Advanced LIGO subsystems
are the organizational units of the overall project. Follow the links below to view the mission and progress of each subsystem.
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Advanced LIGO News
LIGO Makes Final Preparations for the O1 Run
As LIGO looks toward the mid-September 2015 launch of the O1 observation run, detector commissioning teams continue to push the L1 (Livingston) and H1 (Hanford) detectors toward better sensitivities and increased up times. Commissioners expect that H1 and L1 will enter the O1 run with binary neutron star inspiral ranges of 65-70 Megaparsecs, an improvement over the previous run (S6 in 2009-2010) by a factor of roughly 3.5. LIGO anticipates that the detectors initially will operate in detection mode at least 75% of the time, yielding a combined "double coincidence" duty factor of more than 50%. Late in August LIGO will freeze the configurations of both detectors and undertake an extensive set of calibration exercises. After sustaining O1 from Mid-September through December 2015, LIGO will resume commissioning to make more sensitivity progress prior to the O2 run later in 2016.
In the runup to O1, much of the commissioning focus rests on noise mitigation. Teams continue to identify and suppress both narrow-band and broadband noise sources in the detectors. An example of this program is the recently installed black glass shroud visible in the adjacent photo that now encases the output mode cleaner (OMC) of each detector in order to absorb stray light (photo: Stuart Aston). As early as December 2014, commissioners at Livingston noticed variable and excessive noise in L1's DARM channel in the low hundreds of Hertz. DARM is the differential arm length readout, the channel in which a gravitational wave signal would appear. The team began to understand that the noise in question was coupling into the DARM channel through environmental vibrations. Several scientists spent a February night tapping on various parts of the HAM 6 vacuum chamber (the home of the OMC) while simultaneously monitoring the L1 output spectrum and the spectra of external accelerometers and microphones. They even did some shouting and saw that their voices leaked into the DARM channel. The transfer mechanism appeared to involve stray light reflecting from the walls of HAM 6 and its neighbor HAM 5. The mitigation involved installation of the black glass plates visible in the photo. The plates act as absorbers, shielding the OMC from scatter. As anticipated, the L1 DARM spectrum showed reduced coupling following the shroud's installation, a procedure that also occurred on H1.
In August 2015, personnel at LIGO Hanford noticed broadband DARM contamination coming from a different path. Commissioners noted excessive amplitude noise and soon discovered that the source appeared to be the 45MHz oscillations that add sidebands to the main laser beam. Inside the laser, the beam passes through an Electro-optic Modulator (EOM -- a crystal). The adjacent photo shows LIGO's EOM housing and the electrical leads that attach to the crystal inside the housing. When the crystal receives an oscillating voltage, a modulated beam is generated that travels along with the main beam and that differs from the main beam by the oscillation frequency. In this case it appeared that the modulation process was creating noise. The EOM driver circuit was exchanged in the hope that the new driver would exert better amplitude stability on the modulated beam. But the noise largely persisted after the exchange, suggesting that the actual mechanism is phase noise, and perhaps indicating the need for a revised strategy for producing the 45MHz voltage oscillation.
These brief case histories illustrate the importance of the thorough documentation of the detectors' configurations along with detailed scrutiny of their noise signatures. As the O1 run draws near, the LIGO Scientific Collaboration's Detector Characterization working group (LSC DetChar) continues to finalize the methods it will use to provide this scrutiny during the run. DetChar group members from a number of LSC institutions continue to integrate their software monitors deeply into detector operations. A DetChar meeting at LIGO Hanford in late July 2015 (see photo) provided a key late-stage face-to-face opportunity for personnel from several different camps (detector commissioning, detector operations and detector characterization) to come together over procedures and practices that will be implemented in O1.
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aLIGO News Archive
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