Abstract: Methods and apparatus for optimising, improving or maximising the efficiency of an acousto-optic lens (AOL) system are disclosed. Data relating to efficiency is used to select drive frequencies of the acousto-optic devices (AODs) forming the AOL, thereby both increasing the usable field of view and reducing a prior art patternation problem. Preferably according to the invention, drive frequencies are selected that maximise efficiency of transmission through the AOL. When scanning, the centre of each scan is optimised to be of maximum efficiency.

Abstract: The invention is directed to an acousto-optic lens (AOL) and corresponding method. The AOL of the invention comprises a first and second acousto-optic deflector, each being arranged to support a respective acoustic wave. The AOL includes a driver for synthesizing first and second drive signals for the respective first and second acousto-optic deflectors. The driver is arranged to synthesize the drive signals so as to be phase-modulated by functions that can be expressed as a Taylor series having one or more coefficients greater than second order. The functions are preferably non-sinusoidal and have sufficient magnitude at the higher orders to effect one or more of fixed aberration correction, dynamic aberration correction and Z-scanning.

Abstract: The invention is directed to an acousto-optic lens (AOL) and corresponding method. The AOL of the invention comprises a first and second acousto-optic deflector, each being arranged to support a respective acoustic wave. The AOL includes a driver for synthesizing first and second drive signals for the respective first and second acousto-optic deflectors. The driver is arranged to synthesize the drive signals so as to be phase-modulated by functions that can be expressed as a Taylor series having one or more coefficients greater than second order. The functions are preferably non-sinusoidal and have sufficient magnitude at the higher orders to effect one or more of fixed aberration correction, dynamic aberration correction and Z-scanning.

Abstract: Methods and apparatus for optimising, improving or maximising the efficiency of an acousto-optic lens (AOL) system are disclosed. Data relating to efficiency is used to select drive frequencies of the acousto-optic devices (AODs) forming the AOL, thereby both increasing the usable field of view and reducing a prior art patternation problem. Preferably according to the invention, drive frequencies are selected that maximise efficiency of transmission through the AOL. When scanning, the centre of each scan is optimised to be of maximum efficiency.

Abstract: Admission of rapidly varying traffic flows to a communications network is controlled by sampling the traffic flows each at an ingress, and sampling an aggregate flow of said flows at some or all of the resources used by the aggregate flow. From this sampling, a mean bandwidth requirement for each traffic flow and a measure of the variance from that mean are determined. First and second prices for the mean and variance components of the controlled traffic flows that are calculated. These price calculations are used to determine an admission cost for each said flow so as to regulate the admission of that flow via a feedback price mechanism.

Abstract: A communications network has multiple resource-allocation layers and incorporates a management structure for allocating resources to allocate resources requested by a first layer of said layers from a second of said layers. At a first layer, the management structure provides an indication to a second layer of the required resources that are to be allocated from the second layer. The second layer automatically offers the required resource together with a condition for use of those resources. This condition includes a notional price factor which is dependent on current demand. Under the control of the manager, the first layer determines if the condition for use of the offered resources is acceptable and, if so, automatically accepts the offered resources from the second layer. In a preferred embodiment, ingress to an underlying multi-wavelength transport layer of the network is controlled via a virtual port which allocates traffic to real ports one for each wavelength supported by the transport layer.

Abstract: Internet protocol based transmissions have historically been transmitted on a best efforts basis. There are proposals to provide differentiated services to allow different packets to be delivered with different service qualities depending on the bandwidth requirements of a particular flow. By performing bandwidth monitoring within the core network and performing packet dropping and/or queuing at the network ingress, the need for packet dropping and/or queuing within the core network is removed and furthermore the core network resources are not used by packets which will not be delivered to their destination but will instead be dropped within the core network.

Abstract: A packet scheduling scheme schedules packets from a plurality of queues onto outgoing link. The scheme associates a weight and a virtual start time with each of the queues. Queues are selected, in order of the virtual start time, until a non-empty queue is selected. One or more packets are sent from the selected queue and then the virtual start time is updated, based on the length of the transmitted packet and the weight associated with the selected queue.

Abstract: In order to provide price information to customers quickly a fast reflector switch is used. This helps to ensure that price information provided to customers is as up-to-date as possible. To determine a price for use of a particular route in a communications network, the available resources at each node in that route are assessed. The most congested node in that route is selected and a price determined on the basis of the available capacity at that node. This price is then delivered to the customer via the fast reflector switch. In another example, the average amount of available resources at each node in the route is determined over a specified time period. The standard deviation of the resource level values at each node over the specified time period is also determined. A price is then set on the basis of the sum of the average amounts and the sum of the standard deviations. In this way a price which takes into account recent variation in traffic levels as well as average traffic levels is provided.

Abstract: The invention is directed to an optical grating device which is capable of processing at least two optical signals concurrently, comprising a plurality of input waveguides, a plurality of output waveguides, a first free propagating region and a second free propagating region, and a composite light pathway. These are arranged such that a first input waveguide and a first output waveguide are connected to said first region; a second input waveguide and a second output waveguide are connected to said second region; and said first region and said second region are connected by the composite light pathway. Light input along the first input waveguide is able to pass through the composite light pathway to be output through the second output waveguide and light input along the second input waveguide is able to pass through the composite light pathway to be output through the first output waveguide.

Abstract: An ATM switch has an ATM switch core comprising an N.times.N optical cross-connect core and a set of N port cards, each including a selector. Associated with each port card is a satellite. Wavelength shift keyed signals that are wavelength division multiplexed are employed for routing ATM cells between the satellites via the core by way of optical fibres.

Abstract: In a wavelength multiplexed optical transmission system incorporating a concatenation of optical amplifiers the multiplexed signal passes through a limited number of amplifiers in which all channels are amplified together, after which, at the next amplifier, the signal is demultiplexed, each channel is separately amplified, and then the channels are remultiplexed.

Abstract: A switchable multichannel external cavity injection laser incorporates a diffraction grating between a linear array of n reflectors and a common reflector to define a set of n optical cavities having different frequency bands. Each of the (n+1) reflectors is provided by the optically distal end of an associated semiconductor laser amplifier whose optically proximal end is arranged to be substantially non-reflective.

Abstract: A multi wavelength optical network including a plurality of wavelength multiplexed nodes each having a single frequency optical signal transmitting means (13) and a multi wavelength optical signal receiving means (15), an optical transmission network including a passive optical coupling means (12) interconnecting said nodes whereby single frequency signals transmitted from each node are propagated via the coupling means to all the nodes, each node also including wavelength demultiplexing means (14) through which received signals are applied to the multi wavelength receiving means and timing and control means (17, 18, 19) arranged to control the timing of signals transmitted by the node over the network so that the signals from all the nodes are synchronous at the coupler.

Abstract: A semiconductor laser is constructed such that at least a part of the laser optical feedback is provided by an external reflector spaced from the semiconductive material of the laser so as to form a composite optical cavity bounded by the semiconductive material and the reflector. The external reflector is supported on a mount constructed such that in response to changes in temperature the mount moves the external reflector relative to the adjacent surface of the semiconductive material so as to provide the external part of the optical cavity with a thermal coefficient substantially matched with the mode wavelength expansion coefficient of the internal part of the cavity.

Abstract: An injection laser in which the current flow across the active layer of the device is restricted by confining the current flow across an adjacent interface to a pair of closely spaced parallel stripes. The width and spacing of these stripes is chosen to promote the waveguiding of the zero order mode of the laser in preference to the waveguiding of the first order mode and to waveguiding by the so called "gain guiding" mechanism.