Wavelength division multiplexing (WDM) is a technology that combines two or more optical carrier signals of different wavelengths (carrying various information) at the transmitting end through a multiplexer (also called a combiner, Multiplexer) and couples them to the same optical fiber of the optical line for transmission; at the receiving end, the optical carriers of various wavelengths are separated by a demultiplexer (also called a splitter or demultiplexer), and then further processed by the optical receiver to restore the original signal.
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 (sparse wavelength division multiplexing) and DWDM (dense wavelength division multiplexing). The channel spacing of CWDM is 20nm, while the channel spacing of DWDM ranges from 0.2nm to 1.2nm, so relative to DWDM, CWDM is called sparse wavelength division multiplexing technology.
Among the current technical solutions used in 5G fronthaul, passive wavelength division is undoubtedly the most widely used. The passive wavelength division system consists of color optical modules, multiplexers and optical fibers, among which the multiplexer is the key component.
The multiplexer is a passive device that mainly multiplexes and demultiplexes multiple optical wavelengths. Its appearance and packaging are almost the same as those of the splitter in the PON system. Each port of the multiplexer corresponds to a specific wavelength and is marked with a different color.
So, how does the multiplexer multiplex and demultiplex multiple wavelengths?
Internal Structure of The Multiplexer
Opening the shell of a multiplexer, we can see that in addition to the coupler and connecting optical fiber, the core components of the multiplexer are several yellowish glass rods about 2 cm long.
The yellowish glass rods are multilayer dielectric film filters. If you zoom in, you will see the following picture. The transparent small glass rods tied together above the filter in the picture are the protection points of the optical fiber connector.
Each filter has three connecting pigtails. One end is connected to two pigtails, which are the input end and the reflection end, and the other end is connected to the output end.
Each filter can filter out a specific wavelength from the input multi-channel optical signal, and reflect other wavelengths from the reflection end. The combination is the opposite process.
For example, the figure above shows a filter with a nominal wavelength of 1271nm. When the input signal contains three wavelengths of 1271nm, 1291nm and 1311nm, the output end outputs an optical signal with a wavelength of 1271nm, and the reflection end outputs optical signals with wavelengths of 1291nm and 1311nm.
Filter Connection Relationship
Usually a 6 in 1 multiplexer is composed of 6 filters connected together. The connection relationship between the filters is shown in the figure below.
Since optical signals will attenuate when passing through filters, and optical signals of different wavelengths pass through different numbers of filters, the insertion loss of each wavelength port of the multiplexer is also different.
As shown in the figure above, if λ1~λ6 are 1271nm~1371nm respectively, the insertion loss of the 1271nm wavelength port is the largest. The insertion loss of each wavelength port of the 6 in 1 multiplexer is shown in the table below.
For multiplexers with larger multiplexing (such as 12 in 1, 18 in 1), if each filter still uses a connection relationship similar to the 6 in 1 multiplexer in the figure above, the insertion loss of the wavelength finally filtered out will be very large.
Therefore, multiplexers with larger multiplexing usually use a filter to split the signal into two paths first, and then filter out each wavelength one by one.
Dielectric Film Filter Principle
What is the principle of dielectric film filter? Do you remember the “thin film interference” in high school physics class? The reason why soap bubbles look colorful is the result of thin film interference.
When light is irradiated onto the soap film, it is reflected twice by the front and rear films and then superimposed together. The distance of the light reflected by the rear film is twice the distance of the light reflected by the front film.
If the film thickness is equal to 1/4 of the wavelength of light, the reflected light of the front and rear films is enhanced, and bright lines appear; if twice the film thickness is equal to an odd multiple of half the wavelength, the reflected light of the front and rear films is weakened, and dark lines appear.
The dielectric film filter also uses the thin film interference principle, which makes certain wavelengths of light produce strong reflection at the thin film and cannot pass through the thin film, while other wavelengths of light can pass through the thin film.
In order to improve the reflection efficiency, the dielectric film filter usually uses multi-layer thin films, and through the interference of multi-layer dielectric films, a certain wavelength is transmitted and other wavelengths are blocked.
Advantages of Wavelength Division Multiplexing (WDM)
- Large transmission capacity, which can save precious optical fiber resources. For a single-wavelength optical fiber system, a pair of optical fibers is required to send and receive a signal, while for a WDM system, no matter how many signals there are, the entire multiplexing system only needs a pair of optical fibers. For example, for 16 2.5Gb/s systems, a single-wavelength optical fiber system requires 32 optical fibers, while a WDM system only requires 2 optical fibers.
- It is “transparent” to various types of business signals and can transmit different types of signals, such as digital signals, analog signals, etc., and can synthesize and decompose them.
- When expanding the network, there is no need to lay more optical fibers or use high-speed network components. All you need to do is change the terminal and add an additional optical wavelength to introduce any new service or expand capacity. Therefore, WDM technology is an ideal means of expansion.
- Building a dynamically reconfigurable optical network. Using optical add/drop multiplexers (OADMs) or optical cross-connect devices (OXCs) at network nodes can form an all-optical network with high flexibility, high reliability, and high survivability.
Current Problems of WDM
The optical transmission network based on WDM technology with add/drop multiplexing and cross-connection functions has great advantages such as easy reconstruction and good scalability. It has become the development direction of future high-speed transmission networks.
However, before it can be truly realized, the following problems must be solved.
- Network management
At present, the network management of WDM systems, especially the WDM network management with complex up/down path requirements, is still in its immature stage. If the WDM system cannot perform effective network management, it will be difficult to adopt it 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 is still immature. In terms of performance management, the WDM system uses analog methods to multiplex and amplify optical signals, so the commonly used bit error rate is not suitable for measuring the service quality of WDM. A new parameter must be found to accurately measure the service quality provided by the network to users. If these problems are not solved in time, it will hinder the development of WDM systems.
- Interconnection
Since WDM is a new technology, its industry standards are relatively rough, so the interoperability of WDM products from different vendors is poor, especially in the upper-level 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, research on optical interface equipment should be strengthened.
- Optical devices
The immaturity of some important optical devices, such as tunable lasers, will directly limit the development of future optical transmission networks. For some large operators, it is already very difficult to handle several different lasers in the network, not to mention dozens of optical signals.
Usually, 4 to 6 lasers that can be tuned throughout the network are required in optical networks, but such tunable lasers are not yet commercially available.
In Summary
WDM is a multiplexing technology in the optical domain. It forms an optical layer network, namely ” all-optical network”, which will be the highest stage of optical communication. It will be the future trend to establish an optical network layer based on WDM and OXC (optical cross connection) to realize end-to-end all-optical network connection for users and eliminate the bottleneck of photoelectric conversion with a pure ” all-optical network”.
At present, WDM technology is still based on point-to-point mode, but point-to-point WDM technology is the first and most important step of all-optical network communication. Its application and practice have a decisive impact on the development of all-optical network.
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