Fiber 394 INDEX

FIGURE 9.22 Single-mode fiber index profiles (Step and segmented-core). (Courtesy of Corning Cable Systems LLC and Corning Inc.)... 

The experiment was carried out by a continuously working Nd YAG-laser fabricated by NEC. The laser has a maximum output of 1200 W and is controlled by handling facility with a linear axle. A stage index fiber optical waveguide with a diameter of d=1000 pm was used for the control of the beam. The focusing optics consist of a focusing lens (f=l 16 mm) and a collimation lens (f=70 mm). 

Plastic fibers Plastic films Plastic hardcoats Plasticity Plasticity index... 

Fig. 3. Types of optical fiber (a) multimode stepped index, (b) multimode graded index, and (c) single-mode stepped index.

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. 

B. MacChesney, D. W. Johnson, P.. Lemaire, L. G. Cohen, and E. M. Rabinovich, "FluorosUicate Substrate Tubes to Eliminate Leaky-Mode Losses in MCVD Single-Mode Fibers with Depressed Index Cladding," paper no. WH2 in Technica/ Digest of Optica/ Fiber Communications Conference, San Diego, Ca/if, Optical Society of America, Washington, D.C., 1985. 

In general, textile fibers should be optically opaque so that their refractive indexes need to be significantly different from those of their most common environments, namely, air and water. Luster and color are two optical properties that relate to a fiber s aesthetic quatity and consumer acceptance. 

The elasticity of a fiber describes its abiUty to return to original dimensions upon release of a deforming stress, and is quantitatively described by the stress or tenacity at the yield point. The final fiber quaUty factor is its toughness, which describes its abiUty to absorb work. Toughness may be quantitatively designated by the work required to mpture the fiber, which may be evaluated from the area under the total stress-strain curve. The usual textile unit for this property is mass pet unit linear density. The toughness index, defined as one-half the product of the stress and strain at break also in units of mass pet unit linear density, is frequentiy used as an approximation of the work required to mpture a fiber. The stress-strain curves of some typical textile fibers ate shown in Figure 5. 

Acryhc fibers discolor and decompose rather than melting when heated, but they have very good color and heat stabihty at temperatures less than I20°C. In a study by American Cyanamid (using Federal Test Specification TT-P-I4Ia. Method 425.2) the yeUowness of acryhc fiber was measured as a function of temperature. Compared to a value of 0.0 for a pure white body, the original fiber had a yeUowness index of 0.04—0.10. After 30 minutes of exposure at II5°C the yeUowness increased only slightly to 0.II—0.17. After 6 h at I30°C, however, the yeUowness increased to 0.38—0.41. 

Another property, used to compare the flammabiUty of textile fibers, is the limiting oxygen index (LOI). This measured quantity describes the minimum oxygen content (%) in nitrogen necessary to sustain candle-like burning. Values of LOI, considered a measure of the intrinsic flammabiUty of a fiber, are Hsted in Table 2 in order of decreasing flammabiUty. 

Table 2. Limiting Oxygen Index of Textile Fibers...

Refractive Index. The refractive index parallel to the fiber axis (s) is 1.478 for acetate and 1.472 for triacetate. The index perpendicular to the axis (co) is 1.473 for acetate and 1.471 for triacetate. The birefringence, ie, the difference between S and CO, is very low for acetate fiber and practically undetectable for triacetate. 

Microscopy (qv) plays a key role in examining trace evidence owing to the small size of the evidence and a desire to use nondestmctive testing (qv) techniques whenever possible. Polarizing light microscopy (43,44) is a method of choice for crystalline materials. Microscopy and microchemical analysis techniques (45,46) work well on small samples, are relatively nondestmctive, and are fast. Evidence such as sod, minerals, synthetic fibers, explosive debris, foodstuff, cosmetics (qv), and the like, lend themselves to this technique as do comparison microscopy, refractive index, and density comparisons with known specimens. Other microscopic procedures involving infrared, visible, and ultraviolet spectroscopy (qv) also are used to examine many types of trace evidence. 

Lithium Niobate. Lithium niobate [12031 -64-9], LiNbO, is normally formed by reaction of lithium hydroxide and niobium oxide. The salt has important uses in switches for optical fiber communication systems and is the material of choice in many electrooptic appHcations including waveguide modulators and sound acoustic wave devices. Crystals of lithium niobate ate usually grown by the Czochralski method foUowed by infiltration of wafers by metal vapor to adjust the index of refraction. 

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