What
is WDM?
In the same optical fiber at the same time can let two or more
than two wavelength signal transmit and receive information through different
optical channel, called wavelength division multiplex, referred to as WDM. Wavelength
division multiplexing includes frequency division multiplexing and wavelength
division multiplexing. Optical frequency division multiplexing (FDM) technology
and optical wavelength division multiplexing (WDM) technology has no obvious
difference, because the light is part of the electromagnetic wave, frequency
and wavelength of light have a single correspondence. Usually also can understand so, optical frequency division
multiplexing mean subdivision of optical frequency, very dense optical channel.
Wavelength division multiplexing means divided frequency of light, light
channel far apart, even in the optical fiber with different window.
The general application of division multiplexing wavelength is
respectively using a wavelength division multiplexer and demultiplexer arranged at both ends of the optical fiber, coupling
and separation of different wavelength. The main four types of WDM are fused biconical taper type, dielectric
film type, FBG type and planar waveguide grating type .The main characteristic
is the insertion loss and isolation. Usually, the optical link using wavelength division multiplexing
equipment, increase the amount of optical link loss is called WDM insertion
loss. When the wavelength transmission through the same optical fiber, the
D-value between the splitter input mixed power and the output end of the fiber
power is called isolation.. The following are characteristics and advantages of
optical wavelength division multiplexing technical:
(1) Make full use of low loss band fiber, increase the
transmission capacity of optical fiber, the physical limit of an optical fiber
for transmitting information doubled to several times. At present, we only use the
low loss optical fiber spectrum (1310nm-1550nm) a few, WDM can fully utilize
the huge bandwidth of single-mode fiber is about 25THz, the transmission
bandwidth is sufficient.
(2) There are ability to transmit two or more than two asynchronous signal in
the same optical fiber ,there are compatible for digital and analog signals,
has nothing to do with the data rate and modulation mode, the middle line can
be removed or added channel.
(3) About the optical fiber system that has built, especially
early laying optical cable that core number not much, as long as the original
system power is margin, we can increase the capacity; realize the transmission
of multiple one-way or two-way signals without making big changes to the
original system, so it has strong flexibility.
(4) Due to the large number of reducing use amount of the fiber,
it can greatly reduce the construction cost, because the fiber quantity is
less, when a fault occurs, the recovery is also fast and convenient.
(5) Sharing of active optical devices, the cost of transmission
of multiple signals or increase new business will reduce.
(6)The active devices in the system have been substantially
reduced, which improves the reliability of the system. At present, because of
the light multi carrier division multiplexing of optical transmitter, optical
receiver equipment's requirements higher, technology implementation has certain
difficulty, also multiple core cable used in traditional broadcast television
transmission business does not appear especially shortage, so the practical
application of WDM is still not much. However, with the development of CATV integrated service
development, the growing demand for network bandwidth, all kinds of selective
service upgrade and network implementation economic cost considerations and so
on, the characteristics and advantages of WDM in the CATV transmission system
gradually emerged, showing broad application prospects, even influence the
development pattern of CATV network.
DWDM (dense wavelength division multiplexing) is undoubtedly the
first choice technology in the field of fiber optic applications today, But the cause of high cost make many do not bounteous operators
hesitating. Is there a lower cost
for using the wavelength division multiplexing technology? In the face of this
demand, CWDM (coarse wavelength division multiplexing) emerges as the times
require.
CWDM, just as its name implies, is a dense wavelength division
multiplexing next of kin, the difference between CWDM and DWDM mainly has two
points: first, the CWDM carrier channel spacing is wider, therefore, light in a
single fiber can reuse about 5 to 6 wavelengths, that is where the "dense" and "coarse"
appellation come from; Two,
CWDM modulation laser using uncooled laser, but DWDM is used in cooling laser.
Cooling laser using temperature tuning, uncooled laser adopts electronic
tuning. Because the range of
temperature distribution is nonuniform in a very wide wavelength, so the
temperature tuning is very difficult to realize, the cost is very high. CWDM
avoids this problem, so it greatly reduces the cost; the whole CWDM system cost
only 30% of DWDM.
CWDM provides a very high access bandwidth with a low cost,
suitable for point to point, Ethernet, SONET rings and all kinds of popular
network structure, especially suitable for short distance, high bandwidth,
access point intensive, communication applications, such as network
communication between the building or building. It is particularly worth mentioning is that CWDM with the use of
PON (passive optical network).PON is a cheap, one-point to multi-point optical
fiber communication mode, in combination with the CWDM, each individual
wavelength channel can be used as virtual optical link of PON, Implementation of broadband data transmission between center
node and multiple distributed nodes.
There are several companies are offering CWDM related products
at present. However, CWDM is a
product of cost and performance tradeoffs; inevitably there are some
performance limitations. Industry experts point out, at present the CWDM have four
following disadvantages: first, CWDM in a single fiber support multiplexing
wavelength number is minor, leading to future expansion cost is high; second,
multiplexing, multiplexing equipment cost should also be reduced, the device
cannot be simply modified of DWDM corresponding equipment; third, CWDM does not
apply to metropolitan area network, the distance between metropolitan area
network nodes is short , the money that operators use in CWDM equipment
expansion can be used to laying more fiber, and get better effect; fourth, CWDM
has not yet formed standards.
What’s
more, something about the WDM products.
(1)CWDM
Mux/Demux module
CWDM Mux and CWDM Demux
are designed to multiplex multiple CWDM channels into one or two fibers. The
core of CWDM Module application is the passive MUX DEMUX unit. The common configuration
is 1×4, 1×8, 1×16 channels. Available in 19″ Rack Mount or LGX module package.
Optional wide band port is available to multiplex with CWDM Channels
wavelength.
(2)DWDM
Mux/Demux Modules
DWDM Mux and DWDM DeMux are designed to multiplex multiple DWDM
channels into one or two fibers. The common configuration is 4, 8, 16 and 40
channels. These modules passively multiplex the optical signal outputs from 4
or more electronic devices, send them over a single optical fiber and then
de-multiplex the signals into separate, distinct signals for input into
electronic devices at the other end of the fiber optic link.
(3)Optical
Splitter-- a important component in EPON network
Optical splitter in optical communication era is a component of
EPON network construction, is a connection of OLT and ONU passive device.
Its function is to distribute the downlink data, and focus on
the uplink data. Optical splitter has an upstream optical interface, a
plurality of downlink optical interface. Optical signals from the upstream
optical interface over was assigned to the downstream optical interface out all
transmissions, optical signals from the downlink optical interface over being
allocated to uplink optical interface out transmission only. The light
intensity signal downlink optical interface of each can be same, can also be
different.
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