  Ground-based laser interferometric gravitational wave detectors consist of
complex multiple optical cavity systems. An arm-length stabilization (ALS)
system has played an important role in bringing such complex detector into
operational state and enhance the 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, traditional ALS system with frequency-doubled 532 nm light is no
longer an option with AlGaAs coatings due to the narrow bandgap of GaAs, thus
alternative locking schemes must be developed. In this letter, we describe an
experimental demonstration of a novel ALS scheme which is compatible with
AlGaAs coatings. This ALS scheme will enable the use of AlGaAs coatings and
contribute to improved sensitivity of future detectors.
