Ultra-fast Stokes parameter correlations of true unpolarized thermal light: type-I unpolarized light

We measure Stokes parameter correlations in analogy to the intensity correlation measurements in the original Hanbury-Brown & Twiss configuration by realizing an experimental setup by combining a Schaefer-Collett or Berry-Gabrielse-Livingston polarimeter with a Hanbury-Brown & Twiss intensity interferometer. We investigate true unpolarized light emitted from a broadband thermal light source, which we realize by an erbium-doped fiber amplifier, thus being an ideal source of true unpolarized light. We find that all Stokes parameter correlations ⟨S_nS_n⟩, n∈{1,2,3} are equal to 0.5⟨I⟩². The proven invariance of the Stokes parameter correlations against retardation by wave-plates clearly shows for the first time, to the best of our knowledge, that our true unpolarized thermal light represents type I unpolarized light in accordance with a theoretical prediction for a classification of unpolarized light postulated more than 20 years ago.