1.6 Basic Elements of Optical Fiber Systems
23
two or more distinct wavelengths onto the same fiber (or that separate the wavelengths into individual channels at the receiving end) in multiple-wavelength optical
fiber networks, and couplers used to tap off a certain percentage of light, usually for
performance monitoring purposes. In addition, modern sophisticated optical fiber
networks contain a wide range of active optical components, which require an electronic control for their operation. These include light signal modulators, tunable
(wavelength-selectable) optical filters, reconfigurable elements for adding and dropping wavelengths at intermediate nodes, variable optical attenuators, and optical
switches.
Chapters 11 through 13 address factors associated with implementing optical
telecom networks. After an optical signal has traveled a certain distance along a
fiber, it becomes greatly weakened due to power loss along the fiber. Therefore,
when setting up an optical link, engineers formulate a power budget and add amplifiers or repeaters when the path loss exceeds the available power margin. The periodically placed amplifiers merely give the optical signal a power boost, whereas a
repeater also will attempt to restore the signal to its original shape. Prior to 1990, only
repeaters were available for signal amplification. For an incoming optical signal, a
repeater performs photon-to-electron conversion, electrical amplification, retiming,
pulse shaping, and then electron-to-photon conversion. This process can be fairly
complex for high-speed multiple-wavelength systems. Thus researchers expended a
great deal of effort to develop all-optical amplifiers, which boost the light power level
completely in the optical domain. Optical amplification mechanisms for WDM links
include the use of devices based on rare-earth-doped lengths of fiber and distributed
amplification by means of a stimulated Raman scattering effect.
The installation and operation of an optical fiber communication system require
measurement techniques for verifying that the specified performance characteristics
of the constituent components are satisfied. Chapter 14 addresses these techniques.
In addition to measuring optical fiber parameters, system engineers are interested
in knowing the characteristics of passive splitters, connectors, and couplers, and
electro-optic components, such as sources, photodetectors, and optical amplifiers.
Furthermore, when a link is being installed and tested, operational parameters that
should be measured include bit error rate, timing jitter, and signal-to-noise ratio as
indicated by the eye pattern. During actual operation, measurements are needed for
maintenance and monitoring functions to determine factors such as fault locations
in fibers and the status of remotely located optical amplifiers.
1.7 Evolution of Fiber Optic Networks
Optical networking technology has made tremendous advances since the first basic
links were installed to carry live traffic around 1978. The initial installations operated at 6.3 Mb/s over distances of about 10 km using simple on-off keying (OOK)
modulation in the transmitter. As shown in Fig. 1.12, there has been a steady 40–
50% growth per year in link data rates and transmission distances since then. Until
23
two or more distinct wavelengths onto the same fiber (or that separate the wavelengths into individual channels at the receiving end) in multiple-wavelength optical
fiber networks, and couplers used to tap off a certain percentage of light, usually for
performance monitoring purposes. In addition, modern sophisticated optical fiber
networks contain a wide range of active optical components, which require an electronic control for their operation. These include light signal modulators, tunable
(wavelength-selectable) optical filters, reconfigurable elements for adding and dropping wavelengths at intermediate nodes, variable optical attenuators, and optical
switches.
Chapters 11 through 13 address factors associated with implementing optical
telecom networks. After an optical signal has traveled a certain distance along a
fiber, it becomes greatly weakened due to power loss along the fiber. Therefore,
when setting up an optical link, engineers formulate a power budget and add amplifiers or repeaters when the path loss exceeds the available power margin. The periodically placed amplifiers merely give the optical signal a power boost, whereas a
repeater also will attempt to restore the signal to its original shape. Prior to 1990, only
repeaters were available for signal amplification. For an incoming optical signal, a
repeater performs photon-to-electron conversion, electrical amplification, retiming,
pulse shaping, and then electron-to-photon conversion. This process can be fairly
complex for high-speed multiple-wavelength systems. Thus researchers expended a
great deal of effort to develop all-optical amplifiers, which boost the light power level
completely in the optical domain. Optical amplification mechanisms for WDM links
include the use of devices based on rare-earth-doped lengths of fiber and distributed
amplification by means of a stimulated Raman scattering effect.
The installation and operation of an optical fiber communication system require
measurement techniques for verifying that the specified performance characteristics
of the constituent components are satisfied. Chapter 14 addresses these techniques.
In addition to measuring optical fiber parameters, system engineers are interested
in knowing the characteristics of passive splitters, connectors, and couplers, and
electro-optic components, such as sources, photodetectors, and optical amplifiers.
Furthermore, when a link is being installed and tested, operational parameters that
should be measured include bit error rate, timing jitter, and signal-to-noise ratio as
indicated by the eye pattern. During actual operation, measurements are needed for
maintenance and monitoring functions to determine factors such as fault locations
in fibers and the status of remotely located optical amplifiers.
1.7 Evolution of Fiber Optic Networks
Optical networking technology has made tremendous advances since the first basic
links were installed to carry live traffic around 1978. The initial installations operated at 6.3 Mb/s over distances of about 10 km using simple on-off keying (OOK)
modulation in the transmitter. As shown in Fig. 1.12, there has been a steady 40–
50% growth per year in link data rates and transmission distances since then. Until