• 3f_locking_for_the_Upgrade

3f Locking for the 2009 40m Upgrade

Introduction

With 3f locking people usually refer to a technique to lock an optical cavity of the interferometer. In that the optical cavity is locked by means of the error signal obtained by demodulating the signal from the reflection photodiode at 3 times the modulation frequency of the sideband.

This technique is different from the classical Pound-Drever-Hall locking technique. In that, by demodulating the signal from the reflection photodiode at the same frequency as that used to generate the sidebands, one obtains a signal proportional to the amplitude of the beat between the first order sidebands and the carrier. In the 3f locking, the main component extracted by the triple frequency demodulation is proportional to the beating between the first and the second order sidebands.

3f signals to lock the central part of the interferometer

In the old configuration of the 40m, the locking acquisition of the Dual Recycled Michelson (DRM), is