Plessey Overseas Limited

Radiodetection Limited

University of Southampton

Plessy Overseas Limited

John P Dakin

David J Pratt

Andrew Biggerstaff Lewis

Stuart Russell

Christopher A Wade

Stuart John Russell

Philip B Withers

In order to identify a fiber optic cable ( ) a beam ( ) of polarised light is caused to pass down the cable to a first site (A) at which an electromagnetic field ( ) is applied to the cable ( ). The electromagnetic field ( ) traverses the cable ( ) in an essentially transverse direction and has a time-varying component orientated along the length of the cable ( ) at the first site (A), with the component varying so that the line integral thereof along the cable ( ) is non-zero. This results in a variation in the polarisation of the light, which can then be detected by a polarisation discriminator ( ) at a second site (B), thereby to identify the cable ( ).

Identification and location of fiber optic cables

A Sagnac or loop type interferometer is disclosed which uses a single broadband Erbium-doped fibre source ( ) and a single IngaAs detector ( ) together with a 40 km long sensor loop ( ). A wavelength division multiplexer ( ) spectrally slices the broadband light from the source ( ) into two sub-bands, with different optical paths being defined between the source ( ) and detector ( ) for the light in the different spectrally sliced sub-bands. The two optical paths include separate phase modulators ( ) which modulate the two signals at different frequencies, and also separate delay loops ( ) at different places relative to the sensor loop ( ). Effectively, two separate Sagnac loops are provided with a single sensor loop ( ), source ( ) and detector ( ). Standard phase locked loop techniques can be used to extract information from signals that have passed through the two loops and to determine the location of a mechanical or thermal perturbation applied to the sensor loop ( ).

Fibre optic sensor

In order to identify a fiber optic cable ( ) a beam ( ) of polarized light is caused to pass down the cable to a first site (A) at which an electromagnetic field ( ) is applied to the cable ( ). The electromagnetic field ( ) traverses the cable ( ) in an essentially transverse direction and has a time-varying component orientated along the length of the cable ( ) at the first site (A), with the component varying so that the line integral thereof along the cable ( ) is non-zero. This results in a variation in the polarization of the light, which can then be detected by a polarization discriminator ( ) at a second site (B), thereby to identify the cable ( ).

An optical scanning arrangement for gas sensing, in which scanned light source means produces a spectrum of regularly spaced wavelengths or lines which correspond as regards spacing to the absorption line spectrum of a gas to be monitored and in which the multi-line output from the light source means propagates through a region to be sensed for gas and means for determining the light output from the region for detecting the attenuation of the components of the multi-line output from the light source means due to absorption by said gas being monitored.

Optical sensing arrangements

Optical sensing system with averaging circuit

An optical time domain reflectometry system in which laser level reference signals are derived from the respective laser outputs from the laser sources and fed into a single receiver provided for receiving the back-scattered or returned light along an optical fibre. The receiver is thus continuously calibrated in accordance with the relative intensities of the optical signals at different frequencies being launched into the optical fibre so that the receiver provides an accurate measurement of temperature or other parameter being measured by the system.

Multi-wavelength time domain reflectometer

A Fabry-Perot pressure or displacement sensor comprises an optical fibre which serves to transmit a light beam to the mirrors of the sensor and to receive light which is reflected back from the mirrors. The light beam and the reflected light comprise light at a plurality of wavelengths. The wavelength components of the transmitted light and the wavelength components of the received light are range rated in range gates and are compared in comparators to provide respective ratios of transmitted intensities and received intensities. These ratios are in turn compared in comparator to provide a ratio dependent upon the displacement of the mirrors of sensor due to pressure applied thereto but independent of any changes in the intensities of transmitted light due to reflections from optical components in the pressure or displacement monitoring arrangement.

Optical pressure sensor

A two-wavelength referencing optical sensing arrangement comprises two laser sources for producing simultaneous optical pulse signals of two different wavelengths for transmission over a plurality of discrete optical fibre paths to the sensors terminating such paths. Time-displaced return signals of different wavelengths received over the optical paths from sensors are fed into a single receiver which uses the intensity of signals of one wavelength in order to compensate for unknown losses in the system when indicating measurement of conditions sensed at the locations of the optical sensors.

Optical sensing arrangements with wavelength and time-displacement

Optical fiber sensing systems

A spectral filtering optical sensor comprises a stationary zone plate and stationary co-terminous ends of input and output fibres with the interposition in the optical path between the zone plate and the optical fibre ends of a variable optical focussing arrangement the focussing characteristics of which change with changes in magnitude of the particular parameter being sensed and thereby vary the effective optical path length between the zone plate and the optical fibre ends and the wavelength of light impinging on the end of the output fibre.

Optical sensor having stationary co-terminus ends of the input and

An optical fibre sensing system for monitoring and/or measuring temperatures or pressures distributed around an optical fibre sensing loop. Light source and coupler are provided for applying continuous wave light to the loop so that the light propagates simultaneously in opposite directions around the loop with the resultant light output from the loop being detected by a detector. Light pulse generating source and coupler cause light pulses to propagate in one direction only around the loop and by so doing produce transient variations in the propagation constant along the looped fibre in dependence upon the distributed temperature or pressure around the loop. The detector detects the phase changes occurring in the continuous wave light received from the sensing loop due to variations in temperature or pressure around the loop.

Optical fiber sensing system

An optical transmission system comprises a plurality of transducers located at intervals along a continuous optical fibre data transmission path. Data signals are derived from respective transmitters associated with the transducers and are applied to the transducers to produce mechanical stressing or deformation of the data transmission path optical fibre for imparting corresponding modulation of the polarization state of light being propagated along the optical fibre. A receiver connected to the far end of the transmission path includes means for determining the exact state of polarization of the received light and for extracting the modulation information and hence transmitted data therefrom.

Optical fibre transducer injected data transmission system

A temperature measuring arrangement including a relatively long optical fibre temperature sensor doped along its length with material that absorbs light in dependence upon temperature, launcher for launching into one end of the temperature sensor fibre contemporaneous light pulses of two different wavelengths and comparator for comparing variations with time in back-scattered light of the two different wavelengths returned along the sensor fibre in order to provide an indication of the temperature distribution along the fibre, the two wavelengths of the light pulses launched into the fibre sensor being chosen so that they correspond with different absorption bands of the doped material.

Temperature measuring arrangements using optical fibre sensor

An optical sensing arrangement suitable for the detection of acoustic waves and/or temperature or magnetic field changes, comprises a laser light source for producing polarized light which in operation of the arrangement will be applied for detection purposes to the input end of an optical sensor for transmission therethrough, whereby a relative phase delay is produced between orthogonal components of the polarized light at the output end of the optical sensor, characterized in that a relative phase delay compensation device is optically coupled to and so orientated with respect to one end of the sensor, that the relative phase delay between the orthogonally polarized components transmitted through the compensation device and the corresponding relative phase delay of these components in the optical sensor are mutually opposite, thereby reducing the noise content of an output signal derived from the arrangement and fed to polarization analyzer for producing an indication of structural changes which have occurred in the optical sensor.

Optical fibre sensing arrangements

According to the present invention there is provided an optical detecting, monitoring or measuring arrangement, comprising a movable diffraction grating arranged for angular displacement by means responsive for example to changes in one or more parameters, such as temperature or pressure, whereby the color of light being reflected to a light-receiving point from said grating changes and said light is then directed onto a fixed diffraction grating so that the direction of a light beam consequentially reflected from said fixed diffraction grating changes with change in color, such change in direction being dependent upon and providing an indication of the change in said parameter or parameters.

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Hampshire, United Kingdom

John P Dakin