Optical fiber pressure gradient sensors possess the merits of no suspension and easy installation compared to the co-vibration vector sensors. Recent studies are focused on the discrete structure made by individual pressure sensing units. Because the sensor is extremely sensitive to the difference between the units, improvements should be made in many areas, such as the manufacturing process, calibration method, and error correction. Benefited by the mature manufacturing process, a vector sensor with directivity larger than 40 dB is obtained. Considering that the measurement uncertainty is up to 0.7 dB in standing wave tube, an optimized calibration method is developed, which can eliminate the relative measurement error. Besides, an enhanced demodulation method is carried out to decrease the fluctuation of phase shift to within ±0.5 dB. 2-D and 3-D sensors are fabricated and tested on the lake and on the sea. Results show that the sensors have achieved good acoustic measurements.
A self-calibration method for camera using two views of unknown-structure planar scene is introduced. The planar scene is common in the environment and can be easily identifiable outside the lab. Firstly, two orientation- and scale-covariant features, which can be provided by the SIFT feature detector, is used to estimate the homography of two views. Then the homography is decomposed into the camera parameters. A RANSAC scheme is adapted to cope with the outliers of SIFT correspondences. Finally, the camera parameters are optimized with a non-linear parameter optimization using the inliers of two views. This method calibrates the camera parameters and recovers the planar scenes simultaneously. Real scene data experiment demonstrates that the proposed method is easy to operate and provides the reliable calibration results for non-expert users.
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.
In this paper, nonlinear noises that characterize the performance of a long-haul optical fiber sensing system were investigated. In a 50 km transmission system, when stimulated Brillouin scattering (SBS) occurs seriously, the phase noise of the interferometer increases from -102dB (0dB=1rad/sqrt(Hz)) to -84dB due to the enlargement of the laser linewidth and the deterioration of the signal-to-noise ratio (SNR). While the phase modulation (PM) and the Phase-generated carrier (PGC) modulation to the laser frequency are applied simultaneously, the suppression of SBS is 35dB and 10dB respectively in the backscattering spectra and the interferometric phase noise caused by SBS is completely eliminated. When the input power continues to increase and exceeds the modulation instability (MI) threshold, the system performance also deteriorates significantly. The forward output spectra of the 50 km optical fiber and phase noise of the interferometer are measured. The results show that with the increase of the injection power, the increase trend of the MI component in the total power of the spectrum is approximately consistent with that of the phase noise. It can be concluded that the phase noise introduced by MI is mainly caused by the increase of light intensity noise and the deterioration of optical SNR. Therefore, in order to reduce the impact of MI in the sensor system, it is needed to avoid the generation of serious MI as far as possible, and then the ultra-narrow band filter should be used to filter the MI sideband for the improvement of the system SNR.