| Summary: | Hanbury Brown and Twiss effect of multi-spatial-mode thermal light is obtained at ultrashort timescale by two-photon-absorption (TPA) in a semiconductor photon-counting module. A characteristic spectrum, due to the interference in multi-mode fiber, is observed directly in TPA detector for the first time. To analyze the output of such measurement, a theoretical model based on the Feynman's path-integral theory is built, which makes the contribution of related item and the corresponding spectral component match very well. In addition, by introducing an all-optical method into the current HBT apparatus, one can obtain the concerned second-order coherence spectrum directly without any postprocessing. This result may be helpful to develop ultrafast detection for the intensity interferometer with natural light. Meanwhile, the observation of HBT effect of multi-mode interference offers a new detection method for the optical fiber sensors.
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