Advanced LIGO - Core Optics

The Advanced LIGO COC will involve a significant change from the initial LIGO COC to meet the higher power levels and improved shot-noise and thermal-noise limited sensitivity required of the Advanced LIGO interferometer. Many of the fabrication techniques developed for the fused silica initial LIGO COC will be directly applicable to the optics production. However, sapphire is adopted as the baseline substrate material for the test masses in Advanced LIGO. Sapphire is chosen because of its higher mechanical Q and density, which contribute to a significant reduction in the internal thermal noise leading to an improvement of the detector sensitivity by a factor of 2 over fused silica at 100 Hz, and more at higher frequencies. The larger mass is needed to keep the radiation reaction noise to a level comparable to the suspension thermal noise. Its higher thermal conductivity reduces the thermal lensing due to absorbed laser power. Sapphire does have a greater thermal expansivity, leading to a thermoelastic noise contribution. An R&D effort is underway to develop sapphire in a quality and size appropriate to serve as test mass material. The optical coatings must also undergo development to achieve the combination of low mechanical loss (for thermal noise) while maintaining low optical loss.

The COC subsystem consists of the following optics: power recycling mirror, signal recycling mirror, beam splitter, folding mirror, input test mass, and end test mass. The following general requirements are placed on the optics:

Table 5 lists the COC test mass requirements for both fused silica and sapphire materials.

Table 1 COC test mass requirements

	Sapphire	Silica
Surface figure (deviation from sphere over central 12 cm)	1 nm RMS
Micro-roughness	0.1 nm RMS
Optical homogeneity (in transmission through 15 cm thick substrate, over central 8 cm)	20 nm pk-pk, double pass
Optical absorption	<20 ppm/cm	<0.5 ppm/cm
Substrate mechanical Q	2x10⁸	3x10⁷
Optical coating optical loss	0.5 ppm/bounce
Optical coating mechanical loss	2x10^-5 (goal)

As the table shows, the figure, roughness and homogeneity requirements are the same for both materials. The absorption requirement is reduced for sapphire because its relatively higher thermal conductivity reduces thermal distortion for a given heat input.

Sapphire is the reference design for the input and end test mass material because of its promise of reduced internal thermal noise and due to better thermal distortion properties. Internal thermal noise is a limit to interferometer sensitivity at the noise minimum near 100 Hz. As insurance against the risks involved in the sapphire development effort, the option of using ultra-low optical absorption fused silica for the test masses is being preserved. The final decision to retain sapphire as the critical test mass material is scheduled before production fabrication must begin. Fabrication of fused silica to meet most of the requirements in the above table has already been demonstrated and is not expected to involve research and development; work would be required to ensure acceptable mechanical losses of fused silica in large substrates, although very low losses have been seen in smaller samples. The material properties of fused silica would require significantly more reliance on the thermal compensation system (see Auxiliary Optics Subsystem (AOS)).

The beam splitter requirements are met by the best presently available low absorption fused silica and the power and signal recycling mirrors of LIGO-I class fused silica. These mirrors do not have the same noise or power handling requirements as the test masses, so fused silica, being more readily available, is chosen.

The very long lead time for production of substrates, for polishing, and for coating (for either substrate choice) makes this the critical path item in the Advanced LIGO schedule. Early funding for purchase of the substrates is needed to maintaining the present planned schedule.

Sapphire research and development is well underway. In partnership with industry we are developing the techniques to grow, polish and coat sapphire to the Advanced LIGO requirements; full size boules (which can be tailored to the 32cm diameter testmass size) of sapphire have been produced and are now undergoing an initial polishing phase to allow characterization of the absorption, birefringence and optical homogeneity, demonstrating suitability for the Advanced LIGO test masses. This R&D resembles that employed in initial LIGO, in which a pathfinder process demonstrated that fused silica optics could be brought to the initial LIGO specifications.

Sapphire is a very hard material that requires special polishing. It must be polished to give a smooth surface both on small scales (microroughness), and large scales (surface figure). Samples have been polished to our requirements. In addition, compensation may be needed for the optical inhomogeneities experienced by the wavefront as it is transmitted through the non-uniform optic. Four approaches to optical compensation have been explored; at CSIRO there has been work on ion milling, fluid jet polishing and corrective coating, at Goodrich (see Figure 24) compensation has already been demonstrated on a 250 mm optic using computer controlled corrective polishing. Though ion milling is attractive we have chosen corrective polishing as our baseline since the infrastructure for handling large pieces already exists.

Sapphire substrate optical absorption also is receiving attention. Present measurements of a large set of sapphire test pieces indicate baseline absorption of 50-80 ppm/cm. The R&D effort is aimed at reducing this absorption to 20 ppm/cm. Investigations are underway examining the effect of the purity and preparation of raw material, segregation of impurities during growth, and effects of annealing temperature, duration and atmosphere. These studies have suggested that a simple selection of the best material will not be sufficient and that it will be necessary to do post growth processing, possibly including sample harvesting, regrowth and high temperature purification. Preliminary results, at the time of writing this proposal, indicate that such processing can yield absorption of 50 ppm/cm with regions of 20 ppm/cm. With the use of thermal compensation (see next section), 50 ppm/cm would be acceptable, but 20 ppm/cm gives desirable margin in the design. We will continue to pursue this through the development stage (through early 2004).

A very active program to characterize and reduce the mechanical loss in the coatings has made progress. The principal source of loss in conventional optical coatings has been determined by our research to be associated with the tantalum pentoxide, either due to material losses or due to stresses induced during the coating process. Several alternative materials and processes are being explored with multiple vendors. We have a goal of an approximate factor of ten reduction in the loss, as a coating mechanical loss at this level ensures the coating thermal noise does not significantly reduce the sensitivity of the instrument. We have seen reductions of 2.5 in selected samples of exploratory coatings.

The sapphire R&D effort will culminate in early 2003, when a decision will be made on whether to proceed with production of sapphire test masses, or instead rely on the fallback plan of ultralow absorption fused silica. Following this selection, fabrication will proceed with the plan for first articles to be available in 2006.

The time scale for developing a satisfactory coating, with appropriate optical and mechanical losses, is associated with the commencement of coatings on the production optics at the end of 2005.

This element includes all R&D, design, prototype testing, design, purchase of materials, polishing, coating, metrology, cleaning and preparation and transport of the core optics and spares. It includes preparations of the optic for installation in the suspension, but it does not include physical elements attached to the optics required for suspension fiber attachment.