wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

2-10 Industrial Communication Systems
The optical detector detects the modulated optical signal and feeds that detected signal to a demodulator.
The demodulator usually compares the incoming signal with threshold values and generates the electrical
signal. This electrical signal is further modified to proper data sequence if necessary, and finally it is
reproduced at the destination.
2.3.4 Multiplexing
Wavelength division multiplexing (WDM) technology uses multiple wavelengths to transmit information
over a single fiber. Coarse WDM (CWDM) has wider channel spacing (20.nm) and is of low
cost. Dense WDM (DWDM) has high capacity and dense channel spacing (0.8.nm) that allows simultaneous
transmission of 16+ wavelengths. First WDM networks used just two wavelengths, 1310 and
1550.nm. Today’s DWDM systems utilize 16, 32, 64, 128, or more wavelengths in the 1550.nm window.
Each of these wavelengths provides an independent channel. The range of standardized channel
grids includes 50, 100, 200, and 1000.GHz spacing. Wavelength spacing practically depends on laser line
width and optical filter bandwidth. Each optical channel can carry any transmission format (different
asynchronous bit rates, analog or digital). Wavelength is used as another dimension to time and space.
Multiplexing is used throughout communications because it greatly increases the transmission capacity
and reduces system costs.
2.3.5 Implementations and Standards
The fundamental requirements for making optical fibers sound deceptively simple. The material that is
transparent can be drawn into thin fibers with a distinct core-cladding structure. It should be uniform
along the length of the fibers and can survive in the desired working environment. Glass is the most
common material used in optical fibers. Ordinary glass is a noncrystalline compound of silica. Many
different variations of glass fibers have been developed. Simple glass-clad fibers are made by collapsing
a low-index tube onto a higher-index rod. Refractive indices of most optical glasses are between 1.44
and 1.8. Impurities limit transmission of standard glasses. Fibers are drawn from the bottom of a hot
preform. The preform has a physical make up identical to the final fiber, except that it is much wider
and shorter. The preform is heated very precisely, and a thin strand of glass is pulled off on one end. The
diameter of this strand is controlled very carefully through variances in heating and pulling tension.
This strand is the optical fiber, containing both the core and the cladding. Once the fiber is pulled off
the preform, it must be carefully and slowly cooled, covered with the final coating, and wound on to
reels. All silica fibers are used for communications. Hard-clad silica fibers are used for illumination and
beam delivery. Fibers made of fluoride and chalcogenide glasses transmit infrared wavelength, which
silica fiber absorbs.
Splices are used to join ends of fiber to each other permanently. This is done by fusion and mechanical
means. The critical factor in splicing is the fiber joint; special measures are adopted to guarantee
that the light passes through the joint without loss. The joint is mechanically secure and hard to break
easily. The purpose of a fiber-optic connector is to efficiently convey the optical signal from one link to
the next. Typically, connectors are plugs and are mated to precision couplers or sleeves (female). Some
connectors are designed to join fiber ends, in order to reduce reflections resulting from the glass-to-airto-glass
transition. The optical loss in a fiber-optic connector is the primary measure of device quality.
Return loss is the optical power that is reflected toward the source by a connector. Connector return
loss in a single-mode link can diffuse back into the laser cavity, degrading its stability. In a multimode
link, return loss can cause extraneous signals, reducing overall performance. To make sure that light
is passing through the system properly, testing is guaranteed. This is done with a modified type of
flashlight device and takes only a few minutes to perform. A calibrated light source puts infrared light
into one end of the fiber and a calibrated meter measures the light arriving at the other end of the fiber.
The equipment used for testing is called an optical time domain reflectometer (OTDR). This device
© 2011 by Taylor and Francis Group, LLC