2) Young's modulus (kg/mm2) 12,000 20,000 Elongation (%) 20-30 Coefficient of thermal expansion 1.7 X 10 6 2.3 X 10- 1 x 10-5 Specific heat (cal/°Cg) Reprinted from Ref. [18], p. 53, courtesy of Dekker. CHAPTER 6.1 Optical fiber, cable and connectors DETECTOR TUNGSTEN SAMPLE SPLICE HALOGEN MONOCHROMATOR PIGTAIL CHOPPER CURRENT POINT SENSITIVE PREAMP LOCK-IN AMPLIFIER DESKTOP COMPUTER Fig. 6.1-18 Experimental setup for attenuation measurements by the cutback technique. This same apparatus can be used to measure cutoff wavelength. Reprinted from Ref. [20], p. 336, courtesy of Academic Press. and for a graded-index fiber, with q =2(1-A),as = 6.1.2.4 Mechanical Properties Optical fibers have outstanding signal characteristics such (6.1.36) as low attenuation and small dispersion. For optical fibers to be practically useful they must also have good me- Note that for most fibers N1 = n1 is an excellent ap- chanical properties to withstand environmental strains. proximation [3]. In Table 6.1-5, physical properties for different materials The total fiber disposition in a MM fiber, units nesc/ used in communication cables are compared. km, can be shown to be the following [2]: Silica glass has a very high threshold for breaking when there are no flaws, either on the surface or inside the D2 = Date + mod' (6.1.37) glass. The tensile strength has been estimated to be 20 GPa [19]. Because optical glass fibers have a large where A2 is the spectral bandwidth of the light source. surface-to-volume ratio they are very prone to failure Knowledge of the total dispersion, Dtot, can be trans- from surface flaws that propagate into the glass. The mitted into bandwidth (BW), units MHz . km, as follows actual stress for which the fiber breaks as a function of [17]: the size of the surface flaw is shown in Fig. 6.1-17. When fibers are tested for their mechanical reliability, BW - 3500 (6.1.38) it is common to use short fibers for destructively testing the fibers' breaking threshold for static and dynamic launching system light launching