The core beam splitter for the Sub-SQL Interferometer has been installed into the vacuum system and is now freely hanging in its triple suspension system.
Unlike typical beam splitters used in gravitational wave detectors, the shape of our beam splitter is rectangular rather than round. This is because the beam splitter should be wide enough for secondary internal reflections (ghost beams) to exit the beam splitter through the optical surfaces rather than the barrel of the optic so that those beams can be effectively dumped. A round beam splitter fulfilling this requirement would hit the optical breadboard of the suspension platform interferometer (SPI) located directly underneath. This breadboard is housed in the aluminium box below the beam splitter in the above photos. Additionally, the rectangular shape reduces the mass of the beam splitter by approximately 50% compared to round version. This is critical for reducing the total mass on the optical table, which is limited due to the seismic attenuation system.
The beam splitter side of the optic does not use a uniform coating. Instead it uses a split coating design where only the central part of the surface has a beam splitter coating. The sides are anti-reflection coated to suppress further ghost beams.
More details about the design of the beam splitter and its suspension system are available in chapter 4 of Philip Koch’s PhD thesis.