Nowadays, more and more underwater electricity or communication cables and oil or gas pipelines have been installing. Equipment aging and damages to them have caused series of accidents, resulting in huge economic loss and environmental pollution. This paper proposes a long distance underwater linear object detection method based on range-gated optical imaging, which can help the maintenance and inspections of underwater cables and pipelines. The whole object detection algorithm can be divided into three stages: image enhancement, edge detection and object detection. In the image enhancement step, The system deals with the low contrast, blur and noises characteristics of underwater images by means of contrast normalization, median filtering, wavelet transform, and finally gets high quality images. Then, the Canny operator was used to extract object's edge features. Finally, for the emergence of noise edges, a robust algorithm named Random Sample Consensus was chosen to accurately detect linear object and estimate its parameters such as position and direction. This algorithm has been tested on the experimental data in the boat tank of Huazhong University of Science and Technology, collected with a range-gated imaging system. The results show that the algorithm can effectively detect underwater curved-linear objects, with the detection rate achieving 96%, and the effective detection range can be up to 5 times the length of the underwater decay.
In ocean optics, salinity is an important inherent optical parameter to be measured. In the field of optics, refractive index (RI) is closely related to salinity. Through the real-time detection of the refractive index, we achieved the purpose of underway monitoring the salinity of seawater. We designed a refractive index measurement system based on optical total internal reflection. In this system, the detecting precision of the refractive index of the sea water reached 10-4. Through the conversion of the refractive index, we achieved in-situ measurement of the salinity. In 2016 summer, we accomplished a successful underway measurement in China Yellow Sea. The trends of the results from refractive index are basically agreed with the salinity measurements from electrical conductivity.
We investigated a nanoantenna integrated thermomechanical infrared pixel based on bi-material nanobeam. Three bilayer configurations are numerically studied and optimized towards maximum thermomechanical deflection. The integrated optical nanoantennae are geometrically tuned to reach the highest optical absorption at 6 μm. Thermal time constants and fundamental noise equivalent powers of the three bilayer configurations are also calculated. We also discuss the potential implementation of our detector as an infrared polarimetric sensor.
We present a method based on total internal reflection (TIR) phenomenon for measuring the refractive index (RI) distribution with high-precision. In the field of RI measurement, the method based on TIR phenomenon is usually used to detect the average RI of the sample under tested, but unable to get the detail of RI distribution. We present a new method, using the micro-lens array to split the beam of light and divide the testing surface into a certain number of tiny detection areas. With the detector of area CCD camera, each pixel collects the reflected light from each tiny detection area respectively. By analyzing the reflectivity to each tiny detection area, we can get RI information of each tiny area. Through theoretical calculation and the actual scaling, the RI value of each spot is obtained. After collecting the RI of each tiny area, we can obtain the RI distribution.
We have developed a novel light scattering measurement system based on a microfluidic trap to measure the elastic light scattering of micro-particles. The particles were captured from the sample suspension by a microfluidic chip with a hydrodynamic trapping, which were stably immobilized at the predetermined position by the pressure gradient and friction in the micro-channel. The trapped particles were illuminated by a He-Ne laser after refractive index matching, and a narrow-field photodetector designed by the spatial filter and a photomultiplier mounted on a homocentric rotating platform was used to detecting the scattering light. In this paper, we have improved this measurement system. By reducing the background scattering of microfluidic chip to improve the signal-noise ratio and using precise control, we measured the 23.75μm diameter polystyrene microsphere’s light scattering distribution, the results showed a good agreement on the trend with the curves of theoretical result. At the same time, using the microfluidic trap, we captured two particles (same size and different size) in a fixed orientation with touching components and obtained the light scattering distribution.
We report an integrated fiber-optic detector by dip coating PbS colloidal quantum dots onto a pretreated specialty fiber. We measured the readout current at 1550nm as a function of the optical power, the bias voltage and the distance between the contact electrodes.
Based on optical total reflection critical Angle method, we have designed a refractive index measurement system. It adopted a divergent light source and a CCD camera as the occurrence and receiver of the signal. The divergent light source sent out a bunch of tapered beam, exposure to the interface of optical medium and sulfuric acid solution. Light intensity reflected from the interface could be detected by the CCD camera and then sent to the embedded system. In the DSP embedded system, we could obtain the critical edge position through the light intensity distribution curve and converted it to critical angle. Through experiment, we concluded the relation between liquid refractive index and the critical angle edge position. In this system, the detecting precision of the refractive index of sulfuric acid solution reached 10-4. Finally, through the conversion of the refractive index and density, we achieved high accuracy online measurement of electrolyte density in lead-acid battery.
In this paper, we designed a pint-sized underwater pulsed lidar system for underwater obstacles detection based on a 532nm Nd-YAG pulsed laser as a source and a Hamamatsu photomultiplier tube (PMT) as a detector. In order to acquire the location of the obstacles, an algorithm was devised to handle the echo signal. Through this algorithm, the background noise was suppressed and the accurate range information of the target was obtained. A high-capacity lithium battery was employed to support this lidar system operating as long as eight hours continuously. To ensure our lidar system working steady in the natural underwater environment, a stable waterproof housing was designed for the system which has good water-tightness at 40m depth underwater. This system is small, compact and hand-held. An experiment was conducted in laboratory which proof that the system can achieve target detection within 25m. At last, this lidar system was tested in natural underwater environment of Fuxian Lake in Yunnan Province. There are lots of organic particles and other impurity particles in Fuxian Lake and the attenuation coefficient of the lake is about 0.67m-1. The results showed that this small-size lidar system was able to catch sight of the target within 20 meters and perform smoothly in the natural underwater environment.
Small bubbles are widely present in the marine environment, the presence of small bubbles generated by whitecaps, microorganisms, ships sailing will greatly affect the optical properties of seawater. A lot of work has been carried out around the detection of small bubbles, this article will introduce a method of detecting small bubbles underwater with the way of high-speed imaging underwater. The optical mechanisms to measure the parameters of small bubbles are mainly high-speed photography, laser interferometry and holography. The advantages of high-speed photography are intuitive and low cost, the experimenters can real-time monitor the shooting circumstances, and can obtain more detailed parameters concerning the small bubbles. The paper also discusses an experimental method of high-speed imaging for small bubbles in water, that is to get a cooperative target in the back of the bubbles, then shoot the bubbles, and a lot of experiments with the two methods have been done. In order to compare the imaging quality of the two sets of experiments intuitively, the histograms and the results of edge detecting of the pictures have been given. After compared the results, it is found that the images are clearer and higher in contrast in the case of there is a cooperative target behind the bubbles, and with the imaging rate of the high-speed camera increases, the image quality is significantly reduced.
Multiangle dynamic light scattering (MDLS) compensates for the low information in a single-angle dynamic light scattering (DLS) measurement by combining the light intensity autocorrelation functions from a number of measurement angles. Reliable estimation of PSD from MDLS measurements requires accurate determination of the weighting coefficients and an appropriate inversion method. We propose the Recursion Nonnegative Phillips-Twomey (RNNPT) algorithm, which is insensitive to the noise of correlation function data, for PSD reconstruction from MDLS measurements. The procedure includes two main steps: 1) the calculation of the weighting coefficients by the recursion method, and 2) the PSD estimation through the RNNPT algorithm. And we obtained suitable regularization parameters for the algorithm by using MR-L-curve since the overall computational cost of this method is sensibly less than that of the L-curve for large problems. Furthermore, convergence behavior of the MR-L-curve method is in general superior to that of the L-curve method and the error of MR-L-curve method is monotone decreasing. First, the method was evaluated on simulated unimodal lognormal PSDs and multimodal lognormal PSDs. For comparison, reconstruction results got by a classical regularization method were included. Then, to further study the stability and sensitivity of the proposed method, all examples were analyzed using correlation function data with different levels of noise. The simulated results proved that RNNPT method yields more accurate results in the determination of PSDs from MDLS than those obtained with the classical regulation method for both unimodal and multimodal PSDs.
We present a fast, simple, sub-pixel algorithm on the critical angle refractometer to measure the refractive index of the liquid sample by determining the centroid of the light intensity of the relative reflective curve. The centroid algorithm utilizes a divergent fiber-coupled royal blue LED source to irradiate on the dielectric surface between the prism and the media, which generates the light intensity distribution of the reflectance facula. Instead of the critical angle pixel as the differential algorithm and the threshold algorithm, the sub pixel centroid algorithm is based on calculating the centroid value of the light intensity of the relative reflective curve. In some moderate turbid solutions, the centroid algorithm is less sensitive to the scattering and absorption than the differential algorithm and the threshold algorithm. It is possible to utilize the centroid point of the relative reflective curve to determine the refractive index. Supported by the theoretical analysis and experimental results on saline solutions, we can conclude that the proposed algorithm is effective to get the super resolution and meaningful to the refractive index measurement of the liquid. The critical angle refractometer with this centroid method is potential to be a high-accuracy, high-resolution, and reliable automatic refractometer.
Sizing a small particle from its scattered field has been a long-standing problem. Popular established methods require a priori knowledge of either the refractive index of the particle, or the approximate particle size range. In this paper, the diffraction tomography (DT) theory is studied and a single particle sizing approach using angular optical scattering field is proposed. There is a Fourier relationship between the scattering amplitude in the far zone and the scattering potential of the scatterer, under the 1st-order Born approximation for weakly scattering. Based on this relationship, the distribution of scattering potential can be retrieved from angular resolved scattered field by the use of a fast Fourier transform. Single particle size is estimated from the scattering potential. Numerical simulations for spherical particles are presented and discussed. Simulation results show that in the case of low contrast, the size of the particles can be estimated accurately in the presence of moderate noise. A further variant of this algorithm based on Rytov approximation is also discussed.
It is commonly known that absorption and scattering are the main causes of reducing performance of imaging system, especially imaging distance and resolution. Generally, various techniques are applied to decrease the effect of scattering, such as synchronous scanning and range-gated technique. Continuous-laser imaging technique meets requirements of imaging objects in the large field of view in real time, but imaging distance is less than 2 attenuation lengths in natural water. High-repetition-rate green laser, called quasi-continuous wave (QCW) green laser, is a better light source for underwater imaging. It has 1 kHz-100 kHz modulated rate, and its single pulse peak power is KW magnitude, which can be applied to range-gated imaging as Canadian LUCIE system. In addition, its polarization property is excellent for underwater polarization imaging. Therefore, it has enormous potential to underwater imaging. In order to realize its performance in underwater imaging system, we setup a separated underwater staring imaging system. For this system, a theoretic model is built by the lidar equation and optic transmission theory, and the system is evaluated by modulation transfer function (MTF). The effects of laser and receiver’s parameters for the system are analyzed. Then the comparative experiments are conducted in turbid water in laboratory. The results indicate that high pulse energy improves imaging distance. Aperture and polarization could reduce the effect of scattering effectively in staring system. The result shows that this underwater system performs better by choosing suitable parameters of source and receiver.
An improved calibration method for digital Abbe refractometer is proposed. Based on Fresnel reflection theory, digital
Abbe refractometer measures the index of refraction by processing bright-dark pattern images. For extreme environment
applications, our team has developed a digital Abbe refractometer. By analyzing bright-dark pattern images, it shows
optical aberration may reduce positioning accuracy on critical angle. The main work of this paper is to propose a new
calibrate method for digital Abbe refractometer. An optical system is built to simulate the refractometer. A motorized
micropositioning stage is inserted to precisely control the position of bright-dark boundary. An area CCD captures an
image each time boundary displaced. Get the boundary through entire measuring range to form an image database. The
database indicates the corresponding relations between bright-dark pattern image acquired by CCD camera and boundary
position read by motorized stage. When measuring the refractive index of liquid, match its bright-dark pattern image to
images in database, and get the boundary position from the nearest match. Compared to the former method of computing
boundary position from images with aberration, the proposed method calibrate refractometer by large amount of
experimental data, thus improve stability of the measurement.
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.
In three-dimensional (3-D) imaging measurement, the center location extraction of laser bar is one of
the critical techniques in determining the metrical precision of the system. The width and brightness of
laser bar varying with the change of detection distance is an important factor which influences on the
center location determination of laser bar. The underwater 3-D imaging quality is directly affected by
light scattering effect of water. Under coordinate conditions, the width increase and brightness decrease
of laser bar brings great impact on the metrical precision of the system. In this paper, the beam spread
characteristics of underwater laser transferring procedure is studied and the relationship among the
beam spread characteristics of underwater laser transferring, laser beam characteristics and correlative
parameters of water is constituted. Finally, based on the existing relationship between beam width and
beam characteristics in 3-D imaging measurement, the factors which affect the precision of underwater
3-D measurement are obtained by computing beam spread function of underwater laser transferring
under diverse conditions. Calculation result shows that the metrical precision enhances for the
improvement of water clarity as well as the reduction of detection distance.
Lidar is an important equipment 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.
Lidar is an important equipment for the accurate detection of underwater targets. In this paper, a compact
and remote-controlled lidar system is described. The system contains four modules which are Q-switch
Nd:YAG pulse laser module, photoelectric detector module, data collector module and remote-controlled
module. All of the modules are put into a hermetic container. The lidar system can be in operation under the
water and carried by an underwater carrier such as ROV (Remotely Operated Vehicle). The operator controls
the lidar and downloads the data from the system through a fiber. The system can automatically eliminate the
return signal due to the surface wave. The system has been used in underwater bubble detection. A brief
description of the lidar system and its operation is present in the paper. The experimental results of underwater
bubble measurement are discussed.
Laser will attenuate during its propagation in water and also be backward scattered by water when it is used to detect
bubbles in the ocean. Meanwhile backward scattering intensity of the bubbles is feeble, its dynamic range reaches to the
order of 6, which saturates PMT and its post-treatment circuit. Timely gating system is used to solve the problem. The
system contains pulsed laser and gating PMT receiver. The wavelength of the laser is 532nm, with pulse width of several
nanometers. Its operational delay is matched with the time period between laser traveling forward and back after
scattered by the target. By doing this, the light scattered by other object is eliminated, dynamic range of the signal
reduces, and consequently SNR increases. In order to avoid Signal Induced Noise(SIN), we choose PMT R1333 having
no HA coating. TTL logical level, which is used as gating signal, controls the first dynode voltage of PMT to implement
gating. Gating speed is about 100ns, of which the width is tunable. By carefully designing the electronic system, SNR is
eliminated to a level as low as possible, and the output signal of PMT is fast integrated in order to reduce the influences
of signal induced by opening the gate.
The sensitivity of lidar (light detection and ranging) and the contrast of immersed targets are strongly reduced by the
volume backscattering clutter. To overcome this shortcoming it has been proposed to use a microwave modulation in
association with an optical carrier at 532nm, which is called modulated lidar. This paper develops a Monte Carlo
simulation for application of modulated pulse lidar (light detection and ranging) in bathymetric measurements. First,
modulated light pulse propagation in ocean water is simulated by a Monte Carlo model. Second, the echo signal is
processed by advanced mathematical tools like cross-correlation and numerical filtering. The simulation results reveal
the capability of the modulation approach in suppressing the water backscattering and enhancing the target contrast for
the bathymetric field. Furthermore, more simulation experiments are performed with various ocean depth to study the
detection performance in different environment. Details of this simulation model, in addition to the simulation results are
In the detection of underwater object by air-born lidar, a narrow laser beam reflected by the sea bottom or the underwater
target transfers in the ocean and refracted through the wavy sea surface, then received by the receiver carried by plane
over the sea surface. Because of the scattering by the suspended particles and the effect of the wavy sea surface, the
received signal will to some extent blurred, which will greatly decrease the image quality of the lidar system. Computer
simulation by Monte Carlo method was employed to describe the process of narrow laser beam transferring in the ocean
and refracted by the wavy sea surface. As the result, the intensity distribution on the receiver can be got as well as the
property of image blurring after transferred thought the ocean water and refracted through the wavy sea surface. The
relations between this property and the optical property of seawater, the wind speed above the sea surface were analyzed.
The influence of water backscattering on detecting ability should be considered emphatically when optical methods were
used to detect underwater object. The water backscattering light can be decreased by using a short pulsed laser in the
system and the precise distance of the detected object can be obtained easily. An experimental system made up of short
pulsed laser, received optical system, narrowband light filter, high speed detector, high speed data acquisition and
process system and so on was established. To validate its detection ability, this system was set into a large water pond to
detect several kinds of objects with different reflection index. We processed mass of experimental data get from above
experiment with simple method, and the processing result proved that this system had the ability of detecting objects
whose distance is about sixty meters. Because of its precise ranging ability, this system can be used with the combination
of a right scan device in three dimensional underwater imaging system.
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.
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.
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.
There have been many mature results about reflection and refraction characteristic on medium surface of Gaussian beam, which are verified by practical applications. But, those are limited to regular medium surface such as plane and sphere. When the medium surface is wavy, the reflection and refraction characteristic is greatly different comparing with regular one. In the paper, according to the statistical description of direction distribution on wavy surface by Cox, we have set up a physical model of reflection of laser beam on wavy surface, derived that a beam reflected by wavy surface is also a Gaussian beam when the incident beam is a Gaussian beam, and set up the relationship between Gaussian beam’s light spot size and wind speed over sea surface. According to 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, 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.
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.
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.
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.
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.