Mumbai University > Electronics and telecommunication > Sem 7 > optical communication and networks

Marks: 05

Years: MAY 2016

a. Transmission system Model:

Figure 6.1 shows a block diagram of the various components of a unidirectional WDM link.

• The transmitter consists of a set of DFB lasers, with or without external modulators, one for each wavelength. The signals at the different wavelengths are combined into a single fiber by means of an optical multiplexer.

• An optical power amplifier may be used to increase the transmission power. After some distance along the fiber, the signal is amplified by an optical in-line amplifier. Depending on the distance, bit rate, and type of fiber used, the signal may also be passed through a dispersion-compensating module, usually at each amplifier stage.

• At the receiving end, the signal may be amplified by an optical preamplifier before it is passed through a demultiplexer. Each wavelength is then received by a separate photo detector.

• The physical layer of the system must ensure that bits are transmitted from the source to their destination reliably. The measures of quality are the bit error rate (BER) and the additional power budget margin provided in the system. Usually the required bit error rates are of the order of 10 -9 to 10-15, typically 10-12. The BER depends on the amount of noise as well as other impairments that are present in the system.

• Assume that non-return-to-zero (NRZ) modulation is used. In some specific cases, such as chromatic dispersion, both NRZ and return-to-zero (RZ) modulation will be considered. The physical layer is also responsible for the link initialization and link takedown procedures, which are necessary to prevent exposure to potentially harmful laser radiation.