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|
Advanced LIGO News
LIGO Begins Locking Optical Cavities
Advanced LIGO achieved an important milestone during the summer of 2012 by locking two in-vacuum optical cavities. Project staff at LIGO Livingston (LLO) brought the L1 detector's suspended mode cleaner into lock (cavity light shown in photo), and a team at LIGO Hanford (LHO) obtained the same result in one of the four-kilometer arm cavities on the H1 detector. Each circumstance involved the integration of servo controls for aLIGO active vibration isolation with the controls for multi-stage optic suspensions. These shakedown efforts provided the first good look at the global performance of the new aLIGO systems -- how the components in various chambers communicate with each other over distance. Each lock was achieved with low-power laser light. Commissioning efforts yet to come will seek to optimize cavity performance at higher powers.
The term "cavity" refers to a subsection of the overall interferometer beam path bounded by slightly transmitting suspended mirrors that reflect the laser light back and forth within the section. "Lock" means that the light in the cavity comes into resonance, generating a stable beam that transmits out of the cavity and serves as a reference for the feedback loop that will "lock" the positions and alignments of the cavity optics to sustain resonance.
Why not lock the same cavity at both sites? To date, aLIGO assembly and installation schedules at the Livingston and Hanford sites have focused on different sections of the detectors. Livingston began aLIGO installation by addressing the input arm of L1 -- the path that takes the light from the main laser system toward the beam splitter. Hanford, on the other hand, initially built up a long detector arm. By differentiating the installation schedule in this manner, the project can transfer the lessons learned at one site to the same installation tasks that will occur later at the other site.
The work at LHO provided the first full-length test of the strategy for locking the long arms of the advanced detectors. Light from a 1064nm laser was frequency doubled and injected into H1 at the end of an arm. The injected light made its way through a suspended transmission monitor system (photo below), and then traveled through the body of the end interferometer mirror. Once through the end mirror, the green light flew the 4km distance to the reflective face of the inner mirror (adjacent to the beam splitter) then back to the end. The tiny speck of green in the photo above represents the first instance of green light from the end landing near the test mass 4km away. Improved pointing soon moved the beam from the arm cavity baffle onto the nearby face of the mirror. Lock was achieved by stabilizing the inner and end mirror positions and angles to allow the beam to make dozens of reflections along the arm. When fully implemented, this method will bring the long arms of a detector into lock as the front-end cavities lock separately. Control authority of the long arms will then revert to 1064nm light as the locked cavities begin sharing light with each other.
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