Hongru Yang

Signal-to-noise performance analysis of streak tube imaging lidar systems. I. Cascaded model.

Streak tube imaging lidar (STIL) is an active imaging system using a pulsed laser transmitter and a streak tube receiver to produce 3D range and intensity imagery. The STIL has recently attracted a great deal of interest and attention due to its advantages of wide azimuth field-of-view, high range and angle resolution, and high frame rate. This work investigates the signal-to-noise performance of STIL systems. A theoretical model for characterizing the signal-to-noise performance of the STIL system with an internal or external intensified streak tube receiver is presented, based on the linear cascaded systems theory of signal and noise propagation. The STIL system is decomposed into a series of cascaded imaging chains whose signal and noise transfer properties are described by the general (or the spatial-frequency dependent) noise factors (NFs). Expressions for the general NFs of the cascaded chains (or the main components) in the STIL system are derived. The work presented here is useful for the design and evaluation of STIL systems.

Signal-to-noise performance analysis of streak tube imaging lidar systems. II. Theoretical analysis and discussion

In the preceding paper (referred to here as paper I), we presented a general signal-to-noise performance analysis of a streak tube imaging lidar (STIL) system within the framework of linear cascaded systems theory. A cascaded model is proposed for characterizing the signal-to-noise performance of a STIL system with an internal or external intensified streak tube receiver. The STIL system can be decomposed into a series of cascaded imaging chains whose signal and noise transfer properties are described by the general (or the spatial-frequency dependent) noise factors (NFs). Equations for the general NFs of the cascaded chains (or the main components) in the STIL system are derived. This work investigates the signal-to-noise performance of an external intensified STIL system. The implementation of the cascaded model for predicting and evaluating the signal-to-noise performance of the external intensified STIL system is described. Some factors that limit the signal-to-noise performance of the external intensified STIL system are analyzed and discussed.

Analysis and design of imaging Fabry-Perot interferometers for measurement of Rayleigh-Brillouin scattering spectra in gas flows

Laser Rayleigh-Brillouin scattering is a powerful diagnostic tool for the study of gas flow properties. It provides an effective method for non-intrusive measurement of density, temperature and velocity in the gas flow. The received scattered laser light power is proportional to the gas density, the linewidth of the Rayleigh-Brillouin scattering spectrum is related to the gas temperature, and the Doppler frequency shift of the peak of the Rayleigh-Brillouin scattering spectrum is related to the gas velocity. The Rayleigh-Brillouin scattering spectrum can be measured by a Fabry-Perot interferometer operated in the imaging mode where an intensified CCD camera is frequently used to record the interference patterns of the Fabry-Perot interferometer. The Rayleigh-Brillouin scattering spectrum is then reconstructed from the measured data deconvolved with the Fabry-Perot instrument function. In this paper, the analysis and design of an imaging Fabry-Perot interferometer for the measurement of the Rayleigh-Brillouin scattering spectrum in the gas flow is presented. Some factors that limit the performance of the imaging Fabry-Perot interferometer are analyzed and discussed.

Simulation of underwater objects imaging through rough sea surface based airborne laser range-gated system

A model is currently being developed for simulating images of underwater objects as seen by airborne laser range-gated (LRG) system. The simulation model is based on physically accurate models of wind generated waves on the sea surface, and the optical properties of ocean water. It can take as input any digital image and simulate its degradation due to scattering in water and through the wavy sea surface. This paper describes the model implementation for LRG system and presents the results of simulations. Simulation results presented here allow the visual evaluation of image quality for different depth and optical properties of the sea water. These simulations will also allow us to develop optimal strategies for image processing algorithm of Lidar data.

Target 3-D reconstruction of streak tube imaging lidar based on Gaussian fitting

Streak images obtained by the streak tube imaging lidar (STIL) contain the distance-azimuth-intensity information of a scanned target, and a 3-D reconstruction of the target can be carried out through extracting the characteristic data of multiple streak images. Significant errors will be caused in the reconstruction result by the peak detection method due to noise and other factors. So as to get a more precise 3-D reconstruction, a peak detection method based on Gaussian fitting of trust region is proposed in this work. Gaussian modeling is performed on the returned wave of single time channel of each frame, then the modeling result which can effectively reduce the noise interference and possesses a unique peak could be taken as the new returned waveform, lastly extracting its feature data through peak detection. The experimental data of aerial target is for verifying this method. This work shows that the peak detection method based on Gaussian fitting reduces the extraction error of the feature data to less than 10%; utilizing this method to extract the feature data and reconstruct the target make it possible to realize the spatial resolution with a minimum 30 cm in the depth direction, and improve the 3-D imaging accuracy of the STIL concurrently.

Measurement of Rayleigh-Brillouin spectra in gas flows

Laser Rayleigh-Brillouin scattering is an effective non-intrusive method for measurement of density, temperature and pressure in gas flows. In theory, the power of Rayleigh-Brillouin scattered laser light is proportional to the gas density, the full width at half maximum (FWHM) and the Brillouin shift of the Rayleigh-Brillouin scattering spectrum is related to the gas temperature and pressure, respectively. In this paper, a measurement device based on Fabry-Perot interferometer (FPI) is designed to measure the Rayleigh-Brillouin spectrum of nitrogen gas. The experimental data is obtained at different pressures under room temperature conditions. The L3 model is used to fit the experimental data to obtain the FWHMs and Brillouin shifts of the Rayleigh-Brillouin profiles. The composite Rayleigh-Brillouin profiles which consist of Rayleigh peak, stokes peak and anti-stokes peak are represented by three distinct peaks of Lorentz functions. Fitting results show that the error of FWHMs and Brillouin shifts obtained by L3 model is less than 10% compare with the Tenti S6 model. Some factors that affect the measurement accuracy of the Rayleigh-Brillouin parameters are also analyzed and discussed.

Research on Measurement of Spectral Radiance for (230-1700)nm Transient Radiation Source

A spectral radiance measurement device is presented for (230-1700)nm transient radiation source. This device consists of a high temperature black body, two off-axis parabolic mirror, a reference radiation, a grating spectrograph and array CCD detector system. The measure principle model is described. In order to improve the measure precision for UV and NIR nanosecond transient radiation, we adjust the CCD integral periods and adopt the fixed phase high speed signal collection method to enhance the SNR of the device. Simultaneously the spectral distortion of the spectrograph is eliminated using non-parametric kernel regression de-noising algorithm and convolution algorithm. Some experiment results are also presented and discussed in this paper.

Study on detection and identification model of passive teraherz imaging system for extended target

Detection and identification performance prediction is an important part in designing the passive terahertz imaging system. This paper presents a novel detection and identification model of passive terahertz imaging system for concealed extended target. In the modeling process we take into account main factors such as radiation principle of target and background, the characteristics of atmospheric transmission, and imaging detecting system. Firstly we research interaction principle of terahertz radiation for object based radiation principle of low temperatures terahertz blackbody. We calculate target and background terahertz radiation power. So target-to-background contrast is described in different fields and ranges. Secondly, we research the effect of terahertz beam attenuation in atmospheric environment, and choose the best terahertz atmospheric absorption frequency. So we can account target radiation power in the surface of detector. Lastly we research passive terahertz imaging detecting method for improving THz radiometric resolution. Due to testing target is extended, the two dimensional real-time scanning method is adopted. So we account identification probability of passive terahertz imaging system in different distance. The paper shows results in different distances and is useful to design and evaluate the passive terahertz imaging system for concealed object detection and identification.