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.
Future space missions rely on the availability of space qualified high precision optical metrology instruments like ultra stable laser sources. Here, we present a compact, frequency-doubled, monolithic Nd:YAG laser (non planar ring-oscillator, NPRO), frequency stabilized to a hyperfine transition in molecular iodine, based on the method of modulation transfer spectroscopy. Using a 10 cm long iodine cell cooled to 1±C and a total light power of ~ 5 mW a frequency stability of 1 • 10-12 for an integration time of τ = 1 s and 3 • 10-13 for τ < 100 s was achieved. By use of an active offset compensation (offset compensation by amplitude modulated sidebands, OCAMS), the frequency stability of this setup was furthermore improved to 4 • 10-14 for τ > 5000 s. This setup therefore fulfillls the basic metrological requirements for the LISA and Darwin missions (with potential beyond). Due to very compact construction, it serves as a study and demonstrator for a future space qualified iodine standard.
A variety of future space missions rely on the availability of high-performance optical frequency references with applications in fundamental physics, geoscience, Earth observation and global satellite navigation systems (GNSS). Examples are the gravitational wave detector LISA (Laser Interferometer Space Antenna), the Earth gravity mission NGGM (Next Generation Gravity Mission) and missions, dedicated to tests of Special Relativity, e.g. by performing a Kennedy-Thorndike experiment testing the boost dependence of the speed of light. In this context, we developed optical frequency references based on Doppler-free spectroscopy of molecular iodine where compactness and mechanical and thermal stability are main design criteria. We demonstrated a frequency instability of 6•10-15 at 1 s integration time and 3•10-15 for integration times between 100 s and 1.000 s. Furthermore, a very compact spectroscopy setup was realized for the sounding rocket mission JOKARUS which was successfully flown in May 2018. In a current activity, an integrated high-performance iodine-based frequency reference is developed which serves as a demonstrator for future GNSS using optical technologies.
KEYWORDS: Mirrors, Retroreflectors, Space operations, Optical components, Ranging, Field programmable gate arrays, Beam steering, Satellites, Bragg cells, Digital signal processing
Interferometric laser ranging is an enabling technology for high-precision satellite-to-satellite tracking within the context of earth observation, gravitational wave detection, or formation flying. In orbit, the measurement system is affected by environmental influences, particularly satellite attitude jitter and temperature fluctuations, demanding an instrument design, which has a high level of thermal stability and is insensitive to rotations around the satellite's center of mass. Different design approaches for a heterodyne dynamic laser ranging instrument have been combined to a new improved design concept that involves the inherent beam tracking capabilities of a retroreflector into a mono-axial configuration with nanometer accuracy. In order to facilitate the accommodation onboard a future satellite mission, the design allows for a continuously adjustable flexible phase center position. To cover large inter-spacecraft distances, the instrument design comprises an active transponder system, featuring a two-dimensional beam steering mechanism to align a local, strong laser to the (weak) input beam without affecting the measurement path.
To this end, a dynamic laser ranging instrument is presented, which has compact dimensions and is fully integrated on a single Zerodur baseplate. The instrument performance will be evaluated in a dedicated test setup providing a flat-top beam simulating the laser beam received from a distant spacecraft, including a beam steering subsystem, which allows for monitoring of pathlength variations when the angle of incidence at the optical instrument is changing.
Within the European Space Agency (ESA) activity “Gravitational Wave Observatory Metrology Laser” we designed a laser head to fulfill the LISA laser requirements using a non-NPRO seed laser technology: an external cavity diode laser (ECDL) with resonant optical feedback from an external cavity as master oscillator for further linewidth narrowing. Furthermore, our design features a single-stage fiber amplifier with an amplification factor of about 20 dB. This paper covers the requirements on the laser source for LISA, the design and first results of performance characterization of the laser head breadboard.
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.
The space-based gravitational wave detector LISA (Laser Interferometer Space Antenna) requires a high performance position sensor in order to measure the translation and tilt of the free flying test mass with respect to the LISA optical bench. Here, we present a mechanically highly stable and compact setup of a heterodyne interferometer combined with differential wavefront sensing for the tilt measurement which serves as a demonstrator for an optical readout of the LISA test mass position. First results show noise levels below 1 nm/√Hz and 1 μrad/√Hz, respectively, for frequencies < 10−3 Hz.
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.
Design, integration, test setup, test results, and lessons-learnt of a high precision laser metrology demonstrator for dual absolute and relative laser distance metrology are presented. The different working principles are described and their main subsystems and performance drivers are presented. All subsystems have strong commonalities with flight models as of LTP on LISA Pathfinder and laser communication missions, and different pathways to flight models for varying applications and missions are presented. The setup has initially been realized within the ESA project "High Precision Optical Metrology (HPOM)", originally initiated for DARWIN formation flying optical metrology, though now serves as demonstrator for a variety of future applications. These are sketched and brought into context (PROBA-3, IXO onboard metrology, laser gravimetry earth observation missions, fundamental science missions like LISA and Pioneer anomaly).
We realized ultra-stable optical frequency references on elegant breadboard (EBB) and engineering model (EM) level utilizing Doppler-free spectroscopy of molecular iodine near 532nm. A frequency stability of about 1•10-14 at an integration time of 1 s and below 5•10-15 at integration times between 10 s and 100 s was achieved. These values are comparable to the currently best laboratory setups.
Both setups use a baseplate made of glass material where the optical components are joint using a specific assembly-integration technology. Compared to the EBB setup, the EM setup is further developed with respect to compactness and mechanical and thermal stability. The EM setup uses a baseplate made of fused silica with dimensions of 380 x 180 x 40 mm3 and a specifically designed 100 x 100 x 30 mm3 rectangular iodine cell in nine-pass configuration with a specific robust cold finger design. The EM setup was subjected to thermal cycling and vibrational testing.
Applications of such an optical frequency reference in space can be found in fundamental physics, geoscience, Earth observation, and navigation & ranging. One example is the proposed mSTAR (mini SpaceTime Asymmetry Research) mission, dedicated to perform a Kennedy-Thorndike experiment on a satellite in a sunsynchronous low-Earth orbit. By comparing an iodine standard to a cavity-based frequency reference and integration over 2 year mission lifetime, the Kennedy-Thorndike coefficient will be determined with up to two orders of magnitude higher accuracy than the current best ground experiment. In a current study, the compatibility of the payload with the SaudiSat-4 host vehicle is investigated.
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.
KEYWORDS: Mirrors, Semiconducting wafers, Micromirrors, Electrodes, Finite element methods, Silicon, Deep reactive ion etching, Laser interferometry, Space operations, Nanoimprint lithography
A silicon micromirror with 3x3 mm² surface area and a thickness of 100 μm has been designed and realized for the
future space mission LISA (Laser Interferometer Space Antenna). The mirror is electrostatically actuated. The tilt
movement of the mirror is provided by torsional load of the mirror suspension. 3D FEM simulations have been used for
optimization of the layout of the mirror device. A torsion angle of ± 1.9 mrad is achieved at a driving voltage of
U=200V.
The demanding requirements on the laser interferometer in the mission LISA in respect to mechanical stability, noise
performance and especially piston effect, (i.e. the requirement that under rotation of the mirror no significant z-movement
of the reflection surface occurs) are fulfilled with a new design and fabrication concept for the
micromechanical device. The piston-effect is avoided by a rotational axis of the micromirror which coincides exactly
with the surface of the mirror. This is achieved by using a symmetric SOI-wafer (Silicon on Insulator) with handle and
device wafer having exactly the same thickness. The mirror plane is formed by the handle wafer. The suspending beams
are realized from both, the handle and the device wafer of SOI-wafer. Thus the central axis of the beams coincides with
the reflecting plane. In addition, the z-displacement of the mirror under rotation due to the attracting electrostatic force is
minimized by optimization of the beams and the counter electrode using FEM simulation.
Fabricated devices are characterized by special interferometric optical measurements.
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 space mission LISA (Laser Interferometer Space Antenna) aims at detecting gravitational waves in the
frequency range 30 μ Hz to 1Hz. Free flying proof masses inside the satellites act as inertial sensors and represent
the end mirrors of the interferometer. In the current baseline design, LISA utilizes an optical readout of the
position and tilt of the proof mass with respect to the satellite housing. This readout must have ~ 5pm/√Hz
sensitivity for the translation measurement (for frequencies above 2.8mHz with an -2 relaxation down to
30 μHz) and ~ 10 nrad/√Hz sensitivity for the tilt measurement (for frequencies above 0.1mHz with an -1
relaxation down to 30 μHz). The University of Applied Sciences Konstanz (HTWG) ‐ in collaboration with
Astrium GmbH, Friedrichshafen, and the Humboldt-University Berlin ‐ therefore develops a highly symmetric
heterodyne interferometer implementing differential wavefront sensing for the tilt measurement. We realized a
mechanically highly stable and compact setup. In a second, improved setup we measured initial noise levels
below 5 pm/√Hz and 10 nrad/√Hz, respectively, for frequencies above 10mHz.
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.
The ESA/NASA joint space mission LISA (Laser Interferometer Space Antenna), which is planned to be launched around 2015, aims at detecting gravitational waves in the frequency band 3*10-5 Hz to 1 Hz. It consists of three satellites which form an equilateral triangle in space, representing a Michelson-interferometer with an armlength of ~ 5 million kilometer. The end mirrors of the interferometer are realized by free flying proof masses. In the current baseline design--the so-called "strap-down" architecture--the laser light coming from the distant spacecraft is not reflected by the proof mass, but the beat signal with the local oscillator is measured on the optical bench. In addition, the distance between optical bench and its associated proof mass has to be measured with the same sensitivity as in the distant spacecraft interferometer, i. e. below 10 pm/sqrt(Hz) for the translation measurement (for frequencies above 2.8*10-3 Hz with an f-2 relaxation down to 3*10-5 Hz) and below 20 nrad/sqrt(Hz) for the tilt measurement (for frequencies above 10-4 Hz with an f-1 relaxation down to 3*10-5 Hz). Here, we present a compact setup of a heterodyne interferometer which serves as a demonstrator for an optical readout for the LISA proof mass position. We measured initial noise levels below 1 nm/sqrt(Hz) and 1 urad/sqrt(Hz), respectively, for frequencies > 10-3 Hz.
A Darwin precursor breadboard, comprising both fine lateral and longitudinal metrology sensors was designed, built and partially tested. The lateral metrology sensor was designed and built by TNO TPD and more than meets the imposed requirements. The longitudinal metrology sensor consists of a dual wavelength interferometer with an integrated delay line for optical path stabilisation. Here TNO TPD supplied the delay line and implemented the optical path difference stabilisation control. Experiments under ambient conditions show that noise reduction up to five orders of magnitude is achievable.
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