Although gravitational waves-ripples in the space-time continuum due to moving matter-have never been detected, the Large Scale Cryogenic Gravitational Wave Telescope (LCGT; Honshu, Japan) and the Advanced Laser Interferometric Gravitational Wave Observatory (LIGO; Hanford, WA, and Livingston Parish, LA) are just a few of the interferometric systems actively looking for them. These advanced interferometers require low-loss optical mirrors in the Michelson arms of their optical Fabry-Perot cavities to maximize their sensitivity.
To achieve low loss in a mirror designed to have a 10-km-scale radius-of-curvature surface on a cylindrical substrate that is several tens of centimeters in diameter, a 0.1-nm root-mean-square (rms) microroughness and λ/100 waviness is required. Despite the fact that these specifications are systematically and reproducibly obtained by manual polishing, the process is time-consuming and becomes even more difficult if the material is hard and crystalline like the sapphire required for the LCGT mirrors. In sharp contrast, a time-saving technique that applies a curved thin-film coating to a mechanically polished flat surface to produce a 10‑km-scale radius-of-curvature surface has been demonstrated by scientists at the University of Tokyo (Chiba, Japan), the National Astronomical Observatory of Japan (Tokyo, Japan), and the High Energy Accelerator Research Organization (Ibaraki, Japan). 1
Ion-beam-sputtered coating
To fabricate the 10-km-scale curved surface, the scientists used an ion-beam sputter machine to coat a flat silicon dioxide (SiO2) substrate with SiO2 material. The sputter machine used an argon-ion-emitter beam, a target made from SiO2 to be evaporated, a fixed mask set between the substrate, and the target to control the evaporated target material flow, and an evacuation system.
By setting the substrate on a motion plate that has a sun-planet orbital-motion pattern (that is, the SiO2 material is deposited in circular patterns that orbit in helical motion around a ring of larger radius from the center of the substrate), the coating can be applied in layers targeting a 10-km radius of curvature. The deposition process involves varying both radii until the entire surface of the substrate is coated to specification.
Setting the targeted surface shape to be a 10-km spherical radius of curvature, the scientists coated three different substrates to test various optical properties. To test overall performance, the chosen first substrate was a SiO2 disc with 100-mm diameter, 15-mm thickness, and waviness of less than λ/20. The second substrate was a normal glass plate with dimensions 30 × 150 mm, used to measure the coating thickness in one radius vector direction. The third substrate was a superpolished flat plate with 2.54-cm diameter and 5-mm thickness, used to investigate microroughness change before and after the coating process.
Microroughness measurements on the superpolished flat plate over a 10-nm2 area showed an average roughness of 0.4 nm, comparable to the roughness before the coating was applied. A stylus profilometer measured the glass plate and yielded a thickness variation between 520 and 750 nm as a function of radial distance from the center of the plate.
Overall measurement of the coated SiO2 disc using a wavefront-measurement interferometer produced a phase map that yielded a radius of curvature of 10,256 ±120 m, or roughly 10 km. Residual waviness was measured at approximately λ/30.
Although this measured value does not yet meet the λ/100 waviness specification required for gravitational-wave-detecting mirrors, the researchers are continuing to refine the process to improve waviness by adopting an annealing treatment after coating to absorb stresses in the film. They are also planning to experiment with harder materials such as sapphire, being careful to understand and compensate for slight refractive-index differences between the substrate and the coating material. “Based on the obtained data in this work, we would like to obtain lower waviness by using a better flat substrate and to try to make ideal 10-km-scale curvature production using our final target substrate of sapphire,” says researcher Shinji Miyoki from the University of Tokyo. “This coating technique can also be adopted for optical-mode-compensation components.”
REFERENCE
1. S. Miyoki et al., Optics Letters30(11) 1399 (June 1, 2005).