Correlation between birefringence and absorption mapping in large-size Sapphire substrates for gravitational-wave...

Area of Science:

• Optics and Photonics
• Materials Science
• Astrophysics Instrumentation

Background:

• High-sensitivity laser interferometers, such as the KAGRA gravitational-wave detector, require ultra-high-quality mirrors.
• Large-size sapphire crystals are used for KAGRA's main mirrors due to their optical quality and thermal properties at cryogenic temperatures (20 K).
• Meeting stringent optical specifications necessitates precise characterization of substrate homogeneity.

Purpose of the Study:

• To develop and present a dedicated setup for characterizing refractive-index inhomogeneities in large-size uniaxial sapphire samples.
• To achieve high sensitivity (order of 10^-7) and spatial resolution (order of 10^-4 m) in birefringence measurements.
• To simultaneously map residual absorption and compare it with birefringence features.

Main Methods:

• Development of a specialized optical setup for measuring birefringence changes in large sapphire substrates.
• Simultaneous recording of residual absorption maps alongside birefringence measurements.
• Application of the setup to a KAGRA-sized sapphire substrate, comparing results with prior absorption data.

Main Results:

• Demonstration of a setup capable of resolving birefringence changes with high sensitivity and spatial resolution.
• Presentation of the first measurements on a KAGRA-sized sapphire substrate using the developed technique.
• Comparison and discussion of birefringence inhomogeneities and absorption distributions within the sapphire sample.

Conclusions:

• The developed setup enables precise characterization of optical homogeneity in large sapphire substrates.
• Simultaneous measurement of birefringence and absorption provides insights into material properties.
• This technique is vital for implementing the tight optical specifications required for advanced gravitational-wave detectors.