Applications of long storage time optical cavities

Abstract

Optical precision measurements have become one of the most important tools in physics to test the fundamental laws and to probe the universe around us. Often such experiments require high finesse cavities, and optical loss in these cavities is a critical parameter. In particular, for those cavities that deal with quantum systems, minimizing the cavity loss is crucial because any loss can easily degrade the fragile quantum states. One such example is a quantum noise filter cavity for gravitational wave (GW) detectors, where an optical cavity is necessary for producing frequency-dependent squeezed states of light to improve the sensitivity over their broad audioband frequency [1]. To test the feasibility of quantum noise filter cavities for GW detectors, we characterized the optical loss of state-of-the-art mirrors using a 2 m long high-finesse cavity. Using multiple loss measurement techniques, we studied loss dependence on laser beam sizes and positions. Within the 1 to 3 mm beam spot size we measured, we found that the mirror loss is almost constant at around 5 ppm, and that the loss depends more on the beam position on the mirror than on the beam size 121. While intra-cavity optical loss is one of the key parameters informing the design of quantum noise filter cavities, we also need to account for other quantum noise degradation mechanisms such as the phase noise, losses outside of the cavity, and mode-matching. We developed an analytical model of frequency-dependent squeezing with a quantum noise filter cavity to explore the practical degradation mechanisms in detail 131. Finally, by coupling a squeezed light source to the 2 m long high-finesse cavity, we demonstrated frequency-dependent squeezed states where 6 dB of squeezing in the squeezed quadrature was rotated by 90 degrees in the audio frequency band 141. The techniques used are directly applicable to squeezed light sources for GW detectors, and the measurements validated the model. The loss measurement results, the analytical model, and this demonstration, are now the basis for the design of a realistic quantum noise filter cavity for use in GW detectors in the near future to improve their sensitivity.