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Compact, Unattended Lidar System Relies on an Nd:YAG Laser

Photonics Spectra
Jun 1998
Ruth A. Mendonsa

Over the past 30 years, lidar systems have become increasingly useful for the study of air quality and dynamics. Lidar can measure vertical profiles of airborne particulates, wind speed and direction, water vapor, temperature and chemical species such as ozone. But the need for less expensive, smaller and less complex systems has led to the development of a design that could lead to unattended field operation.

Portable lidar
Erim International Inc., formerly the Environmental Institute of Michigan, has produced a portable, ruggedized elastic backscatter lidar based on the CFR-200, a compact, frequency-doubled Nd:YAG laser from Big Sky Laser Technologies Inc. Called the M10 Lidar, the system is designed for easy transport and setup.
It measures direct backscatter for aerosols and molecules, which provide parameters for calibrated scattering coefficient proiles, relative aerosol loading profile, boundary layer mixing height, cloud base height and cloud vertical extent. The system can operate unattended for extended periods, although it usually is checked every 8 to 12 hours to ensure accurate data acquisition.
Erim selected the CFR-200 because of its compact size, stable resonator cavity and durability for field use. The doubled Nd:YAG operates at 532 nm in the green part of the spectrum and produces more than 100 mJ of energy per pulse with a 30-Hz repetition rate at this wavelength.
Besides the Nd:YAG, the M10 Lidar contains transmitting and receiving optics and electronics, including a 10-in. Schmidt-Cassegrain telescope, an interference filter, a Thorn EMI photomultiplier tube and data analysis tools developed for this system using Matlab software from The Math Works. All system controls, including laser firing and data acquisition synchronizing, are computer-controlled. Housed in a weatherproof shipping container, the system operates under varying weather conditions without direct supervision.
The M10 Lidar was tested in the field last summer at the Program for Regional Oxidants, Photochemistry, Emissions and Transport at the University of Michigan Biological Station near Pellston, Mich. The researchers measured more than 500 hours of nearly continuous profiles of aerosols and clouds extending up to approximately 15 km. The data are being used to investigate the chemistry and physics of ozone and fine particulate matter in the region.

Critical role
The lidar aerosol profile data collected during the study played a critical part in understanding the particulate levels at and above the site as well as the boundary layer dynamics. The system provided the ability to map the boundary layer in detail over the course of the day. According to Ken W. Fischer, a research scientist at Erim, these lidar aerosol measurements provided important insight into small-scale mixing and cloud processes that are unavailable from any other sensor type.

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