Two complementary delineation criteria are presented which provide guidelines to the design of relatively short, low-loss tapered fibres and devices. They are used to explain anomalous loss effects in depressed-cladding and W-fibres, as well as the difficulty in fabricating low-loss devices by tapering such fibres. Practical application. of the criteria to couplers, beam expanders and abrupt taper filters is summarised. The accompanying paper provides both experimental and theoretical justification for the delineation criteria.
By developing a direct correspondence between a given whispering gallery mode of a dielectric cylinder and a mode of an equivalent waveguide ring resonator, the evanescent wave coupling characteristics of such modes can be simply modelled. This also allows the conditions for resonant excitation of a single whispering gallery mode to be determined. The model needs a coupled mode theory which accurately describes the coupling between a straight waveguide and a curved waveguide to be developed. This theory is checked against BPM results.
A Gilbert-gamma topology is proposed to model packet-loss processes in UDP connections. The proposed topology introduces state duration modelling with gamma distributions. When compared with the ordinary Gilbert model the proposed topology substantially improves the likelihood of observed packet-loss processes, and gives reductions as high as 70% in the subjective estimation of speech quality transmitted over IP networks. The results presented can be easily applied to other real-time applications such as audio and video streaming.
An L-phase-levelled coherent frequency/phase modulation (LCFPM) scheme is investigated. It is analysed for both spectral characteristics and bit error characteristics on the additive white Gaussian noise channel. Comparison with conventional joint frequency and phase modulations is conducted in terms of power and bandwidth efficiencies. Both uncoded and trellis coded modulation formats are considered. It is found that LCFPM is highly competitive in power and bandwidth efficiencies with conventional dual-frequency joint frequency and phase modulations after experimenting with different uncoded and trellis-coded Pulsed formats.
A quantitative evaluation of the local-mode power evolution along nonadiabatic tapered fibres and the resulting spectral responses are considered. Experimental results are interpreted and analysed for three matched-cladding fibre biconical taper structures, namely a bitaper close to the delineation curves, a bitaper far above and a double bitaper which highlights the effect of multiple cladding-mode coupling.
Optical techniques for microwave signal generation and optical control of microwave devices are described. Optical generation of microwave signals to 35 GHz and above by coherent heterodyne was carried out by FM sideband injection locking of laser diodes and by offset frequency phase locking of solid state lasers. Applications of optically generated microwave signals including optical control of phased array radar and photodetector characterisation are discussed. Optical injection locking, phase control, and phase locking of microwave oscillators are also presented.
A fully quantum-mechanical travelling-wave time-domain model for a Fabry-Perot laser diode is presented. Extending the powerful transmission-line techniques proposed by Lowery, the model treats both the optical field and carrier populations quantum-mechanically. The Glauber function is used to represent the field. and correct correlated carrier and field-noise sources model fluctuations. It is believed to be the first presentation of a realistic quantum model for a laser diode which includes spatial hole burning, multi-moded behaviour and photon fluctuations on a picosecond timescale, together with a quantum-mechanically correct treatment of line width and intensity fluctuations.
New 3-point finite difference formulae with equally spaced grid points are given for the analysis of scalar or semivectorial fields in waveguides with discontinuous refractive index profiles. The new formulae lead to more accurate calculations for the modes than the previously known methods, irrespective of the location of the discontinuities with respect to the grid points and without averaging the permittivity over meshes.
Free-space radiation modes are used to determine a new formula for the main mode reflectivity of a multicoated semiconductor laser facet at normal incidence. The results are shown to compare favourably with exact values, and to provide some unification for the Fresnel methods.
The principal factor in realising a high-performance bandwidth-efficient fibre communication system at an acceptable cost is the choice of modulation format on the optical carrier. In this context, pulse time modulation (PTM) techniques represent an attractive alternative to purely digital or analogue methods. The PTM family is reviewed, a classification system is proposed and their potential for use in high-speed fibre systems intended for the transmission of analogue data is examined.
A novel approach for designing optimal multilayer filters based on a real-coded genetic algorithm is presented. Given the total number of layers in the filter, as well as the electrical properties of the materials constituting each layer, the algorithm iteratively constructs multilayers whose frequency response closely matches a desired frequency response. In contrast to existing iterative techniques, this method does not require a preliminary design using classical techniques. Also, the design procedure is independent of the nature of the multilayer as well as the characteristics of the incident and substrate media. The algorithm is applied to the design of various lowpass and high-pass optical filters, operating between practical terminal conditions. The performance of the resulting designs matches or improves on that for filters that were synthesised using semiclassical techniques.
Quality-of-service (QoS) in ATM networks is largely determined by call admission control (CAC) and switch scheduling algorithms. A CAC algorithm determines if a requested connection can be accepted without violating QoS bounds for the new connection and for existing connections. To support QoS in ATM switches, earliest deadline first (EDF) scheduling can be implemented to ensure that the cells with the tightest time bounds are forwarded before less time critical cells are forwarded. A problem with existing CAC methods is that each node in a network needs to know the traffic parameters of all other nodes, and this requirement induces a high CAC computation and control traffic overhead. A method of aggregating connection parameters into 'super connection' statistics and applying this to a CAC algorithm is developed and proved. This method reduces traffic overhead in an ATM network and enables the design of a simple EDF switch. The design of ain EDF switch that uses multiple input queues, one for each super connection delay class, to sort and send time-stamped cells in EDF order is developed.
It is known that a dielectric waveguide with semiconductor cladding exhibits a maximum attenuation of a given polarisation of light when the cladding thickness corresponds to the cut-off of any guided mode of the same polarisation that the semiconductor layer supports itself. In this paper mode cut-off conditions in the semiconductor layer are analysed and analytic expressions for the layer cut-off thicknesses are developed. The results show good accuracy when compared with more sophisticated numerical analysis techniques consuming a large amount of computer time. Design curves for silicon-clad polarisers on Ti:LiNbO3 and glass waveguides are developed from the presented theory.