Research And Application Of Fiber-optic Low-frequency Acoustic Sensing Technology

As the technology of fiber optic sensing matures, its related applications have begun to extend to many kinds of areas. Fiber acoustic sensor is one important research area of the fiber optic sensor. Fiber acoustic sensor is of great concern for its advantages such as high detection sensitivity, lower manufacturing costs, anti-electromagnetic interference, able to work in harsh environment etc., compared with traditional electrical sonic sensor (capacitive or piezoelectric acoustic sensors). Nowadays, fiber acoustic sensors have been widely used in natural disaster warning, medical diagnostics, geological exploration, even the battlefield and other fields. In recent years, earthquakes and other natural disasters happen frequently. Also modern naval technology has developed rapidly. Due to the special needs of these areas to acoustic detection, fiber acoustic sensor began developed to direction of high-precision, low-frequency, low cost and miniaturization.In this paper, for the higher requirements of fiber acoustic sensors in recent years, studies have been carried out with the combination of key technologies of the double FBG structure, non-standard fiber-coupled, passive mode locking based multi-longitudinal-mode beat frequency and fiber EFPI structure. The focal point lies in high-sensitivity, low frequency, low-cost of the designed structures. Theoretical and experimental analysis have been demonstrated and the main contents of the research results are as follows:First of all, the primary nature of acoustic waves is introduced based on the acoustics related basic theory. Different kinds of elastomer vibration types were classified and described on which the theoretical foundation for acoustic detection schemes for later chapters was laid.Secondly, from the basis theory of the fiber Bragg grating (FBG), a double FBG acoustic sensor was proposed using metal film on the double FBGs vertically to measure the acoustic vibration innovatively. Theoretical analysis and simulation had been carried out for the influence of the output on double FBGs with different structures and different parameters. The experimental results shown that, in range of 200-1000 Hz the acoustic sensitivity up to 90 ?W/Pa, and sound pressure measurement can be implemented within the scope of 100.3 to 118.5 dB. The combination of double FBGs also solved the temperature stability problem that most of the fiber optic sensors have in a certain extent. We implemented the self-demodulation, good temperature stability, accurate and reliable acoustic sensing applications.Thirdly, based on mode coupling theory, under exploring the process of manufacture the optical fiber coupler, using non-standard type optical fiber coupling (multi-cycle type optical fiber coupling and multimode optical fiber coupling), several acoustic sensing methods were demonstrated. Through sound-light conversion of the metal foil, applying laboratory-made non-standard fiber coupler, vibration can be sensed based on the acoustic vibration coupling and light intensity modulation technique. The experimental results showed that the highest sensitivity of 2.63 mW/Pa can be achieved in the ear hearing frequency range 20 Hz-20 kHz. Good shock response experiments showed that sensor performance is great. It can demodulate sound signal without distortion. In addition, the difference treatment of the two outputs, can achieve double sensitivity, also greatly reduce the effects of environmental noise and machine. So we realized high sensitivity, wide dynamic frequency range of acoustic sensing application.Fourthly, in theoretical analysis and on the basis of successful production of DBR laser, using multiple longitudinal mode laser beat frequency demodulation mechanism, and according to the theory that DBR laser resonator cavity length is sensitive to optical fiber axial strain, simulation analysis for specific impact on the laser beat frequency output from the axial strain and experimental verification had been carried out. At the same time, the use of carbon nanotubes embedded in DBR laser cavity is used as the saturable absorber (SA) for passive mode-locking, making the laser stable and reliable. We creatively put forward and verified the possibility that multiple longitudinal mode fiber laser based on passive mode-locked can realize sound pressure measurement. Then the experimental research was carried out, results showed that sensing application worked in 6 GHz, we can achieve sound pressure sensitivity as high as 147.2 Hz/mPa within the scope of 0-23 Pa with good stability. It means that in frequency domain we achieved the high sensitivity, wide dynamic range, good stability sound pressure measurement application.Finally, in view of the special demands for low frequency infrasound detection, combined with the advantages of fiber optic interferometric sensor structure, a novel fiber EFPI sound sensor based on composite diaphragm was designed and proposed. Using combination structure of polymer film and aluminum foil, EFPI interferometer structure is formed between the fiber end face and ceramic ferrule end face of the FC connector. By theoretically simulating and optimally designing the transducer with different materials, sizes and structures, etc., related parameters were selected optimally according to the detection demand. After production and packaging, the contrast test using our EFPI sensor with the standard of Denmark B&K sensor was carried out in the simulation infrasound field environment. Results show that the proposed fiber EFPI infrasound sensor obtained acoustic sensitivity up to 138.3 dB re 1 V/?Pa (?121 mv/Pa) within 1-20 Hz, higher than that of mature commercial acoustic sensor for comparison.