Advanced interferometer arm length stabilisation
LIGO has upgraded their detector to Advanced LIGO. The upgrade include the addition of a signal recycling mirror, improved seismic isolation systems and a higher power laser. These upgrades while improving the sensitivity will also increase the complexity of the instrument, making it more difficult to control.
As such, a new, more deterministic lock acquisition scheme has been developed for Advanced LIGO. Part of this scheme involves locking the Fabry-Perot arm cavities independent of the rest of the interferometer. This system, referred to as Arm Length Stabilisation (ALS), it uses two auxiliary lasers, one in each end station, to lock the arm cavities. The auxiliary lasers will produce light with a wavelength of 532nm, which is half that of the main science laser at 1064nm, so as to avoid interference with the rest of the interferometer.
First the light of each auxiliary ‘acquisition’ laser will be made to resonate within each respective arm cavity by using the Pound Drever Hall technique and feeding back to the frequency actuator of each laser. Next the feedback is handed off to the cavity length actuators, so that the cavity length follows the laser frequency. The auxiliary lasers are phase locked to the main science laser, so that the cavities can be tuned on and off resonance with the main laser beam. The cavities are only tuned onto resonance with the main beam once the rest of the interferometer is locked. Finally the feedback is switched from the auxiliary laser to the main laser.
At CGP, we carried out a variety of work for the ALS such as – testing out this technicolour locking scheme on a smaller cavity (~1m) with suspended mirrors on an optical bench, and phase-locking lasers via a ~4km long optical fibre. When completed, we delivered and installed the system in LIGO.