The demodulation parameter optimization of fiber optic acoustic sensor for broadband large signals is studied in this paper. Firstly, the performance of fiber optic acoustic sensors in demodulating broadband large signals is introduced. The low-frequency energy of the demodulation result is significantly increased and completely submerge the effective signal. By deducing and analyzing the demodulation algorithm of fiber optic acoustic sensors, the reason why it is impossible to demodulate broadband large signals is theoretically explained. An arctangent demodulation algorithm is realized in MATLAB. White noise signals with limited frequency bands are used as simulation signal sources for demodulation testing. Through simulation analysis, the main criteria for determining the upper limit of the system's dynamic range under a certain set of demodulation parameters are provided. Finally, the arctangent demodulation algorithm is implemented in FPGA. Detection experiments on the noise signal of a certain equipment are carried out using the fiber optic microphone system developed by our institute. The experimental results prove that the dynamic range analysis of the demodulation algorithm in this paper is accurate. The fiber optic microphone system designed through parameter optimization can correctly demodulate broadband large signals.
The impact of attitude deformation of towed linear array and the fitting error of point attitude sensors on beamforming. We studied All-fiber acoustic-posture integrated towed linear array based on distributed feedback fiber laser (DFB-FL) hydrophone and optical frequency domain reflectometry (OFDR) distributed attitude measurement. Analyzed the principle of towed array direction finding, and the influence of towed array attitude deformation on directional error was analyzed. Designed an 8-element hydrophone towed linear array structure with a diameter of 16mm, effectively combining fiber laser hydrophones with attitude sensing multi-core optical fiber. Finally, the effectiveness of attitude correction was verified through static direction finding experiments in anechoic tank, and the accuracy of direction finding was better than 2° under the extreme "S-bend" shape conditions of the drag array attitude. It can effectively improve the accuracy of direction finding.
In order to reduce the phase noise introduced by the unbalanced interferometer in the distributed feedback (DFB) fiber microphone modulation system, a phase noise compensation method based on a stable laser source as an optical reference is proposed. The narrow linewidth laser source is incident on the unbalanced Michelson interferometer shared by the microphone, so that it has the same optical path difference. This optical phase noise information is used to offset the noise introduced by the interferometer, so as to achieve the purpose of noise suppression. The principle of the unbalanced interferometer composition and suppression are theoretically analyzed and experimentally verified. The experimental results show that under the quiet conditions of the laboratory, compared with the structure that has not been suppressed, the noise of the microphone primitive is reduced from about ± 0.2rad to within ± 0.04rad, the power spectral density is reduced to less than -58.95dB/Hz1/2. It can be suppressed by more than 32dB at the noise peak, and the suppression effect of the phase noise is very obvious.
A zero drift suppression method of arctangent demodulation algorithm for fiber optic acoustic sensors is studied in this paper. Firstly, the principle of the arctangent demodulation algorithm is introduced, with emphasis on the principle and implementation method of the phase unwrapping. Then, the design and implementation idea of the zero drift suppression method are described in detail. A PID method is used to get the compensation value, which will be fed back to the next phase unwrapping stage, so as to gradually eliminate the zero drift. By adding a decimation unit and combining the low-pass filter unit and the PID unit, the complexity of the algorithm and the design difficulty of the filter are greatly reduced. The designed zero drift suppression method is implemented in FPGA. An actual fiber optic microphone is demodulated under laboratory conditions. The experiment results prove that the zero drift suppression method designed in this paper can stabilize the static operating point of the fiber optic microphone effectively. As the zero drift problem is solved in algorithm, there are no any hardware modification and additional cost.
Hydrogen is an important source used as an energy carrier and a chemical reactant or industrial material. However, if not handled properly, hydrogen content as low as 4% can lead to a life-threatening catastrophe. Hydrogen sensors with higher sensitivity, better selectivity, faster response, and wider dynamic range are of increasing importance in connection with the development. In this paper, a scheme of hydrogen sensor that satisfies these requirements with a single sensing element is proposed which is centered on a nanofiber. The sensor is based on stimulated Raman scattering spectroscopy but the tightly confined evanescent field associated with the nanofiber enhances the Raman gain per unitlength by a factor of more than 104 over free-space beams. In addition, the homemade signal processing circuit plays an important role in the whole sensing system instead of the commercial instruments, which makes it possible to develop a principle prototype. The circuit intergrates tne DFB laser driver circuit, the photoelectric detection circuit and the main control circuit which outputs modulation signal and acts as a digital lock-in amplifier. Several silica nanofibers operating in the telecom wavelength band has been manufactured and measured in an experiment that demonstrates hydrogen detection from hundreds parts per million to 100%. The reported sensor could be used in the field of new energy, electric power and aerospace for detection of hydrogen leakage or monitoring of transformer health conditions with advantages of low cost, small size and outstanding performance.
Natural gas and biogas, as commonly used combustible gases, are widely used in urban residents and industrial enterprises. Gas leakage accidents occur frequently. In order to more accurately distinguish whether it is a natural gas leak or a biogas leak, this paper proposes a method for simultaneous detection of methane and ethane based on TDLAS, and develops a set of methane and ethane dual gas detection devices. Firstly, this paper studied the gas absorption lines of methane and ethane. According to the absorption lines, a 1680nm DFB laser beam was used to scan methane and ethane at the same time. Secondly, the phase-locked amplifier module of TDLAS technology is used to obtain the second harmonic signals of methane and ethane concentration detection at the same time, and identify the components of methane and ethane based on the peak position information and FWHM information of the second harmonic signal. Finally, the concentration calculation function is obtained by fitting the peak and valley difference of the second harmonic. Experimental results show that the detection method and device proposed in this paper can achieve simultaneous detection of methane and ethane
The characteristic absorption lines of carbon monoxide gas cross with that of other gases in near-infrared and midinfrared bands, the detection results are easily disturbed. Especially in some environments such as coal mines and petrochemicals, it is more important to avoid cross interference, in order to achieve accurate measurement of carbon monoxide. In this paper, a DFB laser beam at 2330nm was selected and scanned for absorption line of carbon monoxide. Although interference of other gas components was avoided, the overlap of carbon monoxide and methane spectra still existed. Firstly, the absorption lines of methane and carbon monoxide were studied. According to the theory of molecular spectroscopy, the FWHM of methane and carbon monoxide absorption lines are different. Once, there is methane gas in the background, the harmonic signals will be different due to different line shape. The second harmonic signals with different modulation coefficients are simulated. So the troughs width is selected as the evaluation function of the harmonic signal characteristics for component judgement. Secondly, after component determination, when mixed gas is present, adjacent methane absorption peaks are also scanned for deduction. The influence of temperature is also calculated and the evaluation function is modified again. Finally, the free calibration measurement of carbon monoxide concentration in the mixture is realized. In summary, for the working conditions of complex background gas such as coal mine and petrochemical. This paper presents a calibration-free method for reliable carbon monoxide concentration detection based on a TDLAS technique.
An algorithm of Rayleigh noise compensation in dual-end configured distributed temperature sensor (DE-DTS) is proposed in this paper. A 2 km long multi-mode fiber is used to calculate the attenuation of the light within Anti-Stokes and Stokes bandwidth, and figure out the isolation of the wavelength division multiplex through both Anti-Stokes channel and Stokes channel. Experiments are taken out to validate the proposed Rayleigh compensation algorithm. As a result, the Rayleigh noise in both Anti-Stokes component and Stokes component is compensated, and the temperature error of the dual-end configured distributed temperature sensor is revised. With the help of the proposed algorithm, a dual-end DTS can reach to the absolute accuracy of 1.09°C (RMSE) between 180°C to 300°C, which significantly monishes the temperature error compared to the sensor without Rayleigh noise compensation.
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