In a year filled with important milestones, perhaps the most exciting event for LIGO in 2000 was achieving first lock on its 2 km interferometer at the Hanford Observatory. To achieve the sensitivity required for searching for gravitational waves the mirrors in the LIGO interferometers must be held in the correct positions for the laser light to resonate properly, a process known as "locking the interferometer". The required accuracies range from 0.1 to 100 picometers depending on which optic, a formidable task when one realizes that the ground is continually in motion with an rms displacement of about 1 micron! Over the past year, we have been building toward the goal of locking the full interferometer by locking simpler configurations (for example a single arm cavity last spring) to test and tune the photodetectors, electronics and software that are the heart of the locking system. Finally, last fall we began attempting to lock the full interferometer. To make the task a little easier in the beginning, we introduced some additional losses into the optical path, either by misaligning the optics slightly or by using a gate valve to clip the beams in the arms. After successfully locking the interferometer with the added losses, we used the locked sessions to characterize the servos and finally in January we were able to lock the interferometer without the aid of the added losses. This marks an important transition in our commissioning effort-the beginning of working with a fully functioning interferometer, and not just subsets of the full instrument ( http://www.ligo.caltech.edu/LIGO_web/firstlock/).
Just a couple weeks after the final step in locking the 2 km interferometer, we achieved another important milestone, this time on the interferometer at the Livingston Observatory-the first 4 km arm cavity has been locked to the laser. This initiates the debugging and commissioning of the control systems for lengths and angles on the second interferometer.
Each of LIGO's three interferometers has a well-defined role in the commissioning. The 2 km interferometer is the pathfinder-the place where things are tried first and where problems are found. The Livingston 4 km interferometer is where systematic characterization and resolution of problems takes place. Installation of the Hanford 4 km interferometer (still on-going) is paced so that it takes maximum advantage of the experience with the other two interferometers, but is still completed on schedule for the Science Run.
To help the LIGO Laboratory staff move toward round the clock operation and to help the LIGO Scientific Collaboration (LSC) prepare to analyze the LIGO data, we are carrying out a series of "engineering runs". These runs have durations of a few days to a few weeks and can involve either or both sites, depending on the goals of a particular run. The interferometers and other measurement equipment are operated in a well-defined configuration, and the data taken are archived and made available to LSC members. In early November 2000, the second of these engineering runs was held, with the 2 km interferometer operating as a recombined Michelson interferometer with Fabry-Perot arms (but no recycling). The high percentage of time in lock, about 90 third engineering run is scheduled for March. The data from these engineering runs both help us improve the detectors and prepare for the task of analyzing the full LIGO data to come.
The remainder of 2001 has a busy schedule, bringing the three interferometers into full operation at their design sensitivity. Everything is on track for the initiation of the LIGO Science Run in early 2002.