Sensitivity of Laguerre-Gaussian Modes to Misalignment and Mode Mismatch in Gravitational-Wave Detectors

Abstract

Higher-order Laguerre-Gaussian (LG) modes have broader and more uniform transverse intensity distributions than the fundamental Gaussian mode, making them attractive for precision optical applications. In gravitational-wave detectors, their enhanced spatial averaging of thermally driven test-mass fluctuations can reduce thermal noise. Their implementation, however, requires efficient coupling of the injected beam to the target cavity eigenmode. Residual misalignment and mode mismatch couple power out of the desired mode, reducing intracavity power buildup and degrading detector sensitivity. Here, we analytically and numerically evaluate the coupling loss induced by misalignment and mode mismatch for a generic $\mathrm{LG}_{p,\ell}$ beam. We show that the leading-order loss due to angular or lateral misalignment scales as $2p+|\ell|+1$, whereas that due to waist size or waist position mismatch scales as $2p^2+2p+(2p+1)|\ell|+1$. Although sensitivity to imperfect coupling generally increases with mode order, the donut-shaped $\mathrm{LG}_{0,\ell}$ family exhibits favorable mode mismatch scaling: its loss factor reduces to $|\ell|+1$ and therefore increases only linearly with the azimuthal index. This robustness, together with their broad intensity profiles and central dark regions that permit selective mirror masking, provides additional practical motivation for using $\mathrm{LG}_{0,\ell}$ modes in precision interferometers, including gravitational-wave detectors.