Core Optics R&D Activities

Optics

We made significant progress in producing and characterizing sapphire as the preferred test mass material for Advanced LIGO. Our industrial partner fabricated full-sized boules (see Figure 1) that will now be polished to allow a more complete characterization.

Figure 1 Sapphire substrate pathfinder. This piece, fabricated by Crystal Systems, Inc., is the full size of Advanced LIGO test mass, 32 cm dia., 40 kg mass. (Courtesy Insaco)

To address absorption of the substrate of the 1-micron laser light, annealing processes were refined in collaboration with Stanford, resulting in promising reductions. Industrial partners successfully pursued approaches to compensating for inhomogeneity. The notion is to polish a surface, which has features complementing the defects in transmission, on the anti-reflection side of the optic using two different approaches. One (Goodrich) involves a small rotary abrasive tip and an x-y table; the other (CSIRO) uses an ion-milling technique. Both can bring the net optical path seen by the light to an acceptable level. In parallel, manufacturers were able to produce material with improved homogeneity.

The Thermal Noise Interferometer (TNI) research at Caltech produced its first preliminary results with fused silica test masses, and noise hunting and noise reduction is underway.

Optical Coatings

One important measure of the optical coatings is the optical absorption. Acceptable (sub-ppm) losses have been demonstrated this year with conventional coatings by several vendors.

We pursued a strong LSC/LIGO Laboratory program this year to identify the magnitude and source of coating mechanical losses, and to improve the model of the coating thermal noise. The mechanical losses lead to thermal noise; the coating is an important contributor due to the geometry of the test mass, coating, and laser beam, There is a limited choice of materials and of processes which lead to both low mechanical and low optical losses. We are executing a program to identify the source of loss and to explore alternative coating materials and processes that meet the combined optical and mechanical requirements.

Significant progress has been made: We were able to demonstrate the source of the mechanical losses in the coating. The high-index tantalum material, rather than the low-index material or interfaces, is responsible. We are now pursuing alternative coating materials with several vendors with incremental progress in reducing losses.