A method using wavelength division multiplexing for reducing light diffusion and light reflection interference in an interference path, comprising: connecting a wavelength division multiplexer (10) serially at an end of a sensing optical fiber (6); using the wavelength division multiplexer (10) to extract a wavelength component from a working path for measuring an interfering signal caused by light diffusion and light reflection; using the signal as a reference to extract an effective signal component that has been interfered by light diffusion and light reflection, and obtaining a pure effective signal. Because the device connected at the end of the sensing optical fiber (6) is passive and requires no power, the system is easy to implement and is particularly suitable for situations in which power provision is difficult at the end of the sensing optical fiber (6). The method is suitable for long distance pipeline monitoring and a wide-range optical of fiber perimeter security. Also provided is a system using wavelength division multiplexing for reducing light diffusion and light reflection interference in an interference path.

A measurement system for an aircraft includes an interrogator and an optical sensor assembly, remotely connected to the interrogator. The optical sensor assembly includes an optical sensor connected along a sensor path to the interrogator, a data storage device configured to store operational data, and a control circuit operationally connected to the data storage component, and connected along a power path to the interrogator. The sensor assembly also includes an opto-electric device for converting an optical signal to an electric current, connected along the power path between the interrogator and the control circuit, to power the data storage component through the control circuit, and a transmitter operationally connected to the control circuit and connected along a data path to the interrogator, to provide operational data along the data path to the interrogator. The data storage component is powered across the power path from the interrogator, and provides operational data to the interrogator across the data path, and the interrogator is configured to receive sensor data from the sensor across the sensor path, and manipulate the data in light of the operational data for analysis of the sensor data.

A system and method for dynamically calibrating a distributed fibre optic sensing system is disclosed. The calibration system includes a light source for generating pulses of coherent light, an optical fibre arranged at least partly in a ground soil region to guide the light and a photo detector for detecting scattered light returning from the optical fibre in dependence of time. The method includes obtaining information from which a temporal change of an acoustic transfer characteristic of the ground soil region is derivable and calibrating a distributed acoustic sensing system based on the changed acoustic transfer characteristic.

A method of processing data from a distributed fibre-optic interferometric sensor system for measuring a measurand, the system comprising multiple interferometric sensors. The method comprises interrogating two or more of the multiple interferometric sensors to record a raw measurement time series for each of the sensors. The method further comprises calculating a common reference time series as a measure of central tendency of the raw measurement time series from two or more reference sensors, the reference sensors being selected from the multiple interferometric sensors. Finally, the method comprises compensating at least one raw measurement time series from a measurement sensor selected from the multiple interferometric sensors with the common reference time series to produce a compensated measurement time series, the measurement sensor being configured to be sensitive to the measurand. The invention further relates to a distributed fibre-optic interferometric sensor system.

A system and method for discriminately measuring the response of a plurality of spatially separated fiber sensors positioned along an optical fiber using a sweep of electromagnetic radiation. Each fiber sensor affects the transmission of a particular wavelength of electromagnetic radiation and the particular wavelength affected by a given fiber sensor is dependent on at least one environmental property of the given fiber sensor. By detecting the particular wavelength affected by a given sensor, it is possible to determine the environmental property of the given sensor.

Methods and apparatus for fast sweeping a spectral bandwidth in order to distinguish among signals received from effectively wavelength division multiplexed (WDMed) and time division multiplexed (TDMed) optical components on a single fiber. For some embodiments, a method for interrogating optical elements having characteristic wavelengths spanning a sweep range is provided. The method generally includes introducing a pulse of light, by an optical source, into an optical waveguide to interrogate at least a first set of optical elements having different characteristic wavelengths by performing a sweep of wavelengths over a period of the pulse, wherein the period is less than a round-trip time for light reflected from an optical element closest to the optical source to reach a receiver and processing the reflected light to determine a parameter based on the times at which signals are received.

The application describes methods and apparatus for distributed fiber sensing, especially distributed acoustic/strain sensing. The method involves launching interrogating radiation in to an optical fiber and sampling radiation backscattered from within said fiber at a rate so as to acquire a plurality of samples corresponding to each sensing portion of interest. The plurality of samples are divided into separate processing channels and processed to determine a phase value for that channel. A quality metric is then applied to the processed phase data and the data combined to provide an overall phase value for the sensing portion based on the quality metric. The quality metric may be a measure of the degree of similarity of the processed data from the channels. The interrogating radiation may comprise two relatively narrow pulses separated by a relatively wide gap and the sampling rate may be set such that a plurality of substantially independent diversity samples are acquired.

The invention provides a measurement system to interrogate at least one birefringent optical sensor and a method to interrogate birefringent optical sensors at high speed and high resolution. The system and method detects, at least, a first and a second spectral feature that are polarization dependent, wherein the detected first and second spectral features correspond to different responses of at least one birefringent optical sensor.

The application describes methods and apparatus for distributed fiber sensing, especially distributed acoustic/strain sensing. The method involves launching at least first and second pulse pairs into an optical fiber, the first and second pulse pairs having the same frequency configuration as one another and being generated such that the phase relationship of the pulses of the first pulse pair has a predetermined relative phase difference to the phase relationship of the pulses of the second pulse pair. In one embodiment there is a frequency difference between the pulses in a pulse pair which is related to the launch rate of the pulse pairs. In another embodiment the phase difference between the pulses in a pair is varied between successive launches. In this way an analytic version of the backscatter interference signal can be generated within the baseband of the sensor.

A system and method for simultaneously addressing multiple parallel distributed fiber optic sensors using a single interrogation instrument is disclosed. One or more of the fiber optic sensors are provided with a non-reflective delay element to prevent an overlap in time between backscatter returns from the distributed fiber optic sensors, thereby allowing the backscatter returns from each sensor to be distinguished based on round-trip transit time.