Abstract: A transmitter for an optical transmission system transmits an optical sub carrier multiplexed signal comprising number of sub-carriers, onto an optical transmission path, and provides spectral shaping by different magnitudes of the sub-carriers, or different modulation formats for different sub carriers. This spectral shaping can reduce performance degradation by Kerr effect optical non linearities. This can mean higher input powers may be launched. The magnitudes can provide a signal spectrum which is lower near a center of a band of sub carriers than near an edge of the band. Such spectral shaping can be provided in the receiver either to undo the pre emphasis in the transmitter, or to reduce non linearities from components at the receiving side.

Abstract: A coherent receiver receives a set of signals which are spaced in phase, or phase and polarization, and a reference signal. The receiver processes the set of signals to determine which of the set of signals has a predetermined association (e.g. closest in phase) with the reference signal and selects that signal as an optimum output. The receiver has the effect of de-rotating the phase slip between the input signal and reference signal in discrete steps. The selecting occurs on a repeated basis to select an optimum output. The set of signals can be processed in the analogue or digital domains, by such techniques as: comparing the amplitude of each of the set of signals with a threshold, comparing signals with each other or cross-correlation. The processing can be implemented, if desired, without the need for complicated components, which would either be expensive or become unreliable at the extreme high operating frequencies used for optical communication.

Abstract: Use in an optical communications system of multiple detectors to separately detect respective multiple spectral modes of a received optical signal. The invention also provides for corresponding multi-channel, dispersion-tolerant optical receivers. Embodiments are presented both for direct detection and for coherent detection of optical signals.

Abstract: A polarization diversity receiver has an optical section for converting the received optical signal into four or five polarization diverse component optical signals that substantially represent amplitude and polarization state information of the received optical signal, by respective polarization transformations to respective points on a Poincaré sphere, the points being equally spaced apart to maximize polarization diversity, even in the worst case input polarization state. Detectors produce component electrical signals from each of the component optical signals, for electronic processing to compensate for PMD. By reducing the number of component optical signals significant cost and size reductions are enabled. The need for precise polarization tracking in the receiver can be reduced or eliminated completely. Balanced detectors can be used to reduce the number of electrical signals. The electrical processing can use sequence detection.

Abstract: A coherent optical intradyne receiver is described which comprises an optical image reject mixer capable of dividing, mixing and recombining signals from three inputs so as to produce an inphase and a quadrature channel. On each of the channels a same set of image products is cancelled and from which channels, phase modulated signals can be recovered.

Abstract: A transmitter for an optical transmission system transmits an optical sub carrier multiplexed signal comprising number of sub-carriers, onto an optical transmission path, and provides spectral shaping by different magnitudes of the sub-carriers, or different modulation formats for different sub carriers. This spectral shaping can reduce performance degradation by Kerr effect optical non linearities. This can mean higher input powers may be launched. The magnitudes can provide a signal spectrum which is lower near a centre of a band of sub carriers than near an edge of the band. Such spectral shaping can be provided in the receiver either to undo the pre emphasis in the transmitter, or to reduce non linearities from components at the receiving side.

Abstract: Apparatus for generating and receiving optical sub-carrier multiplexed signals has a digital signal processor for performing a Fourier transform, to generate or receive the optical sub-carrier multiplexed signal. This apparatus enables the generation and reception of the optical sub-carrier multiplexed signal in a single apparatus, giving cost and complexity savings over conventional methods of receiving each sub-carrier in an independent apparatus.

Abstract: An optical coupler 10 comprising at least three fibres 12, 14 and 16, portions of the fibres being fused to each other to form a coupling region 30 operable to couple light therebetween. At least two of the fibres have inputs 18 and/or 20 and/or 22 and at least two of the fibres have outputs 24 and/or 26 and/or 28. In use, on illumination of at least two of the inputs light intensity is emitted from a plurality of the outputs having a substantially asymmetric phase relationship, such as, for example, a quadrature phase relationship between two of the outputs. The invention also provides for methods of making such a device.

Abstract: Use in an optical communications system of multiple detectors to separately detect respective multiple spectral modes of a received optical signal. The invention also provides for corresponding multi-channel, dispersion-tolerant optical receivers. Embodiments are presented both for direct detection and for coherent detection of optical signals.

Abstract: An optical coupler 10 comprising at least three fibres 12, 14 and 16, portions of the fibres being fused to each other to form a coupling region 30 operable to couple light therebetween. At least two of the fibres have inputs 18 and/or 20 and/or 22 and at least two of the fibres have outputs 24 and/or 26 and/or 28. In use, on illumination of at least two of the inputs light intensity is emitted from a plurality of the outputs having a substantially asymmetric phase relationship, such as, for example, a quadrature phase relationship between two of the outputs, The invention also provides for methods of making such a device.

Abstract: An optical packet is provided with an amplitude modulated (ASK) payload and a differential phase shift keyed (DPSK) header so that a routing address in the header can be read and modified at a network node without having to inject light at this node to do this modification. With ASK, though it is easy to convert a high-level bit to a low-level bit by simple attenuation, the conversion of low-level bit to high-level bit requires light injection. With DPSK, bit conversion can be effected without light injection, merely requiring passage through a phase modulator.

Abstract: A hybrid optical fiber comprises a first fiber section comprising a fiber or a concatenation of fibers having first dispersion characteristics at a predetermined operating wavelength and a second fiber section coupled to the first fiber section to form the hybrid fiber. The first fiber dispersion characteristics are selected to maintain the signal dispersion within desired limits, whereas the second fiber is optimised for low loss. Dispersion compensation is provided in the first section of the fiber span where the signal intensity is highest, and therefore the region of the fiber span where these non-linearities have greatest effect. In the second section of the fiber span, the non-linear effects can be ignored, so it is optimised for low loss rather than for dispersion compensation.

Abstract: Conventional quadratically chirped fiber Bragg gratings are typically apodized at both their high and low chirp ends. The present specification describes an improved Bragg grating reflector in which a second quadratically chirped region is arranged in front on the high chirp end of a substantially conventional quadratically chirped portion. The high chirp end of the first portion is not apodized; instead this apodization takes place in the second portion, and so enables the first portion to exhibit appreciable reflectivity to signals having wavelengths extending to the Bragg wavelength corresponding to the high chirp end of the first portion. The present invention thus enables the useable bandwidth of a quadratically chirped grating to be increased, and so enables an increased tuning range to be achieved in adjustable dispersion (and adjustable dispersion compensation) apparatus incorporating such gratings.

Abstract: Conventional quadratically chirped fiber Bragg gratings are typically apodized at both their high and low chirp ends. The present specification describes an improved Bragg grating reflector in which a second quadratically chirped region is arranged in front on the high chirp end of a substantially conventional quadratically chirped portion. The high chirp end of the first portion is not apodized; instead this apodization takes place in the second portion, and so enables the first portion to exhibit appreciable reflectivity to signals having wavelengths extending to the Bragg wavelength corresponding to the high chirp end of the first portion.

Abstract: A PMD (polarisation mode dispersion) emulator which provides at least first and second order emulation of PMD effects observed in fiber installed in the field consists of a small number of concatenated lengths of high birefringence fiber having an appropriate distribution of relative DGDs (differential group delays), and connected with random relative orientation. Varying some parameter, such as temperature distribution, allows the emulator to provide accelerated exploration of random fluctuations in PMD which, in the field, may require many weeks, or longer, to observe.

Abstract: An optical waveguide provided with a linearly chirped Bragg reflective grating can be employed as a device that provides linear dispersion compensation. The amount of the linear dispersion thereby provided can be rendered adjustable by adjustment of the magnitude of axial strain imposed upon the grating. If the chirp is purely linear, and if also, the strain is at all times uniform along the length of the grating, adjustment of the strain magnitude will have no such effect. This requires the presence of a quadratic chirp term, but such a term introduces its own transmission penalty. This penalty is compensated at least in part by causing the light to make a reflection in a second Bragg reflection grating identical with the first, but oriented to provide a quadratic component of chirp that has the opposite sign to that of the first Bragg reflection grating.

Abstract: Method and apparatus for controlling the frequency of a laser in an optical transmission system which involves applying a frequency modulation to the laser, determining an amplitude modulation present in the signal downstream of the laser and attributable to the frequency modulation, and discriminating a portion of the amplitude modulation which is substantially in phase quadrature with the applied frequency modulation. The frequency of the laser is controlled on the basis of the discriminated quadrature portion. An optical component downstream of the laser may have a response which converts FM to AM. The response and the frequency output of the laser can be locked to one another, without errors caused by unwanted AM in the transmitted signal modulation frequency.

Abstract: A method of consolidating a body in any of initially powdered, sintered, fibrous, sponge, or other form capable of compaction, including the steps: providing a bed of flowable particles within a contained zone, the particulate including flowable and resiliently compressible carbonaceous particles; positioning the body in the bed, to be surrounded by the particles; effecting pressurization of the bed to cause pressure transmission via the particles to said body, thereby to compact the body into desired shape, increasing its density; the particles being heated to elevated temperature prior to compacting of the body into desired shape; and the heating of the particles being effected by passing electric current through same, with heat generated in the particles also to be transferred to the body.The electrically heated mass of particles may be fluidized; the particles may consist of graphite; and the body may consist of metal, ceramic, or synthetic resin.