Distinguishing thermal and pseudothermal light by testing the Siegert relation

Abstract

Thermal light, including blackbody radiation and spontaneous emission, exhibits photon bunching. Thermal light sources, however, typically yield low spectral densities, limiting their practical utility. Pseudothermal light sources with higher brightness and longer coherence time are often employed instead. While pseudothermal light also exhibits photon bunching, this property may not suffice to fully replicate the behavior of genuine thermal light. Here we demonstrate a method to directly test the Siegert relation for two sources of photon-bunched light, laser light scattered from a rotating ground glass and spontaneously emitted light from a gas discharge lamp, probing a fundamental criterion expected of thermal light.

Figures (4)

• Figure 1: (a) Thermal light source based on a Mercury (Hg) vapor gas discharge lamp, (b) pseudothermal light generated by laser light scattered off a rotating ground glass diffuser. (c) Hanbury-Brown--Twiss interferometer to measure the second-order photodetection time correlations $g^{(2)}(\tau)$. (BPF: bandpass filter, LP: linear polariser, RGG: rotating ground glass, BS: fibre-based beamsplitter, APD: avalanche photodetectors, SMF: single mode fibre)
• Figure 2: Photon bunching signatures. (a) The Hg lamp exhibits photon bunching with $g_{\text{Hg}}^{(2)}(\tau=0)=1.142\pm0.007$ and coherence timescale $\tau_{\text{Hg}}=0.35\pm0.02\,$ns (both numeric values are results of fits, solid line). (b) Results for scattered light from the rotating ground glass yield $g_{\text{RGG}}^{(2)}(0)=1.859\pm0.002$ and $\tau_{\text{RGG}}=277.5\pm0.6\,$ns.
• Figure 3: Experimental setup for measuring interferometric photon correlations $g^{(2X)}(\tau)$. The delay fibers are chosen to introduce an optical delay of $\Delta=2.22\,\mu$s for testing scattered light from the rotating ground glass, and $\Delta=10\,$ns for testing light from the Hg lamp. (BS: Beamsplitter, APD: avalanche photodetectors)
• Figure 4: Experimental results for interferometric photon correlations. (a) Light from the Hg lamp shows two bunching side peaks $g_{\text{Hg}}^{(2X)}(\tau=\pm\Delta)=1.044\pm0.003$ at reduced amplitudes, with $\tau_{\text{Hg}}=0.41\pm0.03\,$ns. (b) Scattered light from the rotating ground glass exhibits a central bunching peak (blue dashed lines) $g_{\text{RGG}}^{(2X)}(\tau=0)=1.412\pm0.002$ and two smaller side peaks $g_{\text{RGG}}^{(2X)}(\tau=\pm\Delta)=1.206\pm0.002$, with $\tau_{\text{RGG}}=172.8\pm0.6\,$ns. The solid line shows a fit to Eq. (\ref{['eqn:zerotime']}).