Ultra-low-frequency fiber optic hydrophone array based on push-pull unbalanced Michelson interferometers.

A four-element fiber optic hydrophone array Based on 3×3 coupler terminated Mach-Zehnder interferometer is developed. The 3×3 coupler terminated Mach-Zehnder interferometer is constructed to sense the underwater acoustic signal. Spatial multiplexing of fiber optic hydrophone array is achieved by separating the laser into four channels utilizing combined couplers. For each hydrophone element, a demodulator is used to recover the acoustic signal. This configuration is easily implemented and can detect weak signal in a high noisy water environment. A 4-element, spatially multiplexed system is constructed with a noise equivalent sound pressure of 1.5×10^-4Pa/Hz at 1kHz and the frequency range of 80Hz to 1.5 KHz. It has an almost flat response with an average sensitivity of -133dB,and dynamic range of 110dB .

Signal detection stability is very important for vector fiber-optic hydrophone and hydrophone array, because the instability of the demodulated signal directly leads to the target azimuth estimation error and the degradation of system performance. In this paper, a method to achieve high-stability signal demodulation for interferometric vector fiber-optic hydrophone is studied. A parameter estimation and demodulation parameter compensation method for phase generated carrier demodulation system is proposed based on elliptic curve parameter fitting algorithm. An elliptic curve is constructed using the second and third frequency of the reference interference signal. The ellipse curve fitting algorithm is introduced to estimate the distortion parameter of the modulation and demodulation system by fitting the value of each elliptic curve parameter. By compensating the PGC demodulation for the tested signal with the estimated parameters, the instability of demodulation system caused by PGC modulation depth variation and additional modulation intensity of the light source can be effectively reduced. The feasibility of the method is verified by simulation experiments and actual system experiments. High stability signal detection is realized using the proposed method, which can effectively improves the detection effect of the vector fiber-optic hydrophone array.

The alignment coupling between optical waveguide chips and optical fiber arrays is the basis of the alignment coupling of planar optical waveguide devices, and the precise position detection with angle and spacing adjustments is one of the key steps of alignment coupling. A methodology for position detection, and angle and spacing adjustment was proposed for optical waveguide chips and optical fiber arrays based on machine vision. The experimental results show angle detection precision levels higher than 0.05°, line detection precision levels higher than 0.1 μm, and detection time less than 2 s. Therefore, the system developed herein meets the precise requirements necessary for position detection, and angle and spacing adjustments for optical waveguide chips and optical fiber arrays.

In this paper we propose a novel, improved, phase generated carrier (PGC) demodulation algorithm based on the PGC-differential-cross-multiplying approach (PGC-DCM). The influence of phase modulation amplitude variation and light intensity disturbance (LID) on traditional PGC demodulation algorithms is analyzed theoretically and experimentally. An experimental system for remote no-contact microvibration measurement is set up to confirm the stability of the improved PGC algorithm with LID. In the experiment, when the LID with a frequency of 50 Hz and the depth of 0.3 is applied, the signal-to-noise and distortion ratio (SINAD) of the improved PGC algorithm is 19 dB, higher than the SINAD of the PGC-DCM algorithm, which is 8.7 dB.

The phase generated carrier (PGC) demodulation technique for phase-sensitive optical time-domain reflectometry (Ф-OTDR) has significant potential in applications involving distributed optical fiber sensing and imaging. An improved PGC demodulation algorithm for a Ф-OTDR combined with an unbalanced Michelson interferometer system is presented in this letter. This modified PGC technique uses the one output signal as a fundamental carrier frequency signal with mathematical transformations, and another output is the modulated signal, which enable the two signals to be output synchronously from the interferometer. Theoretical simulations and experiments demonstrate that the linear and nonlinear distortion caused by phase modulation depth and carrier phase delay are well suppressed by the proposed algorithm with high stability and low distortion. The signal-to-noise-and-distortion ratio of sensors using the proposed method in the same conditions achieve gains of 12.98 dB, 30.82 dB and 6.93 dB over PGC-Arctan, PGC-differential-and-cross-multiplying and PGC-single-path differential division method, respectively. Experimental verification shows that the proposed demodulation system can exactly recover the original signals of multiple vibration positions simultaneously, and the system employing the ameliorated PGC demodulation algorithm has the noise level of about 1.83 × 10−4 rad (√Hz)−1 and a large dynamic range of 65.72 dB @ 800 Hz, which provides a new choice for distributed optical fiber sensing technology.

This paper reviews recent work on the use of optical fiber arrays to enhance laser generation of ultrasound. Experimental and numerical directivity patterns are presented, obtained using optical fiber arrays to generate longitudinal, shear, and surface waves. Comparisons of the directivity patterns for a single light source and for the fiber array will be presented and discussed for each of the above waves. Also shown will be some experimental results on array gains. This discussion will be limited to sound generation in the thermoelastic (linear) range. The optical fiber array can be used to control both the directivity and the type of elastic wave generated by the laser light. The noncontact fiber array generation can be combined with a noncontact EMAT or laser Doppler receiver to achieve a system for ultrasonic on-line inspection and control of manufacturing processes. The above technique for sound generation and reception is particularly useful in hostile and hard to reach environments. [Work supported by the National Science Foundation.]

An amplified 16 channel dense wavelength division multiplexing (DWDM) array architecture is presented for interferometric fibre optic sensor array systems. This architecture employs a distributed Erbium doped fibre amplifier (EDFA) scheme to decrease the array insertion loss, and employs time division multiplexing (TDM) at each wavelength to increase the number of sensors that can be supported. The first experimental demonstration of this system is reported including results which show the potential for multiplexing and interrogating up to 4096 sensors using a single telemetry fibre pair with good system performance.

The deep-sea riser is an important equipment in marine engineering. It is responsible for transporting minerals from subsea oil wells to offshore platforms. The complex environment of the marine is easy to damage the riser. Aiming at the uncertainty of the deep-sea riser shape in the harsh service environment and the large deformation and vulnerability characteristics of the riser during the laying process, a method of riser shape reconstruction based on deep neural network was proposed. In this paper, the form of a piggyback pipeline is used to monitor the shape of the riser. An optical fiber array is laid inside the sub-tube, and the nonlinear logical relationship between the strain data measured by the optical fiber array and the deformation data of the main-tube is explored through a deep neural network model. Finally, the shape of the riser is obtained through the curve reconstruction algorithm. The results show that the predicted shape of riser obtained by this method is basically consistent with the preset shape, which can be used as a new method for long-term health monitoring of deep-sea riser.

Phase-generated carrier (PGC) demodulation algorithm is the main demodulation methods in Fiber-optic interferometric sensors (FOISs).The conventional PGC demodulation algorithms are influenced by the carrier phase delay between the interference signal and the carrier signal.In this paper, an automatic carrier phase delay synchronization (CPDS) algorithm based on orthogonal phase-locked technique is proposed.The proposed algorithm can calculate the carrier phase delay value.Then the carrier phase delay can be compensated by adjusting the initial phase of the fundamental carrier and the second-harmonic carrier.The simulation results demonstrate the influence of the carrier phase delay on the demodulation performance.PGC-Arctan demodulation system based on CPDS algorithm is implemented on SoC.The experimental results show that the proposed algorithm is able to obtain and eliminate the carrier phase delay.In comparison to the conventional demodulation algorithm, the signal-to-noise and distortion ratio (SINAD) of the proposed algorithm increases 55.99dB.

The inline time division multiplexing (TDM) fiber Fabry–Pérot (FFP) sensor array based on fiber Bragg gratings (FBGs) is attractive for many applications. But the intrinsic multi-reflection (MR) induced crosstalk limits applications especially those needing high resolution. In this paper we proposed an expandable method for MR-induced crosstalk reduction. The method is based on complexing-exponent synthesis using the phase-generated carrier (PGC) scheme and the special common character of the impulse responses. The method could promote demodulation stability simultaneously with the reduction of MR-induced crosstalk. A polarization-maintaining 3-TDM experimental system with an FBG reflectivity of about 5 was set up to validate the method. The experimental results showed that crosstalk reduction of 13 dB and 15 dB was achieved for sensor 2 and sensor 3 respectively when a signal was applied to the first sensor and crosstalk reduction of 8 dB was achieved for sensor 3 when a signal was applied to sensor 2. The demodulation stability of the applied signal was promoted as well. The standard deviations of the amplitude distributions of the demodulated signals were reduced from 0.0046 to 0.0021 for sensor 2 and from 0.0114 to 0.0044 for sensor 3. Because of the convenience of the linear operation of the complexing-exponent and according to the common character of the impulse response we found, the method can be effectively extended to the array with more TDM channels if the impulse response of the inline FFP sensor array with more TDM channels is derived. It offers potential to develop a low-crosstalk inline FFP sensor array using the PGC interrogation technique with relatively high reflectivity FBGs which can guarantee enough light power received by the photo-detector.

In this paper, we demonstrate an improved dynamic demodulation system for acoustic sensors based on distributed feedback-fiber laser (DFB-FL). The signal is demodulated using the phase-generated carrier (PGC) demodulation technology based on unbalanced Michelson interferometer (MI). An improved PGC demodulation algorithm is introduced, which solves the problems of light intensity disturbance (LID) and phase modulation depth drift in the traditional PGC demodulation algorithm that affect the demodulation results. The acoustic pressure sensitivity test of a packaged DFB-FL acoustic sensor is carried out to evaluate the performances of the demodulation system, a flat acoustic pressure sensitivity and frequency response curve was achieved in the frequency range of 200 Hz~18 kHz, the wavelength-acoustic sensitivity was about -40.22 dB re. pm/Pa. The amplitude changes of demodulation signal with light intensity and modulation depth are also tested, and it shows that the stability of the demodulation system has been significantly improved.

Fiber optic inetrferometric sensors have advantages of high sensitivity and can be employed to measure very small disturbance. In this paper, a novel demodulation method of fiber optic interferometer sensor is presented, which is based on the 3 × 3 coupler to demodulate the phase shift. By employing two output signals of the 3 × 3 coupler, it can eliminate the insensitive areas. The optical structure and the demodulation algorithm are easy to deploy, compared with the PGC (phase generated carrier) technique and other 3 × 3 coupler based demodulators. This algorithm can effectively overcome the phase distortion caused by the instability of the light output power. The simulation and experiment demonstrate that the high demodulate precision has been achieved.

Phase generated carrier (PGC) demodulation technology has been widely used in fiber-optic interferometric sensors while nonlinearity is always accompanying with the scheme. We propose a highly stable PGC demodulation algorithm combining internal modulation, ellipse fitting algorithm (EFA), phase modulation depth ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> ) self-calibration and differential-cross-multiplying (DCM). A low frequency triangular signal is added to the laser modulation, which ensures the effectiveness of the EFA under small signal condition. And the real-time <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> value is calculated by using the ellipse fitting parameters and calibrated to the optimal value of 2.63 rad by a closed loop proportion integration differentiation (PID) module. Furthermore, nonlinear distortions are suppressed by the EFA. Experimental results show that the signal-to-noise and distortion ratio (SINAD) and total harmonic distortion (THD) of the proposed scheme are improved by 13.61 dB and 0.12% than the conventional PGC-DCM algorithm. The standard deviation of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> decreased from 0.0178 rad to 0.0058 rad in 1024 seconds. The dynamic range, linearity and phase resolution of the system reach 117.73 dB @ 1 kHz, 99.99% and 4.6 μrad/√Hz, respectively.

This paper describes the experimental application of a phased optical fiber array to transmit pulsed laser light to the free surface of Carborundum CS-3 ceramic samples for ultrasonic nondestructive evaluation (NDE). The pulsed laser beams heat the surface of the ceramic rapidly, and can thermoelastically generate longitudinal, shear, and surface (Rayleigh) waves. Without the optical fiber array all three wave modes are generated simultaneously, and no control over directivity or beam-width is possible. The experimental results presented here indicate that the wave modes, beam-width, and directivity of the generated ultrasound can be controlled by using the phased optical fiber array. Experimental directivity patterns for the laser phased array are compared to a simple theoretical model based on Rose’s point source theory. The model predicts shear wave directivities better than either longitudinal or surface waves. This work demonstrates some of the capabilities and limitations of using laser-phased arrays on ceramics.

We introduce a partially adaptive subarray focused beamformer for measuring the distribution image of ship's noise sources in a reduced-dimensional space. When only a small number of snapshots is available for estimating the spatial correlation matrix, reduced-rank solutions provide high performance to common methods. We study the detection performance of the partially adaptive subarray detector compared with the full array detector and a reduced-rank method called Krylov subspace. It is demonstrated, via simulation, that the minimum-variance, partially adaptive subarrays beamformer is a solid approach in small snapshots, low signal-to-noise (SNR) situations. In addition, data of a fiber optic hydrophone array experiment has been used to validate the method.