Synthetic aperture ladar imaging via synthetic frequency-stepped linearly-chirping signal

In order to achieve a coherent light source with large modulation bandwidth and high repetition rate for synthetic aperture ladar (SAL), a Synthetic Frequency-Stepped linearly-Chirping Signal (SFSCS) is proposed and a SAL using SFSCS illumination (SFSCS-SAL) is investigated both theoretically and experimentally. The SFSCS is synthesized by multiple independent lasers with relatively narrow chirping bandwidth and high repetition rate. Data equations, as well as the image formation theory, on the SFSCS-SAL are given. The theoretical studies show that under proper conditions, the SFSCS-SAL follows the same image formation theory as a conventional SAL; whereas, the range resolution in an SFSCS-SAL will be raised at least N times compared to that of a SAL with only one piece of sub-pulse laser (N is the number of independent lasers in the SFSCS). In the experimental demonstration, an SFSCS of N=3 was produced by optically chopping a linearly wavelength-scanning laser output into 3 sub-pulses; an SFSCS-SAL was set up using the SFSCS and high resolution images were obtained with either retro-reflective or diffusive targets. The experimental results are in well agreement with the theoretical analysis.