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P. Pfeiffer* L. Perret** N. Schuhler ***

Instrumentation Procédés Photoniques. European Southern Observatory. Absolute distance metrology: - sweeping wavelength - frequency comb referenced 2 l interferometric system. P. Pfeiffer* L. Perret** N. Schuhler ***. * Université de Strasbourg ** Université de Strasbourg Sagem

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P. Pfeiffer* L. Perret** N. Schuhler ***

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  1. Instrumentation Procédés Photoniques European Southern Observatory Absolute distance metrology:- sweeping wavelength- frequency comb referenced 2l interferometric system P. Pfeiffer* L. Perret** N. Schuhler*** • * Université de Strasbourg • ** Université de Strasbourg Sagem • *** Europeen Southern Observatory

  2. Outline • Wavelength sweepingAbsoluteDistanceMetrology • Signal processing • Tunable laser source • Non-linearities of the tuning speed

  3. ADM with wavelength sweeping • Distance : 0 - 30m • 2 or more targets simultaneously • Accuracy, resolution: some ppm • Portable • 10 maesurements per second • Cost N. Pfeiffer L. Perret UdS

  4. Experimental Setup Object Interferometer Target A Tunable Laser SC PDmeas PDref ISO a sweeping speed Reference Interferometer Target B

  5. Tunable wavelength laser • External Cavity Laser Diode • Coherence length >> 1km • Central wavelength ~ 1.5µm • Continuous tuning range up to ~ 5nm • Sweeping speed up to 40nm/s • Large ranges and high sweeping speeds without mode hopping to reduce error magnifications. N. Pfeiffer L. Perret UdS

  6. Tunable laser source xt xl Réseau ro Diode Laser qi na Lentille M' q-1 Miroir • Externalcavity laser diode: • LittmanMetcalf configuration • LittmanShoshan configuration N. Pfeiffer L. Perret UdS

  7. Fringe processing • Autoregressive method • Frequency resolution for N samples: N- 3/2 • AR Burg method • Sensitive to non-linearities of the the sweeping speed • Fourier Transform technique • Eliminates low frequencies like drifts N. Pfeiffer L. Perret UdS

  8. Fringe processing 1 I(t) = a(t)+b(t) cos(f(t)) I(t) = a(t)+1/2[b(t) e if(t)+ b*(t) e -if(t)] Blackman window B(f-fs) A(f) 2 B*(-(f+fs)) Fast Fourier Transform 3 4 Spectral filtering Gaussian filter Inverse Fourier Transform 1/2[b(t) e if(t)] 5 Extraction of the instantaneous frequency N. Pfeiffer L. Perret UdS

  9. FTT results for 1017 samples Target A at 2.2m Target B at 8m 6 records/pos. sweeping speed 20nm/s. 9

  10. Non-linearities in wavelength sweeping Variations in fringes size Spectral modulation Results in an overlap of spectral peaks in the multi-target configuration.

  11. Extracted instantaneous beat frequency Sweeping speed

  12. Quasi-periodical variation of the beat frequency. FFT analysis and reconstruction through sinusoidal signals. • mf : modulation rate • Ai : component’s weight (normalized) • fmi : component’s frequency • φi : component’s dephasage Modeling parameters:

  13. Periodical non-linear influence Simulation of different wavelength sweeps Linear sweep 10nm/s model (5 components) + Single sinusoid : mf=2.2e-4 fm=94.5Hz Single sinusoid : mf /2 fm /2 Single sinusoid : mf x2 fm x2 Optimal sinusoidal modulation

  14. Averaging of the instantaneous frequency ratio minimizes errors due to FFT limited resolution. However, modulation still introduces peak overlapping in a multi-target configuration… • Reduces by a factor 20 the mean error (increases precision) • Reduces by a factor 1000 the error dispersion (increases resolution) … compared to a linear sweep. N. Pfeiffer L. Perret UdS

  15. Frequency comb referenced two wavelength interferometry N. Schuhler ADM Laser system form the VLT at Paranal European Southern Observatory

  16. Outline • Frequency comb stabilized 2 wavelength laser interferometry for ADM • Absolute frequency stabilization of PRIMET Nd:YAG laser • Two wavelength laser source • Calibration of the system

  17. Phased Reference Imaging and Micro-arcsecond Astrometry facility DOPD I OPD 2 objects generate 2 fringe patterns related through: where: • B is the baseline; • DS the angular separation of the two objects; • A noise due to the atmosphere; • F phase which depends on the nature of the object (0 for a point like source); • DL instrumental noise (vibrations, internal turbulence). N. Schuhler ESO

  18. Specifications The detection of Exo-planet with PRIMA in astrometric mode requires 10 mas accuracy over several years. Observable: differential optical path difference between to Michelson interferometers, DOPD • Propagation distance:<500 m • OPL for an interferometer: <250 m • Maximum DOPD:60 mm • Accuracy:5 nm (relative accuracy ~ 10-8) • Resolution:1 nm • Measurement:time<30 min • Sampling frequency:>8 kHz N. Schuhler ESO

  19. Proposed solution • Incremental interferometry for the ultimate resolution • 2 wavelength interferometry for increasing the NAR

  20. Architecture Two heterodyne interferometers : • Nd:YAG laser at l = 1.319 mm; • Frequency shifting by Acousto-Optic Modulators; • Electronic differential phase measurement (superheterodyne phasemeter) (IMP Neuchatel)

  21. Error and Non Ambiguity Range Differential OPD measured: Error on DOPD due to the wavelength uncertainty: Non Ambiguity Range: m: fringe order M: fringe number f(m): fractional part F: phase (-p<F<p) N. Schuhler ESO

  22. Stabilization of the Nd:YAG PI CNA + Pound-Drever-Hall method applied to a frequency doubled Nd:YAG, the frequency reference is an I2 transition at 659.5nm To the interferometers 75% Nd:YAG I2 EOM PPLN Lock-in Amplifier T 25% Pz CNA PI CAN P(49)6-6 N. Schuhler ESO

  23. Residual error in closed loop N. Schuhler ESO

  24. Measurements with an optical frequency comb • Self-referenced optical frequency comb based on a fibered fs pulsed laser at the • Max Planck Institute for Quantum Optics (MPQ Munich, Germany) • Provides thousand of modes separated by 100 MHz over one octave (1mm -2mm) • Reference radio frequency signal (10 MHz) derived from a cesium atomic clock • Relative inaccuracy on the frequency of one mode of the comb < 10-12 • Frequency of Nd:YAG is deduced from the beat signal with one mode of the comb I(n) nNd:YAG nrep nnr+n0 2nnr +n0 n0 2(nnr+n0) n0 N. Schuhler ESO

  25. Measurements with an optical frequency comb (3) • Peak-to-valley = 1.45 MHz • Standard deviation = 226 kHz The discrepancy is due to: • the error in the calibration of the error signal; • detection noise. N. Schuhler ESO

  26. Absolute frequency stabilization of PRIMET Nd:YAG laserConclusion Use of the temperature of the laser cavity to enable long-term (weeks) locking; Full automation of the laser frequency stabilization; Accurate characterization of the system performance by the use of a self-referenced optical frequency comb (with the help of MPQ) as an independent sensor : locking frequency n0 = 227 257 330 623 020 Hz ± 94 kHz; frequency noise (rms) over bandwidth 5 mHz- 8 kHz : s<2.27 MHz (PRIMET specifications); Demonstration that the system performance are limited by detection noise; Demonstration that the laser frequency cannot be calibrated with an accuracy better than 10-8 by comparison with a commercial HP interferometer The system will be tested in Paranal with a self-referenced frequency comb from Menlo Systems. N. Schuhler ESO

  27. Principle of two-wavelength interferometry Multiple-wavelength interferometry (Benoit 1895) with the excess fraction method Synthetic wavelength technique for two-wavelength laser interferometry (Wyant in 1971) A Michelson interferometer is used with two wavelength simultaneously: L is the synthetic wavelength The NAR of the system is L/2 L ≈ 90 µm ↔Dl ≈ 20 nm N. Schuhler ESO

  28. Architecture of the source Absolute frequency stabilization System l1 Beat detection + PLL Comb modes nrep fs laser (with stabilized repetition rate) Beat detection + PLL n n2=c/l2 n1=c/l1 l2 ECLD tunable Dn=c/n=n2-n1=Nnrep to the interferometer Two lasers can be stabilized on different modes of the comb to generate a custom and highly stable synthetic wavelength: mm < L < m dn/n < reference radio signal (10-12 GPS based clock) to the interferometer N. Schuhler ESO

  29. Architecture of the prototype PLL Fs-laser TC 1500 Menlo Systems BD 1319 ± 2.5 nm 1300 ± 2.5 nm PLL BD gratings ECLD Thorlabs Intun 1300 n2 1.300 mm +40.65MHz n1 AOM n2 -40MHz n1 + 450 kHz AOM n2 + 650 kHz 1.319 mm AOM Nd:YAG Lightwave 125 +40.45MHz n1 10 MHz source with accuracy < 10-11 N. Schuhler ESO

  30. Performances of the prototype Signal Mean frequency Instability (peak-to-valley) Relative instability Repetition rate 100 MHz 1 mHz 10-11 Beat signal Nd:YAG/comb 20 MHz 10 mHz 0.5×10-10 Beat signal ECLD/Comb 20 MHz 1 Hz 0.5×10-7 nECLD-nNd:YAG ~3.3 THz 35 Hz 10-11 Dn=N×nrep~3.3THz nrep=100MHz Nd:YAG ECLD n fb=20MHz fb=20MHz The relative stability of the synthetic wavelength in vacuum is 10-11. N. Schuhler ESO

  31. Set-up for the calibration of L in air corner cube Reference Interferometer PBS lref=0.633 mm Translation stage probe Phasemeter LP reference BS 2-wavelength Light source l1=1.319 mm l2~1.30 mm N. Schuhler ESO

  32. Result of the calibration of L Slope=139.541582 rad/mm L=90.054666 mm Taking into account the dispersion: Dn=3.32899949 ±0.00000067 Thz N= 33290 modes of the comb Residuals: sF=22 mrad=2p/285 sOPD=160 nm<l1/2 N. Schuhler ESO

  33. Merci de votre attention

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