N.G. Walker

Polarization control for coherent communications

Two practical implementations of endless polarization control are described. The first approach uses polarization maintaining fiber transducer, while the second uses an integrated optic lithium niobate device. In order to characterize the lithium niobate device in detail, a technique for analyzing the birefringence as a function of the applied voltages was developed, and the measured performance of the devices is presented. Fractional-wave controllers for both static and endless control applications are considered, and design rules for maximum wavelength-window and minimum loss fiber-loop polarization adjusters are given. Endless control using various configurations of the three types of transducers is discussed, and it is shown how apparently different schemes are closely related. >

Sensitivity of optically preamplified receivers with optical filtering

The noise spectral densities resulting from detection of preamplified signals using an optical filter are discussed. The authors also carried out a simplified analysis of preamplified receiver sensitivities with ASK and FSK signals. The analysis shows that total optical bandwidths of less than 50 times the data rate result in (10/sup -9/ BER) sensitivities close to the limit set by the amplifier noise figure. The development of low-noise high-gain amplifiers in conjunction with narrowband tunable optical filters enable high-sensitivity FDM systems to be constructed, without the complication of a local oscillator or polarization control/diversity networks which are required for heterodyne coherent detection systems.<<ETX>>

Erbium-doped fiber amplifier cascade for multichannel coherent optical transmission

The requirements and practicalities of amplifier cascades for multichannel coherent transmission are discussed. A four-amplifier cascade which maintains an overall noise figure of less than 13 dB across a 20-nm optical bandwidth for total input powers up to -10 dBm is described. A potential capacity of 100 wavelength division multiplexing channels is demonstrated in a 300-km 622-Mb/s DPSK (differential phase shift keying) coherent transmission experiment. >

Investigating link service infrastructures

Variations on the Distributed Link Service have now been deployed across a spectrum of hypermedia and multimedia projects. Although some implementations have utilised standard database technologies and hypermedia tools behind the scenes, most of the network services have been proprietary implementations. In this paper we discuss the motivation and requirements for a large scale, dynamic and open distributed link service using third party components, and explore the use of off-the-shelf services to provide the distributed infrastructure for link services. In particular we investigate HTTP, LDAP and Whois++ as candidate technologies.

Efficient harmonic generation using an electroabsorption modulator

A multiple quantum-well electroabsorption modulator has been assessed as a harmonic generator of millimeter-wave signals for radio-over-fiber systems. It is shown experimentally that the steep nonlinear absorption characteristics of electroabsorption modulators lead to efficient generation of high order harmonics when driven by a sinusoidal voltage. Photocurrent harmonic power levels generated using electroabsorption and Mach-Zehnder modulators are compared with theory.<<ETX>>

Progress towards the field deployment of coherent optical fiber systems

A description is given of the first demonstration of a coherent optical transmission system deployed in an operational network. The miniaturized external cavity laser and the automated endless polarization control scheme which made it possible to mount this demonstration are described in detail, together with specific system results. A discussion is presented of the possible field deployment of other coherent system options, based on FSK modulation of DFB and DBR lasers, and polarisation diversity reception. The considerable potential afforded by coherent techniques for meeting future network requirements is highlighted. The demonstrated advantages of greater power budget and wavelength selectivity, combined with the use of optical amplifier multiwavelength repeaters, make coherent techniques particularly relevant to the growing demand for greater transmission capacity, transparency, and network flexibility. An increasingly urgent need for robust coherent optical technology can thus be anticipated, for wideband transmission and passive routing, for both telecommunications and computer networks of the near future. >

Optical PSK Transmission Experiment At 565Mbit/S Incorporating An Endless Polarisation Control System

The successful application of an endless polarisation control scheme was demonstrated by its use in a high sensitivity PSK heterodyne optical communication system. The synchronous demodulation of PSK signals was investigated and the practical implementation of a phase locked loop which is able to track the phase noise of optical signals is discussed. A large loop bandwidth is shown to be necessary and a loop filter of modified first order type was found to have advantages over a second order type. A synchronous demodulation system not using a phase locked loop is also proposed.

Assume-guarantee reasoning with local specifications

We investigate assume-guarantee reasoning for global specifications consisting of conjunctions of local specifications. We present a sound and complete assume-guarantee rule that permits reasoning about individual modules for local specifications and draws conclusions on global specifications. We illustrate our approach with an example from the field of network congestion control, where different agents are responsible for controlling packet flow across a shared infrastructure. In this context, we derive an assume-guarantee rule for system stability, and show that this rule is valuable to reason about any number of agents, any initial flow configuration, and any topology of bounded degree.

Model Checking Optimisation Based Congestion Control Algorithms

Model checking has been widely applied to the verification of network protocols. Alternatively, optimisation based approaches have been proposed to reason about the large scale dynamics of networks, particularly with regard to congestion and rate control protocols such as TCP. This paper intends to provide a first bridge and explore synergies between these two approaches. We consider a series of discrete approximations to the optimisation based congestion control algorithms. Then we use branching time temporal logic to specify formally the convergence criteria for the system dynamics and present results from implementing these algorithms on a state-of-the-art model checker. We report on our experiences in using the abstraction of model checking to capture features of the continuous dynamics typical of optimisation based approaches.

Lyapunov convergence for Lagrangian models of network control

Many network control problems can be formulated and studied using the machinery of optimisation theory and Lagrange duality. The goal of the control process is to find the saddle point of the Lagrangian. We present a stability result for a class of dynamic processes for this problem. Our formulation automatically derives a Lyapunov function from the form of the dynamic equations. We show how several stability results from the literature of distributed flow control in networks fit into this formalism.