Medical Lasers and Delivery Systems Flashcards
Why are lasers good for medical treatment?
- Spatial and temporal coherence
- Collimation
- Specific narrow wavelength bands
- High power and can be pulses
Disadvantages of lasers?
- Expensive
- Hazardous
- Largely non-portable
- Only target specific areas not larger
Alternatives to lasers?
- LEDs
- Halogen lamps
- Fluorescent lights
- ARC lamps
What is the beam waist?
For a Gaussian beam, the narrowest point along the beam and the focal point (2w0)
What is z0 in laser geometry?
The distance from the waist where the area has doubled:
z0 = πw02/λ
What is the Rayleigh Range / Depth of Focus?
A measure of the range of waist region where spot size is smallest:
Δ = 2z0
The distance over which the beam diameter increases by √2
Equation for beam divergence?
θ = w0 / z0
What causes speckle patterns?
Spatially coherent lasers reflecting of slightly rough surfaces, interfering with each beam.
Classic equation for beam divergence?
θ = 1.22 λ/d
Beam spot size?
rmin = Fλ = fθ
F = f-number
What does the f-number describe?
F = f/d - the focusing ability of the lens.
Depth of focus for a Gaussian beam?
Δ = 8λf2/πD2 ~2λF2
What is the M2 parameter?
M2 = (π/λ)θrw0r
It characterises the quality of a beam compared to that of a ‘perfect’ theoretical beam. <1.2 is considered very good.
What is temporal coherence for a laser?
Over a large distance, the peaks and troughs of the waves do not shift much with respect to each other.
What is coherence time?
The time needed for the phase to switch to antiphase
- tc = 1/Δf
