Editor in chief：Jun-Hao CHU
International standard number：ISSN 1001-9014
Unified domestic issue：CN 31-1577
Domestic postal code：4-335
- Most Read
- Most Cited
- Most Downloaded
Abstract:Properties of quaternary InAlGaAs alloys prepared by gas source MBE growth have been investigated with high resolution X-ray diffraction rocking curves, photoluminescence and Hall measurements. X-ray rocking curves show that all the samples are well matched to InP substrate according to calibration data. The photoluminescence and Hall measurement show that the PL intensity, electron concentration and mobility decrease distinctly as Al composition increases. The group III compositions are determined from both photoluminescence and x-ray diffraction measurements, and agreed well with the designed values. The relationship between the designed Al compositions and measured values provides us a practical way for the precise composition control.
Abstract:Taking Cr4+:YAGs as saturable absorbers and reflective volume Bragg gratings (RVBG) as output mirror, passively Q-switched operation of flash-lamp pumped Nd:YAG single-longitudinal-mode (SLM) laser was demonstrated. Single-frequency operation is accomplished for all output powers by twisted mode cavity and RVBG. This method can bring higher energy and stability. The maximum linewidth of SLM laser is about 78 MHz. The curve of energy and pulse width have been obtained under conditions of several Cr4+:YAGs with different initial transmissivity. The maximum SLM laser output energy is 20.8 mJ, minimum pulse duration is 13.2 ns and peak power is 1.18 MW.
Abstract:An ultra-high-speed 2∶1 dynamic frequency divider based on clocked-inverter was designed and fabricated using our own fT =214 GHz, fmax=193 GHz InGaAs/InP heterojunction bipolar transistor technology. The frequency divider was designed to operate from 60 GHz to 100 GHz. However, it was only demonstrated from 62 GHz to 83 GHz, due to the limitation of the measurement system. The circuit consumed 1060.8 mW with a supply voltage of -5.2 V and 596.4 mW with a reduced supply voltage of -4.2 V. The successful fabrication of the divider was of great importance on building a phase-locked loop operating at W band.
Abstract:The frequency-dependent absorption characteristics of diesels with different sulfur content have been studied in the spectral range of 0.2-1.5 THz by the terahertz time-domain spectroscopy (THz-TDS). The absorption coefficient presented a regular change with the THz frequency and sulfur content, and the absorption coefficient increases with the frequency at the same concentration, vice versa. A nonlinear multivariate model was established and the sulfur content in diesel can be confirmed easily by means of the measured THz frequency-dependent absorption coefficient. The results made the quantitative analysis of sulfur content in diesel possible by THz-TDS technology and indicated the bright future in practical application.
Abstract:Long-wavelength (λc~12.5 μm) 256×1 infrared photodiode arrays based on liquid phase epitaxy (LPE) grown arsenic-doped HgCdTe were fabricated. The performances of the devices were investigated in this work. The profile of implanted pn junction on arsenic-doped HgCdTe was determined by a novel method. The results show that the junction depth is about 3.6~5.3 μm and the largest lateral size is about 130% of the designed value. In the experiment, HgCdTe long-wavelength photodiode arrays were fabricated by traditional and modified surface processing techniques. The modified process can improve the electrical properties of the devices significantly. The peak resistance and dynamic resistance at-05 V in the R-V curves of the devices by modified process is about 100 and 30 larger than those of the devices by traditional process, respectively. The reason of the improvement is assumed to be the suppression of surface leakage current.
Abstract:Electrical and optical properties of HgCdTe are crucial for detectors. Infrared photoluminescence (PL) spectra in the temperature range of 11-300K and Hall data were recorded on the arsenic-doped narrow-gap HgCdTe epilayers. Curve fittings of PL spectra indicate that AsTe、 VHg、 TeHg-VHg and TeHg exist in the arsenic-doped HgCdTe epilayers after the two-step annealing. More TeHg-VHg pairs are created when the dopant concentration is increased. Analysis of temperature-dependent Hall data verifies the existence of TeHg, which lowers the mobility of the material.
Abstract:Heyd-Scuseria-Ernzerh of screened hybrid density functional was used to calculate the band gap and the formation enthalpy of Cd1-xZnxTe alloys, which are much more accurate than the traditional GGA methods. Special quasirandom structure was used to describe the disordered Cd1-xZnxTe alloys. The band gap bowing parameter of Cd1-xZnxTe alloys is 0.266eV, which is consistent with the experimental value of about 0.254eV. The calculated results show that the formation enthalpy of Cd1-xZnxTe alloys is relatively high, especially for Cd0.5Zn0.5Te alloy (25.60meV/atom). The difference between local bond lengths of Cd-Te and Zn-Te that are very close to those in bulk CdTe and ZnTe is quite large, indicating that the large relaxation exists in these alloys and this relaxation may lead to the large formation enthalpy described above.
Abstract:A series of Ge quantum dot samples were grown by ion beam sputtering on Si (100) substrates with a Si buffer layer. The evolution of the topography and dimension of Ge/Si quantum dot were characterized using AFM and Raman spectra. The results show that the density of the quantum dots increased to a maximum and then decreased with the thickness of Si buffer layers, the maximum is up to 1.9×1010 cm-2 due to the influence of the thickness and growth patterns of Si buffer layers. Growth interruption is beneficial to improve the crystallization of Si buffer layer and the density of the quantum dot. The effects of Si buffer layers which manipulate the growth and shape of the Ge quantum dots are discussed in details. In addition, a growth model of the quantum dots is proposed.
Abstract:An infrared continuous zoom system is designed. The simple system has a large relative aperture with high transmittance, large continuous zoom ratio, and wide wave band. By introducing a diffractive surface and an aspheric surface, the modulation transfer function (MTF) of the system is above 0.6 for all continuous zoom ratios at the spatial frequency of 14lp/mm. As the system only contains 4 lenses, the transmittance is higher than 80%, and the zoom ratio of the system could also reach 8 continuously. As the color and the off-axis aberration are well corrected, the experiment indicates that the image quality of the continuous zoom system is pretty well.
Abstract:T1 weighted magnetic resonance imaging of the brain was segmented into scalp, skull, cerebrospinal fluid, gray matter and white matter, and then meshed in volume with different tissues. The volumetric mesh based on subject specific anatomy was used to build forward optical model and reconstruct image in the diffuse optical tomography for solving low position-accuracy, ill-condition and under-determined problems caused by hemisphere model or brain atlas. In the visual experiments of counter-clockwise wedge stimulus and expanding ring stimulus, the diffuse optical tomography guided by magnetic resonance imaging can obtain higher space resolution and more accurate position for activated oxygenated haemoglobin signals.
Abstract:A cavity ring-down spectroscopy apparatus employing a pulsed quantum cascade laser operating near 1027 cm-1 (9.7 μm) is investigated for trace ammonia detection. The trace ammonia in human breath and clean room atmosphere are measured and analyzed. A sensitivity of 10 ppb has been demonstrated for ammonia detection. The effects of the spectral line-width on the measurement accuracy and sensitivity of the system are analyzed. The measurement accuracy and sensitivity of the apparatus decrease with the increasing laser spectral line-width. The measurement results can be corrected by taking into account of the line-width effect in data analysis.
Abstract:A directional coupler composed of two vertical multiple-slotted silicon wires with trapezoidal cross-section is characterized using a full-vectorial finite element method in terms of the electric fields, where the perfectly matched layer absorbing boundary conditions are adopted. The effects of the structural or material parameters including the sidewall angle, the gap between the coupled waveguides, and the width and index profiles in the slot regions on the behaviors of the directional coupler are taken into account. The effective indexes of the even and odd modes and the corresponding coupling length, both in quasi-TE and quasi-TM modes, are presented. The strongly-hybrid nature of the guided-modes is effectively demonstrated. The results show that polarization-independent directional couplers can be realized by properly choosing the structural and material parameters.
Abstract:Multiband drought indices (DIs) have been developed for quantitatively characterizing the degree of region drought-hit and crop yield loss. Nowadays, drought indices are currently one of the hot topics with potential applications in agriculture, forestry and ecological environment monitoring. However, it is still lack of a systematic research for the multiband drought indices. This paper made a review of latest remote sensing derived drought indices. The concept of multi-dimensional spectral space was extended, where multi-dimensional variables are expanded from the remote sensing spectral bands to all the satellites derived products concerned with drought. Over 30 drought indices developed during the past years were categorized by constitution elements into four groups. Advantages and limitations of each group were analyzed. Further more, the family of drought indices was viewed as a “pyramid”, and the whole family’s composition were deconstructed from the bottom to the top. Environmental impacts like atmosphere, sensor calibration and soil effects on the drought monitoring are analyzed and expressed as improved factors into the models.
Abstract:Spectral information is essential for objects recognition in remote sensing imagery. However, objects which have particular indices are rather few, and spectra types of spectral library and their universality are limited either. Therefore, an adaptive spectral representation method of remote sensing objects based on endmember matching is proposed. Proper endmember of imagery itself is selected. Spectral angle and distance, which is between the characteristic vectors of spectra of the interested pixel and a specific endmember, are both considered to form a new way for comprehensive spectral matching. Experiments of vegetation and water were adopted in ETM+ (Enhanced Thematic Mapper) images, and were compared to those using USGS (United States Geological Survey) library and normalized difference vegetation index (NDVI) /normalized difference water index(NDWI). Moreover, validations of shadow and bareland images were also carried out to test the effectiveness and universality of the proposed method.
Abstract:Carbon dioxide has an absorption band of solar spectra in SWIR, and temperature is a key factor to the absorption effect. Researches on the CO2 observation sensitivity of temperature are done in this paper. First, the dependences of spectral line parameters on temperature are discussed. Then, according to the accuracy level of atmosphere temperature profile product, we simulate the change of measurement in nadir mode by importing 1 K temperature uncertainty, using line-by-line radiative transfer mode under 6 different atmosphere modes, and calculate the corresponding change of retrieval result. In contrast, simulations of measurement change resulted from 1 ppm and 2 ppm CO2 column density differences are also done. After comparison, we come to a conclusion: 1 K uncertainty of atmosphere temperature profile is an important factor which has significant influence to high precise atmospheric CO2 observation.
Abstract:The component temperatures are key parameters in the environment problems. The multi-angle thermal infrared data are important for the retrieval of component temperature. MODIS (Moderate Resolution Imaging Spectroradiometer) is one of the sensors onboard EOS-Terra and EOS-Aqua, two sun-synchronous polar orbiting satellites. The sensitivity analysis shows that the same MODIS bands of Terra and Aqua satellites measures in the same behavior under the same variations of atmospheric water vapor, solar and viewing geometries. These MODIS bands may be considered to be the observations of a satellite at two viewing angles and two viewing times. In the study, the foliage and soil component temperature were retrieved by integrating these MODIS data. Based on linear thermal infrared radiation model, the bands 32 and 33 in Terra-MODIS, or the band 32 in Terra-MODIS and Aqua-MODIS, were separately used to retrieve the vegetation cover ratio, soil component emissivity and temperature, foliage component emissivity and temperature by employing genetic algorithm in Huailai area, Hebei Province. By comparing with the synchronous field measurements, the maximum error in the component temperatures is less than 1℃ for the integrated Terra/ Aqua MODIS data, 0.4℃ smaller than that by just Terra-MODIS data. However, for a pixel, its component temperatures are different by such two group data.
Abstract:The highly detailed information of objects can be provided in multi-scale by high-resolution remotely sensed imagery. When edge feature are detected in high-resolution image effectively, the internal geometric details also come to light but as noise form. In order to detect multi-scale edge feature and suppress noise, a novel method to detect the edge feature integrated spectral difference with wavelet transform was developed. Firstly, based on the theory of spectral angle, spectral difference normalized model (NSD) was defined to picture the contour of the object. Secondly, the dyadic wavelet transform was applied for each band to produce the multi-scale edge detail coefficients which actually are the gradient, and then weight the gradient magnitude of each band by using the cosine of gradient direction to enlarge the edge feature in the main gradient direction. Thirdly, combined with NSD, first fundamental form was used for detecting the gradient magnitude and orientation of multispectral images at different levels. Experiment by using QuickBird multispectral images are presented to demonstrated the representation efficiently. Compared with the results from wavelet transform and traditional edge detection operator, the proposed method can guarantee the edge without distortion, depict edge points more accurately and suppress more noise.
Abstract:Track-before-detect (TBD) technology based on the probability hypothesis density (PHD) filter can effectively solve the problem of tracking dim varying number multitarget. The existing PHD-TBD algorithm has two shortcomings, lack of accuracy in the number of targets and long time delay in responding to the targets being detected. The paper studied the PHD-TBD method, deduced the accurate expression of the updated particle weight of the PHD-TBD algorithm, and achieved the precise estimate of the number of targets. Simultaneously, by using Bayesian theory, it deduced the probability density sampling function of new born particles based on measurement, which can quickly and effectively find the targets. In addition, the simulation results demonstrate that the proposed algorithm can effectively estimate the number of targets, detect the targets and accurately estimate their positions with a more rapid speed compared with the existing PHD-TBD algorithm.
Editor in chief：Jun-Hao CHU
International standard number：ISSN 1001-9014
Unified domestic issue：CN 31-1577
Domestic postal code：4-335