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LIGO Hanford Observatory NewsFirst Large Mirror Suspensions In Assembly at Hanford: 150 lbs of Welded Steel and Optical Appeal!
In the early days of January, the first of the large optics for LIGO was mounted into its suspension cage at the LIGO Hanford Observatory (LHO). This was the culmination of years of work for many of our engineers and scientists. For months, Doug Cook (of Hanford) and Helena Armandula (Caltech) have been preparing the optics lab and cleaning procedures at LHO for these key optical components. Meanwhile, Garilynn Billingsley and her crew at Caltech have been working the global manufacturing pipeline and quality control processes for acquiring the optics, and ensuring that the quality is up to LIGO's exacting standards. Numerous scientists and students at both Caltech and MIT have spent countless hours of effort and computer time modeling the effects of various mirror defects to determine the specifications for the various optical components. At the same time, Janeen Romie (Caltech) has been working the manufacturing pipeline for the suspension hardware, which she developed together with Seiji Kawamura (formerly with Caltech, and now a professor working with the Japanese TAMA project), Mark Barton (now with Caltech, but formerly of TAMA!), and myself.
In Figure 1 at left, Helena Armandula proudly displays one of the optics mounted in its suspension cage.
Optics similar to the one pictured here will be spaced miles apart in LIGO interferometers with tens of thousands of Watts of laser power bouncing between them. A passing gravitational wave will cause the mirror separations seen by the laser beams to vary by about 1/1000-th of the diameter of a proton (or less than 1/10,000,000,000,000-th the diameter of a human hair). This change ultimately is read out by monitoring the "interference" of the laser beams at a beam splitter (hence the name interferometer) with sensitive photodetectors. Eventually this produces a "voiceprint" that can be analyzed by computers or played out on headphones. Imagine yourself listening to these headphones. A gravitational wave might sound similar to the songs sung by whales, amid a background of sounds generated by the vibrating atoms in the mirrors and suspension fibers, as well as the noise of light photons interfering at the beam splitter. Proper design of the mirror surfaces, optical coatings and the attached hardware is essential to keep that background noise from drowning out the faint whispers of gravitational waves generated in deep space.
The mirror itself is a 10"-diameter by 4"-thick piece of ultra-pure synthetic fused silica. The purplish coating is the mirror surface, a multilayer stack of alternating dielectric materials that is highly reflective to infrared light from our lasers. Getting good interference and keeping high-power light beams trapped between the interferometer mirrors requires that the mirror surfaces and coatings be shaped accurately and uniformly to about a millionth of a millimeter. Reflection losses, due to the absorption and scattering of light, are measured in parts per million, necessitating extreme caution to prevent exposure of mirrors to dust and hydrocarbon vapors. The mechanical design criteria for this mirror and its attachments to best manage the noise produced by vibrating atoms in these structures was the subject of a Ph.D. thesis by Aaron Gillespie--a thesis built on research done over many decades in many countries.
As seen in the silhouette and side-view photos of Figures 2 and 3 at left and right above, the mirror is hung in a sling formed by a single loop of steel music wire. A grooved glass rod is used to minimize rubbing produced where the wire meets the glass. To apply small forces to the mirror for alignment and control, a magnet is glued to an aluminum standoff that is glued to the mirror. Sensor/actuator heads will be fitted into holes in the suspension cage to detect the positions of these magnets (using shadow sensors), and to push or pull on the magnets (using magnetic fields generated by coil in the heads). The suspension cage itself is a unibody structure formed by welding machined stainless steel tubes and plates. Silver-plated screws lock down the optic to the cage for transport to the vacuum chambers.
The mirror pictured was mounted and trussed up by Janeen, Helena and Betsy Weaver in preparation for Mark Barton to do the delicate deed of balancing the mirror. The mirror must be balanced so that it hangs true to within about 10 minutes of arc to get within range of the sensor/actuator heads and suspension controllers. Once balanced, the final epoxy bond is formed to "lock in" the alignment and the mirror gets removed for a vacuum bakeout to remove any trace contaminants. Once out of the oven, the mirror surface receives a final cleaning, gets re-inserted into its cage, and is ready for insertion into a vacuum chamber. Once the mirror is installed, Jay Heefner and colleagues in the control group--who designed and built the mirror control electronics--can wire it up and exert control over the mirror.