Proof-of-Concept of a Short-Range High Spectral Resolution Lidar using a Compact High Repetition Rate Fiber Laser

In recent years, several climate and air quality applications have required to understand the impact of aerosols close to their source, leading to the development of novel Short-Range Elastic Backscatter Lidars (SR-EBLs), which enable measuring the radiative properties of aerosols at high spatiotemporal resolutions (<10cm, 1s) in the short-range (3 to 500m). However, the elastic lidar equation is an ill-posed problem, having one equation for two atmospheric variables: the backscatter β(r) and extinction α(r) coefficients. Solving this equation requires assuming a value for the lidar ratio, i.e., a linear relationship between β and α, reducing the accuracy of the retrievals. Advanced lidar techniques, like the High Spectral Resolution Lidar (HSRL), measure molecular and particle scattering separately. Having a direct measurement of the molecular component allows for solving the lidar problem without assumptions about the lidar ratio. However, the existing atmospheric HSRLs cannot perform short-range measurements because i) they are usually blind in the first hundredths of meters (overlap restrictions), and ii) they prioritize spectral performance using ultranarrow band (and thus long-pulse) lasers, resulting in an insufficient spatiotemporal resolution.

This work presents a proof-of-concept of a Short-Range High Spectral Resolution Lidar (SR-HSRL) optimized for aerosol characterization in the first kilometer of the atmosphere. This SR-HSRL uses a compact high-repetition rate fiber laser source with a 300 MHz linewidth and 5 ns pulse length. Since these two parameters are inversely proportional, and both are required for performing SR-HSRL measurements, a compromise had to be found to optimize the overall performance. The main challenge was to prove that, despite its relatively large linewidth, this laser has a satisfactory spectral performance so that it can be used for future implementations of the short-range HSRL. We chose this model after evaluating several laser sources because it has the right compromise between pulse length, linewidth, spectral stability, and size. The laser housing is 270 x 270 x 40 mm and weighs 2.9 kg, making it ideal for future integration on a portable short-range HSRL system.

In the receiver part, a 10:90 beam splitter transmits 10% of the backscattered light to the total channel and reflects 90% of it to the HSR channel. A 40-cm-long iodine cell is used as the spectral filter for separating the Mie and Rayleigh aerosol components. We used two thermoelectrically cooled SiPM Multi-Pixel Photon Counter (MPPC) sensors and a 160MHz analog-to-digital converter to measure the signals. The spatiotemporal resolution, limited by the acquisition system, is 7.5 m and 1 s.

To test the lidar, a two-day measurement campaign was performed at NIES in Tsukuba, Japan, in July 2024. We demonstrate that, despite having a relatively large laser linewidth, we can successfully remove the Mie aerosol component, retrieving aerosol backscatter coefficient profiles from as low as 80 m. We also compare the HSRL retrieval method to a non-conventional forward Fernald inversion method previously reported for SR-EBL. We found that the forward method normally sub-estimates β (up to 30% discrepancy) in aerosol layers and overestimates it in cloud zones (60 to >100% difference).