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Lidar is an important equipmen t for the accurate detection of underwater targets.In lidar measurements of underwater target, the energy and width of the laser pulse will affect the results when using PMT as receiver, which has been proved by data processing and analysyses. To avoid the effect and enhance the accuragy of underwater, many methods such as controling the energy of the laser pulse, reducing the pulse width are taken. The experiment results of underwater target measurement are discussed. Keywords: underwater;Lidar;energy;pulse width; accuracy 1 introduction In nature environment, there are a lot of underwater target such as fish, reef and bubbles resulting from the breaking of the wave, the ship’s wakes and the shoals of sea animals [1 2]. When a particulate in water is illuminated by an incident beam, the incident beam will be scattered in all directions [3]. Lights whose wavelength are within the blue-green region are useful for detecting submerged objects. The difficulty in underwater optical detecting is the scattering and absorbing of the water and the dissolved subs tances.When laser beam propagati ng in the turbid water, the backscattering light from the water and the dissolved substance reachi ng the optical receiver will be the dominant noise source. Due to the special envi ronment in the water, the energy and the pulse width of the laser beam will affect the backscattering observedly. The experimentally research of effects of energy and pulse width on underwater Lidar system is desc ribed and results are given in this paper. 2 setup of the experiment lidar system Figure 1 shows the diagram of experimental syst em used to investiga ting the performance of

Genetic diversity of 54 maize landraces from southwest China was tested using bulk DNA samples and 42 microsatellite (SSR) loci distributed on 10 chromosomes of maize. A total of 256 alleles were detected among the landraces. At each locus, the number of alleles varied from 2 to 9, with an average of 6.1. On the basis of the genetic similarity coefficients, clustering analysis separated the landraces into four groups. The landraces collected from the same region were mostly grouped together. To reveal the genetic structure and genetic diversity within landraces, 165 individuals from 11 landraces were analyzed. Individual DNA samples proved to be superior to bulk DNA samples in identifying genetic diversity of landraces. A total of 330 alleles were detected in the 11 landraces. According to the results of the individual DNA sampling analysis, estimates of the mean number of alleles 'A', the effective allelic number 'A(e)', the observed heterozygosity 'H(o)', and expected heterozygosity 'H(e)' were 7.86, 3.90, 0.69, and 0.37, respectively. An obvious genetic deviation from Hardy-Weinberg expectation was observed both among and within landraces and a considerable genetic variation was revealed within rather than among landraces. In addition, genetic diversity of landraces was greater in Sichuan than in the other three regions. It can be concluded that maize landraces in southwest China were initially introduced to Sichuan and from there to adjacent areas.

The transportation of lidar’s laser beam in seawater is simulated by Monte Carlo method, which combined with statistic, estimate method and weigh method. It should be hypothesized that the incidence laser beam is vertical down and the beam is infinitude thin and vertical. The edge of atmosphere and seawater is located as cone. The axial line of the cone is same to the axial line of the laser impulse spread. It shows that the FOV (fields of view) of lidar’s detector have some influence to the waveform of echo signals. The influence is quite clear when the fields of view is quite small. The larger of the fields of view is, the slower the attenuation speed is. The trend goes to saturation when the fields of view add to a certainty. The conclusion is that the best receiving FOV is between 50mrad ~ 70mrad to the on-board lidar system which located in height at about 500 meters.

A bubble in water is an example of a scatter for which the refractive index of the core (gas) is less than that of the surroundings. So bubbles in water exhibit scattering phenomena which differs significantly from those for drops in air or solid particles in water. The scattering of bubbles in water as a means to detective the size of bubble has been investigated in many ways such as Mie theory and Davis’s geometric-optics theory. In this paper, a new physics-optics method was applied to manifest the scattering properties of a spherical bubble in water. The angular distribution of intensity of light scattered from a collimated beam that is incident upon a spherical air bubble in water is determined for any bubble with radius greater than a few wavelengths of the incident light. One external reflection, four internal reflections and four refractions are considered. The intensity of scattering light is tabulated and plotted as a function of the observing angle, the effects of the bubble’s radius, the electric field’s polarization of the incidence light and the wavelength of the incidence light on the scattering intensity distribution are also discussed.

In this paper, an improved semi-analytic Monte Carlo method is used to simulate the lidar received backscattering signals. The H-G function is used to approximate the scattering phase function of seawater, from which we can derive the scattering angle directly, and a modified H-G function is used to calculate the probability of the photons received by the receiver at each scattering point, which greatly improves the accuracy of the simulation. The simulation result shows that the different parameters of air-sea system of lidar, such as lidar’s field of view, attenuation coefficient and single scattering albedo of seawater, greatly influence the lidar received backscattering signal waveform. Multiple scattering is studied to explain these phenomena.

In this paper, we adopt YAG Q-switch double frequency solid state laser to drill hole in the water and air on three kinds of material: stainless steel, yellow copper and purple copper separately. The results show the differences in the water and air by comparing. At last, we qualitatively and quantitatively analyze the mechanism of interaction for laser processing in the water. Put forward the feasibility and superiority of the ultra short pulse laser processing underwater. The primary parameters of the laser: the width of pulse 10—100ns; wavelength 1.06m, 0.53?m; the energy of single pulse 0---100mJ; The work piece is placed underwater close to the water surface about 0. 1—2cm, focus distance lens f=60mm, the water is tap water, the thickness of work piece is less than 1mm. The laser with different laser parameters processes the work pieces. The results of the above experinlents indicate that the efficiency of drilling hole in the water is much higher than that in the air. And the quality and sharp of the hole processed is much better than in the air. The best power density of drilled hole in the by laser is mainly determined by material characteristics and the parameters of laser.

The paper analyzes the performance of plane-concave resonators with internal thermal lenses, and gives out stability and instability regions in terms of dioptric power. Based on stability region's symmetry and stable and unstable resonator's characteristics, relationship between resonators' loss and pump power is obtained, and then, a detailed analysis for output power of cw plane-concave Nd:YAG laser is presented. Experimental results clearly show the existence of two-peak structure predicted by the theoretical analysis.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

From the laser cavity theory, we analyze the stable properties of the cavity in the conditions of various parameters, and then the output power curves of the laser are fully analyzed. The results show that the double-peak shape of the curves does not appear in three special cases of cavity parameters, in two of which there exist wide regions for stable output. Based on heretical analysis, we design a laser with proper cavity parameters and obtain some experimental results, which coincide with the theoretical expectations.

We present a practical algorithm based on the Fourier power spectrum for expanding the measuring range of a critical-angle refractometer (CAR). Refractive index (RI) information is extracted from the gradual change in the relative reflective ratio. The Abbe-type refractometer has a limited measurement range based on Snell's reflective law. When the RI of a liquid sample is beyond the upper limit of the Abbe-type refractometer, it can be obtained by using the Fourier power spectrum of reflectivity. The maximal value of the Fourier power spectrum of the reflective ratio is associated with the RI of the liquid. Supported by theoretical analysis and experimental results using sugar solutions with RI that varied from 1.359 to 1.3766 tested through the reflective CAR, the algorithm is found to be computationally effective and robust to expand the measuring range of the Abbe-type refractometer.

In airborne laser bathymetry, it is very important that narrow band interference filter matches with central wavelength of Q-switched, frequency-doubled laser. A method of fast match of filter and laser central wavelength is described. The results of airborne laser bathymetry system experiments in the pool is gained under the state of match of interference filter with laser central wavelength.

An experimental airborne laser bathymetry system has been developed and field trial has been conducted. The Q-switched and frequency-doubled Nd:YAG laser output is of 100 HZ pulse repetition rate, 2 MW peak power, 8 ns pulse width. The green light receiving telescope is transmissive with 1400 mm focal length and 200 mm aperture. The varying-gain control of PMT and logarithmic amplifier are used to compress the 105 dynamic range of received signals. The main features of data real-time processing subsystem are of 200 Ms/s sampling rate, 8 bit resolution, adjacent average treatment of return waveforms with high noise, and pseudo-color display of sea depth.

According the wave model on the water surface, the returned laser power expression for the airborne laser bathymetry is derived from. The influence of the wavy water surface, the field of view for the IR received system on returned laser power is discussed.

In this paper, investigations have been made on a CO2 laser with the quasi-tube-plate discharge electrodes under a new gas flow condition. A theoretical model for calculating the stable operation parameter of the new laser was present. A preliminary calculation on small signal gain under many operation conditions has been made. A stable discharge with the input power density of 20 W/cm3 had been obtained in the experiment.

We report on sizing spherical particles from far-field scattering light based on optical diffraction tomography. The distribution of the scattering potential, i.e. the distribution of the refractive index can be retrieved from an angular-resolved scattered field via an inverse Fourier transform. An algorithm based on this method was implemented and tested on a class of low-contrast particles. The results demonstrate that this method provides an accurate estimate of the particle size in the presence of moderate noise.

The utilization of a simple focused optical cell to bring to light the competition between wideband stimulated Brillouin scattering (WSBS) and forward stimulated Raman scattering (FSRS) is investigated experimentally. A pulsed, wide bandwidth second-harmonic Nd:YAG laser is used as the pump source. We found that, the competition between WSBS and FSRS is an alternate process, which one dominated depends on the linewidth and energy of the pump laser, focal length, and optical breakdown.

An improved curve-fitting method for a simultaneous measurement of the real and imaginary parts of the complex refractive index of turbid media is demonstrated. Based on an introduction of angle-dependent light penetration into turbid media by Calhoun et al (2010 Opt. Lett. 35 1224), the fitting is performed on the rapid change part of the measured curve around the critical angle. Moreover, a technology to locate this part of the curve by differentiation is introduced. Experimental results on Intralipid solutions show that this method can significantly reduce the computation time as well as the fitting error between the measured curve and theoretical one.

In order to improve underwater range-gated imagery quality, image restoration and super-resolution reconstruction techniques based on maximum a posteriori framework are combined with the point spread function (PSF) of the system. A model based on beam propagation considering time-dependent backscatter is established including the extreme attenuation properties of water and the diffraction limit of sensor for obtaining the PSF. The proposed method is applied for an underwater range-gated imaging system. Performance of the approach is compared with the traditional restoration and reconstruction algorithms. Objective image quality metrics are used to quantify the effectiveness of the restoration and reconstruction. Experimental results showed that the proposed method can effectively improve the resolution and quality of underwater range-gated imaging.

A new critical angle refractometer (CAR) for high accuracy refractive index measurement of liquid has been developed. The instrument improves the accuracy by two reflections in an elongated parallelogram prism, and acquires the angular reflectivity without any angle scanning parts through introduction of a point source with a divergent beam and a charge coupled device. In addition, it employs a simple and robust measurement method that gets the critical angle by differentiating the angular reflectivity. Through investigating absorbing media with absorption index κ (the imaginary part of refractive index) from 0 to 10(-2.1), the theoretical calculation shows that the proposed two-reflection CAR would outperform the traditional one-reflection CAR on lowering the principal error from the differentiation method and improving the ability of getting the critical angle. By testing two typical liquids-salt-water solution and milk, the preliminary experiment indicates that this two-reflection divergent differentiating critical angle refractometer is feasible and of high accuracy.

In this paper, the stimulated Brillouin scattering (SBS) and the backward stimulated Raman scattering (BSRS) excited by a focused Gaussian laser in liquid water with different attenuation coefficients are investigated experimentally. Experimental results indicate that the relationships between SBS and BSRS are not merely competitive; the former has an obvious amplifying effect on the latter. Also, two different physical mechanisms were discussed in order to explain these phenomena.

We propose a novel and simple all-optical 160-Gb/s orthogonal frequency division multiplexing (OFDM) symbol generator which is based on discrete triangle waveform driving-LiNbO3 modulators to realize largerange linear optical shift. The entire system needs 64 discrete modulators: at the transmitter, a 2.5-Gb/s optical duobinary (ODB) modulator for data modulation and a 2.5-Gb/s triangle waveform driving-LiNbO3 phase modulator for phase shift to generate each subcarrier; and at the receiver, a 2.5-GHz optical band pass filter (OBPF) using Faraday anomalous dispersion optical effect to separate them. Excellent bit error rate (BER) is observed after 1060 km of transmission without any dispersion compensation.

Marine micro-bubbles are one of those important constituents that influence scattering characteristics of water column. Monte Carlo Based simulations show that a water entrained bubble cloud generate a characteristic backscatter of incident laser light [M. Xia, J. Opt. A: Pure Appl. Opt. 8, 350 (2006)]. This characteristic can be used to detect and localize bubble clouds, leading to wide ranging applications, especially in optical remote sensing. This paper describes tests of an underwater lidar system applied to detecting cloud of micro-bubbles. Laboratory experiments demonstrate that the system is capable of detecting bubbles ranging from diameter 10 microm approximately 200 microm, over a distance of 7-12 m from the detector. The dependence of the lidar return signal on size distribution of bubbles, concentration, thickness and location of bubble clouds is studied and compared with simulation results.