Wavelength division multiplexer is a communication technology that combines a series of optical signals carrying information but with different wavelengths into a bundle and transmits them along a single fiber; at the receiving end, a certain method is used to separate the optical signals of different wavelengths. The wavelength division multiplexer uses this technology.
1. Types of wavelength division multiplexers
1.1 WDM(wavelength division multiplexer)
WDM is a communication technology that combines a series of optical signals carrying information but with different wavelengths into a bundle and transmits them along a single fiber; and then uses a certain method at the receiving end to separate the optical signals of different wavelengths. This technology can transmit multiple signals on a fiber at the same time, and each signal is transmitted by a certain wavelength of light, which is a wavelength channel.
In the same fiber, two or more optical wavelength signals transmit information through different optical channels at the same time, which is called optical wavelength division multiplexing technology, or wavelength division multiplexer for short. Optical wavelength division multiplexing includes frequency division multiplexing and wavelength division multiplexing. There is no obvious difference between optical frequency division multiplexing (FDM) technology and optical wavelength division multiplexing (WDM) technology, because light waves are part of electromagnetic waves, and the frequency of light has a single correspondence with wavelength. Usually it can also be understood in this way, optical frequency division multiplexing refers to the subdivision of optical frequencies, and the optical channels are very dense. Optical wavelength division multiplexing refers to the rough division of optical frequencies, and the optical channels are far apart, even in different windows of the optical fiber.
Optical wavelength division multiplexing generally uses wavelength division multiplexers and demultiplexers (also called multiplexers/demultiplexers) to be placed at both ends of the fiber to realize the coupling and separation of different light waves. The principle of these two devices is the same. The main types of optical wavelength division multiplexers are fused taper type, dielectric film type, grating type and flat type. Its main characteristic indicators are insertion loss and isolation. Usually, the increase in the loss of the optical link is called the insertion loss of the wavelength division multiplexing due to the use of wavelength division multiplexing equipment in the optical link. When the wavelengths 11 and l2 are transmitted through the same fiber, the difference between the power at the input end l2 of the demultiplexer and the power mixed in the fiber at the output end of 11 is called isolation.
Products using WDM technology mainly include CWDM and DWDM.
1.2 CWDM
CWDM(coarse wavelength division multiplexer) is a low-cost wavelength division multiplexer transmission technology for the access layer of the metropolitan area network. In principle, CWDM uses an optical multiplexer to multiplex optical signals of different wavelengths into a single fiber for transmission. signal, connect to the corresponding receiving device. Its principle is shown in Figure 1. The main difference from DWDM is that compared with the wavelength interval of 0.2nm to 1.2nm in the DWDM system, coarse wavelength division multiplexer has a wider wavelength interval, and the industry standard wavelength interval is 20nm. The wavelengths specified in ITU-T G.694.2 are shown in Table 1. The band to which each wavelength belongs is shown in Figure 2, covering the five bands of O, E, S, C, and L of the single-mode fiber system.
Due to the wide wavelength interval of the coarse wavelength division multiplexer system, the requirements for the technical indicators of the laser are relatively low. Since the wavelength interval reaches 20nm, the maximum wavelength shift of the system can reach -6.5℃～+6.5℃, the emission wavelength accuracy of the laser can be relaxed to ±3nm, and within the operating temperature range (-5℃～70℃), the temperature The wavelength drift caused by the change is still within the allowable range, and the laser does not need a temperature control mechanism, so the structure of the laser is greatly simplified and the yield is improved.
In addition, the larger wavelength separation means that the structure of the optical multiplexer/demultiplexer is greatly simplified. For example, the number of filter coating layers in a coarse wavelength division multiplexer system can be reduced to about 50 layers, while the number of coating layers for a 100GHz filter in a DWDM system is about 150 layers, which leads to higher yields, lower costs, and a large increase in filter suppliers. conducive to competition. The cost of CWDM filter is more than 50% less than that of DWDM filter, and it will be further reduced with the increase of automatic production technology and batch.
1.2.1 Advantages of coarse wavelength division multiplexer system
The most important advantage of coarse wavelength division multiplexer is the low cost of equipment. The specific situation has been introduced before. In addition to this, another advantage of CWDM is that it can reduce the operating cost of the network. Due to the small size, low power consumption, easy maintenance and convenient power supply of CWDM equipment, 220V AC power can be used. Due to its small number of wavelengths, the amount of board backup is small. Using 8-wave CWDM equipment has no special requirements for optical fibers, G.652, G.653, and G.655 optical fibers can be used, and existing optical cables can be used. The CWDM system can significantly improve the transmission capacity of the optical fiber and improve the utilization rate of the optical fiber resources. The construction of the metropolitan area network is faced with a certain degree of shortage of optical fiber resources or the high price of leased optical fibers. A typical coarse wavelength division multiplexing system can provide 8 optical channels, and can reach up to 18 optical channels according to the G.694.2 specification of ITU-T. Another advantage of CWDM is its small size and low power consumption. The laser of the CWDM system does not need semiconductor coolers and temperature control functions, so the power consumption can be significantly reduced. For example, each laser of the DWDM system consumes about 4W of power, while the CWDM laser without a cooler consumes only 0.5W of power. The simplified laser module in the coarse wavelength division multiplexer system reduces the volume of the integrated optical transceiver module, and the simplification of the equipment structure also reduces the volume of the equipment and saves the space of the equipment room. Compared with the traditional TDM method, CWDM has the transparency of speed and protocol, which makes it more suitable for the development of high-speed data services in the metropolitan area network. There are many services with different protocols and different rates in the metropolitan area network. CWDM provides transmission channels with different rates on one optical fiber that are transparent to protocols, such as Ethernet, ATM, POS, SDH, etc., and CWDM is transparent. Multiplexing and add/drop multiplexing functions allow users to add and drop a wavelength directly without converting the format of the original signal. That is, the optical layer provides a transport structure independent of the service layer. CWDM has good flexibility and scalability. For metro services, the flexibility of service provision, especially the speed of service provision and the ability to expand along with service development, is very important. Using CWDM technology, users can open services in one day or several hours, and with the increase of business volume, the capacity can be expanded by inserting new OTU boards. Improve business quality. Applying the CWDM system in the metropolitan area network can make it possible to restore the optical layer. Optical layer recovery is much more economical than electrical layer recovery. Considering that the optical layer recovery is independent of services and rates, some systems with no protection function (such as Gigabit Ethernet) can be protected by CWDM. Due to the above-mentioned advantages of coarse wavelength division multiplexer technology, CWDM has gained more and more applications in the fields of telecommunications, radio and television, enterprise networks, and campus networks.
1.2.2 Deficiencies of CWDM products
The biggest problem with coarse wavelength division multiplexer technology is that its cost advantage over DWDM equipment is still not obvious enough. Optical transceiver modules and optical components are the key to reducing costs. However, due to the small size of the market and the small shipments of suppliers, the device cost advantage is not obvious. Another way to reduce costs is to simplify equipment functionality, which results in a reduction in system reliability and manageability. DWDM products with decreasing prices also put great pressure on coarse wavelength division multiplexer technology, and using DWDM technology can form a complete metro DWDM network, so the scalability is good, and the pressure on CWDM is relatively large. The number of optical channels (wavelengths) supported by CWDM equipment does not exceed 8, mainly because the manufacturing process of optical transceiver modules in the E-band is not mature. In addition, the G.652C optical cable that eliminates the water absorption peak is rarely used in the existing network, so There is little market demand for E-band optical transceiver modules. There are still many technical problems in the CWDM system with higher speed and longer transmission distance. Such as the chromatic dispersion problem of 10G system, ultra-broadband optical amplification technology, etc. In addition, the standardization process needs to be accelerated, especially the guidance of operators is required in terms of business interface functions.
1.2.3 Development Direction of CWDM
One of the key factors restricting the development of CWDM products is the price of optical transceiver modules and multiplexing and demultiplexing devices. With the development of the market and the progress of the manufacturing process, further reducing the cost of equipment is an important development direction. Develop E-band optical device technology to make it mature as soon as possible. Develop 10G rate optical channel technology to improve the capacity and upgradeability of coarse wavelength division multiplexer systems. Supporting various business interfaces is the development direction of coarse wavelength division multiplexer. The demand for multi-service interfaces at the access layer of the metropolitan area network is the driving force for manufacturers to further develop multi-service interfaces. CWDM equipment will provide FE, GE, SDH, ESCON, FC and other service interfaces. Another development direction is that it can be combined with MSTP or high-performance routing switching equipment as a means for MSTP equipment or high-speed routers to expand line-side capacity. It is also a development direction to provide multi-layer optical layer and business layer protection functions to meet the needs of different customers. The network management technology and equipment security and reliability are further improved to enhance the competitiveness in the market. For the newly launched G.652C optical fiber, since the price of G.652C optical cable is twice that of G.652B, and the technology of CWDM optical transceiver module in E-band is not yet mature, full-band CWDM will be applied in the short term (1-2 years). The possibility of equipment is not high, and the use of G.652C optical cable has the problem of large investment and no benefit in the short term, so the application of G.652C optical fiber in the metropolitan user optical cable network is limited to a certain extent.
1.3 DWDM
DWDM(dense wavelength division multiplexer) technology utilizes the bandwidth and low loss characteristics of single-mode fiber, and uses multiple wavelengths as carriers, allowing each carrier channel to be transmitted simultaneously in the fiber.
Compared with the general single-channel system, dense wavelength division multiplexer (DWDM) not only greatly improves the communication capacity of the network system and makes full use of the bandwidth of the optical fiber, but also has many advantages such as simple expansion and reliable performance, especially it can directly connect Entering a variety of businesses makes its application prospects very bright.
Dense wavelength division multiplexer is divided from the structure, there are integrated system and open system at present. Integrated system: The optical signal of the terminal of the single optical transmission equipment required to be connected is to meet the requirements of G. 692 standard light source. The open system is to add a wavelength transfer unit OTU to the front end of the combiner and the back end of the demultiplexer, and convert the currently commonly used G. 957 interface wavelength conversion to G. 692 standard wavelength optical interface. In this way, the open system uses wavelength conversion technology? Let any satisfy G. The optical signal required by Recommendation 957 can be converted to meet the requirements of G. 692 standard wavelength optical signal, and then through wavelength division multiplexing, so as to transmit on the dense wavelength division multiplexer system.
Dense wavelength division multiplexer system can provide 16/20 wave or 32/40 wave single-fiber transmission capacity, up to 160 wave, with flexible expansion capability. Users can build a 16/20 wave system at the initial stage, and then upgrade to 32/40 wave according to their needs, which can save the initial investment. The principle of the upgrade scheme: one is to upgrade the C-band red band 16 waves and the blue band 16 waves to 32 waves; the other is to use an interleaver, and the C-band is upgraded from 200GHz interval 16/32 waves to 100GHz interval 20/ 40 waves. For further capacity expansion, the C+L band expansion plan can be provided to further expand the system transmission capacity to 160 waves.
In the dense wavelength division multiplexer system, an independent 1510nm wavelength (rate of 2Mb/s) is used to carry the Optical Supervisory Channel (OSC) to transmit network management, official business and monitoring information, and the frame structure conforms to G. 704, the actual transmission rate for monitoring information is 1920kb/s. The OSC optical monitoring channel is the information carrier of the working state of the dense wavelength division multiplexer system. In the dense wavelength division multiplexer system, OSC is a relatively independent subsystem, which transmits the maintenance and management information of the optical channel layer, the optical multiplexing section layer and the optical transmission section layer, provides official communication and user access, and can also provide other additional functions. The main subsystem functions of OSC are: OSC channel reception and transmission, clock recovery and regeneration, reception of external clock signals, OSC channel fault detection and processing and performance monitoring, CMI codec, OSC frame positioning and framing processing, monitoring information processing . The performance monitoring (B1, J0, OPM, optical amplifier monitoring) can be completed by the service access terminal. The analog monitoring function and B1 error monitoring function provide multi-channel optical channel performance monitoring (including each channel wavelength, optical power, and optical signal-to-noise ratio) without interrupting services, timely monitor the performance quality of the optical transmission section and optical channel, and provide fault location. effective means. It has the function of monitoring the input optical power, output optical power, PUMP driving current, PUMP cooling current, PUMP temperature and PUMP back optical power of the amplifier. It has the performance of monitoring multi-directional wave number, wavelength of each channel, optical power and optical signal-to-noise ratio. The monitoring wavelength accuracy can be greater than 0.05nm, the optical power accuracy can be greater than 0.5dBm, and the signal-to-noise ratio accuracy can be greater than 0.5dB.
2. Features of wavelength division multiplexer
2.1 Make full use of the low-loss band of the optical fiber, increase the transmission capacity of the optical fiber, and double to several times the physical limit of the information transmitted by one optical fiber. We only use a very small part of the low-loss spectrum (1310nm-1550nm) of the fiber, and the wavelength division multiplexing can make full use of the huge bandwidth of the single-mode fiber about 25THz, and the transmission bandwidth is sufficient.
2.2 It has the ability to transmit two or more asynchronous signals in the same fiber, which is beneficial to the compatibility of digital signals and analog signals. It has nothing to do with data rate and modulation mode, and can flexibly take out or add channels in the middle of the line.
2.3 For the existing optical fiber system, especially the optical fiber cable with a small number of cores laid in the early stage, as long as the original system has a power margin, the capacity can be further increased to realize the transmission of multiple one-way signals or two-way signals without making major changes to the original system. , has strong flexibility.
2.4 Due to a large reduction in the use of optical fibers, the construction cost is greatly reduced. Due to the small number of optical fibers, when a fault occurs, it is also quick and convenient to restore.
2.5 The sharing of active optical equipment reduces costs for the transmission of multiple signals or the addition of new services.
2.6 The active devices in the system are greatly reduced, thus improving the reliability of the system. , Due to the high requirements of optical transmitters, optical receivers and other equipment for multi-carrier optical wavelength division multiplexing, the implementation of the technology is difficult. At the same time, the application of multi-core optical cables does not appear to be particularly scarce for traditional radio and television transmission services. Therefore, there are not many practical applications of wavelength division multiplexer. However, with the development of cable TV integrated services, the increasing demand for network bandwidth, the implementation of various selective services, the consideration of economic costs for network upgrades, etc., the characteristics and advantages of wavelength division multiplexer have gradually emerged in the CATV transmission system. , showing broad application prospects, and will even affect the development pattern of CATV networks.
3. Parameters of the wavelength division multiplexer
3.1 Insertion Loss
Insertion loss is an important indicator to measure the performance of passive optical devices, which represents the influence of the device on the optical power of each channel. It is generally required that the insertion loss of the combiner and demultiplexer is as low as possible.
Insertion loss (dB) = channel input optical power (dBm) - channel output optical power (dBm). For the optical combiner/demultiplexer, the insertion loss requirements of each channel are roughly the same, and the difference cannot be greater than 1 dB.
3.2 Isolation
Isolation is a parameter that specifically describes the demultiplexing unit, and is defined as the ratio of the output optical power of a certain wavelength to the optical power of another wavelength that crosstalks to the channel.
The isolation of the first wave to the second wave (dB) = P1 (dBm) - P2 (dBm), the isolation of the second wave to the first wave = P2 - P1, if there are more wavelengths, the calculation method is analogous. Isolation is generally required to be greater than 25 dB.
