Telecommunication: Fiber Theory
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How does fiber optics transmit?
From a source, information is encoded into a signal (digital or analogue), then a light source pulses light down a fiber optic cable, a decoder then changes the is used to change the pulses into readable information by a device (computer).
- Any material through which energy propagates over a distance
- -Sound through water
- -Light through high purity glass
- -Radio signal through air
Electromagnetic energy in the form of photons
Stream of electromagnetic energy traveling in a sinusoidal wave
Distance between identical points on a wave expressed in nanometers
What spectrum does fiber optic transmitters operate in?
What wavelengths do Visual Fault Locators(VFL) operate in?
What wavelengths does multimode fiber operate in?
850 and 1300nm
What wavelengths does single-mode fiber operate in?
1310 and 1550nm
What are the 3 parts to a fiber optic cable?
Index of Refraction
Comparison between speed of light in a medium and the speed of light in a vacuum. (table is on page 5)
N= Speed of light in a vacuum/Speed of light in a medium
Clear solid glass, light guiding region
Clear solid glass, contains the light in the core
Acrylate material, protects glass during handling
Bouncing of light
Bending of light
Numerical Aperture (NA)
- -The measure of the maximum angle of the light entering the end of the fiber that will propagate within the core of the fiber.
- -Light rays entering the fiber that exceed the angle of the NA will enter the cladding and be lost.
- -The area in which light must enter to travel through the fiber.
- -Any light that hits with in the cone will travel down the fiber and have total internal reflection, any light that misses will be lost in the cladding due to refraction.
List the types of optical sources
- -Light emitting diode (LED)
- -Vertical cavity surface emitting laser (VCSEL)
- -Light amplification from the stimulated emission of radiation (LASER)
What are the different characteristics in the types of optical sources?
Time delay and spectral width
What are characteristics of a LED?
Low cost, operates at 850 or 1300nm
What are characteristics of a VCSEL?
Cost less than a LASER, operates at 850nm
What are the characteristics of a LASER?
High Cost; can operate at 1310, 1490, 1550 or 1625nm
Standard term for band of wavelengths, no source emits a single wavelength. (Picture page 9)
Single stream of photons that travel in a straight line unless acted upon by the environment.
What is the width of the core in a single-mode fiber cable?
What is the width of the core in a multimode fiber cable?
50 or 62.5 micrometers
Capacity of a medium to carry information over a distance in a given amount of time without loss of data. Measured in bits/second
What can affect bandwidth?
- -The optical source- nominal wavelength, spectral width, time delays
- - The guide (fiber)- Attenuation and Dispersion
What is attenuation?
- Loss of signal energy.
- -Expressed in decibels of power lost
- -Impacts the ability to reach the receiver with sufficient power
What is dispersion?
- Spreading of signal pulses.
- -Impacts the ability to distinguish discrete signals
- Can not be affected by outside influences.
- -Absorption- caused by impurities of the glass
- -The manufacturer controls IA
- Caused by outside influences.
- -Imperfect junctions
Caused by incorrect bends of during installation of cable.
A kink or pinch point in the cable.
Spreading of signal pulses as they travel through fiber. May cause pulses to overlap as the arrive at the receiver, this causes bit errors.
- Spreading of signal pulses due to several modes taking different paths.
- -Prevalent in multimode fiber.
Caused by the existence of multiple wavelengths within each signal pulse.
Caused by light traveling in the cladding of the cable.
Why use fiber optics?
- -Superior performance
- -Ease of installation
- -Secure(difficult to hack or tap)
- -Easy to upgrade
List some fiber applications
- -Long distance telephone
- -Cable TV
- -Traffic control
- -Military use
- -Factory automation
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