A method of optical sensing comprises coupling an excitation optical signal into a first optical fiber to induce Rayleigh backscattering, thereby providing a backscattered signal. The backscattered signal is optically amplified in the first optical fiber, thereby providing an amplified backscattered signal. The amplified backscattered signal is coupled into a second optical fiber and is optically re-amplifying in the second optical fiber.

A prefabricated mat-like structure having lengths of fiber mounted thereon or therein in a predetermined deployment pattern that provides a high spatial density of fiber to give increased spatial sensing resolution is described. The prefabricated mat-like structures may be very easily deployed by being placed against and/or wrapped around an object to be monitored, typically being fastened in place by clamps or the like. In addition, easy removal from the object is also obtained, by simply unfastening the mat-like structure, which may then be redeployed elsewhere. The prefabricated mat-like structure having the fiber already mounted thereon or therein therefore provides a very convenient and easily installable and removable solution

This application relates to methods and apparatus for fiber optic sensing which can provide information about the environment in which the fiber optic is deployed. In particular the application relates to fiber optic based sensing of the mechanical impedance of the environment. The method comprises using an interrogator (201) to interrogate an optical fiber (104) which is coupled to a first element (202; 802) which is responsive to electromagnetic fields. In use a varying electric current (I), which may be an alternating current, is applied so as to induce a varying force (F) on said first element. The optical radiation backscattered from within the optical fiber is analyzed to determine a measurement signal indicative of a variation in the backscattered radiation corresponding with said electric current applied. The first element may be a first conductor (202) and the varying current may be supplied to the first conductor, or to a second conductor (701). Alternatively the first element could be a magnetic element (802). By applying a variable force to the first element, and hence the optical fiber, the characteristics of the environment can be determined.

An improved technique for acoustic sensing involves, in one embodiment, launching into a medium, a plurality of groups of pulse-modulated electromagnetic-waves. The frequency of electromagnetic waves in a pulse within a group differs from the frequency of the electromagnetic waves in another pulse within the group. The energy scattered by the medium is detected and, in one embodiment, may be used to determine a characteristic of the environment of the medium. For example, if the medium is a buried optical fiber into which light pulses have been launched in accordance with the invention, the presence of acoustic waves within the region of the buried fiber can be detected.

A computer-implemented event statistic generation method for intrusion detection comprises processing a plurality of return signals from a coherent optical time domain reflectometer into time domain signals for each of a plurality of sensor bins, the plurality of return signals corresponding to a plurality of stimulation pulses injected into an optical sensor fiber during a time period. For each sensor bin, the method comprises transforming the respective time-domain signal into a corresponding frequency-domain signal, calculating, from the respective frequency-domain signal, a first signal power area of a first frequency band expected to contain system noise, calculating, from the respective frequency-domain signal, a second signal power area of a second frequency band expected to contain any energy related to at least a first event; and generating an event statistic proportional to the ratio of the second signal power area to the first signal power area at least in part by dividing the second signal power area by the first signal power area.

A railroad monitoring system according to the present disclosure includes a cable (20) including a communication optical fiber being laid on a railroad (10), a reception unit (331) configured to receive an optical signal from at least one communication optical fiber included in the cable (20), and a detection unit (332) configured to detect a pattern according to a state of the railroad (10), based on the optical signal, and detect an abnormal state of the railroad (10), based on the detected pattern according to the state of the railroad (10).

A road monitoring system according to the present disclosure includes a cable (20) including a communication optical fiber laid on a road (10), a reception unit (331) configured to receive an optical signal from at least one communication optical fiber included in the cable (20), and a detection unit (332) configured to detect, based on the optical signal, a pattern according to a traveling state of a vehicle on the road (10), and detect, based on the detected pattern, a traveling state of a vehicle on the road (10).

A triggering device includes an optical fiber (126) configured for optical shape sensing. A supporting element (104) is configured to support a portion of the optical fiber. An interface element (106) is configured to interact with the optical fiber associated with the supporting element to cause a change in a property of the fiber. A sensing module (115) is configured to interpret an optical signal to determine changes in the property of the fiber and accordingly generate a corresponding trigger signal.

A distributed optical detection system comprising: a broadband optical source; and a phase and amplitude receiver for measuring phases and amplitudes of distributed backscattered signals from a sensing medium. Methods of quantitatively sensing optical path length changes along a sensing medium in a distributed manner are also disclosed.

Fiber optic cable structures suitable for distributed acoustic sensing that are capable of discriminating between stimuli acting on the cable in different directions, the cable structure including a core structure (202, 203, 204) with an optical fiber wound around the periphery of the core structure, the core further including a mass (203) which is movable in a preferred direction within the cable such that movement of said mass in said preferred direction causes a change in length of the fiber wound around the periphery of the core.