Xiaoying Cao

Characterization of the OPAL obscurant penetrating LiDAR in various degraded visual environments

The OPAL obscurant penetrating LiDAR was developed by Neptec and characterized in various degraded visual environments (DVE) over the past five years. Quantitative evaluations of obscurant penetration were performed using the Defence RD Canada – Valcartier (DRDC Valcartier) instrumented aerosol chamber for obscurants such as dust and fog. Experiments were done with the sensor both at a standoff distance and totally engulfed in the obscurants. Field trials were also done to characterize the sensor in snow conditions and in smoke. Finally, the OPAL was also mounted on a Bell 412 helicopter to characterize its dust penetration capabilities, in environment such as Yuma Proving Ground. The paper provides a summary of the results of the OPAL evaluations demonstrating it to be a true “see through” obscurant penetrating LiDAR and explores commercial applications of the technology.

Comparison of the relationships between lidar integrated backscattered light and accumulated depolarization ratios for linear and circular polarization for water droplets, fog oil, and dust

Recently, an empirical relationship between the layer integrated backscattered light and the layer accumulated depolarization ratio has been established for linear polarization for the case of water droplet clouds. This is a powerful relation, allowing calibration of space lidar and correction of the lidar signal for multiple scattering effects. The relationship is strongly based on Monte Carlo simulations with some experimental evidence. We support the empirical relationship with strong experimental data and then show experimentally and via second order scattering theoretical calculations that a modified relationship can be obtained for circular polarization. Also, we demonstrate that other empirical relationships exist between the layer accumulated linear and circular depolarization ratios and the layer integrated backscattered light for submicrometer particles and nonspherical particles.

On linear and circular depolarization LIDAR signatures in remote sensing of bioaerosols: experimental validation of the Mueller matrix for randomly oriented particles

Strong experimental data supporting the theoretical relationship between linear and circular depolarizations for randomly oriented particles are presented. The analysis of the data leads to the first experimental validation of the theoretical representation of the scattering Mueller matrix as having indeed only one free parameter for randomly oriented particles, which is the depolarization parameter, d. Consequently, there is no added information on the nature of the aerosols when the four Stokes parameters are measured for randomly oriented aerosols, as opposed to measuring only the linear polarization or only the circular polarization related parameters. This conclusion has a direct impact on the analysis of the level of complexity of the systems that are required to analyze aerosols based on their depolarization signatures.

Enhanced scanning agility using a double pair of Risley prisms.

Scanners with one pair of Risley prisms are robust and precise and they can be operated continuously. In this paper, we present a new scanner based on the use of two pairs of Risley prisms. The concept was driven by the need to add flexibility to Risley prism scanners used for lidar 3D mapping applications, while maintaining compactness and robustness. The first pair covers a FOV narrower than the second pair. The second pair is used to position the first Risley pair scan pattern anywhere within its own, larger, FOV. Doing so, it becomes possible, without additional scanner components, to increase the sampling point density at a specific location, to increase the sampling uniformity of the scanned area, and, while in motion, to maintain the sampling of a specific area of interest.

The effect of dense aerosol cloud on the 3D information contain of flash Lidar

A laser pulse propagating through dense clouds suffers from spatial and temporal distortion caused by multiple scattering of light. Both distortions are function of the optical depth, the particle size of the aerosols and the cloud distance relative to the target and receiver. In order to study the effects of all theses parameters, 3-D Monte-Carlo (MC) simulations were performed. The Monte Carlo developed for this purpose has the unique capability to produce both 2D and 3D images of the scenes. For the 2D images we calculated the MTF using the Fourier transform of the system PSF. For the 3D images gratings with rectangular grooves of various frequencies and height were used and the concept of contrast applied as for the calculation of MTF for 2 D images. We found that 3D temporal distortion effects are significantly reduced when the reconnaissance algorithm is base on the shape of the raising pulse.

Scanning Lidar Measurements of the Full-Scale RDD Field Trial Puff Plumes.

AbstractA vertically scanning lidar (light/radar) was used to measure the time evolution of clouds generated by a small explosive device. Vertical sweeps were performed at a downwind distance of 105 m from the detonation. The measured quantity obtained from the lidar was the light extinction coefficient. This quantity is directly proportional to the aerosol concentration. The background aerosol value was set to 0.0001 m−1 (assuming a visibility of 40 km), and assuming the scattering properties of the explosively generated cloud is the same as the background aerosol, the authors found that the instantaneous maximal local concentration of aerosol in the cloud did not exceed 500 times the background aerosol value, and the instantaneous concentration was typically less than five times the background aerosol value. In the two trials that were done, the volumes of the clouds were reasonably close at 2,700 m3 and 4,000 m3, respectively.

Development of an underwater fiber-optic lidar for the characterization of sea water and ice properties

DRDC Valcartier has developed a unique underwater lidar for the measurement of different sea water and ice properties. The lidar head is designed for underwater operation and consists of four telescopes that are connected to the detection and emission unit via five 42 m fused silica optical fibers. Three telescopes are used for data collection, while the fourth is used for laser emission. The laser source and the detection unit are located on a surface vessel. The laser beam is injected into a 100 μm diameter optical fiber. The collimation of the laser beam is done in the lidar head via a lens with 25 mm diameter and 45 mm focal length; the laser beam is linearly polarized using a polarization beamsplitter. A 50 mm receiving telescope co-aligned with the laser beam is used for linear depolarization measurements. A second 50 mm telescope is used to collect off-axis scattered light while a third 50 mm telescope is used to collect inelastic scattered radiation (Raman and induced fluorescence signal). The laser source and detection units are mounted on a small optical table for easy access/modification. Various laser sources and lidar detection techniques (Q-switched pulses or frequency modulated) could be easily implemented. The lidar head can be deployed underwater or mounted on an airborne platform. In this work, the lidar system will be described in detail and preliminary results obtained with a Q-Switch, 532 nm, 1 ns pulse laser source will be presented and compared with the anticipated performance for different water bodies.

On the information content of linear and circular depolarization signatures of bioaerosols

Cao et al.1 published a paper where differentiating bioaerosols (pollens) appeared feasible when linear depolarization ratio signature at multiple wavelengths could be obtained. The measurements were performed at 4 wavelengths. The bioaerosols were disseminated in a controlled environment and the discrimination analysis was based on Mahalanobis distances. Poor discrimination was obtained for single wavelength measurement while acceptable and good discrimination was reported for two and three wavelengths. This innovative work has raised the following question: to which extent does the addition of circular polarization signature to the existing linear polarization increase the overall discrimination capability? In order to answer that question, the measurements of Cao et al. were repeated for linear and circular depolarization ratios. We demonstrate experimentally that the linear and circular depolarization ratios are related to each other via a known simple theoretical mathematical expression in the case of randomly oriented particles. Hence, by measuring one, you obtain the other and consequently there is no additional information that is gained by doing measurements with the two polarization states. This suggests that there is no need for full Mueller matrix measurement systems for detection and discrimination of bioaerosols.

Lidar polarization discrimination of bioaerosols

Lidar bioaerosols discrimination based on depolarization signature is studied. The measurements were performed over 25 pollens and 2 dusts under controlled environment at a distance of 100 m, at wavelengths of 355 nm, 532 nm, 1064 nm and 1570 nm, and both linear and circular polarizations were used. It is found that discrimination of bioaerosols using single wavelength linear depolarization ratios is difficult because most of them are quite alike. However, two or more wavelengths measurements make it possible to discriminate different bioaerosols against others, especially when a depolarization ratio cumulative distribution is available.

Inversion of water cloud lidar signals based on accumulated depolarization ratio.

The relation between the accumulated single scattering factor and the layer accumulated depolarization ratio appears to be independent of the geometry of the measurements and contains information on the optical depth and thus on the extinction coefficient. A simple equation is developed to retrieve the extinction coefficient from the total integrated signal and the integrated depolarization ratio measurements. The results compare well with Klett and Weinman lidar inversion techniques. The results from the measurements of the integrated depolarization ratio can be used to set the far end initial extinction coefficient value required for Klett and Weinman lidar inversion or can be used directly.