Intensity Stabilization of NPRO Lasers in the LIGO 40m Lab

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Abstract:: Interferometric gravitational wave detectors such as the Laser Interferometer Gravitational Wave Observatory (LIGO), are some of the most precise instruments used today, detecting gravitational strains as small as 10-22. However, there are many significant sources of noise when measuring such small displacements, including fluctuations in the output power of LIGO’s cavity lasers. In an effort to reduce these fluctuations, this project aims to design and construct an Intensity Stabilization Servo (ISS), a feedback control system that reduces laser intensity noise. By comparing the unstabilized laser power spectrum to the noise spectrum set by the thermal fluctuations in the mirror coatings used on the interferometer’s test masses (considered a theoretical limit), it was possible to calculate an open-loop gain requirement for the servo. To meet this requirement, an analog circuit was designed and prototyped on a breadboard while its transfer function was determined using a spectrum analyzer. With the filtering behavior of the analog servo verified, the design schematics were transferred to a printed circuit board (PCB) layout. The completed board will be tested, and its behavior optimized, in both LIGO’s 40m prototyping interferometer as well as other LIGO facilities on the Caltech campus

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LIGO Document T1400038-v1