In this paper, a method of detecting the central wavelength of ultra-weak chirped fiber Bragg grating is proposed, based on correlation coefficient to solve the problem of calculating the central wavelength, in the case of wideband reflection spectrum jitter. A mathematical model of the jitter spectrum of grating with piecewise step function superimposed by Gaussian noise is built. By moving the relative positions, the curve of their correlation coefficients is obtained, and the horizontal coordinate corresponding to the maximum correlation coefficient is found, to determine the central wavelength it. Then, the variation trajectories of the correlation coefficient between the odd function and the original signal at different relative positions by simulation are analyzed. Further, a variety of algorithms are used to compare and analyze the calculation results of the central wavelength of a single grating with spectral bandwidth greater than 3 nm and reflectance less than -30 dB. Finally, the ultra-weak chirped fiber Bragg grating array with 284 base cables is compared and analyzed, and the results show that the calculated range of the central wavelength is 0.17 nm. The variance of 0.05 proves that the proposed algorithm is simpler and more robust. This method has potential significance for calibration and engineering application of ultra-weak chirped fiber Bragg grating arrays.
In order to realize the real-time on-line monitoring of motor rotor temperature, a Fiber Bragg Grating (FBG) temperature sensing system using fiber optic rotary joint to transmit signals was built. The effects of the acceleration of the rotor and the mounting angle of the sensors chain on the central wavelength of the FBG were studied. The temperature measurement test of a rotating shaft was carried out. The following conclusions can be drawn. The central wavelength variation of the FBG is related to the linear acceleration of the rotor surface, the density and thickness of the grating coating. For the rotor whose surface linear acceleration is within 100m/s 2 , when the thickness of the coating is within 100μm, the effect of acceleration on the center wavelength of the polyimide-coated or gold-coated FBG can be ignored. For the motors with higher linear acceleration, in order to prevent the impact on the temperature measurement accuracy, the angle between the FBG sensors chain and the rotating shaft is preferably controlled within 20°. For the motors with lower linear acceleration, the installation angle can be compromised according to the rotor surface structure. The experiment shows that the FBG temperature measurement system can run stably and measure the temperature change accurately during the variable speed process of the rotor. Compared with other rotor temperature measurement systems, the FBG temperature measurement system has a simple composition, and the sensors chain is light and small. It is a very promising rotor temperature monitoring system.
The flank array of fiber laser hydrophone takes the fiber laser hydrophone as the sensitive element. A new type of fiber laser hydrophone with double diaphragms is designed, the results of anechoic tank test show that the average sound pressure sensitivity of fiber laser hydrophone is -136.9dB in the middle and high frequency range of 2.5 ~ 10kHz, and the fluctuation amplitude of sensitivity is not more than ±0.5dB. Taking the hydrophone as the sensing unit, a 64-element fiber laser hydrophone flank array is constructed based on wavelength division / time division multiplexing technology and passive homodyne interference detection technology. The lake test shows that the beam forming of the flank array is stable, the target bearing estimation is correct, and the root mean square error of horizontal target bearing estimation is 1.68°.
In order to design and fabricate high-performance fiber gratings and distributed feedback fiber lasers, the monitoring of AC and DC components of the core refractive index modulation during grating engraving was studied. Based on the Bragg equation and the coupled mode theory, the average variation and fringe visibility of the core refractive index modulation were calculated by monitoring the grating center wavelength and transmission spectrum depth. The grating engraving experiment of boron-germanium co-doped fiber, germanium-doped fiber, and erbium-doped photosensitive fiber was carried out and their refractive index modulation data was obtained. Experiment also showed that the larger pulse energy would make the average variation of core refractive index modulation increases and the fringe visibility decreases.
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.
A fiber laser hydrophone enhancing sensitivity through polyurethane end surface pulling is presented. After the relation between acoustic pressure sensitivity, dynamic performance and polyurethane material of the hydrophone are respectively described by theoretical model and ANSYS simulation, the polyurethane used for enhancing whose parameters are optimized is prepared. Prototypes of the hydrophone are fabricated and tested in vibration liquid column and Qiandao Lake experimental field. The average acoustic pressure sensitivity is -142.70dB(0dB=1rad/μPa)with the fluctuation less than ±2dB in the frequency range of 10~2000Hz. The result shows that DFB fiber laser encapsulated by polyurethane end surface ensures the high sensitivity, its fluctuation used in underwater sound detection is restrained at the same time, which is of great significance to promote the engineering application of fiber laser hydrophone.
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.
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.
Due to the limit of the channel isolation in the TDM system or the WDM system, the crosstalk usually exists between the fiber laser sensors in the multiplexing system. In this paper, the relationship between the channel crosstalk and the channel isolation is investigated. By means of studying on the phase generated carrier (PGC) demodulation and the passive homodyne demodulation based on the 3×3 coupler, which are usually used in the fiber laser sensor system, the crosstalk between two channels were discussed separately. The output mathematic models of the demodulation system were analyzed, when the sensing channel including the leak laser from the other channel. The mathematic models were set up used for theoretical analyses of the non-coherent crosstalk between two channels in the system, and the mathematic models show that the channel crosstalk between the two channels have relation to the power of the leak laser and have a slowly varying random factor which represents an environmentally induced variation. The acoustic sensing experiment shows that the models are suitable to calculate the crosstalk between two channels of the fiber laser sensor in the multiplexing system. In the acoustic sensing experiment, the channel crosstalk of the experiment is -18.7 dB, and the channel crosstalk should be -19.36 dB according to the mathematic models.
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.
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.
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.
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.
A method using fiber Fabry-Perot tunable filter to achieve FBG sensor's 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 sensor's absolute strain value
and achieves sub-microstrain resolution at 300Hz interrogating rate.
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