Laser line shape and spectral density of frequency noise

Abstract

Published experimental results show that single-mode laser light is characterized in the microwave range by a frequency noise which essentially includes a white part and a $1∕f$ (flicker) part. We theoretically show that the spectral density (the line shape) which is compatible with these results is a Voigt profile whose Lorentzian part or homogeneous component is linked to the white noise and the Gaussian part to the $1∕f$ noise. We measure semiconductor laser line profiles and verify that they can be fit with Voigt functions. It is also verified that the width of the Lorentzian part varies like $1∕P$ where $P$ is the laser power while the width of the Gaussian part is more of a constant. Finally, we theoretically show from first principles that laser line shapes are also described by Voigt functions where the Lorentzian part is the laser Airy function and the Gaussian part originates from population noise.