10
1 Perspectives on Lightwave Communications
peak around 1400 nm. This process opens the E-band (1360-to-1460 nm) transmission region to provide around 100 nm more spectral bandwidth in these specially
fabricated fibers than in conventional single-mode fibers.
Systems operating at 1550 nm provide the lowest attenuation, but the signal dispersion as a function of distance in a standard silica fiber is larger at 1550 nm than at
1310 nm. Manufacturers overcame this limitation first by creating dispersion-shifted
fibers for single-wavelength operation and then by devising non-zero dispersionshifted fiber (NZDSF) for use with multiple-wavelength implementations. The latter
fiber type has led to the widespread use of multiple-wavelength S-band and C-band
systems for high-capacity, long-span terrestrial and undersea transmission links.
These links routinely carry traffic at 10 Gb/s over nominally 90 km distances between
amplifiers or repeaters. By 2010 links operating at 100 Gb/s were being installed and
in 2017 the IEEE P802.3bs Task Force ratified the 400GbE (Gigabit Ethernet) standard. This standard established the foundation for industrial deployment of 400GbE
in the global network [26–28].
1.3 Decibel Notation
As the following chapters of this book describe, a critical consideration when
designing and implementing an optical fiber link is to establish, measure, and/or
interrelate the optical signal levels at each of the elements of a transmission link.
Thus it is necessary to know parameter values such as the optical output power from
a light source, the power level needed at the receiver to properly detect a signal,
and the amount of optical power lost at each of the constituent elements of the
transmission link.
Reduction or attenuation of signal strength arises from various loss mechanisms in
a transmission medium. For example, electric power is lost through heat generation
as an electric signal flows along a wire, and optical power is attenuated through
scattering and absorption processes in a glass or plastic fiber or in an atmospheric
channel. To compensate for these energy losses, amplifiers are used periodically
along a channel path to boost the signal level, as shown in Fig. 1.5.
A standard and convenient method for measuring attenuation through a link or
a device is to reference the output signal level to the input level. For guided media
such as an optical fiber, the signal strength normally decays exponentially. Thus
for convenience one can designate signal attenuation or amplification in terms of a
logarithmic power ratio measured in decibels (dB). The dB unit is defined by
Power ratio in dB = 10 log
P 2
P 1
(1.4)
where P 1 and P 2 are the electrical or optical power levels of a signal at points 1 and
2 in Fig. 1.6, and log is the base-10 logarithm. The logarithmic nature of the decibel
allows a large ratio to be expressed in a fairly simple manner. Power levels differing
1 Perspectives on Lightwave Communications
peak around 1400 nm. This process opens the E-band (1360-to-1460 nm) transmission region to provide around 100 nm more spectral bandwidth in these specially
fabricated fibers than in conventional single-mode fibers.
Systems operating at 1550 nm provide the lowest attenuation, but the signal dispersion as a function of distance in a standard silica fiber is larger at 1550 nm than at
1310 nm. Manufacturers overcame this limitation first by creating dispersion-shifted
fibers for single-wavelength operation and then by devising non-zero dispersionshifted fiber (NZDSF) for use with multiple-wavelength implementations. The latter
fiber type has led to the widespread use of multiple-wavelength S-band and C-band
systems for high-capacity, long-span terrestrial and undersea transmission links.
These links routinely carry traffic at 10 Gb/s over nominally 90 km distances between
amplifiers or repeaters. By 2010 links operating at 100 Gb/s were being installed and
in 2017 the IEEE P802.3bs Task Force ratified the 400GbE (Gigabit Ethernet) standard. This standard established the foundation for industrial deployment of 400GbE
in the global network [26–28].
1.3 Decibel Notation
As the following chapters of this book describe, a critical consideration when
designing and implementing an optical fiber link is to establish, measure, and/or
interrelate the optical signal levels at each of the elements of a transmission link.
Thus it is necessary to know parameter values such as the optical output power from
a light source, the power level needed at the receiver to properly detect a signal,
and the amount of optical power lost at each of the constituent elements of the
transmission link.
Reduction or attenuation of signal strength arises from various loss mechanisms in
a transmission medium. For example, electric power is lost through heat generation
as an electric signal flows along a wire, and optical power is attenuated through
scattering and absorption processes in a glass or plastic fiber or in an atmospheric
channel. To compensate for these energy losses, amplifiers are used periodically
along a channel path to boost the signal level, as shown in Fig. 1.5.
A standard and convenient method for measuring attenuation through a link or
a device is to reference the output signal level to the input level. For guided media
such as an optical fiber, the signal strength normally decays exponentially. Thus
for convenience one can designate signal attenuation or amplification in terms of a
logarithmic power ratio measured in decibels (dB). The dB unit is defined by
Power ratio in dB = 10 log
P 2
P 1
(1.4)
where P 1 and P 2 are the electrical or optical power levels of a signal at points 1 and
2 in Fig. 1.6, and log is the base-10 logarithm. The logarithmic nature of the decibel
allows a large ratio to be expressed in a fairly simple manner. Power levels differing