Energy-flux noise induced by imperfections of the Gaussian beam in the Li-Baker HFGW detector

Abstract

This paper investigates the energy-flux noise induced by imperfections of the Gaussian beam (GB) in the Li-Baker high-frequency gravitational wave (HFGW) detector. Modeling these imperfections with higher-order Laguerre-Gaussian (LG) beams, we analytically derive the resulting noise-to-signal ratio for both co- and cross-polarized $\mathrm{LG}_{nl}$ modes. It is found that higher-order co-polarized LG beams generate no tangential energy-flux noise at the optimal detection plane ($ϕ=π/2$ or $3π/2$), whereas higher-order cross-polarized LG beams introduce non-negligible noise. Specifically, for an individual higher-order cross-polarized $\mathrm{LG}_{nl}$ mode under our representative parameters, maintaining the noise-to-signal ratio below unity typically requires the power fraction to satisfy $σ_{nl}<10^{-46}$, with the $\mathrm{LG}_{01}$ mode imposing the most stringent constraint of $σ_{01}<6.0\times10^{-49}$. Monte Carlo simulations suggest that the total power fraction $σ_{\mathrm{total}}$ of multiple higher-order cross-polarized $\mathrm{LG}_{nl}$ modes needs to remain below $10^{-49}$. Consequently, we propose employing a polarization filter to retain only the co-polarized component of the GB adopted in the practical experiment, thereby suppressing the noise while preserving signal integrity. These results provide theoretical guidance for noise control in the Li-Baker HFGW detector.