Snippets on cladding from Hecht's Understanding Fiber Optics.
The key development in making optical fibers usable was a cladding to keep the light from leaking out.
Leakage became a serious problem when engineers realized that a bundle of fibers could carry images from one end to the other as long as the fibers formed the same pattern on each end. Clarence W. Hansell, an American electrical engineer and prolific inventor, patented the concept in the late 1920's. Heinrich Lamm, a German medical student, made the first image-transmitting bundle in 1930. But the images were faint and hazy. When many bare fibers are bundled together, their surfaces touch, so light can leak from one into the others. The fibers also can scratch each other, and light leaks out at the scratches. Light even leaks out where fingerprint oils cling to the glass. . . .
Everyone started by looking at total internal reflection at the boundary between glass and air. However, total internal reflection can occur at any surface where light tries to go from a material with a high refractive index to one with a lower refractive index. Air is convenient, and its refractive index of 1.00029 is much lower than 1.5, that of ordinary glass. But total internal reflection occurs as long as the material covering the glass has a refractive index smaller than the glass. . . Moller Hansen produced total internal reflection by coating glass fibers with margarine, but the results were impractically messy.
Brian O'Brien, a noted American optical physicist, separately suggested the cladding to Abraham van Heel in 1951. Van Heel used beeswax and plastic, which were more practical than margarine. In December 1956, Larry Curtiss, an undergraduate student at the University of Michigan, made the first good glass-clad fibers by melting a tube of low-index glass onto a rod of high-index glass. Glass cladding soon became standard, although a few fibers continue to be plastic-clad, and plastic is used to coat fibers to protect them mechanically. . . .
Virtually all fibers share the same fundamental structure. The center of the fiber is the core, which has a higher refractive index than the cladding that surrounds it . . . The difference in refractive index causes total internal reflection that guides light along the core. As we will see later, this is an oversimplified picture, but it remains the central concept of fiber optics.
The size of core cladding can vary widely. If the goal is to transmit images or light for illumination, the cores are made large and the claddings are thin. The cores typically are much smaller and the claddings are thicker in communication fibers. The boundary between core and cladding may be abrupt or gradual, with the core glass grading into the cladding. Sometimes multiple layers are used. As we will see, adjusting these structures changes the properties of the fibers.
The standard diameter of telecommunication fibers is 125 micrometers, or 0.005 in. A plastic coating increases diameter to about 250 micrometers, easing handling and protecting fiber surfaces from scratches and other mechanical damage. Fibers used for imaging may be as small as several micrometers; some special-purpose fibers may be more than a millimeter (0.04 in.) thick. . .
The two key elements of an optical fiber --- from an optical standpoint --- are its core and cladding. The core is the inner part of the fiber, through which light is guided. The cladding surrounds it completely. The refractive index of the core is higher than that of the cladding, so light in the core that strikes the boundary with the cladding at a glancing angle is confined in the core by total internal reflection. . .
The difference in refractive index between core and cladding need not be large. In practice, it is only about 1%. This still allows light guiding in fibers. Thus, light is confined in the core if it strikes the interface with the cladding at an angle of 8 degrees or less to the surface. The upper limit can be considered the confinement angle in the fiber.
Another way to look at light guiding in a fiber is to measure the fiber's acceptance angle -- the angle over which light rays entering the fiber will be guided along its core. . . The acceptance angle normally is measured as numerical aperture (NA), which for light entering a fiber from air is approximately where no [technical symbol] is the refractive index of the core and n1[another symbol] is the index of the cladding. For a fiber with core index of 1.50 and cladding index of 1.485 (a 1% difference), NA=0.21. . . .
Simple fiber amplifiers have the same core-cladding structure as standard step-index fibers, but the core is doped with erbium (or another light-amplifying species) as well as dopants that change its refractive index.
In fiber amplifiers designed for communication systems, the fiber generally has a small core and transmits only a single mode. Both the pump light at [?pump] and the signal to be amplified at [?Er] are coupled into the core of the fiber. . . Amplifiers are used exclusively with single-mode communication systems, so using single-mode fiber for amplification helps couple light efficiently into and out of the rest of the communication system. In addition, concentrating both pump light and stimulated emission in a small area increases efficiency of light amplification.
As the signal travels along the segment of amplifying fiber, its strength increases. Meanwhile, the pump light is absorbed, so it becomes weaker as the signal grows. In practice,. . . fiber amplifiers include additional elements to make them work better in a communication system.
Other types of fiber structures can be used for applications where fiber amplifiers (or lasers) must generate more power than required for communications. For example, the light to be amplified can be confined in a high-index inner core, and the pump light can be inserted in a larger outer core with refractive index higher than the cladding but lower than the inner core. This guides the pump light along what is in effect a larger waveguide than the guide containing the amplified light. The pump light also passes through the core, so it can excite the light-amplifying atoms there. . .
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Okay, that's enough for now. There's more, so if anyone wants it, just ask.
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