Continuously-Operating Laser Range-Finder Based on Incoherent Compression of Periodic Sequences
Abstract: Lidar (or LADAR) is a remote sensing technology that measures distance by illuminating a target with laser light and analyzing reflections. The term lidar was created as a portmanteau of "light" and "radar". There are two kinds of ladar detection schemes: incoherent or direct energy detection (which is a measurement of intensity only), and coherent detection which makes use of both the amplitude and the phase information of the optical wave. Coherent systems are more sensitive and operate at a much lower incident optical power, at the expense of more complex transceiver implementation. Ladar systems can operate based on the transmission of a single intense pulse, or an extended sequence of weak pulses that are compressed at the receiver to form a single 'virtual pulse'. Today, ladars are widely used as a technology for making high-resolution maps, with applications in geomatics, archaeology, geography, geology, geomorphology, seismology, forestry, remote sensing, atmospheric physics, airborne laser swath mapping (ALSM), laser altimetry, and contour mapping.
In this work, a continuously operating laser range finder (LRF) setup, based on the incoherent compression of periodic sequences, was realized. An analytic model was established for compression quality in a real-world system. While the compression protocol is theoretically characterized by zero ranging sidelobes, additive noise of a real world system introduces finite sidelobes which in turn restrict the system performance. The analytic model can predict the ratio between the power of the main peak and that of the highest sidelobe, based on the measurement signal-to-noise ratio. The LRF was build out of telecom 'off the shelf' components. It was used in outdoor ranging measurements at 270 meters distance. The peak transmittance power was as low as 600 mW, traces were acquired over 50 micro-seconds, with no averaging over repeating sequences. The separation between two targets with a distance offset of 1 m, both in partial overlap with a single beam, was demonstrated as well.
* The work was carried out towards the M.Sc. degree in the Faculty of Engineering, Bar-Ilan University, with the supervision of Prof. Avi Zadok