2015光纤通信填空

1The main constituents of an optical fiber communications link . The key sections are a transmitter consisting of a light source and its associated drive circuitry, a cable offering mechanical and environmental protection to the optical fibers contained inside, and a receiver consisting of a photodector plus amplification and signal-restoring circuitry.
2 Attenuation of a light signal as it propagates along a fiber is an important consideration in the design of an optical communication system, the basic attenuation mechanisms in a fiber are absorption, scattering, and radiative losses of the optical energy.
3 Intermodal dispersion or modal delay appears only in multimode fibers. This signal-distorting mechanism is a result of each mode having a different value of the group velocity at a single frequency.
4 In general, LEDs are used with multimode fibers, since normally t is only into a multimode fiber that the incoherent optical output power from an LED can be coupled in sufficient quantities to be useful.
5 At the output end of an optical transmission line, there must be a receiving device that interprets the information contained in the optical signal. The first element of this receiver is a photodetecor. It senses the luminescent power falling upon it and converts the variation of this optical power into a corresponding varying electric current.
6 An optical receiver consists of a photodetector , an amplifier, and signal-processing circuitry. The receiver has task of first converting the optical energy emerging from the end of a fiber into an electric signal, and then amplifying this signal to a large enough level.
7 A characteristic of WDM is that the discrete wavelengths form an orthogonal set of carriers that can be separated, routed ,and switched without interfering with each other.
8 Chromatic dispersion causes pulse broadening , which leads bit-error rates.
9 The function of an OADM is to insert or extract one or more selected wavelengths at a designated point in an optical network.
10 Although the structure of such an optical amplifier is similar to that of
a laser, it does not have the optical feedback mechanism. thus an amplifier can boost incoming signal levels, but it cannot generate a coherent optical output by itself.
11 The pump light is usually injected from the same direction as the signal flow, this is known as codirectional pumping
12 In any fiber transmission system using optical amplifiers, the dynamic characteristics of network traffic, particularly in a metro network, can create large, rapid fluctuations in the input optical power level to an EDFA.
13 The SONET and SDH rings are commonly called self-healing rings because the traffic flowing along a certain path can automatically be switched to an alternate or standby path following failure or degradation of the link segment.
14 The performance characteristics of OBS lie between those of a wavelength-routed network and an optical packet-switched network
15 In single-mode fibers, chromatic and polarization-mode dispersions are important factors that can limit the transmission distance or data rate 16 Polarization-mode dispersion is important for data rates above 10Gb/s, since its statistical behavior ultimately can limit the highest achievable data rate.
17 When a link is being installed and tested, the operational parameters of interest include bit-error rate, timing jitter, and signal-to-noise ratio as indicated by the eye pattern.。