Two-photon superbunching effect of broadband chaotic light at the femtosecond timescale based on a cascaded Michelson interferometer

It is challenging to observe the superbunching effect with true chaotic light. Here we propose and demonstrate a method to achieve the superbunching effect of $g^{\left(2\right)}\left(0\right)=2.42\pm 0.02$ with broadband stationary chaotic light based on a cascaded Michelson interferometer (CMI), exceeding the theoretical upper limit of 2 for the two-photon bunching effect of chaotic light. The superbunching correlation peak is measured with an ultrafast two-photon absorption detector whose full width at half-maximum reaches about 95 fs. Two-photon superbunching theory in a CMI is developed to interpret the effect and is in agreement with experimental results. The theory also predicts that the degree of second-order coherence can be much greater than 2 if chaotic light propagates $N$ times in a CMI. Finally, a type of weak signal detection setup that employs broadband chaotic light circulating in a CMI is proposed. Theoretically, it can increase the detection sensitivity of weak signals 79 times after the chaotic light circulates 100 times in the CMI.

Figure 1

Two-photon interference path diagram and the counts of simulated results of the chaotic light in MI. (a) A schematic diagram of the principle of two photons from chaotic light passing through MI once to trigger a TPA detector. (b) A schematic diagram of the principle of two photons from chaotic light passing through the MI twice to trigger a TPA detector. The solid and dashed lines represent the photon's propagation path. PMT is a TPA detector that simultaneously receives photons from different propagation paths. Part (c) shows the result of the normalization of the constant terms, corresponding to the first term in Eq. (5). Part (d) shows the superbunching effect of chaotic light, corresponding to the sum of the second, third, and fourth terms in Eq. (5). Part (e) shows the sum of the high-frequency oscillation terms, corresponding to the term $F\left(\tau \right)$ in Eq. (5). Part (f) shows the sum of all the terms in Eq. (5).

Figure 2

Experimental setup of superbunching thermal light based on CMI. $M_0, M_1, M_2$, and $M_3$ are 1550 nm dielectric mirrors, where $M_1$ is fixed on a precise motorized linear translation stage (MS). PMT is a GaAs photomultiplier tube.

Figure 3

(a) The measurement results of the broadband chaotic light bunching effect when light passes through the Michelson interferometer. (b) The measurement results based on the broadband chaotic light superbunching effect when light passes through the Michelson interferometer twice in succession. The red line is a graph of all counting results of the TPA detector. The black solid line is the result of filtering out high-frequency terms after numerical signal processing. The insets in (a) and (b) illustrate the enlarged detail of the interferogram near 0 fs, respectively.

Figure 4

Ratio of two-photon interference terms of thermal light after transferring $N$ times through the Michelson interferometer under different conditions. The black rectangle represents $R_{P/B}$. The red circle represents $F_{P/B}$. The blue triangle represents $g^{\left(2\right)}\left(0\right)$, which is the degree of the second-order coherence of the two-photon superbunching.

Figure 5

The oscillatory behaviors of a TPA interferogram under different propagation times. The black, red, and blue circles represent details of the TPA detection event when chaotic light transfers through the CMI once, twice, and thrice, respectively. The inset in Fig. 5 represents the trend of the TPA peak count as propagation time increases.