Got Beam! LIGO Laser Reaches Y-Midstation

- Contributed by Fred Raab

A lot of hard work by a lot of people culminated in a major achievement at  LIGO Hanford Observatory on November 30, 1999 when gate valves on a 2-kilometer section of evacuated beam tube were opened and the corner station optics hurled a near-perfect strikeball of photons to the Y midstation,  2 km (~1.25 miles) away. 
An excited crew can be seen watching the drama unfold in the LIGO control room at Hanford as Mike Zucker (MIT) and Jay Heefner(Caltech) steer the laser beam by modifying control instructions to the suspended mirrors.

The success of this effort required coordination among scores of people working for many different employers with many handoffs. There was going to be no partial credit - due to fears about injecting noise,  the mirror controllers were designed with only enough range to steer the beam by less than 40 seconds of arc. (About the angle subtended by a pumpkin pie at a distance of a mile.) If we were off by 2-3 times that distance, the beam would not even clear the beam tube and we would not know how to realign by mechanical adjustments.

Launching the beam in the right direction required a fancy surveying job to place monuments inside the LIGO corner station accurately referenced to the outside monuments that had previously been used to locate the buildings and beam tubes accurately to within half an inch over the 5-mile LIGO campus. These initial monuments were placed using a combination of the global positioning satellite system and more traditional surveying techniques. Once the buildings and the vacuum  system were complete, some fancy optical work was undertaken to derive directions from these marks and then place and align mirrors within the vacuum system to align the laser beam to the beam tube direction.

Meanwhile the labor of building the vibration isolation platforms inside the vacuum chambers, stabilizing the laser system and installing optics progressed. Each of the critical mirrors was balanced on a single wire slung about its diameter within a cage that holds shadow sensors to monitor the coarse mirror position and alignment. Each cage in turn was mounted from an optical table that floated on a vibration isolation system, gently bobbing with a period of about half a second like a canoe on a calm lake. Counterweights carefully balanced the payload of the mirror to keep the optical table floating level. Outside the buildings, the 5 miles of beam tube were baked to remove water molecules gently stuck to the steel surface of the tube.

With final preparations completed the chambers were sealed and pumps started in early November. Final checkout was completed after Thanksgiving holiday and late afternoon on Tuesday, November 30 the gate valves were opened and closed-circuit television cameras were turned on to look for the beam in the darkened vacuum chambers. The baseball-sized beam could be seen scattering off the mirror's suspension cage in the midstation! The initial alignment was only about 25 cm (10 inches) high and about 10 cm (4 inches) wide of its target. The photo close-up of the TV monitor shows the faint beam, circled in yellow, after it had been centered horizontally on the suspension cage. The pattern of five bright lights are the 5 shadow sensors circling the mirror - about the size of a catcher's mitt. The center of those 5 lights is the intended target. Not bad for dead reckoning!

The beam's image was eerily still, bobbing around by only about 1/4 of the beam diameter, which should help in our next task of locking the world's longest resonant optical cavity.