A Continuous Wave Aerosol Lidar System

Abstract:

A low cost aerosol lidar system has been developed based on continuous wave solid state lasers and a CCD camera detector. Atmospheric aerosols affect air quality and precipitation. The system detects clouds and aerosols by measuring side-scattered intensity of laser light. It can be used with either a single CW laser or a pair of CW lasers as the transmitter. Located far from the transmitter, a CCD camera detector with wide angle lens and laser line filter images the entire laser beam from ground to zenith. It records the intensity of side scattered light at each pixel. Due to the bistatic configuration of the system, the altitude of the scattering particle can be determined by the geometry of the transmitter and detector placement, thus pulse time of flight is not necessary. This allows inexpensive continuous wave lasers to be used as the active source. With the assumption of an aerosol scattering phase function and single scattering albedo, these data can be used to derive aerosol extinction profiles. The system can also be used to track heights of cloud and aerosol layers. The system is capable of providing data all the way to ground level without field of view overlap issues.

The system was tested at night in an urban environment in New Britain, CT. Data images were obtained using 2 Watt and 5 Watt CW NdYAG lasers. Substantial aerosol and some clouds were present. In some experiments the 5W laser was fitted with a simple beam expander for enhanced safety. A SBIG STF-8300 CCD camera was cooled to 0 C. Light was integrated on the CCD chip for 100 seconds. Using code written in IDL, the beam center was found and a function consisting of a Gaussian plus a constant was fitted perpendicular to the beam center at each altitude. The constant represents the sky background and the area under the Gaussian represents the side-scattered beam signal.

The signal to noise characteristics were evaluated for the imaged beams. The system showed sufficient SNR for aerosol studies under these conditions. Lasers may be used together for stronger signal if desired or lasers may be separated physically so the beams are visible in the same image but do not overlap. When two beams are used they could be set at the same polarization for studies of small scale spatial variations or set for differing polarizations to investigate aerosol scattering differences based on polarization.