Theoretical and Monte Carlo analyses of the range-Doppler imaging capabilities of mode-locked CO 2 ladars

Abstract

Mode-locked CO2 lasers have been developed which can produce long coherent pulse trains consisting of many narrow subpulses. This laser waveform may be used to numerically generate range-Doppler images (inverse synthetic aperture radar images) of a target wherein the Doppler spread of a spinning target is used to create a synthetic cross-range target dimension. The narrow micro-pulse temporal width provides good range resolution, and the long coherent pulse train provides good frequency resolution of the (cross-range) target Doppler spread. In this paper we examine the algorithms and imaging capabilities of this waveform as implemented for the FLD and Hi-CLASS laser radar (ladar) systems which are now being installed in the AMOS facility on Mt. Haleakala, Maui and in an aircraft testbed. INTRODUCTIONThe U.S. Army, Space and Strategic Defense Command, and the U.S. Air Force, Phillips Laboratory, are developing high power CO2 lasers under the Field Ladar Demonstration (FLD) and High-performance CO2 Ladar Surveillance Sensor (Hi-CLASS) programs. These lasers may be operated in either the natural 'pulse-tone" waveform or in a mode-locked "pulse-burst" waveform depending on the desired measurements and targets. A first generation ladar system is being installed in an aircraft, and a second generation ladar system is being installed at the Air Force Maui Optical Station (AMOS) on Maui using a 60 cm telescope with beam director. This paper addresses the algorithms and capabilities of the FLD/Hi-CLASS mode-locked laser waveform (pulseburst) mode of operation: the narrow mode-locked laser pulses of less than 2 ns duration provide good range resolution, and the long coherent pulse train of greater than 10 gus, with selectable duration, provides good frequency resolution. This good frequency resolution allows a measurement of the target's Doppler spread as a function of the target's range extent and can thus be used to form a synthetic image of a spinning target in a single pulse. Images of non-spinning or slowly spinning targets can be constructed using tomographic imaging techniques, which requires many "looks", has been previously discussed in reference one. GENERATION OF THE MODE-LOCKED CO, LASER WAVEFORMWhen a laser is mode-locked, many longitudinal laser modes are lasing simultaneously inside the laser resonator, and these modes are all in-phase or "phase-locked". As a result, outside the resonator the E-field is the sum of the individual mode E-fields which results in a string of narrow pulses. The instantaneous E-field is given by the real part of the summation 40 ISPIE Vol. 2702 0-8194-2076-X/96/$6.00Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/23/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

High-fidelity ladar simulation

“…We have implemented several Short-Time Fourier Transforms [4]; the most important these being the sliding Gaussian window (Gabor transform), and Short-Time Cosine Transform. In addition we have implemented the pseudo-Wigner and Wigner-Ville transforms -members of the general Cohen class transforms, and the Empirical Distribution Modes (EMD).…”

“…The basic device parameters just described are related to the minimum resolvable Doppler shift and range increment, and to the maximum, unambiguous Doppler shift and range extent by a series of simple equations [1][2][3][4] shown in figure 3. Velocity is related to Doppler shift as shown in the table.…”