Advanced LIGO subsystems
are the organizational units of the overall project. Follow the links below to view the mission and progress of each subsystem.
Auxiliary Optics Core Optics
 
Data Acquisition Data and
Computing
Systems
 
Facilities
Modifications		 Input Optics
 
Interferometer
Control		 Pre-Stabilized
Laser
 
Seismic
Isolation		 Suspensions

Advanced LIGO News

O1 Results Highlight the Capabilities of Advanced LIGO

August 2016

[spec]LIGO has completed its analysis of the O1 data set for gravitational wave signals from binary black hole mergers, and the results illuminate the success of the Advanced LIGO program. The O1 run, which spanned September 2015 through January 2016, produced two detections that were reported by the LIGO-Virgo collaboration. The remarkable signal GW150914, announced in February 2016, and the "Boxing Day" signal, GW151226, announced in June 2016, appeared in LIGO's H1 and L1 detectors with combined signal-to-noise (SNR) ratios of 24 and 13, respectively. A third event, LVT151012, appeared in the detectors on October 12, 2015 with a lower SNR and was not advanced as an unambiguous detection.

What are the features of the Advanced LIGO interferometers that facilitated these detections? [isi] Much of the credit goes to the multi-stage optic suspensions and their attached internal seismic isolation platforms (adjacent photo). These subsystems produced a significant improvement on the low-frequency side of LIGO's noise spectrum. The strain plot in the paragraph above shows that LIGO's sensitivity at 30 Hz improved by more than an order of magnitude in comparison to the S6 data run that occurred in 2009-2010. The very loud GW150914 signal appeared to be tailor-made to leverage Advanced LIGO's O1 sensitivity profile. Entering the detectors near 30 Hz, the signal swept upward over the course of several wave cycles to a peak frequency of roughly 300 Hz at the time of the merger, leaving the merger nicely visible in the frequency regime that has always been LIGO's "sweet spot" -- approximately 100 to 300 Hz.

The 100 Hz to 300 Hz band roughly overlaps the spectral region in which LIGO's sensitivity is limited by thermal noise in the test mass coatings and substrates. The Advanced LIGO glass-fiber test mass suspensions (adjacent photo) [ears]are designed to lower LIGO's susceptibility to thermal noise, and the fiber suspensions delivered as expected. The GW151226 signal peaked near 600 Hz, a frequency at which quantum shot noise has replaced thermal noise as the limiting detector noise source. LIGO utilized about 23 W of input laser power during O1, a small increase from the 20 W used in the S6 run, but the detectors gained additional improvement against shot noise through the use of the dual recycling technique. Shot noise gains will continue to accrue as future commissioning efforts bring more light into the detectors. In the upcoming O2 run, for instance, H1 will operate with 50 W of input light. Added light power will further suppress shot noise and increase the visibility of black hole mergers and ringdowns in the data.

LIGO will pursue the transition to high-power operations in the post-O2 commissioning program and beyond. Ultimately LIGO plans to send 120 W or more into the vacuum from the pre-stabilized laser system. High power will come at the cost of increased parametric instabilities, more radiation pressure noise and an increased tendency to saturate sensing photodiodes. Parametric instabilities (PI's) occur when excited mechanical modes in the mirrors feed back on the laser light to create kilohertz oscillations of increasing amplitude that can leak excess light into the interferometer's detection port and push the interferometer from its operating point. [rh]The strategies for mitigating PI's involve the tuning of the thermal compensation system's ring heaters (adjacent photo) to adjust the frequencies of the mirror modes, and the addition of mechanical damping capability for these modes. Radiation pressure noise and sensor saturation are being addressed through continued optimization of the detector's angular control system. In the very lowest portion of LIGO's detection band, between 10 Hz and 40 Hz, the presence of unmodeled noise remains an important challenge to solve. Black hole binary parameters such as the spins of the inspiraling black holes are more precisely estimated when the signal persists for numerous cycles in the detector, a circumstance that places a premium of LIGO's low-frequency sensitivity.

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