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arXiv:2610.26082

LIDAR with Optically Amplified Thermal Light

Darren Ming Zhi Koh, Peng Kian Tan, Christian Kurtsiefer

physics.opticsquant-phSubmitted Oct 19, 2026• Announced Oct 20, 2026
arXivHTMLPDF

Abstract

Thermal light exhibits intensity fluctuations or photon bunching that has been used as a source of entropy or temporally correlated light. However, its low spectral density limits its use in practical applications like LIDAR. One method of remedying this is optical amplification to boost its brightness. In this paper, we show that the normalized second order coherence function $g^{(2)}(τ)$ of a thermal light source is preserved after amplification. We then use amplified thermal light to demonstrate a LIDAR raster scan of a building at 1.3 km with a depth resolution of 2 cm.

Comments
6 pages, 5 figures

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Thermal light exhibits intensity fluctuations or photon bunching that has been used as a source of entropy or temporally correlated light. However, its low spectral density limits its use in practical applications like LIDAR. One method of remedying this is optical amplification to boost its brightness. In this paper, we show that the normalized second order coherence function g(2)(τ)g^{(2)}(τ)g(2)(τ) of a thermal light source is preserved after amplification. We then use amplified thermal light to demonstrate a LIDAR raster scan of a building at 1.3 km with a depth resolution of 2 cm.
Abstract Traditional Light Detection and Ranging (LIDAR) systems rely on a modulated light source to encode information in the probe signal to measure distances. Alternatively, thermal LIDAR uses non-modulated thermal sources of bunched light. In this work, we demonstrate a bunched photon source at telecom wavelength, incorporated in a photon-counting LIDAR system, which determines distances by measuring the second-order correlation. The source is a sub-threshold laser spectrally filtered to extend coherence time, producing an oscillating g (2) curve with an enhanced bunching peak. Utilizing this oscillating bunching increases the signal-to-noise ratio and decreases the number of correlation events required to perform a measurement. This system achieves 2 ps resolution in both fiber-based and free-space measurements with up to 65 dB attenuation and ranges up to 87 km in optical fibers. Our approach demonstrates the possibility of increasing the bunching of classical light sources and improving their usability for LIDAR.
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.
The counterintuitive emergence of second-order interference beyond the coherence length has been observed and reported in quantum optical systems [1], [2]. This phenomenon can lead to potential remote sensing applications by overcoming the key limitation of the coherent LIDAR (Light Detection and Ranging), such as the source coherence length, which defines the maximum detection range. In this work, we present an experimental demonstration of coherent two-photon LIDAR beyond the coherence length with thermal light.
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.
Coherent light detection and ranging (LIDAR) offers exceptional sensitivity and precision in measuring the distance of remote objects by employing first-order interference. However, the ranging capability of coherent LIDAR is principally constrained by the coherence time of the light source determined by the spectral bandwidth. Here, we introduce coherent two-photon LIDAR, which eliminates the range limitation of coherent LIDAR due to the coherence time. Our scheme capitalizes on the counterintuitive phenomenon of two-photon interference of thermal light, in which the second-order interference fringe remains impervious to the short coherence time of the light source determined by the spectral bandwidth. By combining this feature with transverse two-photon interference of thermal light, we demonstrate distance ranging beyond the coherence time without relying on time-domain interference fringes. Moreover, we show that our coherent two-photon LIDAR scheme is robust to turbulence and ambient noise. This work opens up novel applications of two-photon correlation in classical light.
Temporal photon bunching is the phenomena in which photons tend to cluster together on a short time scale. This effect can be quantified using the second order coherenceg(2)(τ)g^{(2)}(\tau)g(2)(τ)of the light. Photon bunching has been utilized in LIDAR, ghost imaging and simulating star light. In these applications, bunched light is typically generated inefficiently by sending coherent laser light through a phase randomization media, such as a rotating ground glass [1] or a liquid suspension of microspheres [2].