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What is the difference between Gaussian and Lorentzian linewidth?

Many active electronic and photonic devices exhibit flicker like (1/fα, where α is >2) noise caused by instabilities such as in driver electronics and thermal and vibration effects of resonant cavities. The flicker like noise dominates at lower (Fourier) frequencies of the noise spectrum while other fundamental noise sources such as spontaneous emission and shot noise dominate at higher frequencies. The contribution of the 1/f noise to laser linewidth is that the fundamental Lorentzian line shape transforms to a Gaussian line shape where its numerical value is dependent on the corresponding measurement time scale. Neither Lorentzian or Gaussian linewidth fully describes the overall laser linewidth shape and the convolution of the two linewidth profiles, also referred to as Voigt profile, more accurately describes the overall laser line shape.

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