The development of all-optical network technology

Absrtact: All optical network related technologies include all-optical switching technology, optical crossover technology, All-optical relay based on optical amplifier, multiplexing/multiplexing technology and optical separation technology. The principle, research progress and development prospect of these technologies are described and analyzed in this paper.

Key words: All optical network optical switching optical relay multiplexing/multiplexing OXC

1 All-Optical Network overview

All-Optical Network (all-optical Communication network) refers to the optical information flow in the network transmission and exchange is always in the form of light, without the need for optical/electrical, electrical/optical transformation. That is, the information is always in the light domain from the source node to the destination node's transmission process. The signal transmission in all optical networks is carried out in the optical domain. Therefore, all optical networks have the transparency of the signal. It realizes routing by wavelength selection device. All optical networks should also be scalable, reconfigurable and operable.

All optical networks have 3 basic types, such as Star Network, bus network and tree-shaped network.

2 All-Optical network related technology

The technology of All-optical network mainly includes all-optical switching, optical crossover, all-optical relaying and multiplexing.

2.1 All-Optical exchange

The traditional optical switching has the power of light and electricity, and the switching capacity is limited by the working speed of the electronic device, which limits the bandwidth of the optical communication system. Direct optical switching eliminates the switching process of light/electricity and electricity/light, making full use of the broadband characteristics of optical communication. Therefore, optical switching is considered to be the most potential new generation switching technology in the future broadband Communication network. The exploration of light exchange began in the 70 's, and the mid-80 development was relatively rapid. In general, optical switching is still in the early stages of development and is unlikely to have any form of extensive optical exchange applications until 2000. At the beginning of 21st century, the optical switching technology will reach the practical level and the commercial optical switch will enter the market.

Optical switching technology has such types as space (SD), Time Division (TD) and wavelength/frequency division (WD/FD). Its principle, structure characteristic and research progress status are as follows.

2.1.1 Air Separation Light Exchange

The optical exchange is realized by the switch matrix, the switch matrix node can be controlled by mechanical, electric or light, and the physical channel is established according to the requirement, so that any channel of the input port is connected with any channel of the output end, and the exchange of information is completed. All kinds of mechanical, electrical or optical control related devices can constitute the air separation light exchange. The switches that make up the light matrix are most noticeable with lithium niobate directional coupler.

2.1.2 Time Light Exchange

The time Division optical switching system can be combined with optical transmission system to form all optical network, so the research and development of time division optical switching technology is very fast, and the exchange rate is increased by almost one times every year, at present, several cent optical switching systems have been developed. 1985 NEC successfully implemented the 256MB/S (4-way 64mb/s) color image coding signal of the optical time-division switching system. It uses the 1x4 lithium niobate directional coupler matrix switch as the selector, the Bistable laser diode as the memory (the switching speed 1gb/s), composes the Single-stage switching module. The 512MB/S test system was introduced in the early 90.

The key to realize the optical time-division switching system is to develop high speed optical logic devices, which are being studied by research institutes all over the world.

2.1.3 Wavelength Division/Frequency Division optical switching

Wavelength Division exchange is the signal through different wavelength, select a different network path to achieve, by switching to switch to exchange. WDM networks consist of wavelength multiplexer/multiplexer, wavelength-selective space switch and wavelength switch (wavelength switches).

At present, Wavelength division multiplexing (WDM) system is developed, which is about 10. Recently, a kind of 1.2tb/s wavelength division switching system has been developed, it adopts the wavelength division multiplexing number of 128, the maximum terminal number is 2048, and the multiplexing level is equivalent to the exchange throughput of the switch.

2.2 Optical Cross Connection (OXC)

OXC is a device for optical fiber network nodes, it is an important means to realize reliable network protection/recovery and automatic wiring and monitoring by cross connection between optical signals and flexible and effective management of optical fiber transmission network. Oxc is mainly composed of optical cross connection matrix, input interface, output interface, management control unit and other modules. In order to increase the reliability of OXC, each module has the main and standby redundancy structure, and OXC automatically makes the main standby switching. The input interface and the output interface are directly connected with the optical fiber link to fit and enlarge the input and output signals respectively. The Management control unit is programmed to monitor and control the cross linking matrix, input interface, output interface module, and the optical cross connection matrix is the core of OXC, which requires non-blocking, low latency, wide band and high reliability, and has one-way, bidirectional and broadcast functions.

There are 3 types of OXC, Time Division and Wave Division. At present, the more mature technology is wavelength division multiplexing and air separation technology, time technology is not mature. By combining wavelength division multiplexing with air separation technology, the capacity and flexibility of cross join matrices can be greatly improved.

The 8x8 electrodeless LiNbO3 optical crossover matrix developed by NEC is composed of 64 directional coupled switching units, all switching units are integrated into the LIN-BO3 chip in the form of simple tree structure (STS). OXC, developed by BT Laboratory, combines WDM technology with air separation technology and has been used in wavelength division multiplexing system. Field experiments were conducted on the local network in London, with a 622mb/s rate of transmission. In addition, Siemens, NTT and Ericsson and other large foreign companies in the laboratory of OXC structure, application technology has also carried out similar research and experiments.

2.3 All-Optical relay

The traditional optical fiber transmission system adopts the optical-electric-optical regenerative repeater, which is very complicated, which affects the stability and reliability of the system. Over the years, people have been exploring to remove the light-electricity-light conversion process, directly on the light path to the signal amplification transmission, that is, a full-optical transmission repeater to replace the current regenerative repeater. Scientists and technicians have developed semiconductor optical amplifier (SOA) and fiber amplifiers (Erbium-doped fiber amplifier--EDFA, praseodymium-doped fiber amplifier-PDFA, niobium-doped fiber amplifier-NDFA)

EDFA has a series of advantages such as high gain, high output, wide band, low noise, gain characteristic and polarization independent etc., which can promote the development of a batch of new transmission technologies such as super large capacity, ultra-high speed and all-optical transmission. The main features of the all-optical communication system composed of optical amplifiers are: The working wavelength is the 1.55μm wavelength with the lowest optical fiber loss, the coupling loss of the line is very small, the noise is low (4~8db), the bandwidth is wide (30~40nm), and it is suitable for WDM transmission. However, in WDM transmission, due to the different wavelength of each channel, there is gain deviation, after multistage amplification, gain deviation accumulation, low electro-mail signal Snr deteriorated, high frequency signal is also due to optical fiber nonlinear effect of the signal characteristics of deterioration. In order to make EDFA gain flat, the main use of "gain equalization technology" and "optical fiber technology." The gain equalization technique balances the gain inhomogeneity with the principle that the loss characteristic is opposite to the gain wavelength characteristic of the amplifier. At present, the main use of fiber Bragg grating, dielectric multilayer film filter, planar optical waveguide as a equalizer. "Optical fiber technology" is to improve the characteristics of the EDFA by changing the fiber material or using the combination of different optical fibers to change the characteristics of EDF. The technology includes the following aspects: (1) The development of Erbium doped tellurium glass fiber. The EDFA made of this kind of fiber can make the gain characteristic flat and the frequency band widen. And the frequency bands move toward the long wavelength side. According to NTT in OFC ' 97, its maximum bandwidth of up to 80nm. Between the 1535~1561nm, the gain is basically flat and the maximum deviation is not more than 1.5dB. (2) Multi-core EDFA. Multi-core EDFA uses EDF with the most fiber cores. The excitation light energy is roughly evenly distributed into the first core, and the optical signals in each core are amplified by small signals, thus obtaining a near-flat gain in a wide wavelength range. (3) The development of erbium-doped fluoride fiber amplifier can obtain flat gain in the show wide frequency band. (4) by adding aluminum in erbium-doped fiber, changing the amplification level distribution of erbium, widening the amplified frequency band. (5) Combining different doped materials and doped fiber, making hybrid EDFA. There are mainly (A1-EDF) and (P-A1-EDF) combinations, A1-EDF and P-YB-EDF combinations, erbium-doped quartz fiber and erbium-doped fluoride fiber combinations. In this way, the gain flatness, noise characteristics and amplification efficiency can be achieved best.

The maximum output power of EDFA has reached 27dBm, the optical fiber amplifier can be applied to dense wavelength division multiplexing transmission system over 100 channels, optical image signal distribution system and space optical communication in the access network.

At present, the optical amplification technology is mainly using EDFA. Although SOA has been developed earlier, it has not been applied to the effect of noise and polarization dependence. However, the success of the SOA development of strain quantum well material has aroused people's interest, and SOA has the advantages of simple structure, low cost, mass production and so on, people are eager to develop a 1310nm and 1550nm SOA covering EDFA, PDFA application window.

For the Pdfa of 1310nm window, the research progress is slow and not practical due to the difficulty of making fluoride fiber and the limitation of the characteristic of fluoride fiber.