Laser power stabilization via radiation pressure

Abstract

This thesis reports a new active power stabilization scheme which can be implemented in high precision experiments, such as gravitational wave detectors. The novel aspect of the scheme is sensing laser power fluctuations via the radiation pressure driven motion they induce on a movable mirror. The mirror position and its fluctuations are determined by means of a weak auxiliary beam and a Michelson interferometer, which form an in-loop sensor for the proposed stabilization scheme. This sensing technique exploits the concept of a nondemolition measurement, since the power fluctuations are inferred by measuring the fluctuations in the phase observable of the auxiliary beam. This process results in higher in-loop signals for power fluctuations than what would be achieved by a direct detection, e.g. via the traditional scheme where a fraction of the laser power is picked off and sensed directly by a photodetector. Other advantages of this scheme are that the full beam power is preserved and available for further use, and that it enables the generation of a strong bright squeezed out-of-loop beam. An extensive theoretical investigation on the concept of the new sensing scheme is presented. In this investigation, different schemes in which power fluctuations are transferred to another observable of the light field, e.g. phase or polarization, are compared to each other, and the advantages of the radiation pressure scheme are highlighted. Furthermore, a complete calculation of the fundamental limit of the proposed radiation pressure scheme, set by the quantum noise in the interferometer and the thermal noise of the movable mirror, is performed. The calculations show that a bright squeezed beam with a power of 4W and up to 11 dB of squeezing might be achievable in the near future. Based on the results of the theoretical investigation, a proof-of-principle experiment was realized with microoscillator mirrors with masses ranging from 25 to 250 ng, and fundamental resonance frequencies from 150 to 210 Hz. Power stabilization in the frequency range from 1 Hz to 10 kHz was demonstrated. The results for the out-of-loop power stability are presented for different beam powers, and a relative power noise of 3.7 * 10^−7 Hz^−1/2 was achieved at 250 Hz for 267 mW. The stability performance was limited by the structural thermal noise of the micro-oscillators, which was particularly high due to operation at room temperature. The results from the investigations conducted in this thesis are a promising step towards generation of a strong bright squeezed beam, and towards an improved stabilization scheme to be used in the future generation of gravitational wave detectors.