The key of all optical networks: Optical Division Multiplexing (OADM) node technology

Optical communication has the characteristics of large bandwidth, high reliability and low cost, and the rapid development of optical communication system and optical Network brings new revolution to the information age. The application of OADM node in optical network makes the routing operation in the loop not affected by the type and rate of transmission signal, thus realizes the transparency of the local network and lays the foundation for providing end-to-end wavelength business. That is to say, the user can carry any form, any rate of information according to their needs at a certain wavelength, and the network through the wavelength identification route to the destination.

An overview

Brief introduction of WDM Optical Network

With the exponential growth of data services, especially the rapid popularization of the Internet, the existing network technology has not been able to adapt to the vast number of users of the network speed and bandwidth requirements. In the 90 's, the practical Wavelength division multiplexing (WDM) technology can better utilize the broadband capability of optical fiber, it is a comparatively economical and practical method to enlarge the transmission capacity, so it has been developed rapidly in recent years, the current commercialized system has reached 400gb/s, and the experimental system is 10tb/s.

However, at present, optical fiber transmission information to the node must also be all through the optical/electrical conversion, rely on electronic devices to interconnect and exchange, and then convert the electrical signals into light signals downward transmission. The speed of photoelectric conversion and electronic equipment limits the increase of exchange capacity, namely, the formation of so-called "electronic bottlenecks".

It can be expected that WDM all-optical networks (Wdm-aons) based on WDM transmission and OADM and OXC optical nodes will become the dominant new generation of optical fiber communication networks, with high transparency, compatibility, reconfiguration and scalability to meet the rapid growth of information communication capacity.

Oadm is one of the key devices in Wavelength division multiplexing (WDM) optical network, which can receive and transmit some wavelength channels selectively and locally from the transmission optical path, without affecting the transmission of other wavelength channels. In other words, oadm in the optical domain to achieve the traditional SDH (electro-synchronous digital hierarchy) plug-in multiplexer in the time domain function, and transparent, can handle any format and speed signal, which is superior to the electric Adm.

Research progress and technical level of OADM

In view of the important role of OADM in backbone nodes and local access, universities, companies and groups at home and abroad have carried out more in-depth research, which has greatly promoted the commercialization of OADM. The Monet program, which began in the United States in 1994, contains a 8-wavelength channel OADM node based on acousto-optic tunable filter structure. The EU's acts plan, which began in 1995, has cobnet (United Optical Trunk Communication Network) and Meton (Optical Metropolitan Communication Network), which are all two projects related to Oadm, and the plan has conducted extensive and in-depth research on OADM devices.

From the commercial level, Lucent Company has developed 40X10GB/S with perfect network interface Oadm node, and successfully to the market. Other companies, such as Alcatel,siemens,nec, also have mature products to launch. At present, the domestic research on OAMD has made great progress, in the 863-300 project "China High-speed Information demonstration network", Datang, Wu-mail, ZTE completed 8-channel wavelength, arbitrary up and down OADM nodes, with a sound network management interface, according to network requirements, OADM flexible configuration.

The technical principle of the second Oadm

Physical model of Oadm

The general OADM nodes can be represented by the four-port model, and the basic functions include three kinds: the wavelength channel needed for the next road, multiplexing the road signal, so as to make the other wavelength channels as far as possible without being affected. Oadm specific work process is as follows: The WDM signal from the line contains n wavelength channel, enter the Oadm "Main Input" end, according to business requirements, from the N-wavelength channel, selectively from the lower end (drop) output the required wavelength channel, Enter the desired wavelength channel from the Road End (ADD) accordingly. and other local-independent wavelength channels are directly passed through the Oadm, and the road wavelength channel multiplexing together, from the Oadm line output end (Main output) output.

The requirements of network design for OADM

According to the different network design, business requirements and resource allocation, optical network has some requirements for the OADM nodes, mainly in the performance requirements, embodied in the following aspects: reconfiguration, scalability, transparency and multi-channel processing capacity.

In addition, the introduction of OADM to network management pros and cons. Although OADM allows for flexible management of optical channels, its flexibility is not entirely unconstrained, and the signal deterioration caused by OADM requires serious consideration. There is a balance point of technology selection in the optical performance of network target and Oadm.

The main parameters in OADM

The main parameters are: Channel spacing, channel bandwidth, central wavelength, channel isolation, wavelength temperature stability, channel differential loss uniformity.

Classification and comparison of OADM node technology

The core device of the OADM node is the optical filter device, the wavelength of the upper/lower path is selected by the filter device, and the wavelength routing is realized. At present, it is applied to the mature filter in oadm, such as audio-optical tunable filter, body grating, arrayed waveguide grating (AWG), Fiber Bragg grating (FBG), multilayer dielectric film, etc.

According to the flexibility of the upper and lower wavelength, the oadm can be divided into fixed wavelength oadm, semi reconfigurable oadm and fully reconfigurable oadm. The fixed wavelength oadm and semi reconfigurable oadm can be applied in the system, but it is difficult to realize the complete reconfigurable oadm of the upper and lower arbitrary wavelength channels in large network nodes.

From the concrete form of OADM realization, it mainly includes the two types of the light switch array and the fiber Bragg grating light switch.

1) Split-wave filter Light switch array

The wavelength routing of this structure is based on the Oadm, and the direct and Up-down switching is realized by the optical switch or optical switch array. The crosstalk between the branch of the structure and the group is determined by the optical switch, and the crosstalk between the wavelengths is determined by the wave-filter. Because the loss of the wave-filter is generally relatively large, the main problem of this structure is the large insertion loss. At present, many devices such as body grating, multilayer dielectric film and arrayed waveguide grating are used in the wave-filter. From a physical point of view, the Wave splitter will be used to become a wave-filter, of course, in the actual design of the filter and the wave-filter considerations are slightly different, the following from the composition of the angle of the wave splitter to the three devices are briefly introduced respectively.

Multilayer Dielectric Film

Multiple FP cavity cascade form Multilayer dielectric film, according to each FP cavity through different wavelength to achieve the function of multiplexing, this is the working principle of multilayer dielectric membrane. The utility model has the advantages that the top strip is flat, the wavelength response is sharp, the temperature stability is good, the loss is low, the polarization of the signal is not sensitive, and it is widely used in the commercial system. However, because it is connected to the optical fiber through the lens, so the fiber coupling needs to be calibrated accurately, and the stability is also affected by the ambient temperature, so it is difficult to ensure the precise control of the central wavelength of the bandpass in the process of production and reproduction.

Body Grating

The body grating belongs to the role-scattered device. The diffraction grating is deposited on the glass substrate by depositing the epoxy resin on which the grating line is fabricated and the reflective blazed grating is formed. After the incident light is illuminated on the grating, the light of different wavelengths is reflected at different angles because of the role of the grating, and then the lens is assembled into different output fibers, thus the wavelength selection function is completed. Because the body grating is the body type installment, is not easy to manufacture, the price is expensive.

Arrayed waveguide gratings

Any input from the ordinary NXN star coupler will be transmitted to all output terminals without any wavelength selectivity. In the array waveguide grating (AWG), the input light in any working frequency band will be output from a certain port, so the function of multiplexing and multiplexing can be realized. Compared with the multilayer dielectric films commonly used, AWG is characterized by compact structure, low cost and narrower channel spacing, which is suitable for large multi-channel nodes.

The problems that AWG needs to solve are: the influence of polarization, the influence of temperature, the connection and coupling of optical fiber.

2) Fiber Bragg grating

Fiber Bragg Grating (FBG) is a optical device which uses ultraviolet interference to form a periodic refractive index change (grating) in optical fiber. The advantage is that it can be directly written into the communication fiber, low cost, high repeatability, can be mass-produced, easy to connect with various optical fiber systems, small connection loss, wavelength, bandwidth, dispersion can be flexibly controlled. The main problem is the influence of the environment, such as temperature, strain and other factors of small changes will lead to the drift of the central wavelength.

The main line WDM signal is selected by the switch, each grating is aligned to a wavelength, the wavelength reflected by the grating passes through the circulator to the local, and the other main signal wavelength is combined with the signal wavelength of the local node through the grating, and continues to transmit forward on the main line. This scheme can choose the wavelength of the upper and lower channel according to the switch and grating, and make the configuration of network resources more flexible. Because each FBG can only be the next wavelength channel, due to the cost of production, this structure can only be applied to small nodes with little or no upper and lower paths.

Third, summary

With the development of semiconductor technology and photon integration technology, it can be expected that the future oadm will be smaller and integrated, and its price will be further reduced to meet the practical needs.