2017 Vol. 38, No. 2
In recent years, a lot of new optical phenomena have been discovered based on research of metasurfaces. Among them, geometric phase modulation is one of the most attractive directions. In this paper, some innovation research works in our group on accurate controlling of optical phase based on metasurfaces have been introduced, including anisotropy in electromagnetic response and electromagnetic resonances. Based on these principles, a series of new-concept optical elements and devices have been proposed and investigated. Firstly, a gold nanorod-based CGH (Computer Generated Hologram) was realized. Experimental results show that it can work in broad bandwidth of 630 nm~1 050 nm and the maximum diffraction efficiency reaches 80% at a wavelength of 825 nm. Secondly, a silicon nanorod beam splitter was proposed and it can generate uniform 4 × 4 spot arrays with an extending angle of 59° × 59° in far field. For such device, diffraction efficiency exceeds 50% in ranges of 1530 nm~1 565 nm. At last, a polarising beam splitter was designed with reflectivity of 98.5% along long axis of nanorod, whilst transmissivity along short axis is 94.3%. More interesting, only by changing width of nanorod, we can shift peak response wavelength from 1 460 nm to 1 625 nm. Above research results show that, with advantages such as single-step nanofabrication, continuous, arbitrary, accurate and efficient phase controlling, metasurfaces have brilliant commercial prospects. We predict that metasurfaces will be applied to develop new generation of chip-scale optoelectronic components and devices with high performance. Its application fields can cover optical telecommunication, military defense, industry and consumer electronics.
Ultra-precision turning technology is suitable for machining KDP optical elements which are used as frequency conversion unit in high-power laser system, but stripes are easy to come into being in machining process, and this will result in reduction of laser induced damage threshold. Taking magnitude and spectrum distribution of machining errors as researching object, machining features and error morphology of KDP elements are analyzed after ultra-precision turning in this paper, and correlation between error spectrum and machining parameters is studied based on power spectral density (PSD). Results show that feeding rate and spindle speed will change distance between spiral ripples and affect error spectrums of KDP machining surface, cutting depth has hardly any influence on spectrum of errors, but it will change magnitude of PSD. KDP surface can be accurately manufactured with rms value is smaller than 20nm when spindle speed is more than 500r/min, feeding rate is less than 2mm/min and cutting depth is less than 2 μm. According to a typical KDP element, PSD of low frequency errors is controlled within 300 nm2·mm by using ultra-precision compensating turning method, PSD of mid-to-high frequency errors is under "Not-to-Exceed" line applied in national ignition facility, and working requirements for high-power laser system are satisfied on the whole.
Based on principle of laser stimulated emission deplection, far field optical super-resolution imaging technique, modulated by vortex phase plate, incident Gaussian laser will be transformed into a donut-shaped light beam with zero central intensity. Functional nanoscale structures will be obtained when laser beam with donut-shaped laser beam irradiates onto photosensitive polymer.This paper presents laser direct lithographing system based on CW laser beam, as well as its applications in preparation of composite nanostructures. When light source is 532 nm CW laser output, a composite nanopillar with pillar's diameter less than 50 nm and pillar arrays is fabricated with a positive photo resist. While composite structure of nanochannel with < 100 nm inner diameter in center of micropillar, corresponding arrays are fabricated with a negative photo resist. In addition, when laser source is continuous 405 nm fiber laser, composite nanopillar with 153 nm diameter and various arrays are also achieved with positive photo resist. Basic units of these composite nanostructures are all broken through optical Abbe's diffraction limit. and have practical potential.
In order to improve reconnaissance capability of UAVs, an image fusion technology of unmanned aerial vehicle(UAV) borne opto-electronic and SAR based on wavelet transform is proposed. Firstly image registration source is generated by time registration algorithm, and image feature points are extracted by SIFT algorithm. Then BBF algorithm is used to calculate matching point set, and image registration is done according to perspective of matching point set. Finally registration image fusion is achieved by wavelet transform algorithm. Results show that fusion image preserves 95.7% of details (entropy) of opto-electronic image, which is 1.52 times higher than average gradient of opto-electronic image, and opto-electronic image target area contrast is enhanced and random noise is reduced. Compared to SAR image, fusion image information is increased by 1.44 times.
Zoom projection lens are designed to meet requirements of different screen size with same optical engine and screen position. Focal length of zoom projection lens ranges from 22 mm to 37 mm, viewing angle is 46° to 75°, and F number is 2.8. Considering design of optical system requiring a relatively large aperture, with large field of view and small zoom ratio, according to zoom theory, positive mechanical compensation configuration is adopted, reasonable magnifications of zoom configuration are chosen and Gauss roots are derived. Appropriate primal configurations based on each subassembly are chosen. Optimization design is carried out with Zemax optical design software, and added proper aspheric surface. Mechanical compensation method of two and four component motion is used to solve problem that distortion of large field of view is difficult to control, and whole optical system is evaluated synthetically by using modulation transfer function (MTF). Design results show that optical structure and image quality of zoom projection objective system meet design requirements. Spatial modulation transfer function (MTF) value is more than 0.3 at spatial frequency of 64 lp·mm-1, and distortion is less than 1%.
In order to improve detection precision based on optical axis parallelism of two-dimensional galvanometer and simplify measurement procedure, a multi-spectral integrated target based on two-dimensional galvanometer scanning system is designed. Through integration of multi-spectral target plate and LED lighting source, problems of frequent replacement, repeated adjustment of target plate and light source in a special target are solved. Centering accuracy of CCD detection method is improved by using two-dimensional galvanometer, and Cassegrain collimation system with optimized aberration is used to improve quality of cross-sectional imaging. With optical axis parallelism test, calibration accuracy of multi-spectral integrated target reaches 0.10mard, which meets requirements of high precision and automatic detection of optical axis parallelism of multi-spectral opto-electronic systems.
In order to enhance robustness of target tracking algorithm under conditions of motion displacement, occlusion, deformation and similar object disturbance, it is proposed to construct target appearance model by using super pixel, and match appearance model with candidate region to obtain candidate region target super pixel, and use Meanshift algorithm to determine target center point tracking algorithm. Simulation experiments select representative of video Girl and FaceOcc1 from Benchmark library, which represent video scene in terms of movement displacement, occlusion, deformation, interference of similar objects. Tracking success rate and tracking accuracy of algorithm are 0.601 and 0.856 in video Girl, and success rate and accuracy of KCF algorithm with best tracking performance are higher than normal algorithm of 0.059 and 0.084 respectively. In video FaceOcc1, tracking success rate and accuracy of proposed algorithm only ranked second to KCF, suggesting a fine robustness even when target is blocked or interfered by analogues.
Due to lack of navigation facilities of current airport runway closure and relevant standards disunified, it is difficult to pilots in harsh visual conditions and safe distance to provide eye-catching and effective runway closed information, which causes hidden dangers to airport safe operation. Aiming at this problem, this paper establishes a temporary airport runway shutdown warning light model based on human visual characteristics, designs and analyzes light intensity of warning light lamp by point by point method. Then warning process is simulated. Simulation results show that X-shaped warning light can be recognized at 2 km, and runway closed information can be distinguished by pilots clearly, accurately and rapidly. This plays a warning role for pilot and also provides theory basis for development of warning light.
Emission optical system plays an important role in improving performance of surface layer UV communication system. UV communication emission optical system in surface layer is designed. Through analysis and comparison, this system adopts Cassegrain structure. Its main mirror is paraboloid and secondary mirror is hyperboloid, working at wavelength 265 nm~270 nm and ambient temperature -60 ℃~ 60 ℃. After optimization design, results are as followings: system focal length is 450 mm, total length of system is 200 mm, emission angle is 2°, MTF value is greater than 0.31, and working temperature is stable.
Standard GaAs/AlGaAs quantum well infrared photodetectors (QWIP) have been seriously considered as atechnological choice for the 3rd generation of thermal imagers in the long wave infrared band (LWIR) for some time. Alternative technology like MCT (HgCdTe) was the technology choice of the 2nd generation because of its high quantum efficiency. In the paper, measurements on the QWIP technology will be presented and a comparison with alternative technology will be done.
For infrared focal-plane array imaging system, scene-based non-uniformity correction is key technique to deal with fixed pattern noise. Existing algorithms are mainly restricted by convergence speed and ghosting artifacts. In this paper, a novel adaptive scene-based non-uniformity correction technique is presented, which is based on constant-statistics method (CS). Utilizing temporal statistics of infrared image sequences, the proposed method applies an alpha-trimmed mean filter to estimate detector parameters and minimize sample asymptotic variance estimate. Performance of proposed technique is evaluated by simulation and real non-uniformity image. Experimental results show the proposed method inherits characteristics of fast convergence of CS method and increases peak signal to noise ratio by 44.5% and 32.9% respectively, and image ghost problem is improved obviously.
Traditional imaging methods can not capture axial information through one shot, which lead to information deficiency and incapability of three dimentional surface reconstruction. Unlike traditional camera, lytro's first generation pocket-sized camera using light field sensor can capture multi-dimensional light field information through one single exposure, and at the same time it has advantages such as portable. By using light field camera, with digital refocusing method, defocusing and correspondence cues, acquirement of high quality depth map can be realized. By using matlab platform to process acquired depth map matrix, reconstruction of three dimensional surface can be realized. Normalized depth data of object space is obtained. In this experiment, three dimensional surface reconstruction is realized with average error of 5.47% in depth range of 100mm to 1 500 mm, and maximum reconstruction error is 8.30%.
To satisfy needs of high resolution three-dimension(3-D) display for big data, an angular multiplexing based volume holographic display technology is proposed. Wavefront of 3-D scene is encoded as computer-generated holograms. Holograms are recorded in gold nanoparticles doped photopolymer with angular multiplexing. With recorded holograms, 3-D scenes could be viewed at different angles. In volume holographic three-dimensional display experiment system, total number of single-point pixels in volume holographic material recording area is 120×1 920×1 080, and spatial bandwidth product is 2.5×108, which is 120 times higher than that of spatial light modulator.
Speckle noise is a common phenomenon in laser interferometry, which covers shape information of corresponding area of measured surface, resulting in measurement error. Aiming at characteristics of speckle noise in oblique laser interferometry, a recognition method of speckle noise based on object image is proposed. In this method, upper and lower thresholds of speckle noise are calculated automatically by statistic characteristics of gray scale distribution in effective measurement area and background area in object image. Position of speckle noise in interference fringe image is obtained based on mapping relation between object image and interference fringe image. A correlation experiment is designed to repair speckle noise region identified in interference fringe image, and eliminate phase transition between adjacent pixel points in a fringe period of wrapped phase diagram.
In order to realize the purpose of surface roughness detection for turning parts, a new image processing method for surface roughness detection based on machine vision is proposed. Firstly delete part of collected image that severely affected by diffraction according to corresponding algorithm, and then optimize regions according to gray distribution, so as to obtain image grey feature parameters, which can reflect effective feature areas of surface roughness value. Five turning samples with Ra nominal value ranging from 0.8 μm to 12.5 μm are tested using this method. Feature parameters such as mean value, variance, energy and entropy of processed image have a remarkable monotonic relationship with Ra nominal value. The nonlinear error of each feature parameters relationship curves are all within 1.5%. Contrast experiment results show the method can be applied to distinguish and detect surface roughness.
Nonlinear waveguide is an important photon quantum device. Using Marcatili approximation, mode field distribution characteristics of waveguide can be approximately described. For periodically poled LiNbO3 waveguide with ee-e quasi-phase matching, fundamental mode field distribution of fundamental wave with a wavelength of 1 064 nm and second harmonic of 532 nm is analyzed. Results show that if waveguide is an embedded square waveguide, fundamental and second harmonic are basically circular, and range of fundamental wave field is larger. When length of square side of section is 5 μm and core area is slightly 0.02 higher than cladding refractive index, waveguide can basically confine two kinds of light field in core region, but constraint ability becomes very poor when waveguide length is 0.5 μm. If difference between core region and cladding layer is large, waveguide with 0.5 μm side length can get two kinds of field distribution constrained in core area basically. Results are of significance to analysis of modes of other types of nonlinear waveguides.
Aimed at improving performance of automatic focusing algorithm in dynamic environment, this paper describes recent studies of focusing evaluation function and search strategy. On the basis of analysis of characteristics of human visual system, new focusing evaluation function based on eight direction Sobel operator edge weighting is proposed. At the same time, adaptive variable step search strategy is used in order to overcome the disadvantage of slow speed of traditional climbing method. Simulation experiment results show eight direction Sobel edge detection operator has good edge detection effect, and focusing evaluation function has better anti-interference ability than traditional two direction Sobel operator focusing evaluation function, which is calculated by this operator combined with characteristics of human visual system giving the edges with different weight coefficients. Finally, an experimental platform based on liquid lens is set up, which verifies the performance of improved auto focus algorithm in dynamic environment. Experimental results show focusing accuracy with proposed algorithm can achieve 97.5% in dynamic environment.
Image distortion correction is a prerequisite for distributed aperture system (DAS). Brown model and its aberration (fitness) metric describing radial and tangential distortions of image are introduced. As one of intelligent optimization algorithms, shuffled frog leaping algorithm (SFLA) and its initial population Latin hypercube sampling (LHS), are discussed. Image aberration model (from ray tracing method) and Brown distortion model are simulated and corrected respectively. Results show that characteristic point's positions between origional and corrected are within 2 pixels, which verifies effectiveness of algorithm. Finally, this correction method is applied to correction of target paper by wide-angle CCD camera, and satisfactory results are obtained. The algorithms and simulation results have certain theoretical and experimental significance to development of DAS equipment.
In view of real-time high-precision spacecraft pose measurement is not only an important technical assurance of real-time monitoring and tracking of spacecraft, but also main basis for real-time adjustment and compensation of spacecraft attitude, this paper propose a monocular vision-based real-time pose measurement method, which utilizes a high precision rotating platform geometric camera to carry out pose solution to the measured object. This method does not need to geometrically constrain measured object, and does not need to carry on measurement of station, so it can carry on dynamic real-time on-line pose detection. Experimental results show that attitude measurement error of monocular vision measurement is less than 0.05° and position measurement error is less than 0.02 m, which satisfies accuracy requirements of practical engineering of 0.1 ° and 0.05 m.
Compared with traditional measurement methods, digital speckle correlation method(DSCM) can better meet measurement requirements of small angle displacement in different occasions due to flexibility of its target feature unit meshing. In order to solve problem of sub-pixel small-angle displacement measurement, nine-point quadratic surface fitting method of sub-pixel measurement image is studied in this paper, and optimal error is obtained by computer simulating experiment. Optimal speckle size of sub-pixel small angle displacement is calculated by surface fitting method under condition of efficiency optimization. Calculation window size is 41×41 pixel and window size is 3×3 pixel. Experimental results verify validity of above parameters, and provide reference for further measurement of angular displacement of digital speckle image by surface fitting method.
Harmonic detection is widely used in laser spectroscopy, which can improve detection sensitivity. In this paper, a set of tunable diode laser absorption spectroscopy(TDLAS) methane detection device is established by using 1.653 μm distributed feedback (DFB) diode laser as light source. The device utilizes two circular cylindrical mirror composed of optical multi-cell to increase absorption of optical path and improve detection sensitivity. Absorption length of multi-pass optical cell is 15 cm, and effective absorption path length is 16.8 m in 112 reflex cases, and detection limit of methane 0.60×10-6(2 s sampling time) is realized, which can be applied to trace detection of actual atmospheric methane.
Combining fringe projection profilometry and stereomicroscope, a system is setup which can measure three-dimensional topography of micro devices. This system consists of Greenough-type stereomicroscope, projector, CCD camera and mechanical translation table. After unambiguous phase is obtained by using phase shift algorithm and descrambling method, which is modulated by surface topography, height value is obtained by phase and height mapping relationship, so that surface morphology of micro devices can be reconstructed. At the time of calibration, ceramic calibration plate is set at different heights by means of a precision mechanical translation table, and several groups of corresponding phase and height values are obtained. Then polynomial fitting of each pixel is performed to determine phase-height mapping. Last calibrated plane and micro ball grid array are measured. Results show that measurement error is within 10μm, and spherical structure is clearly seen from three-dimensional reconstruction of micro-ball grid array. It is proved that system can accurately measure both planar and complex three-dimensional structures.
Zoom optics system must also meet requirements of image stabilization while correcting aberration, compensating or eliminating offset from receiver due to movement of components in optical system. Based on principle of dynamic optical image stabilization, stabilization equation of zoom optical system is deduced, mathematical model of zoom optical system is established, and cam curve of optical system is designed. Dynamic analysis process of zoom lens is given. Optical design software CODE V is used to finally obtain zoom objective with zoom magnification of 8.15× and focal length range of 27 mm to 220 mm, fixed F# number 4.2 and radial fields 4.12°~33.56°. Calculation method of cam curve, CODE V image quality analysis and MTF are given.
Basic concept and cause of ghost image are introduced and analyzed. Ghost image simulation method is put forward, which is based on qualitative analysis of optical design software CODE V and light machine fitting software LightTools quantitative analysis. Comparing lightTools software simulation results with normalized true response illumination value of system detector, ghost path is found and improvement measures are given. Ghost path of TV imaging optical lens given in verification experiment is calculated and simulated. Results show that normalized response illumination of ghost path on lens axis is 3.5×10-5, which is less than 9.85×10-5 ~ 1 of the detector and detector does not respond and produce ghosts. Simulation analysis method is proved to be correct and feasible.
In order to meet requirements of photometric performance of vehicle retroreflector, logn-range projection lighting source is designed to meet requirement of high uniformity spot on long-range illumination surface of test light source. Using reflective bowl xenon light source, light guide tube and collimator lens as design type, optical system can achieve effective use of light and long-range transmission, and can be formed at a distance close to parallel light irradiation surface. Besides, optical system is equipped with a rotary adjustment mechanism to complete continuous focusing function of lighting system and can realize continuous adjustment of uniform spot size on long-range illumination surface. On different irradiation surfaces with illumination distance of 30 m~40 m, this system can obtain different sizes of uniform spots, and spot unevenness is less than 5% in irradiation surface Φ250 mm ~ Φ300 mm.
With deep research, iris recognition technology is expected to be widely used in field of mobile payment. In view of big volume and quality of current iris recognition lens, Zemax optical design software is used to design compact 5 mega-pixel iris recognition lens composed of three-piece non-spherical structure. Lens working distance is 170 mm ~ 250 mm, F number is 3, total length of lens is 4.43 mm, optical distortion is less than 2%, TV distortion is less than 0.5%, relative illumination is less than 0.6. Matching 1/4" CMOS made by APTINA, Nyquist frequency of 357 lp / mm, at 1/2 Nyquist frequency, MTF are greater than 0.2, total pixel can reach 5 mega-pixel. Image quality evaluation and tolerance are analyzed. Results show that lens meet optical requirements.
Output characteristics of continuous wave(CW)orange-red laser with all-solid wavelength tunable based on composite cavity structure are introduced. Composite cavity is composed of an s-polarized signal singly resonant optical parametric oscillator (SRO) using an MgO-doped periodically-poled LiNbO3 (MgO: PPLN) crystal and a p-polarized 1 062.9 nm fundamental frequency light cavity. Independent oscillations of signal light pumped by an s-polarized 1 062.9 nm laser and p-polarized 1062.9 nm fundamental frequency are generated in respective cavity, which produces orange-red laser using a type-Ⅱ critical phase-matched KTP crystal in intracavity sum-frequency generation process. When tuning temperature of MgO:PPLN crystal rises from 30℃ to 200℃, CW orange-red laser beams are obtained with tunable wavelengths from 620.2 nm to 628.9 nm based on wavelength red-shift of s-polarized signal light, corresponding to a measured waveband of mid-infrared idler light from 3 714.2 nm to 3 438.3 nm. At minimum temperature of 30℃, maximum CW output power of orange-red laser is obtained to be 2.0 W at 620.2 nm, corresponding to CW output power of idler light is 2.9 W at 3 714.2 nm.
This paper presents He-Ne laser nanometer scale measurement system based on orthogonal polarization dual longitudinal mode. With inserted birefringence element, frequency splitting effect will occur inside He-Ne resonant cavity, turning the laser into dual-frequency laser with adjustable frequency difference. By using effect of laser physics such as frequency splitting effect, mode competition effect and double longitudinal mode power tuning, and setting floating threshold, a new type of laser nanometer measurement ruler is proposed. The laser wavelength is applied as a ruler with traceability, leading to nanometer scale resolution without electrical subdivision. Experimental results of comparing with laser interferometer show that system optical resolution is 79 nm, measuring range is 15 mm, linearity is 5.4×10-5, and standard deviation is 380 nm.
In femtosecond laser random scanning two-photon microscopic imaging system, using a broadband two-dimensional acousto-optic deflector scanning femtosecond laser can increase scanning angle to 74 mrad, increasing two-photon microscopic imaging range. But broadband two-dimensional sound and light deflector has large dispersion in large angle scan, resulting in serious distortion of imaging edge and spot diameter of 2.3 μm. In order to improve image quality, a new dispersion compensation method is designed. Kepler's telescope system based on diffraction lenses can compensate different dispersions of different scanning angles. After dispersion compensation, imaging edge of spot diameter is less than 1 μm, so that wide range of scanning imaging is obtained and dispersions of all scanning angle are well compensated. Spot diameter throughout entire field of view is less than 1 μm and more uniform fluorescence excitation and imaging are achieved.
A laser alignment measurement method based on a focusing objective and PSD is proposed in order to solve the problem of mathematical model in traditional laser alignment method, which is complex and difficult to work out. Misalignment angle deviation and position deviation are acquired using relations between field of view and focal length of the optical system, and the principle of similar triangle. The measurement principle is simple and easy solving. The experiment results show that when position measurement range is 0.5 mm~1.5 mm and angle measurement range is 0.047 7°~0.143 2°, accuracy of position deviation measurement and angle deviation measurement are 0.035 mm~0.207 mm and 0.000 2°~0.012 8° respectively. Finally, error of accuracy is analyzed and a method considered for eliminating influence of installation error is proposed, and a theoretical formula is given.
By combining ends of a multimode fiber with a length of 17 mm and single-mode fiber(SMF), multimode fiber is then subjected to a tapered treatment to obtain a tapered multimode fiber with a wavelength and intensity sensor. Temperature sensing characteristics of sensor in range of 30 ℃ ~ 80 ℃ were studied experimentally. Experimental results show that when ambient temperature changes, response sensitivity of sensor to temperature of interference trough near 1 542 nm can reach 0. 041 nm / ℃ and 0. 106 dB / ℃ respectively, and there is a good linear relationship between wavelength response and intensity response of sensor interference fringe. Based on this linear relationship between sensor wavelength response and intensity response, phase demodulation of interferometric fiber optic sensor can be realized by detecting intensity of sensor signal. This method provides a new idea for sensing and demodulating signal value.
A bending target type flow sensor based on fiber Bragg grating(FBG) is designed. The strain distribution of bending target is simulated by COMSOL software and measured by two FBGs adhered on outside and inside surface of the target. Simulation and experiment results show that outer and inner sides of the target produce tensile strain and compressive strain respectively, and FBGs reflection wavelength have red shift and blue shift under external force. At the same time, temperature characteristics of the structure are tested and two FBGs reflection wavelength and temperature are linear at 20 ℃~40 ℃. Thereby the method can eliminate the effect of temperature in practical applications. Then measuring device of bending target type flow sensor is built, results show that two FBGs reflection wavelength and water flow have a good linearity within 0 L/H~800 L/H, and corresponding flow sensitivity is 48 L/H.