Laser Lineshape and Frequency Fluctuations: Low-Pass Filtered White Noise

Frequency fluctuations cause the optical field of a real laser to deviate from a pure sine wave. Thus the laser is not purely monochromatic, its power spectrum showing a broadened lineshape rather than a Dirac delta function. The lineshape and linewidth depend on the spectral properties of the frequency fluctuations. This Demonstration calculates the lineshape of a laser having a constant frequency noise level below a cutoff frequency but zero above this threshold. When , the lineshape is Gaussian and the linewidth increases as . When , the lineshape becomes Lorentzian and the linewidth is independent of . These observations lead us to separate the frequency noise spectrum into two regions where the effect of noise on the lineshape is radically different: 1) the slow modulation area (left side of the red line) where the noise contributes to the laser linewidth, with a Gaussian shape, and 2) the fast modulation area (right side of the red line) where the noise contributes only to the wings of the line, and not to the linewidth, thus transforming the lineshape from Gaussian to Lorentzian.