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A two-mode interference measurement for high accuracy absolute laser ranging in space Duy Hà PHUNG, Alain BRILLET, Michel LINTZ, ARTEMIS, Observatoire de la côte d'Azur, Nice Christophe ALEXANDRE CEDRIC-LAETITIA, CNAM, Paris M.Lintz, ICSO 2012 11/10/2012 michel.lintz @ oca.eu 1 Recent evolution in the field of laser ranging of long distances: the femtosecond laser frequency comb Coddington et al. Nature Phot. 3, 2008, 351 Optics Express 19 (2011), 18501 "Optical sampling" method (pulse interference) M.Lintz, ICSO 2012 11/10/2012 2 Recent evolution in the field of laser ranging of long distances: the femtosecond laser frequency comb S. van den Berg et al. PRL 108, 183901 (2012) "spectral (dispersive) interferometry" M.Lintz, ICSO 2012 11/10/2012 3 Yet we believe that femtosecond laser frequency combs are not mandatory for high accuracy laser ranging ! M.Lintz, ICSO 2012 11/10/2012 4 Naive idea Optical interference 1 cycle = opt= "non-ambiguity range" Larger path lengths are wrapped into [0, opt] Ideally: Combination of 3 Ideally: Combination of 2 measurements - optical interference (=> accuracy/resolution << opt ) - time-of-flight measurement - and "synthetic wavelength" measurement (E. Samain's proposal) using microwave-modulated optical source => Requirement: accuracy of the "synthetic phase" measurement better than opt/ =10-4 But 20 GHz phase measurements are NOT stable enough on the long term! => optical and microwave phase measurements are merged into 1 single measurement => two-mode interference measurement solves the problem time of flight 1km M.Lintz, ICSO 2012 11/10/2012 1m "synthetic wavelength" 1 cycle = = 15mm for frequency 20GHz. Larger path lengths are wrapped into [0, ] Optical interference ? 1mm long-term phase drifts in µw chains 1µm 1nm 1pm 5 Ranging with a simple set-up: Two-mode optical interference Meas 2-mode laser beam 20GHz beat Ref L, 1- l, l0 reference photodiode signal photodiode micro-wave modulated terms at F=c/ (= synthetic wavelength) : Meas Ref (1 ) cos 2 (ct L) / cos 2 (ct l ) / Ll 2 cos 2 ( L l ) / opt cos 2 (ct ) / 2 Int L scan opt scan segment "hedgehog" Int L = two-mode interference signal straight! (ideally!) Ref Meas phase M.Lintz, ICSO 2012 11/10/2012 2 ( L l0 ) / 2 ei 2 l0 / 6 The recorded two-mode interference signal synth =15mm opt =1.5µm 10.000 "spikes" ! target moved by /2 =7,5mm dead zones: acquisition is blocked (data transfer; slow link, replaced by TCP/IP link) Difficulties: - stray interference (<10-8 power) - 20 GHz signal measurement: accuracy, noise <10-4 - complex procedure M.Lintz, ICSO 2012 11/10/2012 7 Complex signal : How to deal with it ? If time-of-flight data are taken beforehand => (L-l) already known to better than 10-5 => 3 successive measurements of the 2-mode interference signal, at 1= opt, a 2= opt+ FSR/4 3= opt+ FSR/2 1 a2 a3 => extraction of all the parameters ! 2 3 = opt+FSR/2 synthetic phase is measured in 100 µs as a geometrical property (an angle) of the 2-mode signal => not affected any more by the long term phase drifts M.Lintz, ICSO 2012 11/10/2012 ½(synth. phase) <= 2 ( L l ) / 2 a3 a2 2 = opt+FSR/4 opt a1 phase 2 ( L l0 ) / 8 ToF: Non-linear shaping: S. Pitois (Univ. Dijon) Control: Etienne Samain (OCA) Set-up two-mode beat pulse train! 8km two-mode 1, 2 MI two-mode beat fast frequency update target L pol. contr. Glan 45° BS l PBS 20 GHz PhD0 PhD1 20 MHz complex procedure: Acquisition (phase & amplitude) M.Lintz, ICSO 2012 11/10/2012 FPGA-based electronics 9 Typical length measurements ( raw data ) ( slow target movement: 20µm in 10 minutes ) Raw data: considerable systematic effects! M.Lintz, ICSO 2012 11/10/2012 Origin: electronic distorsion of the signals - electronic cross-talk - saturation - amplitude-to-phase coupling => have to be corrected for 10 Typical length recording measurements data) " Iliade ": typical (target is(corrected moved 20µm in 10 minutes) ( slow target movement: 20µm in 10 minutes) 7,49278 measurement length (m) 7,49277 10µm with correction for electronic distorsion : much less systematics! 7,49276 7,49275 7,49274 10 mn 7,49273 M.Lintz, ICSO 2012 11/10/2012 11 Typical length measurements (target is moved 20µm in 10 minutes) 7,49278 measurement length (m) 10µm 1µm 7,49277 correct values 7,49276 7,49275 7,49274 2mn 7,49273 M.Lintz, ICSO 2012 11/10/2012 12 Typical length measurements (target is moved 20µm in 10 minutes) 1 measurement = 320 elementary measurements (1/130µs) 7,49278 measurement length (m) 7,49277 7.4926988829 m 10µm 10 nm 1µm 7.5 m path in air: noise @ 0.1s 10 nm 200nm 7,49276 10 ms 7,49275 Zoom x25 each measurement cycle (43ms) is a series of 320 10 elementary measurements over 130 µs nm 7,49274 7,49273 M.Lintz, ICSO 2012 11/10/2012 400pm interference blocked 2mn 5 s (=>noise only; no signal) nombre de mesures élémentaires 13 " Iliade ": typical recording (target is moved 20µm in 10 minutes) 1µm What's wrong with the data ? 25% of wrong values ( shifted by opt ) M.Lintz, ICSO 2012 11/10/2012 14 â3 â3 Electronic distorsion: instantaneous and retarded AM-to-PM â1 amplitude â1 60µs 60µs (data processing) (data processing) EM4 20 GHz photodiode â4 â5 â5 Electronic distorsion of theâ4signals â2 cross-talk ( 2x10-3 ) - electronic - easy to measure - easy to correct for => no real problem phase - amplitude-to-phase coupling optical power varies by up to X3 65 µs during the â1=>â2=>â3 sequence! Acq 0 opt scan Acq 15µs Acq Acq Acq â2 5µs depends on PhD bias voltage Acq 75µs 135µs "straight" segment... is curved! Acq Acq Acq Acq hysteresys ! retarded AM-to-PM ! instantaneous AM-to-PM amplitude 0.5 M.Lintz, ICSO 2012 11/10/2012 1 ratio time in µs 210µs amplitude 1.5 0.5 1 ratio 1.5 15 Electronic distorsion: retarded AM-to-PM, thermal origin Retarded AM-to-PM coupling: very likely of thermal origin power (VbiasX photocurrent) dissipated in the photodiode junction => several kW/cm² ! Retarded AM-to-PM is difficult to model and to correct for (it involves the whole history of the signal amplitude) opt scan "segment" 5µs 65µs amplitude 0.5 M.Lintz, ICSO 2012 11/10/2012 1 ratio amplitude 1.5 0.5 1 ratio 1.5 16 Length measurement results (target movement 2µm/min) with "slow acquisition" => solve retarded AM-to-PM coupling! <= µm L-7.492717m µm better convergence (94%) slow acquisition reduces retarded AM/PM M.Lintz, ICSO 2012 11/10/2012 17 How to reduce retarded AM-to-PM transfer? Characterization of AM/PM in the photodiodes ( Phung et al., submitted ) => depends on the applied voltage => depends on the photodiode (smaller in New Focus than in EM4 photodiodes) AM/PM also differs with differents amplifiers AM/PM larger when amplifier gain is larger? => still to be optimized M.Lintz, ICSO 2012 11/10/2012 18 Conclusion : Laser vs incoherent C. Courde et al.,ICSO 2010 two-mode interference Resolution measured (expected) @ 10s incoherent source obtained (expected) gain 103 10nm (1nm) 20nm (3nm) | @ 1s @ 0,1s 3nm (10pm) 40nm (10nm) @10ms 3nm (15pm) not yet measured @ 1ms 5nm (30pm) " @100µs 1nm (100pm) Pol. Controller? System simple Source 2 phase-locked CW lasers wideband+modulator Signature AOM polarization switch Procedure complex simple Measurt scheme M.Lintz, ICSO 2012 11/10/2012 OK ... with less retarded AM/PM! simple OK 19 Thanks to ... ANR " ILIADE " (grant 07-BLAN-0309-01), collab. with Etienne Samain (OCA, Nice), S. Pitois, C. Finot, J. Fatome (Univ. Dijon) CNES (R&T R-S06/SU-0001-013) PhD Grants: Thalès, région PACA and Univ. Nice-Sophia michel.lintz @ oca.eu M.Lintz, ICSO 2012 11/10/2012 20