CLOUDSYMP Development of a High Resolution Time-Correlated Single Photon Lidar for Cloud Chamber Observations at the Centimeter Scale

Time correlated single photon counting (TCSPC) techniques have been developed and applied in many fields, such as single-photon imaging, forestry mapping, archaeology site surveillance, fluorescence lifetime measurement, remote sensing, and non-line-of-sight imaging. TCSPC technique can record the arrival time of each individual photon at picosecond resolution with respect to a reference clock thus has been recently used in remote sensing as a tool to derive properties of the atmosphere from backscattered intensity profiles with unprecedented resolution. We design a TCSPC lidar for cloud observations in a laboratory chamber facility. The TCSPC lidar operates with a customized, passively mode locked frequency-doubled Ytterbium fiber laser with a built-in high-speed biased photodetector to provide an electrical reference signal. The laser emits at a wavelength of 515 nm with a pulse width of 8 ps at a repetition rate of 30 MHz. This corresponds to a range resolution down to 0.75 cm and a sampling volume of 0.046 cm³, considering the collimated beam diameter of 2.5 mm. The TCSPC lidar probes the target volume by using a single mode fiber-coupled optical transceiver, allowing alignment-free scanning of 32 by 32 pixels within 2.8 s. By acquiring the backscattered photon arrival time histogram using a Geiger-mode Silicon Avalanche Photodiode and time-tagger, we demonstrate a 3D intensity map (0.675 million voxels) of backscattered photons from a water mist produced via ultrasonic mist generator. The TCSPC lidar demonstrates here would enable chamber observations of cloud microphysical properties at the centimeter scale--a capability needed to advance our understanding of cloud processes at a fundamental level. We envision that this TCSPC technique can be potentially adopted in implementation of highly time-resolved Raman spectroscopy, fluorescence and doppler lidar for cloud chamber observations.