COATS: compact optical 5DoF attitude sensor for space applications

COATS: compact optical 5DoF attitude sensor for space applications M. Pisani 1, M. Zucco 1 and S. Mottini 2 1 Istituto Nazionale di Ricerca Metrologica, INRIM 2 Thales Alenia Space-Italia, Torino, Italy ICSO, Biarritz 20 October 2016

Why attitude sensors? Missions with remote instruments (e.g. magnetometers) Magsat 2

Why attitude sensors? ( ) Ørsted Astrid-2 Messenger CHAMP MOLA 3

Why attitude sensors? Mission with remote detectors Gamma-Ray Imager (Formation Flying) IXO Athena 4

Why attitude sensors? Missions with large or distributed instruments (e.g. interferometers) SIM Lite SIM 5

Why attitude sensors? A synthetic aperture radar where the position of the two antennas must be known to the micrometer level 6

Aim of the work Compact Optical Attitude Transfer System COATS. ESA Contract 4000105051/11/NL/CP. Realization of a device to measure attitude and spatial co-ordinates of a body with respect to a principal co-ordinate system The device must be compact and lightweight and able to work at relatively large distances Wavemill metrology system requirements Parameter Accuracy Range Rate Comment Longitudinal displacement 7 m 5 cm 0.1 Hz Baseline: 2.5-7.5 m Lateral displacement (pitch) 5 m ±1 cm @ 6m 0.1 Hz Baseline: 2.5-7.5 m Most critical parameter Lateral displacement (yaw) 5 m ±1 cm @ 6m 0.1 Hz Baseline: 2.5-7.5 m 7

Conceptual Design 8

2D angular measurement A laser beam is reflected by a plane mirror in front of the passive target. A 2D Position Sensitive Detector placed in the focal plane of the lens measures the angle of the incoming beam. 9

2D lateral measurement X 2d x F A laser beam is laterally shifted by the displacement of the retro reflector. The shift is measured by a 2D Position Sensitive Detector placed out of the focal plane of the lens. 10

1D longitudinal measurement fixed mirror moving mirror Relative incremental Interferometer Absolute Interferometer Laser λ Laser fixed mirror L Detector λi Detector fixed mirror signal at the detector L = n λ/2 L = (N 1 + N 1 )λ 1 /2 L = (N 2 + N 2 )λ 2 /2 L = (N 3 + N 3 )λ 3 /2 L = (N 4 + N 4 )λ 4 /2 L = λ/2 Underdetermined system λ i laser wavelength, known variables N i decimal fraction of the fringe, known variables N i integer number of fringes, unknown variables

1D longitudinal measurement The distance information is now written in the phase of the synthetic frequency, fixed mirror Increasing the synthetic frequency (tens of GHz), decreases the distance resolution. λ 1 Laser Laser λ 2 Reference detector Measurement signal Reference signal Measurement detector example: synthetic frequency of 20 GHz; synthetic fringe of 7.5 mm; distance resolution of 7.5/1000 mm = 7.5 µm Since it is difficult to detect and condition signal at tens of GHz we have implemented the superheterodyne detection scheme that down-converts the signal at the khz level. f synthetic = f 2 f 2 = c/f synthetic = c f synthetic = λ 1 λ 2 λ 1 λ 2

1D longitudinal measurement D1 Offset frequency D3 Laser 1 D2 S2 Laser 2 φ Phase Simplified schematic of the synthetic wavelength interferometer 13

PI 20 MHz 1/40 7<f<20 GHz fsynthesizer Synthesizer 10 MHz Time Base SA f 1 v 1 D v 1 -ν 2 0<f<20 GHz telescope corner cube v 1 L1 FI S 90% 10% AOM C v 1, ν 2 50% S 10% v 1, ν 2 v 1 -ν 2 =f synthesizer L2 FI ν 2 S ν v 1 +f 1 90% 2 v 1, ν 2 v 1, v 2 v AOM C 50% 1 +f 1, v 2 +f 2 10% v 2 +f 2 50% L v 1 +f 1, v 2 +f 2 L1, L2 EC Diode Laser FI Faraday Isolator S Splitter 90-10% C D AOM n 1, n 2 f 1 = 80 MHz f 2 =80 MHz +, Fiber Coupler Detector AcustoOptic Modulator Laser frequency = 120 khz f 2 L = I = A cos 2π f 2 f 1 t = A cos 2π t c 4π (f synth +800MHz) Massimo Zucco, Final meeting, ESA-ESTEC,17/03/2014 C D C D f 1, f 2 f 1, f 2 f 1 - f 2 f 1 - f 2 DAC PC I = A cos(2π t + ) Mixers

Practical realization: combining the three metrology systems 15 Massimo Zucco, Final meeting, ESA-ESTEC,17/03/2014

Distance and lateral metrology layout 2D PSD X 2X 16 Massimo Zucco, Final meeting, ESA-ESTEC,17/03/2014

Angular metrology layout 2D PSD 17 Massimo Zucco, Final meeting, ESA-ESTEC,17/03/2014

Combining the three systems Beams of the three metrologic systems are separated and recombined by means of dichroic mirrors and share the same output lens Lens are AR coated for the three wavelengths (1550, 852, 785 nm) 18

Dichroic filters and AR coatings AR coating 19

The passive target + 20

Practical realization of the optical head 21

ì 22

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Optical Head assembled 24

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Optical Head assembled 26

PSDs and fiber launchers Detail of the lateral PSD with frontend electronics attached on the back Lateral and angular fiber launchers and angular PSD 27

Mass and dimensions Total mass of the complete OH: 938 g 28

Optical power budget 100 uw 1 mw 0.5 mw 10 uw 15 mw 29

Laser and fiber assembly Amplifiers AOM drivers Lasers Photodiode AOMs Splitter Circulator 30

PERFORMANCE TEST 31

Interferometer test set-up Retro reflector on a 28 m long rail Corner-cube positioned at 0m, 5m, 10 m, 15 m, 20 m, 25 m on the rail, the distance of the IR interferometer with synthetic frequency 20 GHz is compared with the reference displacement measured with our calibrated interferometer. The displacement are compared after the correction of the index of refraction of air

Test results 40 - accuracy limited by the stability of the fiber interferometer (the fibers are not thermally isolated) - it takes 15 to move the retro-reflector from 0 m to 25 m, without loosing the fringe counting of the reference interferometer 20 difference ( m) 0-20 -40 0 5 10 15 20 25 distance (m)

Repeatability test SET-UP: fixed Corner cube at 62.25 m in a controlled environment standard deviation of 4 μm 5 synthetic wavelengths f = 20 860 000 000 GHz f = 20 841 730 000 GHz f = 20 803 430 000 GHz f = 20761730000 GHz f = 20711378000 GHz 1 set of measurement takes 5 minutes

76 m distance test

135 m distance test

Interferometer noise synthetic f 21 GHz, synthetic λ 14 mm 100 target at 0.5m CC at 78 m in air CC at 137 m in air noise limit 10 PSD( m/ Hz) 1 0.1 0.01 1E-3 0.01 0.1 1 10 100 1000 10000 frequency (Hz)

Lateral metrology test Setup: measurement on the long bench: the OH has been tested at various distances ranging from 0.5 to 7.5 m. The passive target is mounted on a precision motorized X-Y stage 2D MOTORIZED STAGE OPTICAL HEAD 38

Test set-up The retroreflector is moved with a precision x-y stage having micrometer resolution 39

Mapping of the whole area An area of 22x22 mm has been explored with a 1 mm step. The points out of the sensitive area have been eliminated. The circle has 1 cm radius 40

Repeatability test The same grid has bee repeated 8 consecutive times. The whole process takes long time and includes effects of mechanical and thermal drifts. 41

4 points repeatability test A square pattern having 1 mm to 10 mm side is repeated hundreds of times in different zones of the working area. The standard deviation of each point is calculated. 42

Long term repeatability test 4000 cycles in 10000 s. 43

Angular MetrologyTest Same set-up as for the lateral metrology The passive target is mounted on a precision tilter. A mirror fixed at the back of the PT is seen by a calibrated autocollimator TILT MOUNT OPTICAL HEAD 44

Angular metrology test set-up Back side of the passive target Autocollimator 45

Angular metrology test at 0.5 m X Y posx posy angx angy -976-992 -0,30175-0,14851 0,706694-0,63809-514 -977-0,15364-0,27348 0,398992-0,63526 0-960 -0,06768-0,35532 0,050359-0,63371 512-943 0,018239-0,21436-0,28151-0,63332 968-928 0,096326-0,23585-0,56762-0,62842 958-496 -0,052-0,48358-0,57965-0,34511 y = 0,0007x + 0,0067 angy 489-512 -0,00149-0,19766-0,28077-0,34758 R² = 0,9992-7 -529-0,05406-0,16359 0,8 0,040968-0,34942-504 -544-0,1337-0,48177 0,6 0,382841-0,34759-984 -559-0,21975-0,46171 0,703036-0,34897 0,4-1004 3-0,19691-0,42107 0,705768 0,027702-504 18-0,10727-0,22497 0,36958 0,026811 4 34-0,02973-0,08514 0 0,022044 0,02878-1500 496-1000 53 0,109505-500-0,11881 0-0,29977 500 0,030115-0,2 1007 70 0,039036-0,09862-0,6265 0,033218 1000 1500 994 502 0,113341-0,08118-0,4-0,63185 0,323113 513 486-0,03625-0,11662-0,3261 0,32307-0,6-4 470-0,05353-0,14024 0,013335 0,322427-504 457-0,11527-0,12077-0,8 0,360634 0,320284-995 439-0,17446-0,14026 0,689586 0,321959 angx 0,8 0,6 0,4 0,2-0,4-0,6-0,8 y = -0,0007x + 0,0324 R² = 0,9981 0-1500 -1000-500 -0,2 0 500 1000 1500 Calibration of the angle sensor over a square angle 1000 x 1000 arcsec At short distances the angular sensor is linear 46

Angular metrology test at 7.5 m Calibration curve at 7.5 m distance. Cubic nonlinearity is evident. Range is ± 350 arcsec 47

PSD (arcseconds/ Hz) PSD (micrometers/ Hz) Noise analysis Power Spectral Densities of lateral and angular sensors at short distances Noise spectral density (in arcsec/ Hz) of the angular sensor at short distance. Sub arcsec sensitivity is demonstrated. Noise spectral density (in µm/ Hz) of the lateral metrology. Sub micrometer sensitivity is demonstrated. 48

3DoF Combined Test A scan of 20x20 mm area in 25 points has been registered to show the capability of the interferometer to work in misaligned conditions. A series of four repeated points has been recorded in 3 dimensions (X, Y, Z) and the statistical dispersion of the data has been recorded. 49

distance (um) 3DoF repeatability test x-y repeatability z repeatability posx posy PUNTO1X PUNTO1Y PUNTO2X PUNTO2Y PUNTO3X PUNTO3Y PUNTO4X PUNTO4Y interf pos 1 pos 2 pos 3 pos 4 DEV ST (V) 0,0001 0,0002 0,0003 0,0004 0,0008 0,0004 0,0012 0,0003 DEV ST (um) 1,5 2,6 4,8 5,5 10,9 5,5 16,5 3,6 4,6 3,9 5,2 2,9 0,7885 0,0242-0,4627 0,0242-0,4627 2,6954 925,5018 0,0249 0,2218 0,0249 0,2218 0,8364 844,4937-0,6749 0,2213-0,6749 0,2213 1,3023 1148,997-0,6716-0,4703-0,6716-0,4703 0,754 852,1426 0,0244-0,4626 0,0244-0,4626 2,7087 926,5398 0,0247 0,222 posy 0,0247 0,222 1200 0,8422 838,9845-0,6755 0,2214 0,3-0,6755 0,2214 1,2375 1151,121-0,6713-0,4703 0,2 1100-0,6713-0,4703 0,693 853,0687 0,0244-0,4626 0,0244-0,4626 2,7173 916,1922 0,1 0,0248 0,2218 0,0248 0,2218 1000 0,8437 829,2438 0-0,6733 0,2204-0,6733 0,2204 1,2446 1152,494-0,8-0,7-0,6-0,5-0,4-0,3-0,2-0,1 0 0,1 900-0,6712-0,4702-0,1-0,6712-0,4702 0,7096 853,3083 0,0244-0,4625 0,0244-0,2-0,4625 800 2,6701 917,326 0,0249 0,2217 0,0249 0,2217 0,7728 831,8945-0,3-0,6754 0,2214-0,6754 0,2214 700 1,2597-0,4-0,6705-0,4701-0,6705-0,4701 0,6979 841,9866 1144,957 0,0244-0,4625 0,0244-0,5-0,4625 600 2,6737 919,7372 0 10 20 30 40 50 0,0244 0,2221-0,6 0,0244 0,2221 0,829 830,0262 # measurement -0,674 0,2206-0,674 0,2206 1,2733 1145,532-0,6703-0,4701-0,6703-0,4701 0,726 850,9609 0,0243-0,4624 0,0243-0,4624 2,6716 921,9089 Repeatability: <7 µm over 10 measurements 50

Conclusions and next steps A 5 DoF compact and lightweight optical sensor has been built and tested The specs have been tested at distances from 0.5 to 7.5 m in air The resolution achieved are within the required specs limited, at long distances, by the turbulence of air Next Work: Adaptation of the COATS optical metrology design to the OSCM mission metrology requirements OSCM 1-3 51

Thank you 52

Synthetic wavelength interferometry with super-heterodyne detection λ 1 Laser The distance information is now written in the phase of the synthetic frequency, Laser λ 2 Reference detector Measurement detector Increasing the synthetic frequency (tens of GHz), decreases the distance resolution. example: synthetic frequency of 20 GHz; Measurement signal synthetic fringe of 7.5 mm; distance resolution of 7.5/1000 mm = 7.5 µm Reference signal Since it is difficult to detect and condition signal at tens of GHz we have implemented the superheterodyne detection scheme that down-converts the signal at the khz level. f synthetic = f 2 f 1, = c f synthetic = λ 1 λ 2 λ 1 λ 2 Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

All-Fiber Synthetic-Wavelength Interferometer with super-heterodyne detection Timebase L1 ECDLs at 1.5 µm PM fibers L2 n 1 PLL AOM f 1 n 1 + f 1 n 2 AOM n 2 + f 2 n 2 - n 1 n 1, n 2 telescope corner cube L f 2 reference measurement synthetic frequency n 2 - n 1 up to 40 GHZ super-heterodyne frequency f 2 - f 1 = 120 khz f 2 -f 1 f 2 -f 1 DAC Phase measured with IQ demodulation PC L = (N + N) /2 Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

PLL for locking two lasers in frequency difference traceable to the SI metre PI 20 MHz 1/40 7<f<21 GHz Synthesizer 10 MHz Time Base SA 0<f<20 GHz 0.1<f<40 GHz L1 n 1 L2 n 2 n 2 -n 1 =Synthetic frequency = f synthesizer + 20 MHz x 40 = f synthesizer + 800 MHz Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Absolute Interferometry with a synthetic frequency scan without mode jumps scan of laser synthetic wavelength 1 2, = 1-2 scan of laser synthetic frequency n 1 n 2, n =n 1 - n 2 scan of interference fringes N 1 N 2, N= N 1 - N 2 Laser fixed mirror Detector L fixed mirror L = 1 2 N 1 2 = 1 N c 1 2 2 n δ L = 1 2 c n δ N + 1 c N 2 n 2 δ n λ N *not taking into account the index of refraction Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Realization of the prototype opto-electronic components in fibers polystyrene foam box not temperature controlled Launcher in air plus beam-expander Corner cube retro reflector x10 beam expander The expanded beam has a diameter of about 2 cm corresponding to a Rayleigh distance of about 400 m. Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Realization of the prototype Portable electronics with a crane low noise preamplifiers synthesizers replaced by DDS AD9854 boards Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Characterization of the longitudinal metrology in air in terms of: resolution, the target is displacement PSD < 1 μm/ Hz in the 300 Hz- 10 khz range accuracy, the target is displacement accuracy < 7 μm repeatability, the target is displacement repeatability < 7 μm laser stability, frequency fluctuations of free running laser smaller than 200 MHz synthetic frequency limited to 7 21 GHz due to the mixer and synthesizer Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Accuracy: calibration with a reference He-Ne interferometer SET-UP Retro reflector Corner cube Corner-cube positioned at 0m, 5m, 10 m, 15 m, 20 m, 25 m on the rail, the distance of the IR interferometer with synthetic frequency 20 GHz is compared with the reference displacement measured with our calibrated interferometer. The displacements are compared after the correction of the index of refraction of air Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Accuracy: calibration with a reference He-Ne interferometer 40 Results - accuracy limited by the stability of the fiber interferometer (the temperature of the fiber breadboard is not controlled) - the measurement are corrected for the index of refraction of the air - it takes 15 to move the retro-reflector from 0 m to 25 m, without loosing the fringe counting of the reference interferometer 20 difference ( m) 0-20 d = 1 ± 13 µm -40 0 5 10 15 20 25 distance (m) Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Resolution in terms of displacement PSD for synthetic f 20 GHz, synthetic λ 15 mm 100 target at 0.5m CC at 78 m in air CC at 137 m in air noise limit target PSD < 1 μm/ Hz in the 300 Hz- 10 khz range PSD( m/ Hz) 10 1 0.1 ESA requirement 0.01 Corner cube 1E-3 0.01 0.1 1 10 100 1000 10000 frequency (Hz) fiber length drift and seismic noise acoustic noise detection noise Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Resolution in terms of Allan Deviation of displacement for synthetic f 20 GHz, synthetic λ 15 mm Corner cube 100 noise limit cc at 78m (out) cc at 137 m (out) target at 0.5 m 10 ADEV( m) 1 0.1 0.1 1 10 100 1000 time (s) Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Repeatability in terms of dispersion of absolute distance measurement in air, d 76 m d=75 m Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Absolute distance measurement, 5 synthetic wavelengths. Two corner-cubes at two different positions f 1 =20 860 000 000 Hz f 2 =20 841 730 000 Hz f 3 =20 803 430 000 Hz f 4 =20 761 730 000 Hz f 5 =20 711 378 000 Hz 1 set of measurement measures 5 synthetic phases and it takes 5 minutes pos a pos b 76.253920 L L=(76 = (76.253 900 3) 3 ) µm µm absolute distance (m) 76.253900 dt = 0.1 C -> dl = 7.6 µm index of refraction changes by 10-6 per C L L = 10 6 T 76.253880 1 2 3 4 5 measurement Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Absolute distance measurement in air, d 137 m, window closed! Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Absolute distance measurement, 7 synthetic wavelengths. Two corner-cubes at two different positions pos a pos b 137.102610 137.102600 L=(137.102 102 563 20) ) µm f 1 =20 860 000 000 Hz f 2 =20 841 730 000 Hz f 3 =20 803 430 000 Hz f 4 =20 761 730 000 Hz f 5 =20 711 378 000 Hz f 6 =13 233 151 000 Hz f 7 = 7 765 073 000 Hz 1 set of measurement takes 5 minutes absolute distance (m) 137.102590 137.102580 137.102570 137.102560 137.102550 137.102540 137.102530 137.102520 dt = 0.1 C-> dl = 14 µm index of refraction changes by 10-6 per C 137.102510 0 1 2 3 4 5 6 7 measurement Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Comparison of air temperature measurements on a 76 m path using three different techniques for synthetic f 21 GHz, synthetic λ 14 mm T = 10 6 L L 2 h Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Laser stability, in terms of free running laser frequency fluctuations measured with an optical comb traceable to the SI second The synthetic frequency is locked but the average laser frequency is free-running Fluctuations of the Free running laser frequency PSD of free running laser frequency fluctuations Group refractive index (dotted lines) of silica 1MHz λ=1542 nm T= 0 C T= 200 C Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016 [1] M. Medhat et al., J. Opt. A: Pure Appl. Opt. 4, 174 (2002).

Absolute distance measurement in an underground corridor, L 260m d 260 m INRIM Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Absolute distance measurement in an underground corridor, L 260m d 250 m INRIM Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Absolute distance measurement over 260 m in an underground corridor d 260 m retroreflector dt = 0.1 C absolute distance (m) 258.421200 258.421100 258.421000 258.420900 d 250 m Absolute measurement Synthetic frequency scan of n = 10 GHz 258.420800 0 100 200 300 400 500 600 time (s) Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Absolute distance measurement over 520 m in an underground corridor d 260 m retroreflector mirror 513.766250 absolute distance (m) 513.766200 513.766150 513.766100 513.766050 d 250 m turbulence caused by people passing by 513.766000 INRIM -100 0 100 200 300 400 500 time (s) Absolute measurement Synthetic frequency scan of n = 10 GHz Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Uncertainty budget: Synthetic frequency u( n) 0 Fringe resolution,u L = 1 2 Cyclic errors < 5 µm u L = 1 2 c N u n 0 n2 c u N, u(l)=7 µm, n=20 GHz -> u( N)<10-3 n Free running stability u(n L )=2 MHz, u(l)<100 nm Temperature stability of the non compensated fiber length L=5m, with a u(l)=7 µm -> u(t)<10-1 C Limited by the index of refraction of air u(l) = 10 7 L Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016

Next steps: - added an internal optical switch to generate the reference signal - shorten the fiber cables - control the temperature of the fiber set-up - create a long vacuum setup to test the interferometer in vacuum Massimo Zucco, INRIM, MetroAeroSpace, Firenze, 23/06/2016