The kilohertz gravitational-wave band provides a unique window into the postmerger physics of binary neutron-star coalescences. In Fabry-Perot-Michelson interferometers, however, high-frequency signals are averaged over the arm cavities, limiting sensitivity in the presence of optical loss. A recently proposed topology overcomes this limitation by directly amplifying high-frequency signals using an L-shaped optical arm cavity pumped through a Sagnac-like vortex. Despite its apparent departure from the Michelson configuration, the topology exhibits a remarkable low-frequency property: when the laser is resonant with the L-shaped cavity, the input coupler becomes effectively transparent and the interferometer reduces to a simple Michelson interferometer. Because this property underpins the proposed sensing and control scheme, its experimental validation is an important step toward practical implementation. We report a tabletop measurement of the near-dc optical response that confirms this effective-transparency feature. The result connects the new topology to the familiar Michelson intuition and provides guidance for lock acquisition in future high-frequency gravitational-wave detectors.

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• Gravitational wave detection
• Gravitational waves