This paper addresses current design improvement issues of aerosol sensing Micro-Pulse Lidars (MPL). MPLs are designed to adhere to eye-safety restrictions while achieving acceptable signal to noise ratios (SNR). This method is realized by reducing the per pulse energy of the laser and employing a narrow receiver field-of-view (FOV). Due to the narrow FOV requirement, only a partial return signal is measured until the laser beam propagates a distance where the receiver FOV fully overlaps the laser beam. This is called the full overlap distance and is usually 4 km or more for reasonable MPL parameters. Accurate MPL measurements are typically only possible beyond this distance. The fraction of laser beam energy that is within the receiver FOV versus range is called the overlap function. The causes of the overlap function are discussed. An overlap related problem with current MPL designs is that the majority of the atmospheric aerosols are located below an altitude of 4 km to 5 km, within the partial overlap region. Another problem is that the overlap function is not thermally constant. This introduces errors in the experimentally derived overlap function and system constant factor, ultimately leading to errors in the retrieved lidar signal.
By combining the capability of a differential absorption lidar (DIAL) and the excellent characteristics of a micro pulse lidar (MPL) we have designed and tested a micro pulse DIAL system, which could be operated from the ground or airborne platform, to monitor the atmospheric water vapor mixing ratio. To maintain the compact and rugged optical frame work of an MPL it employs a diode pumped tunable Cr:LiSAF laser operating at 825 - 840 nm range, a fiber optic beam delivery system, and an APD photon counting detector. The system parameters were optimized through extensive DIAL simulations, and the design concept was tested by building a breadboard lidar system. Based on the results of the simulations and the performance of the breadboard lidar the Micro Pulse DIAL system design has been refined to (1) minimize scattered laser light -- the major source of signal induced bias, (2) permit near field measurements from less than 400 m, (3) produce a compact, rugged, eye-safe instrument with a day and night operating capability. The lidar system is expected to provide 150 m vertical resolution, high accuracy (approximately 5%), and 3 km range looking up from the ground.
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