A long-distance high-precision optical fiber interference sensing and positioning system includes a sensing light path, a control terminal, a light source module, a front-end optical processing module, a back-end optical processing module, a photoelectric conversion module, a digital signal collection module and a MCU central processing module. The control terminal is provided with a signal processing and positioning device for calculating and positioning sensing signal positioning information. The present system can solve the security problem of an ultra-long-distance optical cable, can send alarm information in a timely manner with regard to an intrusion event by means of vibration positioning, and can also solve the problem of occupation of an optical cable channel, and improves the utilization efficiency of the optical cable by means of wavelength division multiplexing technology.

This application described methods and apparatus for distributed fibre optic sensing. A sensing apparatus has a modulator which modulates radiation from an optical source to interrogate a sensing optical fibre with a first interrogation pulse at a first frequency (F1) and a second interrogation pulse at a second, different, frequency (F2), both different in frequency from a local oscillator (LO). A mixer mixes backscatter from the sensing optical fibre with the local oscillator and supplies the mixed signal to a detector that provides a corresponding digital signal. A processor processes the digital signal (DX, DY) in a first and second processing channels to demodulate respective first and second phase signals based on the respective frequency difference between the first and second frequency and the local oscillator and determines a temporal difference between the first and second phase signals.

Methods and systems for characterizing fractures in a subterranean formation are provided. The method includes introducing an encapsulated explosive unit into a casing located in a wellbore within the subterranean formation and maintaining the encapsulated explosive unit in a stage of the casing. The method also includes detonating the encapsulated explosive unit within the stage to generate a pressure wave that passes through a group of perforations and into the fractures and measuring a reflected pressure wave using a pressure sensor coupled to the bridge plug to produce a pressure measurement. The method further includes converting the pressure measurement into an acoustic signal correlated with the pressure measurement by an acoustic signal generator contained in the bridge plug and transmitting the acoustic signal to apply acoustic pressure on a fiber optic cable coupled to an exterior surface of the casing.

A distributed acoustic system may comprise an interrogator which includes a single photon detector, an umbilical line comprising a first fiber optic cable and a second fiber optic cable attached at one end to the interrogator, and a downhole fiber attached to the umbilical line at the end opposite the interrogator. A method for optimizing a sampling frequency may comprise identifying a length of a fiber optic cable connected to an interrogator, identifying one or more regions on the fiber optic cable in which a backscatter is received, and optimizing a sampling frequency of a distributed acoustic system by identifying a minimum time interval that is between an emission of a light pulse such that at no point in time the backscatter arrives back at the interrogator that corresponds to more than one spatial location along a sensing portion of the fiber optic cable.

A distributed acoustic sensing (DAS) system for determining an acoustic event may include an interferometer and an acoustic event detection processing device. The interferometer may measure DAS data from sensed signals from a sensing fiber deployed in a wellbore. The acoustic event detection processing device may determine an acoustic event in the wellbore from an out-of-band signal using the DAS data by performing operations. The operations can include determining a first acoustic event and a second acoustic event from the DAS data. The operations can include determining a first set of aliased frequencies from the first acoustic event and a second set of aliased frequencies form the second acoustic event. The operations can include determining, using an intersection of the first set of aliased frequencies and the second set of aliased frequencies, a frequency or amplitude of out-of-band signals that are usable to determine the at least one acoustic event.

A non-blind area real-time monitoring and alarming system for an accident on a freeway is provided, which belongs to the field of photoelectric technology and can solve the existing problems in whole journey information monitoring for freeways, such as failure to cover all freeway sections and lack of all-weather and prompt monitoring, and being subject to severe environments such as rain, fog and snow, or conditions such as poor visibility at night. The system includes a distributed sound wave detection fiber-optic cable, a sound wave signal demodulator, a network switch, a workstation, and a monitoring terminal. A fiber-optic sensing network composed of a series of reflection nodes distributed at equal distances is utilized to monitor sound wave signals from traffic accidents efficiently in real time without blind areas, accurately locate a traffic incident by analyzing frequency components, and transmit alarming information in time.

A thermo-fluorescent optical fiber including a core able to propagate light and a sheath, the fiber carrying at least at one of its ends or at one site along its length, devoid of sheath, the optical fiber being provided with a probe consisting of a matrix having thermo-fluorescent particles, the matrix being photo-polymerized. Also provided is a method that relates to manufacturing such an optical fiber as well as to its applications.

A computer-readable, non-transitory medium storing a program that causes a computer to execute a process is provided. The process includes acquiring a backward Rayleigh scattered light from an optical fiber composite overhead ground wire which an electrical power transmission facility has; generating vibration information of a frequency band including a natural frequency of the optical fiber composite overhead ground wire, on a basis of the backward Rayleigh scattered light; and detecting abnormality of the electrical power transmission facility, on a basis of the vibration information.

A computer-readable, non-transitory medium storing a program that causes a computer to execute a process is provided. The process includes acquiring a backward Rayleigh scattered light from an optical fiber composite overhead ground wire provided along an electrical power transmission line, determining each of spectral densities of each of frequencies of vibration of the optical fiber composite overhead ground wire, on a basis of the backward Rayleigh scattered light, estimating a wind speed of a wind hitting the electrical power transmission line, on a basis of a first spectral density of a first frequency band including a natural frequency of the optical fiber composite overhead ground wire, and estimating a wind direction of the wind, on a basis of a second spectral density of a second frequency band which does not include the natural frequency of the optical fiber composite overhead ground wire.

Described herein are systems, methods, and articles of manufacture for a spatially nonuniform scattering profile along its length, whose backscattering signal can be used for sensing even after fiber attenuation increases due to the conditions in the sensing environment. In one embodiment, the fiber has been pre-exposed to the conditions that produce attenuation, and the spatially nonuniform profile compensates for this. Subsequent exposure then results in very little or at least acceptable levels of additional attenuation. An exemplary fiber comprises a fiber length and an optical back scatter along the fiber length greater than a Rayleigh back scattering over the fiber length, wherein the optical back scatter does not decrease along the fiber length by more than 3 dB after exposure to a hydrogen-rich first environment having a given pressure and temperature. An exemplary method comprises drawing a fiber, applying a UV coating, post-processing the fiber using an interferogram, measuring optical back scatter enhancement dependence based on a UV dosage, incrementally increasing the reflectivity, exposing the fiber to a hydrogen-rich first environment.