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LIGO MIT NewsISC Components, Tools and Software Under Construction at MIT
LIGO's sensors and feedback control systems unify the basic optical, mechanical and electronic components of LIGO into a single instrument, allowing these elements to collaborate as a readout for gravitational waves. We call the distributed network of sensors and measuring tools--and their associated interconnections and algorithms--the Interferometer Sensing and Control (ISC) system.
To start up, LIGO interferometer mirrors must be aligned at least well enough so that the laser beam will fall back on itself after each traversal of the 4 km arms. This means we need to hang each test mass in a precise orientation with respect to its mate at the far end, to an accuracy of a few centimeters per 4 km (about a thousandth of a degree). To find the correct direction in the first place, ISC uses a high-precision optical transit square to sight outdoor survey markers, precisely located along each Beam Tube using the satellite-based Global Positioning System (GPS).
To compare the LIGO mirror orientation to this "master" transit, we use a custom-modified electronic distance measuring theodolite fitted with a piggyback laser autocollimator (a telescopic instrument for measuring the angle of a reflective surface). Using precision adjustment fixtures, the suspended optic is rotated in its cradle within the vacuum chamber until the autocollimator readout indicates it's hanging true to the nearest arcsecond. The theodolite then also indicates the axial and lateral position of the mirror, which must be accurate within millimeters. In late November, Ken Mason, Myron McInnes and Matt Smith of MIT participated with Caltech and Hanford personnel in a highly successful alignment "dry-run" in a test mass chamber at the Hanford site. Figure 1 at left shows them measuring the orientation of the test mass mockup used for this experiment (the aluminum disk in the background with four holes in it) using the modified theodolite/autocollimator (visible in the foreground), which was designed, integrated and tested at MIT before being shipped to Hanford.
Once in operation, LIGO must achieve and maintain alignment about three thousand times better than this to attain its target sensitivity, and must also stabilize the mirror separation to better than a thousandth of an angstrom (i.e., a ten-billionth of a millimeter). The sensing part--determining whether the interferometer is on resonance and if its alignment is optimized in real time--is handled by highly specialized length- and alignment-sensing photodetectors, with a supporting cast of protection shutters, power splitters, beam handling telescopes, filters and diagnostic sensors. These components are integrated into "ISC tables," which are installed near the LIGO vacuum envelope near the four viewports which serve up the interferometer's main and auxiliary output beams. Figure 2 at left shows Rana Adkikari (first-year grad student) and Peter Fritschel (research physicist) putting the finishing touches on IOT7, the first of these ISC tables, shortly before packing it in its padded crate for shipment to Hanford. In the foreground are two special ISC high-power length sensing photodetectors, initially developed in the MIT lab and engineered for production by the Caltech electronics group. A close-up of one of these units is shown in Figure 3 at right.
To maintain the alignment and length within such miniscule tolerances in the face of seismic, thermal and acoustical environmental disturbances, one needs not only state-of-the-art sensors but cutting-edge feedback control systems. Digital signal processing technology allows LIGO's length and alignment controls to achieve peak performance. At left, Figure 4 shows MIT physicist Ed Daw, who is developing real-time control and diagnostic software at MIT on the testbed VME crate development system. Ed was a key member of the MIT team which first successfully applied digital controls to a suspended interferometer, the Phase Noise Interferometer, earlier this year.
The ISC team is eager for the moment next spring when we will activate our sensors and controls to "lock up" the interferometer. This moment has always been a thrill in our laboratory prototype experiments, since it is at this instant that the collection of carefully prepared parts suddenly comes to life as a single instrument. The "birth" of the first full-scale LIGO interferometer promises to be the biggest thrill yet.