Length sensing and control of a Michelson interferometer with Power   Recycling and Twin Signal Recycling cavities
  The techniques of power recycling and signal recycling have proven as key concepts to increase the sensitivity of large-scale gravitational wave detectors by independent resonant enhancement of light power and signal sidebands within the interferometer. Developing the latter concept further, twin signal recycling was proposed as an alternative to conventional detuned signal recycling. Twin signal recycling features the narrow-band sensitivity gain of conventional detuned signal recycling but furthermore facilitates the injection of squeezed states of light, increases the detector sensitivity over a wide frequency band and requires a less complex detection scheme for optimal signal readout. These benefits come at the expense of an additional recycling mirror, thus increasing the number of degrees of freedom in the interferometer which need to be controlled.   In this article we describe the development of a length sensing and control scheme and its successful application to a tabletop-scale power recycled Michelson interferometer with twin signal recycling. We were able to lock the interferometer in all relevant longitudinal degrees of freedom, enabling the long-term stable operation of the experiment. We thus laid the foundation for further investigations of this interferometer topology to evaluate its viability for the application in gravitational wave detectors. 
Christian Gr\"af, Andr\'e Th\"uring, Henning Vahlbruch, Karsten   Danzmann and Roman Schnabel
physics.optics gr-qc quant-ph