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Ground-based laser interferometric gravitational wave detectors (GWDs) consist of multiple optical cavity systems whose lengths need to be interferometrically controlled. An arm-length stabilization (ALS) system has played an important role in bringing these interferometers into an operational state and enhancing their duty cycle. The sensitivity of these detectors can be improved if the thermal noise of their test mass mirror coatings is reduced. Crystalline AlGaAs coatings are a promising candidate for this. However, the current ALS system with a frequency-doubled 532 nm light is no longer an option with AlGaAs coatings because the 532 nm light is absorbed by AlGaAs coatings due to the narrow bandgap of GaAs. Therefore, alternative locking schemes must be developed. In this Letter, we describe an experimental demonstration of a novel ALS scheme, to the best of our knowledge, which is compatible with AlGaAs coatings. This ALS scheme will enable the use of AlGaAs coatings in current and future terrestrial gravitational wave detectors. 

Thermal noise in high-reflectivity mirror coatings is a limiting factor in ground-based gravita-tional wave detectors. Reducing this coating thermal noise improves the sensitivity of detectorsand enriches the scientific outcome of observing runs. Crystalline gallium arsenide and aluminum-alloyed gallium arsenide (referred to as AlGaAs) coatings are promising coating candidates forfuture upgrades of gravitational wave detectors because of their low coating thermal noise. How-ever, AlGaAs-based crystalline coatings may be susceptible to an electro-optic noise induced byfluctuations in an electric field. We investigated the electro-optic effect in an AlGaAs coating byusing a Fabry-Perot cavity, and concluded that the noise level is well below the sensitivity of currentand planned gravitational-wave detectors.