LIGO Document T1700331-v1

Development of Remote Controls for the Motorized Polarization Controller in LIGO’s Arm Length Stabilization System

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T - Technical notes
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The arm length stabilization (ALS) system allows the arms of the interferometer to be locked separately, decoupling these two degrees of freedom from the Fabry-Perot and recycling cavities; this system faces issues with polarization drift along fiber optic cables, however, due to factors such as thermal stress, mechanical stress, and irregularities in the shape of the core. If this drift is not corrected, the mismatch in polarization can prevent the interferometer from observing. Currently, this drift is corrected by a motorized polarization controller (MPC) that must be adjusted manually on a regular basis. This project aims to develop comprehensive, user-friendly, and robust remote controls for the polarization controller to streamline the drift correction. The controller was connected through a serial port to TwinCAT Programmable Logic Controller software, on which the controls were written in IEC-1131 structured text. User input through a graphic interface is interpreted and written to this code, allowing the user to remotely control the polarization. In the future, this program could be used to automate the correction process.
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