2017 Vol. 38, No. 5

Oe System and Engineering
Study on performance of multi spectral reconnaissance of unmanned aerial vehicle
Cheng Gang, Fang Fan, Ning Fei, Yan Ming, Gao Zedong, Ding Na
2017, 38(5): 685-688. doi: 10.5768/JAO201738.0501001
Spectral imaging detection technology combines imaging technology and spectral technology, which can provide rich information of the target scene. The multi-spectral imaging detection technology is studied in detail, and multi-spectral camera of unmanned aerial vehicle is designed for the camouflage target reconnaissance. The calculation method of multi-spectral reconnaissance detection and recognition is analyzed emphatically. Experimental results show that the efficiency of multi-spectral reconnaissance performance is improved by 20%~50%.
Optimization method of electro-optical imaging systembased on information distortion
Ren Zhibin, Hu Jiasheng, Tang Honglang, Sun Yahui
2017, 38(5): 689-693. doi: 10.5768/JAO201738.0501002
A novel method was presented to optimize the matching of photoelectric imaging system by using information distortion parameters. The problem of spectral aliasing in photoelectric imaging system was analyzed. The object light intensity function was decomposed into a series of non-negative harmonic waves and the image harmonic waves were calculated. Thus, the information models of object, image and electro-optical system were built up. Moreover, the information distortions with different optical system cutoff frequencies were calculated, and the matching relationship between the optical system and the detector based on information distortion was described. Results show that, when the detector pixel width is 10 μm, the Gaussian spectrum should choose the diffraction-limited optical system with the cut-off frequency of 100 mm-1 in order to obtain the minimum distortion 0.144 1.
Azimuth correction of optoelectronic mast based on EO calibrated INS
Liang Qingqian, Liu Haining, Yu Shuai, Zhao Jianzhao, Liu Yu
2017, 38(5): 694-699. doi: 10.5768/JAO201738.0501003
According to the special situation of optoelectronic mast system working in parking state, a method for correcting azimuth of optoelectronic mast based on inertial navigation system (INS) calibrated by optoelectronic system was researched. The optoelectronic system was utilized to regularly aim a static target, and the reference azimuth was got to correct the azimuth error of the INS of optoelectronic system.Then the divergence of the azimuth error of the INS was restrained to improve the azimuth accuracy of the photoelectric system under a long time work. The results of experiment show that, under the situation of 0.06° equivalent azimuth error caused by the mast swing interference when it aiming the target and using STIM300 as the IMU, the INS azimuth error is 0.059° in 60 min using the correction method proposed and 1.14° in 31 min without correction.
Analysis of friction characteristics at low temperature of interiororientation motor assembly in an electro-optical pod
Ning Fei, Zheng Fengzhu, Wang Huilin, Zhao Zhicao, Huo Liye, Wang Peiqun
2017, 38(5): 700-705. doi: 10.5768/JAO201738.0501004
In order to improve product reliability and environmental adaptability, it is necessary to improve friction characteristics at -50℃ of interior orientation motor assembly in an electro-optical pod. Possible factors influencing friction characteristics of motor assembly are analysed, and corresponding improvement measures for these factors are put forward. Effectiveness of the improvement measures are verified with combination of simulation and experiment. Results show that at -50℃, increasing friction torque is both the result of the role of low temperature alone, and the results of low temperature and potential factors of motor assembly together. The micro deformation of motor assembly at low temperature is not sufficient to cause the increasing of friction torque. The friction torque of the motor can reduce to 0.063 N·m by adding a protective cover of the motor and a soft magnetic alloy sheet with a thickness of 1 mm between motor and motor seat. These improvements apply to the design of motor assembly of electro-optical pod and research of similar products.
Analysis and calculation of tracking target based on rotating mirror
Zhang Wenbo, Feng Bin, Wu Yaoxia
2017, 38(5): 706-712. doi: 10.5768/JAO201738.0501005
Based on the rotating mirror ballistic synchronization tracking system of the high-speed CCD camera main axis, the flight attitude and speed of the high-speed flight projectile were studied, the time-space relation model of the rotating mirror moving with the projectile and math model of the mirror motion parameters were established by using the reflection principle and motion law of the rotating mirror, and the mathematics relationship among the rotating mirror motion parameters, the projectile flight speed and the vertical height between the trajectory and the center of rotating mirror was derived.Based on the theoretical analysis, the laws of ballistic width in the field of rotating mirror changing with time, as well as the rules of the rotation angle and angular acceleration of turned rotating mirror changing with time were analyzed by using MATLAB, the field size and the parameter changing curve of rotating mirror at a certain moment were obtained. In addition, the calculating methods for main parameters, such as mirror size, rating curve of scanning, maximum discrete speed, and etc, were provided, and the influences of the factors on the whole tracking system were analyzed. The simulation results show that the system can realize the synchronous tracking of high-speed projectile with the designed parameter h=150 m, v=300 m/s.
Auto-tracking control method against target occlusion
Li Chao, Wu Yujing, Xu Feifei, Zhao Chuangshe, Zhang Hui, Meng Lixin
2017, 38(5): 713-718. doi: 10.5768/JAO201738.0501006
In order to solve the problem that the target is transiently occluded during the automatic tracking process, an automatic tracking control strategy on electro-optical targeting system (EOTS) was presented. In the normal tracking state, the current tracking control command is recorded, and when the target is partially or completely occluded, the control system enters the memory tracking state. The method can predict the trajectory of the target by conducting quadratic curve fitting for the recorded control command, so as to ensure the stability and reliability of automatic tracking.When the target suddenly changes the direction or speed, the prediction error of the quadratic curve fitting is no more than 6%, which is much smaller than the prediction error of linear fitting of 73%. This method was verified by flight test on a certain kind of EOTS. The result shows that when the target is occluded, the EOTS tracks the target smoothly, and the sight line has no obvious shaking.
Application of low-altitude photogrammetry in test ofprojectile falling point coordinates
Yu Guodong, Wang Shizan, Wang Chunyang
2017, 38(5): 719-724. doi: 10.5768/JAO201738.0501007
Based on ground observation platform, most testing devices for projectile falling point (FP) positioning have limitation of visibility and cannot predict outline coordinates of FP, and cannot distinguish ballistic sequence for bursts of intensive shooting test.According to geometric position relation between object point and image point, three methods for measuring projectile falling point of UAV in the air are proposed:interpolation method (IM), distance ratio method (DRM) and attitude angle solved method (AASM).Basic principle and mathematic model of three algorithms are introduced firstly. Then three methods are verified by test data of simulation test. Minimum technical requirements of camera are calculated finally. Experiments show that accuracy of IM and DRM is less than 3 m and accuracy of AASM is less than 0.3 m.
Design of optical imaging system with high resolution, long-focus, wide angle and low distortion
Zhang Jianlong, He Lei, Yang Zhen
2017, 38(5): 725-731. doi: 10.5768/JAO201738.0501008
Current optical imaging system in the three-dimensional measuring instrument with high precision and full view cannot satisfy the technological demands of large viewing angle, high resolution and low distortion simultaneously, we therefore designed a kind of optical imaging and distortion correction system, which could overcome above defects. To improve the imaging quality of the system, the design of f-θ lens was performed with a complicated double-Gauss structure, and the aspheric surface was introduced. The optical imaging system was machined and tested.Results show that the optical system designed has the characteristics of long focal length, wide angle, low distortion and high resolution, it can achieve the field-of-view of 90°, the distortion of < -0.001 67% and the modulation transfer function(MTF) of 0.4@100 lp/mm without reducing the image quality obviously in the large working range of 3 m~100 m under the ambient temperature of -10℃~70℃. Moreover, it is found that the surface figure accuracy of aspheric lens in optical system has a great influence on the imaging quality, tolerance analysis shows that the imaging quality of optical system can meet the requirement, when the surface accuracy peak-valley(PV) value of aspheric surface is less than 0.17 μm. In actual machining process, the PV value of aspheric surface can reach 0.158 μm, and the MTF can also meet the design requirements. Hence, the imaging quality of the system can be improved.
Status and outlook of infrared semiconductor quantum dots
Geng Rui, Chen Qingshan, Lyu Yong
2017, 38(5): 732-739. doi: 10.5768/JAO201738.0501009
Semiconductor quantum dots have unique optical and electrical properties. Especially the infrared quantum dots(QDs), of which the good optical stability and biocompatibility, make them attractive in fields of photoelectric devices, biological medicine and etc. We summarized the status and outlook of the infrared quantum dots of absorption or emission spectra in applications of laser, energy, photoelectric detection and biomedicine, summed up the preparation methods suitable for infrared quantum dots and compared their respective advantages in applications. There are rich material selections and application forms for the infrared semiconductor QDs. InAs QDs passive mode-locked laser can generate 7.3 GHz near-diffraction limit pulses at 1.3 μm wavelength. InAs/GaAs QDs dual-wavelengtlaser can be used as pump to generate 0.6 nW THz wave. PbS QDs doped fiber amplifier can realize 10.5 dB optical gain at 1.53 μm central wavelength with a bandwidth of 160 nm. CdSeTe QDs sensitized solar cells and Si heterojunction QDs solar cells can achieve a total conversion efficiency of 8% and 14.8%, respectively. Colloidal HgTe quantum dot infrared photodetector (QDIP) can achieve 3 μm~5 μm medium wave detection while Ge/Si QDIP can realize 3 μm~7 μm infrared detection. CdTe/ZnSe core-shell QDs can be used for the detection of DNA damage and mutation. The development of the applications mentioned above for the infrared semiconductor quantum dots can further promote the infrared optoelectronic systems to develop toward the direction of efficient, fast and large scale integration and can also greatly promote the popularization of in vivo imaging detection in clinical medicine.
Design of optical system for broad-band micro-spectrometer
You Zezhang, Wang Xianpei, Tian Meng, Hu Mingyu, Shen Bin
2017, 38(5): 740-745. doi: 10.5768/JAO201738.05010010
Aiming at the situation that there have been less researches on the wide-band micro-spectrometer based on the M type Czerny-Turner structure, the wide-band micro-spectrometer was designed and the complete design flow was put forward. According to the principle of geometrical optics, the constraint relations among the parameters of optics system were analyzed, then the main parameters were obtained through design and calculation, and simulation was conducted by using Zemax.The measurement results for actual production of the spectrometer show that the resolution reaches 1.5 nm in 200 nm~1100 nm full band and 1 nm in mid, which meets the demands of the design.
Oe Information Acquisition and Processing
Single station pose measurement method for range basedon axis image length matching
Hu Xiaoli, Tang Minggang, Zhang Sanxi, Zhang Weiguang, Zhang Yulun, Yu Chunxiang
2017, 38(5): 746-750. doi: 10.5768/JAO201738.0502001
To solve the question of no effective solution for single station pose measurement, a single station attitude measurement method based on length matching of central axis is proposed to measure projected-axisymmetric targets. Perspective projection is extended into two equivalent forms, which will reflect central axis of target attitude state. Image length of center axis or equivalent length of equivalent surface can be obtained by projection of image plane. According to a priori length of target axis, internal and external parameters of the camera and imaging information, pitching and yaw angle can be obtained through image matching by axis image length matching. Practical engineering test verifies the feasibility of this algorithm. Precision of pitch and yaw angle is 1.7° and 1° respectively, which can meet the needs of single station pose measurement in range. Finally, key factors concerning error of pose measurement model are analyzed. This method can be adapted to the non-projected-axisymmetric targets.
RS code performance analysis of visible lightcommunication system based on H-PPM
Hu Xiaoli, Wang Lixue, Qian Yongjie, Jiang Jie, Du Yongxing
2017, 38(5): 751-757. doi: 10.5768/JAO201738.0502002
In order to improve the light intensity scintillation effects on signals under the turbulent channel of complex traffic environment, we studied the system model of the atmospheric turbulence intensity scintillation in theory, and put forward a new kind of modulation method of free space optics(FSO), head pulse position modulation(H-PPM), aiming at the existing problem of pulse position modulation (PPM) and differential pulse?position modulation (DPIM) methods.According to the characteristics of turbulent channel, we derived the slot error rate formula of visible light communication system based on H-PPM when encoded with Reed-Solomon(RS) code or not.The results of numerical simulation show that the RS(15, 7) code can improve about 18 dB coding gain when the system bit error rate is 10-10.Therefore, the combination of H-PPM modulation and RS coding can improve the quality of the signal and facilitate the visible light communication in complex traffic environments.
Adaptive slicing algorithm to keep model characteristicsfor rapid prototyping
Zheng Hualin, Wang Haoyu
2017, 38(5): 758-763. doi: 10.5768/JAO201738.0502003
Adaptive slicing algorithms have been emerged endlessly at present; however, a few of them have good performance in the aspect of model characteristics reservation and deviation's reduction. In order to solve this problem, an adaptive slicing algorithm based on volumetric error was proposed. First, the mathematic models were established for 3 different slicing situations which divided into not cross, across to the adjacent and across to the nonadjacent triangle on the prototyping direction. Then, the reservation performance of model characteristics was analyzed and a specific algorithm flow combined the 3 slicing situations and volumetric error formula was given out. Finally, 3 different slicing methods were applied on a component to confirm the validity of volumetric method. Furthermore, the corresponding virtual components were got and compared with the CAD original model. It is concluded that, comparing to the other 2 methods, the volumetric method most fits the CAD contour line and has the most form accuracy with the deviation distances of 0.09 mm and 0.10 mm at the sampling points.
Localization and ego-velocity estimation for vehiclebased on binocular image sequences
Tong Zhixue, Zhao Tao, Wang Xiaowei
2017, 38(5): 764-769. doi: 10.5768/JAO201738.0502004
A real-time ranging location and ego-velocity estimation method based on binocular image sequences was presented to obtain the relative location and velocity of the vehicle in the running process. This method used the vehicle-borne binocular vision sensor to collect the image sequence of the surrounding environment. Then the depth of field of environmental feature points was obtained through matching feature points of left and right images at the same time based on speeded up robust features(SURF) in order to achieve vehicle location. Meanwhile, the feature points of two adjacent frames were tracked and matched based on SURF.The transformation parameter of the camera coordinate system before and after vehicle movement was computed through the 3D coordinates of the corresponding matching points in the two adjacent frames. And the velocity of the vehicle was estimated according to the transformation parameter. The simulation experiment results show that the method is feasible, and the speed calculation results are relatively stable, the average error is less than 6%.
Optical system design of fingerprint and finger veinimages compound acquisition
Ji Zijuan, Bao Jiaqi, Yin Juanjuan
2017, 38(5): 770-776. doi: 10.5768/JAO201738.0502005
Due to the biological multi-mode identification technology, an optical system for compound acquisition of fingerprint and finger vein multiple images was designed and described.The optic camera for collecting fingerprint and finger vein images contains respectively 3 and 12 spherical lenses, which works at 650 nm and 850 nm wavelength with a 640×480 CCD and a 640×512 uncooled infrared(IR)detector as the imaging devices, respectively.With the aid of Zemax software, the optical system was designed and optimized to cause a good imaging quality. Image quality evaluation results show that the modulation transfer function(MTF)value of the fingerprint imaging lens is above 0.6 at 67 lp/mm, while the finger vein imaging lens is above 0.8 at 30 lp/mm.The view field spots'root-mean-square (RMS) radii of the 2 lenses are both much smaller than the detectors'pixel size, which are close to the diffraction limit, and the distortions are both less than 0.5%.Experiments prove that images acquired by this optical system have the merits of fine image quality and high resolution.
Optical Metrology and Measurement
Dynamic detection of object deformation based onshearing speckle interferometry
Guo Yuan, Liu Dandan, Mao Qi
2017, 38(5): 777-783. doi: 10.5768/JAO201738.0503001
In order to make the spatial phase-shift shearing speckle-pattern interferometry system for static measurement only be used for dynamic measurement of object deformation, a new dynamic detection method based on improved traditional shear speckle interferometry is proposed.The piezoelectric ceramic transducer (PZT) in testing system is replaced with reference mirror to reduce the control and execute time.For the same state of the distortion of object, only one interferogram needs to be collected to meet later calculation.With two-dimensional continuous wavelet filtering and least-squares phase unpack algorithm, this system can meet the needs of dynamic on-line detection of object deformation.Theoretical and experimental results show that the proposed method can detect the dynamic deformation of the object quickly and reliably, and has high precision and practicability.The maximum error of whole system is in the range of -1.5 rad~1.5 rad and the maximum percentage error of whole detection process is 6.4%.It can provide a reference method for the improvement and design of the new shear speckle interferometry measurement system.
Measurement of aperture based on fractional derivativefor eliminating local strong reflection
Tang Ruiyin, Wang Quan, He Hongkun, Hu Lianjun
2017, 38(5): 784-789. doi: 10.5768/JAO201738.0503002
In order to solve the problem that the hole diameter of the hard disk is easy to be affected by the local strong reflection and the high reflective surface, a method for measuring the diameter of circular parts to removal the noise based on the fractional differential was presented.The fractional-order differential algorithm was used to process the images of the hard disk aperture images with strong reflection and high reflection, so as to eliminate the influence of irrelevant information in strong reflection surface around circular hole on the extraction of the edge of circular hole.Experiments were carried out to compare with the Prewitt, Soble and Laplacian operators.It is proved that the fractional differential can effectively reduce the amount of image information to be analyzed and achieve a better visual effect.Moreover the Canny edge detection was performed for the circular hole image after processed by fractional-order differential algorithm to extract the available circular hole edge.Finally, the least square method was used to measure the hole size.Experimental results show that, compared with other algorithms, in the case of accurately preserving the edge information of the round hole and effectively suppressing the surrounding noise, the error can be controlled around 0.05 mm, and the the measurement accuracy is guaranteed.
3D measurement method based on binary fringe plusphase coding fringe defocus projection
Zeng Zhuohuan, Huang Chao, Qu Guoli, Fu Yanjun
2017, 38(5): 790-797. doi: 10.5768/JAO201738.0503003
Precision three-dimensional measurement of complex surface is very significant in industrial non-destructive testing. The method of binary fringe defocus projection has important application prospects in fast three-dimensional measurement; however, it is difficult to realize high precision three-dimensional measurement on complex surface.For this reason, a complex surface 3D measurement method based on binary fringe plus phase coding fringe defocus projection is proposed. Because of defocus, the high harmonics and high-frequency noise are filtered out, the nonlinear Gamma effect of the projector can be overcome, its measurement accuracy can be improved, compared with the traditional projection sinusoidal fringe method. For the out-of-focus projection, as the frequency of the phase coding fringe increases, the fringe order decision becomes difficult, the periodic dislocation occurs which can lead to the errors of phase unwrapping.In order to solve the problem, the principle of phase-shift coding method is proposed to correct the periodic dislocation and make the fringe order decision accurate, so the measurement accuracy is further improved. The experimental results show that the accuracy of this method can reach 0.044 mm, which verifies the validity and practicability of the method.
Three dimensional shape restoration method with parallellight interference projection
Wang Yi, Liu Huiyan, Song Baogen
2017, 38(5): 798-803. doi: 10.5768/JAO201738.0503004
In traditional Fourier transform profilometry, the astigmatism projection is used and 4 constraints are needed to obtain accurate mapping relationship between phase and height. Moreover, the phase nonlinear quadratic error caused by astigmatic projection can also reduce the topographic recovery accuracy. In order to reduce the influence of phase nonlinearity in less restricted conditions, a universal model based on parallel optical interference projection was established. The reference plane phase under the parallel optical interference projection changes linearly along x-axis, the phase distribution is more accurate than the astigmatic projection or oblique projection. The restoration of the measured object under astigmatism and parallel light projection was compared and analyzed. The experimental results show that the new model has high flexibility and operability under less restricted conditions, and the recovery accuracy is good, the relative error is 1.1%.
Screening of light environment evaluation indices for ship night navigation
Huang Cheng, Zhu Jinshan
2017, 38(5): 804-809. doi: 10.5768/JAO201738.0503005
In order to objectively and comprehensively evaluate the light environmental quality of the ship navigation at night and ensure the safety of the ship navigation at night, the concept and source of the light pollution at sea were analyzed, the light environment evaluation indices of the ship navigation at night were proposed, which should follow the principle of "dominance, operability and wide coverage".On the basis of the principle for selecting indices, the theoretics of optics, chromaticity and system engineering were used, 19 evaluation indices were selected from 3 aspects including navigator's visual performance, ship collision avoidance and light environment condition of ship night navigation, and the light environment evaluation system at sea was established; furthermore, the evaluation criteria of the brightness of background light, the chromaticity of background light, the glare of pollution light and the flickering degree of pollution light were given in the navigation light environment condition, which laid foundation for the measurement and evaluation of light environment of ship night navigation.
Optical Element and Fabrication
High-resolution X-ray CCD camera based on the first generation image intensifier
Liu Shulin, Yang Luping, Dong Yuhui, Huang Mingju, Su Detan, Yan Baojun, Wen Kaile, Qi Huirong, Yang Yuzhen, Yu Yang
2017, 38(5): 810-814. doi: 10.5768/JAO201738.0505001
In order to achieve a higher resolution at lower X-ray doses, a low-cost X-ray CCD camera is developed. The camera uses gadolinium sulfuric acid as a conversion screen, and images enhanced by the first generation image intensifier is coupled to CCD camera through fiber optical taper. In order to improve optical transmission efficiency of the system, a scheme is proposed in which output of image intensifier is directly replaced with fiber optical taper, and its small end is directly coupled with CCD camera. The contrast ratio curve is calculated by gray analysis, and the intrinsic spatial resolution of the system is fitted with 13 Lp/mm. By shooting the actual scene, internal details can be clearly observed, showing a better image quality.
Flexible support structure design of lightweight thin reflector
Yang Jianli, Qi Yuan, Yang Xiaoqiang, Zhu Lei, Teng Guoqi, Shi Daoyun
2017, 38(5): 815-819. doi: 10.5768/JAO201738.0505002
The shape of optical lightweight thin reflector possessing large flakiness ratio is always effected by wide-range temperature and harsh environment(especially at -40℃), seriously influencing the system image quality. Aiming at this problem, based on flexible support principle and thermodynamics theory, a new support structure was designed for small lightweight thin splitting reflector. The temperature adaptability of the support structure was analyzed by using finite element method, and the low temperature tests for assembled mirror components were conducted. The result shows that the accuracy of reflector surface figure remains 0.25λ(λ=632.8 nm, peak-to-valley)at -40℃. These results demonstrate that the novel flexible supporting structure design satisfies the application environment requirements.
Design of stage lamp with zoom lens based on RGBW light source
Dong Wenjuan, Shi Lin, Meng Shuang, Xia Lei, Zuo Hui, Ding Guilin
2017, 38(5): 820-825. doi: 10.5768/JAO201738.0505003
For the shortcomings of stage lamps, such as a small changing range of beam angle and a poor uniformity of illuminance, here we presented a single zoom lens system based on the structure of total reflection lens and the principles of zoom lens group. The system was consisted of a collimating lens and a mobile focusing plate. By using SolidWorks software, the mirror plate of collimating lens was filled with circular arrays of irregular pentagon fly's-eye lens, and a circular array with regular hexagon fly's-eye lens was built on both side of the focusing plate. After matched with the type of S2WP full color red-green-blue-white(RGBW) LED light source, the system was simulated by using LightTools and the actual illumination efficiency was detected. The results show that, these structures set above can solve the poor performance of uniformity and color mixing on the stage lamp effectively. What's more, the light utilization efficiency of the stage lamp zoom lens system we presented above can reach up to 87%, and the changing range of beam angle(1/2 light intensity angle) can be from 4° to 53° under the focusing distance extent from 0 mm to 15 mm.
Laser Technology
Effect of working angle on system sensitivity in laser triangulation
Bai Fuzhong, Zhang Xiaoyan, Liu Zhen, Gao Xiaojuan, Wang Ying
2017, 38(5): 826-830. doi: 10.5768/JAO201738.0507001
Through analyzing sensitivity formula of laser triangulation measuring system, effect of optical structure parameters on system sensitivity is analyzed, which mainly include measured surface fluctuation, magnification of imaging system and working angle as well. Additionally, laser triangulation system of surface roughness is established, and influence of working angle on sensitivity is experimentally studied. Results show that larger working angle can achieve higher system sensitivity. However, when working angle is large, it needs to consider optical magnification and working angle on sensitivity of integrated influence and ensure that intensity of light receiver is greater than its sensitivity. For measuring system used in this paper, working angle corresponding to highest sensitivity equals to about 70°.
Effective ablation focus determination of femtosecondlaser by using spot image feature
Wang Fubin, Liu Tongle, Wu Chen, Hu Lianjun, Chen Zhikun, Paul Tu
2017, 38(5): 831-836. doi: 10.5768/JAO201738.0507002
In order to determine the influence of femtosecond laser machining center on ablation depth with micro-scale structure, the effective ablation focal length of laser beam corresponding to the given power was researched, a method for getting the ablated spot array image at laser focus, extracting the image feature in defocus stage and obtaining the effective ablation focus range of laser beam by analyzing the relationship between the image features and the defocus distance was proposed. Firstly, the spot array was ablated on the surface of silicon wafer near the laser beam focus. Secondly, the focal length was gradually reduced downward to capture the spot image of silicon wafer, the average pixel area of spot image and the gray difference of R component between spot target and background were extracted, and the change curves of spot pixel area and gray difference with focal length of laser beam were obtained. Again, the focal length was gradually increased upward, the change curve of spot pixel area and gray difference with focal length of laser beam was extracted and obtained similarly. Finally, combined with the downward defocus threshold (633 μm) and upward defocus threshold (993 μm) of laser beam, under the condition of 20 mW output power, the effective ablation depth of femtosecond laser on silicon wafer surface was determined to be 360 μm. By using median value method, the focal length on the surface of silicon wafer was determined to be 0.823 mm while defocused. Experimental results show that the relationship among spot pixel area, gray difference and laser beam focal length can objectively reflect the variation range of effective ablation focal length of laser beam.
Effect of static/dynamic etching on surface quality andlaser-induced damage of fused silica
Wang Zhiqiang, Yang Ke, Li Yuan, Yan Hongwei, Yuan Xiaodong, Zhang Lijuan, Liu Taixiang
2017, 38(5): 837-843. doi: 10.5768/JAO201738.0507003
In order to investigate evolution rules of surface quality and laser damage resistance performance during static/dynamic etching process, and optimize chemical etching technologies, fused silica were treated by static/dynamic etching for different time respectively using the buffered hydrofluoric(HF) acid.Experiment results show that the dynamic etching rate of fused silica is higher than the static etching rate due to megasonic agitation. The surface root-mean-square(RMS) roughness and reflection profile peak valley (PV) value of dynamically etching surface are < 1 nm and 0.46λ, respectively, and 355 nm transmission of dynamically etched surface first increases in a small range (0.1%) and subsequently remains steady. Moreover, the surface RMS roughness and reflection profile PV of statically etched surface increase to ~5 nm and 0.82λ, respectively, and the 355 nm transmission of statically etched fused silica first remains basically steady and subsequently decreases by 0.4% aproximately. Damage thresholds of fused silica treated by static/dynamic etching present obviously different changement laws:the damage thresholds of fused silica increase by about 30% and then decrease gradually during static etching, while that increase by about 100% and then keep relatively steady during dynamic etching. The results above show that the properties of fused silica optics treated by dynamic etching are significantly better than that of static etching.
Fiber Sensing and Optical Communication
Broadband THz single-mode single-polarization hollow core fiber
Hou Yu
2017, 38(5): 844-847. doi: 10.5768/JAO201738.0508001
Traditional terahertz (THz) single-mode single-polarization (SMSP) fibers are mostly designed on the basis of solid fibers, which increases the THz wave transmission loss. In recent years, there have been a few reports about THz SMSP hollow core fibers; however, the fibers have narrow bandwidth, they can only work around a single frequency, which reduces the application value and technical advantages of single-polarization devices in THz networks. In order to solve these problems, a broadband THz SMSP fiber device was proposed. The fiber can achieve SMSP transmission from 1.63 THz to 1.73 THz with a bandwidth more than 0.1 THz. It has great significance for the establishment and application of THz communication network.
High sensitive fiber strain sensor
Lu Weitao, Zhang Shuanmin, Chen Anfeng, Qiang Wei, Wang Cheng, Wang Jian
2017, 38(5): 848-851. doi: 10.5768/JAO201738.0508002
A fiber-optics strain sensor based on the composite structure of tapered fiber and fiber Fabry-Pérot(F-P) interferometer was proposed and experimentally demonstrated, which comprised 2 strain sensitive regions, the tapered region formed by single mode fiber taper and the F-P cavity based on the quarts capillary tube. The propagation process of light waves in the sensor was analyzed theoretically, and the light intensity transfer function was obtained. As the cladding high-order mode excited by tapered fiber participated in interference, the interference spectrum of sensor had modulation characteristics.The interference spectrum of sensor was obtained by experiment and the independent measurement was realized by analyzing the resonant wavelength shift or extinction ratio change. Experimental results indicate that the proposed sensor presents a sensitivity of 14.6 pm/με in the measurement range from 0 to 500 με Using the modulated interference spectrum formed by the tapered fiber induced mode interference and the double-beam interference of the F-P cavity to conduct strain sensing, the strain sensitivity is high, and 2 kinds of independent strain detection methods (resonant wavelength and extinction ratio detection) can be provided.
Wavelength calibration method of temperature FBG basedon FPGA and DS18B20
Tang Chenfei, Wan Shengpeng, Jia Peng, Chen Ruilin
2017, 38(5): 852-856. doi: 10.5768/JAO201738.0508003
Based on real-time response of DS18B20 to external temperature, digital temperature sensor DS18B20 is placed in the same temperature field as the temperature fiber Bragg grating (FBG) connected with the spectrometer. Taking field-programable-gate-array(FPGA) as a data processing and control chip, real-time monitoring and display system on the temperature field change is designed through serial port to send intelligent instructions. This method does not need thermostat to control temperature, which reduces the cost and power consumption of FBG wavelength calibration. Experimental results show that the linearity of wavelength-temperature curve is 0.999, and the temperature sensitivity coefficient of FBG is 9.899 pm/℃, which differs from the result 10.468 pm/℃ measured with thermostat to 0.569 pm/℃. The error falls in the range of 0.619 pm/℃ allowed by the system, which verifies the accuracy of the method.