This article will include these subject. What does WDM stand for? The basic structure of WDM system Advantages of WDM technology What does Mux and Demux stand for? The difference between WDM and optical splitter The indicators that affect the WDM devices How to understand the O, E, S, C, L, U band What does CWDM stand for vs. DWDM, FWDM, LWDM, MWDM? HYC can provide customers with a one-stop optical network device and low-cost optical communication products, supplying a range of WDM products. HYC Co.,Ltd（HYC）is a national Hi-tech optoelectronics company engaged in R&D, manufacture and marketing of fiber optical products. Providing professional product and service for fiber connectivity,WDM, PLC splitter and high density datacom cabling. HYC products and solutions widely applied in 4G/5G, Data Center and Cloud Computing industry etc.

1. What does WDM (Wavelength Division Multiplexing )stand for?
This article will include these subject.
What does WDM stand for?
The basic structure of WDM system
Advantages of WDM technology
What does Mux and Demux stand for?
The difference between WDM and optical splitter
The indicators that affect the WDM devices
How to understand the O, E, S, C, L, U band
What does CWDM stand for vs. DWDM, FWDM, LWDM, MWDM?
What does WDM stand for?
Wavelength Division Multiplexing(WDM) is one of the most common way of using wavelengths to
increase bandwidth by multiplexing various optical carrier signals onto a single optical fiber. It
combines a series of optical carrier signals with different wavelengths carrying various
information and coupled to the same optical fiber for transmission at the transmitting end. At the
receiving end, optical signals of various wavelengths are separated by a demultiplexer. This
technique of simultaneously transmitting two or many different wavelengths in the same fiber is
called wavelength division multiplexing, or WDM.
As shown in the figure below, the traditional optical transmission method is that one fiber can
only transmit one wavelengths of signal in a single time. If you want different services, you need
countless different and independent optical fibers for transmission. However, if there is a large
amount of services, a large number of optical fibers need to be laid for transmission, which poses
a great challenge to cabling space and cost. The application of a WDM system can quickly solve
the above problems. The WDM system can carry multiple signals through multiplexing and
demultiplexing technologies, such as ATM, IP, etc., and multiple service signals can be transmitted
through a single optical fiber, which greatly reduces the amount of optical fiber. The WDM system
can carry multiple signals, such as ATM, IP, etc., through multiplexing and demultiplexing
technology, the multiple service signals can be transmitted through a single optical fiber, which
greatly reduces the amount of optical fiber. This is an ideal technology for capacity expansion.
When introducing new broadband services such as CATV, HDTV, B-ISDN, etc., only one additional
wavelength needs to be added.
The basic structure of WDM system

2. The basic structure of the WDM system is mainly divided into two modes: dual-fiber
unidirectional transmission and single-fiber bidirectional transmission.
Unidirectional WDM is the transmission of all optical channels on a fiber propagating
simultaneously in the same direction. Different wavelengths carry different optical signals, which
are combined at the transmitting end for transmission through an optical fiber, and
demultiplexed at the receiving end to complete multiple paths. In the opposite direction, a
second optical fiber is needed. The transmission in the two directions is completed by two optical
fibers respectively.
Bidirectional WDM is the transmission of optical channels on a fiber propagating simultaneously
in both directions, and the wavelengths used are separated from each other to achieve
full-duplex communication between the two parties.
Unidirectional WDM
Bidirectional WDM
The general WDM system is mainly composed of five parts: network management system, optical
transmitter, optical relay amplifier, optical receiver, and optical monitoring channel.

3. The overall structure of the WDM system
The simple WDM system mainly includes transceivers, WDM wavelength division multiplexers,
patch cord, and dark fiber components.
WDM system
In the entire WDM system, the multiplexer and demultiplexer are key components in the WDM
technology, and their performance is decisive for the transmission quality of the system.
Advantages of WDM technology
Large capacity
An important feature of WDM is that it can make full use of the bandwidth resources of the
optical fiber and increase the data transmission capacity without changing the existing network
infrastructure, so that the transmission capacity of an optical fiber is multiple times that of a
single wavelength. For example, the DWDM system can support up to 192 wavelengths in a pair
of optical fibers, and the transmission capacity of each wavelength is as high as 100Gbit/s ~
400Gbit/s and one Terabit/s.
Good compatibility
WDM has good compatibility with different signals. When transmitting signals with different
properties such as image, data and voice, each wavelength is independent from each other and
does not interfere with each other to ensure the transparency of transmission.

4. Flexibility, economy and reliability
WDM technology allows new channels to be connected as needed without changing the existing
network, which makes upgrades easier. When upgrading and expanding the network, there is no
need to renovate the optical cable line, and new businesses can be opened or superimposed by
adding wavelengths. Optical fibers and 3R regenerators can be saved during large-capacity
long-distance transmission, and the transmission cost is significantly reduced.
Wavelength routing
WDM technology is one of the key technologies for realizing all-optical networks. In the
all-optical network that is expected to be realized in the future, by changing and adjusting the
wavelength of the optical signal on the optical path, the up/down and cross-connection of
various telecommunication services can be realized.
What does Mux and Demux stand for?
MUX
The main function of the combiner MUX is to combine multiple signal wavelengths into one fiber
for transmission. At the transmitting end, the N optical transmitters operate on N different
wavelengths respectively, and the N wavelengths are separated by appropriate intervals, which
are respectively recorded as λ 1, λ 2, ... λ n. A multiplexer combines these optical
wavelengths into a single-mode fiber. Since
optical carrier signals of different wavelengths can be regarded as independent of each other
(regardless of fiber nonlinearity), multiplexing transmission of multiple optical signals can be
realized in one optical fiber. Through multiplexing, communication carriers can avoid maintaining
multiple lines and effectively save operating costs.
DEMUX
The main function of DEMUX is to separate the multiple wavelength signals transmitted in one
fiber. In the receiving part, the optical carrier signals of different wavelengths are separated by a
Demux and further processed by the optical receiver to restore the original signal. A multiplexer
(Demux) is a device that reverses the processing of a multiplexer.
In principle, the device is reciprocal (two-way reversible), that is, as long as the output and input
of the demultiplexer are used in reverse, it is a multiplexer.
The difference between WDM and optical splitter
Many people cannot understand the difference between wavelength division multiplexing and
optical splitters. In short, WDM separates and transmits light of multiple wavelengths in the line.
Of course, it can also transmit light of multiple wavelengths together. The optical splitter divides
the light of one wavelength into multiple beams according to the purpose. The power of the light
depends on the specifications of the splitter used. The most important difference between the
two is that the former can compositely transmit optical signals of various service wavelengths,
while the latter can only transmit light of one wavelength to split light according to a specific
splitting ratio.

5. The indicators that affect the WDM devices
Working band
Working bands of the multiplexer/demultiplexer. For example, there is three bands of 1550
wavelength: S-band (short-wavelength 1460~1528nm), C band (conventional band
1530~1565nm), L band (long-wavelength band 1565~1625nm).
Number of channels & channel spacing
The number of channels is the number of channels the device has to send information. This
number can range from 4 to 160 with design enhancements adding more channels. The normal
channels are 4, 8, 16, 32, 40, 48, etc. Channel spacing is the center-to-center difference in
frequency between neighboring channels. It can be used to prevent inter-channel interference.
Insertion loss
Insertion loss is the attenuation caused by the insertion of wavelength division multiplexers
(WDM) in an optical transmission system. The attenuation effect of wavelength division
multiplexer directly affects the transmission distance of the system. In general, the lower the
insertion loss, the less the signal attenuation.
Isolation
Isolation refers to the degree of isolation between individual channel signals. High isolation
values can effectively prevent crosstalk between signals and cause distortion of the transmission
signal.
Polarization dependent loss（PDL）
Polarization-dependent loss is the maximum deviation in insertion loss across all input
polarization states.
In addition to the above, there are of course other performance parameters that affect the
multiplexing/demultiplexing devices, such as operating temperature, bandwidth, etc. Generally, a
multiplexer and a demultiplexer are combined into a single device allowing the device to process
both incoming and outgoing signals. Or a single output of a multiplexer can be connected

6. through a single channel to a single input of a demultiplexer. But mostly is the combined and
complex devices for both directions transmission.
How to understand the O, E, S, C, L, U band
What is O band?
The O band is the original band with wavelength from 1260 to 1360nm. The O-band is the first
wavelength band used in optical communications in history, and the signal distortion (due to
dispersion) is minimal.
What is E band?
The E-band (extended wavelength band: 1360-1460 nm) is the least common of these bands. The
E-band is mainly used for the expansion of the O-band, but it is rarely used, mainly because many
existing optical cables show high attenuation in the E-band and the manufacturing process is very
energy-intensive, so the use in optical communication is limited.
What is S band?
The optical fiber loss in the S-band (Short-wavelength Band, 1460-1530 nm) is lower than the loss
in the O-band. The S-band is used as many PON (passive optical network) systems.
What is C band?
The C-band (Conventional Band) ranges from 1530 nm to 1565nm and represents the
conventional band. Optical fiber shows the lowest loss in the C-band and occupies a large
advantage in long-distance transmission systems. It is usually used in many metropolitan areas
combined with WDM, long-distance, ultra-long-distance and submarine optical transmission
systems and EDFA technology. As the transmission distance becomes longer, and fiber optic
amplifiers are used instead of optical-to-electronic-to-optical repeaters, the C-band becomes
more and more important. With the advent of DWDM (Dense Wavelength Division Multiplexing)
that allows multiple signals to share a single fiber, the use of the C-band has been expanded.
What is L band?

7. The L-band (Long-wavelength Band, 1565-1625nm) is the second lowest-loss wavelength band,
and is often used when the C-band is insufficient to meet the bandwidth requirements. With the
wide availability of b-doped fiber amplifiers (EDFAs), DWDM systems have expanded upward to
the L-band, and were initially used to expand the capacity of terrestrial DWDM optical networks.
Now, it has been introduced to submarine cable operators to do the same thing-to expand the
total capacity of submarine cables.
Due to its low transmission attenuation loss, C-band and L-band is usually selected to use in the
DWDM system. Except for the O-band and L-band, there are two other bands, 850nm band and
the U band (ultra-long band: 1625-1675 nm). The 850nm band is the main wavelength of the
multimode optical fiber communication system, which combines VCSEL (Vertical Cavity Surface
Emitting Laser). The U frequency band is mainly used for network monitoring.
WDM technology can be divided into WDM, CWDM, DWDM according to different wavelength
modes. The wavelength range stipulated by ITU for CWDM (ITU-T G.694.2) is 1271 to 1611nm,
but considering the attenuation of the 1270-1470nm band in the application, the band of
1470~1610nm is usually used. The channel space of DWDM is more closeness, so choose the
C-band (1530 nm-1565 nm) and L-band (1570 nm-1610 nm) transmission windows. Ordinary
WDM generally uses 1310 and 1550nm wavelengths.
What does CWDM stand for vs. DWDM, FWDM, LWDM, MWDM?
WDM solutions include coarse wavelength division multiplexing (CWDM), dense wavelength
division multiplexing (DWDM), medium wavelength division multiplexing (MWDM), and Lan
wavelength division multiplexing (LWDM).
CWDM (Coarse Wavelength Division Multiplexing)
The CWDM wavelength set consists of a series of 18 wavelengths spaced 20nm apart, from
1270nm to 1610nm. The biggest advantage of CWDM systems is the low cost, and the
component cost is mainly reflected in filters and lasers. The wide wavelength spacing of 20 nm
also gives CWDM the advantage of low specification of the laser and simplified structure of the
optical multiplexer/demultiplexer. The structure is simplified, the yield is improved, so the cost is
reduced.
Band Description Wavelength range Bandwidth
O band original 1260-1360nm 100
E band extended 1360-1460nm 100
S band Short wavelength 1460-1530nm 65
C band conventional 1530-1565nm 40
L band Long wavelength 1565-1625nm 60
U band Ultralong wavelength 1625-1675nm 50

8. DWDM（Dense Wavelength Division Multiplexer）
DWDM can carry 40, 80 or up to 160 wavelengths with a narrower spacing of 1.6/0.8/0.4nm
(200/100/50GHz). The DWDM module further increases the system bandwidth and capacity by
using tightly spaced wavelengths to carry more signals on the same fiber. DWDM is mainly due to
the high cost of laser diodes and the cooling laser technology used to maintain wavelength
stability. Compared with CWDM, DWDM with tighter wavelength spacing can carry 8 to 160
wavelengths on an optical fiber, which is more suitable for long-distance transmission. With the
help of EDFA, DWDM system can work within thousands of kilometers.
FWDM（Filter Wavelength Division Multiplexing）
The filter type WDM is based on mature membrane filter technology. Filter-type WDM can
combine or separate light of different wavelengths in a wide wavelength range, and are widely
used in erbium-doped optical amplifiers, Raman amplifiers and WDM optical fiber networks.
MWDM（Medium Wavelength Division Multiplexing）
MWDM is proposed based on mature CWDM technology. CWDM has 18 wavelengths
(1271~1611nm), but due to the relatively large attenuation of the 1270~1470nm band and cost
considerations, usually only 6 wavelengths (1271nm, 1291nm, 1311nm, 1351nm, 1371nm) are
used. MWDM reuses the first 6 wavelengths of CWDM, compresses the 20nm wavelength
interval of CWDM to 7nm, and uses Thermal Electronic Cooler (TEC) temperature control
technology to expand 1 wave into 2 waves. In this way, an increase in capacity can be achieved

9. while further saving optical fibers. MWDM is based on the 6 wavelengths of CWDM, shifted by
3.5nm left and right to expand to 12 waves (1267.5, 1274.5, 1287.5, 1294.5, 1307.5, 1314.5,
1327.5, 1334.5, 1347.5, 1354.5, 1367.5, 1374.5nm).
LWDM（Lan Wavelength Division Multiplexing）
LWDM is based on the Ethernet channel wavelength division multiplexing Lan-WDM technology,
also known as dense wavelength division multiplexing. Its channel interval is 200~800GHz, this
range is between DWDM (100GHz, 50GHz) and CWDM (about 3THz). LWDM uses 12 wavelengths
in the O-band range from 1269nm to 1332nm, with a wavelength interval of 4nm (Wavelengths
including 1269.23, 1273.54, 1277.89, 1282.26, 1286.66, 1291.1, 1295.56, 1300.05, 1304.58,
1309.14, 1313.73 , 1318.35nm). The characteristic of LWDM working wavelength is that it is
located near zero dispersion, with small dispersion and good stability. At the same time, LWDM
can support 12-wave 25G to increase the capacity and save fiber.

10. CWDM DWDM
MWDM LWDM
Channel
Spacing
20nm 1.6, 0.8, 0.4 nm 7nm 4nm
Wavelengths
1270~1610nm
O/E/S+C+L
band
1525~1565nm(C band)
1570~1610nm(L band)
1267.5、
1274.5、
1287.5、
1294.5、
1307.5、
1314.5、
1327.5、
1334.5、
1347.5、
1354.5、
1367.5、
1374.5nm
1269.23、
1273.54、
1277.89、
1282.26、
1286.66、1291.1、
1295.56、
1300.05、
1304.58、
1309.14、
1313.73、
1318.35nm
Number of
channels
Up to 18
channels
40,80, up to 160 wavelength
12 channels 12 channels
Transmission
Distance
for short
distance
< 160km
longer haul transmit
over 1,000 kilometers can be
achieved
Cost cost-effective more expensive
Application
Man access
layer,
telecommunica
tions,
enterprise
network,
campus
network, etc.
Suitable for long distance,
large capacity long-distance
trunk networks, or ultra large
capacity man core node, or
large-capacity metropolitan
area network core nodes,
telecom 5G, metropolitan
area networks, backbone
networks, data centers, etc.
HYC can provide customers with a one-stop optical network device and low-cost optical
communication products, supplying a range of WDM products. HYC Co.,Ltd（HYC）is a national
Hi-tech optoelectronics company engaged in R&D, manufacture and marketing of fiber optical
products. Providing professional product and service for fiber connectivity,WDM, PLC splitter and
high density datacom cabling. HYC products and solutions widely applied in 4G/5G, Data Center
and Cloud Computing industry etc.
http://www.hyc-system.com
sales@hyc-system.com