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Fiber Fundamentals

Nov 21, 2011 10:00 PM, By Bennett Liles

A refresher course on the latest in fiber-optic technology.


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Most fiber-optic communication is sent on wavelengths that are above the visible light spectrum. As wavelength increases, frequency decreases.

Bend Insensitive Fiber-Optic Cable

While fiber is being pulled during installation, the bend radius should be greater than 20 times the cable diameter. After installation, under no tension, 10 times the cable diameter is OK because excessive bending also causes signal attenuation, although bend radius has been the source of ongoing improvement in newer fiber-optic cables. This “bend insensitive” fiberoptic cable introduced in 2009 uses a ring of lower index optical material called an “optical trench” to surround the core and reflect light loss back into it. The factors that affect bending loss are the fiber type (single mode or multimode), the core diameter, numerical aperture, transmission wavelength (longer wavelengths are more sensitive to stress), and cable design.

Fiber-optic cable is also classified as to the material of which it is made. Glass fiber-optic cable has a glass core and cladding, and it is the most widely used, but plastic-clad silica (PCS) offers less performance for less money. For home-theater applications where high loss and short-range capability is not a problem, plastic fiber has a plastic core and cladding. This is the most economical variety. PCS and plastic fiber also have fewer protective layers than glass fiber. The loss characteristic can vary from plastic at 300dB/km to single-mode glass at 0.2dB/km.

Receivers

Also known as detectors, the receiver in fiber-optic transmission is normally a PIN- or Avalanche-type photodiode mounted in a case similar to that used for the transmitter. These have a much larger sensitive detecting area than the emitting area of transmitters, so the stringent alignment precautions necessary on the sending end are usually not necessary on the receiving end. The size of the receiver, however, must be matched to the size of the fiber so as not to overload the detector's high-gain amplifier and cause a distorted output. If the fiber is too small for the receiver, too little light will reach the detector, and the eventual signal-to-noise ratio will suffer. The signal originally received is tiny, and voltage converted from it has to be amplified in steps to make it useable. Most fiber-optic receivers employ an analog preamplifier followed by either an analog or digital output stage. This is normally followed by additional stages to boost the signal before it is fed into other equipment or transmitted over other media. The receiver sensitivity is another important parameter in the fiber-optic system; this is the minimum optical power that is required for the receiver circuitry to deliver the signal correctly. A typical receiver sensitivity figure is -28dBm or 0.00158mW.

Once the transmitter power, the loss on the line, the receiver sensitivity, and the signal-to-noise ratio that is required are known, the optical budget of the fiber system may be calculated. The optical budget, referenced to 1mW or 0dBm, is basically the difference between the transmitter power and the receiver sensitivity. This figure will represent the maximum loss that can be tolerated along the line for the system to work.

TRANSMISSION WINDOWS

As was mentioned previously, there are some standard windows in the fiber transmission spectrum and these exist between several large absorption areas. The general rule is: The higher the wavelength, the lower the attenuation. The original window that was used in fiber transmission was the 850nm region, which is known as the “first window.” It provides about a 3dB/km loss, and as longer wavelength signals came into use, they came into favor. The second window, at 1310nm, exhibits an attenuation of about 0.5dB/km; the third window, at 1550nm, offers a loss of 0.2dB/km. All these are located within the overall spectrum just above visible light in the infrared region, but some low-end, short-distance systems use the 660nm range in the visible light area. As one might expect, higher-wavelength systems provide better performance but at higher cost. The attenuation factor can, of course, be offset by the fiber-cable diameter. Once again, narrower cable means less optical attenuation. Multimode fiber operates at 850nm or 1300nm while singlemode fiber operates at 1310nm or 1550nm.

A number of methods have arisen for combining multiple signals onto one single-mode fiber, and this effort has been aided by the development of flat-spectrum fiber that eliminates the high-loss region around 1380nm. The technique of Coarse Wave Division Multiplexing (CWDM) can combine up to 18 separate signals on one single-mode fiber. When CWDM is used with systems providing up to eight separate video channels, a total of 144 video channels on one fiber are possible. An even more densely packed multiplexing scheme is Dense Wave Division Multiplexing (DWDM), in which up to 128 separate wavelengths can be simultaneously transmitted. DWDM systems are far more expensive than CDWM systems.



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