Superheterodyne Laser Metrology for the Very Large Telescope Interferometer (VLTI) Y. Salvadé, R. Dändliker Institute of Microtechnology, University of Neuchâtel, Switzerland S. Lévêque European Southern Observatory, Garching bei München, Germany IMT, University of Neuchâtel European Southern Observatory
Table of contents ➢ Short description of VLTI-PRIMA ➢ Metrology requirements ➢ Superheterodyne metrology ➢ Phase-meter prototype ➢ Test of accuracy ➢ Foreseen tests at the VLT observatory ➢ Conclusions IMT, University of Neuchâtel European Southern Observatory
Very Large Telescope Interferometer (VLTI) ➢ Four 8-m Unit Telescopes (UT) ➢ Three moveable 1.8-m Auxiliary Telescopes (AT) IMT, University of Neuchâtel European Southern Observatory
Phase-referenced imaging and µas astrometry (PRIMA) R S Ref. star R, θ R ➢ Goals Science star S, θ S T 2 ❏ Observation and imaging of faint Baseline B objects Telescope T 1 ❏ Micro-arcsecond astrometry Star Sep. SS1 SS2 ➢ Principle DL2 Delay Line DL1 ❏ Bright star as reference star (fringe tracking) Reference Beam ❏ Laser metrology for controlling Combiner (PRIMA) OPL R1 OPL R2 internal optical path lengths ❏ Angular separation of the two OPL S1 OPL S2 objects: Diff. Delay Line • OPD R – OPD S = ∆ S B + ∆ L DDL2 DDL1 Science Beam Combiner (AMBER, MIDI, PRIMA camera) IMT, University of Neuchâtel European Southern Observatory
PRIMA metrology - requirements Range a nda c cura c y Ma x . pro p a g at i o n p a t h (re t u rn wa y ) 550m In d i v id u a l O PDL 1 , L 2 (return wa y ) 240m l OPD, ∆ L (1 arcm i n 60m m Differen t i a ) ∆ L (µ a Ac c u rac y o n s a c c u rac y ) < 5 n m ∆ L R e so l u ti o n o n < 1 n m e variations ( λ λ λ λ = 1 µ m Expected dynamic pha s ) on indi v idua lOPD T y pi c a l v a lue Trac k i n g of DL & STS ( ∂ L ∂ t = 11 m m /s) 22 k H z Variab l e c u rv a ture mirror a b o u t 4 k H z on differ e nti a l OPD of DDL & STS ( ∂∆ L ∂ t Trac k i n g 20 H z Sl e wi n g of DDL & STS ( ∂∆ L ∂ t 30 k H z IMT, University of Neuchâtel European Southern Observatory
PRIMA metrology - additional requirements ➢ Laser source ❏ Coherence length: > 500 m ❏ Frequency stability: < 10 -8 (same laser is used for both interferometers) ❏ Wavelength between 1.1 µm (bandgap of Si) and 1.45 µm (H band), to avoid straylight on existing stellar detectors ➠ Frequency stabilized Nd:YAG laser @ 1.319 µm (to be developed) ➢ Phase detection technique ❏ High-resolution technique (2 π /660 phase resolution) ❏ Suppression of crosstalks between reference and science channels (Calibration mode: Star separator inject the same star in both channels) ➟ Two heterodyne interferometers: ➤ Different heterodyne frequencies f 1 and f 2 ➤ Frequency offset ∆ν between the two interferometers IMT, University of Neuchâtel European Southern Observatory
Heterodyne interferometers Telescope 1 Telescope 2 Retroreflectors on secondary mirrror DL ν+∆ν+ f2 ν+∆ν Reference object PRIMARY FSU DDL I Frequency shifter module ν+ f1 2 ν Science object Secondary FSU Frequency shifter LASER I module 1 Nd:YAG, 1.3 µm ❏ Interference signals: I 1 (t) = cos(2 π f 1 t + φ 1 ) I 2 (t) = cos(2 π f 2 t + φ 2 ) φ 1 = 4 π φ 2 = 4 π c ν L 1 c ( ν+ ∆ν )L 2 IMT, University of Neuchâtel European Southern Observatory
Superheterodyne detection ➢ Electronic mixing + low-pass filtering [ ] I mes (t) = I 12 cos 2 π (f 1 − f 2 )t + φ 1 − φ 2 φ 1 − φ 2 = 4 π ν ∆ L − 4 π ∆ν L 2 photodetectors c c f1 ≈ 4 π ν f1–f2 ∆ L c f2 φ 1 – φ 2 ➢ Advantages f1 f1–f2 ❏ Direct access to ∆ L ❏ Slower phase variations f2 → enable longer integration times ❏ Phase noise less important IMT, University of Neuchâtel European Southern Observatory
Frequency shifters ➢ Fiber pigtailed acousto-optic modulators (IntraAction Corp.) AOM3 AOM4 +39.35 MHz +40 MHz AOM1 AOM2 –38 MHz –38.45 MHz PM fiber couplers Laser Nd:YAG ❏ Heterodyne frequencies: f 1 = 650 kHz and f 2 = 450 kHz ❏ Frequency offset: ∆ν = 78 MHz IMT, University of Neuchâtel European Southern Observatory
Electronic prototype ➢ VME boards ❏ Low-noise photodetectors + preamplifiers • Sensitivity of 0.9 V/µW • NEP of 0.2 pW/Hz 0.5 ➟ Required optical power: 10 nW ❏ Superheterodyne modules ❏ Limiting amplifiers ❏ Digital phase-meter • Zero-crossing phasemeter • On board averaging capability IMT, University of Neuchâtel European Southern Observatory
Superheterodyne modules ➢ Superheterodyne modules f 1 ❏ Input bandpass filters • 450 kHz and 650 kHz f 1 – f 2 • Bandwidth > 50 kHz • Minimized phase shifts (!) f 2 ❏ Ouput bandpass filters • 200 kHz • 50 kHz bandwidth IMT, University of Neuchâtel European Southern Observatory
Digital phasemeter ➢ Digital zero-crossing ❏ FPGAs (Altera) to measure the « instantaneous » phase and the number of 2 π cycles ❏ On-board averaging (Average over 2 n periods) ❏ PLL to generate a clock frequency of 200 MHz Fractional fringe ➨ 2 π /1000 phase resolution counter A Output Reference limiting Start ampl. Integer fringe counter Stop Clock Error (quadrature 1 Probe limiting counter) instantaneous integer ampl. Output 0 number i Fractional fringe -1 counter -2 Output B Phase 4000 Start Quadrant shifter detector Stop Clock 3000 PLL xN 1 Timebase signa l instantaneous fractional number f 2000 1000 Adder Summed integer i I number I 0 f Summation start Summed fractional F 0 100 200 300 400 500 600 700 number F Summation stop Phase [deg] IMT, University of Neuchâtel European Southern Observatory
Test of accuracy ➢ Two-wavelength interferometry I(t) = a 1 cos(2 π f 1 t+ φ 1 ) + a 2 cos(2 π f 2 t+ φ 2 ) ❏ Interference signals: φ 1 – φ 2 = 4 π ( ν 2 – ν 1 )L/c ➨ Reduced sensitivity: IMT, University of Neuchâtel European Southern Observatory
Results ➢ Two-wavelength interferometry ❏ ν 2 – ν 1 = 1.5 GHz ➙ Λ = 200 mm (stability of 10 –5 ) ❏ Required mechanical stability > 100 µm ❏ Measured accuracy: • Standard deviation of 2p/300 Error [digit] • Corresponding to 2.3 nm accuracy 5 0 ❏ Bandwidth: 50 kHz -5 ❏ Optical power: 100 nW 600 ❏ Improvement by averaging over several periods 400 Digital Output 200 0 -200 0 20 40 60 80 Distance [mm] IMT, University of Neuchâtel European Southern Observatory
PRIMA metrology - Test Campain at Paranal- Q1 2002 ➢ Main Objectives ❏ Quantify the influence of environmental parameters (OPD and Tilt Disturbance) ❏ Quantify the influence of the VLTI optical train (transmission, polarization) ❏ Determine straylight levels ❏ Retro-fit results to the Design of the PRIMA metrology system. ➢ Infrastructure ❏ VLTI Instrument “VINCI” for injection in the stellar path ❏ full VLTI optical train up to Retro-reflectors mounted on 2 UT ’s (optical path ≈ 350m) Picture of VINCI Instrument (Courtesy of P. Kervella) IMT, University of Neuchâtel European Southern Observatory
Conclusion ➢ Concept based on superheterodyne detection for PRIMA ➢ Electronic prototype: ❏ Manufacture and preliminary tests ❏ Accuracy better than 5 nm for optical power of 100 nW and 50 kHz bandwidth • Good hopes to improve this performance ❏ Suitable for two-wavelength interferometry (absolute distance measurement) ➢ Next step: full scale tests at the VLTI ❏ Retro-fit results to the Design of the PRIMA metrology system. IMT, University of Neuchâtel European Southern Observatory
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