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.
In this paper a novel optical fiber interferometer structure for ultra-large dynamic range detection is proposed. The structure combines conventional 3×3 interferometer with optical differential 3×3 interferometer. And the sensing fiber of the conventional interferometer is used as the transmission fiber of the differential interferometer while sensing. When the external signal acts on the sensing fiber, the conventional coherent detection and differential coherent detection can be carried out simultaneously. Conventional interferometer is used to detect the normal phase change of interferometric signals. However differential interferometer can detect the difference of the phase change, that is, the undistorted phase compression signal. Then the actual signal waveform can be obtained by integrating the compressed signal, so that the detection of large signal can be realized. The simulation analysis and experimental results show that the dynamic range of 200dB can be obtained within 20Hz-10kHz band. The structure of combined interferometer uses continuous light injection and has the advantages of simple structure and low cost. It can be used to detect wide-band and ultra-large signal and has good application prospects.
The stability features of the miniature fiber optic extrinsic Fabry-Perot interferometric (EFPI) sensors are important to signal recovery. In this paper, we explore the stabilization mechanism of the micromachined diaphragmbased EFPI sensors by means of the workpoint drift. Three sensors with the same sensing units but different encapsulation structures were fabricated to achieve comprehensive demonstration. Experiments were carried out separately in air and in water within a wavelength-tunable workpoint feedback control (WPFC) configuration. Results show that these three different sensors present distinguished workpoint drift rates. The main disturbance which cause the workpoint drift comes from the thermal exchange of the sensing diaphragm with the surroundings. Whereas a fast drift rate could disable WPFC, a totally enclosed encapsulation structure could help to improve the stability of the EFPI sensors.
The conventional electronic accelerometer meets electromagnetic compatibility problem in environments with strong electromagnetic filed. We herein design an all-optical accelerometer to solve this problem. A series of miniature plane spring-mass components were micromachined on silicon wafer by means of lithography and reactive ion etching. These components were served as sensitive structures. The fiber-optic extrinsic Fabry-Perot interferometer is adopted as the sensing structure. Two reflectors, one of which is cleaved fiber end while the other is sensitive structure with Au film, are used to constitute the F-P cavity. The proposed structure did not require high-precision alignment. Therefore, it is easily fabricated. The assembled sensor possesses small volume, which is 5 mm in radical and 12 mm in longitudinal. High-precision interferometric optical phase detection technique is used for signal recovery. The sensitivity of the fabricated sensor is about -11.2 dB re. rad/g with the resonance frequency at 2530 Hz. The equivalent noise acceleration is about 31.2 μg/√Hz. All these experimental results indicated a high-performance accelerometer. The fabricated accelerometer has potentials in large engine testing.
We designed and constructed a 400km interrogated fiber-optics hydrophone (FOH) array for the bottom mounted applications. The experimental results show that the phase noise level of the remote array is -97dB re 1rad/sqrt(Hz) @1kHz, the maximum time division multiplexing (TDM) crosstalk level is -54dB, the wavelength division multiplexing (WDM) crosstalk level is better than -73dB and the signal correlation between two sensors is better than 0.98. The results demonstrate that the performance of this remote array has reached to an acceptable high level, so this architecture would be of great importance in the practical applications.
Fiber Bragg grating written in polarization-maintaining fiber is strongly polarization-dependent, and its spectral character depends on the polarization state of light that interrogates the grating. We propose a novel and practical method to test the spectra character of the polarization-maintaining fiber Bragg grating (PM-FBG) interrogated by a linearly polarized light with a different azimuth for the first time. We specially designed a splice with a different angle in the system to alter the azimuth of the input linearly polarized light. Using this method, we got the transmission and reflection spectra of the PM-FBG measured at different splice angles and then the experimental results were analyzed originally. The analysis result shows that the relationship between the transmission dip and the splice angle obeys the Gaussian distribution. The reflectivity of the two wavelengths of the PM-FBG depends on the azimuth of the input linearly polarized light.
A distributed optical fiber vibration sensing system based on compensating interferometer is established and a distributed
optical fiber vibration waveform detecting technology based on a MZ compensating interferometer and compensating
interference of Rayleigh backscattering lights in adjacent areas is put forward. In laboratory experiment, the sensing fiber
is a 2500m SM fiber. By exerting 500Hz PZT vibration signals on the fiber at 40m, 430m, and 2500m, the locations of
the signals are obtained by phase demodulation. But there is a crosstalk at 40m because of multiple scatterings. The
spatial resolution is 40m and the SNR is 18dB.
Polarization fluctuation in polarization maintaining fiber (PMF) resonator is one of the major noise sources in resonant fiber optic gyroscope (R-FOG). 90-deg polarization-axis rotated splicing in R-FOG is an effective way to suppress the polarization-fluctuation induced noise. 90-deg polarization-axis rotated splicing error influences the noise suppression effect. Here, a polarization-coupling testing system based on white-light interferometry is designed to control 90-deg splicing error in double-coupler PMF resonator first time and a result of 0.37-deg splicing error is obtained for the first time. Then the resonant characteristics of the double-coupler PMF resonator are tested using the saw-tooth waveform scanning method. The finesse of this double-coupler PMF resonator is 24.0 and the phase interval of the two eigenstates of polarization (ESOPs) is π.
Two DFB semiconductor lasers are adopted as master and slave lasers to investigate the properties of the weak injection
locked DFB laser, such as the stable locking range, the phase noise and the power stability. A Brillouin/erbium fiber laser
pumped by the master DFB laser is injected into the slave DFB laser to validate the improvements of power stability and
spectral purity through the laser injection locking technology, which also demonstrates the feasibility of single frequency
extraction. The locked laser acts like a tunable narrow-band optical filter with central frequency and bandwidth decided
by the input signal. The experimental results give rise to some potential configurations useful for Brillouin distributed
fiber sensing and signal processing in microwave photonics.
A signal demodulation scheme based on heterodyne technique is demonstrated, which can help fiber-optic hydrophone
system meet the demand of large dynamic range and large scale multiplexing. Optical system based on the heterodyne
demodulation is given. Reference signal in the demodulation procedure is obtained from the optical heterodyne output,
which is quite different from other reported heterodyne techniques. This method not only simplifies RF electronics, but
also eliminates the effects of frequency shift vibration generated by the acoustic-optic modulators. Demodulation
algorithm is presented and the maximum signal processing capability is analyze. Experiments are carried out and the
results show that a signal with a frequency of 1kHz and amplitude of 40rad could be demodulated without distortion
when the heterodyne frequency is 64kHz. The same signal is demodulated using PGC scheme with a modulating
frequency equal to the heterodyne frequency, and the result shows distortion. Comparing the two demodulation methods
verifies that the system we have designed works well and is more suitable for the detection of large signal. Besides, this
architecture of system offering advantage of easy to be time-division multiplexed. Together with wavelength-division
multiplexing, it demonstrates the potential for the fiber-optic hydrophone to achieve large-scale arrays with high dynamic
A kind of fiber optical temperature variety sensor based on fiber optical ring resonator is presented. The temperature variety causes the variety of the round-trip phase of the ring and the movement of the resonance dips. The magnitude and rate of the movement are proportional to the magnitude and rate of temperature variety, respectively. Through the measurement of the dips movement, the temperature variety can be sensed. In this paper, we make a theoretic analysis about the sensor, propose the system configuration, present a simple and effective signal detection method, and analyze the performance of the sensor. This kind of temperature sensor has such characteristics as high sensitivity, simple detection method, flexibly changeable sensitivity for different applications and temperature variety detection. Combined with the multiplex technology such as time division multiplexing (TDM), the sensor can form temperature variety sensor array for distributed fiber optic temperature detection.