Deming Ren

Scattering properties of ultrashort laser pulses by air bubbles in the sea water

The Generalized Lorenz-Mie theory(GLMT), which describes the interaction between spherical particles and laser pulses, is firstly introduced in this paper. Then scattering properties of ultrashort laser pulses by air bubbles in the sea water are studied, including the scattering efficiency factor, extinction efficiency factor, phase function and degree of polarization, and influences of the bubble size, pulse duration and scattering angle on pulse scattering properties are discussed. Finally, pulse scattering properties of bubbles are compared with those of suspended particles in the sea water. The results show that laser pulses have the smoothing effect, which strengthens with decrease of the pulse duration, but have no influences on the critical angle scattering and Brewster angle scattering. So they could be measured to distinguish between air bubbles and suspended particles and thus to detect air bubbles. This will have certain significances on pulsed laser detection for wake bubbles. The results also show that small bubbles could enhance the backscattering of the sea water, which is advantageous for pulsed laser detection for wake bubbles.

An improved method of new image decomposition technique: two-dimensional EMD

An improved algorithm of 2-D Empirical Mode Decomposition (EMD) in image processing has been presented. It contains selecting extrema of the pixels and interpolation of them in the course of EMD, A variance phenomenon has been discovered in the interpolation of boundary pixels. Delaunay Triangulation has been used to partition the selected extrema, a pixel that not contained in the Delaunay polygon can be processed through symmetry principle, It can restrain the variance phenomenon that appeared in the cubic spline interpolation. An image has been processed with the improved algorithm, The calculation result of standard deviation between the original image and the reconstructed image is 6.667×10-6 . A slight fluctuation can be seen from the calculation result. The reconstructed image is consistent with the original image. It demonstrates that the improved algorithm been presented is accurate and feasible. The method of EMD is used in image compression and de-noising more and more popular, therefore, the improved algorithm in the paper will be useful in the course of calculation speed enhancement of image processing based on EMD.

Laser linewidth measurement based on image processing and non-air gap F-P etalon

Laser linewidth measurement has been realized in the paper through image processing and on non-air gap F-P . Firstly , The expression of linewidth measurement founded on non-air gap F-P has been obtained from the interference theory of multi-beam of light. Secondly, The practical linewidth of pulse Nd:YAG laser has been measured with the method above. An interference pattern produced by CCD was used for digital image processing. In the course of processing, Canny operator was applied for the sake of picking-up the edge of interference circle, then the inner and external radius of the interference circle can be acquired in the form of pixels. The actual physical size can be calculated through relative transformation according to the radius of the interference circle. At last, Nd: YAG laser with 28ns pulsed-width was used as the emission source, The experimental results based on the method above is 36.8 MHz. This data is in agreement with 34.2 MHz through Discrete Fourier Transform(DFT), As we all know, the frequency resolving power of DFT is depend on the effective acquisition points, So the cubic spline interpolation was introduced after the course of DFT, and the better result has been achieved.

Processing methods in frequency domain for bubble laser scattering signals

For the detection of the bubble-laser-scattering property, it is the key to improve the signal to noise ratio. The various noises in the bubble-laser-scattering signal, the background light, power frequency of 50Hz, radio frequency noise and inherent noise within the electronic system, are analyzed in detailed. Two frequency-domain methods used to process the bubble scattering signal, the Fourier transform and the power spectral estimation, are focused on. These two methods can reduce the noise, isolate the DC component and eliminate the impact of the power frequency of 50Hz, thus the pulse of bubble laser scattering could be detected. The physical meaning for the power spectral estimation is much clearer than that of the Fourier transform. The power spectrum curve obtained by using the Yule-Walker AR (regression model) method is much smoother and its frequency resolution is higher than the other methods. The results show that the Yule- Walker AR spectrum estimation is the most reasonable and most effective method.

Nd:YAG laser with passive-active mode-locking

All solid-state flash-lamp pumped passive-active mode-locked Nd3+:YAG laser is designed and experimentally studded. Saturation absorber Cr4+:YAG with initial transparency 25 and 47% are used as a passive Q-switcher and acousto-optical fused quartz modulator as an active mode-locker. Efficient length of the laser cavity with fixed mirror positions (1.45 m spaced) is droved by changes of 100% flat mirror for concave mirrors with different focus lengths. Changeable output mirrors with transparencies of 15 and 50% are used. Driving of the cavity parameters, laser and acousto-optical modulator power supply voltages let us to control output pulse train and single pulse parameters. As it goes from the analyses of oscillograms fixed with pyroelectric detector (τ=0.5 ns) and 1 GHz oscilloscope, over 95% of pulse output energy has been mode-locked. Average duration of the pulse train envelope of 5 to 50 single pulses at FWHM has been droved within 50 to 600 ns. When this single pulse duration is controlled but did not exceed 2 ns.

The backscattering property experiment research of wake bubbles

The bubble backscattering intensity between 140 and 160 degree is achieved utilizing bistatic method, and the bubble scattering signal is detected, analyzed and processed. The photo multiplier tube is used to detect the weak signal. Threshold detection distinguishes between the bubble backscattering and water backscattering. Bubble scattering pulses are stationary random signals and have statistic properties. So the sample average for an enough long time may be as the scattering intensity at a certain angle. Experimental results have the same trend as the theoretical results. The experimental research on the bubble backscattering properties is importance to the ocean sensing and wake homing.

Optimization of discharge circuit of the TEA CO2 laser with two discharge channels

In order to achieve the highest peak power of radiation pulse and highest output energy, the primary circuit parameters are investigated to optimize the discharge circuit performance of the laser. The structure and the discharge circuit of the laser are discussed at first. To realize synchronous discharge in two discharge channels, the conjunct electrode device for two pairs of discharge electrodes is designed. Finally, the results of the experiments on the primary circuit parameters are given. The discharge is most stable at a pressure of 5.33×104Pa when the pressure of gaseous mixture CO2:N2:He=1:1:3 is changed from 2.67×104 Pa to 6.67×104 Pa. The ratio of storage capacitance to peak capacitance is chosen to be about 1.5-7/3, because residual voltage is lower on this condition and residual voltage is adverse to discharge. When the inductance 330&mgr;H is used, the homogeneous glow discharge in a widest voltage range is obtained. The duration of when the stimuli voltage is increased in homogeneous glow discharge condition. The discharge circuit allows charge and discharge and the magnitude of residual voltage decrease the homogeneous glow discharge in a wide range of pressure of gaseous mixture when these circuit parameters are used. Thus it offers reference to the improvement of output characteristic of TEA CO2 laser with two discharge channels.

Influence of organic film for bubbles on scattering properties of ship wakes

Mie theory is used to study the influence of organic film coatings for bubbles on optical scattering properties of ship wakes. The influence of organic film coatings on the normalized volume scattering function of single bubble is researched. Based on the fractal data of ship wakes, analysis and comparison are made on the scattering properties of clean and coated bubbles, including the normalized volume scattering function, scattering coefficient, backscattering coefficient and backscattering ratio. Organic film coatings have little influence on the critical angle scattering. Contributions of the organic film coatings to the scattering properties of ship wakes relates to the thickness and refractive index of them, in addition, the normalized volume scattering function also relates to the scattering angle. Scattering properties of ship wakes are of importance to wake homing and optical remote sensing of ocean.