Page 358 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition
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350 CHAPTER 13 Communications
Absorption
850 1300 1550
Wavelength
FIGURE 13.23
Combined attenuation within the typical optical fiber.
BER of a fiber-optic link
Receiver optical power
FIGURE 13.24
Bit error rate (BER) as a function of receiver optical power.
the optical power begins to overpower the receiver (Figure 13.24). For our purposes, BER decreases proportionally with the increase in received optical power.
Typical fiber types and specifications are provided in Table 13.2.
13.2.5.1 Enough with the theory! What’s important?
Fiber-optical data communication systems used in ROVs (Figure 13.25) consist of the following components:
1. The optical transmitter (LED or laser)
2. The cable (MM-GI, MM-SI, SM, POF)
3. Connectors (temporarily join strands of fiber)
4. Splices (permanently join strands of fiber)
5. Hardware (for mounting of connectors, splices, and routing cables)
6. Test equipment (OTDR, test light, etc. used to test continuity and performance)
The transmitter (Figure 13.26) comes in two varieties—light emitting diodes (LED) and semi-
conductor lasers (laser). The LED transmitter produces incoherent light with a fairly wide spectral width (30 2 60 nm) and is of a much lower performance than the laser and is generally used only with MM fiber systems. As the SM fiber technique will typically be employed in the subsea indus- try, the focus will be on the laser transmitter with the use of SM fiber. In the laser diode, an electric current is injected (as opposed to optically pumped for the LED) into a pn junction, thus emitting light at a very narrow-band and discrete frequency/wavelength in a coherent fashion.
BER
Attenuation
Scattering
