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Register a new userA High Precision Fabry-Perot Cavity Polarimeter at HERA
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A Fabry-Perot cavity polarimeter, installed in 2003 at HERA for the second phase of its operation, is described. The cavity polarimeter was designed to measure the longitudinal polarisation of the HERA electron beam with high precision for each electron bunch spaced with a time interval of 96ns. Within the cavity the laser intensity was routinely enhanced up to a few kW from its original value of 0.7W in a stable and controllable way. By interacting such a high intensity laser beam with the HERA electron beam it is possible to measure its polarisation with a relative statistical precision of 2% per bunch per minute. Detailed systematic studies have also been performed resulting in a systematic uncertainty of 1%.
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A precise and fast Fabry-Perot cavity polarimeter, installed in the HERA tunnel in the summer of 2003, was used to measure the longitudinal polarisation of the lepton beam. A complete theoretical model has been developed in order to control at the per mill level the degree of circular polarisation of the laser beam. The transport of this quantity within the whole optical setup has also been performed and controlled at the same level of precision. This is the first time that such a precision is achieved in the difficult, hostile and noisy environment of a particle collider.
A high-finesse Fabry-Perot cavity with a frequency-doubled continuous wave green laser (532~nm) has been built and installed in Hall A of Jefferson Lab for high precision Compton polarimetry. The infrared (1064~nm) beam from a ytterbium-doped fiber amplifier seeded by a Nd:YAG nonplanar ring oscillator laser is frequency doubled in a single-pass periodically poled MgO:LiNbO$_{3}$ crystal. The maximum achieved green power at 5 W IR pump power is 1.74 W with a total conversion efficiency of 34.8%. The green beam is injected into the optical resonant cavity and enhanced up to 3.7~kW with a corresponding enhancement of 3800. The polarization transfer function has been measured in order to determine the intra-cavity circular laser polarization within a measurement uncertainty of 0.7%. The PREx experiment at Jefferson Lab used this system for the first time and achieved 1.0% precision in polarization measurements of an electron beam with energy and current of 1.0~GeV and 50~$mu$A.
Fiber-based optical microcavities exhibit high quality factor and low mode volume resonances that make them attractive for coupling light to individual atoms or other microscopic systems. Moreover, their low mass should lead to excellent mechanical response up to high frequencies, opening the possibility for high bandwidth stabilization of the cavity length. Here, we demonstrate a locking bandwidth of 44 kHz achieved using a simple, compact design that exploits these properties. Owing to the simplicity of fiber feedthroughs and lack of free-space alignment, this design is inherently compatible with vacuum and cryogenic environments. We measure the transfer function of the feedback circuit (closed-loop) and the cavity mount itself (open-loop), which, combined with simulations of the mechanical response of our device, provide insight into underlying limitations of the design as well as further improvements that can be made.
We show how to analyze the motion of very low dissipation suspended mirrors in a Fabry-Perot. The very precise measurements of the mirrors motion can be determined, also in the presence of a disturbing noise, by means of the sudden reflectivity changes in special points of the mirrors positions. When the mirrors cross such positions, the effective opto-mechanical potential that arises in the device is (roughly) at a maximum. We show that the motion cross such potential maxima is not only confused by the presence of noise, but also favoured by noise itself that induces hoppings. Thus, the measurements of the times at which the crossings occur can be exploited to identify the properties of the applied signal. We also show how to circumvent the difficulty of the extremely long transient that occur in the system analyzing the escape average time with two different methods: a direct sample average and the indirect estimate from the tail distribution. Numerical simulations and physical insight suggest that the indirect estimate, through the analysis of the distribution tails with an appropriated cut off is robust against the disturbances that arise from the presence of transient dynamics.
Ultrahigh-resolution fiber-optic sensing has been demonstrated with a meter-long, high-finesse fiber Fabry-Perot interferometer (FFPI). The main technical challenge of large, environment-induced resonance frequency drift is addressed by locking the interrogation laser to a similar meter-long FFPI, which, along with the FFPI sensor, is thermally and mechanically isolated from the ambient. A nominal, noise-limited strain resolution of 800 f{epsilon} /sqrt(Hz) has been achieved within 1 to 100 Hz. Strain resolution further improves to 75 f{epsilon} /sqrt(Hz) at 1 kHz, 60 f{epsilon} /sqrt(Hz) at 2 kHz and 40 f{epsilon} /sqrt(Hz) at 23 kHz, demonstrating comparable or even better resolutions than proven techniques such as {pi}-phase-shifted and slow-light fiber Bragg gratings. Limitations of the current system are analyzed and improvement strategies are presented. The work lays out a feasible path toward ultrahigh-resolution fiber-optic sensing based on long FFPIs.
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