All-fiber amplifier based on chirally-coupled-core fibers for gravitational wave detectors

authored by
Sven Hochheim
supervised by
Benno Willke

High-power lasers are required for the development of novel gravitational wave detectors based on interferometric measurements. The design study for the European ’Einstein Telescope’ revealed that the optical performance of current used laser systems is no longer sufficient for the desired specifications of advanced detector designs. The development of such powerful laser systems is therefore a wide field of research and is of current interest. In particular, fiber amplifiers have increasingly come to the fore in recent years due to their excellent beam quality, even at higher power levels. The power scaling of such laser systems beyond current limitations is the concept of this work. In the area of single-frequency fiber-based laser systems, the non-linear effect of stimulated Brillouin scattering represents the fundamental limitation. In particular, a significant excess noise level of the laser system can be measured above this threshold. In this context, the intensity noise was investigated in more detail. Here, the conversion of phase to intensity noise plays a crucial role. For the first time, the characteristics of the additional broadband noise was reconstructed based on the parameters of an asymmetric Brillouin gain spectrum. The optical output power of fiber-based single-frequency laser systems is scaled, for example, by increasing the effective mode area of the fiber. For the compensation of the resulting reduction of the beam quality in fiber amplifiers, the special concept of the ’chirally-coupled core’ (3C®) fiber was developed. This fiber type reduces the content of guided higher order modes through additional side cores rotating around the actual signal core. Compared to other specialty fiber types with micro-structures inside, this fiber features an all-solid design. The light-guiding properties of the 3C®-fiber were examined with a specially developed S2-setup and the high content of the guided fundamental mode in the fiber core was experimentally confirmed. For the first time, optical fiber components were integrated directly into such a 3C®-fiber. Especially, the manufacturing of signal and pump light couplers opens up new possibilities for the development of advanced fiber amplifier systems. In this work, a first monolithic single-frequency fiber amplifier design was realized, which achieves a power level of over 300 W. This prototype is based on a 3C®-fiber without the need for additional fusion splices, because the optical fiber components were integrated directly into the Ytterbium-doped 3C®-fiber. So, the optimal beam parameters were realized after the amplification process. In particular with regard to the guided fundamental mode content of over 90%, this work emphasizes the high potential of fiber amplifiers based on 3C®-fibers as laser sources for the special requirements of gravitational wave detectors.

QUEST-Leibniz Research School
Doctoral thesis
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