2013 Vol. 6, No. 3
In this paper, the electromagnetic responses and potential applications of THz metamaterials are reviewed through the focus on fabrication, unit structure design, and material selection, respectively. It describes different kinds of fabrication technologies for obtaining metamaterials with special electromagnetic responses such as magnetic resonance and reconfigurable tunability, which is helpful for further understanding of electromagnetic resonances in metamaterials. The paper analyzes the electromagnetic response characteristics in detail and points out that the unit structure design can be used to obtain desired electromagnetic characteristics, such as anisotropy, bianisotropy, polarization modulation, multiband response, broadband response, asymmetric transmission, optical activity, and perfect absorption, etc. The dependence of electromagnetic responses upon surrounding dielectrics can be used not only to control resonant frequency by a proper substrate selection, but also for sensing applications. Furthermore, the introduction of functional materials with controllable dielectric properties by external optical field, electrical field, magnetic field and temperature has the potential to achieve tunable metamaterials, which is highly desirable for THz functional devices. Finally, the opportunities and challenges for further developments of THz metamaterials are briefly introduced.
In this article, we introduce several main Focal Plane Array(FPA) structures used nowadays and try to find their advantages and shortcomings. Several main parameters used to describe the performance of FPA and their influences on system performance are described. On the analysis and comparison for existing FPA technologies, it suggests that the integrated detecting scheme based on a Avalanche Photodiode(APD) and a Read-out Integrated Circuit(ROIC) will be an ideal real-time three dimensional imaging technology due to its advantages in sensitivity and operation ranges. It also indicates that in the near future, the scale of FPA can exceed 1 024 pixel1 024 pixel with a pixel pitch of 15 m.
This article introduces a systematic review of recent progress in applications of Fiber Bragg Grating(FBG) sensors to Structural Health Monitoring(SHM), oil industry, electrical power industry, geotechnical engineering, aerospace,mine, blasting, traffic, medicine, etc. It points out that the lacks of unified theories and standard technolgies lead to different product specifications for the Bragg grating sensors from different manufacturers, which limits the industrialization and large scale application of the sensing technology. Furthermore, because of their complex and fine manufacturing technologies, the Bragg grating sensors have a higher production cost. Most importantly, the demodulation products are far from enough although many demodulation methods have already been published. On the analysis above, the paper presents its future work.
We analyze the fixed point method with Tikhonov regularization under the periodic boundary conditions, and propose a changable regularization parameter method. Firstly, we choose a bigger one to restrain the noise in the reconstructed image, and get a convergent result to modify the initial gradient. Secondly, we choose a smaller one to increase the details in the image. Experimental results show that compared with other popular algorithms which solve the L1 norm regularization function and Total Variation(TV) regularization function, the improved fixed point method performs favorably in solving the problem of the motion degradation and Gaussian degradation.
As interference pixels influence the background features in traditional methods, a new anomaly detection algorithm for hyperspectral imagery is proposed based on the summation of spectral angles. The anomaly degree of each pixel in the imagery is acquired by calculating the angles between the spectral vector of test pixel and other spectral vectors in the test region, and accumulating the angles. Then, the pretreatment method of band selection is used to further improve detection performance. HyMap hyperspectral data experiments show that the detection probability reaches 0.73 when the probability of false alarm is set to be 0.008. The reliability of anomaly detection is improved, while the probability of false alarm is reduced.
On the basis of elastic bending theory, the bending deflection expression by both quadrate and circinal facula irradiations is deduced for burn-through effect with no-melting of metal heated by a laser caused by tangential airflows. A rupture criterion for burn-through with no-melting is given out by the Mises theory. The study indicates that the material softened by laser heating is the primary mechanism for burn-through effect with no-melting. The maximal deflection of thin plate in the spot area is related to the airflow speed, spot diameter, plate thickness and the elasticity modulus(expression is U2a4/Eh3), the damage temperature is related to the airflow speed, spot diameter and the plate thickness(expression is (Ua/h)2), and the damage threshold under the circinal facula irradiated is higher than that under the quadrate facula. The numerical result indicates that for aluminum alloy shell, the damage threshold of laser energy can decrease much up to 40%-50% under 0.8 Ma tangential airflows, but it can decrease less(about 20%) for a stainless steel shell. The damage threshold decrease under airflows needs more attention.
This paper attempts to look for a new method to obtain a square spot whose brightness distribution is homogeneous and the value of BPP is equal in two directions. Moreover, a spherical spot may be obtained too, if it is coupled with a fiber. The method only adopts the cylindrical and the spherical lenses and no other kinds of micro optical elements unlike the methods in foreign countries. An exit pupil of the system is taken as a spot to solve the problem of brightness homogeneous distribution, and the spacing influence of the light sources upon the value of BPP is clean up to decrease the value of BPP of the slow axis. Furthermore, the characteristic of the point light source is utilized to allow the BPP values of the fast axis and the slow axis to be equaled. Finally, this new method is used to design the coupling optical systems with different demands and specifications. The design example in this paper shows that there are 19 light sources at a bar, and the number of the bars is 21, then the total number of the light sources is 399. The size of the square spot obtained is 0.6 mm0.6 mm and NA is 0.22. The spherical spot can be obtained if the system is coupled with the fiber with a diameter of 1 mm. Compared to the foreign methods, the construction of the new method is simpler and the demand for crafts is lower. The new method is suitable for the semiconductor lasers with high power and low values of BPP.
The composition and principle of a vehicle-borne laser mapping system are described in this paper. A visible light source is designed for a laser scanner in the vehicle-borne laser mapping system, by which the laser measuring center and scanning direction can be determined effectively and a parts of calibration parameters are able to be measured directly. A calibration scheme based on the visible light source is designed, then static calibration experiment and dynamic measurement are performed. Obtained results show that the calibrating error is 0.028 1 m and plane positioning error is 0.288 m. The experiment results and analysis prove the correctness and validity of calibration project.
Influence of exciting laser sheet intensity distribution on the fluorescence intensity distribution in a Planar Laser-induced Fluorescence(PLIF) experiment was analyzed. Based on cylindrical lens array, a laser sheet shaping system was designed. According to the requirement of PLIF, several parameters were optimized with ray tracing method. The laser beam shaping experiment system was established and applied to the PLIF experiment for dye laser beam shaping and acquiring smoothing laser sheet with the nonuniformity no more than 4%. With the uniformity laser sheet beam, PLIF was applied to the measurement of fluorescence imaging of OH in alcohol flame and CH4/air premixed combustion.
The variable-focus cylindrical lens based on a Polydimethylsiloxane(PDMS) chip is designed and fabricated, which is composed mainly by a glass capillary embedded in a soft PDMS chip, and the focal length varies by selecting different liquids filled into the glass capillary. When the refractive index of liquid changes in the range of 1.451 8 to 1.550 2, the focal length varies from 21.369 to 3.362 mm, which reaches a 6.4 zoom ratio. With light-scattering imaging method, the ray traces of parallel light passing through various lenses are viewed and photographed. The imaging processes are simulated by using the ZEMAX optical design software, and the simulated results are consistent with the experimental photographs. The focal length formula of a glass capillary buried in PDMS chip is derived with the Gaussian optical successive imaging method, and is verified by the simulated method and experimental results. The successful fabrication of the variable-focus cylindrical lens based on PDMS substrate provides an important optical imaging element for lab on a chip.
A temporal model was proposed for simulating the atmospheric turbulence varying with the time in the adaptive optics system testing. The relationship between the number of interpolated frames for the random phase screen, refurbish frequency and the mean wind speed was analyzed. The analysis result demonstrates that it is necessary to smooth the random phase screen for characterizing the temporal gradual variation of the random wavefront in order to make the change of the random wavefront better aligned with the influence of atmospheric turbulence on continuous smoothing gradients of incident wavefront. The interpolated frames of the random phase screen is only related to the aperture diameter and the atmospheric coherence length, but not related to the wind speed, and the refurbish frequency of the random phase screen increases with the mean wind speed, and the refurbish frequency smoothed increases with the number of interpolated frames. A atmospheric turbulence simulator was constructed in laboratory and the analysis of the power spectrum density of experimental result demonstrates that temporal model of the atmospheric turbulence simulation is valid.
To overcome the instability of the video sequences caused by the undesirable shakes of a camera, and to reduce the missing of edge information in the process of compensation, a full-frame video stabilization system based on the Speed-Up Robust Feature(SURF) was proposed. Firstly, the SURF was employed to extract the features in the images of the current and the reference frames and to match the features between the two images, so that the correspondence could be established. As a few features were extracted and the features of some areas in the scene were similar, a method of consistency restrain of the features' displacement was proposed to ameliorate the RANSAC. The motion vector precision is less than 1 pixel. Secondly, by determining the reference frame update strategy, the smoothed inter-frame global motion vector was obtained. Finally, mosaic was used to implement the motion compensation, and the corresponding pixels of the reference frame were filled with a stabilized frame to compensate the unstable motion and to output a stabilized full frame video. The Peak Signal to Noise Ratio(PSNR) is improved by 33.1 percent.
To improve the Pixel Response Non-uniformity(PRNU) in a TDICCD mosaic camera, an algorithm was proposed to correct the PRNU between the channels and among the pixels in channels at different gains, deviations and integrated time. Then, a design method was given to implement the algorithm on a Field Programmable Gate Array(FPGA) platform. The experimental results indicate that the problems of PRNU could be solved at different gains, deviations and integrated time in the TDICCD mosaic camera. The PRNU of a single TDICCD could be decreased from 4.72% to 0.27%. In bad circumstance, the PRNU of the single TDICCD could be decreased from 4.68% to 2.13%. The actual image also proves that the pixel correction algorithm has high real time performance, high reliability and stability, and a satisfied result to the requirements of space-projects. It solves the real-time pixel correction problem of the PRNU in TDICCD mosaic cameras.
In order to clearly reconstruct targets in a dim and deep space under the condition of the Lower Signal to Noise(SNR), a new time domain signal acquisition method of Fourier Telescopy(FT) based on Non-uniform Periodic Sampling(NUPS) was proposed. The simulation experiments on the proposed method were performed and the reconstructed images by NUPS method and traditional uniform sample method were compared. With the NUPS method, 100 points were respectively collected at the sampling frequency 1 MHz and 5 MHz. Then the signals of two sequences were respectively fast Fourier transformed and the concern frequency information was averaged. With traditional uniform sampling methods, 200 points were collected at 1 MHz and 5 MHz sampling frequency and then demodulated average was developed. Compared results show that the Strehl ratio of reconstructed image with diffraction-limited image is enhanced 0.03 compared to the original method when SNR is 50, and the Strehl ratio is 0.531 1 that is improved 0.223 3 compared with uniform sampling when SNR is 20. Obviously, imaging quality is improved by the NUPS method under conditions of low SNRs, thus the laser power is reduced. The NUPS lays the technical foundation for the implementation of the FT engineering system.
In order to satisfy the requirements of a three-linear CCD camera for stiffness and stability and to improve its the mapping accuracy, the structure design of the mapping foundation support of the camera was accomplished based on finite element analysis. The mode, self-weight deformation and light axis variation due to self-weight deformation were analyzed. A special detection tool was developed based on the qualifications of the mapping foundation support. The tracking detection to the mapping foundation support in the process was developed to verify the feasibility of the special detection tool and to assure its geometric accuracy.
When traditional Digital Particle Image Velocimetry(DPIV) is used to measure water spray particles, the elongated image from a common CCD will effect the measuring results. To eliminate the effect of the elongated image of water spray particles, an Improved Image Method(IIM) was built based on DPIV. An experiment system was designed to make a real-time test for the water spray, by which the difference of measured particle sizes obtained by the improved image method and the direct equal method was contrasted. The results show that the algorithm used in this paper is more available to measure the radius of water spray particle. The radius distribution curve is fitted by least square method, which shows that both the lognormal function and Weibull function can describe the radius distribution well.
To measure the micro-vibration parameters of a solid target, the laser Doppler signal model of microvibration for the solid target was established. First, the laser Doppler vibration instantaneous samples were converted into signal spectral samples based on Hilbert transform. Then, the mean value of the instantaneous frequency per vibration period was determined by spectral method, the value of vibration frequency was obtained by the quadrature procession of the difference sample sequences, and the amplitude was estimated according to the relation between frequency deviation and amplitude of vibration signal. Finally, experimental results were verified according to the Hilbert measuring method and the error source was analyzed. Experimental results indicate that the vibration amplitude of the target is about 1.8510-4 m, and the rotation frequency is about 170 Hz. It is feasible to obtain the motion parameters by using Hilbert transform to measure target micro-vibration.