Thermal tweak
Improving GEO600's beam splitter thermal compensation system
The GEO600 gravitational-wave detector serves as a testbed and key technology development center for the international research community. Due to its unique design, the beam splitter in the interferometer's 1200-meter long arms is exposed to a high circulating laser power of approximately 3 kilowatts. This high power is minimally absorbed in the beam splitter, resulting in a locally increased temperature, a change in refractive index, and the formation of a thermal lens. Consequently, the fundamental laser mode is converted into higher-order modes, which negatively impact the detector's sensitivity to gravitational waves. To address this issue, a research team at GEO600 has developed an improved version of the beam splitter thermal compensation system. This innovative design projects a tunable heat pattern onto the beam splitter, effectively counteracting the thermal lens and restoring optimal operating conditions. The new system offers several advantages, including safe and continuous operation at the highest temperature, a larger numerical aperture, capabilities that ensure optimal heat distribution of the projected pattern on the beam splitter, and a noise level well below the current total detector noise. Future updates and improvements to the system will be implemented in parallel with increasing the operating laser power of GEO600. This development is crucial for understanding the thermal lensing challenges expected in third-generation detectors, such as the Einstein Telescope and the Cosmic Explorer.
Paper abstract
Gravitational waves have revolutionised the field of astronomy by providing scientists with a new way to observe the universe and gain a better understanding of exotic objects like black holes. Several large-scale laser interferometric gravitational wave detectors (GWDs) have been constructed worldwide, with a focus on achieving the best sensitivity possible. However, in order for a detector to operate at its intended sensitivity, its optics must be free from imperfections such as thermal lensing effects. In the GEO600 gravitational wave detector, the beam splitter (BS) experiences a significant thermal lensing effect due to the high power build-up in the Power Recycling Cavity (PRC) combined with a very small beam waist. This causes the fundamental mode to be converted into higher order modes (HOMs), subsequently impacting the detector’s performance. To address this issue, the GEO600 detector is equipped with a thermal compensation system (TCS) applied to the BS. This involves projecting a spatially tunable heating pattern through an optical system onto the beam splitter. The main objective of the TCS is to counteract the thermal lens at the BS and restore the detector to its ideal operating condition. This paper presents the new beam splitter TCS in GEO600, its commissioning, and its effect on strain sensitivity. It also outlines the planned upgrade to further enhance the performance of the TCS.
