Optical Breakdown Acoustic Sources for Broadband Underwater Sensing
Abstract
Current methods of testing underwater structures for fatigue, damage and foreign objects areslow, costly, and often require divers"" to hold an acoustic transducer in direct contact with thetarget under investigation. Maritime inspection tasks would become safer,"" more cost-effective,and more reliable if underwater vehicles could independently scan a target with dense coveragethat reveals ma""terial properties and health. Unfortunately, marine corrosion, biofouling, andsensing geometry limit the ability of existing remote" sensing techniques to identify materialproperties remotely and consistently. Optical techniques are limited by their inability to penetratebeyond surface coatings or fouling layers to characterize the target of interest. Acoustictechniques are limited by low" bandwidth excitation and excitation geometry, which cannotsufficiently produce a measurable elastic response that encodes target m""aterial properties.We propose a mixed optical and acoustic technique for remote material testing underwater,which combines (1) the" high resolution of laser illumination with (2) the ability to exciteinformation-bearing acoustic waves in the target. In our techn"ique, a high-power pulsed laser isfocused to a distant point in the water, where nonlinear optical interactions generate a plasma.""This plasma expands explosively, inducing a high-intensity ultra-broadband acoustic wave in thewater, which can be used to probe th""e elastic response of an underwater target.Because the process is driven by a laser pulse, the acoustic source can be displaced fro""m theinspection vehicle for multistatic sensing, and can produce compact acoustic signals, ultimatelyproviding optically-limited s""canning resolution. Optical breakdown generates a wider bandwidthacoustic signal than conventional underwater transducers, and the"" nonlinear behavior of theacoustic shock can improve target coupling for near-field interactions. Finally, opticalbreakdown genera""tes acoustic source levels comparable to current underwater transducers, withlevels on the order of 180-210dB re 1uPa reported in t""he literature.To analyze the target response, we are building off of signal processing techniques originallydeveloped for seismic"" imaging and industrial nondestructive testing. By using full waveformanalysis, we expect to recover the response of the target acr""oss a broad range of frequencies,allowing us to isolate the effects of underlying target structures and ignore the response of afo""uled surface. By increasing the distance between the optical breakdown acoustic source and thereceiving transducer, we can also pro""be a single point on the target from many differentexcitation angles, allowing us to characterize shallow surface properties using" interface wavepropagation analysis techniques that have been developed for seismology and industrial nondestructivetesting. By ex"perimentally testing these techniques in a controlled laboratoryenvironment, we will compile a database of material responses to be" used for empirical materialclassification while we build up analytical models for more robust arbitrary-materialclassification as" a long-term project goal.In summary, we are exploring the hypothesis that optical breakdown can generate an acousticsource useful" for broadband mapping of material properties and structural health. This techniquewould enhance traditional narrow-band SONAR meas"urements with high-resolution,multispectral target information, in order to generate material property maps of diverse andcomplex" underwater targets.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jan 23, 2018
- Source ID
- N000141812066
Entities
People
- Doug Hart
Organizations
- Massachusetts Institute of Technology
- Office of Naval Research
- United States Navy