LIGO Document P1400190-v1
- Advanced LIGO uses fiber-coupled diode lasers as input to its optical lever system. The optical levers make up an auxiliary system that provides error signals in pitch and yaw in the first step of the lock of the interferometer. Currently, the optical levers contain noise which makes it harder to bring the interferometer into lock and sometimes provide false positives of seismic noise. A possible source of that noise is in the optical fibers; if excess light is coupled into the cladding of the otherwise single-mode fiber, the fluctuating higher-order mode could appear as beam wiggle and be interpreted as test mass motion. In addition, a small ring appears around the core of the laser spot at the end of its throw, decreasing the precision of the optical levers. In this experiment, we examined whether the single-mode optical fibers used could really be carrying higher-order modes, and if the ring around the core of the beam could be reduced or eliminated. Our analysis indicates that the optical fibers do not contain excess mode content, and the ring may be caused by diffraction in the optics of the optical lever telescopes.
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