Speaker
Description
The use of composite silicon substrates — assembled from multiple bonded components — offers a practical path to realising large test masses for the Einstein Telescope Low-Frequency (ET-LF) interferometer, while relaxing constraints on boule diameter and availability. However, these designs introduce bond lines across the optical aperture, which are expected to exhibit excess absorption and scattering. To mitigate this, we propose the use of Hermite-Gaussian (HG) optical modes, specifically HG$_{33}$, whose transverse intensity nulls can be aligned with the bond lines, suppressing their optical impact.
In this talk, I will assess the compatibility of the HG$_{33}$ mode with ET-LF infrastructure, focusing on constraints imposed by the vacuum envelope and mirror apertures. Based on my previous work, I will quantify the impact of optical mode mismatch on interferometer performance, showing that HG$_{33}$ is approximately twenty times more sensitive to mismatch than the fundamental Gaussian. However, this increased sensitivity is accompanied by significantly stronger spatial error signals — as shown in the work of Fulda and collaborators — which may enable effective control. I will reinterpret their results in the context of ET-LF to clarify the transverse control requirements should composite substrates be adopted.
Finally, I will present recent technical progress from the ET-OPT platform, including the assembly of a laser system and mode converter capable of generating high-purity HG modes, as well as the conceptual design of a full-scale test facility. This facility will demonstrate the use of HG modes at scale, validating mode control strategies and confirming their suitability for ET-LF. By enabling the use of large-diameter boules with lower absorption, this approach reopens a promising design path previously abandoned due to technical risk.
