Laser Principles and Applications Quiz

Laser Physics and Safety

• Laser physics examines light, its properties, and manipulation.
• Socrates observed the thermal damage of the eye by sunlight.
• Max Planck linked the energy emitted from a black body to its frequency (Planck's Constant).
• In the 1940s, Meyer-Schwickerath used sunlight for precision retinal burns.
• Maiman created the first working laser (Ruby Laser) in 1960.

What is Light?

• Light is electromagnetic radiation.
• It can be considered as a wave or particle.
• Described by frequency or wavelength.
• Frequency is inversely proportional to wavelength.
• Wavelength is used in this study.

Light as a Wave

• Describes light's movement through space.
• Light sources create wavelengths through oscillations.
• Two types of wavelengths are:
  • Longitudinal (compression waves) that move in the same plane as the direction of travel.
  • Transverse waves that move perpendicular to the direction of travel.
• Light's wave properties are mathematically expressed as: v = c/λ where:
  • v = frequency
  • c = velocity
  • λ = wavelength (nm).

Interference and Coherence

• Light waves have amplitude, frequency, and phase.
• Combining waves is called interference.
• Constructive interference occurs when waves in the same phase combine, amplifying the wave.
• This amplification is desired in lasers.
• Destructive interference occurs when waves with 180° phase difference cancel out.
• Coherent waves have the same amplitude, frequency, and phase.
• Lasers are the only coherent light source.

Light as a Particle

• Light acts as a packet of electromagnetic radiation (quantum) called a photon.
• Energy transfer between particles or atoms is described in terms of photons.
• Mathematically, the energy of a photon is E = hv where:
  • E = photon energy (Joules)
  • h = Planck's constant (6.626 x 10^-34 Joule seconds)
  • v = frequency (s^-1)

Atom Anatomy

• Atoms have a nucleus (neutrons + protons) surrounded by electrons.

Electron Energy Levels

• Electrons occupy distinct energy levels (orbitals).
• Electrons typically occupy the lowest orbital (ground state).
• Orbitals further from the nucleus have greater energy (excited state).

Absorption

• Electrons move to higher energy levels (excited states) by absorbing photon energy.
• Photon energy exactly matches the difference in energy between orbitals (ΔE).

Emission

• Light is produced when electrons transition from higher to lower energy levels.
• The light emitted is classified based on its production methods:
  • Incandescence (produced by heating matter - inefficient and produces various wavelengths of light).
  • Luminescence (produced by direct electron excitation - producing single wavelengths of light). Lasers are a form of luminescence.
• Spontaneously emitted photons are emitted when electrons return to ground state. This is a random process.

Spontaneous Emission

• Electrons in excited states will spontaneously return to the ground state, emitting a photon.
• This process is random in terms of direction and emission time.

Simulated Emission

• Incoming photons of the appropriate energy can induce electrons to spontaneously emit a photon. These photons have the same properties as initial light.
• Electrons must remain excited for a longer period in order to achieve stimulated emission.

What Conditions Are Needed for Simulated Emission?

• Electron population inversion: Most electrons must be in an excited state, ideally a metastable state. This is because electrons in metastable states remain in the excited state for longer periods.
• Metastable State:
  • Electrons in a metastable state are stable and less likely to spontaneously emit photons.
  • Electrons in this state remain long enough to be stimulated by an incoming photon to emit a photon that is identical to the incoming light. This leads to photon amplification.

Laser Characteristics and Components

• Laser lights are highly focused and coherent monochromatic beams.
• Powerful: More photons emitted per second per radiating surface area, relative to other light sources.
• Monochromatic: Light of a single wavelength amplified through simulated emission.
• Temporally coherent: Has a nearly infinite length.
• Collimated: Goes in the same direction with minimal divergence, creating a highly concentrated beam of light.

Power Density

• Power density is the ratio of power to the area where photons are transferred. In essence, it is power per unit area.
• J equals joules or number of photons emitted
• W equals watts or joules/second
• Cm^2 equals the specific area where photons are transferred to (spot size).

Laser Components

• Lasing medium (gas, solid, dye, or semiconductor that provides atoms for simulated emission).
• Excitation mechanism, or pump (supplies energy to excite atoms to higher energy states for stimulated emission, this method could employ electricity or light to energise the electrons.)
• Optical resonator
  • Using mirrors to amplify the light, using highly aligned mirrors, photons are kept within the cavity for amplification purposes and determine the output wavelength length.

How Does a Laser Work?

• Exciting atoms increases their energy levels to their excited state and subsequently to metastable states within the lasing media.
• A photon, emitted as a result of an exciting mechanism, induces a similar photon from a metastable atom in this process of stimulated emission. This results in amplification of light.
• Within the optical resonator and mirrors, light's directionality and intensity is harmonized.
• The output beam emerges from the laser as it escapes the resonator through the output coupler.

Laser Pulse Types

• Continuous: Laser light is emitted as long as activated, and frequently causes thermal effects to tissues.
• Long Pulse: Emits laser light in milliseconds, also causes thermal effects to tissues.
• Quotient (Q)-Switching: Emits laser light in nanosecond pulses, achieved by using a shutter to build-up energy levels and does not cause thermal effects, suitable for precise procedures.
• Mode Locking: Emits laser light in femtosecond pulses, achieved by adding an absorber to release high-frequency laser light, and it is suitable for non-thermal procedures.

Spot Size Can Also Be Modified

• Spot size is an important characteristic of the laser beam. A narrower spot size means higher power density.
• Modifying the spot size can alter the effectiveness of the laser.

Laser-Tissue Interactions

• The interaction between a laser and tissue is involved with three components: transmission, absorption, and degradation.
• Transmission, absorption and degradation depend on factors such as wavelength, power, and pulse duration.

Laser Transmission

• Transmission of light depends on the ocular media's transparency.
• Light transmits in the ultraviolet, visible, and infrared spectrums.

Absorption

• Absorption of light depends on the wavelength of emitted light and the absorbing tissue's characteristics.
• Tissue components like melanin, xanthophyll, and hemoglobin absorb light at different wavelengths.

Degradation

• Photocoagulation: Absorbed light raises the tissue temperature causing proteins to denature, creating scarring and contractions.
• Photovaperization: Infrared light vaporizes water within tissues.
• Photoablation: Ultraviolet light breaks down polymers in tissues.
• Photoradiation/Photostimulation: light induces the production of cytotoxic free radicals in tissues. Selective photothermolysis triggers a response involving macrophages that causes tissue degredation. Photodisruption uses high-energy pulses of light to strip electrons from atoms, creating a plasma shock wave.

Table of Laser-Tissue Interactions

• Different types of lasers and their associated tissue reactions are tabulated as a helpful guide for tissue treatments.

Safety Considerations

• ANSI: Develops laser classification and safety standards.
• OSHA: Develops classification system and workplace safety standards.
• IEC:Develops classification system.
• FDA: Develops classification system and safety standards.

Laser Classification System

• Based on hazard level and usage.
• Classes I, II, IIa, III, and IV lasers are classified based on hazard characteristics.

Required Safety Features on Class IIIB & IV Lasers

• Protective Housing
• Safety Interlocks
• Key Activation
• Emission Indicator
• Warning Labels

Other Safety Measures

• Protective eyewear, appropriate for specific wavelengths.
• Nominal hazard zone: Area of high eye damage risk.
• Posted warning signs.
• Safety manual, important to ensure safe use.

Laser-Induced Ocular Disease

• Various wavelengths of light can cause distinct ocular effects.
• UV, visible, and infrared light can lead to ocular diseases.

Negative Health Effects of Lasers

• Negative health effects tabulated, associated with specific tissue damage.
• Wavelengths of light and the potential effects on the eyes and skin are listed.

Photokeratitis

• Acute and painful condition due to excessive UV light exposure.
• Typical causes include exposure from the direct sun and welding.

Retinal Injury from Lasers

• Typically caused by accidental high-energy laser exposure.
• Foveal photocoagulation, macular holes, retinal hemes, RPE discoloration are potential outcomes.

Laser Pointer Injuries

• Risks from prolonged exposure and misuse.
• Risk factors, including young age, clear ocular media.
• Some symptoms include TVO and visual field defects.

Laser Pointer Induced Macular Injury

• Images of macular injuries are presented as clinical examples.