The Gravity Recovery and Climate Experiment (GRACE) is a successful Earth observation mission launched in 2002 and consisting of two identical satellites in a polar low-Earth orbit. The distance variations between these two satellites are measured with a microwave ranging system located in the central axis with a precision down to several μm/. In data post-processing the spatial and temporal variations of the Earth’s gravitational field are recovered. More than 1200 articles where published in the context of GRACE , yielding significant improvement in the understanding e.g. of the seasonal and long term development of Earth’s water basins. Examples are the loss of ice masses on the north pole , articles on Ground water decrease in India  and California , and long term disaster prediction capability .
On GRACE Follow-On a laser ranging interferometer (LRI) will fly as a technology demonstrator to provide about 2 orders of magnitude higher ranging measurement precision, down to 80 nm/ in the measurement band between 2 mHz and 0.1 Hz.
The optical bench is a key unit of the LRI on-board the GRACE follow-on mission which will be launched in 2017 by the joint collaboration between USA (NASA) and Germany (GFZ).
The design, the performance analyses as well as the functional, performance and environmental tests results of the optical bench engineering model are discussed in this paper.
The LRI is based on a laser interferometer evaluating the heterodyne signal of two near-infrared single frequency lasers (λ=1064 nm) with a frequency offset in the MHz range. The two lasers are located on the two GRACE FO satellites, which follow each other in a near polar orbit at 450 km height with about 220 km distance to each other.
The instrument hardware on both spacecraft is identical. It consists of a single frequency laser, a fabry perot reference cavity, a triple mirror assembly (TMA) and an optical bench subsystem (OBS, consisting of the optical bench assembly (OBA) and associated electronics (OBE)) and the instrument electronics, the so called laser ranging processor (LRP). Fig. 1 shows an overview sketch of the whole instrument configuration.
On one spacecraft (the master S/C), the laser is frequency locked to the reference cavity via a fibre link and the beam is launched via the fibre injector onto the OBA. On the OBA it is directed to the quadrant photo receiver as well as to the other spacecraft by means of a beam splitter.
The beam then passes the TMA, which enables the range measurement from CoM to CoM of the two spacecraft, due to its retro reflector properties and the placement of its vertex at the CoM of the respective S/C. While in principle a retroreflector is in general required to achieve this sort of ‘racetrack configuration’, on GRACE FO the TMA has the further purpose to route the beam around the Ka-Band Horn and the coldgas tank, which requires it to have a displacement of about 600 mm between the receive and transmit beam path. The very constrained space on the GRACE FO platform together with the required beam co-alignment in the range of 50 µrad makes this a challenge on its own. In Fig. 2 the approximate placement of the LRI units is shown exemplary in the GRACE BUS, which is very similar to the one GRACE FO will use. On the second spacecraft (the slave S/C) the hardware configuration is identical, but the laser is not locked to the reference cavity but frequency offset locked by 10 MHz to the received signal from the master S/C. The heterodyne signal of the superposition of the local oscillator of the master S/C and the received signal from the slave S/C yields two times the Doppler shift of the relative motion of the two S/C. This phase shift signal is the main science signal from which the distance variation is derived and ultimately the gravity field is reconstructed. For a more detailed description of the instrument principle see [6,7].
The top level instrument requirement is the ranging noise requirement of
From this requirement several key and driving design requirements for the free space optics part of the LRI (which is the OBA, the TMA and the instrument baffles) and the OBA are derived and listed in Table 1.
key and driving LRI optical design requirements
|Parameter||Requirement LRI||Requirement OBA|
|Beam Diameter||5 +/- 0.5 mm||5 +/- 0.5 mm|
|Free Aperture (over FoR)||> 3 x beam diameter||> 3 x beam diameter|
|Beam propagation Factor M2||< 1.2||< 1.2|
|Wavefront planarity (1/e2)||< λ/8||< λ/12|
|Beam alignment error||< 50 µrad||< 10 µrad|
|Field of regard (1/e2)||> 4.5 mrad||> 4.5 mrad|
|Beam walk on QPR (over FoR)||-||< 10 µm|
|Ranging noise||< 80 nm/√Hz x NSF(f)||< 5 nm/√Hz x NSF(f)|
|Rotational coupling factor||< 200 µm/rad (in pitch an yaw)||< 80 µm/rad (in pitch an yaw)|
|Pointing stability||< 20 µrad over 1 orbit||< 15 µrad over 1 orbit|
The beam diameter requirement is derived from the overall power budget, taking required power at the distant spacecraft and heterodyne signal loss under the influence of co-alignment errors into account. The same is true for M², the required wavefront planarity and the beam co-alignment. The field of regard defines the required laser beam steering range relative to the S/C because of pointing misalignments after launch (caused by uncertainty of the S/C pointing due to star camera errors, launch settling and optimized pointing for the microwave ranging system) and S/C AOCS pointing performance of about +/- 300 µrad.
The optical bench features the ultra-stable fibre injector (FIA), the beam fine steering mirror (FSM), superposition of the laser signal, imaging optics as well as the photoreceiver frontends (PRFs) with the quadrant photodiodes. Its key functions are to
• launch the laser beam to other S/C (via TMA)
• superimpose the received beam and the local oscillator on the PRFs
• convert the optical heterodyne signal to electrical signal and provide to LRP
• enable the beam steering (by LRP command) to cover the field of regard
• enable closed loop beam steering (by LRP) to keep the received beam and the transmitted beam co-aligned
• image the OBA entrance aperture and the FSM onto the PRFs (for beam size adaption, avoidance of beam walk, avoidance of diffraction rings)
Fig. 3 shows the setup of the optical bench and the beam path and Fig. 4 shows the actual EM hardware during the assembly and test campaign at STI. The individual components are either placed in or attached to the optical bench body out of titanium. This approach has been selected to achieve a low thermal noise (due to the high thermal mass enclosing the components) and high beam pointing stability (due to the symmetry of the titanium body with respect to the plane containing the beam). The optical components are out of BK7G18, which is well CTE-matched to titanium, further improving the thermal stability of the OBA.
The key requirements for the fibre injector are the 5 mm collimated beam diameter with a wavefront planarity of better than λ/15 for the full operational temperature range of 10 to 40 °C, a free aperture of 3 times the beam diameter and a pointing stability of better than 5 µrad/K. Furthermore the fibre injector return loss for signals entering the injector in the reverse direction shall be higher than 50 dB. An optically fully monolithic design, with the fibre directly spliced to the aspheric lens body by CO2 splicing technology , has been developed to achieve these requirements. The design combines an extremely high thermal stability with high return losses, but requires very precise manufacturing capability. It has been performance tested over the full operational temperature range and successfully passed thermal cycling, vibration and shock testing. The engineering model met all requirements, except for the beam diameter, due to a conversion error of the fiber numerical aperture. The flight model design has been modified to achieve the required 5 mm beam diameter.
The design is adaptable to other fibre NAs and beam diameters and considered an attractive design of various collimator applications with high stability requirements. Fig. 5 shows the fibre injector EM, the wavefront measurement in vacuum and the beam profile in the focus.
The internal optics consists of a polarizer after the FIA, a beamsplitter and compensation plate and the imaging optics.
The beam splitter is a wedged plane optic with a 90:10 splitting ratio and a surface figure error of better than 3/0.2(0.2), corresponding to approx. λ/18 at the design wavelength of 1064 nm. The beamsplitter is the element, where the received beam and the local oscillator are superimposed. The planarity of the beamsplitter is of key importance, as any deviation in planarity leads to a loss of heterodyne signal due to a wavefront mismatch between the received beam and the local oscillator on the local spacecraft as well as a loss of signal on the distant spacecraft due to a change in the far field divergence. The submount design and the gluing process had to be optimized to ensure the required planarity of about λ/15.
The compensation plate has identical dimensions like the beam splitter, but is coated with AR coatings on both sides. Its function is to reduce the linear rotation-to-pathlength coupling by about three orders of magnitude (from about 2300 µm/rad to less than 40 µm/rad).
The imaging optics are comprised of 3 lenses and a beam splitter. The function of the imaging optics is to image the entrance aperture and the plane of the steering mirror onto the two redundant quadrant photodiodes of the PRFs. They provide a demagnification factor of 8.4 to reduce the 8 mm entrance aperture to about 0.95 mm to fit the 1 mm diode aperture and eliminate any beam walk on the PRFs during rotation of the individual spacecrafts.
Fine Steering Mirror
The fine steering mirror is a flight proven design from LCT. It provides an optical steering range of +/-8.1 mrad and allows a fast initial link acquisition over a range of +/- 3 mrad at 150 Hz, which is the initial alignment uncertainty of optical axes of the LRIs on the two spacecraft with respect to each other. The steering mirror will be operated throughout the whole mission to cancel out the spacecraft angular movement in pitch and yaw (in the order of +/- 300 µrad) with an accuracy of a few µrad.
The photoreceivers are comprised of a frontend (PRF) and a backend(PRB) and has been designed by DLR Adlershofen. The PRF contains the quadrant photodiode and the preamplifier electronics, while the PRB contains the main amplifier and the power. The whole design has been optimized to ensure a high sensitivity and low noise in the heterodyne measurement band between 4 and 16 MHz. The PRF is positioned laterally to the optical beam to µm accuracy by carefully hammering and to 30 µm accuracy longitudinally by shimming.
OPTICAL DESIGN AND ANALYSIS
The major part of the optical design is based on a detailed nonsequential optical raytracing model incorporating a full representation of OBA optics, including aberrations and the local oscillator as well as the received beam. It has been used to derive PRF shimming accuracy by worst case element misalignments, applied to derive single element tolerances (driven by beam walk) and delivers the qualitative heterodyne signal efficiency behaviour under alignment changes. Fig. 6 shows the optical design and tolerance analysis flow, the optical raytracing model and some evaluation features.
THERMAL DESIGN AND ANALYSIS
The thermal design of the OBA has to ensure three constraints: 1. the allowable operational temperature limits for all components are kept during all times on orbit. 2. the thermal noise is low enough to support the allocated ranging noise of less than 5 nm/√Hz x NSF(f). 3. the angular alignment of the beam out of optical bench is better than 15 µrad over one orbit (for initial acquisition reasons). For 2. and 3. the design approach with the titanium structure with high thermal capacity into which the optical components are inserted or attached to has been selected. It provides high thermal mass and a good thermal symmetry with respect to the plane of the optical axes.
To first order the thermal noise couples into the ranging noise only via the thermal dependency of the optical properties of the beamsplitter and the compensation plate and electrical noise of the PRFs and PRB with a coupling factor of 2.1 nm/K. The coupling factor of the BS and CP is 74.4 nm/K and of the PRFs and PRB is 2.1 nm/K. From this one calculates a required thermal noise of less than 0.065 K/√Hz x NSF(f). The assumed worst case thermal environment for GRACE FO is based on actual thermal noise measurement data from GRACE, which yielded 0.3 K/√Hz x NSF(f) and a +/-3K sinusoidal temperature variation at orbit frequency.
Fig. 7 shows the assumed temperature time series for the analysis and the resulting ranging noise from the detailed transient thermal analysis of the OBA.
The analysis predicts thermal noise performance of less than 1 nm/√Hz x NSF(f), significantly better than required. The PRB shows up as the greatest contributor, which is caused by the worst case assumption of no thermal mass of the PRB in comparison to the detailed transient thermal modelling of the OBA.
Extensive tests have been performed with the engineering model of the optical bench and its subsystems during assembly and in the final configuration. For most tests a single frequency laser signal was applied either via the FIA or the RX OGSE serving as far field simulator, providing plane wavefronts as would be received from the distant satellite. Especially for the wavefront planarity measurements a careful calibration of the Shack Hartmann sensor is required to achieve an absolute accuracy of better than λ/5. This calibration is repeated for each measurement with a point source setup providing a spherical wavefront with 1 m radius. Fig. 8 shows the test setup schemes and some photos of the integration and tests during integration.
Fig. 9 shows a measurement of the beam position on the Photodiode when the steering mirror driving over the full field of regard and a measurement of the tilt to length coupling verifying that the compensation plate reduces the coupling by several orders of magnitude.
Table 2 summarizes the achieved results of the key and driving optical requirements. Apart from the about 16 % too small beam diameter out of the FIA, all requirements are fulfilled with the EM. The ranging noise and the rotational coupling factors are currently based on analysis. The corresponding tests as well as the vibration and shock tests will be performed in the next months.
test results of the key and driving optical design requirement for the optical bench
|Parameter||Requirement OBA||Test/analysis result|
|Beam Diameter (4sigma)||5 +/- 0.5 mm||4.3 mm|
|Free Aperture||> 3 x beam diameter||> 3 x beam diameter|
|Beam propagation Factor M2||< 1.2||M2x=1.16M2y=1.15|
|Wavefront planarity (1/e2)||< λ/12||< λ/14|
|Beam coalignment error||< 10 µrad||α =1.1 µrad (yaw)β=0.7 µrad (pitch)|
|Field of regard (1/e2)||>4.5 mrad||> 4.5 mrad (will be 8 mrad for PFM)|
|Beam walk on QPR (over FoR)||< +/-10 µm (over 2 mrad)||< +/- 3 µm (over 4.5 mrad)|
|Ranging noise||< 5 nm/√Hz x NSF(f)||< 1 nm/√Hz x NSF(f)*|
|Rotational coupling factor||< 80 µm/rad (in pitch and yaw)||Yaw: <20 µm/rad (< 42 µm/rad*) Pitch: <30 µm/rad (<23 µm/rad*|
|Pointing stability||< 15 µrad over one orbit||Yaw: <2.41 µrad**Pitch: < 1.31 µrad**|
measurement supported by analysis, without OBA isostatic mounts
In addition to the requirements listed below the heterodyne efficiency of the optical bench is a direct measure to check the quality of the optical and electrical system. The heterodyne efficiency γhet is defined as:
With Pmax, Pmin being the maximum respectively minimum power detected in the coherent superposition of the two laser sources with individual powers P1, P2. The theoretical maximum of the heterodyne efficiency of the superposition of an 8 mm tophat beam (the received beam) and a 4.3 mm Gaussian beam (the local oscillator) on the used quadrant photodiode with the same power (P1=P2) is γhet,id=0.54. A homodyne measurement with one single frequency laser signal split and applied to the entrance aperture as a 50 mm diameter Gaussian beam and to the FIA has been performed and a γhet,meas=0.56 has been achieved in average over the 4 segments. That the measured value is actually slightly higher than the theoretically expected value is attributed to the non-perfect representation of a tophat beam by the 50 mm Gaussian beam and minor deviations from the perfect Gaussian beam out of the FIA. Nonetheless the result shows that the optical bench system is basically achieving the maximum signal efficiency.
The design of the optical bench for the laser ranging interferometer on GRACE FO and test results of the key and driving optical requirements achieved with the already nearly flight like engineering model have been presented. Apart from the slightly too small beam diameter, which will be adapted in the PFM design, all requirements have been met. The engineering model is currently used in further tests on subsystem level and in combination with the LRP provided by JPL/NASA for risk mitigation for the Protoflight Model(PFM) in development. The formal qualification will happen with the PFM begin of 2015.
As part of the development a new highly stable fibre collimator (here called fibre injector) has been developed that has the potential to be adapted to a variety of other systems with the demand for a highly thermally stable diffraction limited output beam with excellent wavefront planarity. Furthermore a low noise quadrant photoreceiver and low stress optical mounting technology has been developed and is available for future projects.
The LRI on GRACE FO is expected to successfully demonstrate a significantly reduced ranging noise compared to the microwave instrument and to enable even higher accuracy measurements of Earth’s gravity field in future missions such as GRACE 2 or NGGM.
The development and implementation of the German contributions to the GRACE-FO Laser Ranging Interferometer (LRI) is funded (FKZ 03F0654) by the German Ministry for Education and Research (BMBF) and includes in kind contributions of the German Aerospace Center (DLR).
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