WDM coding for high-capacity lightwave systems

An interchannel parallel coding scheme in the wavelength domain-the WDM coding system-is proposed. The system differs from the usual serial coding systems and provides many advantages. First, data channels are completely unaltered in the coding process, rendering it very suitable for practical lightwave systems with standard bit rate. Second, parallel encoding/decoding systems are simpler than those of serial coding systems, being easier to be implemented in high-speed optical systems. Third, compared with serial coding, WDM coding is able to reduce heavily the number of encoding/decoding pairs. For example, a (15, 11) Hamming coded WDM system reduces the number from 12×11=132 to 1 at the line rate of STS-12. Fourth, the WDM coding system could offer infinite coding gain in dispersion-limited lightwave systems. Finally, WDM coding systems could correct single-channel burst error. The system performance was evaluated and the system limitation imposed by bit-skew among wavelength channels was analyzed. The results indicated that a 15-channel Hamming coded WDM system can reduce the uncoded BER from 10^-9 to 3×10^-17 and the distance limitation imposed by bit-skew is 250 km if a dispersion-shifted fiber is used and a channel span of 30 nm is assumed     

Dispersion of cascaded fiber gratings in WDM lightwave systems

Fiber gratings operating in the transmission mode can provide high dispersion at wavelengths close to the Bragg resonance. When multiple gratings are cascaded for wavelength division multiplexing (WDM) applications, the net dispersion between the stop bands of any two consecutive gratings is significantly modified. We discuss the dispersion characteristics of such cascaded fiber gratings and propose a dispersion compensator for simultaneous compensation of group-velocity dispersion (GVD) for multiple channels of a WDM lightwave system. We also discuss the impact of the dispersion possessed by cascaded gratings on grating based add-drop multiplexers     

Optical components for WDM lightwave networks

Recently, there has been growing interest in developing optical fiber networks to support the increasing bandwidth demands of multimedia applications, such as video conferencing and World Wide Web browsing. One technique for accessing the huge bandwidth available in an optical fiber is wavelength-division multiplexing (WDM). Under WDM, the optical fiber bandwidth is divided into a number of nonoverlapping wavelength bands, each of which may be accessed at peak electronic rates by an end user. By utilizing WDM in optical networks, we can achieve link capacities on the order of 50 THz. The success of WDM networks depends heavily on the available optical device technology. This paper is intended as a tutorial on some of the optical device issues in WDM networks. It discusses the basic principles of optical transmission in fiber and reviews the current state of the art in optical device technology. It introduces some of the basic components in WDM networks, discusses various implementations of these components, and provides insights into their capabilities and limitations. Then, this paper demonstrates how various optical components can be incorporated into WDM optical networks for both local and wide-area applications. Finally, the paper provides a brief review of experimental WDM networks that have been implemented     

Microwave-multiplexed lightwave systems: A new approach to wideband networks

For the last fifteen years the attention of the fibre-optic community has been principally focused on developing baseband digital transmission system for transmitting time-division-multiplexed voice channels over long distance and interoffice routes. Attention has now shifted to extending the fibreoptic network to the subscriber loop and to providing a multitude of new services through lightwave networks. Based on the results reviewed here, it is apparent that microwave-multiplexing techniques provide a new and attractive way to transmit wideband services over optical fibre. By allowing lightwave systems designers to use and combine the complete range of available electronics, including both analog and digital electronics, and both baseband and microwave techniques, a great number of new possibilities arise for exploiting the bandwidth of lightwave components and for designing wideband lightwave networks. Over the next few years the microwave-multiplexing techniques described in this article may fundamentally change the way we use lightwave technology.     

An algebraic approach to network coding

We take a new look at the issue of network capacity. It is shown that network coding is an essential ingredient in achieving the capacity of a network. Building on recent work by Li et al.(see Proc. 2001 IEEE Int. Symp. Information Theory, p.102), who examined the network capacity of multicast networks, we extend the network coding framework to arbitrary networks and robust networking. For networks which are restricted to using linear network codes, we find necessary and sufficient conditions for the feasibility of any given set of connections over a given network. We also consider the problem of network recovery for nonergodic link failures. For the multicast setup we prove that there exist coding strategies that provide maximally robust networks and that do not require adaptation of the network interior to the failure pattern in question. The results are derived for both delay-free networks and networks with delays.     

Optimization of WDM lightwave systems (BAC) design using error control coding

In a binary asymmetric channel (BAC) it may be necessary to correct only those errors which result from incorrect transmission of one of the two code elements. In optical fiber multichannel systems, the optical amplifiers are critical components and amplified spontaneous emission noise in the optical amplifiers is the major source of noise in it. The property of erbium doped fiber amplifier is nearly ideal for application in lightwave long haul transmission. We investigate performance of error correcting codes in such systems in presence of stimulated Raman scattering and amplified spontaneous emission noise with asymmetric channel statistics. Performance of some best known concatenated coding schemes is reported.     

Fading in lightwave systems due to polarization-mode dispersion

System fading caused by polarization-mode dispersion is investigated at 1.7 Gb/s using highly-birefringent, dispersion-shifted fiber at 1.55 μm. The observed fading, which is manifested by random fluctuations of the bit error rate for a fixed receiver power, is observed to depend on the environmental conditions of the fiber, with the time constant for fading varying from minutes to hours depending on the rate of change of the ambient temperature. The mean dispersion penalty inferred from the observed fluctuations in the bit error rate is consistent with a square-law dependence on the polarization-mode dispersion for small penalties     

A WDM cross-connected star topology for multihop lightwave networks

The authors propose a star topology for multihop lightwave networks in which the conventional N×N passive star coupler is replaced by fixed wavelength division multiplexed (WDM) cross connects. The proposed topology overcomes three major limitations of the conventional star topology. First, it reduces the number of wavelengths needed in a (p,k) ShuffleNet from kp ^k+1 wavelengths in the conventional topology to p wavelengths in the proposed one. Second, the signal power loss due to the 1/N power splitting at the star coupler no longer exists in the WDM cross connects and, therefore, the restriction on the supported number of users by the star network is alleviated. Third, it completely eliminates the need for wavelength filtering at the input to the receivers as is the case in the conventional star topology     

A process algebraic approach to the specification and analysis ofresource-bound real-time systems

Recently, significant progress has been made in the development of timed process algebras for the specification and analysis of real-time systems. This paper describes a timed process algebra called ACSR, which supports synchronous timed actions and asynchronous instantaneous events. Timed actions are used to represent the usage of resources and to model the passage of time. Events are used to capture synchronization between processes. To be able to specify real systems accurately, ACSR supports a notion of priority that can be used to arbitrate among timed actions competing for the use of resources and among events that are ready for synchronization. The paper also includes a brief overview of other timed process algebras and discusses similarities and differences between them and ACSR     

High-capacity coherent lightwave systems

Recent theoretical work on coherent optical detection systems is reviewed and experimental results in high-speed coherent transmission are summarized. The theoretical advantages and limitations of the various modulation and detection formats are discussed and experimental progress towards the implementation of these systems is reviewed. The most significant obstacles to the attainment of quantum-noise limited detection at higher speeds are seen to be the requirement of uniform frequency response from electronic components and the local oscillator laser power requirement, which increases as the square of the bit rate. To make full use of the single-mode fiber bandwidth, frequency-division multiplexing of many moderate-rate channels is a very promising technique for local systems. For long-distance applications, frequency multiplexing is still possible but is limited by the need for optical amplifiers or wavelength-selective multiplexers     

Repeaterless undersea lightwave systems

A repeaterless undersea lightwave system connects two terrestrial locations separated by the sea without the need for undersea regeneration of the optical signal. To achieve the longest span length possible, the system combines terrestrial terminals containing superior-performance optoelectronic devices with ultrareliable undersea cable technology. The lightwave technology used to achieve cost-effective, long-span repeaterless undersea lightwave systems is discussed. This includes undersea fiber and cable, lasers and receivers, and terminal equipment. The first application of this technology, Taiwan's Tainan to Peng-Hu system, is described. The possibilities for increasing the maximum attainable span length of high-capacity repeaterless undersea systems are examined. Key elements are higher-output transmitters, more sensitive receivers, and improved optical fibers     

使用李代数法计算光学系统的三级象差

借助于李代数和辛映射推导出厚透镜成象系统的Seidel三级象差系数的解析公式。我们证明了这是普遍的表示式,通过选择合适的几何参数可用于分析薄透镜、凹面(或凸面)-平面透镜、球透镜等常用光学成象系统的象差。     

Wavelength-discriminating photodetector for lightwave systems

We demonstrate that a dual-wavelength photodetector can be used to determine simultaneously the wavelength and the power level of a lightwave signal. The signal wavelength is determined by monitoring the ratio of the two outputs from this photodetector at the crossover of its responsivity curves; the signal level is determined from the sum of the two outputs. Possible applications for this type of photodetector include mode-control of single-frequency injection lasers, direct demodulation for lightwave systems utilising frequency-shift-keyed transmission, wavelength-division multiplexing and optical logic.     

Noise reduction in long-haul lightwave all-amplifier systems

The overall gain of a chain of erbium-doped fiber amplifiers in a long-haul all-amplifier system would be automatically stabilized if each amplifier were operated slightly into saturation. However, with the required low level of amplifier output power which is imposed by nonlinearity in the transmission fiber, the resulting pump power becomes too low to effectively invert the gain medium of the amplifiers. Consequently, the amplifier output noise level becomes too high for proper system operation. This problem is solved by pumping the amplifiers harder so that a higher gain and higher output power are achieved. The excess gain is then counteracted by an appropriate value of post-amplifier loss. Because of the higher pump power in this case, the amplifier noise is reduced significantly. This technique is investigated theoretically, and experimental work that verifies it is reported     

Subcarrier multiplexed lightwave systems for broad-banddistribution

Microwave subcarrier multiplexing (SCM) is an important approach to the design of lightwave systems for broadband distribution. Recent progress in the design and performance of both analog and digital multichannel SCM systems is reviewed. The application of broadband SCM systems to both passive and optically amplified distribution networks is discussed. The discussion covers the general features of SCM systems; the electrooptic components that have been used in the system experiments described here, including laser intensity noise and noise due to intermodulation products; the carrier-to-noise ratio requirements; some multichannel FM systems experiments; and a 20-channel digital systems experiment; and a hybrid system carrying 60 FM SCM channels plus a 100-Mb/s baseband digital channel. Several approaches to broadband subscriber distribution networks are analyzed     

Forward error correction in dispersion-limited lightwave systems

A rate 0.964 forward error correcting (FEC) code is integrated into the low-speed tributaries of a 565-Mb/s lightwave system as an exploratory system design approach toward relaxing requirements on laser sources in dispersion-limited operation. By virtually removing error rate floors, regardless of their cause, FEC is shown to provide an increasing advantage in conditions of greater degradation and to be effective against mode partition noise (MPN), mode jumping, and reflection impairments.The experimental FEC code is implemented in a standard gate array. The FEC code is described and its performance is analyzed. A new system design strategy is suggested for low-cost gigabit lightwave systems using FEC     

Improved robustness in long-haul lightwave all-amplifier systems

The issue of system robustness of a long-haul lightwave communication system that uses erbium-doped fiber amplifiers to periodically boost the signal power level is addressed. The signal power level at the output of each amplifier is the focus. It is found that this power level is the same throughout the system, and that it depends only on the amplifiers' gain characteristic and the span loss and is independent of the launched power at the input to the chain, provided it is close to the design value. As such, the systemwide signal power level is sensitive to systematic design and/or manufacturing errors. The sensitivity of the signal power level to errors in amplifier design and span loss is studied, and a simple technique is proposed that reduces this sensitivity, thus making the system more robust     

Amplifier-induced crosstalk in multichannel coherent lightwave systems

When an in line semiconductor laser amplifier is used to amplify several channels simultaneously, it can induce inter channel crosstalk if the amplifier gain is channel-dependent. It is shown that modulation of the carrier density at the beat frequency of two neighbouring channels can lead to considerable crosstalk even when the amplifier is operated well below the saturation level. An analytic expression for the channel gains of a travelling-wave amplifier is used to discuss and compare the crosstalk for ASK and FSK systems. The relatively short carrier lifetime in high-gain amplifiers may ultimately limit the channel spacing of such multichannel systems.     

Equalization in coherent lightwave systems using microwavewaveguides

The maximum bit rate/ distance product in recent single-frequency laser direct-detection lightwave system experiments has been limited by dispersion. An equalization technique, appropriate for coherent lightwave systems, that uses a microwave waveguide for overcoming the delay dispersion problem is considered. Results show that small low-loss waveguides can be used to greatly reduce dispersion. For example an 8 GHz bandwidth signal transmitted over 68 km of fiber can be equalized by a waveguide with a cross section of 6 mm×3 mm and a length of only 17 cm. With the waveguide equalizer, the dispersion-limited maximum bit rate/distance product for a standard fiber system can be increased to that of a dispersion-shifted fiber system at 1.55 μm, e.g., a 16-fold increase in maximum bit rate for 100 km transmission