Page 357 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

Input pulse
Step index
Graded index
Single mode
Output pulse
(a)
(b)
(c)
FIGURE 13.22
Index of reflection
200 n μm
50–100 n μm
<10 n μm
13.2 Transmission 349
Light paths (termed “modes”) of optical fiber for (a) multimode, (b) graded index, and (c) single mode.
sensitivity of the receiver. The two general types of losses (termed “attenuation”) are due to the combi- nation of two factors—scattering and absorption. Scattering is the largest of the attenuation factors and occurs when the photons of light collide with the individual atoms in the fiber. The photons of light can be deflected slightly, thus bouncing off the cladding and continuing down the line, or they can be bounced back to the transmitter. Scattering is a function of the wavelength and the longer the wave- length, the lower the scattering loss. Absorption loss, however, is due to the absorption of light into the molecules within the glass and the light energy is converted into heat energy (which cannot be sensed by the optical receiver). The main components in the fiber that absorb the light energy are the dopants and residual OH1 impurities used during the fiber manufacturing process to manipulate the refractivity index of the fiber; therefore, the absorption (Figure 13.23) is reactive within a narrow range of wavelengths (1000, 1400, and .1600 nm). As a result of this absorption phenomenon, standard wavelengths used in fiber-optic data communications equipment are 850 or 1300 nm (for multimode (MM)) and 1310 or 1550 nm (for single mode (SM)).
“Dispersion limitation” involves the accumulation of errors (excessive BER) due to signal cor- ruption as the length of the fiber conductor within the vehicle’s umbilical/tether increases, thus degrading the optical wave pattern. The higher the transmitted optical power, the more likely it will be received on the far end. But the receiver has a certain minimum optical power for sensing the coherent light signals; therefore, BER will decrease as the received optical power is increased until
380 μm 125 μm 125 μm