Laser Noise Reduction in Air

Fluctuations of the white-light supercontinuum produced by ultrashort laser pulses in selfguided filaments (spatio-temporal solitons) in air are investigated. We demonstrate that correlations exist within the white-light supercontinuum, and that they can be used to significantly reduce the laser intensity noise by filtering the spectrum. More precisely, the fundamental wavelength is anticorrelated with the wings of the continuum, while conjugated wavelength pairs on both sides of the continuum are strongly correlated. Spectral filtering of the continuum reduces the laser intensity noise by 1.2 dB, showing that fluctuations are rejected to the edges of the spectrum. Considerable interest has recently been devoted to quantum optics and non-linear effects in transparent media. Second order nonlinearity (χ) processes in parametric generators have been the model of choice in this respect, where both photon correlation and squeezing were first demonstrated. Recent studies showed that both phenomena also occurred for temporal solitons in optical fibers [1,2]. The origin of the correlations in the generated continuum is intrinsic to the (χ) self-phase-modulation process [3]. For sufficient laser intensity, selforganized spatio-temporal solitons form even in the air. These self-generated light filaments propagate over several hundreds of meters and give rise to an exceptionally broad continuum [4]. In this Letter, we report the observation of both spectral correlations in the continuum and laser noise reduction for filaments propagating in air. These results, which remain in the classical domain of correlations and noise reduction, open perspectives for high precision remote measurements of atmospheric molecules or for the transmission of encrypted information [5]. For instance, water vapour concentration and atmospheric temperature profile