The use of modulated radiation pressure in target detection and classification

In the past few years, we have experimentally demonstrated that modulated radiation pressure (MRP) can be used to extract classification clues such as resonant frequencies, mode shapes and sizes for targets of interest.  These experiments were carried out at Washington State University (WSU) between (2015-2020) in water tanks involving scaled targets.  Using a 15-cm acoustic lens, MRP was applied to a variety of metallic targets, including circular plates, solid aluminum cylinders, open-ended cylindrical shells, and even an aluminum replica of a UXO, and their resonant frequencies and mode shapes were measured.   Modulated radiation pressure exploits the phenomenon of the acoustic radiation force, which is produced due to the rate of change of momentum that an incoming wave experiences when it is scattered by a target.  The acoustic radiation force has no obvious use for a regular incident signal, but if the incident field is a double-sided suppressed carrier modulated (DSB-SCM) signal, the way in which a physical system responds to such an excitation, results in a signal that oscillates at twice the frequency of the modulation oscillator. This creates a process by which a target can be interrogated by the surgical accuracy of a narrow beam, produced by the high carrier frequency, while ‘being shaken’ at a single low modulation frequency. A modulation frequency sweep (frequency scan) can reveal target resonances, while a physical scan along its length can reveal its mode shapes corresponding to those resonances, which can be used to estimate target size and content.  The above experiments were conducted in highly controlled laboratory environments at short standoff distances due to the short (15 cm) focal lengths of the available acoustic lenses.  Prior to the start of this STTR, an acoustic lens with a focal length of one meter was designed and fabricated at WSU.  During Phase I of this STTR, this lens was used to conduct MRP experiments involving a variety of targets including a solid aluminum cylinder and an open-ended, thin aluminum cylindrical shell.  It was demonstrated that MRP could excite these targets at a standoff distance of one meter and for the case of the cylindrical shell it was also demonstrated that it could extract its mode shape by a physical scan along its length. The focus of the Phase II work is 1) to demonstrate that MRP can be used in a realistic ocean environment and 2) that it is capable of extracting classification clues for real-size targets at longer standoff distances of up to three meters.