2012 Vol. 33, No. 4

OE System and Engineering
Numerical simulation of thermal radiation effect about optical dome in aerodynamic thermal environment
FAN Zhi-gang, YU Cui-ping, XIAO Hao-su, ZHANG Wang
2012, 33(4): 649-653.
The temperature of the high speed aircraft optical dome rapidly rises due to aerodynamic heating, and causes severe thermal radiation effect to detection system in the aerodynamic thermal environment. To assess the performance degradation as a result of aerodynamic heating of the dome flying at high speed, the thermal radiation transfer of the optical dome was modeled by introducing the concept of heat pupil. The thermal radiation transmission of the dome through the optical system was simulated numerically, and the irradiance distribution of the dome on the detector-s receiving surface was obtained according to the nonhomogeneous temperature distribution. The research results show that the concept of heat pupil suits for the dome and reduces the time in ray tracing enormously, and according to the trend of the dome-s radiation receiving by detector following time, we can see after a few seconds in flight the interference radiation from dome would flood the target signal. Therefore, the thermal radiation of the dome in aerodynamic thermal environment cannot be neglected.
High-resolution X-ray digital radiography of electronic industry
LI Wei
2012, 33(4): 654-659.
This paper introduced a design of high-resolution digital X-ray detection system used in electronic industry. According to the requirement of the design, this paper put forward the overall scheme of the system,researched and selected the micro-focal spot X-ray source and double proximity focusing X-ray intensifier as the key devices in the system, analyzed the influence on the final resolution caused by their own resolution of micro-focal spot X-ray source, double proximity X-ray intensifier and the system magnification. It designed the light path, image acquisition device and acquisition software, developed a complete imaging system prototype, and carried out debugging and experiment on it ,then completed the practical X-ray image processing software by using VC 6.0. Results show that the effective vision of this imaging system achieves up to 75 mm, and the resolution is 160 lp/cm.
Illuminant system with high extinction ratio and poly polarization
YAN Li-rong, GAO Ai-hua, LIU Wei-guo
2012, 33(4): 660-665.
In modern engineering measurement, higher performance of illuminants is required to adapt the present engineering technical specifications for which have been demanded increasingly. An illuminant system with high extinction ratio and poly polarization was designed, based on the influence of the crystal birefringence and phase retardation on polarization state, as well as the relationship between the light refraction angle and the vibration direction, and a test program to verify the polarization state and extinction ratio of the emitting light in the system was designed. The illuminant system could easily output the polarized light including S polarization, P polarization and circular polarization by the computer-precisely-controlled stepping motor, and the system obtained the polarized light with high extinction ratio using the polarizing prism which was controlled by the translation and rotation device, and the extinction ratio of emitting light was no more than 510-6.
Optimal design for Sagnac interferometer of imaging spectrometer
LU Xue-feng, BAI Qing-lan
2012, 33(4): 666-669.
The optimal design of Sagnac interferometer was studied to meet the lightweight design requirements of imaging spectrometer. The relationship between Sagnac and the magnification of system-s front telescope was discussed. The interferometer-s structure and optical paths were analyzed by unfolding it to flat glass. The experiential formula of the geometry dimensions of Sagnac that was changed with the field angle and the aperture of incident light was given. The optimal geometry dimension of Sagnac and the corresponding magnification of system-s front telescope could be calculated by the formula, which was called the optimal design. The optimal design result of a short wave spectrometer was calculated as an example using the formula.
Calculation of initial configuration for 15× double-linkage infrared continuous zoom system
WANG Nan-xi, JIAO Ming-yin, JIANG Xiao-bai, HE Yu-lan, YANG Hua-mei, QIANG hua
2012, 33(4): 670-674.
Double-linkage zoom structure is composed of two groups connected together which is the zoom group for doing linear motion and the compensation group for doing non-linear motion. This structure has the characteristics of both the optical compensation and the mechanical compensation. This structure is easy to realize large zoom ratio, the system length is short and the cam curve is smooth. The radii and materials of components were established by using the PW method and the optical structure parameters relationships, and the initial structure meeting the design requirement was designed by CODEV software.
Freeform surface secondary lens for LED streetlight
QIAO Qing-fei, LIN Feng
2012, 33(4): 675-679.
Light distribution is a critical issue for using LED streetlight. Roadway illumination requires the LED streetlight to have a rectangular facular with a good uniformity on the roadway and without any light pollution and glare out of the road. The freeform surface equation was deduced according to the Snells law and etendue conservation. Then Pro/Engineer was applied for the freeform surface lens construction. With the Matlab tool, the partial differential equation was solved with the method of finite difference and the data array of the freeform surface was got. Finally, TracePro was applied to simulate this optical system, test and analyze the design result. For this design, the LED streetlight lens has an ellipsoid inner surface and a freeform outer surface. The results show the total uniformity of illumination on the roadway reaches 0.75, while the longitudinal uniformity reaches 0.85.
Far-field distance analysis of LED array luminaries
WANG Hang-wei, LIU Xian-ming, CHEN Wei-min, LEI Xiao-hua, LI Cun-long
2012, 33(4): 680-685.
Light emitting diode (LED) has been the most popular lighting source around the world for its varieties of advantages such as energy-saving, environmental friendly, seismic and longeval. LED luminaries are usually assembled by aligned packaged LED dies, which are surface sources. In order to measure the light distribution accurately, the detector should be placed far enough away from the luminary. The measurement distance is called far-field distance which makes the surface source be considered as the point source. There are a variety of national standards defining the value of far-field distance; however, the defined distances are different from each other and only the impacts of the maximum source size are considered. In this study, the far-field distance of LED array luminary was investigated. The farfield distance of lamps with different intensity distributions, LED array sizes, LED array arrangement densities and LED array arrangement styles were calculated first. Then, the impacts of these parameters on far-field distance were analyzed. Finally, suggests to determine the far-field distance of LED array luminaries were given. This research could offer reference for the accurate measurement of the far-field distance intensity distribution of LED array luminary.
Ray tracing method of optic free-form surface based on segmentation and iteration of sub-surface
ZHU Jin, GAO Zhi-shan, YUAN Qun, YE Jing-fei, ZHANG Cong-yang
2012, 33(4): 686-692.
A novel method based on segmentation and iteration of sub-surface is presented as the description of optic free-form surface, aiming at the complex and relatively smooth character of its figure. Three steps are executed in the whole process of ray tracing of free-form surface. Firstly the free-form surface is divided into some sub-surfaces by discrete points. Then the target sub-surface is determined based on the angle sum of a triangle theorem. And finally the iterative algorithm is utilized on the target sub-surface. Even asphere, biconic surface, extended polynomial surface and a single lens system with a freeform surface are set into simulation. The comparison between the result of the method presented and the result of ZEMAX shows that the method presented is of high precision, good versatility and high speed.
Reflection characteristics of metal-dielectric-metal reflective-type color filter
DING Qiu-lan, ZHOU Yun, YE Yan, CHEN Lin-sen
2012, 33(4): 693-697.
Reflection filtering characteristics of metal-dielectric-metal (MDM) structure at visible region are investigated theoretically. The structure consists of 3 parts: the flexible transparent substrate, the metal layer and the dielectric-metal grating layer. By using rigorous couple-wave analysis (RCWA), the reflection characteristics of transverse magnetic (TM) polarized light are analyzed as a function of the duty cycle, the period and the thickness of the dielectric-metal grating. It is found that different colors can be obtained by adjusting the parameters of the structure. Based on the simulation results and the cyan-magentayellow (CMY) color scheme, a high-performance color filter is proposed by optimizing the structural parameters and the 3 prime colors including cyan, magenta and yellow are obtained. The reflection valley and the bandwidths remain almost unchanged over a wide incident angle range of 0~45. The proposed color filter is suitable for filtering used in reflective-type color displays.
SiO2 photonic crystal grating based on 2-D rectangular
LU Hui-dong, ZHANG Rui, SHEN Hong-jun
2012, 33(4): 698-702.
In order to improve the light transmittance and solve the Fresnel reflection problem which is common in optical systems, we designed a two-dimensional rectangular SiO2 photonic crystal surface grating structure. Using the Legendre polynomial expansion method to calculate the transmittance in the visiblenear infrared wavelength range of the incident light wave of the structure, we found the appropriate duty cycle e, the grating depth h, and the substrate thickness d, which could stabilize the change of transmission rate and make the average rate reach over 96% when the incident angle varied in the range of 0~60 and the incident wave length in the range of 400 nm~1200 nm.
OE Information Acquisition and
Particle filter object tracking algorithm based on dynamic feature fusion
YANG Heng, QIAN Jun, JI Ming, SUN Xiao-wei, LU Yang, SONG Jin-hong
2012, 33(4): 703-710.
A particle filter object tracking algorithm based on dynamic feature fusion is proposed. The presented algorithm uses the complementary features, which are gray histogram and gradient histogram, to represent the object model. In the tracking procession, the confidence for each feature is adjusted according to the discrimination between the object and the background, and the object model is established and updated onlinely. The presented method can improve the accuracy of the object modeling and furthermore improve the accuracy of the particle filter tracking algorithm. Experimental results show that, in the representative object tracking scenes, the proposed algorithm can gain more accurate and more reliable tracking performance.
No-reference image SNR assessment under visual masking
FAN Yuan-yuan, , SANG Ying-jun, SHEN Xiang-heng
2012, 33(4): 711-716.
A novel no reference image signal to noise ratio(SNR) assessment method based on the property of human visual system (HVS) was proposed to evaluate the quality of noisy image. Firstly, the method adopted Hosaka plots idea and quadtree decomposition which wiped out the limits on image size. Secondly, this improved method was applied to the image to make plots and was employed to abstract sub-blocks noises. Then, the no reference image signal to noise ratio (NRPSNR) was computed according to the pollution degree. The method was tested and verified by using the LIVE Database Release2 image database. The experiment results show that this no reference assessment method which CC reaches -0.895 0, SROCC reaches -0.923 6, OR reaches 0.042 9,is superior to other three kinds of methods on the whole, it is more consistent with subjective image quality assessment, suitable for other noisy styles and embodies the HVS visual masking effect. It can give accurate and reliable quality assessment of no reference noisy images.
Ground multi-target positioning algorithm for airborne optoelectronic system
YAN Ming, DU Pei, WANG Hui-lin, GAO Xian-juan, ZHANG Zheng, LIU Dong
2012, 33(4): 717-720.
Traditional single target position finding method can not fit the condition that the situation of modern war field changes in real time and has many targets. In order to enhance the efficiency of target positioning, this paper developed a new semi-active multi-target positioning algorithm for airborne optoelectronic system on the basis of single target position finding. Through establishing the initial east-north-up (ENU) coordinates, aeroplane ENU coordinates, crankshaft coordinates, EO stabilized-aiming coordinates and multi-target projection coordinates, a series of coordinates transform formulas using the relative angles of target were derived, which could calculate the longitudes and latitudes of multi-target. The result of computer emulation showed that this algorithm could calculate the latitudes and longitudes of multi-target at the same time. The positioning precision of hypo-targets was lower than that of the main target because of the increased accumulated error of coordinates transforming by the pixel deviation.
Selection and extraction of difference statistical features based on dual color MWIR images
ZHOU Xiao, YANG Feng-bao, LIN Su-zhen, AN Fu
2012, 33(4): 721-726.
Feature space dimension of the dual-color mid-wave infrared (MWIR) image is large and difficult to conduct a comprehensive analysis of its dualband imaging differences. The three-level feature selection model based on the statistical features was proposed. This model firstly standardized the dual-bands original statistical features, then with the distribution of four-group dual-band images, through the analyses of the significant-difference level, consistentdifference level and special-difference level, chose the effective statistical features which satisfied the dual-band image differences distribution regularity. Based on the feature selection, this paper used locally linear embedding (LLE) to reduce the dimension of the effective statistical features. The experimental data indicate that the one-dimensional features after feature selecting and extracting can show the differences in rate of dual-band image, achieving the purpose of using small amount features to comprehensively analyze the differences in dual-band.
Vehicle infrared video colorization based on contour tracking
QIAO Shuai, SUN Shao-yuan, GU Xiao-jing, GE Man
2012, 33(4): 727-732.
A fast algorithm of vehicle infrared video colorization was proposed. Contour feature points tracking was utilized to get the contours and areas of different object classes in each frame. Characteristic colors of different classes were transferred to the corresponding areas in each frame. Firstly, we constructed the characteristic color sets of different classes of scenes. The characteristic colors of different scenes in the natural color images were extracted as the corresponding scenes-colors in the infrared images. Secondly, we clustered and segment the scenes in the key infrared video frame using the improved efficient KMeans clustering method, and the contour feature points were extracted. Thirdly, we tracked the contour feature points using the Kanade-LucasTomasi (KLT) algorithm to get and correct their positions in the next frame. The B-spline interpolation method was used to reconstruct the contours. The contours and areas of different object classes in each frame were obtained. Finally, characteristic colors of different classes were transferred to the corresponding areas in each frame. Infrared video colorization was realized and the scenes were with reasonable colors. Experimental results show that the processing speed of the proposed method is nearly 5 times as fast as that of the algorithm based on motion estimation and it can give a natural color look to the infrared video.
Infrared imaging colorizing method based on local linear embedding
WU Qiang, WANG Xin-sai, WANG Wei-ping, MEI Jing-rui, HE Ming
2012, 33(4): 733-737.
Aiming at the problem that the infrared image has the weakness of dim targets and poor details, is generally black-white image and not used for chroma subsampling, the paper introduces a new infrared imaging colorization algorithm based on local linear embedding(LLE). The method seeks the mapping for the gray space to the color space, achieves the transfer from infrared image to colorized infrared image. Firstly this paper transfers the object infrared image and source color image to YUV color space, separates the luminance and hue information, secondly it makes each pixel and the neighbor pixels connected to vector, chooses equally part pixels of reference color image and makes them become vector using the same principle, then accomplish matching pixels between the object and reference images, and highlights the brightest parts of the resulted infrared image based on auto-threshold pseudocoding. Applying the method on infrared thermal imager of lab, it has color information that improves the visual effect, avoids visual fatigue, and makes the hot targets more noticeable using the sensitive color such as red and yellow, the experiment results show that it is advantageous to long-time observation and target distinguishing.
Optical Metrology and Measurement
Waveband optimization method for enhancing precision ofmeasuring space object-s temperature
WANG Guo-qiang, WU Yuan-hao, CAO Jing-tai
2012, 33(4): 738-742.
This paper proposed a method to enhance the sensitivity and the resolving power of the space object-s temperature measurement through optimizing the waveband. We built the mathematical model for measuring the temperature, in terms of the signal-to-noise ratio(SNR) and the relationship among the relative temperature measurement sensitivity, the temperature resolving power,the linearity of the metrical curve and the working waveband, and optimized the central wavelength and the bandwidth. The optimal results show that the wavebands fit for measuring is 8.0 m ~9.6 m, the bandwidth is 60 nm, the biggest resolving power of the measurement temperature is 0.06 K, and an SNR above than 5 is achieved. The proposed method improves the resolving power of temperature measurement and the measurement sensitivity of the system by 8.7% and 11.3% respectively.
Image measuring system of engine tip clearance
FAN Xiao-hu, ZHU Mu-cheng, NIE Shi-liang
2012, 33(4): 743-746.
The gap from the engine rotor blade tip to the open half machine magazine is an important parameter that inspects whether the engine is qualified. And its value is of prime importance to the engine on high efficient and safe operation. Based on stereo vision principle, automatic focus technology, edge detection algorithm, this paper designed a blade tip clearance measurement device used in the process of engine assembling. This device realized the non-contact measurement to aero-engine rip clearance with simple operation and high-accuracy measurement. Experiment result shows that the device accuracy can reach to 20 m.
Methane gas sensor based on infrared difference detection
Lu Yu-xiang, DONG Xiao-jie, GUO Feng
2012, 33(4): 747-751.
In infrared absorption detection of methane, the key to improve detecting precision is to compensate the errors. Various differential absorption techniques are usually adopted to compensate the errors, which reduce different kinds of interferences. On the basis of infrared differential detection, a dual-wavelength difference methane sensor was designed. A rotating filter was used to control the filtering and passing through of the light, which made the detecting light and reference light pass through the optical path in a time-sharing way. The sensor structure of single-source single-light-path and single-detector was implemented, eliminating the errors caused by the source power fluctuations, the light-path loss and the detector instability. The experiments show that the sensor has high measurement precision: in the concentration range of 0~6%, the maximum relative error is less than 1%.
Pyroelectric laser energy meter used in non-regular ambient environment
YANG Ye-ping, HOU Min, LI Gao-ping, YANG Bin, YU Shuai
2012, 33(4): 752-755.
The sensor sensitivity of laser energy meter at present is related to the ambient temperature and varies considerably in the range of -50℃~70℃ which influences the measurements directly. To eliminate the influence of ambient temperature,this paper describes the sensitivity calibration method for pyroelectric energy meter at different ambient temperatures. First of all,the energy meter is put into the environment test chamber and the temperature point is set every 10℃ in the range of -50℃~70℃.Next the sensitivity correction coefficients at different set temperature points are calculated. The function between the sensitivity correction coefficients gotten above and the ambient temperatures is established and the other sensitivity correction coefficients at different temperature points are deduced by least square method. In real ambient environment,the sensitivity coefficient is calculated according to the function between sensitivity coefficient and ambient temperature to realize accurate measurements of laser energy in non-regular environment. The developed pyroelectric laser energy meter which can work in non-regular environment solves the problem of laser energy measurement in field.
Digital camera synthetic evaluation system
LIAN Li-quan, ZHOU Tao-geng, ZHANG Peng-song, SHA Ding-guo
2012, 33(4): 756-760.
With the development of the charged coupled device (CCD), complementary metal oxide semiconductor (CMOS), digital image processing and electronic components, the image quality of the digital camera has also developed a lot in the past ten years. The decline of the machining cost makes electro-optical imaging system used extensively in all kinds of fields, so the test and evaluation of the imaging capacity of the imaging system are also paid a lot attention. Digital camera(DC) can put out the digital image which can be identified and automaticly processed by the computer, and make it possible that the image quality of the digital camera can be evaluated objectively.Based on the DC standards,this paper studies methods of the objective evaluation and calculation of the DC parameters.Finishing the evaluation of single parameters,this paper makes a integrated grade from the DC using attribute math.
Curvature radius measurement by three-dimension profilometry
DING Xiang, LI Fei, HONG Bao-yu
2012, 33(4): 761-765.
Accurate measurement of curvature radius can be realized by three-dimension profilometry by scanning and fitting the surface topography of lenses. It takes the advantages of high accuracy, variable scanning area, tiny contact force and neglectable scratch to optical surface. Precision of curvature radius measurement by three-dimension profilometry is investigated, as well the influence of scanning area size and scanning intervals on the precision are discussed. Experimental results indicate that the standard deviation of relative repeatability can achieve as high as 110-6 in curvature radius measurement in both convex and concave lenses.
IR target detection based on improved K-means clustering
JIANG Bin, SHI Feng, CUI Dong-xu, ZHANG Peng-hui, YUAN Yi-hui, ZHANG Jun-ju
2012, 33(4): 766-769.
Considering the variance of image was a very good response for edge information, a target detection algorithm by K-means clustering algorithm based on variance was presented. First, this paper prepressed the infrared image by morphological method, and calculated the corresponding variance image by using a specific template, then gathered each difference image class by using the K-means clustering method, finally the different target edge information was got. Experimental results show that the algorithm can effectively extract the IR target edge.
Infrared pedestrian detection method using series combination classification
JIA Jun-tao
2012, 33(4): 770-773.
An effective method using series combination classification was proposed to solve the problem of high false alarm ratio in infrared pedestrian detection. After obtaining the region of interest, two kinds of characters were computed, ratio of width to height and histograms of oriented gradients (HOG). The series combination strategy was used to detect the pedestrian. In the experimental results, the detection ratio reached 90.5% and the false alarm ratio reduced by 43.7%. The results show that the proposed method is effective to detect the infrared pedestrian and reduce the false alarm ratio.
Laser particle size distribution test based on CCD sensor technology
LI Cai-rong, LIU Chan-lao, YAN Si-si
2012, 33(4): 774-778.
Particle testing is widely involved in scientific research and industry.The frequently used examination method for particle size and distribution is laser particle size distribution test method,which owns especial advantages such as high precision,high speed, good repeatability and wide measuring range.Its a common trend that using CCD as the sensor of laser particle analyzer can improve the intensity resolution,because CCD sensor has the merits of high sensibility,high resolution, low noise and great dynamic range.For enhancing the measuring accuracy, using image processing technique to design detector was presented based on the study of CCD sensor technology,and a laser particle size analysing system was realized based on Mie scattering theory.The experimental results indicate that using the CCD sensor to capture the light scattering spectra of image and then using image processing techniques to design photodetector enhance the measuring precision and reduce the repetition error effectively.
Measurement of aircraft attitude angles based on vanishing point theory
HU Bing-hua, ZHANG Hu-long, YAN Hui
2012, 33(4): 779-783.
In order to find a better solution for the current problem of accurate measurement of the attitude angles parameters in the flight landing test, a new method using parallel lines acquired from the images of the runway surface obtained by airborne camera during landing to calculate the aircraft attitude angles is proposed based on vanishing point theory. The functional relationship between vanish point and orientation angle elements is discussed, and the models of calculating the attitude angles parameters based on vanishing point geometry and line geometric constrains are presented. Experimental results show that the proposed approach is feasible in practice, and can obtain accurate and reliable measurement of aircraft attitude angles in landing.
Optical Element and Fabrication
Processing technology of aspheric silicon lens in IR tracking system
MA Fang, FU Xiu-hua, JIA Zong-he, XIE Ying, TANG Hao-long
2012, 33(4): 784-787.
With the rapid development of military optical instruments, IR optical materials have been widely used in IR optical elements and semiconductor industry, especially single-crystal silicon. To meet the precision instruments requirement for the optical system, the surface figure of the optical components has been increasingly demanded. Aiming at the processing technology requirement of high-precision silicon lens in the IR tracking system, taking a Ф26mm silicon lens as an example, by repeatedly grinding, repair/polishing, testing analysis, continuous optimization of the process parameters and designing a special fixture, this paper solved the problem of the silicon surface smoothness and the difficult control of the center deviation, and achieved mass production. After testing, the precision of the single-crystal silicon aspheric surface figure got 0.2m, the surface smoothness achieved above the grade Ⅲ and the center deviation was less than 1m. All the parameters met the requirements of the infrared tracking system.
Phase modulation properties of digital micromirror device in UV beam
YUAN Xiao-feng, PU Dong-lin, SHEN Su, CHEN Lin-sen
2012, 33(4): 788-792.
In order to improve the optical energy utilization of the maskless lithography system based on digital micromirror device (DMD), the phase modulation properties of DMD at UV laser illumination were investigated. The impact on the phase modulation properties caused by the processing error of DMD was analyzed. The diffraction patterns of three DMDs of the same model at the UV laser illumination were tested, and results showed that their light field distributions were significantly different, the DMD processing error had great impact on the phase modulation properties. This conclusion has great reference significance for choosing DMDs with high utilization efficiency.
Laser Technology
Excimer laser power standard detector
XU Tao, YU Jing, DENG Yu-qiang, SUN Qing, ZHANG Yun-peng
2012, 33(4): 793-798.
Thermal detector of 10 mW~1 W with SiC ceramic absorbing material and thick film electric heater for excimer laser power measurement is reported. Uncertainty for value reproduce reaches 0.26% (k=1). Laser pulse energy damage threshold of SiC ceramic and other common absorbing materials were studied. Threshold of the cavity used in the detector at 248 nm excimer laser condition is 2.2 J/cm2. Cavity absorbance under different wavelengths was also investigated through integrating sphere system and method for absorbance measurement at 248 nm excimer laser condition is presented. Absorbance of designed cavity is more than 0.99 at tested wavelength. The detector-s opto-electric equivalence was tested based on electric substitute and the unequivalence factor for sensitivity is 1.0026.
Thermal blooming of repetitively pulsed laser propagationin different atmospheric models
LIAO Tian-he, LIU Wei, GAO Qiong
2012, 33(4): 799-803.
Considering the variety of absorption coefficient along with different altitudes in different climates, this paper gets the absorption coefficient in a certain altitude by interpolation, sets up the model of transient thermal blooming of collimated repetitively pulsed laser from ground to air, and analyzes the transient thermal blooming by four-dimensional computer code. What is more, the paper simulates the model in 5 atmospheric models and simulates thermal blooming of continuous wave numerically. The results are as follows: In the same condition, after the laser transmits 10 km, in the region close to the North Pole, the nearest motion of the peak at the horizontal axis is 0.0569m in winter; the farthest motion of the peak at the horizontal axis is 0.224m in tropic. In the region close to the North Pole, the power in winter is 10.5 times as many as the power in tropic. Thermal blooming in tropic is more serious than that in mid-latitude region; thermal blooming in mid-latitude region is more severe than that in the region close to North Pole in the same season; thermal blooming in summer is more serious than that in winter in the same region; thermal blooming in summer in the region close to North Pole is more severe than that in winter in mid-latitude region.
LD-pumped all-solid-state 355 nm ultraviolet picosecond pulse laser
BAI Zhen-ao, BAI Zhen-xu, CHEN Meng, LI Gang
2012, 33(4): 804-807.
A laser diode (LD) end-pumped Nd∶YVO4 all-solid-state ultraviolet picosecond(PS) pulse laser was demonstrated under the semiconductor saturable absorption mirror(SESAM) mode-locking technology and the PS pulse laser regeneration amplifier technology, by using the type-Ⅰ-phase-matched LiB3O5(LBO) as the second harmonic generation(SHG) crystal and the type Ⅱ-phase-matched LBO as the third harmonic generation(THG) crystal. Finally, the 355 nm UV laser was obtained with 129.6 J single pulse energy, 17 ps pulse width at 1Hz repetition rate while the pump power was 5 W. The third harmonics were up to 60.3% conversion to 532nm and 16.6% to 355nm and the single pulse energy fluctuation was less than 0.58% in 3h operation.
Fiber Sensing and Optical Communication
4×10 Gbit/s all-optical wavelength conversion based on four-wavemixing in photonic crystal fiber
GONG Jia-min, LI Rui, PING Lu
2012, 33(4): 808-814.
Multiple wavelength conversion is very important for improving the flexibility of wavelength division multiplexing optical networks. A 410 Gbit/s all optical wavelength conversion is demonstrated based on multiple four-wave mixing in high nonlinear photonic crystal fiber. The impact of parameter variation such as pump power, fiber length and polarization state on wavelength converted signal quality is investigated. The simulation results show that the Q factor of converted signal increases with the increasing of the pump power (from 6 dBm to 20 dBm) and the fiber length (from 50 m to 120 m), and the maximum Q factor is 82.01 and 57.41, respectively. Furthermore, the Q factor of converted signal decreases with the increasing of the polarization mismatch angle, and the Q factor decreases rapidly especially when the polarization mismatch angle is more than 60.
Robust time delay estimation of distributed optical fiber sensor system
ZHANG Yan, QIU Tian-shuang, REN Fu-quan
2012, 33(4): 815-820.
In the positioning problem of distributed optical fiber sensor system, traditional time delay estimation (TDE) algorithms usually fail due to strongly correlated noise.A modified general cross correlation method was used to remove the correlated noise. According to the system features, a new TDE scheme was proposed to further improve the distributed optical fiber sensor system performance on accuracy and stability. The received data were first divided into many frames on the basis of event signal and then Kalman filter was applied to track the TDE results of the data frames. Simulation experiments and real data tests indicated that the proposed TDE scheme had good performance to suppress the correlated noise and improve the TDE accuracy and stability. Lots of on-site tests proved the TDE error was stably controlled below the sampling interval of 0.2, which satisfied the system accuracy requirement.
Spectral characteristics of cascaded long-period gratings
WANG Xia-yuan, HUANG Yong-lin
2012, 33(4): 821-826.
The transmission spectrum of cascaded long-period fiber grating has many advantages, such as large amplitude of diffraction peak, various spectral types and so on. The transmission spectra of cascaded long-period grating (LPG) were analyzed by using transfer matrix method. The effects of distance, position and phase-shift related to the LPGs on the cascaded spectral characteristics were discussed. And the cascaded and phase-shifted long-period grating spectra were compared. The results show that the spectra of phase-shifted LPG and cascaded LPG have great consistency when the cascading position is near the end of the cascaded LPG or the distance between the cascading fibers is small. And for phase-shifted LPG, when several phase-shifts are inserted, the interval of the two main-stop-bands broadens with the increase of the number of phase-shifts inserted.