Progress of optoelectronic devices in automatic switched optical networks

(Department of Electrical Engineering, Tsinghua University, Beijing 100084)

Optical Network is developing in the direction of high speed, large capacity, good expansibility and intellectualization. The appearance of automatic switched Optical Network is an important breakthrough of optical transmission network technology, and the enhancement of the function of optoelectronic devices will play a decisive role in its realization and development. This paper introduces the important characteristics and development trend of some optoelectronic devices in the automatic Switched Optical network.

Keywords Automatic switched Optical network optical transmission Network optoelectronic devices

1 Introduction

In recent years, with the Internet business as the main data services, the rapid growth of Internet services has led to the urgent need for the expansion of the telecommunications network, and because of the IP traffic itself in the burst, self-similarity and asymmetric, the network bandwidth dynamic distribution requirements are more and more urgent. Therefore, the industry strongly recommends defining a new network concept that automates network connectivity-intelligent automatic switching Network (ASTN). This is an independent control layer to implement dynamic configuration and connection management network, in which the optical transport network (OTN) based on the ASTN also known as Automatic Switched Optical Network (ASON), is the main direction of development astn. The introduction of dynamic switching in traditional optical networks is a major breakthrough in the concept and technology of OTN, so that OTN has the higher intelligence of automatic route selection and management. In addition to meeting the general requirements of ASTN, the main functions of the Ason control layer are to simplify the fast and efficient connection allocation within the transmission network to support switched connections and soft permanent switching connections, to support the modification and reconfiguration of established call connections, and to implement recovery functions. At present, many ISO have joined in the research work in this field. In the study of Ason, the development of related technologies and devices is critical in addition to these well-defined and developed architectures and the implementation of the Protocols. Besides the general requirements of OTN, the optoelectronic devices in Ason in the network performance detection, control, a variety of tunable devices, optical switches and switch arrays have high requirements, the advanced, reliability and economy of these new devices will directly affect the system equipment and even the entire network vitality and market competitiveness.

Development trend of optical electronic devices in 2 optical network

The basic feature of next generation optical transport network is super capacity, from the current development of various multiplexing technologies, dense wavelength division multiplexing (DWDM) is considered to be the most effective way to enlarge network capacity and improve its flexibility. The use of DWDM can rapidly expand capacity by dozens of times to hundreds of times times. Because of the influence of market drive and technology breakthrough in recent years, Wavelength division multiplexing system develops extremely rapidly. Therefore, all kinds of newly developed optical devices are more or less related to wavelength division multiplexing. The development of DWDM has been the pursuit of a higher frequency spectrum efficiency, on the one hand, improve the speed of each channel, on the other hand increase channel density. At the rate, the current commercial systems are mostly 2.5gbit/s or 10gbit/s, the higher rate 40gbit/s systems are being applied, and commercial applications are expected to commence by 2004, and some telecommunications companies such as Alcatel Laboratories have carried out 160gbit/s transmission experiments. In the channel density, the wavelength gap between the channels has been small to 25GHz, but also to 12.5GHz efforts, making the total number of commercial system channels are now 160~240, the laboratory up to 1022. In order to get larger capacity, it is sometimes necessary to compromise between the above two, while also taking measures to restrain the dispersion and non-linear effects in optical fibers. All these requirements relate to the high speed, flexibility and reliability of the device, and ultimately to the low cost, which makes the current new principles, new structures and new features of the device continue to emerge.

According to the function, the optical electronic devices in the optical network can be divided into two kinds: active devices and passive devices. Active devices are mainly responsible for the production, amplification and reception of optical signals, complete the signal of optical/electrical, electrical/optical conversion and amplification functions. Passive devices are mainly used to control the optical signal flow direction, pass and break and light signal integral type. Active optical devices provide most of the functions of optical network, and also occupy most of the cost of the whole system, including lasers, optical receivers, optical modulators, optical amplifiers and wavelength converters. DFB laser and high speed modulator are the most important devices in DWDM optical networks and are widely used in 2.5~40gbit/s high-speed systems. Pin and APD photodiode are essential for optical receiving devices. The receiver with a rate of 10gbit/s has been applied, and the 100GBIT/S device has a laboratory sample. Optical amplifiers mainly include fiber amplifiers and semiconductor optical amplifiers (SOA), in which erbium-doped fiber amplifier (EDFA) has been widely used in practical projects, the current research hotspot is bandwidth over 80nm UWB EDFA and distributed Raman fiber amplifiers. Due to its small size, gain width and amplification, switch and other functions, SOA is expected to be a counterweight to optical fiber amplifiers in the metropolitan area Network and has irreplaceable function in all optical wavelength converters. More rapid than active devices are a wide variety of passive optical devices, they will play a more and more important role in optical networks, the proportion of the total system costs would continue to rise. Mainly include: dense wavelength division multiplexing/Zhei, optical isolator, Ring line, optical coupler and connectors, optical switches and optical switching arrays, fixed or reconfigurable optical splitter multiplexer (OADM), optical crossover connectors (OXC), dispersion and dispersion slope compensator, polarization mode dispersion compensator and optical amplifier gain flattening, etc.

In recent years, with the "network economy" bubble burst, the optical communications industry's capital expenditure greatly reduced, as the optical communications industry chain at the bottom of the optical electronic devices industry is facing a very big challenge. It is estimated that 2002 U.S. capital spending on optical devices will continue to fall by 24% in 2001, by a 29% plunge. On the other hand, the early estimate of the market blind optimism caused a large backlog of optical electronic devices, it is estimated that the situation will continue to 2003 years. In this kind of market environment, the research and development trend of optoelectronic devices is mainly shown in the following aspects:

(1) from the optical electronic devices to achieve the function, so that optical network capacity is larger, more intelligent is still the development direction of optoelectronic devices, but the focus of the study has changed. In the aspect of system transmission capacity, the research direction of optoelectronic devices will focus on reducing the cost of each bit per kilometer of transmission system, instead of pursuing the breakthrough of single fiber transmission rate. There are three ways to improve the optical fiber transmission capacity: Extending the optical band, increasing the optical channel density and increasing the channel rate. In device-level research, the broadband amplifiers combined with EDFA are considered to be the most promising optoelectronic devices when the system is extended to the L-band, and the wavelength-locked lasers, high-power cladding pumped EDFA and high-density group filters will reduce the optical channel spacing to 50GHz, A key device in a high optical channel density transmission system of 25GHz or even 12.5GHz. 40GBIT/S High Speed optical modulator and receiver, dynamic dispersion compensator and polarization mode dispersion compensator will be the key devices in the system with Channel rate of 40gbit/s. The performance and price of these key optoelectronic devices will directly affect the selection of future optical transmission system design, but the key products are still on the 10GBIT/S series, and the 2.5gbit/s products will gradually decline.

(2) miniaturization and integration are becoming a new trend in the competitiveness of optoelectronic devices. With the increasing proportion of optoelectronic devices in optical transmission equipment, the miniaturization requirements of optical electronic devices are becoming more and more obvious. So that the equipment can take up less area of the room and less energy consumption, can effectively reduce the operating costs of the network. The miniaturization requirements of optoelectronic devices also promote the development of integrated technology. The Optoelectronic integration technology can integrate the photon element with its drive electronic chip. The planar waveguide integration technology can integrate the optical switch, the tunable attenuator and the Wavelength division multiplexing/Zhei device and so on, the system which realizes the subsystem function in a chip is compared with the discrete component system, which greatly reduces the volume and reduces the cost of the package. In the development of miniaturized optical devices, laser/detector devices and microelectronic chips are assembled into one, and the development trend of forming a variety of functional modules is obviously accelerated. Modularization can eliminate parasitic parameters to improve performance, and can save the process and cost of post assembly. It has also facilitated the cooperation and standardization of relevant industries, such as the agreement reached by several enterprises on the 10GBIT/S transponder's optical, electrical and mechanical performance standards a year ago, greatly boosting the performance-price ratio of such devices. In function, forward error correction (FEC), hot plug has been widely adopted for high-end products. in size, compared with the traditional disk, with the integrated transponder module to reduce the volume to the original 1/10, power loss of 2/3 and the price is only 1/3 of the original. The main optical transceiver module used in metropolitan area network and access network is also developed from duplex SC type to the SFF module of smaller package. Compared with the Duplex SC package, it reduces the size of the disk by 1/2. In the light amplifier aspect, the new EDFA module size only 7cm′9cm′1.2cm (long is the width is high), but can provide the 24dB gain and the 15dBm power output. Modularity has further facilitated the progress of micro-encapsulation lasers and refrigeration-free lasers. Now not only is the optical signal source with the laser, power pump laser also achieved no refrigeration technology breakthrough. 120mW of the following 980nm without refrigeration laser has been provided for goods, because of the skip cooler, EDFA module power loss from 4.5W to less 1W, the volume is greatly reduced. It is noteworthy that erbium-doped waveguide optical amplifiers (EDWA) have been integrated into planar waveguide recently to overcome the disadvantages of large insertion loss of planar waveguide devices, making it possible to make a more complex planar waveguide device with newer functions.

(3) The automation technology of OPTOELECTRONIC Devices Assembly will be the key to reduce the cost of optical electronic devices. Manual assembly is the main factor limiting the further decrease of the cost of optical electronic devices. Automatic assembly can reduce human cost, increase production and save production space, so the research of automation technology of optoelectronic Device Assembly will be the key to reduce the cost of optical electronic devices. Due to the precision of automatic assembly of optoelectronic devices in Submicron scale, automated assembly production has been considered to be very difficult, but recently a great breakthrough. Foreign academic journals have repeatedly reported on the basis of Vcsel, new optical collimator and self aligning technology, the breakthrough of automatic assembly of optical devices, and the design of optoelectronic devices specially designed for automated assembly. 2002 OFC Exhibition has more than 10 automatic packaging, automatic welding equipment manufacturers exhibiting, welding, alignment, pressure welding and many of the past that can only be manually operated by the process can now be carried out by the manipulator. According to Electronicast forecast, by 2005, automated assembly and test equipment sales will reach 1.71 billion U.S. dollars, optical electronic components in the output value of 70%~80% will be automatic or semi-automatic assembly production, can be said that the emergence of automated production lines is the light electronics industry began to mature signs and development of the inevitable.

New optical electronic Devices in Ason 3

In the current economic environment, communications operators to enhance the capacity of optical networks, will pay more attention to optical network flexibility and scalability, to reduce operating costs and respond to rapid changes in the market environment. On the device level, based on the development trend of the devices described above, the development of tunable devices, multi-function integrated optical switch devices or components and network performance monitoring devices will be the cornerstone of building an intelligent optical network.

(1) Tunable devices are currently a very popular topic, it can be said that it will be a necessary part of Ason. The tunable devices currently under development include tunable wavelength light sources, tunable optical filters, reconfigurable oadm and OXC, dynamic gain equalization and transient effect suppression optical amplifiers, adaptive dynamically tunable dispersion compensator, and PMD compensator. Compared with the similar devices with fixed parameters, the tunable devices can realize remote control and automatic bandwidth management, which makes the network more flexible, reliable and efficient. such as wavelength tunable laser can greatly reduce the DWDM system in the light source configuration, backup and maintenance of the great pressure. The tunable optical filter, reconfigurable oadm and OXC make it possible to adjust the optical signal path and destination so as to realize real-time operation of the network traffic.

The tunable laser light source has the greatest effect and difficulty in all kinds of tunable devices, so it is also the most concerned device. At present, some manufacturers have introduced products, they use different principles, performance and price respectively have their own strengths and weaknesses. The DFB laser with temperature tuning is first commercialized, its principle and structure are simple, but the wavelength tuning speed is slow and the adjustable range is narrow, only 3~9nm. Multi-segment structure DBR lasers adopt current injection to achieve wavelength tuning, the speed of tuning is fast, but the range is still limited, about 8~10nm. The use of complex grating structure can greatly extend wavelength tuning range, such as the United States ADC launched GCSR (Grating assisted co-directional coupler with sampled) lasers typical tuning range for Reflector 40nm, the best result covers the whole C and L band, adjustable range reaches 114nm. Marconi and other companies using sampled grating SG-DBR laser structure, wavelength tuning to 40nm, but this kind of complex grating structure device is difficult to make, high cost, the use of control is also very complex. Recently, Nortel and Bandwidth9 have released the products of a tunable light source integrated with a vertical cavity surface emitting laser and a micro-mechanical mirror, the wavelength tuning range reaches 20~30nm, and the external cavity structure of the micro-mechanical reflector makes the spectral characteristic of the device superior and has a promising future. But it has to compromise on the complexity of output power and tuning range and encapsulation. In addition to continuing to improve the basic performance of tunable lasers, manufacturers are further integrating electronic or optoelectronic devices into tunable lasers, making them a light source module with multi-function integration.

Generally speaking, the wide application of tunable lasers in optical networks still needs to improve the performance and price ratio. But its development and application of the future is indisputable, with the development of intelligent Optical Network, it will quickly form a large industry, according to the U.S. Yankee Group forecast, tunable laser market share in 2001 years to start, by 2005 will reach 2.3 billion U.S. dollars.

(2) The importance of monitoring technology and equipment is growing. It is noteworthy that with the application of tunable devices and the improvement of optical network flexibility, monitoring technology and equipment are getting more and more attention. At present, the monitoring of network performance is not only to detect the existence of a WDM wavelength, but also to probe the signal to noise ratio or even ber of optical signals, in order to optimize the management of optical network. This has led to a need for a range of new devices, such as a light detector array that can simultaneously monitor hundreds of cells in all DWDM channels. Denmark's Ibsen company recently announced the performance of its D-mon products, it uses holographic diffraction grating to the DWDM system in various wavelengths of light signals to the light detector array of different units, can simultaneously measure 80 50GHz spacing of the optical channel, the device's size is 70mm′46mm′18 mm (long High), and the size of the credit card, including all the circuit components, in the 6ms can record all the optical power of the channel. The company says the D-mon product is currently designed in either an optical amplifier or a dynamically tunable oadm to achieve gain-slope detection, dynamic gain equalization, and channel-wavelength resource control. More multi-functional products have been in the declaration of Patents and will be introduced in succession.

(3) The development of multifunctional integrated optical switch devices will promote the evolution of automatic switched optical networks. In Ason, optical switch is the key device to realize optical transmission path transformation, and it is widely used in the aspects of routing selection, wavelength selection, optical cross connection (OXC), Optical Division multiplexer (OADM), optical network monitoring, device testing and self-healing protection. Micro-electro-mechanical (MEMS) technology has been considered to be the best solution to the modularization and miniaturization of optical switching arrays, and is now in the midst of a period of adjustment and seeking new breakthroughs after a wave of 2 years ago. Its research and development work is still being intensified, and the 32′32 of the following smaller MEMS optical switching array modules has been available, and its reliability has been telcordia demonstrated. In order to solve the problems such as the complexity of the manufacture and the use of the three-dimensional MEMS switch array, the new principle and the research and development of new devices are very active, and several new MEMS schemes have been put forward successively. The CMEMS (compliant MEMS) uses the high elasticity silicone rubber material to improve the problem that the silicon material is too hard and too brittle, while greatly improving the MEMS performance, it can reduce the cost and improve the yield. Dmems (diffractive MEMS) uses diffraction instead of reflection, and its simple capacitor-like structure determines its intrinsic solidity and reliability, and is not degraded by 50 billion operations in the laboratory. It avoids the problem of high driving voltage and complex control circuit of the silicon mirror in general MEMS switches, and can be used to manufacture the typical 1.0μm CMOS process line, which can greatly reduce the cost and improve the yield, and the Dmems technology has a good application prospect in the manufacture of optical switch and tunable optical attenuator. Different companies have adopted different solutions to address the scale of optical switching arrays. Some companies are still using three-dimensional MEMS technology to develop a large array of more than 000 of 1 000x1, but some manufacturers are already looking to use small arrays as modules and build blocks to form large arrays. They believe that this can be used some of the same basic modules to expand the user needs to the corresponding scale, low cost, flexibility, and can only use two-dimensional MEMS devices to avoid the three-dimensional technology unresolved problems. In addition, it is also proposed to make full use of the advantages of DWDM, a wavelength-selective crossover (WSXC) way to replace a part of the usual OXC mode, such optical network with one-dimensional MEMS array can be achieved, so as to achieve complexity, power consumption and cost of a substantial decline.

Based on the development of MEMS optical switch, multifunctional integrated optical switch has been developed and utilized gradually with the evolution of optical network technology. For example, in the Intelligent Optical Network based on broadband video, HDTV and multimedia service, the use of all-optical point-to-point connection can greatly expand the network capability and improve the efficiency. Compared with traditional optical/electric/optical networks with optical point to point connection, all optical points can be optimized by using the least wavelength and wavelength backup and the least optical transceiver to achieve the most virtual connections between network nodes. Its realization should be built on a number of functions of the optical switch devices. The device shall have a 1xN switching function and a control over the access gain, which can be realized by integrating the light switch with multiple single pole and the variable optical attenuator and related detection and control circuits. This device is also widely used in intelligent optical amplifiers, reconfigurable oadm and network detection devices. With the development of the intelligent Optical Network, it will be the focus of the research and development to integrate all kinds of multifunctional optical switching modules with active and passive devices. Some companies in the industry are gradually introducing integrated optical switching modules or photonic switching platforms with different integration levels and functions to suit the needs of different manufacturers. May 2002, NEC of Japan and the United States Tellium Company announced the joint development of integrated optical transfer/switching unit. These include Tellium's optical switching array and starnet wavelength management system (WMS) and NEC's DWDM optical transponder and Spectrawave Network Management System (NWS). It is said to be the first commercially available integrated optical transfer/switching unit that can save telecom operators ' investment and operating costs, reduce power consumption and volume of equipment. At the International conference, many people discussed this integrated device, resulting in the artificial "transwitch" word to emphasize the importance of optical transmission and switching function integration. It should be said that the integration technology of optical devices is still in the initial development period, compared with microelectronic integrated circuit technology, the integration of optoelectronic devices still has a long way to go, but from separation to integration, from simple devices to component modules, to subsystems and even the system chip is the optical electronic devices to adapt to the current optical network market demand of the inevitable direction of development.

4 concluding remarks

With the concept of automatic switched Optical network, tunable devices, multi-functional integrated optical switch devices, network performance monitoring devices are becoming the focus of attention. The development of these new optoelectronic devices will play an important role in the automatic Switched Optical network. From a historical perspective, optical communication system of each leap and the breakthrough of optoelectronic devices are inseparable, we have reason to believe that the current development of automatic switched Optical Network is also looking forward to and Yun-Electronics in the new Materials development, novel photonics principle device research and integration technology and other new breakthroughs.

[Author profile] Sheshijong, is a professor of electronic engineering, Tsinghua University, PhD tutor, director of Information Optoelectronics Institute, national "863" plan information field of Experts Committee members, the United States IEEE senior member and senior member of the China Electronic Society. The research field is optoelectronic technology, high speed and large capacity optical fiber communication system and broadband fiber network.