Koji Arai, Masaki Ando, Shigenori Moriwaki, Keita Kawabe, and Kimio Tsubono

Robust extraction of length-control signal is a crucial issue to realize stable operation of power-recycled Fabry-Perot Michelson interferometers. Since length deviations of the arms are greatly enhanced by the cavities, it becomes hard to obtain the signals for control of the recycling mirror and the beamsplitter independently from those of the arm-cavity motions.

We present a new signal-extraction scheme based on harmonic demodulation.  The control signals for the recycling mirror and the beamsplitter are obtained at the reflection port by demodulating at the third-harmonic frequency of phase modulation. Since these signals are generated by second-order modulation sidebands, which do not resonate with the interferometer, the amplitudes and the signs of the signals are robust against changes in the optical parameters. In addition, the extracted signals are inherently less sensitive to the common motion of the arm lengths. In the presentation of the paper, the principle and the advantages of this technique are discussed. We also report the experimental results of this technique on a 3-m prototype interferometer at University of Tokyo.