System description

Use of a diode laser (semiconductor active medium) in a lidar has distinct benefits. The lasers are considerably more compact, reliable and less expensive than typical lasers used for lidar instrumentation.

The diode-laser-based (DLB) lidar architecture developed by NCAR in collaboration with MSU uses continuous wave seed lasers that are amplified into pulses (5-10 µJ/pulse) at high repetition rates (5-10 kHz).  For high quality daytime operation, suppression of the solar background is achieved with a narrow receiver field of view (100 µrad) and extremely narrow-band (10-20 pm full width half max) optical filters.  The transmitted laser beam is eye-safe and invisible (Class 1M) and the receiver uses single photon counting detectors.

The differential absorption lidar (DIAL) technique uses two separate laser wavelengths: an absorbing wavelength (online) and a non-absorbing wavelength (offline).  A ratio of the range-resolved backscattered signals between the online and offline wavelengths is proportional to the amount of water vapor in the atmosphere.  The technique requires knowledge of the absorption feature (obtained from molecular absorption database) and estimates of the atmospheric temperature and pressure (obtained from surface measurements and standard atmosphere models). The technique also requires the laser wavelength to be stable and confined to a narrow band or “single frequency” so some type of diffraction grating is used for feedback to the seed laser.