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Abstract

Modern ground-based gravitational wave (GW) detectors require a complex
interferometer configuration with multiple coupled optical cavities. Since
achieving the resonances of the arm cavities is the most challenging among the
lock acquisition processes, the scheme called arm length stabilization (ALS)
had been employed for lock acquisition of the arm cavities. We designed a new
type of the ALS, which is compatible with the interferometers having long arms
like the next generation GW detectors. The features of the new ALS are that the
control configuration is simpler than those of previous ones and that it is not
necessary to lay optical fibers for the ALS along the kilometer-long arms of
the detector. Along with simulations of its noise performance, an experimental
test of the new ALS was performed utilizing a single arm cavity of KAGRA. This
paper presents the first results of the test where we demonstrated that lock
acquisition of the arm cavity was achieved using the new ALS and residual noise
was measured to be $8.2\,\mathrm{Hz}$ in units of frequency, which is smaller
than the linewidth of the arm cavity and thus low enough to lock the full
interferometer of KAGRA in a repeatable and reliable manner.