1.5 Multiplexing of Wavelength Channels
19
levels completely in the optical domain. Chapter 10 presents further details on WDM
concepts and components
1.5.2 Polarization Division Multiplexing
As Sect. 3.2.8 describes, signal energy at a given wavelength occupies two orthogonal
polarization modes. The basis of polarization division multiplexing (PDM) is to
impose independent optical signal streams on the two orthogonal polarization states,
thereby doubling the transmission capacity of an optical fiber. The PDM method
generally is used with phase modulation or optical quadrature amplitude modulation
(QAM) techniques thus allowing data rates of 100 Gb/s or more to be sent over a single
optical fiber in a WDM link. A challenge with implementing PDM is to mitigate the
problems of polarization-mode dispersion (see Sect. 3.2.8), polarization-dependent
loss, and cross-polarization modulation. This challenge is addressed through the
use of advanced coding techniques, such as polarization-multiplexed differential
quadrature phase-shift keying (PM-DQPSK) modulation formats (see Sect. 13.4.2).
1.5.3 Optical Fibers with Multiple Cores
Another concept for increasing optical fiber capacity is the technique of space division
multiplexing (SDM) through the use of fibers with multiple cores. In such fibers, each
core provides a spatially isolated transmission path for independent groups of WDM
optical signals. SDM simply multiplies the transmission capacity per fiber by the
number of fiber cores. This condition holds, provided that each SDM channel (each
fiber core) acts independently and has transmission characteristics that are equivalent
to the performance of conventional single-core fibers. For example, the capacity of
a seven-core fiber would be seven times that of a single-core fiber. Section 3.6 gives
some examples of multiple-core optical fibers.
1.6 Basic Elements of Optical Fiber Systems
Similar to electrical communication systems, the basic function of an optical fiber link
is to transport a signal from communication equipment (e.g., a computer, telephone,
or video device) at one location to corresponding equipment at another location with
a high degree of reliability and accuracy. Figure 1.10 shows the 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
19
levels completely in the optical domain. Chapter 10 presents further details on WDM
concepts and components
1.5.2 Polarization Division Multiplexing
As Sect. 3.2.8 describes, signal energy at a given wavelength occupies two orthogonal
polarization modes. The basis of polarization division multiplexing (PDM) is to
impose independent optical signal streams on the two orthogonal polarization states,
thereby doubling the transmission capacity of an optical fiber. The PDM method
generally is used with phase modulation or optical quadrature amplitude modulation
(QAM) techniques thus allowing data rates of 100 Gb/s or more to be sent over a single
optical fiber in a WDM link. A challenge with implementing PDM is to mitigate the
problems of polarization-mode dispersion (see Sect. 3.2.8), polarization-dependent
loss, and cross-polarization modulation. This challenge is addressed through the
use of advanced coding techniques, such as polarization-multiplexed differential
quadrature phase-shift keying (PM-DQPSK) modulation formats (see Sect. 13.4.2).
1.5.3 Optical Fibers with Multiple Cores
Another concept for increasing optical fiber capacity is the technique of space division
multiplexing (SDM) through the use of fibers with multiple cores. In such fibers, each
core provides a spatially isolated transmission path for independent groups of WDM
optical signals. SDM simply multiplies the transmission capacity per fiber by the
number of fiber cores. This condition holds, provided that each SDM channel (each
fiber core) acts independently and has transmission characteristics that are equivalent
to the performance of conventional single-core fibers. For example, the capacity of
a seven-core fiber would be seven times that of a single-core fiber. Section 3.6 gives
some examples of multiple-core optical fibers.
1.6 Basic Elements of Optical Fiber Systems
Similar to electrical communication systems, the basic function of an optical fiber link
is to transport a signal from communication equipment (e.g., a computer, telephone,
or video device) at one location to corresponding equipment at another location with
a high degree of reliability and accuracy. Figure 1.10 shows the 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