Photon-count fluctuations exhibit inverse-square baseband spectral behavior that extends to $< 1\;\mu$Hz

Abstract

A broad variety of light sources exhibit photon-count fluctuations that display inverse-square spectral behavior at extremely low frequencies. These sources include light-emitting diodes, superluminescent diodes, laser diodes, incandescent sources, and betaluminescent sources. In a series of experiments carried out over an 18-month period, the photon-count fluctuations for these sources were found to exhibit a $1/f^2$ spectral signature over the frequency range $1.0 \times 10^{-6} \le f \le 5.0 \times 10^{-4}$ Hz, corresponding to $33$ min $ \le T_f \le 11.6$ d, where $T_f \equiv 1/f$. The lower time limit is established by the photodetector noise floor while the upper time limit is determined by the duration of the individual experiments. Scalograms computed from our data are consistent with the periodograms. The universal character of this inverse-square baseband spectral behavior stands in sharp contrast to the optical spectra of these sources, which differ markedly. Time traces of the photon-count fluctuations are examined to shed light on the origin of this enigmatic spectral behavior.