PHYS 798E and ENEE 698D *Speaker: Ted Jones, Naval Research Laboratory, Plasma Physics Div.*
*Abstract:* Intense underwater laser propagation, filamentation, and ionization are being investigated at NRL for many Navy applications, including remote undersea laser acoustic generation for low-frequency long-range sonar, and advanced micromachining. The key to low-frequency laser acoustic generation is the shaping of the heated underwater volume, where more elongated volumes produce longer-duration acoustic pulses with more energy at low-frequencies.
A patented scheme for generating elongated, meter-scale, high-energy-density underwater plasmas [USP 9,088,123] is under study, in which optically active electrons associated with an “igniter” laser filament could serve as a target for a second energetic “heater” laser pulse. Previous NRL experiments revealed 100 µm diameter filament-like structures generated by ns pulses1. Separate NRL time-resolved absorption spectroscopy measurements, during intense underwater propagation of 400 nm, 170 ps laser pulses, revealed hydrated electron densities up to 5.4 x 1018 cm-3 with a lifetime of 350 ps. Together, these results suggest improved ionization efficiency using a 100 ps UV (266 or 355 nm) igniter pulse. An energetic 532 nm, 10 ns pulse could then serve as the heater pulse. Analytical and 2-D numerical modeling is underway to understand and predict beam propagation and filamentation in the presence of strong stimulated Raman scattering (SRS) and thermal lensing2,3. Results from recent experiments and simulations will be discussed.
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*This work is supported by NRL 6.1 Base Funds and direct ONR 6.2 Funds.
1M. Helle, et al, “Formation and Propagation of Meter-Scale Laser Filaments in Water,”Appl. Phys. Lett. *103*, 121101 (2012).
2B. Hafizi, et al., “Stimulated Raman Scattering and Nonlinear Focusing of High-Power Laser Beams Propagating in Water,” Opt. Lett. *40*, 1556 (2015).
3B. Hafizi, et al., “Stimulated Raman & Brillouin Scattering, Nonlinear Focusing, Thermal Blooming, & Optical Breakdown of a Laser Beam Propagating in Water,” JOSA B *33*, 2062 (2016). pending.
*Bio:* Dr. Jones presently leads research as PI on intense underwater laser propagation and interactions for many Navy applications, including underwater laser acoustic generation for sonar search, communications, and navigation. Dr. Jones’ research interests also include nonlinear optics, laser particle acceleration, and magnetic fusion. He has worked in the NRL Plasma Physics Division since receiving his Ph.D. in plasma physics from Princeton University in 1995.