8
1 Perspectives on Lightwave Communications
on using the 1260–1675 nm spectral region, because optical fibers exhibit low optical
power losses in this range. Then in 2012 the National Institute of Information and
Communications Technology (NICT) of Japan proposed using the 1000–1260 nm
spectral band, which is designated as the T-band. Here the symbol “T” stands for
“thousand.” The motivation for examining this spectral band was for addressing the
rapidly increasing need for moderate distance links within data centers, the developments of Internet of Things (IoT), and the support of fifth-generation (5G) wireless
systems [25]. Although the optical signal attenuation in standard telecom fibers is
higher in the T-band compared to the ITU-T bands, the losses are tolerable for the
relatively short transmission distances of up to several kilometers used for applications in the T-band. However, other fiber types with potentially lower losses in the
T-band are being considered.
The 770-to-910 nm band is used for shorter-wavelength multimode fiber systems.
Thus this region is designated as the short-wavelength or multimode fiber band.
Later chapters describe the operational performance characteristics and applications
of optical fibers, electro-optic components, and other passive optical devices for use
in the short- and long-wavelength bands.
1.2.2 Optical Windows and Spectral Bands
Figure 1.4 shows the operating range of optical fiber systems and the characteristics
of the four key components of a link: the optical fiber, light sources, photodetectors,
and optical amplifiers. Here the dashed vertical lines indicate the centers of the three
main legacy operating wavelength bands of optical fiber systems, which are the shortwavelength region, the O-band, and the C-band. One of the principal characteristics
of an optical fiber is its attenuation as a function of wavelength, as shown at the top
in Fig. 1.4. Early applications in the late 1970s made exclusive use of the 770-to910 nm wavelength band where there was a low-loss window and GaAlAs optical
sources and silicon photodetectors operating at these wavelengths were available.
Originally this region was referred to as the first window because around 1000 nm
there was a large attenuation spike due to absorption by water molecules. As a result
of this spike, early fibers exhibited a local minimum in the attenuation curve around
850 nm.
By reducing the concentration of hydroxyl ions (OH–) and metallic impurities
in the fiber material, in the 1980s manufacturers could fabricate optical fibers with
very low losses in the 1260-to-1675 nm region. This spectral band is called the longwavelength region. Because the glass still contained some water molecules, initially
a third-order absorption spike remained around 1400 nm. This spike defined two
low-loss windows, these being the second window centered at 1310 nm and the third
window centered at 1550 nm. These two windows now are called the O-band and
C-band, respectively.
The desire to use the low-loss long-wavelength regions prompted the development of InGaAsP-based light sources, InGaAs photodetectors, and InGaAsP optical
1 Perspectives on Lightwave Communications
on using the 1260–1675 nm spectral region, because optical fibers exhibit low optical
power losses in this range. Then in 2012 the National Institute of Information and
Communications Technology (NICT) of Japan proposed using the 1000–1260 nm
spectral band, which is designated as the T-band. Here the symbol “T” stands for
“thousand.” The motivation for examining this spectral band was for addressing the
rapidly increasing need for moderate distance links within data centers, the developments of Internet of Things (IoT), and the support of fifth-generation (5G) wireless
systems [25]. Although the optical signal attenuation in standard telecom fibers is
higher in the T-band compared to the ITU-T bands, the losses are tolerable for the
relatively short transmission distances of up to several kilometers used for applications in the T-band. However, other fiber types with potentially lower losses in the
T-band are being considered.
The 770-to-910 nm band is used for shorter-wavelength multimode fiber systems.
Thus this region is designated as the short-wavelength or multimode fiber band.
Later chapters describe the operational performance characteristics and applications
of optical fibers, electro-optic components, and other passive optical devices for use
in the short- and long-wavelength bands.
1.2.2 Optical Windows and Spectral Bands
Figure 1.4 shows the operating range of optical fiber systems and the characteristics
of the four key components of a link: the optical fiber, light sources, photodetectors,
and optical amplifiers. Here the dashed vertical lines indicate the centers of the three
main legacy operating wavelength bands of optical fiber systems, which are the shortwavelength region, the O-band, and the C-band. One of the principal characteristics
of an optical fiber is its attenuation as a function of wavelength, as shown at the top
in Fig. 1.4. Early applications in the late 1970s made exclusive use of the 770-to910 nm wavelength band where there was a low-loss window and GaAlAs optical
sources and silicon photodetectors operating at these wavelengths were available.
Originally this region was referred to as the first window because around 1000 nm
there was a large attenuation spike due to absorption by water molecules. As a result
of this spike, early fibers exhibited a local minimum in the attenuation curve around
850 nm.
By reducing the concentration of hydroxyl ions (OH–) and metallic impurities
in the fiber material, in the 1980s manufacturers could fabricate optical fibers with
very low losses in the 1260-to-1675 nm region. This spectral band is called the longwavelength region. Because the glass still contained some water molecules, initially
a third-order absorption spike remained around 1400 nm. This spike defined two
low-loss windows, these being the second window centered at 1310 nm and the third
window centered at 1550 nm. These two windows now are called the O-band and
C-band, respectively.
The desire to use the low-loss long-wavelength regions prompted the development of InGaAsP-based light sources, InGaAs photodetectors, and InGaAsP optical