Modeling thermal phenomena and searching for new thermally induced monitor signals in large sale gravitational wave detectors

Abstract

The Laser Interferometer Gravitational-wave Observatory (LIGO) array’s 4 km detectors have transitioned from an initial configuration (iLIGO) to an enhanced configuration (eLIGO) [1]. Both configurations relied on high circulating laser powers to achieve sensitivity goals between 150 Hz and 8 kHz. These power levels were sufficient to induce thermally driven focal affects in the primary optics. Since the detectors were designed to achieve maximum sensitivity when laser light was optimally coupled (mode matched) into the antenna, small deviations in focal parameters influenced performance. A laser based thermal compensation system (TCS) was installed for use in both configurations to counteract excessive or insufficient thermal lensing. Consequently a toy model has been studied to search for detector derived parameters that might be used to monitor the focal state of the two most affected optics. Additional thermal behaviors induced by the TCS were investigated and modeled