Fiber-Optic Communications James N. Downing. Chapter 2 Principles of Optics.

Fiber-Optic Communications

James N. Downing

Chapter 2

Principles of Optics

Chapter 22.1 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 22.1 Geometrical Optics

Snell’s Law– Mathematical determination of the index of

refraction at the interface of two media

– Critical angle is the angle at which the refracted ray is at 900 to the normal

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Chapter 22.1 Geometrical Optics

Reflection– Bouncing off of rays from a material interface– Depends on the smoothness of the surface and

the refractive indices of the mediaFresnel 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 22.2 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 22.2 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 22.2 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 22.2 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 22.2 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 22.2 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 22.3 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 22.3 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.

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Chapter 22.4 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 22.4 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 22.4 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 22.5 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