Optical fiber
Optical fiber consists of thin glass fibers that can carry information at frequencies in the visible light spectrum and beyond. The typical optical fiber consists of a very narrow strand of glass called the core. Around the core is a concentric layer of glass called the cladding. A typical core diameter is 62.5 microns (1 micron = 10-6 meters). Typically Cladding has a diameter of 125 microns. Coating the cladding is a protective coating consisting of plastic, it is called the Jacket.
An important characteristic of fiber optics is refraction. Refraction is the characteristic of a material to either pass or reflect light. When light passes through a medium, it "bends" as it passes from one medium to the other. An example of this is when we look into a pond of water.
If the angle of incidence is small, the light rays are reflected and do not pass into the water. If the angle of incident is great, light passes through the media but is bent or refracted.
Optical fibers work on the principle that the core refracts the light and the cladding reflects the light. The core refracts the light and guides the light along its path. The cladding reflects any light back into the core and stops light from escaping through it - it bounds the medium!
Optical Transmission Modes
There are three primary types of transmission modes using optical fiber. They are
Step Index
Graded Index
Single Mode
Step index has a large core, so the light rays tend to bounce around inside the core, reflecting off the cladding. This causes some rays to take a longer or shorter path through the core. Some take the direct path with hardly any reflections while others bounce back and forth taking a longer path. The result is that the light rays arrive at the receiver at different times. The signal becomes longer than the original signal. LED light sources are used. Typical Core: 62.5 microns.
Step Index Mode
Graded index has a gradual change in the core's refractive index. This causes the light rays to be gradually bent back into the core path. This is represented by a curved reflective path in the attached drawing. The result is a better receive signal than with step index. LED light sources are used. Typical Core: 62.5 microns.
Graded Index Mode
Note: Both step index and graded index allow more than one light source to be used (different colors simultaneously), so multiple channels of data can be run at the same time!
Single mode has separate distinct refractive indexes for the cladding and core. The light ray passes through the core with relatively few reflections off the cladding. Single mode is used for a single source of light (one color) operation. It requires a laser and the core is very small: 9 microns.
Single Mode
Comparison of Optical Fibers
We don't use frequency to talk about speed any more, we use wavelengths instead. The wavelength of light sources is measured in nanometers or 1 billionth of a meter.
Indoor cable specifications:
LED (Light Emitting Diode) light source
3.5 dB/Km Attenuation (loses 3.5 dB of signal per kilometer)
850 nM - wavelength of light source
Typically 62.5/125 (core dia/cladding dia)
Multimode - can run many light sources.
Outdoor cable specifications:
Laser Light Source
1 dB/Km Attenuation (loses 1 dB of signal per kilometer)
1170 nM - wavelength of light source
Monomode (single mode)
Advantages of Optical Fiber:
Noise immunity: RFI and EMI immune (RFI - Radio Frequency Interference, EMI -Electromagnetic Interference)
Security: cannot tap into cable.
Large Capacity due to BW (bandwidth)
No corrosion
Longer distances than copper wire
Smaller and lighter than copper wire
Faster transmission rate
Disadvantages of optical fiber:
Physical vibration will show up as signal noise!
Limited physical arc of cable. Bend it too much and it will break!
Difficult to splice
The cost of optical fiber is a trade-off between capacity and cost. At higher transmission capacity, it is cheaper than copper. At lower transmission capacity, it is more expensive.
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