Transverse dynamics and boundary conditions of resonance self-action in frequency-modulated cw laser beams

Abstract

ABSTRACT In present paper we investigate the transverse dynamics of frequency-modulated cw laser beam propagated in resonance conditions. At modulation periods comparable with the atomic relaxation times the time dependence of the output intensity exhibits the manifestations of resonance sel f-action effects. The influence of resonance self-induced lens and aperture on cw laser beam transverse profile and the boundary conditions of self-action are analyzed by numerical simulation. Keywords: resonance self-action, cw laser beam propagation, numerical simulation, frequency modulation, propagation length, transverse profile of laser beam, Gauss-Laguerre modes. 1. INTRODUCTION Self-action of cw laser beams due to saturated refraction in resonance media has been studied for a long time (see, e.g. [1-4]), most of investigations was related to observation of lensing properties of the media. Under extremely high absorption and saturation conditions the manifestations of self-induced diffraction have been found to mimic self-focusing and self-channelization at zero frequency detuning when the optical strength of the saturation-induced lens is zero [2-4]. From the theoretical point of view it means that the medium response can be described by means of the nonlinear susceptibility. Obviously, the same approach is valid for time-dependent field whose variation is slow compared with atomic relaxation. Faster field changes are expected to cause transient response of the medium and, therefore, transient self-focusing and self-aperturing effects. Experimental manifestations of non-stationary near-resonant self-focusing of frequency-modulated beams have been reported in saturation spectroscopy [5]. The present paper is a continuation of our previous work [6], where we inve stigate the beams initially modulated in frequency. Passing through resonant absorbing medium, the beam gradually acquires intensity modulation, which is simply caused by different absorption at different frequencies. This effect is neither nonlinear, nor transient, it takes place even at low intensity and low frequency of modulation. Note, that in this case it is not affected by the lensing properties of the medium, since they are due to saturation and not to frequency-dependent refraction as such. First, we study how the saturation of absorption and refraction affects the transmission of modulation from phase at the input to the intensity at the output. We take the frequency of modulation low enough to consider the saturation as adiabatically following the field variation in time. The most prominent manifestations of the induced lens can be seen in moderate saturation regime. In this regime we focus our attention on the resonant self-action dynamics and boundary conditions and on the self-action influence on laser beam transverse profile.