Fiber optic transmission systems

In 1980, video signals were carried by optical fibers 2 4 miles (4 kilometers) for the Winter Olympic Games in Lake Placid, New Yotk. The first long-haul intercity installations (AT T, Washington-New York New York-Boston) were made in 1983. After that, the capacity of fiber optic transmission systems increased exponentially. Despite this progress, the fundamental limits predicted by the physics of photonics materials, devices, and systems have not yet been approached (Kogelnik). The challenge of future research and development continues to be a fuller exploitation of the ultimate capacity of optical fibers. 

Today fiber-optic transmission systems offer several advantages over conventional copper wire and coaxial cable systems. Among these are increased bandwidth, smaller size, lower weight, lack of crosstalk, and a very low susceptibility to electromagnetic interference. It is to be expected that these advantages will open widespread application of fiber-optic transmission systems in the future. This seems to be supported by the fact that a great number of public and in-house trial systems are under test all over the world. 

Lithium niobate modulators have been used for a number of years in high capacity fiber optic transmission systems. The combination of an external modulator and a CW laser produces a more noise-free signal than a diode laser modulated by current drive. In spite of this advantage, large-scale use of modulators was not realized until recently because of earlier stability problems (Ko-rotky and Veselka, 1996). 

This is a development of the above where a fiber-optic linked hqnid sample transmission cell is integrated with the sample fast loop cabinet (Figures 5.24 and 5.25). There can be multiple sample streams, take-offs and fast loops, each with its own separate fiber-optic transmission cell. The analyzer can either be local with short fiber-optic runs to the sampling cabinet(s), or remote, where a safe area location for the analyzer module may be feasible, but at the cost of longer, potentially less stable fiber-optic runs. This system avoids physical stream switching. 

In January 1992. E. Desurvire (Columbia University Center for Telecommunications Research) reported that optical fibers made from silica glass and traces of erbium can amplify light signals when they are energized by infrared radiation. Desurvire developed an efficient radiation source (referred (o as a laser diode chip) that, when integrated into a fiber optic communication system, can increase transmission capacity by a factor of 10(1. 

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The major elements required for fiber-optics transmission include long flexible fibers made of transparent materials such as glass, plastic, or plastic-clad silica a light-transmittal source such as a laser of light-emitting diode (LED) cables or rods lined with a reflective core medium to direct light and a receiver to capture the signal. Many systems... 

Viable glass fibers for optical communication are made from glass of an extremely high purity as well as a precise refractive index stmcture. The first fibers produced for this purpose in the 1960s attempted to improve on the quahty of traditional optical glasses, which at that time exhibited losses on the order of 1000 dB/km. To achieve optical transmission over sufficient distance to be competitive with existing systems, the optical losses had to be reduced to below 20 dB/km. It was realized that impurities such as transition-metal ion contamination in this glass must be reduced to unprecedented levels (see Fig. 

The medium used for the transmission of information and data over distances has evolved from copper wire to optical fiber. It is quite likely that no wire-based information transmission systems will be installed in the future. The manufacture of optical fibers, like that of microcircuits, is almost entirely a chemical process. 

Because optical fibers are nonconducting, fiber optic systems provide excellent electrical isolation and immunity from electrical interference. Signal losses are much lower in fibers (as low as 0.20 dB/km) compared to other guided transmission media, such as twisted copper pairs, coaxial cable, and metallic waveguides. In addition, the bandwidth or information carrying capacity of fibers is far greater. When one or more optical fibers are packaged into cables, the cables are smaller and more flexible than their metallic counterparts. 

Various types of optical fibers are used for specific applications. For example, multimode fibers are used primarily in enterprise systems buildings, offices, campuses. Special single-mode transmission fibers exist for submarine applications, and for metropolitan and long-haul terrestrial applications. And in addition to these transmission fibers, there are various specialty fibers for performing dispersion compensation (dispersion compensating fiber), optical amplification (erbium-doped fiber), and other special functions. 

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