Monopulse ladar: super-resolved 3D localization with Si-photonic serpentine optical phased arrays

Abstract

We present an optical ranging and super-resolution object localization method, monopulse ladar, used to determine the angle of a point target in two dimensions to a few percent of an optical beam width from differential measurements of four just-resolved waveform-encoded beams while simultaneously providing target range via either coherent or incoherent coded waveform correlation. A common optical carrier is shifted by four GHz-scale tones, each modulated with distinct ranging waveforms, which when transmitted from a Si-photonic 2D wavelength-steered serpentine optical phased array (SOPA) aperture form an encoded rectangular beam cluster that propagates to and scatters from a distant point target. Superposed backscattered target returns from each beam are decoded by correlation with reference waveforms at the receiver. The angular position of the target along the two orthogonal axes is calculated from pairwise ratios of beam amplitudes, while target range is determined from the round-trip time delay of each beam as measured with a wideband correlation peak. The analysis of coherent and incoherent monopulse ladar architectures presented herein indicates that a 50-fold increase in angular resolution—to the tens of arcseconds level—of a point target located within a wide field of regard is achievable while maintaining cm-scale resolution-limited ranging using a single SOPA tile transmitter, with further improvement in angular resolution possible through arrayed tiling of SOPAs. Implementation of monopulse ladar with a SOPA aperture enables non-mechanically steered high-resolution 3D object localization in a compact, low-control complexity form factor.