Review of Fiber-optic Distributed Acoustic Sensing Technology

A distributed optical-fiber acoustic sensor is an acoustic sensor that uses the optical fiber itself as a photosensitive medium,and is based on Rayleigh backscattering in an optical fiber.The sensor is widely used in the safety monitoring of oil and gas pipelines,the classification of weak acoustic signals,defense,seismic prospecting,and other fields.In the field of seismic prospecting,distributed optical-fiber acoustic sensing(DAS)will gradually replace the use of the traditional geophone.The present paper mainly expounds the recent application of DAS,and summarizes recent research achievements of DAS in resource exploration,intrusion monitoring,pattern recognition,and other fields and various DAS system structures.It is found that the high-sensitivity and long-distance sensing capabilities of DAS play a role in the extensive monitoring applications of DAS in engineering.The future application and development of DAS technology are examined,with the hope of promoting the wider application of the DAS technology,which benefits engineering and society.

Fabry-Perot Temperature Sensor for Quasi-Distributed Measurement Utilizing OTDR

A quasi-distributed Fabry-Perot fiber optic temperature sensor array using optical time domain reflectometry （OTDR） technique is presented. The F-P sensor is made by two face to face single-mode optical fibers and their surfaces have been polished. Due to the low reflectivity of the fiber surfaces, the sensor is described as low Fresnel Fabry-Perot interferometer （FPI）. The working principle is analyzed using twobeam optical interference approximation. To measure the temperature, a certain temperature sensitive material is filled in the cavity. The slight changes of the reflective intensity which is induced by the refractive index of the material was caught by OTDR. The length of the cavity is obtained by monitoring the interference spectrum which is used for the setting of the sensor static characteristics within the quasi-linear range. Based on our design, a three point sensor array are fabricated and characterized. The experimental results show that with the temperature increasing from -30℃ to 80℃, the reflectivity increase in a good linear manner. The sensitivity was approximate 0.074 dB℃. For the low transmission loss, more sensors can be integrated.

Dielectric spectroscopy and hydrogen bonding studies of 1-chloropropane-ethanol mixture using TDR technique

The time-domain reflectometry technique was exercised to assess the complex permittivities of 1-chloropropane and ethanol along with their binary mixture in the frequency range of 10MHz–50 GHz and in the temperature range of 10–25℃.Dielectric parameters like static dielectric permittivity(ε_(0))and relaxation time(τ)have been acquired by the least-square-fit method.The obtained assessments of static permittivity and relaxation time were used to compute various dielectric parameters like excess dielectric constant,excess relaxation time,Kirkwood correlation factor and thermodynamic parameters.The experimental outcome illustrates the nonlinear variations in dielectric permittivity,relaxation time and authenticates the structural formation due to intermolecular interaction between 1-chloropropane and ethanol.Molecular rotary motion and dipole reorientation movements of these complex systems are presented in agreement with the molar entropy and enthalpy.