Kerr-free propagation of a near-resonant laser in an atomic vapor

In this paper, we address the propagation of a near resonant laser inside an atomic vapor in the case when, due to the Kerr effect, the laser beam would either self-focus or self-defocus. We show, both theoretically and experimentally, how to get rid of such an alteration in the transverse beam profile without changing any of the characteristics of the laser light under consideration (wavelength, intensity, etc.), nor of the atomic vapor. Moreover, our proposed method offers a lot of control on the beam profile, whose transverse size after propagation may be chosen at will by making use of a second, copropagating laser, whose required wavelength and intensity may be derived analytically.

Figure 1

$XY$ surface plots of laser 1 at the output of a 1-m-long barium vapor in three situations illustrating the control of the Kerr effect by a copropagating laser, laser 2. (a) Laser 1 is alone and its frequency is far enough from the atomic transition for its space profile to remain nearly unperturbed. (b) Laser 1 is still sent alone in the vapor but its frequency is tuned near the atomic resonance (its detuning is ${\Delta }_1=-25.84$ GHz, and its intensity is $I_1=3.2\times {10}^3$ W/cm${}^2$) which leads to self-focusing due to the Kerr effect. (c) The input characteristics of laser 1 are the same as those in panel (b), but the Kerr effect is almost canceled out by making use of a second copropagating laser whose detuning with respect to resonance and intensity are, respectively, ${\Delta }_2=-34.17$ GHz and $I_2=1.2\times {10}^6$ W/cm${}^2$.

Figure 2

Energy ladder of the three-level atom.

Figure 3

Hyperfine levels coupled to the ground level $|0,F_0,m_0\rangle$ in the case when $J_0=0$. Arrows show which levels are coupled.

Figure 4

Hyperfine structure levels in the case of the chosen transitions in ${}^{135}$Ba or ${}^{137}$Ba.

Figure 5

Experimental setup.

Figure 6

Example of the monitoring of the space profile of laser 1 obtained by tuning the frequency of laser 2. The other laser characteristics are identical to those of Fig. 1.