A form of frequency division multiplexing is used as well as TDM to harness
the tremendous bandwidth of fiber optic channels. It is called WDM (Wavelength
Division Multiplexing). Here four fibers come together at an optical combiner, each
with its energy present at a different wavelength. The four beams are combined
onto a single shared fiber for transmission to a distant destination. At the far end,
the beam is split up over as many fibers as there were on the input side. Each output
fiber contains a short, specially constructed core that filters out all but one
wavelength. The resulting signals can be routed to their destination or recombined
in different ways for additional multiplexed transport.
There is really nothing new here. This way of operating is just frequency division multiplexing at very high frequencies, with the term WDM owing to the description of fiber optic channels by their wavelength or ‘‘color’’ rather than frequency. As long as each channel has its own frequency (i.e., wavelength) range and all the ranges are disjoint, they can be multiplexed together on the long-haul fiber. The only difference with electrical FDM is that an optical system using a diffraction grating is completely passive and thus highly reliable.
The reason WDM is popular is that the energy on a single channel is typically only a few gigahertz wide because that is the current limit of how fast we can convert between electrical and optical signals. By running many channels in parallel on different wavelengths, the aggregate bandwidth is increased linearly with the number of channels. Since the bandwidth of a single fiber band is about 25,000 GHz , there is theoretically room for 2500 10-Gbps channels even at 1 bit/Hz (and higher rates are also possible).
WDM technology has been progressing at a rate that puts computer technology to shame. WDM was invented around 1990. The first commercial systems had eight channels of 2.5 Gbps per channel. By 1998, systems with 40 channels of 2.5 Gbps were on the market. By 2006, there were products with 192 channels of 10 Gbps and 64 channels of 40 Gbps, capable of moving up to 2.56 Tbps. This bandwidth is enough to transmit 80 full-length DVD movies per second. The channels are also packed tightly on the fiber, with 200, 100, or as little as 50 GHz of separation. Technology demonstrations by companies after bragging rights have shown 10 times this capacity in the lab, but going from the lab to the field usually takes at least a few years. When the number of channels is very large and the wavelengths are spaced close together, the system is referred to as DWDM (Dense WDM).
There is really nothing new here. This way of operating is just frequency division multiplexing at very high frequencies, with the term WDM owing to the description of fiber optic channels by their wavelength or ‘‘color’’ rather than frequency. As long as each channel has its own frequency (i.e., wavelength) range and all the ranges are disjoint, they can be multiplexed together on the long-haul fiber. The only difference with electrical FDM is that an optical system using a diffraction grating is completely passive and thus highly reliable.
The reason WDM is popular is that the energy on a single channel is typically only a few gigahertz wide because that is the current limit of how fast we can convert between electrical and optical signals. By running many channels in parallel on different wavelengths, the aggregate bandwidth is increased linearly with the number of channels. Since the bandwidth of a single fiber band is about 25,000 GHz , there is theoretically room for 2500 10-Gbps channels even at 1 bit/Hz (and higher rates are also possible).
WDM technology has been progressing at a rate that puts computer technology to shame. WDM was invented around 1990. The first commercial systems had eight channels of 2.5 Gbps per channel. By 1998, systems with 40 channels of 2.5 Gbps were on the market. By 2006, there were products with 192 channels of 10 Gbps and 64 channels of 40 Gbps, capable of moving up to 2.56 Tbps. This bandwidth is enough to transmit 80 full-length DVD movies per second. The channels are also packed tightly on the fiber, with 200, 100, or as little as 50 GHz of separation. Technology demonstrations by companies after bragging rights have shown 10 times this capacity in the lab, but going from the lab to the field usually takes at least a few years. When the number of channels is very large and the wavelengths are spaced close together, the system is referred to as DWDM (Dense WDM).
Wavelength division multiplexing.
One of the drivers of WDM technology is the development of all-optical components.
Previously, every 100 km it was necessary to split up all the channels
and convert each one to an electrical signal for amplification separately before
reconverting them to optical signals and combining them. Nowadays, all-optical
amplifiers can regenerate the entire signal once every 1000 km without the need
for multiple opto-electrical conversions.
We have a fixed-wavelength system. Bits from
input fiber 1 go to output fiber 3, bits from input fiber 2 go to output fiber 1, etc.
However, it is also possible to build WDM systems that are switched in the optical
domain. In such a device, the output filters are tunable using Fabry-Perot or
Mach-Zehnder interferometers. These devices allow the selected frequencies to
be changed dynamically by a control computer. This ability provides a large
amount of flexibility to provision many different wavelength paths through the
telephone network from a fixed set of fibers. For more information about optical
networks and WDM, see Ramaswami et al. (2009).
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