Overview: Sound wave as the carrier of information and energy is the natural phenomena which has been paid attention to study for the first time. With the rapid development of information technology, the detection of acoustic signals with high sensitivity and large bandwidth becomes more and more important. In recent years, the research focus of acoustic sensors has been developed from electro-acoustic sensing technology to photoacoustic sensing technology. Optical fiber sensing technology was developed rapidly in the 70 s of last century. Due to the high precision of light detection and the advantages of fiber working frequency band and small transmission loss, optical fiber acoustic sensor features high sensitivity, broad-band frequency response, high signal to noise ratio and high dynamic range compared with space optical microphones and traditional electroacoustic sensors. Furthermore, because of its immune electromagnetic interference and miniaturization, it can be used in environments with high temperature, high pressure, strong corrosion and strong radiation that traditional electroacoustic sensors can not work normally. Therefore, optical fiber acoustic sensor has been valued and widely used in the national security, industrial non-destructive testing, medical diagnostics, consumer electronics etc. In this paper, from the perspective of the frequency response curve of optical fiber acoustic sensors, optical fiber acoustic sensors are classified as indirect and direct coupling types by introducing a new classification method based on whether acoustic coupling materials are used or not.
As shown in the figure above, indirect coupling type, including light intensity modulation, phase modulation and wavelength modulation, present some problems such as uneven frequency response, narrower bandwidth, and smaller dynamic range due to the frequency response features of the acoustic coupling materials. While direct coupling type, including the self-coupling effect and Fabry-Perot etalon (FPE) optical fiber sensors, have overcome these shortcomings and thus have very good linear frequency response characteristics, large bandwidth and large dynamic range. Direct coupling optical fiber acoustic sensor technology has got rid of limitations from acoustic coupling materials by using direct coupling of light and sound field, especially FPE optical fiber sensing technology can make full use of light multi-reflection characteristic of the cavity to further enhance the detection sensitivity, which is a new technology with great development value.