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Fiber-Optic Communications
James N. Downing
Chapter 2
Principles of Optics
Chapter 2
2.1 Geometrical Optics
– A model by which the nature of light is used to explain
refraction, reflection, and propagation of light
Refraction:
– The bending of light as it passes through a medium
– Index of refraction: The ratio of the speed of light in a
vacuum to the speed of light in the medium
– Phase velocity: The speed of light in a medium
– Optical path length: apparent length of an optical
element
Chapter 2
2.1 Geometrical Optics
Snell’s Law
– Mathematical determination of the index of
refraction at the interface of two media
n1 sin 1  n2 sin 2
– Critical angle is the angle at which the refracted
ray is at 900 to the normal
Chapter 2
2.1 Geometrical Optics
Reflection
– Bouncing off of rays from a material interface
– Depends on the smoothness of the surface and
the refractive indices of the media
Fresnel reflection law
– Determines the fraction of light reflected as a
function of the incident ray as well as the amount
of light refracted or transmitted into the medium
Chapter 2
2.2 Wave Optics
Electromagnetic Waves
– Result of the dual properties of electricity and magnetism
and their relationship
– Derived from Maxwell’s equations
– Electric waves and magnetic equations are perpendicular to
each other
– Function of both space and time
– Electromagnetic spectrum consists of all forms of
electromagnetic energy
Chapter 2
2.2 Wave Optics
Polarization
– Describes the direction of the electric field
oscillations
– Induced by preferential reflection, transmission,
scattering, or passing light through a birefringent
material
– May be either perpendicular, horizontal, z-axis,
circular, or elliptical
Chapter 2
2.2 Wave Optics
Coherence
– Phase difference is the shift between two waves
along their axis of propagation
– Coherent light—no phase shift
– Incoherent light—phase is continually shifting
– Temporal coherence —waves are equal
– Spatial coherence—waves are in phase at a point
in space
Chapter 2
2.2 Wave Optics
Interference
– Due to the linear superposition of electromagnetic waves
such that the amplitude at any point is equal to the sum of
the individual amplitudes at that point
Constructive interference
– Phase shift is zero
Destructive interference
– Phase shift is 1800
Chapter 2
2.2 Wave Optics
Diffraction
– Diffraction describes how light can spread out after
going through a small aperture.
– Diffraction grating is the separation of the
diffracted light into different bands of different
colors.
Chapter 2
2.2 Wave Optics
Scattering
– Scattering is the spreading apart of light caused
by interaction with matter.
– Rayleigh scattering, or molecular scattering, is
caused by small particles of matter (less than or
equal to 1/10 wavelength) interacting with light.
– Mie scattering is due to interaction with matter
larger than 1/10 wavelength of light.
Chapter 2
2.3 Quantum Optics
Bohr Model
– Consists of nucleus and orbitals
– Nucleus contains the protons and neutrons
– The orbital contains the electrons
Chapter 2
2.3 Quantum Optics
Absorption
– Ground state is the minimum level of energy needed to keep
an electron associated with its orbit.
– Excited state is that in which the electron has absorbed
some energy.
– Absorption is the process in which light energy is converted
into electrical energy.
– Beer’s Law describes the absorption transfer function.
Chapter 2
2.3 Quantum Optics
Emission
– Emission is the process by which electrical energy
is converted to light.
– Spontaneous emission occurs naturally.
– Stimulated emission occurs when an external
photon causes a photon to lose energy.
– Linewidth is the length of a wavelength of light
(defined at the 50% power level).
Chapter 2
2.3 Quantum Optics
Planck’s Law
– This law describes the energy released when an
electron moves from one energy level to another.
E  E2  E1  h  h
c

Chapter 2
2.4 Nonlinear Optics
Four-Wave Mixing
– Four-wave mixing results in a fourth frequency
when three frequency signals are combined.
– Can be used to generate a fourth frequency, if
needed.
– Problems arise when the fourth frequency is
already in use.
Chapter 2
2.4 Nonlinear Optics
Phase Modulation
– The result of a change in the refractive index with a change
in light intensity
– Self-phase results in a broadening of the linewidth of a
particular signal
– Cross-phase occurs when self-phase modulation causes
phase changes in another signal. which results in a linewidth
broadening at another wavelength.
Chapter 2
2.4 Nonlinear Optics
Brillouin Scattering
– Occurs at optical powers high enough to generate
small acoustic waves in the material
– Alters the refractive index, and shifts the frequency
– Scattering increases as power increases
Chapter 2
2.4 Nonlinear Optics
Raman Scattering
– Light is absorbed and some energy is lost or
gained from molecular vibrations.
– Can be used to transfer energy from one
wavelength to another resulting in signal
amplification.
– Cross-talk may be enhanced if more than one
wavelength is used.
Chapter 2
2.5 Optical Power
Radiometric and Photometric Quantities
– Photometric quantities describe the visual
brightness of a light and exist only between
400nm and 700nm with a peak at 550nm.
– Radiometric quantities are consistent throughout
the spectrum and are proportional to the square of
the energy.
Chapter 2
2.5 Optical Power
Power
– The ratio of energy per unit time (measured in
watts or dBm)
– Transfer function: TdB = Pout-dBm – Pin-dBm