Objectives: In this chapter, you will learn:
How fiber optic data links and transmission systems work.
What components are used in transceivers?
What types of sources and detectors are used in transceivers?
The performance parameters of fiber optic transmission systems.
Fiber optic data links
Fiber optic transmission systems use data links that work similarly to the one illustrated in the diagram above. Each fiber link consists of a transmitter at one end of the fiber and a receiver at the other. Most systems operate by transmitting in one direction through one fiber and in the opposite direction through another fiber in order to have a two-way transmission. It is possible to transmit in both directions through a single fiber but couplers are needed to do so, and the fiber is less expensive than them.
Most systems use a " transceiver " that includes both a transmitter and a receiver in a single module. The transmitter takes an electrical pulse and converts it into an optical output from a laser diode or LED. The light from the transmitter is coupled to the fiber with a connector and transmitted through the network of fiber optic cables. The fiber end light is coupled to the receiver, where a detector converts the light into an electrical signal that is then conditioned so that it can be used in the receiving equipment.
Analog or digital
The analog signals are continuously variable and the information contained in them is in the amplitude of the signal with respect to time. The digital signals are sampled at regular time intervals and the amplitude is converted to digital bytes, therefore the information is a digital number. Analog signals are the most common form of data transmission, but suffer degradation by the noise present in the transmission system. Because the analog signal is attenuated in a cable, the signal-to-noise ratio worsens and consequently, the signal quality degrades.
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