The fiber-optic hydrophone, with the characteristics of high sensitivity, easy multiplexing and long transmission distance, is becoming a new direction for the development of the underwater detection system. The overflow cavity type optical fiber hydrophone is composed of a sensitizing inner shell, a rigid outer housing, sensing optical fiber and other auxiliary structures. It is a small-size and high-sensitivity hydrophone in the form of the air-backed sensitized structure, which could be applied to new towed array and fixed array system. In this paper, the acoustic performance of the hydrophones in different sound fields was simulated using the COMSOL finite element analysis. In the light of the design of the overflow cavity type optical fiber hydrophone, the acoustic field distribution of the hydrophone surface and the strain distribution of the optical sensing fiber were analyzed and compared with the free sound field model and the standing wave field model. And the sensitivity response of the hydrophone was predicted. On this basis, the sample of the overflow cavity type optical fiber hydrophone with the same design parameters was developed, and the sensitivity test was carried out in the standing wave tube. The experimental results show that the test frequency is 315 Hz to 2 kHz, the phase shift sensitivity of the hydrophones is -143 dB ref.1 rad/μPa. The difference between the test results and the simulation results is less than 1.5 dB under 1kHz.
The photodetection intensity noise to demodulated phase noise conversion process of fiber optic sensors using phase generated carrier (PGC) scheme is investigated through theoretical calculation and experimental verification. Several categories of intensity noise are calculated, according to their relation between intensity noise power spectral density and photodetection current. The results revealed that demodulated phase noise power level increases by several decibels over the relative intensity noise power level of detected light power signal. Phase noise power level could also fluctuate with demodulated phase signal. Increase of phase noise power level varies according to the noise type, as well as the fluctuation amplitude. For noises that intensity noise power level unrelated to detected light power, such as electronic noise of detector circuit, phase noise power level increases by about 3.7 dB and do not fluctuate with demodulated phase signal. For noises that intensity noise power level proportional to detected light power, such as signal-amplified spontaneous emission(ASE) beat noise of optical amplifiers, phase noise power level fluctuates with demodulated phase signal by about 5.7 decibels and averagely increases about 3.7 decibels over a 2π period of demodulated phase signal. For noises that intensity noise power level proportional to square of detected light power, such as light source relative intensity noise, phase noise power level fluctuates with demodulated phase signal by about 9.0 decibels and averagely increases about 4.8 decibels over a 2π period of demodulated phase signal. Verification experiments are demonstrated on electronic noise, ASE-signal beat noise and light source relative intensity noise separately.
A fiber-optic hydrogen sensor based on tunable diode laser spectroscopy is designed. The fiber-optic hydrogen sensor is composed of a fiber-optic Fabry-Perot interferometer, where one mirror is made of a hydrogen sensitive Pd thin film. By optimizing the structure parameters of the fiber-optic hydrogen sensor, a reflectance spectrum with the similar curve to a gas absorption spectrum is obtained. The reflectance spectrum of the fiber-optic hydrogen sensor is calculated at different hydrogen concentrations and the relation between the intensity minimums of the spectrum and the hydrogen concentrations is explored. The wavelength modulation spectroscopy is used to analyze the reflectance spectra, the relation between the second harmonic component and hydrogen concentrations is obtained, and the feasibility of the application of TDLAS on the fiber-optic hydrogen sensor is verified. This hydrogen sensing method has great potential in industrial application due to the advantages of high sensitivity, low cost, system’s simplicity.
In this paper we propose a low-cost and stable configuration of Brillouin Optical Time Domain Analysis (BOTDA). Both pump and probe are generated by one single laser source for steady frequency beating. Polarization-maintaining modulators and amplifiers have been applied into the system in order to suppress the ground noise and to control the stability of the pump pulse. The probe is filtered and amplified to obtain the Stokes wave. The bias voltages of modulators are carefully controlled. We implement the prototype of interrogator by using this method and compare it with commercial products. The result shows that the long-term stability of the prototype is three times higher than that of commercial product.
In this paper, Brillouin Optical Time Domain Analysis (BOTDA) is proposed to solve the problem that the traditional point sensor is difficult to realize the comprehensive safety monitoring of bridges and so on. This technology not only breaks through the bottleneck of traditional monitoring point sensor, realize the distributed measurement of temperature and strain on a transmission path; can also be used for bridge and other structures of the damage identification, fracture positioning, settlement monitoring. The effectiveness and frontier of the technology are proved by comparing the test of the indoor model beam and the external field bridge, and the significance of the distributed optical fiber sensing technology to the monitoring of the important structure of the bridge is fully explained.