Synthetic aperture ladar imaging demonstrations and information at very low return levels.

We present synthetic aperture ladar (SAL) imaging demonstrations where the return-signal level from the target is near the single-photon level per resolved pixel. Scenes consisting of both specular-point targets and diffuse-reflection, fully speckled targets are studied. Artificial retro-reflector-based phase references and/or phase-gradient-autofocus (PGA) algorithms were utilized for compensation of phase errors during the aperture motion. It was found that SAL images could reliably be formed with both methods even when the final max pixel intensity was at the few photon level, which means the SNR before azimuth compression is below unity. Mutual information-based comparison of SAL images show that average mutual information is reduced when the PGA is utilized for image-based phase compensation. The photon information efficiency of SAL and coherent imaging is discussed.

Shot noise statistics and information theory of sensitivity limits in frequency-modulated continuous-wave ladar.

A theoretical analysis and experimental verification of the sensitivity limits of frequency-modulated continuous-wave (FMCW) ladar in the limit of a strong local oscillator is presented. The single-photon sensitivity of coherent heterodyne detection in this shot-noise dominated limit is verified to extend to linearly chirped waveforms. An information theoretic analysis is presented to estimate the information efficiency of received photons for the task of locating the range to single and multiple targets. It is found that the optimum receive signal level is proportional to the logarithm of the number of resolvable range locations and the maximum theoretical photon information efficiency for FMCW ranging with coherent fields is log(e)≈1.44 bits per received photon.

Characterization of an actively linearized ultrabroadband chirped laser with a fiber-laser optical frequency comb.

The optical frequency sweep of an actively linearized, ultrabroadband, chirped laser source is characterized through optical heterodyne detection against a fiber-laser frequency comb. Frequency sweeps were measured over approximately 5 THz bandwidths from 1530 nm to 1570 nm. The dominant deviation from linearity resulted from the nonzero dispersion of the fiber delay used as a reference for the sweep linearization. Removing the low-order dispersion effects, the residual sweep nonlinearity was less than 60 kHz rms, corresponding to a constant chirp with less than 15 ppb deviation across the 5 THz sweep.