An Optical Readout for the LISA Gravitational Reference Sensor

Abstract

The space-based gravitational wave detector LISA (Laser Interferometer Space Antenna) consists of three identical satellites. Each satellite accommodates two free-flying proof masses whose distance and tilt with respect to its corresponding optical bench must be measured with at least 1 pm/ √ Hz sensitivity in translation and at least 10 nrad/ √ Hz sensitivity in tilt measurement. In this thesis, a compact optical readout system – consisting of an optomechatronic setup together with associated electronics, data acquisition and software – is presented, which serves as a prototype for the LISA proof mass attitude metrology. We developed a polarizing heterodyne interferometer with spatially separated frequencies. For optimum common mode rejection, it is based on a highly symmetric design, where measurement and reference beam have the same frequency and polarization, and similar optical pathlengths. The method of differential wavefront sensing (DWS) is utilized for the tilt measurement. An intrinsically highly stable Nd:YAG laser at a wavelength of 1064 nm is used as light source; the heterodyne frequencies are generated by use of two acousto-optic modulators (AOMs). In a first prototype setup noise levels below 100 pm/ √ Hz in translation and below 100 nrad/ √ Hz in tilt measurement (both for frequencies above 10−1 Hz) are achieved. A second prototype was developed with additional intensity stabilization and phaselock of the two heterodyne frequencies. The analog phase measurement is replaced by a digital one, based on a Field Programmable Gate Array (FPGA). With this setup, noise levels below 5 pm/ √ Hz in translation measurement and below 10 nrad/ √ Hz in tilt measurement, both for frequencies above 10−2 Hz, are demonstrated. A noise analysis was carried out and the nonlinearities of the interferometer were measured. The interferometer was developed for the LISA mission, but it also finds its application in characterizing the dimensional stability of ultra-stable materials such as carbon-fiber reinforced plastic (CFRP) and in optical profilometry. The adaptation of the interferometer and first results in both applications are presented in this work. DBR (Distributed Bragg-Reflector) laser diodes represent a promising alternative laser source. In a first test, laser diodes of this type with a wavelength near 1064 nm are characterized with respect to their spectral properties and are used as light source in the profilometer setup.

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Cite this paper

@inproceedings{Schuldt2010AnOR, title={An Optical Readout for the LISA Gravitational Reference Sensor}, author={Thilo Schuldt and H W{\"{u}nsche}, year={2010} }