Superheterodyne Laser Metrology for the Very Large Telescope - - PowerPoint PPT Presentation

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Superheterodyne Laser Metrology for the Very Large Telescope - - PowerPoint PPT Presentation

Sep 22, 2022 193 likes •363 views

Superheterodyne Laser Metrology for the Very Large Telescope Interferometer (VLTI) Y. Salvad, R. Dndliker Institute of Microtechnology, University of Neuchtel, Switzerland S. Lvque European Southern Observatory, Garching bei

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IMT, University of Neuchâtel European Southern Observatory

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

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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

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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)

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IMT, University of Neuchâtel European Southern Observatory

Phase-referenced imaging and µas astrometry (PRIMA)

➢ Goals

❏ Observation and imaging of faint

  • bjects

❏ Micro-arcsecond astrometry

➢ Principle

❏ Bright star as reference star

(fringe tracking)

❏ Laser metrology for controlling

internal optical path lengths

❏ Angular separation of the two

  • bjects:
  • OPDR – OPDS = ∆S B + ∆L

Science star S, θS

  • Ref. star R, θR

R S Telescope T1 T2 Star Sep. SS1 SS2 Delay Line DL1 DL2 DDL2

  • Diff. Delay Line

DDL1 Reference Beam Combiner (PRIMA) Science Beam Combiner (AMBER, MIDI, PRIMA camera) OPLR1 OPLR2 OPLS1 OPLS2 Baseline B

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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 PDL1, L

2(return wa y

) 240m Differen t i a l OPD, ∆L (1 arcm i n ) 60m m Ac c u rac y o n ∆L (µ a s a c c u rac y ) < 5 n m R e so l u ti o n o n ∆L < 1 n m Expected dynamic pha s e variations (λ λ λ λ = 1 µ m )

  • n indi v

idua lOPD T y pi c a l v a lue Trac k i n g

  • f 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

  • n differ e

nti a l OPD Trac k i n g

  • f DDL & STS (∂∆L ∂t

20 H z Sl e wi n g

  • f DDL & STS (∂∆L ∂t

30 k H z

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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 (2π/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 f1 and f2 ➤Frequency offset ∆ν between the two interferometers

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IMT, University of Neuchâtel European Southern Observatory

Heterodyne interferometers

DL DDL I 2 Telescope 1 Telescope 2

Frequency shifter module

Secondary FSU

I 1

Reference object Science object PRIMARY FSU

Nd:YAG, 1.3 µm ν ν+f1 ν+∆ν+f2 ν+∆ν

Retroreflectors on secondary mirrror

Frequency shifter module

LASER

φ1 = 4π c ν L1 φ2 = 4π c (ν+ ∆ν)L2

❏ Interference signals: I1(t) = cos(2πf1t + φ1)

I2(t) = cos(2πf2t + φ2)

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IMT, University of Neuchâtel European Southern Observatory

Superheterodyne detection

➢ Electronic mixing + low-pass filtering ➢ Advantages

❏ Direct access to ∆L ❏ Slower phase variations

→ enable longer integration times

❏ Phase noise less important

φ1 – φ2 f1 f2 f1 f2 f1–f2

photodetectors

f1–f2

Imes(t) = I12 cos 2π(f1 − f2)t + φ1 − φ2

[ ]

φ1 − φ2 = 4π ν c ∆L − 4π ∆ν c L2 ≈ 4π ν c ∆L

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IMT, University of Neuchâtel European Southern Observatory

Frequency shifters

➢ Fiber pigtailed acousto-optic modulators (IntraAction Corp.)

❏ Heterodyne frequencies: f1 = 650 kHz and f2 = 450 kHz ❏ Frequency offset: ∆ν = 78 MHz

AOM3 AOM4 Laser Nd:YAG +40 MHz +39.35 MHz PM fiber couplers AOM1 AOM2 –38 MHz –38.45 MHz

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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/Hz0.5

➟Required optical power: 10 nW

❏ Superheterodyne modules ❏ Limiting amplifiers ❏ Digital phase-meter

  • Zero-crossing phasemeter
  • On board averaging capability
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IMT, University of Neuchâtel European Southern Observatory

Superheterodyne modules

➢ Superheterodyne modules

❏ Input bandpass filters

  • 450 kHz and 650 kHz
  • Bandwidth > 50 kHz
  • Minimized phase shifts (!)

❏ Ouput bandpass filters

  • 200 kHz
  • 50 kHz bandwidth

f1 f2 f1 – f2

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IMT, University of Neuchâtel European Southern Observatory

Digital phasemeter

➢ Digital zero-crossing

❏ FPGAs (Altera) to measure the « instantaneous » phase and the number of 2π cycles ❏ On-board averaging (Average over 2n periods) ❏ PLL to generate a clock frequency of 200 MHz ➨ 2π/1000 phase resolution

Reference Probe Fractional fringe counter Start Stop Clock Output

xN1

Timebase signal PLL Adder i f I F Summation start Summation stop Summed fractional number F Summed integer number I Fractional fringe counter Start Stop Clock Output

Phase shifter

Integer fringe counter (quadrature counter) Output

A

instantaneous integer number i instantaneous fractional number f Quadrant detector

B

limiting ampl. limiting ampl.

4000 3000 2000 1000 700 600 500 400 300 200 100 Phase [deg]

  • 2
  • 1

1

Error

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IMT, University of Neuchâtel European Southern Observatory

Test of accuracy

➢ Two-wavelength interferometry

❏ Interference signals:

I(t) = a1cos(2πf1t+φ1) + a2cos(2πf2t+φ2)

➨ Reduced sensitivity:

φ1 – φ2 = 4π(ν2 – ν1)L/c

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IMT, University of Neuchâtel European Southern Observatory

Results

➢ Two-wavelength interferometry

❏ ν2 – ν1 = 1.5 GHz ➙ Λ = 200 mm

(stability of 10–5)

❏ Required mechanical stability > 100 µm ❏ Measured accuracy:

  • Standard deviation of 2p/300
  • Corresponding to 2.3 nm accuracy

❏ Bandwidth: 50 kHz ❏ Optical power: 100 nW ❏ Improvement by averaging

  • ver several periods

600 400 200

  • 200

Digital Output

80 60 40 20

Distance [mm]

5

  • 5

Error [digit]

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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)

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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.