Medical Laser Power Measurement

Power Measurement and Laser Delivery Systems

• Medical physics and INIR specifications are covered.
• Measurement tools, fiber optic delivery, articulated arm delivery, hollow wave-guide delivery, and hand-pieces are included in the lecture outline.
• Safety signs and signals, guidance on health and safety regulations, are referenced.

Laser Power Meter

• All medical lasers feature integrated displays estimating power based on selected factors.
• Engineers need to measure power when servicing or employing new techniques, and more regularly for specialist procedures like PDT.
• Thermopiles or photodiodes are common power measurement devices.

Thermopile

• Consists of a bank of thermocouples.
• Measures temperature using the Seebeck Effect.
• Indicates laser power.
• Two dissimilar metals, joined together, form junctions heated or cooled.
• Voltage change indicates temperature.
• The cold end is maintained at ambient temperature.

Photodiode Detectors

• Utilizes a silicon photodiode.
• Measures current to indicate power.
• Voltage relates to temperature.
• Highly specialized and wavelength-specific.
• Quick response time.
• Limited to a few mW continuous power.

Questions Relating to Thermopiles

• Sensor size is typically around 10mm diameter.
• Matte black surfaces absorb approximately 98% of incident light.
• Good wavelength response, typically a few percent from 200–20,000nm.
• Maximum power handling is usually 10W.
• Maximum average power density is ~10kW/cm².
• Damage threshold is ~ 10 GW/cm².
• Typically, response time is 1–2 seconds.

Integrating Sphere

• Diffuse reflections from the inner surface of a sphere measure laser power from a broad beam source.
• Output is independent of beam shape and geometry.

Spectro-Radiometer

• Simple power outputs can suffice for lasers with well-known properties.
• Unknown or novel broadband sources (LEDs, Xenon lamps) may yield inconsistent or unsatisfactory results.
• Precise data (like for PDT) needs spectro-radiometer investigation.

Fiber Optics: Total Internal Reflection

• Reflection/refraction occur when optical radiation encounters a boundary with differing wave velocities.
• Refractive index (n) is the ratio of velocity in a vacuum to that of the medium (e.g., water = 1.33; glass ≈ 1.5).
• For air to medium with refractive index n : reflected/incident ≈ [(n-1)/(n+1)]².
• Fresnel equations explain wave behavior at interfaces.
• Incident, reflected, and transmitted beams lie in the same plane.
• Reflection/refraction angle obeys Snell's Law.
• Fiber optics are formed by a transparent core surrounded by a cladding with a slightly lower refractive index.
• Maximum acceptance angle, θ, is defined by the numerical aperture (NA) as n₁sinθ = √(n₁² - n₂²), where n₁ is the core refractive index and n₂ is the cladding refractive index.
• Numerical aperture is normally referred to as the NA.
• The output cone for a plane fibre delivering into air equals the input cone's acceptance cone.
• In urological procedures, NA remains constant, but the delivery cone differs.

Modes for Fibres

• Multimode fibres (often used in surgery) allow a range of harmonics, improving illumination.
• Communication fibres are typically graded or single-mode.
• Graded index fibres reduce signal noise and enable longer distances.

Materials for Fibres and Lenses

• Common materials include silica glass (for near-infrared, low loss), and specialized materials (e.g., ZBLAN, for specific lasers).
• Plastic optical fibre (POF) is inexpensive, but has high loss and seldom offers a gradable index.
• Suitability for CO2 and other materials depends on losses, cost, and fabrication difficulties.

Hollow Waveguides

• Far-infrared transmission can be challenging due to high Fresnel loss (over 25%) within the core.
• Hollow waveguides (air cores) are under development, potentially with lower loss.

Coupling into Fibres

• Illuminating beams need to match the fiber's acceptance cone and focus to roughly the same size as the fiber core.

CO₂ Beam Delivery through Articulated Arms

• Hollow tubes with mirrors at each turn deliver the CO₂ beam.

CO₂ Beam Delivery through Articulated Arm Mirrors

• Copper or gold mirrors are selected for their reflectivity and ability to maintain flat surfaces.

Lenses

• Zinc Selenide lenses are the only material that is transparent to both treatment and aiming CO₂ beams at treatment sites.

CO₂ Handpieces

• Various handpieces cater to different tasks, including positioning spacers to regulate target distance.

Fibres Bundles in Endoscopy

• Endoscopy uses fibre bundles consisting of filaments (3-4 microns) of Ge-doped silica glass for illumination and imaging.

Imaging and Illumination Parameters in Endoscopy

• Crosstalk (light and signal leakage) influences image clarity.
• Illumination level depends on core diameters.
• Image resolution depends inversely on diameter (1/2d).
• Contrast is affected by light intensity.
• Core area ratio to whole bundle is approximately 50%.
• Defects and ordering errors also reduce image quality.

Laser-Induced Fluorescence Imaging

• Topical and in-vivo laser illumination induce wavelength-specific fluorescence from tagged agents.
• These agents are "fluoro-pharmaceuticals," aiding imaging and analysis externally, and also can be used endoscopically.

Laser-Initiated Raman Spectroscopy

• Inelastic Raman scattering identifies spectral patterns of pre-cancerous proteins.
• Special Raman nanoparticles target specific tumour types.

Case Study

• You need to analyze a commercially available novel laser application enabling either diagnosis or therapy.