Page 355 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

FIGURE 13.20
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2
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13.2 Transmission 347
Table 13.1 Refractive Indexes of Various Translucent Materials
Medium Refractive Index (n) Critical Angle (θc) with Air
Vacuum
Air
Water
Ethanol Benzene Fused silica Crown glass Polycarbonate Diamond
1.000 (per definition) 1.0003
1.33
1.36
1.501 1.458 1.52 1.5849 2.42
Undefined 90 48.77 47.35 41.79 43.32 41.15 39.13 24.41
Components of a typical single mode optical fiber (1, core at 9 μm; 2, cladding at 125 μm; 3, buffer at 250 μm; 4, jacket at 400 μm).
The glass or plastic core is surrounded by a cladding of optical material (Figure 13.20) with differ- ing refractivity index than the core, allowing the light to be “trapped” (termed “total internal reflec- tance”) within the core of the fiber. The outer buffer protects the core and cladding from abrasion and moisture and is the coating that is stripped off during cable termination. The cladding is of a different refractivity index so as to either bend the light back toward the core (thus “steering” the light down the core’s “channel”) with single or graded index fiber or to totally reflect it within the core (for multimode fiber) to keep it encapsulated within the core.
The term “mode” with regard to optical fiber describes the path taken by a photon of light as it transits down the fiber. In multimode fiber, the photon will take any number of paths as it is bounced around within the core toward the receiver. This multimode technique is much cheaper than the other techniques as the fiber need only be coated with an imprecise reflective coating for “bouncing” the reflections down the fiber. The core and cladding sizes are typically 50 and 125 μm, respectively (Figure 13.21). With graded index fiber, the cladding is layered optical mate- rial with gradually changing characteristics to bend the light back down the channel in a pattern