The water has a strong absorption and scattering of light radiation, resulting in a certain depth of the underwater world in the dark. Therefore, underwater optical detection technology based on active lighting has become the main mode of deep-sea photo detection. Using a 532nm narrow-pulse laser and a self-built gain CCD system to form a underwater laser rang-gated imaging system, the forward and backward scattering are modeled and analyzed. Underwater laser rang-gated imaging system, was studied by laser pulse jitter, thus lead to the door of the control signal, the open time (earlier/later), the relationship between secondary scattering and image contrast curve, puts forward an optimal pulse with door control strategy. The correctness of the model is verified by the method of simulation and calculation of the relative ratio of the images acquired by the actual underwater laser distance gating imaging system. The results show that when the imaging system is not saturated, the image quality is best when the gate and the laser pulse are optimally matched. The effects of early opening and delayed opening on the image quality are different, and the duration of the gate opening is equal to the laser pulse width. At that time, the image quality is not optimal, and the effects of laser pulse jitter and secondary scattering are not all unfavorable. Based on the model, the opening time and the gate opening time are determined according to the contrast curve. It is generally advantageous to open the door lagging behind, and the duration of the gate opening should be 1- 3 times the laser pulse width.
Towed linear array array based on fiber laser hydrophone which had much research potentiality owing to its higher acoustic pressure sensitivity, smaller size and lower difficulty of multiplexing. Flow noise is one of the main sources of noise in the towed linear array system. It will compress the dynamic range of fiber laser hydrophone and reduce the detection ability to small signal. In this paper, the structure of a 4 channel fiber laser hydrophone towed linear array was presented, and the data of sea trial of flow noise was analyzed. The results shows that the influence of flow noise mainly concentrate on the low frequency band which under 500Hz, and the background noise level has increased nearly 70-80dB when the array is towed at the speed of 3 kn.
Optical fiber laser hydrophone which has more research potentiality and value owing to higher acoustic pressure sensitivity, smaller size and lower difficulty of multiplexing. However the detecting capacity to low frequency signal of optical fiber laser hydrophone will be limited because of the low frequency noise such as 1/f noise and thermal noise of optical fiber laser and pumped laser. In order to suppress these noises, the iterative discrete wavelet transformation algorithm was designed which used multi-scale trait of wavelet transform. The different spectral components of underwater acoustic signal were separated and the noises below 1kHz were eliminated on the basis of the target signal amplitude would not be weakened. The measured data acquisitive from anechoic tank showed that the algorithm reduced the noise below 1kHz nearly 50dB and the Signal to Noise Ratio(SNR) is improved from 55.23dB to 84.05dB.
It is a key problem of measuring warp making use of collimated laser how to accurately calculate laser beam's center. A new
method, named double geometrical center method, is put forward. In this method, plane wave perpendicularly irradiates an aperture stop,
and a charge couple device (CCD) is employed to receive diffraction-beam spots, then the geometrical centers of the first and second
diffraction-beam spots are working-out respectively, and their mean value is regarded as the center of datum beam. In face of such
adverse instances as intensity fluctuations of the laser beam spots, part of the image being saturated, this method can still work well.
What's more, this method can detect whether an unacceptable error is exits in the courses of image receiving, processing and calculating.
Experimental results indicate this method can remarkably improve the measurement accuracy.