How does Wavelength Division Multiplexing(WDM) Work?
WDM is a technology that converges multiple optical signals of different wavelengths through a combiner and couples them into the same fiber for data transmission.
Working principle of WDM
Wavelength x frequency = speed of light (constant value), so WDM is actually the same with frequency division multiplexing.
To put it simply, we can think of WDM as a highway——where different types of vehicles rush in and then go their separate ways when they get to their destination.
The role of wavelength division multiplexing is to improve the transmission capacity of optical fiber and the utilization efficiency of optical fiber resources. For the WDM system, to make it work normally, it is obvious that the wavelength (frequency) of each optical signal must be controlled. If the wavelength interval is too short, it is easy to “crash”; If the wavelength interval is too long, the utilization rate will be very low.
The Advantages of WDM Technology:
WDM technology has been developing rapidly in recent years because of the following advantages.
(1) Large transmission capacity, which can save valuable fiber resources. For a single-wavelength fiber system, a pair of fibers is needed to send and receive a signal, while for a WDM system, only one pair of fibers is needed for the entire multiplexing system, regardless of the number of signals. For example, for a sixteen 2.5Gb/s system, a single-wavelength fiber system requires 32 fibers, while a WDM system requires only two fibers.
(2) Transparent to all kinds of service signals, it can transmit different types of signals, such as digital signals, and analog signals, and can synthesize and decompose them.
(3) There is no need to lay more optical fibers or use high-speed network components during network expansion. Any new services can be introduced or capacity can be expanded only by changing the terminal and adding an additional optical wavelength. Therefore, WDM technology is an ideal means of expansion.
(4) Build a dynamically reconfigurable optical network, and use optical add-drop multiplexers (OADM) or optical cross-connect equipment (OXC) at network nodes to form a highly flexible, highly reliable, and highly survivable all-optical network.
the WDM system
Problems Existing in WDM Technology:
The optical transmission network based on WDM technology, with an add-drop multiplexing function and cross-connect function, has great advantages such as easy reconfiguration and good scalability. It has become the development trend of the high-speed transmission network in the future. But before it can be realized, the following problems must be solved.
At present, the network management of the WDM system, especially that with complex up/down path requirements, is still immature. If the WDM system cannot carry out effective network management, it will be difficult to adopt on a large scale in the network. For example, in terms of fault management, since the WDM system can support different types of service signals on the optical channel, once the WDM system fails, the operating system should be able to detect the fault in time and find out the cause of the fault.
But so far, the relevant operation and maintenance software are still immature. In terms of performance management, WDM systems use analog methods to multiplex and amplify optical signals, so the commonly used bit error rate is not suitable for measuring the quality of WDM services. A new parameter must be found to accurately measure the quality of service provided by the network to users. If these problems are not solved in time, they will hinder the development of the WDM system.
Interconnection and Intercommunication
Since WDM is a new technology, its industry standard is relatively rough, so the interoperability of WDM products in different businesses is poor, especially in the aspect of upper-layer network management. In order to ensure the large-scale implementation of WDM systems in the network, it is necessary to ensure the interoperability between WDM systems and the interconnection and intercommunication between WDM systems and traditional systems. Therefore, the research on optical interface equipment should be strengthened.
The immaturity of some important optical devices such as tunable lasers will directly limit the development of optical transmission networks. For some large operating companies, it is already very tricky to deal with several different lasers in the network, let alone dozens of optical signals. In most cases, 4 to 6 lasers that can be tuned in the entire network are required to be used in an optical network, but such tunable lasers are not yet commercially available.
The design of the communication system is different, and the spacing width between each wavelength is also different. According to the different channel spacing, WDM can be subdivided into CWDM (coarse wavelength division multiplexing) and DWDM (dense wavelength division multiplexing). The channel spacing of CWDM is 20nm, while the channel spacing of DWDM is from 0.2nm to 1.2nm.
setup of a tunable laser
CWDM vs DWDM
At first, the technical conditions were limited, and the wavelength spacing would be controlled within tens of nanometers. This type of WDM is called Coarse Wavelength Division Multiplexing(CWDM).
Later, the technology became more and more advanced, and the wavelength interval became shorter and shorter. It was called Dense Wavelength Division Multiplexing(DWDM) when it reached a level within a few nanometers.
In addition, CWDM-modulated lasers use uncooled lasers, while DWDMs use cooled lasers. Cooled lasers are temperature tuned and uncooled lasers are electronically tuned. It is difficult and expensive to implement temperature tuning because the temperature distribution is highly non-uniform over a wide range of wavelengths. CWDM avoids this difficulty and thus greatly reduces the cost. The cost of the entire CWDM system is only 30% of that of DWDM. CWDM is achieved by combining wavelengths transmitted in different fibers into one fiber for transmission using an optical multiplexer. At the receiving end of the link, the demultiplexer is used to send the decomposed wavelengths to different fibers and then to different receivers.
CWDM has a wavelength spacing of 20nm and 18 wavebands from 1270nm to 1610nm.
|Wavelength Number||Central Wavelength||Wavelength Number||Central Wavelength|
However, due to the obvious attenuation increase in the wavebands from 1270nm to 1470nm, many old-type optical fibers cannot be used normally, so CWDM generally gives priority to the use of the 8 wavebands from 1470nm to 1610nm.
CWDM to DWDM
The wavelength spacing of DWDM can be 1.6nm, 0.8nm, 0.4nm, and 0.2nm, which can accommodate 40/80/160 waves (up to 192 waves). The wave range of DWDM is 1525nm to 1565nm (C band) and 1570nm to 1610nm (L band).
CWDM to DWDM
DWDM is commonly used in C-band, with a wavelength interval of 0.4nm and a channel frequency interval of 50GHz.
Other differences Between CWDM and DWDM:
CWDM has a simpler structure
CWDM system does not contain OLA, namely Optical Line Amplifier. In addition, since the CWDM channel spacing is relatively large, there is no need to consider power balancing compared to DWDM.
CWDM consumes less power
The operating cost of an optical transmission system depends on the maintenance of the system and the power consumed by the system. Even if the maintenance costs of both DWDM and CWDM systems are acceptable, the power consumption of a DWDM system is much higher than that of a CWDM system. In DWDM systems, with the increase in the total number of multiplexed wavelengths and single-channel transmission rates, power loss and temperature management have become key issues in circuit board design. Lasers without coolers are used in CWDM systems, resulting in low system power consumption, which is beneficial for system operators to save money.
CWDM devices have smaller physical size
CWDM lasers are much smaller than DWDM lasers, and uncooled lasers generally consist of a laser sheet and a monitoring photodiode sealed in a metal container with a glass window. The size of the DWDM laser transmitter is about five times the volume of the CWDM laser transmitter. That is, if the volume of the DWDM laser transmitter is 100cm3, the volume of the CWDM laser without the cooler is only 20cm3.
CWDM has lower requirements on the transmission medium
When DWDM runs services above 10G, G.655 optical fibers are required. However, CWDM has no special requirements for optical fiber. G.652, G.653, and G.655 optical fibers can use CWDM technology, so they can make a lot of use of the old fiber optic cable previously laid.
Comparison of application environments
Most of the DWDM suitable for metro networks inherit the characteristics of long-haul backbone networks, such as end-to-end logical connections, inflexible topology, no support for mesh structure, and no adaptation to the complex and mobile multi-logical topology in metro networks. The cost of DWDM equipment for a long-haul backbone network is much lower than the cost of laying new fibers and adding optical amplification. However, within the scope of the metropolitan area network, the network cost mainly comes from the cost of the access end equipment rather than the cost of the transmission line, so DWDM does not have a great advantage in terms of price. CWDM realizes wavelength division multiplexing in the full wavelength range (1260-1620nm) by reducing the window requirements for wavelengths. It also greatly reduces the cost of optical devices and can achieve a higher cost performance within 0-80km.
A summary comparison of CWDM and DWDM：
Full Name Coarse Wavelength Division Multiplexing Dense Wavelength Division Multiplexing
Wave Interval 20nm in general 0.8nm/0.4nm/0.2nm/1.6nm
Wave Range 1270nm to 1610nm 1525nm to 1565nm (C band)
1570nm to 1610nm (L band)
Numbers of Wavebands 18 40/80/160 (up to 192)
Optical Modulation Form Uncooled laser, electronically tuned Cooled laser, tuned by temperature
Cost Low High
Communication Distance Short (Optical amplifiers unsupported) Long
Structure Simple Complex
Power consumption Low High
Physical size Small Big
Requirement for transmission medium Low High
MWDM vs LWDM
Nowadays, the 5G network is blooming. When the Communications Service Providers (CSP) build a 5G fronthaul network, they always fall into a dilemma: if they choose the more active WDM with higher operation and maintenance efficiency, the cost will increase; If we choose the low-cost passive WDM mode, it is difficult to improve the operation and maintenance efficiency, and it can not match the business needs in the future. Therefore, CSPs hope to find a way to deploy a 5G fronthaul network to realize both cost and operation efficiency. In this case, open WDM was born.
Application of 5G fronthaul network
The principle of MWDM(Medium Wavelength Division Multiplexing), is to focus on using the first 6 waves of 25G CWDM, by adding TEC (Thermal Electronic Cooler) for temperature control, then left and right offset 3.5nm wavelengths to form 12 wavelengths, this solution can save a lot of fiber resources.
MWDM:6 wavelengths increase to 12 wavelengths
Then about LWDM(Lan Wavelength Division Multiplexing), LWDM is Ethernet channel-based wavelength division multiplexing (LAN WDM), with a channel spacing of 200 to 800 GHz, a range between DWDM (100 GHz, 50 GHz) and CWDM (about 3 THz).
Wavelength Application Scheme Industrial chain
1269.23 DWL+PIN /
1273.54 DWL+PIN Share 400G LR8 industry chain
1291.1 DWL+PIN /
1295.56 DWL+PIN Share 400G LR4 industry chain
1313.73 DWL+PIN /
1318.35 DWL+PIN /
DML (Directly Modulated Laser) is at the transmitting end(TOSA) of the optical module, and its counterpart is the EML (Electro-absorption Modulated Laser), which is more costly. And PIN refers to the light-emitting diode at the receiving end(ROSA) of the optical module.
The internal structure of an optical module
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