2013 Vol. 6, No. 2
With the increasing urgency of the international energy crisis, the concept of the space Solar Power Systems(SPSs) has been widespread concerned in the various countries of the world. Developing SPSs and optimizing energy structure is also a workable way to solve energy problems for our country. This paper elaborates the origin, conception and the course of development of the space SPSs in detail, analyzes and summarizes the key technologies of the space SPSs based on the combination of the roadmap and the typical space SPS models of the major countries in the world. This paper also gives the assumption to develop the SPSs in our courtry by combining with China's current technology and future development of the technology progress. It proposes a new type of space SPS according to the new design concept on the lighting condensing film, and introduces its composition, function and path design. The lightweight structure, low launching costs and emission difficulty are its remarkable characteristics. It will be a useful supplement and reference for the future development of space SPSs in China.
The rotational-double-prism system can change the optical path by rotating separately the two prisms of a common axis, so that it can be used to steer the beams or the Line of Sight(LOS). Compared with the conventional two-axis or three-axis optical electro-optical gimbals, the rotational-double-prism-based beam or LOS steering devices offer tremendous advantages such as high accuracy, compactness and good dynamic performance, and have been proved to be the helpful complementarity for electro-optical gimbals. This paper analyzes the beam steering mechanism of the rotational-double-prism system, and introduces the key problems of the relevant theories for the beam steering, which focus on the beam deflection mechanism, beam scanning mode, prism rotation control, beam deformation, imaging aberration and the imaging distortion induced by a prism. It describes the research progress of engineering technology and gives several kinds of products based the rotational-double-prism system and their application to the laser beam steering and target searching, target recognition and traking imaging. Finally, the technical challenges concluded in scanning patterns, beam quality, chromatic aberration, image distortions and rotation control are discussed and the development trends are evaluated.
Shipborne laser weapons provide complements for traditional shipborne weapons and they have been given much attention by many countries in the world. This paper introduces the history of shipborn laser weapons, and analyzes their compositions, functions and key technologies. Then, it points out that the development of shipborne laser weapons will focus on the free electron lasers and optical fiber lasers, and the low-energy lasers with small weights and volumes are expected to used in shipborne laser weapons. Actually, the shipborne laser weapons will develop toward the direction of miniaturization and high efficiency.
The development of atmospheric trace gas detection technology is elaborated. The design concepts, working principles and apparatus functions, working modes, spectrum setting and the main technical indicators of several kinds of advanced atmospheric trace gas remote sensing instruments are introduced in detail. Then, the present situation of the atmospheric trace gas detection apparatus on remote sensing satellite in China is discussed, and the working models, spectral set and other technical indicators for two typical remote sensing instruments are given. Finally, the future development direction of spaceborne atmosphere trace gas detection instrument is pointed out, which involves the improvements of the spectral resolution, spatial resolution sensetivity and calibration accuracy of the instrument and the development of active remote sensing technology. These provide a reference for the development of our country's satellite atmospheric trace gas detection apparatus.
In order to solve the occlusion problem in multi-object tracking for the complex background of a video image, an approach for the multi-object tracking based on multi-feature joint matching is presented. First, the adaptive Gaussian mixture background model is used for reconstructing and updating the background to achieve the background subtraction of current frame and multi-object detection. Then, the joint matching tracking is developed based on matching color characteristics, positions and objects velocities. Finally, the objects in the scene are divided into entering, exiting, temporarily disappear of the object, the re-emergence of the object and the merge and split of the object, and the predicted position and the occlusion factor of the object are used to improve the accuracy of multi-feature joint matching. Experimental results indicate that the similarities of the same object and the different objects are 0.949 71 and 0.505 73 respectively in the tracking with the proposed approach, which is better than that of matching with the color characteristics. Furthermore, the similarity of object is 0.972 83 after occlusion. The approach is satisfactory for real-time tracking of multi-object with appearance similarity in a complex environment.
To improve communication quality and meet the reliability requirement of data transmission, the Bit Error Rate(BER) performance of Reed-Solomon(RS) codes is analyzed, and a RS(15,9) hardware decoder based on Reformulated Inversionless Berlekamp-Massey(RiBM) algorithm is proposed. The decoder operates in pipeline architecture, and its key equation is resolved by the RiBM algorithm. On that basis, an atmospheric laser communication system is simulated by adding Additive White Gaussian Noise(AWGN) in a fiber, and it is tested on a Field Programmable Gate Array(FPGA) platform. The experimental result shows that decoding speed can be up to 1.11 Gbit/s, as 3.54 times as that of Altera's IP core. The RiBM algorithm has advantages of low hardware complexity and short critical path delay, and can meet the demands of the decoding system.
The image drift phenomenon caused by the dispersion of an acousto-optic crystal is researched when the spectral camera is imaging based on acousto-optic tunable filtering mechanism. The image drift caused by the crystal outside diffraction angle is calculated theoretically and measured experimentally by the dispersion compensation and the image displacement compensation methods when a Acousto-optic Tunable Filter(AOTF) is in the visible band of 488-644 nm, and the experiments are also optimized and analyzed. Using the dispersion compensation method and in the incident light wavelength of 488-644 nm, when we adjust the incident subject as a parallel light and add a wedge of 0.6 in the crystal exit surface, the change of the crystal outside diffraction angle can be reduced from 0.066 50 to 0.004 2, and the image drift is reduced from 162.1 m to 10.9 m. Using the image displacement compensation method without adding the optical wedge, when the incident light wavelength is in 488-644 nm, the image level drift can be reduced from 468 m to 0.658 m within a pixel drift. Experimental results show that the effect of the imaging can be neglected, and the two methods can improve the imaging resolution of the AOTF based on mechanism of the spectral camera.
A number of debris in low earth orbit is exponentially growing although future debris release and mitigation measures have been considered in human space activities. Especially, an already existing population of small and medium debris is a concrete threat to operational satellites. Ground-based DF laser and space-based Nd:YAG laser solutions appear as a highly promising answer, which can remove hazardous debris around the selected space assets at low expenses and in a non-destructive way. This paper introduces a research on the Space Vehicle(CV) protection and the orbit clearing from dangerous Elements of Space Debris(ESD) with diameters from 1 to 10 cm by means of a high-power and high repetition rate P-P Nd:YAG laser with an average power of 100 kW and a DF-laser with an average power about 1.5 MW.
Both tapered structure and broad-stripe semiconductor laser diodes were fabricated based on the AlGaAs/AlGaAs epitaxial layers with a graded-index waveguide separated confinement hetero-structure under the excited wavelength of 850 nm. The temperature characteristics of the devices were investigated and compared at the temperatures between 20 ℃ and 70 ℃. Experiments show that the measured characteristic temperature(164 K) of the tapered devices is much higher than that of the broad devices(96 K) and the wavelength-shift coefficients of the tapered and broad-stripe devices are 0.25 nm/K and 0.28 nm/K respectively under a duty cycle of 0.5%(t=50 s, f=100 Hz) and a pulsed current of 1 000 mA. When the temperature is below 50 ℃, the Full Width at Half Maximums(FWHMs) of the tapered devices and the broad-stripe devices are 1.12 and 1.24 nm, respectively. These results indicate that the tapered lasers have better temperature characteristics than the broad ones with the same epitaxial structure under certain temperature conditions.
It is necessary to confirm the system parameters by theoretical analysis and experiments for designing a optimum fiber-coupling system. By approximating the mode fields of a Single Mode Fiber(SMF) with a step index and a Photonic Crystal Fiber(PCF) with a Large Mode Area(LMA) using the Gaussian mode field, two analytical expressions for fiber-coupling efficiency in an ideal case and in the presences of lateral offset and fiber-end tilt are derived. Based on the two theoretical expressions, the efficiency for free-space laser coupling into the fibers is computed. Moreover, the validity of the two expressions are verified by experiments. As a result, the theoretical computation and experiments demonstrate that the SMF with the step index is more sensitive to the lateral offset compared to the PCF. When the offset is equal to the radius of the fiber core, the efficiency is only 20.25%, which is a quarter of the theoretical maximum efficiency. Besides, the PCF with the LMA is more sensitive to the fiber-end tilt. When the tilt angle is 2, the efficiency is only 40.5%, which is a half of the theoretical maximum efficiency. In conclusion, the proposed theoretical expressions and experiments can provide accurate parameters for designing fiber-coupling systems.
The paper presents the physics and the math models of laser ablation for fiber-reinforced composite materials in consideration of the physical processes including pyrogenation, oxidation, radiation, phase transition, both inner and outer convections. By taking carbon/epoxy composites for examples, the laser ablation process is simulated, and the results agree well with the experimental results. The numerical results indicate that the inner convection plays an important role in the ablation process; the oxidation effect can be ignored during high intense laser irradiation; the ablation mass is nearly linear to laser irradiation time when the laser power density is a constant and the ablation efficiency increases with increasing power density. The parameter sensitivity is analyzed with the loss mass and the rear surface temperature at the end of laser irradiation as the object. The analysis results show that the thermal capacity and thermal conductivity have more influences on the bear surface temperature; the epoxy volume fraction has much influence on ablation mass, but its relative sensitivity decreases with increasing laser power density. Moreover, the radiation coefficient is an important factor when the laser power density is more than 1 kW/cm2, but its relative sensitivity decreases with increasing laser power density.
An optical system of portable fundus angiography in dual-bands is designed based on the Gullstrand-Le Grand eye model. By considering the human vision system and applying the coaxial illumination, the apparatus with 2 mega pixels in all fields of view is designed, which can be used in Fundus Fluorescein Angiography(FFA) and Indocyanine Green Angiography(ICGA) under the wave bands of 525 nm and 826 nm. An annular diaphragm, a hollow mirror and a coaxial illumination are adopted in the illuminating system to avoid the stray light caused by the reflection of the corneal surface. Furthermore, two aspheric surfaces are involved in order to simplify the photographic system. The result shows that with a larger accommodation, this system can adapts to human eyes from -10 m-1 to +8 m-1. With the field of view of 30, the resolution of the eyeground over 108 lp/mm, and the distortion value less than 10% the system can satisfy the requirements as a portable dual-band imaging instrument.
In order to improve the imaging quality of coronagraphs, the ghost image caused by multi-reflection light from an objective lens for the internally occulting refractive ground-based coronagraph is analyzed at a band from 530 nm to 555 nm. The coronagraph with an aperture of 120 nm shows its main specifications in a field of view of 1.1 to 3R⊙, a resolution of 13.5 m, and F number of 8.2. The paper analyzes its imaging theory and establishes a model for the ghost image. Then, it presents the structure masking measures and performs an experimental demonstration. The experiment result shows that the masking structure is effective on the elimination of ghost image. In addition, the intensity of ghost image and the intensity of diffracted light of objective lens edge are basically the same. The experiment also illuminates that the size of ghost image is the same as the simulation result of 0.9 mm.The ghost image suppressing further improves the capability of elimination of stray light and the image quality of the coronagraph achieves the effective observation of the solar corona.
A thermal simulation was established according to sensor parameters and the mission requirements to accomplish the thermal design of a lightweight space remote sensor. Atomic oxygen resistant cloth was chosen as the outmost material to reduce the damage by the space environment approximating to the earth. For some satellite devices installed on the back frame of the remote sensor, the connections with high heat resistance were designed and heat insulators were used to eliminate the heat influence on the devices. The positions and powers of the heaters were distributed according to the remote sensor's structure characters. However, none radiator was set because of such small power and duty factor of the CCD components. Finally, the thermal design was certified by a thermal balance test. Three cases designed according to the orbit parameters and attitudes of the remote sensor were simulated and tested. The experiments show that the temperatures of frames and mirrors are (184) ℃ and (182) ℃, respectively, and the temperatures of the CCD components are lower than 30 ℃. The simulation analysis and the thermal balance test results both indicate that the thermal design is valid and content to the mission requirements.
To realize the high accuracy measurement of optical surfaces by a phase-shifting Fizeau interferometer, a synchronous acquisition phase-shifting system in the interferometer was established, and an accurate phase shifting method was researched. First, the composition and working principle of the phase-shifting system were introduced. Then, the velocity of PZT phase shifter during measurement was calculated. Since there exist defocus errors as well as speed-up and speed-down steps during phase shift, the moving process of phase shifter was detailedly designed. Finally, the phase shifter was calibrated. A verified experiment was carried out on the modified interferometer, and it shows that the RMS simple repeatability is /11 340. Comparative measurement was also carried out with two same components between the modified interferometer and the Verifire XP/D interferometer(Zygo company), and the RMS difference is about 0.9 nm. It can be seen that the repeatability and accuracy of this phase-shifting system and phase-shifting method have satisfied the requirement of nano-scale form measurement, which can provide references for developing high accuracy phase-shifting interferometers.
In order to solve the difficulty of detecting three-dimension wind field oblique upper of a scanning wind lidar, a method of wind field inversion was provided based on multi-point regular acquiring, four-beam packet calculating and multi-area merging. To verify the validity of this method, a target with controllable velocity was studied and synthetic velocity was compared with the velocity in control. The result indicates that the method is feasible and effective. The accuracy test was designed to calculate the actual wind field within 400 m. Measurement data from lidar were compared with that from three-axis anemometer. The results indicate that the wind direction accuracy is better than 5, and the wind speed accuracy is better than 2 m/s, which is enough to describe the wind field of detecting area accurately.
An optical method for the nanogap measurement beyond the optical diffraction limit was reported. The function relationship between the nanogap width and the phase difference was built. Based on the high sensitivity of a surface plasmon resonance(SPR) sensor, the nanogap width between a light transmitting mirror and a reflector could be measured by detecting the phase difference between p- and s- polarizations of the light. Numerical simulation shows that the phase difference shifts from 150 to -150 by changing the nanogap width from -0.5 to 0.5 m and the measured sensitivity of nanogap width is better than 1 nm. This nanogap measurement method can realize the measurement for the smallest gap below 10 nm and provides a simple and real-time operation beyond the physical diffraction limit.
A peak value hold circuit is developed for the energy measurement of a high-repetition-rate pulsed laser. The circuit consisting of four modules including a charge integrator, a 2nd-order Butterworth low-pass filter, a time-delay trigger and a peak-value-holder is designed to convert the photocurrent pulses to voltage pulses which are linear to the energies of incident pulse laser. The experimental measurement results indicate that this circuit is suitable for the pulsed laser with a width less than 10 ns and a repetition rate over 2 kHz. The results show that the circuit has a dynamic range about 140 times and works stably. It can be used to achieve a high spatial resolution in photodiode measurement systems.