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Fiber optic transmission systems and their components

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. 

The transmission of data by optical fiber can be analog or digital, although it is mostly digital. Computer and telephone networks are digital, cable television is currently analog but is migrating to digital, and CCTV systems may also do so.
Both analog and digital transmissions have some common parameters and important differences. For both types of transmission, the optical loss margin or the optical power budget is the most important. Analog data transmissions are tested by measuring the signal-to-noise ratio to determine the link margin, while digital transmissions use the wrong bit rate to measure performance. Both transmissions must be tested over the entire bandwidth specified for the operation; however, currently, most links are specific to a network application,
Design (chassis)
Generally, the design of the transceivers is standard so that multiple sources can be connected to the transmission equipment. The modules connect to a duplex connector on one optical end and a standard electrical interface on the other end. The transceivers receive power from the equipment in which they are integrated.

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