Near-losslesss method for generating thermal photon-bunched light

TL;DR

This work addresses the challenge of generating bright, spectrally narrow, thermally photon-bunched light with timing resolution suitable for sensing. It introduces a wavelength-insensitive method that cascades asymmetric Mach-Zehnder interferometers in single-mode fiber to transform coherent laser light into a phase-uncorrelated ensemble, producing thermal statistics with $g^{(2)}(0)$ approaching 2 as the number of stages increases. Experimental demonstrations at 780 nm and 1550 nm show significant efficiency gains (up to 92.5% conversion) and tunable coherence times, with measured $g^{(2)}(0)$ values rising from 1 to 1.805 at 780 nm and 1.461 at 1550 nm. The approach promises near-unity brightness of photon-bunched light and has potential applications in range finding, clock synchronization, and non-line-of-sight imaging by tolerating higher return losses in practical environments.

Abstract

Thermal light sources exhibiting photon bunching have been suggested for sensing applications that exploit timing correlations of stationary light, including range finding, clock synchronization, and non-line-of-sight imaging. However, these proposals have remained unrealized in practice because available sources of photon bunching either possess coherence times too short to be timing resolved by photodetectors, or produce brightness levels too low to tolerate realistic return losses. In this work, we demonstrate a low-loss method for generating photon bunching with a conversion efficiency nearly 9 orders of magnitude higher than that achieved by many other bunching processes.

Figures (6)

• Figure 1: Power relations of various methods and for generating photon-bunched light from laser light: MS dravins:15 -- liquid suspension of microspheres, RGG zhu:12 -- rotating ground glass, EDFA janassek:18 -- Erbium-doped fiber amplifier. Methods based on parametric conversion: MRR steiner:21 -- cavity enhanced via microring resonator, FWM england:19 -- four-wave mixing, BBO lohrmann:18 -- downconversion in $\beta$-Barium Borate crystal, PPKTP jeong:16 -- downconversion in periodically polled Potassium Titanyl Phosphate crystal
• Figure 2: Spectral densities of stationary light sources exhibiting photon-bunching correlations. Apart from the demonstrations referenced in Fig. \ref{['fig:conversion']}, other demonstrations shown are: STL pk:23 -- subthreshold laser diode, pk:16 -- filtered Sunlight, Hg pk:14 -- Mercury discharge lamp, SLD rahman:20 -- superluminescent diode, PPLN zhang:15 -- parametric down conversion in periodically poled Lithium Niobate.
• Figure 3: Scheme to efficiently generate thermal photon-bunched light from coherent laser light by using a cascaded series of asymmetric Mach-Zehnder fiber interferometers. The input field is first split by a beamsplitter (BS), with one arm acquiring a propagation delay $\Delta_{1}$ before recombination at a subsequent beamsplitter with the non-delayed arm. The interferometric loop can be repeated $n$ times, with each stage introducing progressively longer fiber delays $\Delta_{n}$, providing two single-mode outputs ($A_n, B_n$).
• Figure 4: Photon bunching signatures from a laser light (center wavelength $\lambda=780$ nm) passing through a number $n$ of asymmetric Mach-Zehnder interferometers. A numerical fit to a double-exponential decay reaches a peak value of the second order correlation function at time delay $\tau=0$. We find for $n=0$ (no interferometer): $g^{(2)}(\tau)=1$, for $n=1$: $g^{(2)}(0)=1.471\pm0.003$, for $n=2$: $g^{(2)}(0)=1.665\pm0.003$, and for $n=3$: $g^{(2)}(0)=1.805\pm0.004$. The coherence times extracted for all traces is around $\tau_c\approx135$ ns (see text for details).
• Figure 5: Tunable (a) output photon bunching power and (b) coherence time $\tau_c$ by adjusting the laser injection current $I_{L}$.