KEYWORDS: Crystals, Modulation, Monte Carlo methods, Laser crystals, Modulators, Laser communications, Electro optics, Signal attenuation, Ocean optics, Refractive index
Indirect modulation with electro-optical crystal is a useful way to generate optical signal for underwater blue-green laser communication. However, as crystal surface is not strictly flat in practical application, light intensity distribution in the cross section is non-uniform, which would affect extinction ratio of modulated signal and system performance. In this letter, we study this issue with Monte Carlo method. The result shows that with the increase of crystal flatness, extinction ratio is decreasing dramatically, and it should be smaller than 0.78μm in order to make the extinction ratio greater than 10dB while 0.25μm for 20dB, 0.08μm for 30dB, and 0.025μm for 40dB.
In underwater wireless optical communication (UWOC), one of the key technologies is to generate high-speed communication signal for transmitter. In this paper, we designed such transmitter based on laser diode (LD) arrays, which is composed of three LDs with central wavelength 450nm. The modulation format is non-return-to-zero on-off keying (NRZ-OOK) with data rate up to 50Mbps. Using such transmitter, we established a point-to-point underwater wireless optical communication link in an experimental tank with 20m length, 20m width and 14 depth. The experimental results show that the maximum error-free data rate of the system can reach 50Mbps with 10.7m transmission distance, while the maximum error-free transmission rate is 30Mbps with 14.7m transmission distance. These results verify the feasibility of the LD-based modulation scheme for high-speed UWOC applications.
In view of whether the optical system of a focal length 14~360mm continuous zoom TV can obtain high quality and high reliability images in high and low temperature environment, especially in extreme low temperature environment. The thermal design of zoom TV optical-mechanical system using passive thermal control and active thermal control is proposed. The passive thermal control uses polyimide as insulation material to increase the thermal resistance between the camera interior and the outside. Active thermal control uses electric heating film to dynamically heat the key parts of the camera lens. Under the condition of low temperature, the finite element method is used to establish the heat transfer model of the whole lens assembly in the workbench finite element software, and analyze the heat load composition, including the heating power load, the heat convection load and the heat radiation load, and carry on the steady state thermal analysis. Through thermodynamic analysis and experimental verification, the consistency of focal plane of zoom camera optical system is good after taking active thermal control measures at the extreme low temperature of -45°C. The optical transfer function (OTF) of zoom lens at cut-off frequency (100lp/mm) is 0.25 higher than that before thermal design, which can meet the requirements of thermal control design with better transfer function distribution and higher imaging quality. The correctness of the simulation results and the rationality of the optical-mechanical design are verified.
Based on the principle of the strap-down inertial navigation system and the attitude algorithm, a novel evaluation method is proposed. It could be used to effectively diagnostic the precision of the attitude algorithm. The experimental data including Algorithm 1 (N=2, P=0, optimal two-sample), Algorithm 2 (N=2, P=1) and Algorithm 3 (N=2, P=2) is recorded. The precision of the three algorithms is evaluated by calculating the relative error. In addition, the influence of the sampling frequency on the precision is investigated. The test results show that the precision of Algorithm 2 and Algorithm 3 is approximate; as the sampling frequency is improved from 1Hz to 20Hz, the precision of the attitude algorithm can research 0.08°. When choosing the attitude algorithm of the strap-down inertial navigation system, increasing the number of the sample not always depresses the coning error, improving the sampling frequency and using the former attitude to renew the number of the sample of the period could improve the precision of the attitude algorithm, further.
In order to acquire the geographic location information of sea-surface, aiming at feature that sea-surface elevation is known. A self-location algorithm independent of ranging equipment was developed. The paper uses camera’s position and attitude information measured by position and orientation system which rigid connect with camera. Then the paper compensates the lever arm which comes from GPS Antenna phase center and camera center of photography. So that accuracy position was got. The collinearity equation is built by the camera’s position, attitude and the known sea-surface elevation which equals zero to calculate target geodetic coordinates. The error model is built basing on total differential method. At last the location error of different detection range and angles of strabismus is analysed.
When the aerial camera photograph,a variety of image motion is caused by prior to the flight, pitching, rolling and vibration and other reasons,thus leading to the existence of relative motion of the illuminated objects in the focal plane of a photosensitive medium, the image is blured,and the imaging quality of the camera is seriously affected. Various causes of image motion and effects on image is analyzed by this paper,the necessity of image motion compensation is expounded. By analyzing existed methods of image motion compensation ,and on this basis, a new multi degree of freedom motion compensation method is designed,through the parallel mechanism motion,for image motion compensation by optical image motion compensation principle,a variety of airborne camera to take pictures of the image motion also can be eliminated.
To a high resolution digital camera which works in visible light and is on the space-based platform flying in 500 km orbit, analyzed the principle of image acquisition, established the collinear equation and target location model from the system measured the pose of camera and single image without control points, and researched the method to calculate the geographic coordinate of target point. Analyzed the consisted factors of target location accuracy, and generated the formula for calculating target location accuracy based on the accuracy theory. For the cameras working in this mode, gave the elements of orientation and the parameters of camera, then obtained the target location accuracy is 16.1 meter through the simulation analysis to the model. The analysis to the target location accuracy provides a theoretical base for the practical use of the space camera. Analyzed the impact of the camera parameters and operation mode on the location accuracy, and put forward some measures to improve the target location accuracy.
Thermal control and temperature uniformity are important factors for space remote sensing cameras. This paper describes the problems with existing systems and introduces the thermal design of a space optical remote sensing camera. Firstly, based on the theory of wave-front aberration distribution, the thermal control index of a space remote sensing camera is proposed. Then on the basis of the analysis of the heat flux environment outside the camera space, the thermal optical analysis of the camera is performed by using the finite element analysis method at high and low temperature conditions. The results show that the transfer function of the optical system with the resolution of 50 lp in the full field of view is more than 0.4. The optical design index can be satisfied, and the rationality of the thermal design is verified. The simulation result meets the requirements of optical design very well. Therefore the study in this paper can be used as an important reference for other space optical systems, which has certain engineering significance.
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