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.
The Space-Time Explorer and Quantum Test of the Equivalence Principle mission (STEQUEST) is devoted to a precise measurement of the effect of gravity on time and matter using an atomic clock and an atom interferometer. The mission was selected by ESA as one of four candidates for the M3 mission within the Cosmic Vision Program (launch in 2022).
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 present LISA (Laser Interferometer Space Antenna) gravitational wave detector concept features three satellites in individual earth trailing helio-centric orbits, which are linked by bi-directional monostatic laser interferometry between free-falling inertial reference masses inside the payload. The spacecrafts are maintaining an equilateral triangular constellation with 5 Million km armlength. The optical payload consists in the present configuration out of two assemblies, each one comprising a telescope, an optical bench and an inertial sensor and serving one arm of the adjacent interferometers. Due to orbital distortions, the constellation triangle is not perfectly maintained, but the line of sights offset angle is slowly changing during a one year revolution by 60°±0.75°. This variation is far beyond the diffraction limited beam width (2.5 μrad) and hence requires active compensation presently done by actuation of the complete assemblies. While allowing almost stationary on-axis operation of the optics, the arrangement requires two separate active inertial sensors, a rather sophisticated optical interfacing between the interferometer arms and active electrostatic suspension of the test masses in all but one degree of freedom.
We identified an alternative architecture, characterized by a single operational inertial sensor and a single optical bench serving both adjacent interferometer arms. Both telescopes are rigidly fixed to the optical bench and the angular breathing is accommodated by in-field of view pointing of transmit and receive beams via on-bench actuation mechanisms. Only attitude electrostatic actuation of the test mass is required, which can be kept otherwise in free fall. Such an architecture requires a decoupled inter- and intraspacecraft metrology in two steps linked via optical bench fiducial points (strap-down). Peculiar technical challenges are the actuation mechanism and the inherent metrology to calibrate or compensate within the LISA measurement band –at pm and nrad resolution- for laser phase and pointing changes, respectively, inside the optical assembly.
We present a detailed concept for the realization of the LISA optical metrology subsystem, which employs heterodyne interferometry in a so-called “strap-down” architecture to accomplish a highly sensitive detection of gravity-wave induced displacements of dedicated mass references within the payload of the three LISA satellites. A frequency swap between transmitted and local reference beam is introduced to minimize the impact of stray light on the measurement sensitivity. The performance of the system is demonstrated by first optical modeling.
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 review possible alternatives for the realization of the LISA laser assembly against the requirements and present a concept for a tailored master oscillator fiber power amplifier solution. An all-solid-state, all- fiber coupled approach with a Nd:YAG NPRO seed appears particularly attractive due to the possibility to combine excellent spectral properties and high output powers with superior environmental robustness.
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).
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 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.
In preparation for the Laser Interferometer Space Antenna (LISA) space mission, the prototype engineering model of the LISA-Pathfinder optical bench instrument has been built and tested. The instrument is the central part of an interferometer whose purpose is to measure the separation of two free-floating test masses in the spacecraft, with required accuracy to a noise level of 10 pm/Hz?1/2 between 3 mHz and 30 mHz. This will allow the spacecraft to achieve drag-free flight control to a similar level, as a demonstration of technology capability for detection of gravitational waves in the later LISA mission. The optical bench design, fabrication, and experimental results are described in detail, with attention to the strategies for building and alignment. These are particularly problematic in this instrument due to restrictions on the allowable materials and devices, the limited size, the tight alignment requirements for interferometry and interfaces, and the challenging environment specification for space flight. The finished optical bench was integrated to the complete optical metrology package for system-level tests, which were successful, both in meeting the metrology accuracy and in environmental testing. This verifies the feasibility of the design and build methods demonstrated here for use in the space-flight version.
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.
The LISA Technology Package (LTP) aboard of LISA pathfinder mission is dedicated to demonstrate and verify key technologies for LISA, in particular drag free control, ultra-precise laser interferometry and gravitational sensor. Two inertial sensor, the optical interferometry in between combined with the dimensional stable Glass ceramic Zerodur structure are setting up the LTP. The validation of drag free operation of the spacecraft is planned by measuring laser interferometrically the relative displacement and tilt between two test masses (and the optical bench) with a noise levels of 10pm/√Hz and 10 nrad/√Hz between 3mHz and 30mHz. This performance and additionally overall environmental tests was currently verified on EM level. The OB structure is able to support two inertial sensors (≈17kg each) and to withstand 25 g design loads as well as 0...40°C temperature range. Optical functionality was verified successfully after environmental tests. The engineering model development and manufacturing of the optical bench and interferometry hardware and their verification tests will be presented.
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