First Lock of the 2 Kilometer Interferometer
at LIGO Hanford Observatory
by Fred Raab, Head of LIGO Hanford Observatory
October 20, 2000

 
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"First Lock" Video (requires free RealPlayer Basic)
 
LIGO "First Lock" - transparencies by LIGO Director Barry Barish Get Free Acrobat Reader
 
"First Lock" for the LIGO Detectors - transparencies by LIGO Scientist Stan Whitcomb Get Free Acrobat Reader
 
 
Aerial Photo of Hanford Observatory
Sketch of Interferometer
Aerial Photo of Hanford Observatory and
Sketch of Interferometer

At 10:00 AM, Friday, October 20, 2000, Barry Barish, Director of the LIGO Laboratory and Linde Professor of Physics at Caltech, along with Dr. Stan Whitcomb, leader of the LIGO Commissioning effort, announced that "First Lock" had been achieved with the two-kilometer long interferometer at the LIGO Hanford Observatory. This marks the fulfillment of a major LIGO milestone, the first time laser light has resonated throughout a full LIGO detector. Additionally, all mirrors were "locked" into their proper positions to atomic-scale precision using a sophisticated computer-based control system. First lock validates many aspects of the control system design for the initial LIGO detectors, but it has even greater significance as the beginning of the process of tuning the interferometer to its full sensitivity. Most importantly, as Barish points out, "This achievement brings us closer to our real goal--LIGO's first gravitational-wave observations."

Beam Splitter Chamber for 2K Interferometer
Beam Splitter Chamber for 2K Interferometer

The Washington two-kilometer interferometer, WA2K as it is called, has long optical cavities that span the two kilometer distance from a "mid-station" on each arm of the observatory to the corner station building, which also houses the laser, beam splitter, recycling mirror and photodetectors. Light incident on the beam splitter is divided into beams that enter the optical cavities in each arm. When the interferometer is locked into resonance, this light bounces back and forth up to fifty times in each arm, exactly reproducing its path and spatial pattern on each round trip. This causes a large build up of stored light in the long cavities. Light leaking back from the two arms toward the beam splitter will interfere and return toward the laser because of the careful positioning of the mirrors by the control system. Any imbalance between the two arms is immediately removed by adjusting the positions of the beam splitter and cavity mirror. The adjustment forces can be measured to indicate the presence of any gravitational-wave or other perturbing force acting on the instrument. The recycling mirror traps the light returning toward the laser and sends it back toward the beam splitter, causing the light on the beam splitter to grow much more powerful than the light emitted by the laser.

Interferometer Locked!
Interferometer Locked!

Video cameras monitor the build-up of light in the interferometer, which is displayed live in the control room. In the photo at left, the top two images on the screen show the light built up in the long cavities, while the image on the lower left shows the built up recycled laser light at the beam splitter. These images are dark in the absence of resonance, when only the laser light shining on the recycling mirror (shown at lower right on screen) can be seen. Also present in this photograph is Professor Rainer Weiss of MIT (standing at center in front of screen), who in 1973 originally proposed building a gravitational-wave detector using laser interferometry. He has worked decades toward this accomplishment.

Scientists in Triumphant Repose
Scientists in Triumphant Repose

Achieving first lock required tight coordination between the many scientists and engineers who contributed to the design, construction, installation and commissioning of the detector. Stan Whitcomb, shown at right seated amongst a triumphant group of scientists, directed the successful commissioning effort. Despite the achievement, Stan is mindful of the work yet to be done. "The detector control systems must be carefully characterized and tuned to achieve maximum sensitivity and reliable operation. And, of course, this is just the first of three interferometers that we have to commission." Indeed scientists Matt Evans (in red shirt) and Nergis Mavalvala (kneeling at console) remained focused on improvements to the computer lock-acquisition codes even as this photo was taken.

Currently, the WA2K interferometer is the largest precision optical device in the world. Yet that title will soon be surpassed when the Louisiana four-kilometer (LA4K) interferometer comes on line in Livingston. Then will follow Washington's four-kilometer interferometer, currently being installed next to WA2K at Hanford. In the near future, WA2K will be put through its paces during a week-long second engineering run. Future commissioning efforts and engineering runs are expected to lead to improvements in the sensitivity and reliability of each of the interferometers, climaxing in the first LIGO science runs, slated for 2002. A lot has been accomplished, and much still remains to be accomplished. At the end of his talk on Friday, Stan put everything into perspective by comparing "first lock's" significance for LIGO to the significance the Wright brothers first flight had for aviation. "It doesn't stay up that long. It isn't very far off the ground. But it does fly!"