EINTEL/SPACE-ECHO - Optics Development for Einstein Telescope in the ET-LF cryogenic environment
Participating countries
Duration and status
In progress.
Partners
TNO
Maastricht University
VSL B.V.
NOVA
SRON
Associate partners
University of Twente
Budget
Funding: € 2.5 million
Program and en domain
Domain: Optics
Problem Statement
Existing mirrors for interferometers operate at ambient temperature. The Einstein Telescope will have mirrors that must operate at cryogenic temperatures. To support the wide laser beam, the mirrors must be ±50cm in diameter to minimize diffraction loss on the outside of the mirror. To minimize the optical pressure of the lasers moving the mirrors, the mirrors are made 60cm thick, leading to a weight of ±200 kg. Some mirrors must be concave polished with a focal length of 5km, so that the laser light is centered back each time. This new optical design requires new materials and manufacturing techniques.
Objective
- Achieve the best production method for the cooled Silicon main mirrors of the Einstein Telescope:
- Silicon material recipe
- Cutting & polishing treatments aimed at maximizing reflectivity in Silicon material to 99.999% .
Metrology measurement setups:
- Objects of larger size minimum 50x50cm
- Measurement of roughness <0.1 nm, flatness <2 nm and defects
- Measurement of surface and bulk adsorption by laser light at 1ppm/cm
- Joining techniques to assemble larger Silicon mirrors.
Description of the innovation(s).
Multiple interferometric tests will be performed on Silicon material samples of 2.5x2.5 cm - 3 cm depth. High quality magnetically purified Czochralski silicon samples will be cut from 300 mm diameter ingots. Further operations include CNC machining, cutting and pre-polishing (SPDT and laser-assisted SPDT), various polishing techniques (IBF, MRF and wet etching) to achieve flatness < 2 nm and surface roughness Ra < 0.1 nm. Various metrology steps (PCI photothermal common-path interferometer, AFM Atomic Force Microscopy, WLI White-Light Interference Microscopy and Cavity Ring Down method) are employed to measure bulk absorption for 1550 nm laser light and reflectance of highly reflective coated surfaces with an uncertainty of 1 ppm/cm. Traceable calibrations are used to compare the roughness values of different process steps. Between steps, samples are packaged and transported.
