Subcarrier locking for signal recycling
GEO600 has been designed to enhance its sensitivity to gravitational waves at high frequencies. The Very High Frequency project aims to observe a range of frequencies of up to a few hundred kHz. Key aspects of this include changing the detuning of the detector's signal recycling cavity, increasing the circulating light power, and injecting squeezed states of light to minimise the detector's shot noise.
GEO600 researchers have developed a new locking technique for the signal recycling cavity that is better suited to transitioning the detector to a detuned state. This technique detects changes in the cavity length by injecting an auxiliary sub-carrier field from the dark port of the interferometer. A key challenge is demonstrating the compatibility of the new scheme with squeezing, given that the sub-carrier field takes the same path as the squeezed light.
