Abstract: An optical fiber for communications systems, the fiber being designed to ensure a compensation of Kerr effects. The fiber has a profile which ensures that changes in power produce changes in distribution of power between core and cladding, such that the phase change associated with the changed spatial distribution of the power, is equal and opposite to the phase change due to Kerr Effect.

Abstract: An optical fibre for communications systems, the fibre being designed to ensure a compensation of Kerr effects. The fibre has a profile which ensures that changes in power produce changes in distribution of power between core and cladding, such that the phase change associated with the changed spatial distribution of the power, is equal and opposite to the phase change due to Kerr Effect.

Abstract: An optical signal delay mechanism having a cladding material and a signal conducting core embedded for protection in the cladding material. The core has a longitudinal direction which is the direction of propagation of the optical signal and a transverse direction which is a direction substantially perpendicular to the direction of propagation of the optical signal. The embedding of the core is such that longitudinally spaced apart core sections are transversely separated by a continuous region of cladding material. This provides a more compact, robust and cheaper alternative to known delay mechanisms.

Abstract: During the photo-induced creation of a Bragg grating in a photosensitive optical fibre using the interference fringe pattern generated in the near field of a phase grating, the spacing between fibre and phase grating is modulated.

Abstract: An optical amplifier for a wavelength multiplexed transmission system has a set of optically amplifying fibres (26a, 26b) each provided with an associated spectrally selective Bragg reflector (27a, 27b). The amplifying fibres are optically coupled with a coupler (23) such that an optical signal having different spectral components launched into an input (21a) of the coupler is emitted from an output (21b) of the coupler after the different components have propagated through different ones of the amplifying fibres each after reflection in its associated reflector.

Abstract: A distributed Bragg grating of uniform pitch is created in a length of unstrained optical fibre which is then secured in an arcuate path to a flexible plate so as to be displaced from the neutral surface of bending of that plate. Changing the curvature of the plate imparts axial strain in the fibre which is arranged to be non-uniform as the result of the path of the fibre being arcuate. This non-uniform strain has the effect of converting the uniform grating into a chirped grating, such as may for instance be used as a dispersion equaliser.

Abstract: An existing proposal to limit jitter and ASE noise generation in an optical soliton transmission system by the use of a concatenation of passive optical filters, each with its centre frequency offset in the same direction with respect to that of its predecessor in the transmission path, is modified by the replacement of the passive filters with active ones. In each active filter a feedback control loop operates to tune the centre frequency of the filter to provide a specific offset with respect to the centre frequency of solitons incident upon it.

Abstract: An optical pulse retiming and reshaping circuit for soliton transmission in a transmission system with distributed amplifiers transmits the pulses through an optical modulator, uses a slow photodiode to determine the mean power transmitted (or intercepted) by the modulator. This is used for generating a control signal for use in a phase-locked loop to regulate the phase of an oscillator driving the modulator.

Abstract: In a wavelength division optical transmission system incorporating optical amplifiers, an identifying pilot tone is modulated on to each multiplexed wavelength. Each system amplifier determines from the pilot tones the total number of wavelengths being transmitted and thereby provides a corresponding adjustment of the amplifier gain. This prevents overamplification when one or more wavelengths is missing from the multiplexed signal e.g. as the result of a transmitter fault.

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 method of controlling a laser in an FSK direct detection system employing a two-path unbalanced interferometer in series with the laser output, the method including the steps of sensing when the system output level reaches a predetermined threshold, and superimposing a fixed amplitude step of drive current on the feedback controlled modulated drive current to the laser. The superimposed fixed amplitude step in the laser drive current causes a phase shift in the interferometer sufficient to cause the interferometer to acquire a fresh interference fringe whenever the interferometer is near to running out of range. The superimposed fixed amplitude step may be synchronized with a data bit transition.

Abstract: An injection laser driver regulates the bias and modulation depth by means of two feedback control loops, one deriving its feedback control signal from a measure of the mean optical output of the laser, and the other deriving its feedback control signal from a measure of the spectral purity of that optical output. The spectral purity measure is conveniently effected by measuring the coherence function at a specific value of differential delay using a Mach-Zehnder interferometer configuration with a passive quadrature optical network.

Abstract: The gain of an optical amplifier is maintained constant by employing a portion of the amplified optical signal in a feedback loop. In one arrangement this is used to control the output of a pump signal source accordingly. The portion of the said signal corresponds to spontaneous emission (ASE) at a control wavelength or band of wavelengths different to the wavelength of the optical signal output. In the case of an erbium, for example, optical fibre amplifier the portion is preferably ASE at 980 nm, the amplified signal being at 1.535 .mu.m, i.e. completely different transitions in the amplifying medium are used for gain and ASE feedback in order to lessen requirements on the filter used to extract the portion for the feedback loop.

Abstract: In an optical fiber transmission system, signals are transmitted along a fiber optic path (11) in the form of substantially dispersion-free solitons. The transmission path includes a number of optical amplifiers (12) each of which determines, from the transmitted soliton signal purity, a gain control signal which is fed back to a previous amplifier in the path. This ensures soliton purity and optimizes the transmission power.

Abstract: A device for producing an optical delay in an optical signal having variable optical frequency, the optical delay varying with the optical frequency, comprises a chirped Bragg reflector formed in an optical fiber and a directional coupler for separating the reflected signal from the input signal. One application of the device is for chromatic dispersion equalization. Various methods of manufacturing the chirped Bragg reflector are described.

Abstract: A non-circularly-symmetric intensity distribution is formed on a quadrant photodetector by coherently mixing two optical signals propagating in selected modes of different order. These modes are chosen either such that a change in relative phase of the two signals produces solely a rotation of the orientation of that intensity distribution, or such that a change in relative states of polarization produces this rotation.

Abstract: Optical energy at a predetermined frequency or narrow band of frequencies is tapped from an optical fibre by causing spatially periodic deformation of the fibre such as to produce coupling of optical energy between modes of the fibre, for example between the core and cladding of a single mode fibre, at the predetermined frequency and removing the optical energy at the predetermined frequency, for example by means of a cladding mode tap for a single mode fibre. If the deformation means includes a mechanical grating then tuning of the coupling frequency can be achieved by changing the relative orientation (O) of the fibre and the grating.

Abstract: A distributed optical fiber sensor cable, for sensing perturbations such as pressure, comprises a length of optical fiber (10) and means adapted, in response to the perturbation to be sensed, to mechanically deform separate portions of the length of the fiber at sensing elements (11,12,13) periodically along their length in order to cause optical power launched into a mode of propagation in the core (14) and guided thereby to be coupled to an unguided mode, for example a cladding (15) mode. The coupling is optical wavelength selective and the separate sensing elements are each addressed with a different wavelength (.lambda..sub.11, .lambda..sub.12, .lambda..sub.13) for spatial resolution along the length of the cable.

Abstract: In an optical transmission system in which a plurality of optical signal channels is multiplexed before transmission along a common optical path to a receiver, each channel is produced by means of a respective multimode injection laser, the overall cavity length of each laser being arranged to provide a multimode spectral output with a respective intermode spacing which differs from that provided by the other lasers for channel discrimination purposes. Comb filters are employed for demutliplexing.

Abstract: An optical communication system including at a transmitter a semiconductor laser (2) and at a receiver a photodetector (3). Intermediate the transmitter and the receiver is an optical frequency discrimination (4), e.g. a Fabry-Perot resonator. The laser (2) is operated at a substantially constant average power level above the lasing threshold. Superimposed (5) on the substantially constant bias current is a low level modulation signal. The level of modulation of the bias current is sufficient only to cause an effective change in the optical frequency of the laser output without causing the laser oscillations to change from one longitudinal mode to another. The discriminator (4) is tuned to one of the frequencies thus produced.