Page 368 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

360 CHAPTER 13 Communications
Light source
Light detector
Fiber
Coupler
Coupler
FIGURE 13.34
Light source
Light detector
Example of FDM with a fiber-optic WDM simultaneous transmission.
As the demand for more and more bandwidth in our information-hungry industry expands, there are just two options to meet the higher requirements: install more cabling (thus substantially increasing cost) or pack more bandwidth onto individual lines through multiplexing. Multiplexing is quite common in the ROV industry (especially in fiber-optics communications). The two general methods of multiplexing are:
1. multiplexing in the frequency domain and 2. multiplexing in the time domain
Frequency division multiplexing (FDM) makes use of various nonoverlapping frequency slots in order to carry information simultaneously over a single line. As fiber optics uses the moniker “wavelength” (as opposed to “frequency” in all other medium), the acronym WDM is used. Both are the same concept of varying the frequency for capturing the communications channels since the wavelength of a signal is defined as the inverse of its frequency. Figure 13.34 depicts a WDM dia- gram breaking out two communication channels, coupling them for transmission and then decou- pling them at the receiving end. For an example of a typical industry standard course wave division multiplexer (CWDM) board, the Focal 907-CWDM spaces its channels 20 nm apart allowing for
Frequency Y
Filter
Frequency Y
Frequency X
Filter
Frequency X