd:YAG, 1450 nm diode, 1540 nm Er:glass (heat dermis without destroying epidermis)
• Fractional lasers - ablate microscopic columns of tissue, sparing surrounding skin

Classified by Pulse Mode:

• Continuous wave (CW) - constant beam
• Quasi-CW (chopped) - interrupted CW beam
• Pulsed - discrete pulses (ms, μs, ns, ps range)
• Q-switched - ultra-short pulses (ns/ps) with very high peak power

5. Physiological Effects of Laser on Tissue

1. Reflection - beam bounces off the surface (stratum corneum reflects ~5% even perpendicularly)
2. Scattering - beam deflects when it contacts small molecules; more common with shorter wavelengths; increases the effective irradiated area
3. Transmission - beam passes through to deeper structures without being absorbed; more common with longer wavelengths
4. Absorption - laser energy is converted to heat in the target chromophore - this is the therapeutic effect

Thermal Effects (Temperature-dependent):

• Higher power + shorter pulse = more vaporization, less surrounding coagulation
• Lower power + longer pulse = less vaporization, more coagulative necrosis at edges

Selective Photothermolysis (SPT):

1. Correct wavelength - preferentially absorbed by the target chromophore (melanin, oxyhemoglobin, water, tattoo ink)
2. Sufficient fluence - enough energy to heat the target to destructive temperatures
3. Pulse duration ≤ Thermal Relaxation Time (TRT) - pulse must be shorter than the time the target takes to cool by 50%, so heat is confined to the target

• Oxyhemoglobin: 418, 542, 577 nm
• Melanin: broad absorption, decreasing with increasing wavelength
• Water: peaks at 2940 nm (Er:YAG), 10,600 nm (CO₂)
• Tattoo ink: variable (different dyes absorb different wavelengths)

6. Precautions and Safety

Regulatory Bodies:

• International Electrotechnical Commission (IEC)
• American National Standards Institute (ANSI Z136.3)
• FDA Center for Devices and Radiological Health (CDRH)
• OSHA

Skin Safety:

• Select the correct wavelength for the target chromophore and patient's skin type
• Darker skin types (Fitzpatrick IV-VI) require: lower fluences, longer pulse durations, longer wavelengths, higher cutoff filters
• Apply pre-cooling, intra-procedural cooling, and post-procedural cooling to protect the epidermis
• Ablative lasers carry higher risk of infection and scarring than vascular/pigment-specific or non-ablative types
• Fractional ablative lasers significantly reduce the risk compared to fully ablative

Eye Safety:

• 70% of laser ocular injuries result from lack of or improper eye protection
• Wavelengths 380-1400 nm are transmitted through the eye to the retina; the refractive media focuses them, increasing irradiance >10,000-fold at the retina
• Wavelengths < 380 nm are absorbed by the cornea, causing corneal injury or cataracts
• Wavelengths > 1000-1400 nm are absorbed by water in the cornea, causing corneal thermal damage
• Three types of ocular injury:
  • Thermal (μs to seconds) - protein denaturation, retinal necrosis
  • Mechanical (ps to ns) - plasma formation, pressure wave, tissue explosion
  • Photochemical (> seconds, lower energy) - molecular bond breakage in nucleic acids/proteins
• The blink reflex (150-250 ms) and aversion response are NOT fast enough to protect from most laser pulses
• Safety goggles must have optical density (OD) ≥ 4 at the relevant wavelength and must match the specific laser being used

General Laser Safety Principles (Table 208-4):

Fire and Electrical Hazards:

• Laser-ignited fires are a real risk in OR settings (draping, oxygen-enriched environments)
• Wet towels/drapes around the operative field reduce fire risk
• Limit FiO₂ near the operative field when possible