Photon return on-sky test for by-product Rayleigh plume of sodium laser guide star

TL;DR

This study evaluates the photon return of Rayleigh backscatter from sodium laser guide stars for real-time turbulence correction, demonstrating its potential for wavefront sensing in adaptive optics systems.

Contribution

It provides an empirical assessment of Rayleigh backscatter photon return, comparing field test results with theoretical models to validate its use in turbulence correction.

Findings

Photon return differences less than 20% from models

Optimal range gating at 9 km for various turbulence conditions

Range gate depths depend on subaperture size and turbulence strength

Abstract

The sodium laser guide star (sLGS) adaptive optics (AO) system has become an essential component for large ground-based optical/infrared telescope. The performance of the AO system can be significantly hindered if the on-sky spot profile of its sLGS is degraded by the turbulence along the laser launching uplink path. One effective method to overcome this problem is to perform real-time pre-correction on the laser before it is launched onto the sky to counter this turbulence effect especially for the lower altitude where turbulence is stronger. The by-product Rayleigh backscatter generated by large molecule and aerosols at low altitude when projecting sLGS is a perfect candidate for real time detection of low altitude turbulence. It is therefore important to evaluate whether this by-product of a sodium laser guide star could achieve suitable performance for wavefront sensing. In this paper, we attempt to answer the question regarding to the achievable photon return of such Rayleigh backscattered plume. A comparison between results from our field test and theoretical model of MSISE-90 was presented. We showed that the greatest differences is less than 20% which hints the applicability of using the defocused Rayleigh plume as wavefront sensing signal for different turbulence strengths. The result also showed that in case of strong (Fried parameter (r0) of 5 cm) or moderate (r0 of 10 cm) turbulence, the optimum center for range gating is 9 km, and the corresponding value of range gate depths are 3 km, 1.1 km, 0.56 km and 0.28 km when subaperture sizes are 3 cm, 5 cm, 7 cm and 10 cm, with the maximum pulse repetition frequency at 1500 Hz.