Mode Selectivity in Resonant Delay Lines
Authors:Stefan Ballmer, Antonio Perreca, Robert Wright
Mentor:Joshua Smith, Assistant Professor, California State University Fullerton
Thermal noise has become a limiting-factor in gravitational-wave-interferometry. This noise is caused by microscopic vibrations of the molecules in the optical mirrors at non-zero temperatures. An interferometric readout of the mirror position with a laser beam averages the motion of the individual molecules over the beam cross section. Thus a large reflection area is desired. A resonant delay line configuration is proposed to increase the effective averaging area and hence increase sensitivity. A resonant delay line utilizes the same hardware as cavities already in place at the arms of the interferometer, but creates multiple beam reflections per mirror. This configuration would reduce noise caused by thermal induced vibrations by a factor equal to the square root of the number of reflections per mirror. The challenge of this configuration is gaussian-mode selectivity. It is found that the geometries which allow for multiple beam resonances also cause multiple spatial mode shapes to be resonant at the same time. The addition of 2 “symmetry-breaking” mirrors for one of the beam bounces would break this symmetry and hence provide good mode cleaning.
In our experiment we create a resonant delay line with 3 resonant spots using a 45cm cavity with R=30cm mirrors, and break symmetry with 2 R=50cm mirrors to create desired mode-selectivity.