Remote Underwater Laser Acoustic Source

Summary form only given. Through the mechanisms of nonlinear whole-beam self focusing (NSF) and linear group velocity dispersion (GVD), an ultrashort laser pulse can propagate relatively long distances underwater at moderate intensity (up to distances on the order of the attenuation length, approximately 10 meters in sea water), then quickly converge to an intense focus within a few centimeters at a predetermined remote location. Optical breakdown would then generate a plasma and an acoustic shock at this location. Such an acoustic source could be useful for sonar imaging and other Navy applications. Previous experiments at NRL indicate that ~1 mJ of coupled laser energy will produce a 200 dB, microsecond-timescale acoustic pulse, a source level more than adequate for high resolution acoustic imaging applications. This technique has the capability to improve on previous laser acoustic generation schemes in two important ways: 1) the present scheme allows for laser propagation through many meters of water, and 2) the photoacoustic energy conversion efficiency can be on the order of tens of percent for optical breakdown, versus 10^-4 or less for other schemes relying on thermal expansion of water. The NRL research program aims to study the physics of intense underwater laser propagation and acoustic generation, including: GVD; nonlinear refractive index effects such as NSF, filamentation, and self-phase modulation; scattering; absorption; and laser-induced breakdown. NRL FY05 experiments include efforts to generate and tailor an appropriate frequency-chirped pulse at 400 nm, to measure the GVD of water, and to demonstrate GVD/NSF-induced pulse compression. Broadband 2^nd harmonic generation at 400 nm with conversion efficiency up to 12%, and pulse energies up to 2 mJ has been demonstrated. NRL intense underwater laser propagation simulations predict an upper limit on initial pulse power due to beam filamentation instability of 15 P_nsf, where P_nsf=lambda² /2pin₀n₂. A lens-aided compression scheme is under investigation which may increase this limit. Initial experimental and simulation results will be presented