Improving frequency stabilisation at the 40m with digital controllers

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Abstract:: The LIGO interferometers need to robustly lock its various degrees of freedom to detect
Gravitational Waves. The LIGO 40m prototype uses an auxiliary (AUX) laser as a reference
to lock the main laser to the arm cavity and stabilize it. The AUX laser is stabilized by
locking it to the arm cavity using the Pound-Drever-Hall (PDH) technique. The stability
of the AUX laser is crucial for the stability of the main laser. Mechanical resonances of the
AUX laser’s piezoelectric (PZT) actuator and the rigid nature of the currently implemented
analogue PDH controller limits the performance of the system, hindering noise suppression
especially at low frequencies. This project aims to develop a digital controller to replace the
currently implemented AUX laser locking system. A digital controller will be more robust
and easily customizable. Specifically, the features to be implemented include better controller
performance in the 10Hz-20kHz range, where the AUX laser noise has greater contribution,
delivering an increased bandwidth over the current analogue system. It also enables faster
lock reacquisition when lock is broken and adding resonant gain filters at specific resonant
frequencies to facilitate calibration of the interferometer.

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LIGO Document T2300178-v4