Measurement of a low-absorption sample of OH-reduced fused silica.

Low-absorption optics made of OH-reduced fused silica are a key technology for future gravitational wave detectors such as the Advanced LIGO. We developed a sensitive method to measure the absorption inside the beam splitter of the GEO 600, taking advantage of the effect of thermal lensing. Using this method, we derived a bulk absorption of less than 0.25 ppm/cm for a piece of Suprasil 311 SV at a wavelength of 1064 nm. To the best of our knowledge, this is the lowest value of light absorption inside fused silica reported so far in the literature.