New Thinking on Gravitational and Optical Reference Architecture for LISA and BBO
Ke-Xun Sun
Stanford University
kxsun@grs.stanford.edu
Additional authors: Graham Allen, Dan DeBra, Sasha Buchman, Robert Byer
LISA and the next generation space-born laser interferometers require gravitational reference sensors (GRS) to provide picometer precision for LISA, and femtometer precision for the proposed Big Bang Observatory (BBO). We describe a stand-alone GRS structure, which has the benefits of both higher sensitivity and ease of fabrication. The proposed GRS structure enables high precision interferometric links in 3-dimensional directions. The GRS housing provides the optical reference surface onto which the transmitted laser beam, and the independent received laser beam are referenced. The stand-alone GRS allows balanced optical probing, picometer sensitivity without radiation pressure imbalance, and freedom to select transmitting and receiving wavelengths without consideration for the reflectance of the proof mass. Optical readout from proof mass to housing reference surface may use a different wavelength. Our second consideration was to improve reliability and to reduce complexity by merging the two cubic proof masses into one spherical mass. This eliminates complex back surface fiber coupling from one proof mass to the other, and the inter-proof-mass noise. With proper algorithms, this approach finds the center of mass of proof mass. The single parameter that reduces disturbance of the proof mass is the gap spacing. Optical readout allows the use of a large gap between the GRS housing and proof mass. We propose using rf-modulated optical interferometry to measure both relative displacement and absolute distance. We further propose to use reflective grating in both the GRS and optical bench. The grating reflective optics design eliminates in-path transmissive optical components thus dn/dT effects, and simplifies the optical bench structure. Inside the GRS, near Littrow mounted grating will enable picometer precision measurement. Proper grating design allows aiming at the correct point-ahead angle without having to tilt the proof mass. Tertiary or quaternary telescope adjusts aiming direction by moving small mirrors and avoid heavy mass movement inside spacecraft. The sensitivity scaling law agues for larger telescope diameter and shorter laser wavelength. In a grand scale, BBO may share similar 3-arm interferometer architecture with LISA. However, BBO requires much higher displacement sensitivity than LISA. To measure proof mass position at femtometer precision, optical sensing will necessarily replace electrostatic sensing, which is the LISA baseline design for GRS. The reflective optics approach is particularly suitable for BBO, where laser power is proposed to reach from hundreds watts, producing significant heating effects if transmissive optics is used. The proposed stand-alone or modularized GRS approach is well suited to a variety of space missions requiring high precision gravitational referencing.
