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Novel Devices for Wavelength Routing and Optical Access Networks

Novel Devices for Wavelength Routing and Optical Access Networks

Novel Devices for Wavelength Routing and Optical Access

Novel Devices for Wavelength Routing and Optical Access Networks SPECIAL SECTION L K CHEN PhD MIEEE MOSA Department of Information Engineering, The Chinese University of Hong Kong Email: lkchen@ie.cuhk.edu.hk H K TSANG PhD MHKIE MIEEE MIEE Department of Electronic Engineering, The Chinese University of Hong Kong Email: hktsang@ee.cuhk.edu.hk C K CHAN PhD MIEEE Department of Information Engineering, The Chinese University of Hong Kong Email: ckchan@ie.cuhk.edu.hk C SHU PhD SMIEEE Department of Electronic Engineering, The Chinese University of Hong Kong Email: ctshu@ee.cuhk.edu.hk We report the recent research thrust at the Chinese University of Hong Kong that investigate novel devices to enhance the network flexibility, versatility, and robustness of wavelength routing networks and optical access networks. In particular, we will discuss wavelength and format converter, optical performance monitoring devices, low cost WDM transmitters for access networks, and the associated protection and restoration schemes. Keywords: Wavelength Converter, Format Conversion, Injection-locking, Birefringence Switching, Four-wave-mixing, Optical Performance Monitoring (OPM), Fiber Bragg Grating (FBG), WDM-PON, Network Protection Introduction Growth in internet traffic, perceived by some to be as high as 200% per annum [1], encouraged the widespread deployment of dense wavelength division multiplexing (DWDM) during the boom in optical communications between 1998 and 2001. By simply adding new wavelengths in existing fibers, DWDM was seen as a cost effective means for increasing network capacity to meet the growing demand without requiring additional expenditure in installing new fiber infrastructure. DWDM also has potential advantages for network management, routing and reducing overall system costs [2,3]. Wavelength-routing is a technique which allows high data-rate signals to be routed according to their wavelength, without the need, as in present day systems, for costly (space and power consuming) equipment to perform the optical to electronic conversion, processing with electronics and electronic to optical conversion. All-optical wavelength routing can allow the realisation of all-optical networks which can reduce overall costs for high-speed data networks [2]. Field trials of all-optical networks in North America [4] and Europe [5] have been carried out. All-optical wavelength converters, which allow data at one wavelength to be transferred to another wavelength, are key elements in such all-optical networks. Wavelength converters are needed to resolve collisions which would block the routing of a channel on an already occupied wavelength. The deployment of wavelength converters in a network also reduces the need for precise absolute wavelengths of all lasers and tunable filters in the network and provides added flexibility in network management [6]. The deployment of DWDM in densely packed metropolitan area networks may make it necessary to convert data modulation formats. Spectrallyefficient modulation, (eg non-return to zero (NRZ) modulation), is preferable for DWDM metro networks where there are a large number of closely spaced (in wavelength) channels used for distribution within a metropolitan network. On the other hand, bit rate aggregation to high (10 or 40 Gb/s) data rates and the longer span lengths in long haul networks make it advantageous to use modulation formats which amore tolerant to polarisation mode dispersion (PMD) but not necessarily as efficient in occupying the spectrum. Return-to-zero (RZ) format is more tolerant to PMD at 40 Gb/s [7], and even at 10 Gb/s nonlinear RZ transmission has been shown to be more tolerant to dispersion compared with nonreturn-to-zero (NRZ) format [8]. RZ modulation is less spectrally efficient than NRZ. Hence there may be a need for future networks to be able to employ different data modulation formats [9] in different parts of a network or inter-network. All optical data format conversion can enable different optical networks to employ the data formats that are suited for that network’s coverage, eg the use of RZ for high bit-rate optical time-division-multiplexed (OTDM) long-haul traffic and NRZ for densely packed DWDM access networks. RZ to NRZ format conversion is therefore needed to interface from the ultra-fast OTDM long-haul networks to lower speeds DWDM access and metro networks. The RZ signals (> 40 Gb/s) in long-haul networks will be time division de-multiplexed [10] to lower bit rate (~ 10 Gb/s) and then format converted into NRZ for the local area DWDM networks. On one hand, the advanced wavelength and format converter provides greater flexibility for the next generation reconfigurable and transparent optical networks. On the other hand, the task for network management becomes even more challenging in order to handle the dynamic variation in network configuration and signal format, and the subsequent change in various system parameters. Hence performance monitoring that provides information for overall network management is essential. In addition, tremendous increase in the optical network’s capacity also implies that single fiber cut may lead to catastrophic effects in regional or even national economics. Thus it is also necessary to employ performance monitoring to provide crucial information reflecting the health condition of network elements and transmission links. Performance monitoring also serves an early warning purpose that detects system degradation before it deteriorates to an unacceptable level, causing greater damage. The verification of service-level-agreement (SLA) also requires performance monitoring. Performance monitoring can be implemented in electrical or optical domain. By monitoring in optical domain, certain causes of system degradation originated from the optical layer can be identified. Therefore, optical performance monitoring (OPM) is an indispensable element for the quality assurance of the next generation optical network [11,12]. There are three tiers of performance monitoring, namely, power monitoring, optical signal-to-noise ration (OSNR) monitoring, and finally the bit-error-rate (BER) monitoring. In this paper, we discuss two schemes for the first and the second tier, respectively. Finally, we would like to focus on the recent advances in optical access networks. While the overbuild in the core network is evident, access network is the sector that still has substantial activities and attracts new operators, including public utility companies or municipalities, entering the arena. To support broadband access, passive optical network (PON) is an attractive approach that provides many advantages [13]. Passive cable plant (remote node) reduces maintenance cost. The number of fiber ends and physical units are greatly reduced at the Metro interface, resulting in corresponding reduction in space and easing of fiber handling. The tree-and-branch topology is well suited for broadcast services. In addition, if WDM is employed in PON, it can further provide high bandwidth scalability, privacy, and flexibility for provisioning different services for different areas. The obstacle of the deployment of WDM-PON are the TRANSACTIONS • Volume 11 Number 2 21

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