Company, agency, university collaborate for lidar monitoring

17 Jun 2002

Optical Surfaces Ltd., the Remote Sensing Group of the Defence Evaluation and Research Agency, and the Civil Engineering Department of Salford University in Manchester have collaborated to develop high-performance off-axis beam expanders to be integrated into pulsed coherent lidar systems.

The lidar system will be used to measure local wind fields and the dispersion of atmospheric pollution in urban areas. The lidar directs a pulsed, eye-safe CO2 laser into the atmosphere, whereupon naturally occurring aerosol particles in the air backscatter the laser radiation for detection by the lidar receiver. The Doppler shift between the outgoing and backscattered light is evaluated to give the line-of-sight velocity of the aerosol scatterers which are assumed to move with the localized wind field.

Measuring the dispersion of airborne pollutants in a built-up city environment along a clean air corridor required the development of the pulsed coherent lidar system. Continuous wave lidar offered good spatial and spectral resolution, but the systems typically measure wind velocity and turbulence at a point in space and are limited to a detection range of a few hundred meters. Radar was determined not to be a feasible solution because of clutter which is experienced close to a built-up environment along with the lack of clear air returns.

The pulsed coherent lidar system was theorized to provide clean air detection over a distributed corridor to a range of several kilometers without suffering from terrain-induced clutter. Optical Surfaces felt that the required performance and design compactness for a monostatic system in which the same lidar telescope transmits and receives the laser signal would best be achieved through an off-axis parabolic beam expander.

Optical Surfaces supplied a compact x15 off-axis beam expander unit with a transmitted wavefront accuracy of lambda/5 over the complete 150 mm output aperture measured at 633 nm. The high surface quality eliminated limitations on the performance of the system due to telescope feedback.

The DERA researchers integrated the variable focus beam expander, covering infinity to 20 meters, into the lidar unit. Future operational flexibility was built in due to the off-axis beam expander design which gives achromatic performance from the infrared to the visible.

The lidar system operates at 120 pulses per second with a 500 nanosecond pulse length. It has been able to measure the wind velocity and aerosol concentration to a range of 3 kilometers with a range resolution of approximately 100 meters.

Future plans include exploiting the achromatic performance of the optical system to make multiple wavelength differential absorption lidar measurements in the Doppler mode where species concentrations and the wind field can be simultaneously monitored with the same equipment.