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Advantages of fibers
Abundance of raw material
CheaperLow losses
EMI immunity: Security
Information capacity
Chapter 2: Optics review
Ray theory (geometrical tracing)
Light enters a fiber – critical angle - TIR
Lenses: focus light onto fiberFocal point, distance, planeThin lens, ideal lensFocal length formula
Thin Lens rules
1. Center rays are undeviated
2. Parallel rays pass through focal point3. Rays parallel to center ray intersects it on focal plane
3. Rays pass through focal length emerge parallel to axis
Lenses
Cylindrical lens – fig 2.10Graded-index – fig 2.15Collimating light out of fiber – fig 2.10
ImagingImage location and magnificationAngular spreadExample 2.4
Numerical ApertureIt is important for an optical system to collect light over a wide incident angleLens+Rx combination
Max acceptance angle: tan= d/2fNumerical aperture: NA = nosin
Numerical Aperture
High and low NA fibersLong path fibers: NA (0.1 – 0.3)Short fibers: NA (0.4 – 0.5)
Example 2.5
Derive NA of a fiber
DiffractionTransverse plane
Power = intensity
Central spot diameter
Gaussian beams
Compare to uniform beam
Uniform Beam
d = 2.44f/D
wo = f/w
Beam focusing
Diffraction
Gaussian beam representation
Beam collimation
Beam diverges at at a constant full angle of =2/w
HW #1:2-6,2-8,2-9, due: a week