One, optical wavelength division multiplexing (WDM) technology
Wavelength division multiplexing (Wavelength Division Multiplexing, WDM) is a technology in a single fiber and multi wavelength optical carrier signal at the same time, and each optical carrier by FDM or TDM, each carrying multiple analog or multiplex digital signals. The basic principle is to combine the different wavelengths of light signals in the transmitter (multiplex), and coupled to the transmission of the same fiber cable on line, at the receiving end and the combination separately in different wavelength signals together (demultiplexing), and further processing, different terminal to restore after the original signal. Therefore, this technology is referred to as optical wavelength division multiplexing, which is referred to as optical wavelength division multiplexing technology.
The expansion of WDM technology on the network upgrade, the development of broadband business, mining fiber bandwidth capacity, implementation has important significance for ultra high speed communications, especially with erbium-doped fiber amplifier (EDFA) WDM has a strong attraction to the modern information network.
Two, the basic structure of WDM system
The basic structure of WDM system mainly consists of two fiber unidirectional transmission and bidirectional transmission in two ways. One way WDM refers to all the light path in one fiber at the same time along the same direction of transmission, will contain all kinds of information in the transmitter with different wavelength modulated light signal through the optical device with extended together, and in a fiber unidirectional transmission, because the signal is carried by different wavelengths of light so, each other will not be confused, separated in the optical signal receiver through the multiplexer to light of different wavelengths, transmit multiple light signals, while the opposite direction through another optical fiber transmission. Bidirectional WDM is the optical access in one to two different direction of the transmission of the optical fiber, the wavelength used to separate each other in order to achieve full duplex communication between the two sides. The current WDM system in the development and application of unidirectional and bidirectional WDM is more widespread, due to the influence of factors such as the channel isolation and crosstalk interference, light reflection effect, bi-directional path between the design and application, the actual application.
Three, two fiber unidirectional WDM system composition
As an example, the WDM system is mainly composed of 5 parts: optical transmitter, optical relay amplifier, optical receiver, optical monitoring channel and network management system.
1 optical transmitter
The optical transmitter is the core of WDM system, in addition to the special requirements of the center wavelength of laser emission in WDM system, also should according to the different applications of WDM system (mainly type and transmission distance of optical fiber) to select the transmitter with chromatic dispersion capacity. At the sending end first from the terminal equipment output light signal using the optical transponder light signal non specific wavelength conversion of the signal with a specific wavelength stability, the combiner synthesis of multi-channel optical signal, the optical power amplifier (BA) output.
2 optical repeater amplifier
After long range (80~120km) optical fiber transmission, the need for optical signal to amplify the optical signal, the majority of the current use of optical amplifiers for erbium doped fiber optical amplifier (EDFA). In the WDM system, the gain flattening technique is used to make the EDFA have the same amplification gain to the optical signal of different wavelength, and the gain competition of the optical channel can not affect the transmission performance.
3 optical receiver
In the receiver, optical preamplifier (PA) amplification of main channel signal via the transmission and attenuation of the wave splitter from the main channel optical signal from the light of a specific wavelength channel, the receiver should not only meet the optical signal sensitivity, overload power and other parameters of the requirements, but also can bear a certain signal light noise, have the electric performance of sufficient bandwidth.
4 optical monitoring channel
The main function of the optical monitoring channel is to monitor the transmission of each channel in the monitoring system. At the sending end, the optical monitoring signal is inserted into the optical monitoring signal generated by the node which is a wavelength of S (1550nm), and the output of the optical signal of the main channel is combined with the optical signal. At the receiving end, the received optical signal is divided into waves, respectively, the output wavelength of S (1550nm) of the optical control signal and operational channel light signal. The overhead bytes used by the frame synchronization byte, the official byte, and the network management are delivered through an optical monitoring channel.
5 network management system
The network management system through the optical supervisory channel to transmit overhead bytes to other nodes or receive overhead bytes from other nodes on the WDM management system, realize configuration management, fault management, performance management, security management and other functions.
Four, optical wavelength division multiplexer and multiplexer
In the whole WDM system, the optical wavelength division multiplexer is a key component in the WDM technology, and its performance has a decisive effect on the transmission quality of the system. The signals of different wavelengths of light source are combined together by a device called the transmission fiber output multiplexer; on the other hand, the multi wavelength signal transmission fiber from the same individual wavelengths were decomposed into device output called demultiplexer. In principle, the device is reciprocal (reversible), the output and the input of the demultiplexer is used in turn, multiplexer. Optical wavelength division multiplexer performance indicators are mainly access loss and crosstalk, the requirements of loss and frequency offset is small, the loss of access is less than 1.0~2.5db, the crosstalk between the channels is small, isolation is large, the impact of small signal between different wavelengths. In the current practical application of WDM system, there are mainly grating based optical wavelength division multiplexer and dielectric film filter.
1 grating type optical wavelength division multiplexer
The grating is a groove mark which can be characterized by equal and equal distance in a plane that can be transmitted or reflected,
