Abstract Details
Two-beam coupling in laser beams intersecting in air
Author: Aaron C. Bernstein
Requested Type: Poster Only
Submitted: 2009-04-21 16:53:29
Co-authors:
Contact Info:
University of Texas at Austin
1 University Station, C1510
Austin, TX 78712
USA
Abstract Text:
Two-beam coupling in laser beams intersecting in air
A.C. Bernstein, M.W. McCormick, G.M. Dyer, J.C. Sanders, T. Ditmire
Department of Physics, University of Texas at Austin, Texas 78712, USA
We observed a 7% energy exchange between two nearly identical nonlinearly propagating beams intersecting in air. The extent of the energy exchange was determined by their relative delay, and which pulse (i.e. advanced or delayed) gained energy was determined by the sign of the pulse chirps. The data is well reproduced by two-beam coupling (TBC) theory which incorporated the inertial (non-local) nonlinear impulsive stimulated Raman response in air. Further, the beams were intense enough to form filaments, which are local transverse structures in the beam that propagate with air-ionizing intensity (~10^14 W/cm^2) for long distances.
Many applications for filaments have been suggested, including few cycle pulse generation, lightning and discharge triggering and guiding, remote-sensing, generation of directional terahertz generation, and even power-delivery. However, thus far, filament control has been primarily through optimization of the pulse launch conditions. Our measurements of the optical conical emission of the filaments shows that filaments were strongly influenced by the energy exchange taking place in the beams, indicating that TBC may be a mechanism to control filament propagation during its actual flight.
To make the measurements, an all-reflective technique was devised which effectively produces two nearly identical, nearly Gaussian pulses positively chirped from a bandwidth-limited 42-fs to durations ranging from 90 to 270 fs. After compression, each beam contained approximately 0.8 mJ, struck a 5 m focal-length mirror and was steered with two mirrors to cross in the ambient laboratory air. At an intersection half-angle of 0.3º the whole-beam interaction length was about 20 cm and the filament interaction length was about 1 cm. A motorized stage in one of the beam paths allowed us to adjust the relative delay of the beams.
Our theoretical treatment required TBC in the presence of a delayed Kerr nonlinear response, and air has such a response with a time-constant on the order of our pulse durations. As a verification of the TBC description, our data showed that when reversing the chirp, the sense of the energy exchange also reversed sign. With the magnitude of the nonlinear interaction as the sole fitting parameter, we fit the data for pulse lengths varying from 90 to 270 fs with nonlinear index of refraction values of n_2 = 2.5x10^-19 cm^2/W and 5x10^-19 cm^2/W, close to the accepted value of n_2 = 2x10^-19 W/cm^2.
Immediate applications include shaping and amplification of short pulses in air, control of intense laser propagation through atmosphere, and extension of filament propagation to previously unattained distances.
Comments:
In the above, I use ^ for superscript and _ for subscript. Thanks!