Key technologies of dense wavelength division multiplexing optical Networks

The explosive development of ip-based data Service brings unlimited demand for bandwidth, and dense wavelength division multiplexing (DWDM) system has incomparable networking advantages while satisfying the increasing bandwidth demand. In this paper, the key technologies are discussed.

Key technologies of full optical networking for keywords dense wavelength division multiplexing (DWDM) WDM Optical Networks

WDM Optical Network in the continuous progress at the same time still have a lot of problems to be solved, such as expensive equipment, standardization issues, how to distribute wavelength among different users, how to user authentication and retention issues, crosstalk, integrated network management mechanisms and interoperability issues. The realization of optical networking depends mainly on the maturity of the key network element equipment. Research or invention of new technologies and devices that can be used in WDM is of paramount importance, and a new technology or device can greatly improve the performance of the entire system, sometimes overturning the entire old system. So now there are a lot of big and small new old companies, have invested a greater power to develop WDM technology and new optical devices.

1 Key devices

To construct the future high-speed and high-capacity information network system with optical network, we need to focus on high speed optical transmission, multiplexing and multiplexing technology. Based on optical interpolation multiplexing (OADM) technology, Optical Cross Interconnection (OXC) technology, integrated narrowband, high-speed, wavelength tunable low noise detector technology, and can be used for fiber network backbone transmission, the speed of 4ogbit/s, wavelength tunable, High-stability gain-coupled DFB laser/light modulator integrated light source.

1) fiber transmission is generally considered to be a large dispersion of single-mode fiber SMF, it is beneficial to reduce the disturbance caused by four wave mixing (FWM), but it needs a lot of compensating fiber. The actual experiment shows that SMF (g.652) and DSF (g.653) are used in WDM systems, and their spm,xpm is less harmful than is thought. In the past, theoretical and experimental results show that the FWM interference of DSF optical fiber is serious and not suitable for WDM system. However, when Raman amplification is used, the amplification is distributed rather than concentrated along the optical fiber, so that the transmitted light power can be reduced, so that the FWM interference can be reduced, so the WDM in the DSF fiber transmission can still achieve good results. The bias array mode color (PMD) and dispersion compensation are the inevitable problems in long-distance large capacity WDM system, if you want a wide and flat band. The dispersion and dispersion slope of the dispersion compensation device are also required.

2 DWDM Light source WDM Optical network requirements are high speed (large capacity), low chirp (to improve transmission distance), working wavelength stability, to study and develop high-speed, low chirp, working wavelength adjustable and highly stable light source. From the development trend of the world, integrated light source is the preferred scheme, and the integration of laser and modulator has the features of stable wavelength, adjustable and high speed and low chirp of the modulator. There are several integrated light sources: One is the DFB semiconductor laser and the electronic absorption modulator monolithic integration. The second is the single chip integration of the DFB semiconductor lasers and the m-z modulator: There are also distributed Bragg reflector (DBR) lasers and modulator monolithic integration as well as a hybrid integrated DBR laser composed of semiconductors and fiber gratings.

3 The wavelength tunable narrow-band optical detector of DWDM detector is a kind of optical receiving technology which is efficient and high SNR in WDM Optical network. In order to greatly reduce the size of the system, you can consider the integration of Preamplifier circuit and detector together. Each detector of such device must correspond to different channels, so the detector must be narrowband, and the peak wavelength of response must be to the central wavelength of the notified channel, so the response bandwidth must be tunable within a certain range. Furthermore, the crosstalk between detectors is required to be small. Resonant cavity enhanced (RCE) optical detectors set narrow-band tunable filters and detectors in one, is the preferred solution for such detectors.

4 Wavelength conversion All-optical wavelength conversion module in the access-side application is to the router or other devices from the optical signal conversion, the optical signal on the mismatched wavelength is converted to the standard wavelength of the ITU and inserted into the optical coupler, and when it is used for wavelength switching nodes, it swaps the optical path and performs the function of wavelength reuse. , so it has a great effect on wavelength-routed all-optical networks. Broadband transparency and fast response are the basic requirements of wavelength converters. In the technology of all-optical wavelength switching (including cross gain modulation, cross phase modulation, four mixing, nonlinear optical loop mirror), The most promising all-optical transponder is a wavelength converter, which is composed of an integrated Mach-zehnder interferometer (MZI) or Michelson Interferometer (MI) in a semiconductor optical amplifier (SOAS) based on the cross phase modulation principle, which is recognized as a high speed, The ideal scheme for wavelength conversion in large capacity optical networks.

In large-scale use of WDM networks, especially channel scheduling, one wavelength may need to be converted to another wavelength, or the entire band of the transformation. The optical band converter developed by Lucent is based on the second order nonlinear coefficient x (2): X (2) of LiNbO3 to transform the wavelength. Optical waveguides are periodic polar LiNbO3 optical waveguides (periodicallypoledwaveguide).

5 amplifier is needed to overcome attenuation in optical fiber. Erbium-doped fiber amplifier EDFA has been widely used in long-distance communication system, it can provide 30nm of flat gain bandwidth in 1550nm windows.

For broadband EDFA amplifiers, the gain-flatness characteristics of the entire WDM bandwidth are particularly needed. There has been a dual-band fiber amplifier DBFA (dual-bandfiberamplifier) based on erbium-doped fiber, its bandwidth can cover 1528~1610nm range. It is composed of conventional EDFA and extended band Fiber amplifier EBFA (extendedbandfiberamplifer). Similar products have Belllab Uwoa (ultra-widebandopticalamplifier), which has 80nm of available bandwidth to magnify up to 100 channel wavelength channels in a single fiber. It covers the C-band (1530~1656NM) and L-band (1565~1620nm).

The British Imperial College (Ukimperialcollege) developed a broadband Raman amplifier. Stimulated Raman Amplification (stimulatedramanamplify) is a direct addition of optical pump power in conventional optical fiber, which amplifies the optical signal by the nonlinearity of the optical fiber. The Raman amplification of the single optical channel pump has a narrower gain bandwidth, and a Raman amplifier with a wavelength of 1420nm and 1450nm two optical pumps can obtain a wide bandwidth (1480~1620nm). The gain of Raman amplification is up to 30dB and the noise coefficient is less than 6dB. The optical pump power is 860mW.

6) Optical Division multiplexer (OADM) and optical crossover connector (OXC) Optical Division multiplexer OADMS (OPTICALADDDROPMUXS) to achieve in the WDM optical fiber in the selection of the ground/down (DROPORADD) specific to any rate, format and protocol types of the required optical wavelength channel. It is a high-speed high-capacity WDM fiber network interface with the user interface. Oadm is generally a multiplexer, a multiplexer, a monolithic integrated or hybrid integration of optical switching arrays. Oadm devices with adjustable wavelength work are under development and breakthroughs have been made. In addition, the cross interconnection between WDM optical networks will gradually transition to the full use of light form. There is already a report on the laboratory work of the monolithic integrated OXC, but more work is focused on the key devices, mainly in order to solve the network congestion and reasonable use of network resources wavelength conversion devices. AWG (arraywaveguidegrating) is a new key device which is most suitable for DWDM multiplexing and multiplexing and as a core device to compose Oadm and OXC. Because AWG can be efficiently coupled with quartz fiber, the insertion loss is very low and can achieve low cost integration. In addition, AWG reduces the integration of the light source surface array, the use of multiple single wavelength lasers and their coupling can achieve the DWDM target. The key technique of this research is to master the preparation technology of thick-layer waveguide, try to avoid the polarization dispersion caused by stress and even cause the device to rupture.

7 Light Switch Optical waveguide switch integrated surface array is also a key component of OXC and Oadm, the current practical optical switching array, mostly with LiNbO3 optical waveguide switch to achieve. This kind of optical switch matrix realizes large scale monolithic integration difficulty, in particular, it is difficult to achieve OEIC integration with the Operation Circuit, but also the use of sio2/si hot-light switch, but the response speed is slow, about the millisecond level, only for channel switching, the exchange of the letter/packet, the response speed can not meet the requirements, The minimum response time to achieve a letter/packet Exchange is a microsecond level. Quasi-real-time exchange (such as in the computer network Exchange) to achieve a nanosecond magnitude. The utilization of information resources in the network depends on the integration scale of OXC and the flexibility of operation, so the final OXC should be monolithic integrated. The key technology is to develop the high speed response SI base and light switch, but the SOI type SIO2/SI waveguide light switch, which is composed of the electric injection folding effect, can realize the operation of light switch less than microsecond, and it is expected to realize large-scale monolithic integration.

The Hirsch laboratory has developed a very high speed optical switch, which can be sampled in the 160gbit/s optical data stream. Its working principle is: using the wavelength of 1302nm, 1312nm of two optical pulses in the semiconductor optical amplifiers generated by four wave mixing can be compared to check 155Onm of light signal pulse sampling. This high-speed switch is suitable for future direct extraction of routing addresses from optical IP signals in order to achieve optical IP (ipoveroptical).

Micro-mirror Array optical switch technology based on MEMS (MICRO-ELECROMECHAN-ICALSYSTEMS) technology is also a hotspot in recent development. The use of MEMS technology in optical networks has the advantages of low cost, high speed, small volume, large communication capacity, small volume, flexible variable, transparent bit rate and protocol, and increasing network speed through electronic restriction. But the current switching speed is not up to the requirements. Micro-mechanical technology can also do variable optical attenuator, its working principle is the use of electrostatic force to change the position of the shield in the micro-machinery, to cover the optical fiber guide area, so as to change the light attenuation. The device can be controlled by optical signal, which can be used for making: optical attenuator, optical power stabilizer, optical power equalizer and optical band switch.

Another kind of optical switch is a high molecular digital switching device. Recently, optical waveguide devices using polymer polymer materials are becoming mature. The polymer material is easy to process, the cost is low, the voltage on the electrode can control the light signal through or not through the optical waveguide. At present, the problem is that it is easy to peel off, absorb water and aging on substrate silicon.