Range determination with waveform recording laser systems using a Wiener Filter

Current pulsed laser scanning systems determine the range to an object surface by a time-of-flight measurement. Critical measurement situations occur in discriminating the ranges of surfaces close to their edges or of small objects within the beam footprint which are closely located in range. Capturing the complete waveform of the laser pulse allows discriminating differences in a range smaller than the length of the laser pulse. The capabilities of this technique can be predicted by modeling the emitted pulse, the surface, and the backscattered pulse. Due to the varying waveforms of the emitted pulses each individual emitted pulse is recorded and considered for the determination of the surface features. A deconvolution is used to remove the characteristic of the transmitted waveform from the received waveform to obtain a surface response. A Wiener Filter reduces the noise of the determined surface response. For extraction of temporal position, length, and amplitude the corresponding surface features are approximated by Gaussians using the Levenberg-Marquardt Method. Experiments have shown that a stepped surface within the beam with a step smaller than ten times of the pulse length can be distinguished.