Within the DLR project COMPASSO, optical clock and link technologies will be evaluated in space on the Bartolomeo platform attached to the Columbus module of the ISS. The system utilizes two iodine-based frequency references, a frequency comb, an optical laser communication and ranging terminal and a GNSS disciplined microwave reference. While COMPASSO is specifically dedicated to test optical technologies relevant for future satellite navigation (i.e. Galileo), the technologies are also crucial for future missions related to Earth observation and science. The optical frequency reference is based on modulation transfer spectroscopy (MTS) of molecular iodine near a wavelength of 532 nm. An extended cavity diode laser (ECDL) at a wavelength of 1064 nm is used as light source, together with fiber-optical components for beam preparation and manipulation. The laser light is frequency-doubled and sent to a mechanically and thermally highly stable free-beam spectroscopy board which includes a 20 cm long iodine cell in four-pass configuration. The iodine reference development is lead by the DLR-Institute of Quantum Technologies and includes further DLR institutes, space industry and research institutions. Phase B of the project will be finalized soon and an Engineering Model of the iodine reference, which represents the flight models in form, fit and function, will be realized by mid 2023. The launch of the COMPASSO payload is planned for 2025. Additional presentation content can be accessed on the supplemental content page.
We present the European development of an engineering model Laser Head for LISA. This single box includes a seed laser, an electro-optical phase modulator, a fiber amplifier and all PCBs to operate the Laser Head.
The gravitational wave detector LISA utilizes as current baseline a high sensitivity Optical Readout (ORO) for measuring the relative position and tilt of a free flying proof mass with respect to the satellite housing. The required sensitivities in the frequency band from 30 μHz to 1Hz are ∼ pm/ √ Hz for the translation√ and ~ nrad/√ Hz for the tilt measurement. EADS Astrium, in collaboration with the Humboldt University Berlin and the University of Applied Sciences Konstanz, has realized a prototype ORO over the past years. The interferometer is based on a highly symmetric design where both, measurement and reference beam have a similar optical pathlength, and the same frequency and polarization. The technique of differential wavefront sensing (DWS) for tilt measurement is implemented. With our setup noise levels below 5pm/ √Hz for translation and below 10nrad/ √Hz for tilt measurements – both for frequencies above 10mHz – were demonstrated. We give an overview over the experimental setup, its current performance and the planned improvements. We also discuss the application to first verification of critical LISA aspects. As example we present measurements of the coefficient of thermal expansion (CTE) of various carbon fiber reinforced plastic (CFRP) including a "near-zero-CTE" tube.
We present the development of a compact optical frequency reference with a stability in the 10-15 domain at longer integration times utilizing Doppler-free spectroscopy based on molecular iodine. With respect to its future application in space, a setup on elegant breadboard (EBB) level was realized and successfully implemented and tested. A frequency stability of 5 • 10-15 at an integration time of 200 s was verified in a beat measurement with a ULE cavity setup. For ensuring high thermal and mechanical stability, the EBB utilizes a baseplate made of ultra-low CTE glass ceramics. The optical components are fixed to the baseplate using an adhesive bonding technology. In a current activity, a setup on engineering model (EM) level will be realized with increased compactness and stability compared to the EBB setup utilizing a very compact multipass gas cell.
Path length errors caused by beamwalk over the surface topography of optical components can have a detrimental influence on the accuracy of highly sensitive translational metrology, that is of particular relevance for In-Field Pointing payload concepts, investigated for the LISA space mission. This paper presents the results of our experimental and theoretical investigations in surface induced path length errors with a detailed characterisation of their magnitudes.
Structural materials with extremely low coecient of thermal expansion (CTE) are crucial to enable ultimate
accuracy in terrestrial as well as in space-based optical metrology due to minimized temperature dependency.
Typical materials, in particular in the context of space-based instrumentation are carbon-ber reinforced plastics
(CFRP), C/SiC, and glass ceramics, e.g. Zerodur, ULE or Clearceram. To determine the CTE of various samples
with high accuracy we utilize a highly symmetric heterodyne interferometer with a noise level below 2 pm√Hz at frequencies above 0.1 Hz. A sample tube made out of the material under investigation is vertically mounted in
an ultra-stable support made of Zerodur. Measurement and reference mirrors of the interferometer are supported
inside the tube using thermally compensated mounts made of Invar36. For determination of the CTE, a sinusoidal
temperature variation is radiatively applied to the tube. One of the essential systematic limitations is a tilt of
the entire tube as a result of temperature variation. This tilt can simultaneously be measured by the DWS
technique and can be used to correct the measurement. Using a Zerodur tube as a reference, it is shown that
this eect can be reduced in post processing to achieve a minimum CTE measurement sensitivity <10 ppb/K.
We present a symmetric heterodyne interferometer as a prototype of a highly sensitive translation and tilt
measurement system. This compact optical metrology system was developed over the past several years by
EADS Astrium (Friedrichshafen) in cooperation with the Humboldt-University (Berlin) and the university of applied science Konstanz (HTWG-Konstanz). The noise performance was tested at frequencies between 10-4 and 3 Hz, the noise levels are below 1 nm/Hz 1/2 for translation and below 1 μrad/Hz1/2, for tilt measurements. For
frequencies higher than 10 mHz noise levels below 5pm/Hz1/2 and 4 nrad/Hz1/2 respectively, were demonstrated. Based on this highly sensitive metrology system we also developed a dilatometer for the characterization of the CTE (coefficient of thermal expansion) of various materials, i.e. CFRP (carbon fiber reinforced plastic) or
Zerodur. The currently achieved sensitivity of these measurements is better than 10-7 K-1. Future planned
applications of the interferometer include ultra-high-precision surface profiling and characterization of actuator noise in low-noise opto-mechanics setups. We will give an overview of the current experimental setup and the latest measurement results.
We developed a compact, fiber-coupled heterodyne interferometer for translation and tilt metrology. Noise
levels below 5 pm/√Hz in translation and below 10 nrad/√Hz in tilt measurement, both for frequencies above
10-2 Hz, were demonstrated in lab experiments. While this setup was developed with respect to the LISA
(Laser Interferometer Space Antenna) space mission current activities focus on its adaptation for dimensional
characterization of ultra-stable materials and industrial metrology. The interferometer is used in high-accuracy
dilatometry measuring the coefficient of thermal expansion (CTE) of dimensionally highly stable materials such
as carbon-fiber reinforced plastic (CFRP) and Zerodur. The facility offers the possibility to measure the CTE
with an accuracy better 10-8/K. We also develop a very compact and quasi-monolithic sensor head utilizing
ultra-low expansion glass material which is the basis for a future space-qualifiable interferometer setup and serves
as a prototype for a sensor head used in industrial environment. For high resolution 3D profilometry and surface
property measurements (i. e. roughness, evenness and roundness), a low-noise (≤1nm/√
Hz) actuator will be
implemented which enables a scan of the measurement beam over the surface under investigation.
Highly stable but lightweight structural materials are essential for the realization of spaceborne optical instruments,
for example telescopes. In terms of optical performance, usually tight tolerances on the absolute spacing
between telescope mirrors have to be maintained from integration on ground to operation in final orbit. Furthermore,
a certain stability of the telescope structure must typically be ensured in the measurement band. Particular
challenging requirements have to be met for the LISA Mission (Laser Interferometer Space Antenna), where the
spacing between primary and secondary mirror must be stable to a few picometers. Only few materials offer sufficient
thermal stability to provide such performance. Candidates are for example Zerodur and Carbon-Fiber
Reinforced Plastic (CFRP), where the latter is preferred in terms of mechanical stiffness and robustness. We are
currently investigating the suitability of CFRP with respect to the LISA requirements by characterization of its
dimensional stability with heterodyne laser interferometry. The special, highly symmetric interferometer setup
offers a noise level of 2 pm/√Hz at 0.1Hz and above, and therefore represents a unique tool for this purpose.
Various procedures for the determination of the coefficient of thermal expansion (CTE) have been investigated,
both on a test sample with negative CTE, as well as on a CFRP tube specifically tuned to provide a theoretical
zero expansion in the axial dimension.
For translation and tilt metrology, we developed a compact fiber-coupled polarizing heterodyne interferometer
which is based on a highly symmetric design where both, measurement and reference beam have similar optical
pathlengths and the same frequency and polarization. The method of differential wavefront sensing is implemented
for tilt measurement. With this setup we reached noise levels below 5 pm/square root of Hz;
Hz in translation and below
10 nrad/square root of Hz; in tilt measurement, both for frequencies above 10-2 Hz. While this setup is developed with respect
to the requirements of the LISA (Laser Interferometer Space Antenna) space mission, we here present the current
status of its adoption to industrial applications. We currently design a very compact and quasi-monolithic setup
of the interferometer sensor head based on ultra-low expansion glass material. The resulting compact and robust
sensor head can be used for nano-positioning control. We also plan to implement a scan of the measurement beam
over the surface under investigation enabling high resolution 3D profilometry and surface property measurements
(i. e. roughness, evenness and roundness). The dedicated low-noise (≤1nm/square root of Hz) piezo-electric actuator in the
measurement beam of the interferometer will be realized using integrated micro-system technology and can either
be implemented in one or two dimensions.
The laser interferometer space antenna (LISA) mission utilizes as current baseline a high sensitivity optical
readout for measuring the relative position and tilt of a free flying proof mass with respect to the satellite housing.
The required sensitivities are ~5pm/&sqrt;
Hz for the translation measurement and ~20 nrad/&sqrt;Hz for the tilt
measurement. For this purpose, EADS Astrium GmbH - in collaboration with the Humboldt-University Berlin
and the University of Applied Sciences Konstanz - develops a fiber-coupled heterodyne interferometer including
differential wavefront sensing for the tilt measurement. The interferometer is based on a highly symmetric design
where both, measurement and reference beam have the same optical pathlength, frequency and polarization. We
realized a mechanically highly stable and compact setup which is located in a temperature stabilized vacuum
chamber and utilizes frequency stabilization of the laser and intensity stabilization of the heterodyne frequencies
at the fibre outputs. Noise levels below 5 pm/&sqrt;
Hz in translation movement and below 10 nrad/&sqrt;Hz in tilt
movement (both for frequencies above 10-2 Hz) were measured.
While this setup is developed with respect to the requirements of the LISA space mission, it also has potential
applications beyond: In industry, high precision position measurements - with ever increasing sensitivity - are
needed e.g. for guaranteeing very small tolerances for automobile industry components. While current systems
developed for this purpose use for instance whitelight-interferometry with resulting sensitivities in the nm-range,
our interferometer opens the possibility to further improve the sensitivity. Here, we discuss possible
implementations of our interferometer for industrial applications.
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