An optical fiber ocean bottom vector magnetometer (OOBVM) array based on fiber laser sensing technology is proposed. The fiber laser is modulated at the natural frequency of the sensing structure by the Ampere force in the magnetic field. In this way, the sensitivity is enhanced and the effect of power fluctuation of fiber laser is eliminated. The test result shows that its magnetic field resolution is about 0.86 nT / √ Hz@100 mA. The design and assembly process of the OOBVM array are demonstrated. A field test in South China Sea demonstrated that the OOBVM array can detect the movement of ferromagnetic objects in water, which implies the potential for applications in underwater object detection and marine geophysics.
A fiber optic accelerometer-magnetometer (FLAM), which can measure acceleration and magnetic field simultaneously, based on fiber laser sensing is proposed. The principle of the FLAM and the theory of the decoupling the signals of acceleration and magnetic field are presented. The FLAM was interrogated using phase-generated-carrier (PGC) method. A test of simultaneously measuring acceleration and magnetic field was performed. The results show that the responding mixed signal achieves a good SNR and prove that the function of the sensor has been realized and the theory of decoupling signal is reliable.
In this paper we report the field test of fiber optic ocean bottom seismograph (OOBS) which can be used in the active source seismic research. There are three fiber laser accelerometers (FLAs) and one fiber laser hydrophone (FLH), which is wavelength division multiplexed, in the OOBS. The interrogation system is put on shore and is connected with the OOBS with optical fiber cable. The field test of using an air gun is carried out under water with a depth of 30 m. The results show that the OOBS has similar performance as conventional electric OBS.
In this paper we present the field test of the fiber optic ocean bottom cable (FOOBC). The FOOBC are several ocean bottom seismic stations connected by optical fiber cables. In the ocean bottom seismic station, there are three orthogonal fiber optic accelerometers and one fiber optic hydrophone. The design of the sensors and stations are introduced. The field demonstration of an ocean bottom seismic station is carried out in Yunnan Province, China. The test results show that the three accelerometers has similar response to the seismic signal with traditional MEMS accelerometers. We believe that the fiber optic seismic cable is promising in the field of oil and gas exploration and earthquake monitoring.
Liquid damped fiber laser accelerometer (LD-FLA) was proposed and a kind of LD-FLA structure, of which damping liquid could be easily change, was designed to study its damping characteristic. The principles of a LD-FLA were analyzed comparing with a common FLA, which found that liquid damper could produce additional liquid pressures in vibration so that not only damping responses but sensitivities and frequency responses could be changed at the same time. Four double diaphragm-based LD-FLAs were manufactured and experiments under several kinds of damping liquids were carried out. The experimental results are close to the calculated values and verify the theoretical analysis on LDFLA. The theoretical model provides guidance for further research on damping characteristic.
A fiber optic Fabry-Perot accelerometer (FOFPA) with diaphragm-mass-collimator (DMC) gathered structure is presented. This design makes the structure more compacts and the manufacturing process more controllable. The operation principle based on Fabry-Perot interference is described. Several tests using intensity demodulation scheme which can control the working point of FOFPA were carried out. Experimental results show that: axis sensitivity of the proposed FOFPA is 36.07 dB (re: 0 dB=1 V/g) with a fluctuation less than 0.9 dB in a frequency bandwidth of 10-125 Hz, the resonant frequency is about 350 Hz, measurement range is about 70 dB@100 Hz. which are much close to theoretical values
The paper proposes an accelerometer construction based on 45-degrees Fabry-Perot (F-P) interferometer cavity. The uniform intensity cantilever consists of a mass block in the middle and a 45-degrees F-P cavity fixed inside the mass. The mass block can oscillate freely when the vibrating sensor is subject to the vibration and the F-P cavity length is changing. The G-lens end face and total reflective film make up the two reflective films of the F-P cavity, and the reflectivity are 4% and 90% respectively. In the F-P cavity, a 45-degrees mirror fixed in the middle of the G-lens and total reflective film. The mirror can change the transmission of the light and increase the optical path difference. The total reflective film fixed in the steel tube and the G-lens fixed in the fine tuning bolt. The bolt can fine adjust the F-P cavity in sensor encapsulating. The sensor structure lead to the optical loss in the airborne and tilted mirror, besides the distance of F-P gap in steel tube and the optical coupling efficiency can’t work out accurately, so we did a series deterministic test before encapsulating, for example the selection of the structures, the diameter of the optical fibers and the diameter of the reflective films. At last, 9/125 μm optical fiber, 1.4 mm total reflective film and the structure of total reflective film out of steel tube were used for the accelerometer. The sensitivity can reach 0.042 rad/g and the resonant frequency of the accelerometer is 400 Hz.
Fiber laser hydrophone has attracted more and more attention because of its potential application in novel sonar system. For a towed fiber optic hydrophone array, the flow noise is an important source of the system noise. This paper presents the field test result of the flow noise of an eight-element fiber laser hydrophone array. The structure of the fiber laser hydrophone and the array is also introduced. The field test shows that obvious flow noise can be measured when the array is towed at the speed of 3 kn.
A fiber optic Fabry-Perot (FP) hydrophone with hydrostatic pressure compensation was demonstrated. A polyimide (PI) diaphragm attached on the end of an Acrylonitrile Butadiene Styrene (ABS) tube was used as the sensing element. A pair of grooves was designed in an inner ABS tube to connect the Fabry-Perot cavity with the outside environment, which made the hydrophone hydrostatic pressure compensated. The operation principle, design and testing of polyimide diaphragm-based sensor were described. Experiment results show that it has not only high stability in different hydrostatic pressures, but also flat frequency response of about 158 ±3 dB within 300-3000 Hz.