The fiber cable used in telecommunications for telephony transmission is commonly single-mode cable. Single-mode optical fibers are used in long-haul telephone network interchange and feeder loop installations because they easily span between 50 and 100 kilometers and have capacities exceeding 20,000 voice channels. Single-mode is easily identified by its yellow color and the single-mode label on the cable. As we discussed in the how fiber works analogy, single-mode fibers have a small core, concentrating the laser light signal. This signal concentration results in reduced signal loss in the cable, providing longer cable runs between repeaters and higher bits per second transmission speeds.
Single-mode fibers operate similar to microwave RF wave-guide. The wave-guide's physical dimensions approach the wavelength of the electromagnetic radiation being transmitted. This prevents microwaves from traveling through the wave-guide to escape the wave-guide. The same principal is used to construct see-through door panels on microwave ovens. The wire mesh in the door prevents the microwaves from escaping the microwave oven.
Because core diameters of single-mode fiber are small—8.3 microns or less—their light wave propagation path is nearly straight. The nearly straight path permits the light to travel through the fiber, efficiently giving singlemode fiber a higher signal bandwidth (more bits per second capacity). Single-mode fiber core diameters range from 6 to 10 microns. Cladding diameter for these fibers is 125 microns (about 0.005 inch). Single-mode fiber cables constructed using 8.3/125 micron fiber are identified as 8.3/125 single-mode fiber cables (see Figure 6-42).
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