2013 Vol. 42, No. 5
2013, 42(5): 1113-1116.
The IR spectrum from CO2 in hypersonic flow fields along stagnation line was studied; the influence on total IR radiation from hypersonic target by CO2 was analyzed. Using CFD method, the physical conditions of near-space hypersonic flow fields along stagnation line were calculated for sphere target. According to the flow fields' calculation, the spectrum of CO2 in hypersonic flow fields in different flying conditions was calculated using Line-By-Line method. And the radiation from CO2 was also estimated for ablating condition. According to the theoretical calculation, the IR radiation from CO2 in hypersonic flow fields along stagnation line without ablation is far less than the IR radiations from solid craft self, but in ablating condition, CO2 in shock can not be ignored.
2013, 42(5): 1117-1122.
Infrared scene project (IRSP) is a key technology for infrared imaging hardware in the loop (HWIL) simulation. Real-time quality is very important for IRSP system. A design of IRSP system was presented. The IRSP system was composed of infrared image generation subsystem and infrared scene transformation subsystem. The two subsystems were based on general GPU and resistor array device respectively. Reflective memory network was used for image data transfer and frame synchronization to meet the demand of image size and frame rate for HWIL simulation. The real-time quality was evaluated by testing IR image frame period. The result indicates that significant errors occur unpredictably under Windows system and the frame period is unstable. A new real-time design was introduced based on RTX extension, and the detailed scheme was presented. Test results of the real-time design indicate that the reason for significant frame period error is the unpredictable interrupt response delay under Windows system and the real-time design under RTX extension can ensure a stable frame rate of 200 Hz under the control of frame synchronization.
2013, 42(5): 1123-1130.
Ultrasonic lock-in thermography is a nondestructive testing method, which combines ultrasonic excitation and infrared lock-in thermography. The detection of contact interface defects of sheet metal was studied in this paper using ultrasonic lock-in thermography, and the selective heating advantage of the ultrasonic excitation was analyzed. The finite element simulation model of heat transfer in sheet metal was discussed, and the effect of ultrasonic excitation parameters on the detecting results was illustrated. Experiments were performed on prefabricated defects of sheet metal to validate the built simulation model. The result shows that amplitude decreases and phase is invariant when location of loading departs from crack, amplitude increases and phase is invariant when ultrasonic excitation amplitude increases, amplitude decreases and phase increases when modulated frequency increases. The built finite model accords with the experimental results. Ultrasonic lock-in thermography is fit for test surface defects of sheet metal, and provides guidance for ultrasonic lock-in thermography testing.
2013, 42(5): 1131-1136.
According to the technical traits of Network Centric Airborne Defense Element, an aim point selecting algorithm based on sparse pixel vectorization was presented. On the basis of summarizing the structural and infrared characters of ballistic missile during the boost and ascending phase, the target's plume was segmented by extending the global histogram threshold method to a dual threshold method with the histograms for different areas, then the sparse pixel vectorization was used to find the center axis of the target's infrared image, and the aim point was found in two ways according to the proportion between the skin and the plume, finally the aim point was confirmed with the consistency of direction and distance in the subsequent frames. Experiments show that the algorithm presented here is able to realize the aim point selecting for infrared imaging endgame guidance fast and adaptively without manual work or traversing all the pixels.
2013, 42(5): 1137-1140.
The temperature of space object is influenced by the underlying surface of the earth. In the thermal analysis of the space object, the underlying surface of the earth is regarded as uniform thermal radiation surface. Affected by the landmark and meteorological conditions, the reflectivity and radiation flux intensity changes with the location and the season. The simulation method of the radiant heat flux of the earth was given. Based on the data of the telemetric temperature and different land cover types of the earth, using different simulation method of the radiant heat flux, the influence of the non-uniform underlying surface on the temperature of space object was analyzed. The influence to the light decoys was a little obvious.
2013, 42(5): 1141-1145.
This paper describes the study of very long wavelength infrared(VLWIR, c14 m) HgCdTe detectors, used for the remote atmosphere sounding. The very long wavelength band is particularly rich in information about humidity and CO2 levels and provides additional information about cloud structure and temperature distribution across the atmosphere. According to the results, it is shown that the VLWIR HgCdTe detectors have abnormal current-voltage phenomenon at a low temperature, which may be the results of a parasite pn junction. Besides, through data analysis and curves fitting, it is shown that the surface current of the VLWIR HgCdTe diodes is nearly comparable with the bulk current, which reflects that the surface effect is large in the VLWIR HgCdTe diodes. Through the results, it proves that the passivation of diodes is very important.
2013, 42(5): 1146-1152.
The system of polarized thermal imaging is more suitable to anti-camouflage than the classic thermal imager. If a polarizer inserts in front of a thermal imaging system, the system's operating range will be changed which had mainly studied in the paper in detail. Firstly, the NETD, MRTD and operating range of a polarized thermal imaging system were modeled from first principles of infrared physics. Secondly, the coefficient of the operating range gain of the polarized system compared with that unpolarized one was deduced. Lastly, the polarization and radiation contrast between a camouflage tank and grass and the operation range gain of the polarized thermal imaging system were calculated and analyzed. The results show that the polarization contrast is much larger than the radiation contrast. And the operating range of the polarized thermal imaging system is also larger than that of the unpolarized one when detecting a camouflage tank, which reveals the polarized one's better performance.
2013, 42(5): 1275-1279.
Tracking accuracy and other technical criteria increasing leads to improving of alignment and detecting technology in optical system photoelectric theodolite. A new method of alignment and detecting for photoelectric theodolite main mirror, including assembling, adjustment, coating, axis detection and checking was introduced in this paper. Then, ZYGO interferometer was used in main mirror detecting, this method toke the place of traditional detecting method. Ultimately, ZYGO interferometer technology got clear image both from aviation aircraft 70-80 km away and target in the distant of 5 000 m according to the field experiments. The main mirror meets accuracy of designing and alignment. So the application of interferometer in photoelectric theodolite primary mirror enhances the alignment accuracy.
2013, 42(5): 1280-1284.
It's difficult for both a single pixel scanning detector and a pixel array detector to test minutely in infrared region. An infrared laser receptor was adopted to increase the spatial resolution of infrared laser detection. The receptor consisted of a high infrared sensitive material Carbon Nanotube film which could convert infrared light into visible light. A refracting reflecting optical system was designed to detect the visible light converted by the receptor which can increase the spatial resolution compared with the infrared system. The Cooke Triplet lenses were used to reduce the system field curvature. The refracting reflecting optical system was finally optimized to have high value of MTF and low field curvature, which make the high resolution infrared detection possible.
2013, 42(5): 1285-1290.
The precise testing of large aperture light weighted reflecting mirror in gravity environment is a difficult problem at present. In view of the high rate of light weighting and the absolute low stiffness, in this situation, different forms of support have huge impacts on optical test results in gravity environment. In this paper, a solution was suggested that pre-defining of supporting force was achieved through a series of pulley blocks self-adapting to solve large aperture mirror testing in gravity environment. The result of mechanics model analysis shows that the testing of optical axis horizontal can reduce the Rams variation of surface shape effectively, which is caused by gravity in vertical and horizontal state, and the variation less than 0.003 (=632.8 nm). The method can be extended and applied to the design of testing supporting for other large aperture light weighted reflect mirror, and provid; technology foundation for large aperture mirror.
2013, 42(5): 1291-1297.
The space solar telescope(SST) is designed to obtain its diffraction limit quality with aperture over one meter. It observes the sun with a small view field of 2.8'1.5' to obtain its high spatial resolution imaging of 0.1-0.15. SST observes the sun directly can receive huge heat flow more than 1 000 W that will lead to unacceptable thermal distortion of the optical components. The sunlight enters into the telescope, which is an intense source of both heat and stray light. Based on the special thermal effect and stray light in the solar telescope, a compatibility analysis of the thermal effect of vanes in SST was performed. In the compatibility analysis, the geometric parameter of vane structure became a key point which would affect the geometry composing function(GCF) in stray light analysis and affect the radiation shape factor in thermal analysis synchronously. This paper presented the relationship between the thermal control design and scatting elimination plan on the vane structures in SST. The objective and method of the compatibility analysis were determined. With the thermal analysis software, the temperature fields were calculated for different geometric parameters of vane structure including depth, separation, and angle. A design of the vane structure was put forward in thermal control terms and the suggestion was put out synchronously, which restrains the vanes with less depth, more separation to the primary mirror and angled at 90. The aims of the optimal design of the vane structure of SST were studed reached. The thoughts and methods of the optimal analysis are also useful for similar optical telescopes designed for solar observation.
2013, 42(5): 1298-1301.
In order to optimize the system's total performance, aiming at missile tracking system, a kind of infrared optical system with the principle of conformal optics was designed in this paper. Firstly, according to the missile's flying information, through numerical simulation computation and relative wind tunnel experiments, the outer figure of the conformal dome was gotten. By optimizing the optical path differences, the inside figure of the conformal dome was obtained. By using the Wassermann-wolf equations and the Zernike aberration analyzing method, the fixed two pieces correctors were designed for correcting the aberration induced by conformal dome. The detector used in this system was a F=2 mid-wave infrared detector made by Sofradir in France. The imaging optical system was designed in refraction/reflect structure, then by using software ZEMAX the final system was gotten. Finally, the tolerance analysis was done. The focal length of this conformal optical system was 120 mm, the cold diaphragm efficiency was 100%, the MTF(modulation transfer function) of all fields are all above 0.6. Even after the tolerance analysis, the MFT value was also above 0.4 which is good enough for manufacture. The results of imaging tests show that this kind of conformal optical system is good in imaging.
2013, 42(5): 1302-1305.
The polarization effect is inevitable when light-splitting film is titled. Any effective measures were usually taken to reduce the influence of polarization effect, but only single wavelength or very narrow spectral range can realize non-polarizing, the spectral range was almost no more than 50 nm. An initial formula was designed by resolution method according to the design target, Then, the incidence of angle with 45 light-splitting film from 1260 nm to 1360 nm spectral range was designed by resolution, needle and variable metric optimization methods on the designed initial formula by using TFCalc formula design software. The monitoring transmittance curve of designed formula was analysed to decide whether it could be deposited. The formula was deposited by the type APS1104 deposition machine made in Germany, light-splitting film with good performance was acquired.
2013, 42(5): 1306-1310.
The solar simulator as a crucial equipment is used in simulating the space environment for satellites, testing and calibrating for solar cells. The air mass filter as the crucial component can filtrate the lamp for getting the proper solar spectrum. Through the air mass0(AM0) filter the solar spectrum on the surface of aerosphere could be obtained. The transmittance of AM0 filter could be obtained through the standard AM0 spectrum and xenon lamp spectrum. Then, the design and fabrication of the filter was done. The filter gets the A level standard. At last, the filter was dealt with ultraviolet and high temperature for studying the optical stability. The results show the new filter is better than the traditional filter in optical stability.
2013, 42(5): 1311-1315.
Image motion model on star sensor is an important component of dynamic simulation algorithm of stars, and it lays the foundation for motion blur modeling simulation of high resolution star sensor. Firstly, with the help of the normal distribution of static star image energy model, introducing the star image motion speed, the energy distribution function of moving stars was obtained. Secondly, in order to obtain the star image motion speed, two solution methods were studied: (1) star image motion model based on the coordinate transformation, (2) image motion model based on the stellar rotation vector. Finally, the simulation environment was established. Through adjusting input parameters such as integral time, rate of change of three attitude angle, the launch time, location, important simulation data of stars observed by star sensor in orbit could quickly be obtained, such as navigation stars information, value and direction of image motion, intensity distribution, signal to noise ratio. This work is very important to study and evaluate the star image motion compensation algorithm.
2013, 42(5): 1316-1321.
In order to improve the disturbance rejection of seeker stabilized platform, the servo control system was designed. The disturbance rejection model was proposed based on inertia space and the main factors that influence the disturbance rejection were shown. Then, the friction model was established based on the characteristic of friction and the friction adaptive compensation algorithm was proposed. The velocity controller was designed based on the pole-placement method while the lead-lag compensator was used as position controller. Simulation result indicated that the disturbance rejection of pitch frame improved 97.14 percent while the missile disturbance was 1 and 2 Hz. Experimental results showed that the disturbance rejection of two frame were 1.58%、1.81% and 2.84%、2.15% while the missile disturbance were 1 and 2 Hz and 3 and 3 Hz. The disturbance rejection of seeker stabilized platform was improved by using friction adaptive compensation.
2013, 42(5): 1322-1329.
The extended trajectory shaping guidance law was deduced based on the time-to-go weighted cost function. For the extended trajectory shaping guidance systems with one single lag seeker and autopilot dynamics, the non-dimensional position and angle adjoint models were derived using the non-dimensional technique and adjoint method when the seeker angular and angle zero position errors, angular noise and target glint noise was introduced into the guidance system. The simulation results show that in order to reduce the miss distance introduced by seeker angular zero position error, a large exponent n and long guidance time is necessary and when the missile terminal guidance time is about 15 times of the system total lag time, the position miss distance introduced by seeker angle zero position error approaches to zero while the angle miss distance approaches to an opposite value of the angle zero position error. With the increasing of the terminal guidance time, the position and angle miss of the seeker angular noise and target glint noise approaches to a nonzero stable value and the miss distance is increasing when the exponential number n increases. Finally, it will be seen that the miss distance introduced by angular noise is further increased with a slow guidance dynamics and with the increasing of the filter bandwidth, the miss distance introduced by glint noise is also increased.
2013, 42(5): 1330-1337.
Sub-sampling technique provides a way of fulfilling fully digital measurement and simplifying the electronic system. In order to reduce the interference brought by sub-sampling and improve traditional sub-sampling digital synchronous demodulation method, the sub-sampling spectrum analysis method was introduced. This method was based on spectrum analysis method under Nyquist sampling, obtained the phase information from low frequency signal after sub-sampling and fulfilled a better performance of anti-jamming. Furthermore, measurement performances under the influence of various interferences were compared between these two methods based on simulation. Data shows that the accuracy of sub-sampling spectrum analysis method was 0.13(0.68 mm at 80 MHz modulation frequency) with 1 MHz measuring speed, under the condition of SNR 40 dB, spurious frequency, frequency deviation 0.1 MHz and second harmonic, this method could performance better than the traditional one. Consequently, sub-sampling spectrum analysis method is applicable to high speed and high accuracy fully digital phase-shift measurement.
2013, 42(5): 1338-1344.
To improve the accuracy of droppoint measurement system when airborneweapon attacks over the ground, this article presented utilizing the intersection measurement method to measure the instantaneous space position of target, and completed the function of pick up all dynamic target in image order by employing multiple target dynamic measurement method. The function was realized what dynamic target collection and position measurement in image which contained complex background, and could improve the accuracy and rapidity of dynamic target measurement system. Experiments testify that the system can pick up the laser target exactly. The measurement accuracy of misses of image is better than 1 pixel when detecting the result error of experiment and true-value. The distinguishability of misses of image is 0.5 pixel.
2013, 42(5): 1345-1349.
In order to solve the problem that how to measure solar radiation measure instrument, a kind of high precision meteorological radiation calibration system was put forward. The optimization technique of focusing system and optical integrator was described; integrator aberration which influenced the uniformity of sun simulator in detail and defocusing effect of projector lens were discussed. The optimal defocus distance formula was put forward at the same time. The collimating optical system and simulated the system in LightTools were designed. Finally, solar simulator was tested. The results show that it can simulate one solar constant with irradiation range of 200 mm, working distance of 1 000 mm. Irradiation nonuniformity in 60 mm range is less than 1%, and in(60-200)mm is less than 2%, instability is better than 1%/h, which meets the requirements of meteorological radiation calibration system.
2013, 42(5): 1350-1354.
In the routine infrared inspection signal processing, pulse phase thermography doesn't take into account the measure noise, the experimental data isn't made good use of by the simple Fourier transform, where more information needs to be dig. The non-Gaussianity analysis was introduced to the routine signal processing, the third-order cumulant and bispectrum estimation were used for extracting the characteristic information from the detection signal when damage occurred, therefore the criterion of distinguishing sound area from defective area was obtained. The definitions of third-order cumulants and bispectrum were achieved for the acquired signal, in addition, the advantage using magnitude and phase of slice spectrum was analyzed and the effects were compared in actual detection. The proposed method has the advantages of suppressing Gaussian and symmetry distribution non-Gaussian noise efficiently, small calculation amount, fast computation speed. Moreover, it compensates the demerit which magnitude of spectrum is hard to use in the traditional pulsed phase thermography.
2013, 42(5): 1355-1359.
With regard to the large number of irrelevant and redundant features exist in the near infrared spectra, a novel feature selection method based on random forest and principal component analysis (RF-PCA) was proposed in this paper. By using the RF-PCA, a classification model of cigarettes qualitative evaluation was developed and also compared with other methods. The result shows that RF-PCA effectively classifies the samples of high dimensional data and can be used to evaluate quality and authenticity of the cigarettes. RF feature selection removes irrelevant features of the classification, while PCA further eliminates the influence of redundant features and also reduces the feature dimensionalities. The experiments show that RF-PCA effectively removes noise and redundant features in the NIR spectra and the classification accuracy is improved as well.
2013, 42(5): 1360-1365.
The aim of this paper is to study the practicability of the general and objective fusion image quality assessment method. Based on the analysis of the relationship between image quality assessment and fusion image quality assessment, the general expression of fusion image quality assessment was provided. Four fusion image evaluating metrics were presented based on information theory and structural similarity. Experiments were performed for 36 fusion images generated by 4 fusion algorithms fusing visible and infrared images. Experimental result shows that the objective evaluating method bonding to the human vision system is better than the metrics such as entropy and mutual information. However, the experiment result also show the assessment result is unsatisfied to the concordance subjective and objective assessing result, which explain that it is very difficult to build the fusion image evaluating metrics of the general, efficient and good consistency between subjective and objective result. Furthermore, the possible reason for performance assessment is discussed in the end.
2013, 42(5): 1366-1371.
A novel wedge-ring poor-pixels photoelectric detector is valuable for a micro vision system. However an image acquired by the detector has extremely low resolution and it does not reflect the same or similar shape information of an object in the real world. To enable such a detector and its images available in further object identification, a unique shape recovery framework was presented in this paper. By rotating the wedge-ring detector around its center in a sub-wedge range, original low-resolution images were generated. Then linear interpolation along with a least squares method was applied to preliminarily recover the object shape. After noise removal via a two-stage level set evolution with an edge indicator function, the final high-quality object shape was achieved. Experiments demonstrate the effective performance of the proposed algorithms, in which the shape recovery rate is up to 95%.
2013, 42(5): 1372-1381.
The demand of tracking accuracy is very high in the boost-phase ballistic missile interception, how to fuse the data of many kinds of boost-phase detection equipments to get more precise data has become an urgent problem. To solve this problem, a new fusion tracking algorithm based on missile early warning satellites and radar was studied, a fusion tracking algorithm of boost-phase based on Bayes theory was proposed. The detection model and tracing model of missile early warning satellite and radar for boost-phase were established, the accuracy of this fusion algorithm was improved by the help of POFACETS' RCS simulation of one type of ballistic missile. Simulation results show that the result is more accurate than by using arithmetic mean on the limited sensors, this algorithm has high reliability.
2013, 42(5): 1382-1387.
A new algorithm of minimum enclosing rectangle based on the center-of-gravity principle, searching main axis method, was proposed. Using the center-of-gravity principle the initial positions and the rotating center of the horizontal axis and the vertical axis were obtained. The initial enclosing rectangle was determined by means of the horizontal axis. In the acute angle region between the horizontal axis and the vertical axis the enclosing rectangle was rotated until the enclosing rectangle of minimum area was found. This enclosing rectangle was just the optimization goals. The initial position of the main axis determined with the center-of-gravity principle was so efficient that region of search was shrunk and rotation count was reduced in the whole optimizing procedure. And since only the main axis which could be expressed by linear equation was rotated, calculating complexity was decreased remarkably and computation speed was increased effectively. Minimum enclosing rectangle of the object can be quickly found.
2013, 42(5): 1388-1394.
Point set matching is one of the classical NP problems in computer vision and pattern recognition. Membrane computing is an emergent branch of natural computing, which aims to abstract innovative computing models or computing ideas from the structure and function of a single cell or from complexes of cells, such as tissues and organs. On the basis of membrane optimization algorithms with hierarchical structure and the feature of the point set matching problem, a novel point set matching algorithm was proposed. In this algorithm, three new heuristic search rules were introduced, by which matching rate increased to some extent. Compared to the traditional optimization algorithms, the algorithm exhibited a better global search capability, thus a better solution for point set matching problem was obtained. Experimental results illustrate that the proposed algorithm is effective on both matching rate and stability.
2013, 42(5): 1153-1160.
Based on the analysis of thermo-elastic-plastic effect, the surface temperature of Q235 steel during tensile tests was measured by using an infrared camera. Both the surface temperature field and it versus time for different strain rate were obtained. The true stress and true strain curve was determined, and this curve was used as the constitutive equations of Q235 steel. A numerical procedure was devised to model the thermoplastic effect during the tensile tests by using ANSYS software, and the influence of the strain rate, coefficient and heat-transfer coefficient on the surface temperature were studied. The results show that the heat loss during deformation process will be smaller as the strain rate increase, and the temperature increase on the specimen surface generated by the plastic deformation will be higher. The temperature rise at the middle of specimen near the necking area is maximum, while it declines towards the end of specimen from the necking area. It is concluded that the larger and more concentrative plastic deformation of specimen happens at the same time, the much higher temperature rise can be obtained. The simulation results show that it is a good way to analyse the thermoplastic effect by the commercial finite element software. Smaller as the strain rate increase, and the temperature increase on the specimen surface generated by the plastic deformation will be higher. The temperature rise at the middle of specimen near the necking area was maximum, while it declined towards the end of specimen from the necking area. It is concluded that the larger and more concentrative plastic deformation of specimen happened at the same time, the much higher temperature rise could be obtained. The simulation results matched well with the experiment results showed that it was a good way to analyse the thermoplastic effect by the commercial finite element software.
2013, 42(5): 1161-1165.
The laser beam is the most precise and popular instrument to measure the smoke permeate rate. But in most field testing cases, the scene of laser beam light is too small for the smoke field testing and the synchronization problem is also unavoidable when it works at scanning way. In order to insure the measurement precision for the disturbance effect of smoke to the small scene imaging, it introduces the design of lasers array with multi small power same semiconductor laser after beam homogeneous enlarge shaping. The relative variety rate of room permeating rate can be measured by the imaging sensor. It can measure 2.8 m2.8 m smoke at 113.4 m distance with 33 beam homogenous enlarge shaping laser array during the principle test. The scene of CCD is 1010and the error of relative variety rate is less than 0.5% when grey level is no more than 230.
Continuous wave chemical oxygen-iodine laser can be turned into pulsed operation mode to obtain higher peak power by means of gas discharge. An electrode system with the assistance of surface sliding pre-ionization was proposed to solve the problem of the stable glow discharge with large aperture. The electrode system was applied in pulsed COIL. Laser energy up to 4.4 J with a pulse duration of 58 s was obtained and pulse peak power was 75 kW. The ratio of pulse peak power to continue power was 63. The influence of P and output coupling mirror on laser characteristics was investigated.
2013, 42(5): 1170-1173.
Compared with Nd:YAG, the Yb:YAG laser system is more suitable to produce at several Hz repetition rates high pulsed energy laser beam. A LD pumped Yb:YAG solid state pulsed MOPA laser system at cryogenic temperature was reported. The amplifier chain was composed of three parts: regenerative amplifier seed, pre-amplifier and main amplifier. The thermal management was realized by liquid nitrogen cooling sink on which laser crystal was compressed using indium and gold foil as thermal interface. The coating film damage at cryogenic and vacuum environment was studied, which directly affected the pumped and extracted parameter design. A 1 030 nm pulse 10 Hz, 10 ns laser output energy 3 J was achieved. At last, in order to improve laser energy scalability the further step was analyzed.
2013, 42(5): 1174-1178.
Direct pumping technique of micro-modularization diode side-pumped solid-state laser(DPL) was studied. The pump distribution of laser diode in gain medium was analyzed. The results show that the distribution of the gain relates to the parameter such as the waist radius of the emitter, the number of the bar around, the distance between pump sources and crystal surface, the radius of the crystal rod and the absorption coefficient of the crystal. Through adjusting the above pump parameters the pump distribution can be realized which has the character of strong center and weak edge. An experimental equipment of laser diode symmetrically side-pumped Nd:YAG laser was also designed, and the miniaturization of diode pumped solid state laser was achieved.
2013, 42(5): 1179-1183.
The principle of coherent wind lidar and digital filter banks were introduced briefly at first, and then the theory of cosine modulated filter banks was analyzed. Cosine modulated filter banks have excellent characteristics such as simple realization and less resources occupying. Signal can be decomposed into several narrow sub-band by using cosine modulated filter banks. The Doppler frequency of the target will fall into one of sub-band and the higher SNR can be obtained by processing each sub-band. Based on this method, a 8-channel cosine modulated filter banks were designed with Matlab, which was used in coherent wind lidar signal processing simulations. The results of the simulation and measured signal prove the correctness of the method.
According to the structure of IR detector and the theory of thermal conduction, thermal damage of IR detector irradiated by optical maser of laser jammer was investigated. InSb photovoltaic detector was selected as a research detector. The Thermal Model was developed based on the structure of InSb detector. In this model, the detector was divided into three layers which included InSb chip, glue and copper substrate. The thermal conductivity functions of each layer were different due to various material's conduction coefficient values. The distribution in space of incident laser beam was Gaussian profile, supposing the incident laser irradiated on the top layer, and set the initial value parameters and boundary conditions. The initial temperature of the detector was 77 K and the boundary conditions were Neumann type. The multi-layer thermal models for laser damage were solved by PDE-tool. The plot of temperature rise was acquired by specifying a group of typical laser parameters through simulation. Key factors consisting of the power of optical maser, the irradiated time and the distance between the infrared detectors and optical maser were deeply researched. Through Orthogonal experimental design, a series of contrastive figures were obtained. The thermal damage effects were analyzed on basis of these data. The simulation results confirm that the detector absorbs the energy of the laser and the temperature rise centralized on InSb materials and the glue. Since the glue's conduction coefficient value is low, the glue is poor at thermal transfer, so the temperature drops dramatically in the glue layer. When the power of continuous laser is 50 W, the irradiated distance is 200 m, and the irradiated time is 3 s, the infrared detector suffers soft damage. The power of density continuous laser is lower than the pulsed laser, as a result, the optical maser doesn't bring about long-lasting destructive effects.
2013, 42(5): 1189-1193.
Semiconductor laser with narrow linewidth and high frequency stability has been widely used in precision interference measurements, optical frequency standards, laser communication, optical gyroscope, laser radar, measurements of fundamental physics constants and cold atomic system, et. The frequency drift of a free running semiconductor laser may be up to several GHz per day. Therefore, the research on frequency stabilization of semiconductor laser is extremely necessary. A variety of frequency stabilization techniques was developed. As an example of 780 nm semiconductor laser, the principles and experimental realizations of frequency stabilization were introduced, such as Doppler-free saturated absorption spectroscopy, Doppler-free Dichroic spectroscopy, modulation transfer spectroscopy, frequency modulation spectroscopy and frequency-to-voltage conversion technology, et al. Meanwhile, the performance and characteristics of different frequency stabilization methods were analyzed.
2013, 42(5): 1194-1197.
The DF/HF chemical laser needs the pressure recovery system with huge volume to work successfully. In order to optimize the design of the laser and the pressure recovery system, the flow field and total pressure losing of two types nozzles(standard nozzle and thin neck nozzle) in DF/HF chemical lasers was studied by numerical simulation. The difference of their total pressure losing and interior flow field were compared. Then the influence of the difference on the entire laser system performance was discussed. The computer result shows that the total pressure losing of the thin neck nozzle was as same as the standard nozzle. So, using the thin neck nozzle was benefit to diminishing the volume of gain generator of the DF/HF chemical laser. And it did not take more berden to the pressure recovery system. So, the pressure recovery system at the end of the laser will not have to be enlarged.
2013, 42(5): 1198-1203.
The theory model which studied the interaction between optical activity and elastic-optical effect of crystal quartz in the differential laser gyro was presented. The solving method and steps of the model were proposed. Firstly, the stress-induced transformation matrix between the new and old principal coordinate systems, inverse dielectric tensor and gyrotropic dielectric tensor in the new principal coordinate system were calculated. Then applying the transformation matrix between the new principal coordinate system and laboratory coordinate system, the inverse dielectric tensor and gyrotropic dielectric tensor in the laboratory coordinate system was obtained. According to the normal mode equation and material equation, the stress-induced normal modes and corresponding refractive index along the direction of wave vector were analyzed. Through the calculation example, the change of latent model's ellipticity and refractive index as the function of stress for different directions of wave vector was investigated in detail. This model offers a means to analyze the mechanism of production of differential laser gyro's thermal bias, which had some degree of guiding significance in installation of quartz crystal.
2013, 42(5): 1204-1207.
In this paper, the experiment on damage in K9 glass induced by pulsed CO2 laser under different repetition rates was carried out, which had a pulse width of 90 ns. The laser pulse energy was 10 J and the repetition rate was kept within the range of 100 Hz to 300 Hz. The damage morphologies of two kind repetition rates after laser irradiation were characterized. The experimental results indicate that the effect of laser irradiation on samples can be affected considerably by the change of laser repetition rate, and the intensity of damage morphology on the sample increases with the laser repetition rate, and the damage in K9 glass induced by pulsed CO2 laser is dominated by stress. As a result, the plasma detonation wave induced by laser occured, the material was broken result from the melting and evaporation of K9 glass. It is shown that the plasma detonation wave affected stress damage considerably, and this mechanical effect almost destroyed K9 glass sample. A numerical simulation was performed to calculate temperature and stress distributions in K9 glass sample irradiated by pulsed CO2 laser using finite element method. The model prediction was in line with the experiment data.
2013, 42(5): 1208-1211.
This paper is focused on the experiment of light back scattering characteristics for existing roughness blocks and other typical rough targets. The experimental results show that, when the wavelength of incident light is 632.8 nm, an obvious change of scattering intensity for different targets can be observed as incident angle gradually increases. If the root mean square of surface slope for different targets keeps close, as a whole, the scattering intensity will show almost same tendency when it varies and there will be no obstacle difference between the polarization degrees for different targets. Particularly, the target with relative small root mean square of surface slope will show obviously larger scattering intensity as well as polarization degree than other targets. This means that the intensity of back scattering light for the target which has relative smooth surface is much stronger than those which show relative rough surface. The research results in terms of back scattering characteristics for typical rough targets in this paper are useful for further theoretical and experimental studies on the target recognition.
2013, 42(5): 1212-1217.
The refractive index and absorption features of a novel polymer-cyclic olefin copolymers(trade names Topas COC) were studied in 0.1-3 THz band. With SolidWorks software, a common double-convex spherical lens of 100 mm focal length with effective aperture of 1.75 inches was designed, and then a set of mold for manufacturing the lens was designed. By hot compression molding, the Topas COC terahertz lens was fabricated, and the effects of heating temperature and time on the lens surface morphology was analyzed. The effects of the wavelength, thickness, radius of curvature and refractive index on focal length of the lens in the terahertz wavelength of 150 m, 300 m and 600 m was simulated and analyzed by using Zemax software. Studies have shown that the lens made by this method has surface smooth, high accuracy and defect-free structure, so the performance is very superior and meets linear and compact requirements of the terahertz system, and plays the role of collimating and focusing in the visible region and terahertz band.
2013, 42(5): 1218-1222.
The absorption spectra of folic acid and retinoic acid were measured in 0.2-1.8 THz using terahertz time-domain spectroscopy (THz-TDS) system at room temperature, and the refractive index was obtained as well. Experimental results indicate that folic acid and retinoic acid both have distinct absorption features in the THz region, which can be used as fingerprints for the molecular recognition. At the same time, the Gaussian03 was used for isolated-molecule simulation, the density function B3LYP and Gaussian-type basis set 6-31 G(d,p) were chosen to perform all the theoretical simulations. Experimental results were in good agreement with the simulation. Meanwhile, the vibration modes for different absorption peaks of folic acid and retinoic acid were recognized by using Gaussian-View3.0. This work could be as certain references in kind of vitamin molecules recognition and new medicine research.
2013, 42(5): 1223-1227.
The process that Q switched tunable CO2 laser pumps nonlinear GaSe crystal was studied. Based on the collinear phase match type, the phase matching angle, effective nonlinear coefficient, walk-off angle and acceptant angle were calculated. The results are as follow: THz wave between 73.84 and 3 000 m can be generated using DFG based on GaSe crystal which has high effective nonlinear coefficient of 41-54 pm/V. The walk off angle is big, which is between 1.771 and 10.63. With the small acceptant angle 13.95-94.7 mrad, the requirement in directional property is strict. When the THz wavelength is above 500 m, the two phase matching types, oe-e and oe-o, are very similar except the effective nonlinear coefficient. In experiments, it was up to the experiment conditions to decide which type to choose. The simulation result can guide the experiments about DFG in GaSe crystal.
2013, 42(5): 1228-1235.
Terahertz(THz) radiation has many particular characteristics, which make THz image technology become the hot research area. As the distribution information of the cross section and the 3D reconstruction image of a sample can be obtained, THz computed tomography(CT) also attracts wide extensive attention worldwide. The simulation research was done based on continuous THz CT. The filtered back-projection(FBP) method and the modified simultaneous algebraic reconstruction technique(MSART) method have been utilized to reconstruct cross-section images, and the efficiency of the image processing operations such as the Gaussian low-pass filter(GLPF) and the morphological functions in mathematical morphology like open operation and close operation in improving reconstruction image quality has been compared and analyzed. It can be inferred from the simulation results that the required number of the projection directions using the MSART method and the corresponding image processing operations can only be 9, which is inosculate to the real imaging experiment results.
2013, 42(5): 1236-1240.
Porous silicon thin films were prepared by chemical etching method on monocrystalline silicon surface. P-type morphology of these films exhibited three tapes: crater, quasi nano-pillar-array and porous. Terahertz time domain spectroscopy of these samples from 0.2-10 THz showed that transmitted intensity of these samples was very different. Taking air as the reference, transmission intensity of crater sample from 0.2 to 6 THz dropped by about one half and intensity of other two samples only decreased about by one fifth. For filter characteristics, samples of crater one and quasi nano-pillar-array one had low-pass characteristics and porous sample had cascade band-pass characteristic. Cut-off frequency of quasi nano-pillar-array sample improved 3 THz than others. Furthermore, there were much absorption peaks in samples and positions of these peaks had very closed relationship to the micro-surface-structures of porous silicon films. Experiments showed that, both shape and size of these films appeared to change and control transmission properties of monocrystalline silicon in terahertz such as transmitted intensity, absorption frequency and cut-off frequency. So porous silicon can be a new wide spectral detecting and modulation materials from terahertz to visible light.
2013, 42(5): 1241-1246.
The de-noising algorithms applied in passive THz imaging was studied in this paper. An improved mean filtering algorithm was proposed, in which the threshold was introduced based on the traditional mean filtering algorithm. The shape of filter mask was decided by the characteristics of scan images resulting from passive THz imaging, and furthermore the best size of the kernel was determined according to one of the evaluation parameters of the digital images-entropy. The experimental result shows that the novel de-noising algorithm has good effects on passive THz images. It not only preserves the information of imaging targets, but also removes out the background noises. The algorithm enhances the quality of the images and increases the ability to detect the concealed metal objects underneath people's clothing. Therefore, it provides necessary guarantees for the application of passive THz imaging technology in the future security inspection.
2013, 42(5): 1247-1252.
Theory of the fiber vector-sum microwave photonic phase shifter was investigated, and the new-type structure of the phase shifter was designed. Variable directional coupler and polarization-maintaining fiber were used in the structure, which could reduce the complex and the coherent interference, the precision requirement of the fiber length, which could be decreased, and the realizability and the controllable of The Novel Numerical Control fiber vector-sum microwave photonic phase shifter could be also enhanced. The affections on the phase and amplitude of the signals which from the change of division index and frequency were analyzed. The phase shift above 140 was achieved. The numerical control voltage source was designed to adjust the driving voltage of the directional coupler to change the light intensity in the fiber. The numerical control on the phase of the output signal was achieved finally. This microwave photonic phase shifter can be realized easily and has some novelty.
2013, 42(5): 1253-1257.
A research on the influence of oxidation on laser irradiating iron was done. Based on the temperature field result, oxidation layer thickness and the quantity of heat in the course of oxidation were computed. Besides, by using multilayer reflective theory, the laser energy coupling coefficient was got with the varying oxidation layer thickness, and the computed value was in line with the experiment result very well. The result indicates that the oxidation heat had little effect when laser irradiated the iron and it could be ignored in engineering application. But the laser energy coupling coefficient may be influenced clearly by the oxidation behavior.
2013, 42(5): 1258-1264.
In order to solve the problems of polarization zonal distribution of optical state of polarization (SOP) in the output light of wedge-shaped depolarizer, an optical crystal depolarizer with high disorderly distribution of SOP on the cross section of output light was designed and built. The optical crystal depolarizer combined many more crystal slope structures and optical axes to make the polarization change in light plane and to improve depolarizing efficiency. The simulation and experiments showed that the polarization zonal distribution of output light was weaken obviously and the uniformity of depolarization was improved on the transverse plane of light. The more intricately both crystal slope structures and optical axes were combined, the more disorderly the polarization distribution of the depolarizer would be, and the better the depolarization effect would be.
2013, 42(5): 1265-1268.
A practical heat sink fabrication process for low-cost, high-power and millimeter-wave devices was presented. The uniformly doping epitaxial structure(n n+) was grown by molecular beam epitaxy(MBE) on heavily-doped n++ InP substrate. Also a batch-fabrication technique for mesas with gold heat sink was proposed. A technology was developed to form ohmic contacts to indium phosphide Gunn diodes, and the metallization of cathode and anode was fabricated by Ge/Au/Ni/Au evaporation, and was annealed at different temperature. Results show that the best ohmic contact is formed at 450 ℃. The complete fabrication procedure is described to realize the Gunn devices for low-cost millimeter-wave applications. By the way, wet etching could get approximately vertical MESA structure by HCl-based solution.
2013, 42(5): 1269-1274.
TMT(Thirty Meter Telescope) was a Ritchey Chrtien optical-infrared thirty meter telescope with 3.594 m2.568 m ellipse flat one and 1 800 kg weight. The tertiary mirror system reflected the light coming from the secondary mirror(M2S) to the science instruments which mounted on the telescope Nasmyth platforms. It had tracking and slewing functions. Tertiary mirror support system included axial support system and lateral support system. Based on the mirror surface figure error requirements, the paper proposed axial support system used 18 point whiffletree structure with surface figure RMS of 118.5 nm after optimum. The surface figure RMS can reach 4.7 nm by the lateral support, and the surface figure RMS was less than 77 nm by axial and lateral support. Based on this support program, the mirror support structure is designed to satisfy weighted and volume requirements.
