Nature of light

Ray or geometrical opticsWave or physical opticsQuantum theory: Wp=hf

Advantages of fibers

Abundance of raw material

CheaperLow losses

EMI immunity: Security

Information capacity

Applications

Undersea cables

Along power lines

Aircraft wiring

Sensors

Biomedical

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

Free Space Optics

The Canobeam DT-55 adjusts the beam according to the distance it must travel, maintaining beam focus and signal strength fully over its transmission range.