A high-precision SAR echo simulation method based on FDTD

Synthetic aperture radar (SAR) echo simulation offers a low-cost and convenient way to obtain high-resolution images of targets, and plays an important role in system design and algorithm validation. Although high frequency approximation simulation is widely used, it is considered to be imprecise when calculating scattering field of fine structures, such as exhaust pipes and groove structures, especially in low frequency band. In this paper, a finite-difference time-domain (FDTD) based method is proposed for high-precision SAR echo simulation. In this method, scattering process of electromagnetic wave is accurately simulated to obtain equivalent electric and magnetic current on the surface of the target. Also, a near-to-far-field transformation is applied to the equivalent electric and magnetic current to calculate the field at the receiving antenna. In this transformation, a waveform forming method is introduced to simulate stripmap SAR echoes. By introducing this method, the usage of FDTD in one single simulation can be greatly reduced. The experiments show that proposed method can significantly improve the efficiency of the simulation while maintaining echo accuracy

Road network detection based on improved FLICM-MRF method using high resolution SAR images

The automatic detection of road network from satellite and aerial images is highly significant in many actual applications, for instance, urban traffic measurement, military emergency response, and vehicle target tracking. Compared with other high-resolution satellite remote sensing images, high-resolution synthetic aperture radar (SAR) has become a popular research perspective for road detection owing to its insensitivity to the atmosphere and sun-illumination. However, the method of road network detection is still lagging due to the strong multiplicative speckle noise and complex background interference, causing the loss and break in the road segment extraction results. Aiming to solve this problem, a three-step road network detection framework is proposed. In the first step, the road segment candidates are extracted by the Fuzzy Local Information C-Means (FLICM) algorithm based on the gray-level co-occurrence matrix(GLCM) with Markov Random Fields (MRF), and it contains an adaptive parameter selection procedure which is presented for adjusting joint clustering parameters. In order to reduce false segments, we perform the local processing which combines the morphological operation, linearity index, and local Hough transform in the second step. Finally, as for the global road segment connection, we propose an improved region growing algorithm which fully considering the rationality of road elements to gain the road network. Compared with the traditional region growing algorithm, the proposed method can effectively promote the improvement of the integrity of the road network detection. Moreover, the performance of the proposed method is evaluated by comparing the results with the ground truth road map and the evaluation index including the completeness, correctness, and quality factor. In experiments, the algorithm has been verified with the SAR images from the different resolutions of the GF-3 satellite SAR image. The results of the various real images demonstrate that the proposed algorithm has improved considerably the adaptability and efficiency of road detection compared with other methods

Associations Between Home Dampness-related Indicators and Eczema among Preschool Children in Shanghai, China

AbstractIn recent years, prevalence of eczema has been increasing among preschool children in China. It's urgent to find associated factors. On basis of 13,335 questionnaires from parents or guardians for 4-6 year-old children (response rate 85.3%) in a cross-sectional study from April 2011 to April 2012 in Shanghai, the associations between home dampness related indicators and the prevalence of childhood eczema was investigated. There were 7.8%, 15.3%, 42.1%, 55.7%, and 30.7% of the surveyed residences who had visible mold spots, visible damp stains, damp clothing and/or bedding, water damage, condensation on window in winter, and moldy odor (six home dampness-related indicators), respectively. The prevalence of eczema (ever) and eczema (in the past 12 months) was 22.9% and 13.2%, respectively. These home dampness indicators had significant and strong associations with the increased risk of childhood eczema. With regard to the same indicators, the increased risk of eczema in girls was higher than in boys. Total numbers of home dampness-related indicators had notable dose-response relationships with the prevalence of childhood eczema. In conclusion, home dampness-related exposures are strong risk factors for childhood eczema

Single/dual-wavelength switchable bidirectional Q-switched all-fiber laser using a bidirectional fiber polarizer

A single/dual-wavelength switchable bidirectional Q-switched fiber laser using a bidirectional fiber polarizer is demonstrated. A 45° tilted fiber grating is used as a bidirectional fiber polarizer to induce a bidirectional intracavity birefringence filter in both clockwise (CW) and counter-clockwise (CCW) directions. A carbon nanotube saturable absorber is employed to produce Q-switched pulses. Through adjusting polarization states, switchable single/dual-wavelength lasing at 1551 and 1560 nm can be achieved in both CW and CCW directions. To the best of our knowledge, this is the first demonstration of a wavelength switchable bidirectional passively Q-switched fiber laser

Mode locked Erbium doped fiber lasers using 45º tilted fiber grating

We have systematically studied the 45º tilted fiber grating (45TFG) as a functional device for Erbium doped fiber laser (EDFL) mode locking. A number of 45TFGs with different polarization dependent loss (PDL) have been fabricated. Mode locked Erbium doped fiber laser using these devices have been characterized in terms of threshold, pulse duration, signal to noise ratio (SNR), and spectral width. Our results show that a 45TFG with higher PDL could achieve better laser results. By using a 45TFG with 24dB PDL, the mode locked laser has 8.1% conversion efficiency and a threshold of 200mW

Dual pulse bound states in a dispersion-managed mode-locked all-fiber laser with 101.75MHz repetition rate using 45° tilted fiber grating

We demonstrate a dispersion managed Erbium doped all-fiber laser with 101.75MHz repetition rate using 45°TFG to realize mode locking based on nonlinear polarization rotation technique. In our experiment, stable dual-pulse tightly bound states were observed under the pump power of 505mW

Wavelength-tunable passively mode-locked Erbium-doped fiber laser based on carbon nanotube and a 45° tilted fiber grating

A wavelength-tunable all-fiber Erbium-doped mode-locked fiber laser based on carbon nanotubes and 45° tilted fiber grating (TFG) is demonstrated. We investigated the effect of PDL of 45TFG in the tuning range of a mode locked laser. The central wavelength of the laser can be tuned continuously from 1559.85 nm to 1564.46 nm with a tuning range of 4.6 nm using a weak 45TFG and from 1553.37 nm to 1568.63 nm with a tuning range of 15.26 nm using a strong 45TFG. The laser maintains high signal to noise ratio >50 dB across all the wavelength tuning range

Wavelength-tunable mode-locked fiber laser with birefringence-enhanced cavity

A wavelength-tunable Erbium-doped fiber laser with birefringence-enhanced cavity is proposed. A 45° tiled fiber grating (45°TFG) and a section of polarization maintaining (PM) fiber are placed between two two polarization controllers (PCs). Two PCs and PM fiber are employed to enhance the birefringence effect. The 45° TFG plays the role of an in-fiber polarizer. The fiber laser is mode-locked by single walled carbon nanotube polyvinyl alcohol (SWCNTs-PVA) composite film. Central wavelength of the fiber laser can be tuned from 1552.52nm to 1575.28nm with a tuning range of 22.76nm. The demonstrated laser provided the widest tuning range in a high repetition rate all-fiber Erbium doped laser based on SWCNT