Search

For the special requirements of the exciting source of a 87Rb based atomic clock for the miniaturization and high temperature conditions, a 795 nm Vertical Cavity Surface Emitting Laser(VCSEL)corresponding to the Rb atom energy level transition was designed and fabricated. Firstly, the energy levels and material gains of the InAlGaAs/AlGaAs Multiple Quantum Wells (MQWs) were calculated by the kp method to optimize the compositions and thicknesses of the quantum wells. Then, a Distributed Bragg Reflectors(DBRs) at 795 nm were designed and their reflection characteristics, longitudinal optical fields and averaged doping profiles were calculated and optimized using a one-dimensional transfer matrix method. Finally, the epitaxial structure of the 795 nm VCSEL with optimized MQWs and DBRs were grown on a GaAs substrate by Metal Organic Vapor Phase Epitaxy (MOVPE) and the oxide-confined 795 nm top-emitting VCSELs with unclosed-mesa structures were fabricated and characterized. Experimental results indicate that the packaged VCSELs can keep lasing under a cw current from 25 C to 85 C with power decreasing from 17 mW to 1.8 mW, the far field profiles are circular with a divergence angle of 15 and the temperature-shift of the lasing wavelength is 0.064 nm/C. Moreover, the lasing wavelength moves to the wavelength required by 87Rb atoms at an ambient temperature of 52 C and a current of 100 mA. The 795 nm VCSELs satisfy the basic requirements of 87Rb based miniaturized atom clocks for stable operation in a special wavelength and high-temperatures.

An InAlGaAs/AlGaAs strained quantum-well laser with high temperature stability was designed and grown to overcome the emission wavelength shift occurred in high temperature for a 852 nm laser diode.Based on a comprehensive model,the gains and wavelengths versus the operation temperatures of InAlGaAs,InGaAsP,InGaAs and GaAs quantum-wells were calculated and compared.The results indicate that In0.15Al0.11Ga0.74As quantum-well is the most appropriate candidate for the quantum well of the 852 nm laser diode with the higher gain and better temperature stability simultaneously.Then,Metal-organic Chemical Vapor Deposition(MOCVD) was used to grow compressive-strained In0.15Al0.11Ga0.74As/Al0.3Ga0.7As active region and Reflectance Anisotropy Spectroscopy(RAS) and Photoluminescence Measurements(PL) were applied to the evaluation of crystalline quality for InAlGaAs/AlGaAs interfaces.It is proved that the indium segregation effect can be effectively suppressed by lowering the growth temperature and using the interruption time between InAlGaAs quantum-well and AlGaAs barriers,and an abrupt interface and good crystalline quality for InAlGaAs/AlGaAs quantum-well can be obtained.Finally,an InAlGaAs/AlGaAs strained quantum-well laser was grown with optimized growth conditions.Experimental results indicate that the laser has a Full Width Half Maximum(FWHM) of 1.1 nm,the slope efficiency of 64 W/A and the wavelength shift with temperature of 0.256 nm/K.The theoretical calculation results are in good agreement with experimental results,which verifies that the laser meets the work requirements at a high temperature.

For the peculiar Smear noise occurred in the area CCD imaging,a method to extract the Smear from the interline transfer CCD dark pixel region was proposed by taking an interline transfer area CCD for an example.The working principle of the area CCD was introduced,the creation reason of the Smear was analyzed and the composition of the Smear noise was discussed.Then,the KTC noise and the dark current noise of the Smear were filtered by using the Correction Double Sample(CDS) and the dumb pixel correction.The average filter method based on a middle value was proposed to eliminate the light shot noise.Finally,the Smear was eliminated from a original image by the subtract method,and the Smear image region was compensated by the triangle-based interpolation.A real-time hardware processing system was designed by using Field Programming Gate Array + Digital Signal Processor(FPGA + DSP) as the core processor.When the camera is working at the fastest working mode 3 frame / s,the Smear can be extract and filtered in 1.265 ms and the gradation variance of the image area has decreased to 95.34% by eliminating the Smear.With the imaging experiment verification,the system has a high integration and real-time ability and the Smear noise is eliminated completely.

The growth rate and surface structure of AlGaAs sample were investigated by employing the optical in-situ monitoring technique.In situ monitoring of optical time resolved normalized reflectance(NR) and reflectance anisotropy(RA) in growth process of multilayer AlxGa1-xAs sample was carried out.Oscillation characteristics of NR curve was analyzed.Converged value of NR and RA curve changed monotonously with Al composition at the monitoring optical energy of 1.9 eV.The growth rate was gained from fitting the NR transient.And the deviation of calculated growth rate was lower than 0.02 nm/s compared to the SEM measurement results.From the analysis of time resolved RA curve,surface reconstruction caused by the growth temperature was observed during the epi-growth of GaAs layers.

Due to a CCD and ADC restriction,the dynamic range of the commonly used image sensor is only about 60 dB,which does not coincide with the dynamic range(110 dB)of eyes,leading to the loss of partial details of the image.In this paper,an image extended dynamic range technique based on the wavelet transform would be proposed.By means of taking pictures of the same scene repeatedly,the source images of different exposure time were procured,which were further divided into high frequency part and the low frequency part by using multi-scale wavelet transform.The method based on human visual gray recognition features was applied to the processing of low frequency part.By using different weights for processing,the most background information possible of the image was retained.For the high frequency part,the local maximal variance criteria for fusion was used to maintain the image edges' characteristics to the greatest extent.Reconstruction is proceeded by using inverse wavelet transform to the merged high frequency part and low frequency part.Test results showed that comparing the reconstructive image and moderately exposed image,the details of information in the high brightness area and low light area was recovered,and the dynamic range got significant expansion.

In order to reduce or eliminate the image blur caused by the degradation in imaging process of a space remote sensing camera,the acquired images were restored to enhance the characteristic objects of images.First,a frequency-domain notch filter was adopted to remove noises in the remote sensing images.Then,the degradation function,namely,Point Spread Function(PSF) of the imaging system was estimated by ground characters with the knife-edge shape,and the estimated PSF was corrected with Gaussian fitting.Finally,the images were restored by using adaptive Wiener filtering with the fitted PSF.Experimental results show that almost all strip noises in the image are eliminated.After the denoised image is restored,its variance and gray mean gradient increase by 4.395 and 1.799,respectively,and also Laplacian gradient increases by 10.014.Experiments demonstrate that the restoration with Gaussian fitted PSF is beneficial to interpreting and analyzing the remote sensing images.After restoration,the blur phenomenon of the images is reduced,the characters are highlighted,and the visual effects of the images are clearer.

Healthy forests are closely related to global ecological stability and sustainable development of human society.It is important to study forest ecosystem health including developing theories related to forest health and exploring various assessment methods.Reasonable methods lead to accurate assessments of forest ecosystem health.The definition of forest health is controversial.The concept of health is well understood as applied to humans but the human concept of health may not be appropriate for ecosystems.The difficulties of defining the optimal conditions for ecosystem health have led to a lack of universally accepted indicators to measure ecosystem health.In this paper,a healthy forest ecosystem is defined as a system having good mechanisms for self-regulation,such as beneficial mechanisms that allow an ecosystem to react to external harmful disturbances.Positive ecosystem responses to negative changes keep forest ecosystems intact,stable and sustainable.With this concept of forest ecosystem health in mind,we can discuss assessment methods which can be used to monitor forest ecosystem health.Several forest ecosystem health assessment systems already exist.For example,ecosystem health can be assessed using measures of resilience,vigor and organization.Today,most assessment systems are based on this concept.Based on the concept of forest ecosystem health presented above,a new approach is proposed,which considers four external harmful disturbances: forest fire,forest pest species,air pollution and human-caused disturbances.This method is designed to build assessment indicator systems which can be used to monitor and evaluate forest ecosystem health.It also considers beneficial disturbances in this assessment indicator system.This method includes an analysis of forest fire disturbance,which causes severe damage to forest ecosystems,followed by eight other indicators of forest health including average precipitation,average temperature,proportion of flammable species,crown density,altitude,slope gradient,slope direction and distance between the forest and the nearest road.Disturbance caused by forest pests can also be measured.These include disturbances caused by forest animals,such as rat and rabbit damage,as well as indicators like forest diseases and forest pests,the degree of damage and rate of infection in host trees.Air pollution disturbance can be measure by studying leaf damage as well as soil and contamination analysis.The study of harmful human-caused disturbances includes measurement of forest management techniques such as timber harvest and management of undergrowth vegetation.Measurements of forest ecosystem stability include measurements of biological diversity,forest community structure and measurements documenting how closely a forest is to natural conditions,with natural conditions defined as being free of human-caused disturbances.These types of measurements have been used frequently to measure ecosystem complexity and health.Next,the content and significance of key indicators were also well-analyzed.This forest ecosystem health assessment indicator system abandons traditional assessment ideas and combines the most important factors which have been determined to be closely related to forest ecosystem health in recent years.This assessment indicator system focuses at the ecosystem level and has mostly been applied to forests both in South and North China.Information is presented for use by forest and district resource managers as well as academic experts.This assessment indicator system establishes a scientific basis for conducting forest health projects,provides a context for planning ecosystem restoration,and contributes to the understanding of the physical,biological,and human dimensions of these ecosystems.This research may enrich the theories and methods used in assessing the health of ecological systems.

To improve the output powers of Vertical Cavity Surface Emitting Lasers(VCSELs),a 977 nm VCSEL array with three In0.2Ga0.8As/GaAs0.92P0.08 strained Quantum Wells(QWs) was studied.The structures of the QWs were optimized and GaAsP with a larger band gap was chosen as the barrier material,and the band offsets of In0.2Ga0.8As/GaAs0.92P0.08 were calculated.The output powers of the devices which used In0.2Ga0.8As/GaAs0.92P0.08 and In0.2Ga0.8As/GaAs QWs were simulated theoretically and analyzed comparetively,respectively and the pulsed peak powers of two array devices were measured.Then,the performance of the array device was estimated by a functional method using a p-parameter determined by the turn-on voltage,threshold current,and the differential resistance.Experimental results show that the 4×4 VCSEL array with In0.2Ga0.8As/GaAs0.92P0.08 QWs and an emitting area of 0.005 cm2 can achieve a pulsed peak power of 123 W when the injecting current is 110 A,and its power density and slope efficiency are 45.42 kW/cm2 and 1.11 W/A,respectively.This output power is 13 % larger than that of the array with In0.2Ga0.8As/GaAs QWs and the same emitting area.Furthermore,the values of p parameter are 15 and 13 under CW operation and pulsed operation,respectively,which indicates that the device has relatively good performance.In conclusion,the 4×4 VCSEL array with three In0.2Ga0.8As/GaAs0.92P0.08 strained QWs is able to achieve higher output powers.

This paper put forward an available method aimed at the requirements of aerial camera with multi-mode.Driving schedules of interline transfer progressive scan KAI-2093 CCD sensor produced by Kodak corporation have been examined based on the structure and theory of dual-channel transmission.Three kinds of driving mode have been designed: binning,no-binning and TDI mode.The design takes the FPGA EP1C6Q240 produced by Altera corporation as the schedule generator to realize data cache and mosaics.QUARTUSII7.0 has been chosen as the software design platform.Driving schedule generator is described with VHDL.The design of data cache and mosaics has been fulfilled though simulation by Modelsim AE6.1b.Experimental results indicate that the schedule can output CCD signals at the speed of 60 frames/s,120 frames/s etc in binning mode,which can meet the requirements of moving target tracking with high speed;and can output full frame at the speed of 30 frames/s in no-binning mode;and does not affect image quality for long-time working in TDI mode.The system integration and anti-jamming capability have been improved.

The spatial coherence properties of high power vertical cavity surface emitting lasers (VCSELs) are studied based on the theorem of partially coherent light proposed by Van Cittert-Zernike. The interference stripe patterns are gauged using the Young's double-slit experiment for the devices of 980 nm wavelength VCSEL single emitters. Then interference patterns are switched to grayscale by intensity distribution patterns for data collection. In this paper, the integral average value method is proposed to calculate the degree of spatial coherence. The proposed integral average value method and traditional average method are used respectively to calculate the light intensity of the pattern. The results are compared with those calculated by the theoretical value of Van Cittert-Zernike theorem. The VCSEL optical apertures' influence on coherent characteristics are also discussed. The experimental results show that, a relative error within 2.5%~9.4% is reached by our integral average value method. On the contrary, a relative error of 7.5%~67.4% are got from the traditional average method. Apparently, traditional average value method causes the error generally 1.5~27 times worse than our integral average value method. Moreover, for single emitter VCSELs with optical aperture from 200 m to 500 m, the smaller the optical aperture are, the larger the coherent degrees (between 0.731~0.426) are. The conclusion can be wildly used on theoretical and experimental design for VCSEL coherent arrays.

In order to realize the high-power narrow linewidth of vertical cavity surface emitting laser (VCSEL), the shallow surface relief rectangular structure (SR VCSEL) is designed. The difference of the current density distribution leads to the mode distribution different. Calculation results show that, compared with circular mesa VCSEL, the uniformity of the current density distribution of rectangular mesa VCSEL is not changed when the active area is increased. And theoretical results indicate that the threshold gain of high-order mode increases greater than that of the fundamental mode when etching shallow relief of surface. In the experiment, the power of 5.87 mW when current is continuous, spectral width of 0.1 nm, power polarization degree of 10, and transverse mode suppression ratio more than 30 dB are obtained.