Laser Technology and Applications Quiz

Coefficient Scattering

• Consider a collimated light beam traveling in the z-direction through a thin, non-absorbing medium that scatters light elastically.
• As the beam passes through the medium, some light is scattered out of the beam into different directions.
• The intensity of the beam follows exponential decay, exp(-μsz).
• The probability that a photon will be scattered while traveling a short distance between z and z + dz is μsz.

Tissue Propagation Light

• Transport theory is a statistical mechanics area describing the passage of particles (neutrons, photons, molecules) through a scattering medium.
• It's only possible to give statistics about a large population of particles.

Light Sources and Detectors

• Incoherent light sources: Light emitting diodes and superluminescent diodes
• Coherent light sources: Lasers

Light Emitting Diodes (LEDs) & Superluminescent Diodes (SLDs)

• LEDs are semiconductor p–n junctions that emit light when electrons and holes recombine at the junction.
• LEDs are not narrowband sources, unlike lasers.
• LEDs are inexpensive, very reliable, and can be modulated at high frequencies.
• SLEDs are high-power broadband light sources generated at p–n junctions. Like LEDs, they have some amplified light due to the stimulated emission as it passes through the device.

Lasers - Medical Applications

• Carbon dioxide laser (CO₂): Wavelength λ = 10.6 μm. Active medium vibrational levels of CO₂ molecule, with He and N₂ to improve efficiency. Strong absorption by water. Used for laser scalpels and skin resurfacing. Power typically between 10-50W.
• Nd:YAG laser: Wavelength λ = 1064 nm. Active medium is an yttrium–aluminum–garnet crystal (YAG) doped with neodymium Nd³⁺ ions. Used for photodisruption in posterior capsulotomy, photoacoustic imaging, and coagulation. Generates pulses of tens to a few hundred mJ.
• ArF excimer laser: Wavelength λ = 193 nm. Active medium is a gas of diatomic molecules that are only bound in the excited state. On returning to the ground state, the atoms separate. Useful for corrective laser eye surgery (LASIK) because the short wavelength permits photoablation of corneal tissue without much thermal damage. Pulses are typically ~10ns with energy of 0.2-1J.
• Diode laser: Wavelength λ = 650-2000 nm, semiconductor lasers that operate similar to superluminescent diodes but use a resonance cavity to form the laser. Compact, inexpensive, long lifespan, and have output power up to 50W for Near Infrared (NIR) diode arrays. Can be utilized for NIR spectroscopy, optical imaging, photodynamic therapy, and photocoagulation.

Optics Fiber

• Using light, especially laser light, has a significant advantage over other wavelengths of radiation as light can be coupled into optical fibers and delivered easily and precisely to the desired location.
• This is convenient for endoscopy and minimally invasive surgery.

Photodiodes

• Optical fibers use the principle of total internal reflection to trap light beams, enabling transmission over longer distances and to otherwise inaccessible areas.
• Step-index fibers have a core medium (higher refractive index) and cladding (lower refractive index) to allow total internal reflection. The core diameter is 8-10 µm for single-mode fiber.
• Photodiodes are crucial for applications like imaging.
• Many applications of light, such as destroying tumours and reshaping the cornea, don't need detailed light detection, only dosimetry.