In fiber-optic communications, WDM (wavelength-division multiplexing)
is a technology which multiplexes a number of optical carrier signals
onto a single optical fiber by using different wavelengths (i.e.,
colors) of laser light. This technique enables bidirectional
communications over one strand of fiber as well as multiplication of
capacity. Generally, WDM technology is applied to an optical carrier
which is typically described by its wavelength.
WDM system uses a
multiplexer at the transmitter to join the signals together, and a
demultiplexer at the receiver to split the signals apart (see Figure 1).
WDM system is very popular in the telecommunication industry because it
allows the capacity of the network to be expanded without laying more
fiber. By utilizing WDM and optical amplifiers, users can accommodate
several generations of technology development in their optical
infrastructure without having to overhaul the backbone network.
Moreover, the capacity of a given link can be expanded simply by
upgrading the multiplexers and demultiplexers at each end.
Figure 1
WDM could be divided into CWDM (coarse wavelength
division multiplexing) and DWDM (dense wavelength division
multiplexing). DWDM and CWDM are based on the same concept of using
multiple wavelengths of light on a single fiber but differ in the
spacing of the wavelengths, number of channels, and the ability to
amplify the multiplexed signals in the optical space. Below part will
introduce some differences between CWDM and DWDM system.
Wavelength Spacing
CWDM provides 8 channels with 8 wavelengths (from 1470nm
through 1610nm) with a channel spacing of 20nm. While DWDM can
accommodate 40, 80 or even 160 wavelengths with narrower wavelength
spans which are as small as 0.8nm, 0.4nm or even 0.2nm (see Figure 2).
Figure 2
Transmission Distance
DWDM multiplexing system is capable of having a longer
haul transmittal by keeping the wavelengths tightly packed. It can
transmit more data over a larger run of cable with less interference
than CWDM system. CWDM system cannot transmit data over long distance as
the wavelengths are not amplified. Usually, CWDM can transmit data up
to 100 miles (160km).
Power Requirements
The power requirements for DWDM are significantly
higher. For instance, DWDM lasers are temperature-stabilized with
Peltier coolers integrated into their module package. The cooler along
with associated monitor and control circuitry consumes around 4W per
wavelength. Meanwhile, an uncooled CWDM laser transmitter uses about
0.5W of power.
