2011 Vol. 32, No. 1
According to the design theory of wavefront coding, taking the imaging restorability into consideration, we used the angle in Hilbert space between infocus PSF and several defocus PSFs as an evaluation for making the shape and intensity of point spread function (PSF) insensitive with defocus over a wide range of defocus parameter values. The angle in Hilbert space is capable of evaluating similarity of the PSFs of different defocus parameter values and was used to optimize the design of phase mask in real wavefront coding systems. A doublet-wavefront coding system was designed based on a finite focused doublet system. The simulation results show that the phase mask can extend the depth of focus by an order of magnitude more than that of Hopkins defocus criterion. Better and more stable imaging quality over a large depth of focus is acquired with the optimized pupil phase mask.
Since the most visual tracking can only track face target with displacement but not rotation, this paper presents a displacement and rotation tracking method based on maximum posterior probability. On the basis of maximum posterior probability visual tracking algorithm, this paper uses adaptive angle template to perform rotation tracking of face target. Compared with traditional visual tracking method, this method can accurately obtain the position and direction of human face when the face is slanted. The experimental results show that this method can efficiently track the face target with displacement and rotation.
By using Jones matrix method, it is derived that polarization converter with two variable retarders oriented at 0 and -45 transforms random polarization state to horizontally linear polarization, and the expression of retardation is given. Liquidcrystal variable retarder (LCVR) was developed, and the retardation was linearly proportional to applied voltage ranging from 1V to 2.7V. In addition, polarization converter based on two LCVRs was designed, and it was able to change any given polarization state (Stokes parameters described as (-0.567，0.752，0.36)) to horizontally linear polarization state (Stokes parameters described as (1，0，0)). The theoretical analysis and experimental result are presented.
Optics is the key part of a spectrophotometer, which directly affects system performance and accuracy. With portable, low cost, appropiate spectral range and resolution requirements as design target, the crossed Czerny-Turner structure based on plane diffraction gratings was proposed based on the operation principle of spectrophotometer and the theory of optical design. Comparison of some feasible design solutions were made, which included Littrow system, ebert-Fastie system, Czerny-Turner system and crossed Czerny-Turner system. Optical software was used for simulation and optimization of optics. The system spectral range is 360nm-740nm, spectral resolution is 10nm, F number is 5.25, spectrum width is 44.4nm, and the volume of the optical system is 80mm69mm62mm, which meets the design requirements.
An ultra-thin Fresnel condenser lens was discussed and designed. According to Fema principle, an aspheric surface was designed as central refraction area, and a total internal reflection (TIR) prism was designed as a catadioptric area in saw-teeth part. The optimal condense performance was achieved with ZEMAX. ZEMAX was used to simulate the concentration and distribution of Fresnel condenser, and a Fresnel condenser with thickness of 30mm was designed. The compounded Fresnel lens improves the utilization of solar energy and provides a better uniformity of energy distribution. The design method generates a better compound condenser than conventional Fresnel condenser.
A compact 2X optical zoom lens system for mobile phones was designed with ZEMAX. The system consists of 4 plastic aspheric lenses and an IR filter. The limiting resolution is 285lp/mm, which matches the CMOS image sensor of 1.75m pixel size. In the whole field of view (FOV), the RMS raidus is less than airy disk and the maximum distortion is less than 3%, the MTF at the half Nyquist frequency (285pl/mm)of CMOS in most part of the FOV were greater than 0.5,and the length of the optical zoom lens system is less than 9.2mm. It is compact in structure, high in resolution and low in cost, which meets the zoom lens system requirements of mobile phones.
A large-shearing block prism is designed, which can be used in electronic speckle pattern interferometry (ESPI). A wedge is made of a beam splitter cube on one of its surface. The oblique plane and one of its adjacent surfaces of the block are coated by reflection enhancing film. And other two surfaces are coated by anti-reflection films. When a normal incidence beam arrives at one of the reflect-reducing coating surfaces, the beam is split into two after inner beam-splitter. The two outgoing beams can be separated from each other when they are reflected by a parallel reflector and a tilted reflector respectively. If the large-shearing block prism is placed in front of the lens of a CCD camera, an object can be imaged into two shearing images and two adjacent objects can be imaged at same place on image plane. The shearing amount of the image is determined by the angle of the wedge. A large-shearing electronic speckle pattern interferometry (ESPI) can be realized by using the block prism if the shearing amount is sufficient. The angle can be selected from 1 degree to 10 degree based on the size of test specimen and the object distance. Carrier modulation experiments using a centrally loaded clamped circular plate are completed. The experimental results prove that the large-shearing block prism can be used in ESPI and the displacement fields can be obtained effectively.
The statistical properties of the speckle image from slightly rough cylindrical surfaces in the Fresnel diffraction field are researched. The relation between the correlation function of intensity fluctuations and cylindrical curvature and surface roughness parameter is derived. According to the discretization definition of its intensity fluctuations, the correlation function is obtained in theory and experiment when there is structure difference in radial and axial direction of the cylinder. It is shown that the light speckles are elongated in the direction normal to the cylinder axis for C1,C2, and that the fluctuation of correlation function intensity are differed in the directions parallel and perpendicular to cylindrical axis; the scattering characteristics of the two directions are basically the same for C3,C4. Further experiments show the dimensions of light speckles depend on both the surface roughness and the surface curvature. The result can be used in machinery manufacturing quality control such as column type pipelines, bearing etc.
The uniform cylindrical pulse source is often used as calibration source. Based on Monte Carlo techniques, the distribution of the irradiance at the output plane of the cylinder is simulated by variable weight method and fixed weight method. The distribution of the light is affected by the type of reflection, the position of the light source, and the shape of the cylinder. Simulation and experiment results show that, when light source located at the center of the cylinder with an aspect ratio of 3/2 (length to diameter), and was diffused by the cylinder inner surface in transmission, the required uniformity was achieved.
The role of electro-optical detection for early warning was discussed. The performance and features of infrared search and track system were reviewed. The technology status and development trend of electro-optical warning system in home and abroad were described based on the above mentioned information.
To detect airport runway in SAR image automatically, a new algorithm is proposed based on region segmentation and distance transform. The statistic information is used to enhance the contrast between runways and background，and 2-D histogram method is used to segment the image. Then, distance transform and partial space Hough transform is utilized to detect the center line of runways. Experimental results show that the algorithm can extract runways in large complex SAR images with good real-time performance.
A new method to improve the spatial image resolution of electro-optical sighting system is proposed. The movement of the optical double wedge was used to change the position of the imaging plane slightly. The micro-displacement sampling of the adjacent pixels and the targets in blind areas were realized without decreasing the size of the detection pixel or moving the target. The sampling rate was increased and the resolution of this system is improved using the super-resolution reconstruction technique. The experiments indicate that the method is simple and feasible, and the spatial image resolution is improved.
A new approach for seamless image mosaic based on feature invariant description was proposed. Firstly, the interesting points from two images were extracted by Harris corner detector, and the interesting points were descript by SIFT descriptor. Then the corresponding points were obtained by k-d tree algorithm and RANSAC algorithm. At last, the images were stitched by the corresponding features. The experiments show that this algorithm is invariant to rotation, and it has good robustness to image mosaic.
For estimating time difference of the received signals from two spatial separated sensors, this paper puts forward a special generalized correlation algorithm for the estimation of time delay based on wavelet denoising according to wavelet denoising theory. Since the denoising of the traditional hard and soft threshold is insufficient, the selection of wavelet threshold is improved, a new wavelet thresholding technique is developed. This algorithm is not limited by a priori knowledge of signal and noise needed in the traditional generalized correlation method, and enable direct correlation method with less stringent signal and noise conditions. Simulation and experimental results indicate that the algorithm is still effective at low signal to noise ratio (SNR), and could improve the delay estimation accuracy.
Interferogram processing is one of main techniques in optical interferometry metrology. Modern interferometry for thin-film thickness has the advantages of non-contact, high accuracy and great field of view, etc. Taking advantage of Twyman-Green interferometer, the relationship between interferogram and measured object parameter can be determined by mathematical model, thus the thickness of measured object is measured automatically. A new algorism of phase unwrapping for measuring the material object is proposed based on the FFT method, an algorism of diamond phase unwrapping. This algorism identifies a seed point, seed points spread to four points nearby the fist seed point, the four points will serve as the second group of seed points, these seed points will spread to four points nearby the second group of seed points in turn and pass through all of the effective information points by a diamond path. Seed points will eventually bring about the phase unwrapping in the entire image. In this paper, the thin-film thickness is determined by the method, setting the results of this method against results from ZYGO, the PV error and RMS maximum error are 0.0364 and 0.002 respectively. Although single interferogram was processed in this study, a phase distribution with high accuracy was achieved.
Since traditional measurement can only measure the parameters of information field at the beginning stage and terminal stage，complete information field parameters during whole guiding process is not available and the missile guidance performance can not be accurately evaluated. To solve this problem, the guiding distance measurement of the information field was realized by using large aperture zooming collimator and precision grating ruler. The information field was detected by optical fiber target and PIN detector array. The signal was processed according to the existing mathematical model and the information field parameters during guiding process were obtained. Results indicate that the test accuracy of command test reaches 0.01 unit command, the accuracy of irradiance measurement reaches 5% and the bore-sight accuracy reaches to 4.89.
The performance of polarization interference Fourier transform spectrometer is dependent on the angle error of each prism in Wollaston prism array(WPA). Based on the seamless stitching of interferograms, the maximum optical path difference of 1n WPA was deduced. Taking a 13 WPA as example, the impact of the angle error of each prism on spectral resolution was analyzed. The interferogram model was constructed based on fringe pattern intensity formula. The spectrum of 630nm monochromatic light was simulated with MATLAB program and the impact of angle error on spectrum was analyzed. A false spectral peak occurred when the angle error was larger than the magnitude of 10-3.
Spectral imaging traditional Chinese medicine (TCM) assessing method can be used to evaluate the quality of the medicine. In order to realize the fast, non-destructive and real-time detection and analysis of TCM, software is designed to control the components and take the spectral images. In the system, there are two key components, VariSpec Liquid Crystal Tunable Filter and CCD camera, which can be controlled by the software. The CCD camera obtains a series of spectral images with a specific wavelength interval by controlling the two components simultaneously and automatically. In addition, the software can draw two-dimensional characteristic spectrum curve and spatial distribution curve of three-dimensional of the TCM after processing the original spectral images. The results show that the software is available to control the components to detect the content of the TCM and draw its spatial distribution.
Since traditional edge detection algorithms have the disadvantages of low precision and efficiency in measuring the edge, a sub-pixel edge detection algorithm based on the polynomial fittings is proposed. According to the grey level distribution of the picture, this algorithm uses cubic polynomial to fit the edges to realize sub-pixel localization. Traditional sub-pixel edge detection algorithms detect coarse position at first, and then carry out the sub-divide, therefore the running time is relatively long. This method first fetches some points near the edge, and then carries out the sub-pixel edge detection algorithm, so the running time is reduced. Finally, the experimental results show that the proposed algorithm is very reliable and efficient.
Filter is an indispensable component for optical fiber communication system, especially for filter with all-fiber structure and selectable wavelength spacing due to its unique characteristics. A novel periodic filter with interleaving wavelengths based on high birefringence fiber interferometer was proposed and theoretically analyzed. Simulation results indicate that this filter has unique features. By adjusting the combination of the polarization rotations in this comb filter, filtering channels can be chosen by suppressing interleaving channels. The efficiency of the birefringence fiber is enhanced, as using 12m high-birefringence fiber can obtain the same filtering results with using 16m birefringence fiber in the Sagnac filter. This filter has potential applications for multi-wavelength fiber laser and other optical fiber communication system.
As a referenced spectral detector, the accurate calibration of fiber spectrometer is very important. The absolutely spectral response function of the fiber spectrometer within 550nm-900nm was obtained with a new technique, which avoids using a high temperature blackbody, and the uncertainty of the response function is less than 3.53%. Besides, a novel method is used to obtain the color temperature of the light source with characteristic of gray body.
Using the property of side polished fiber (SPF) sensor's polishing area sensitive to external refractive index, the photorefractive effect of liquid crystal hybrid film mixed with Azobenzene (AZO) and ZLI811 was studied under UV light pumping. The mixture composed of liquid crystal, AZO, and ZLI811 was coated on the polishing area after the SPF was calibrated, and the hybrid film was prepared. The transmission optical power in the SPF changes when photosensitive film is UV irradiated. Experiment shows that the photosensitive film is a negative photorefractive material, and the refractive index changes from 1.474 to 1.470 under the UV light pumping. This photosensitive film material can be used to create a new all-fiber optical-controllable components and sensors.
The sensitivity of long-period grating in refractive index sensing was analyzed by using the coupled-mode theory, and the formula of resonance wavelength shifts of LPFGs which changes relative to the surrounding refractive index was derived. If the period of grating is not changed, when the cladding refractive index is less than and close to the surrounding refractive index, the results show that the resonant wavelength shifts increased. The resonant wavelength shifts are greater for higher order cladding modes and smaller fiber cladding radius and smaller cladding refractive index, namely, the sensitivity to the surrounding refractive index is obviously enhanced. When the surrounding refractive index is greater than the fiber cladding refractive index, the resonant wavelength of grating will remain unchanged.
Two kinds of optical fiber sensors fabricated by fiber side polishing technique are proposed and demonstrated experimentally to measure the concentration of acetic acid. One of them is a side polished fiber Bragg grating sensor, whose cladding in the region of fiber Bragg grating is side polished. Because its reflective Bragg wavelength shifts when polished region is overlaid with different concentrations of acetic acid, the concentration of acetic acid could be measured.The other kind is a side polished single mode fiber sensor, whose part of cladding is side polished. The concentration of acetic acid could be measured by the transmitted optical power when polished region is overlaid with acetic acid. The resolution of side polished fiber Bragg grating sensor with a residual thickness of 0m is about 6.67%. The resolution of side polished single mode fiber sensor with a residual thickness of 0.5m is 0.55%.
Different kinds of Ce-TiO2 films were fabricated by radio frequency reactive magnetron sputtering. Based on the transmittance spectra measured by UVVis spectrophotometer, it was found that optical absorption band exhibited a red shift with the increase of Ce/Ti doping ratio. Calculation results indicates the band gap energy (Eg) decreases from 3.40eV to 2.73eV with the absorption band shift from 366nm to 455nm, which results in more absorption of visual light. The impurity energy level Ce 4f plays an important role in the absorption of visual light.
〖WTBZ] Optical power splitters are essential devices for optical fiber communication system in passive optical networks(PONs) and photonic integrated circuits (PICs). As one of the optical splitters, 12 electro-optical multimode interference (EO-MMI) power splitter fabricated by electro-optic organic polymer was analyzed by the selfimaging effect. The operation principle of the 12 EO-MMI power splitter was discussed. Firstly, the effective index of the three dimensional (3-D) ridge waveguide was computed by the effective index method (EIM). Then the positions of the two images were calculated respectively by using the formulas of the selfimage effect. The power-splitting ratio in the output waveguides was simulated by the guided mode propagation analysis (MPA). Research results show that, for the 12 EO-MMI power splitter operating at 1.55 microns, the special powersplitter ratio can be obtained if there are no electrodes on the ridge waveguide. Furthermore, when the electrodes are put on the top of the waveguide, the power splitting ratio is realized for a tunable scope of 7.8dB, for the electro-optical effect to change the refractive index.
Genetic algorithm based on elite selection strategy has good ability of global optimization and robustness. The non-linear least square method with Levenberg-Marquardt algorithm has fast and efficient local convergence features. The feasibility and effectiveness of reverse engineering of optical constants and thicknesses by the combination of both algorithms were studied. The results indicate that the combined fitting method can be used to calculate the optical constants and thicknesses of multilayer with great feasibility and efficiency under the actual restrictions, especially in finding the global optimum solution. At the same time, based on the theory of the whole spectrum fitting, the combined fitting method was applied to calculate the optical constants and thicknesses of 15-layer high-reflection multilayer and accurate thin films' parameters were obtained. The fitting experiments show that the combined fitting method is practical in reverse engineering of optical constants and thicknesses of multilayer, and it can be used in optical reverse engineering.
Passive athermalization technology of infrared zoom optical system is studied. Athermalization relationship was deduced and the condition expression for zoom system athermalization was obtained. Defocusing and chromatic aberration caused by thermal change can be taken as two kinds of primary aberrations, and can be eliminated with a passive athermalization system. There are four ways to implement athermalization in conventional systems, appropriate focal power arrangement, proper match of optical material, change of lens surface shape, and use of aspherical elements. A dual field of view (FOV) infrared zoom optical model was built, which had 6 lenses and one aspherical surface. It achieved good image quality in the range of -45℃～+55℃ in both FOV, with low Narcissus and less stringent tolerance requirement. Diffraction surface can be easily introduced in this model, which reduces the number of lens used and improve optical performance.
Based on the requirement of high-precision near-infrared data acquisition system and the characteristics of infrared detectors, two different types of data acquisition systems are designed, which can achieve the expected accuracy. In addition, a direct current analog-to-digital acquisition system is designed, which simplifies the circuit structure of the acquisition system and provides a reference for the design of near-infrared spectrum measurement system. Three different designs of data acquisition system for near-infrared detector are provided, and relevant analysis circuit is implemented to analyze each circuit's feature and its application environment.
To meet the performance requirement of infrared materials in battlefield scenarios, the feasibility of composite materials for infrared stealth based on super absorbent polymer (SAP) was analyzed, and its sample was produced. According to the evaluation, the material can change the infrared radiation of military targets and can be used as the surface material in military equipment to lower target signature.
The combination effect was evaluated by combining effect factor defined as the ratio of actual maximum light intensity to the ideal combined light intensity at the target. On this basis，the main factors affecting combination effect and the propagation property of combination effect factor were studied．Good combination effect could be obtained with a small duty ratio，while the main factor influencing the combination effect is the random phase errors，because the beam combination has actually become incoherently combined when the random phase errors are greater than a specific value．With the increase of the propagation distance，the influence of the duty ratio on combination effect becomes smaller, and one of the main factors influencing combination effect is random phase errors.
The relative phase between the neighboring nodes in the adjacent planes of Walker- constellation was introduced as the connecting parameter K. The motion equations in the body-fixed coordinates system of reference satellite were established to obtain the azimuth angle, elevation angle and range (AER) of the inter-plane laser links. Then, the extremums of AER and the variation rates were calculated numerically. Among the AER, the azimuth angle rate, with the maximum value inversely ratio to the minimum range, impacts the stability of laser links significantly. In some extreme cases, it would be too fast to track the laser beam because of large azimuth angle velocities. K=0 is a good trade off between the linksstability and the complexity of the network topology, and is fit for low earth satellite optical networks.
In order to implement blind-pixel compensation for IRFPA in omni-directional laser warning system, a new weighted interpolation algorithm based on neighborhood grey character is presented. The impact of blind-pixel on the system was analyzed, the basic concept and principle of the algorithm were elaborated and analyzed, and the algorithm was verified by experimental simulation. Finally, the blind-pixel compensation result was evaluated subjectively and objectively. Compared with the conventional algorithms of neighborhood averaging and mid-value filter, the new algorithm is more suitable for the blind-pixel compensation of omni-directional laser warning system. The blind-pixel compensation offset of the new algorithm is smaller than those of the other two algorithms, and it reduces the impact of blind-pixel influence on the system, which can improve the system working performance effectively.
In order to achieve high peak power and narrow pulse-width semiconductor laser, a driving system for the laser was designed. The electrical circuit for producing narrow-high current pulses based on MOSFET as a fast switch for the discharge circuit was analyzed. The working procedure of the MOSFET as a fast switch was discussed. In order to make the MOSFET switch as fast as possible, the push-pull driving circuit was brought forward and the circuit of narrow trigger pulses was designed. When the LD was connected to the RC discharge circuit, the peak power of the emission laser could be up to 67.5W and the full width at half maximum (FWHM) could reach 20ns. Finally, the impact factors on the pulse width of the laser were analyzed.
Backscattering sources and their influence on total backscattering in ring laser gyroscope was analyzed based on vector superposition principle. According to the coupling effect of backscattering on counterclockwise and clockwise beam, the relationship between total backscattering coefficient and light intensity of oppositely directed traveling waves was derived. Based on this relationship, the curve of backscattering was obtained through pushing and pulling mirrors to different places to change the shape of resonator, at the same time, and the amplitude of AC portion was measured. The vector superposition principle of backscattering was verified by comparing it with simulated results.
In order to improve the accuracy and recognition efficiency in the process of decoding, laser pulse code was decoded using neural network technology. Firstly, the regular code, such as periodic code and arithmetic code, was decoded using linear neural network. The results showed that, it can recognize PCM code accurately if the pulses of two periods were acquired, and only 4 pulses for arithmetic code. After that, the minimum period of pseudo-random code was recognized using PNN. The results showed that, PNN can identify the minimum period only in limited amount of information.
Atmospheric turbulence induced intensity fluctuations lead to scintillation, beam drift, angle of arrival fluctuation and wavefront aberration. The multi-beam launch technique is one of the effective ways to mitigate the effect of atmospheric turbulence. Calculation of the beam quality parameters through peak intensity, encircled power radius and bulk parameters based on single beam, double beam, four beam and eight beams of light emission was made. Single-beam and multi-beam intensity fluctuation characteristics of atmospheric transmission were summarized. The results show that, with the increase of beam number, the peak intensity increases significantly, the encircled power radius decreases gradually, and the bulk parameters decreases as well. It is concluded that multi-beam emission method can effectively reduce the atmospheric turbulence effects of laser transmission.
The spectra properties of Yb3+∶SrMoO4 crystal were investigated by employing the absorption and emission spectra. The strongest absorption is observed at 976nm in the absorption spectra. The absorption cross-section is 1.7110-20cm2 at 976nm. The absorption band has a FWHM of 71nm. The Stark energy levels of Yb3+ in Yb3+∶SrMoO4 crystal were obtained in the absorption spectra and low temperature emission spectra. The emission peak is at 1021nm. The emission band has a FWHM of 44nm. The emission cross-section calculated by the reciprocity method is 1.2410-20cm2 at 1021nm. The fluorescence lifetime of Yb3+∶SrMoO4 crystal is 878s by fitting the fluorescence decay curve. The laser parameters of Yb3+∶SrMoO4 crystal were calculated by the spectral data, the pump saturation intensity parameter is 4.35kW/cm2, the minimum fraction of Yb3+ ions that must be excited to balance the ground state absorption and the gain exactly is 10.08%, the minimum pump intensity is 0.44kW/cm2. For large absorption and emmision FWHM, long fluorescene lifetime and low laser threshold, Yb3+∶SrMoO4 crystal could be considered as a LD pumped laser gain material and used in femto-laser and tunable laser area.