Hongcan Gu

A 4-element fiber laser hydrophone towed linear array

The hydrophone which use DFB fiber laser as sensing element has many advantages such as thin diameter, high sensitivity, easy to multiplex array, wide dynamic range, immunity to electromagnetic interference and no need of impedance matching. A 4-element fiber laser hydrophone towed linear array is studied. A sleeve with grooves type fiber laser hydrophone is designed and realized. The sensitivity experiment of the hydrophone element shows that the sensitivity is about -135dB（re 1rad/μPa）in the range of acoustic frequency from 20Hz to 1000Hz, and the amplitude fluctuate of the frequency response curve is less than±1.5dB. Based on the hydrophone of this structure and the WDM technology, the 4-element fiber laser hydrophone towed linear array that is encapsulated with polyurethane sleeve is established, and its diameter is 15mm. The experiments of the hydrophone towed linear array in the lake test show that the localization and tracking of the target can be realized by the towed linear array.

Phase generated carrier technique for fiber laser hydrophone

A distributed feedback (DFB) fiber laser is compact, and is very suitable for using as a hydrophone to sense acoustic pressure. A DFB fiber laser hydrophone was researched. In the fiber laser hydrophone signal demodulating system, an unbalanced Michelson fiber interferometer and a Phase Generated Carrier (PGC) method were used. The PGC method can be used to demodulating the acoustic signal from the interference signal. Comparing with the Naval Research Laboratory (NRL) method and Naval Postgraduate School (NPS) method, the digitized PGC method requires a greater amount of computation because of the high signal sampling, but it demands only one interference signal which makes the less fiber connections of the fiber laser hydrophone array. So the fiber laser hydrophone array based on the PGC method has lower complexity and higher reliability than that based on the NRL method or NPS method. The experimental results approve that the PGC method can demodulate acoustic signal between 20~2000 Hz frequency range with good signal-to-noise ratio (SNR) when the PZT driving frequency is 20 kHz.

Improved symmetric signal demodulating technique for DFB laser hydrophone

The disadvantage of the symmetric signal demodulating method for DFB laser hydrophone is showed. Because of the different transmission coefficients of the three signal channels, the symmetric condition is dissatisfied. A model with three unsymmetrical interferometric signals is simulated. The simulated result shows that the output signals amplitude is modulated. In order to correct the unsymmetrical coefficients, PGC (phase generated carrier) technique is introduced to the signal demodulating scheme. The experimental result shows that the PGC technique can improve the signals symmetry and the output signals amplitude-modulated problem does not appear obviously.

64-element fiber laser sensing system with interferometric interrogation

We have developed a 64-element fiber laser sensing system based on 8 wavelength and 8 space division multiplexing. The array consists of four 980nm pump source which illuminate eight group distributed feedback fiber lasers(DFB FL) and eight optical switches which addressing in turns. The wavelength shifts of the DFB FL are converted to phase changes by routing the reflected signal through a nearly path balanced fiber Michelson Interferometer(MI). Through introducing an improved symmetric phase recovering method based on a 3×3 coupler, the influence of light intensity and splitting ratio is decreased. A 64 channels real-time synchronous demodulation system has been realized based on FPGA.

Crosstalk analyse of DFB fiber laser hydrophone array based on time division multiplexing

In this paper, the crosstalk of a time division multiplexed (TDM) system of distributed feedback (DFB) fiber laser (FL)hydrophones based on optical switch using Phase Generated Carrier (PGC) method was quantitatively analyzed. After mathematical deduction, the relationship among crosstalk, multiplexing scale and extinction ratio of optical switch was given. The simulation results show that to realize a TDM system of DFB fiber laser hydrophones with crosstalk lower than -40dB, the average extinction ratio should be higher than 24.78dB for a 4- channel system, while higher than 28.45dB for an 8- channel system. Two experiments to analyze the array crosstalk to a certain channel in an 8- channel array were conducted in this paper. Firstly, by testing the powers of leak laser to a certain channel from others, the array crosstalk to this channel was obtained according to the equation mathematically deduced in this paper. The result shows the array crosstalk to a certain channel of the 8-channel array was -7.6dB. An experiment of underwater acoustic detection was carried out finally to get the real array crosstalk to this certain channel, and the experimental result shows that the array crosstalk to this channel is -8.8dB, which is close to the calculated result.

Calibration test of the temperature and strain sensitivity coefficient in regional reference grating method

In order to verify the validity of the regional reference grating method in solve the strain/temperature cross sensitive problem in the actual ship structural health monitoring system, and to meet the requirements of engineering, for the sensitivity coefficients of regional reference grating method, national standard measurement equipment is used to calibrate the temperature sensitivity coefficient of selected FBG temperature sensor and strain sensitivity coefficient of FBG strain sensor in this modal. And the thermal expansion sensitivity coefficient of the steel for ships is calibrated with water bath method. The calibration results show that the temperature sensitivity coefficient of FBG temperature sensor is 28.16pm/°C within -10~30°C, and its linearity is greater than 0.999, the strain sensitivity coefficient of FBG strain sensor is 1.32pm/με within -2900~2900με whose linearity is almost to 1, the thermal expansion sensitivity coefficient of the steel for ships is 23.438pm/°C within 30~90°C, and its linearity is greater than 0.998. Finally, the calibration parameters are used in the actual ship structure health monitoring system for temperature compensation. The results show that the effect of temperature compensation is good, and the calibration parameters meet the engineering requirements, which provide an important reference for fiber Bragg grating sensor is widely used in engineering.

Homodyne coherent demodulation method for fiber laser hydrophone using a 3×3 coupler with gains adjusted real-time for symmetry

To develop a practical fiber laser hydrophone, demodulation method to get the wavelength shift of the fiber laser hydrophone is a key technology. A novel scheme for demodulating the phase shift is proposed to eliminate the affect of the three unsymmetrical outputs from 3×3 coupler. A special polarization-insensitive unbalanced Michelson fiber interferometer based on the 3×3 coupler is employed to transpose the wavelength shift into phase shift of the interferometer. The interferometer just has two outputs. A certain length of the fiber in an arm of the interferometer is wrapped onto a PZT tube to get a high frequency phase modulation signal whose amplitude is bigger than π rad, so the direct and alternate item of the two outputs from the interferometer can be gotten. The two outputs from the interferometer are detected by photoelectrical detectors, and the digitized acquisition data from the detectors are processed by Labview program to demodulate the phase shift. The gains of the two outputs are adjusted real-time to get two symmetric signals after their direct items were subtracted, and the two symmetric signals are processed through differentiator and cross-multiplier to demodulate the acoustic pressure signal. The sensitivity experiment shows that the new demodulation scheme can get a steady output and the phase output is linear to the pressure applied on the fiber laser sensor when the outputs from the interferometer are unsymmetrical. The amplitude fluctuate of the phase output is less than 5% when the fiber laser hydrophone is under stable acoustic pressure at a frequency point.

Analysis on the amplitude instability of homodyne coherent demodulation method based on a 3×3 coupler for fiber laser hydrophone

To develop a practical fiber laser hydrophone, demodulation method to get the wavelength shift of the fiber laser hydrophone is a key technology. A homodyne coherent demodulation scheme utilizing a 3×3 coupler has the advantage of passive detection without phase or frequency modulation in the reference arm and wide dynamic range. But the amplitude of phase output from the passive homodyne coherent demodulation system based on a 3×3 coupler for fiber laser hydrophone is instable during a lake test. To solve this problem, the principle of the passive homodyne phase demodulation scheme based on a 3x3 coupler for the fiber laser hydrophone is investigated and a digital demodulation system is built. A unbalanced Mach-Zehnder fiber interferometer based on the 3×3 coupler is employed to transpose the wavelength shift into phase shift of the interferometer. Through theoretical analysis and simulation calculation, the relations between amplitude discrepancies and the phase differences with the phase output are finally educed. The theoretical analysis and simulation calculation shows that the amplitude discrepancies and the phase differences of the three outputs from the interferometer result in the amplitude instability of phase output. To solve these problems, we can get three symmetrical outputs by adjusting the gains of the three outputs. Both the simulation calculate and the underwater acoustic experiment show that the phase output of homodyne coherent demodulation system based on a 3x3 coupler can be steady by adjusting the gains of the three outputs, when the 3×3 coupler is asymmetric.

Using FBG sensor array for monitoring structural security

A method based on FBG sensor array to monitoring large structures security state is described. A configuration combing a fiber Fabry-Perot tunable filter and a fiber Fabry-Perot interferometer is used to interrogate FBG sensors. The strain signal and acceleration signal can be obtained from the some one FBG sensor, and the number of DFB sensors is reduced to half. The fiber Fabry-Perot tunable filter-based system has the capability to interrogate large number of DFB sensors, whilst obtaining absolute Bragg wavelength with pm resolution. A demonstration system with 4 DFB sensors is presented, which obtains DFB sensors direct strain value and achieves sub-microstrain resolution at 300Hz interrogating rate. With fast interrogate rate, wide band acceleration signal can also be achieved.

Measuring FBG's Bragg wavelength using fiber Fabry-Perot tunable filter

A method using fiber Fabry-Perot tunable filter to achieve FBG sensors Bragg wavelength is described. The system has the capability to interrogate large number of DFB sensors, whilst obtaining absolute Bragg wavelength with pm resolution. A demonstration system with 4 strain sensors is presented, which obtains DFB sensors absolute strain value and achieves sub-microstrain resolution at 300Hz interrogating rate.