Advances in real time lidar spectroscopy

We summarize the resolution limits and potential sensitivity of existing Hoar-spectrometer systems. The spectral resolution of these systems is limited primarily by the phase space area (entendue), which depends on the width and convergence angle of the laser beam's image on the spectrometer input slit. Decreasing the beam's image width can be achieved with conventional optics at the expense of an increase in convergence angle, which often results in a loss in coupling efficiency due to mismatching with the spectrometer's phase space area. We describe how this limitation has been overcome using suitably designed optical fiber bundles. We also discuss both theoretical and computer modeling predictions of the variation in telescope-to-spectrometer coupling efficiency with range for lidar spectrometers. Finally we compare the performance of photon counters and intensified CCD arrays and discuss methods of range-gating their outputs.