September 2024
Improving GEO600's beam splitter thermal compensation system
Due to the unique design of GEO600, 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.