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.
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.
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.
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 signal's 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 signal's
symmetry and the output signal's amplitude-modulated problem does not appear obviously.
A method based on FBG sensor array to monitoring large structure's 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 sensor's direct strain value and achieves sub-microstrain resolution at 300Hz interrogating
rate. With fast interrogate rate, wide band acceleration signal can also be achieved.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.