Far-Field Acoustic Subwavelength Imaging with Blind Structured Illumination and Joint-Sparsity Reconstruction

The resolution of far-field imaging systems suffers from the diffraction limit. In this work, a blind structured illumination microscopy (blind-SIM) system that achieves subwavelength resolution in acoustic far-field imaging is developed. In blind-SIM, precise knowledge of the structured illumination (SI) patterns is not required. Instead, the acoustic SIs are generated by a randomly located acoustic diffraction grating with a subwavelength grating period. The spatial frequency mixing between the object and the SIs converts evanescent waves to propagating waves that can reach sensors in the far field. The image of the object is then reconstructed from multiple far-field measurements utilizing a joint-sparsity compressive sensing algorithm. Numerical simulations and experiments in kilohertz airborne sound are performed to validate the proposed system. A subwavelength imaging resolution of around 1/3 wavelength is achieved in experiments, and around 1/5 wavelength in simulations. The blind-SIM system largely reduces the imaging hardware complexity compared with existing acoustic subwavelength technologies that mostly rely on metamaterial lenses, time-reversal mirrors, or the precise localization of time-varying contrast agents. The combination of blind-SIM and the joint-sparsity compressive sensing algorithm for image reconstruction reduces the requirements for an object???s sparsity level and enhances the system???s robustness against noise. Our work has potential in improving the practicality of acoustic far-field subwavelength imaging in medical diagnoses, nondestructive testing, and underwater acoustic imaging and communications.