Multipass Cell for Laser Spectroscopy of Muonic Hydrogen
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2022
Publication Type
Doctoral Thesis
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Abstract
The CREMA collaboration currently prepares a laser spectroscopy experiment at Paul Scherrer Institute in Switzerland to measure the ground-state hyperfine splitting in muonic hydrogen with a 1 ppm relative precision. This thesis is devoted to one of elements of the apparatus, a multipass cell employed to concentrate laser radiation on the volume of interest. Two design types are considered: a closed toroidal mirror and a cell made of two opposing toric mirrors.
Within the scope of this work, I developed several analytical tools and experimental methods to evaluate the performance of the cell. These include an approximate description of the light distribution in the cell derived from paraxial phase-space optics of multipass systems, a volumetric definition of light fluence consistent with the usual formulation as energy per unit area, and a ray-trace model based on that definition to compute three-dimensional fluence distributions. A light injection system allowing for precise control of laser beam parameters was designed and built. Three mirror samples were investigated with this system in ring-down experiments with a pulsed laser source. The analysis algorithm is backed by a statistical model of evolution of light energy circulating in the cell and emerging from it as the measured signal.
A circular cell made of uncoated copper shows a lifetime of around 35 ns, corresponding to a mirror reflectivity of 99.2 %, whereas a two-mirror glass cell with a commercial-grade dielectric coating has a lifetime of up to 80 ns, implying a reflectivity of 99.9 % or better. The third sample, a circular cell with an experimental dielectric coating, appears to be nonreflective at the desired wavelength of 6.8 μm and has a reflectivity similar to the bare copper cell at wavelengths below 6.0 μm. Basing on ray-trace simulations, for an injected pulse energy of 1 mJ, we expect an average fluence in a disk-shaped volume of a thickness of 1 mm and a diameter of 10 mm at the level of 1.4 J/cm² in the circular bare copper cell and 2.4–2.9 J/cm² in the two-mirror dielectric cell, which may be sufficient for our experiment. An improvement in coating reflectivity to 99.9 % could boost the average fluence in the circular cell to 5 J/cm².
Methods established in this work will continue to be employed in tests of future samples and will guide the development of the experimental coating. The spatial fluence distributions computed with the ray-trace models are used by other members of the collaboration in calculations of the signal rate in the experiment.
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ETH Zurich
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Subject
classical optics; optical engineering; spectroscopy
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03864 - Kirch, Klaus / Kirch, Klaus
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725039 - Hyperfine splittings in muonic atoms and laser technology (EC)