A bonded structure includes a first member, a second member, an adhesive that bonds the first member and the second member together, and an optical fiber sandwiched between the first member and the second member. When pressure is applied to the optical fiber only from a predetermined direction, the sectional shape of the optical fiber changes to an elliptical shape, so that birefringence occurs, whereby the shape of the light spectrum changes so as to have multiple peaks. The optical fiber is used as a sensor for detecting the bonding condition between the first member and the second member based on the birefringence.

This application relates to methods and apparatus for monitoring power cables (100) carrying multiple AC phases to detect deformation of the power cable. A distributed fibre optic interrogator unit (302) is used to interrogate a sensing optical fibre (301) coupled to the power cable to provide a measurement signal from each of a plurality of longitudinal sensing portions of the sensing optical fibre. An analyser (602) is configured to analyse the measurements signals to detect a characteristic of an imbalance in magnetic fields. The characteristic may be a signal component with a characteristic frequency related to the power frequency and number of AC phases, the sensing optical fibre may be sensitised to magnetic fields and the characteristic frequency may be 2n times the power frequency where n is the number of phases, e.g. six times the power frequency for three phase AC.

A wading structure health sensing distributed optical fiber calibration system and a method includes a heat insulation barrel, and calibration modules located in the inner ring of the barrel, the calibration module comprises a through-shaft, a first electronic thermometer and a second electronic thermometer, an optical cable to be calibrated is twisted on the through-shaft and is connected to an optical fiber temperature demodulator outside the heat insulation barrel, the first and second electronic thermometers are respectively connected to first and second temperature control meters outside the barrel, a temperature source wire for heating water in the barrel arranged in the wall of the barrel, the temperature source wire connected to a power temperature control meter outside the barrel, and the first temperature control meter connected to cooling and heating devices simultaneously through leads of the first temperature control meter.

Various embodiments include methods, apparatus, and systems to operate a tool downhole in a well, where the tool has sensing system to determine different properties of downhole structures. Such an apparatus can include a received signal that is input to a beam splitter with 5 a local oscillator signal. The beam splitter outputs light signals to first and second photodetectors that convert the respective signals to electrical signals. The electrical signals are input to differential amplifier that generates an amplitude representative of the phase difference between the two input signals. A feedback path converts that 10 amplitude to a phase adjustment signal that is couple to the local oscillator.

Various embodiments include apparatus and methods to measure a parameter of interest using a fiber optic cable. The parameters can be provided by a process that provides for multiplexed or distributed measurements. A multiplexed or a distributed architecture can include acoustic sensor units placed selectively along an optical fiber such that the acoustic sensor units effectively modulate the optical fiber with information regarding a parameter to provide the information to an interrogator coupled to the optical fiber that is separate from the acoustic sensor units.

Provided are a spatial sensing device, and the like, that are suitable for spatial sensing. This spatial sensing device (3) comprises a sensing data acquisition means (11) for acquiring sensing data through optical fiber sensing using a plurality of optical fiber cables (1) laid in different directions and an object detection means (12) for using the sensing data to detect the position of an object in a space of interest.

An optical sensor assembly including a diffractive optical element (DOE) responsive to an optical input beam and a refractive lens responsive to a shaped optical beam from the DOE. The lens focuses the shaped optical beam to provide a center beam having a short focal length and an outer beam having a long focal length. A sensor is positioned in front of the long focal length and is responsive to the shaped input beam from the lens. A fiber is positioned within a center opening of the sensor so that an input facet of the fiber faces the DOE and is located at the first focal length. The sensor is positioned relative to the DOE and the position of the input beam is controlled so that the center beam impinges the input facet and the outer beam impinges the sensor.