Advanced LIGO: Research on Mode Matching and Control Systems

A study of a new optical design and a modified length sensing scheme for the planned upgrade of the current LIGO detector will be performed. The very high laser power used in Advanced LIGO will deform the mirrors and degrade the mode matching between the various optical cavities in the interferometer. In the new optical design the beam expander in the input optics chain is moved inside the power recycling cavity. This makes it possible to optimize the mode matching between the various optical cavities inside the interferometer after all optical components are installed and the thermal deformations are measured on site. However, thermal modeling of the new layout is necessary and will be done over the next years to optimize this design. In addition, the current length sensing scheme for Advanced LIGO has some limitations because it requires rather high modulation frequencies but still generates a highly coupled sensing matrix. The modified length sensing scheme which will be derived over the next years will use non-harmonic, low-frequency

modulation signals to reduce these problems significantly.

The Laser Interferometer Gravitational Wave Observatory (LIGO) operates two laser interferometer in Hanford, WA, and one in Livingston, LA, to detect gravitational waves from astrophysical sources. The current LIGO interferometers will have to be upgraded in a few years to improve the

sensitivity to better resolve the waveforms and locate the sources. This research will develop technologies to optimize the spatial mode matching between the various optical cavities in the Advanced LIGO detector. This will increase the signal strength and reduce the requirements on many technical noise sources like laser beam pointing and intensity fluctuations.