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The Nova Fringe Tracker (NFT): a second generation cophasing facility

for up to six telescopes at the VLTI

Jeffrey A. Meisner, Sterrewacht Leiden

Walter J. Jaffe, Sterrewacht Leiden

Rudolf S. Le Poole, Sterrewacht Leiden & TNO Science & Industry

Local Switchyard

Incoming beams from M1 (pick-off)

Polarization Reversers

PRS M4mirrors

M5 mirrors PinholeMask

Spectral prismWollaston prismCamera lensDetector

NFT design (shown for 6 telescopes): simplicity!

Back end of NFT,Not sensitive to OPD, No visibility loss possible due to alignment etc.

Monolithic prism block of polarizing beam splitters

Concept for the NFT proposed to ESO in 2010 by the (former) NOVA consortium which included:

Outline of this talk: Fringe-tracking requirements at the VLTI and the Phase-A

studies towards a second-generation fringe-tracking facility, ESO's (non-)decision, and subsequent (in-)action

Overview of the NFT, its technological choices, and performance features

Strategies for the fringe-tracking control loop and simulation results based on the proposed NFT design.

The particular problem of “photometric crosstalk,” and the solution proposed using the Polarization-Based Collimated Beam Combiner topology: how the NFT works.

The road forward toward a 2nd generation fringe tracker at the VLTI (hopefully....)

So how did we get here?A timeline:

2009: ESO calls for Phase-A studies to propose a concept for a Second Generation Fringe Tracker for the VLTI

Requirements: Accept beams from 4 or 6 telescopes (not just 2) either from science target itself or from off-axis reference star Measure phase for control of VLTI delay lines (fringe locking) for long coherent exposures & phase referenced imaging Sensitive to group delay for dispersion control and fringe-jump detection Tolerant of wavefront and photometric fluctuations Tolerant of different (possibly small) visibilities on some baselines Possibility of combining AT (1.8 meter telescope) with UT (8 meter telescope), 20x brighter! Rapid update rate possible (up to 2 KHz) with best possible limiting sensitivity (of course!)

Two well developed proposals submitted in response to the call by ESO:

• POPS (Planar Optics Phase Sensor) submitted by LAOG

• NFT (NOVA Fringe Tracker) submitted by the NOVA consortium

Concepts presented in May 2010 for ESO's consideration

Meeting at ESO in December 2011 to discuss and compare the concepts, with inconclusive results. It was generally agreed that: Theoretical sensitivity of NFT significantly higher. Technology used in POPS proposal already had been tested and verified in on-sky interferometry, unlike that of the NFT.

The road forward: no agreement at all.

NFT Bulk optics interferometric implementation, using a novel beam-combination topology Only partial spatial filtering using pinholes, adaptable to conditions Ultra-wide-band (1.2 – 2.4 micron) Direct combination of 4 baselines, other two inferred Nominally measures single interference phase in order to generate delay-line correction

POPS Integrated optics beam-combination using a fiber-fed IO device

Full spatial filtering by virtue of using optical waveguide components IO chip built either for K or H band Direct combination of each of the 6 telescope pairs Measures both quadrature phases of interference

Similarities: Both use pair-wise interference. Both based on static measurement of interference phase (like PRIMA) rather than phase stepping/scanning (like FINITO). Both use low-resolution spectral dispersion to detect group-delay offset while co-phasing (again, like PRIMA!).

Differences:

Various concerns raised by ESO, and answered by additions to the NFT design or the NFT analysis: Questioning of only detecting one quadrature phase, with no OPD modulation (whereas POPS detects both quadrature phases, like PRIMA). Questioning of not obtaining photometric information (POPS can obtain the photometry of the individual telescope beams indirectly). Questioning of how the fringe OPD sensor would drive a feedback loop to control the delay lines, as the NFT outputs did not seem compatible with the current fringe tracker paradigms. Questioning the “risk” of employing system technologies that had never been tested on stellar light.

The NFT has carefully addressed the first three concerns, and the fourth one can only be addressed (to the satisfaction of the skeptics) by doing it!

Factors driving the design of the NFT:

Problem: a fringe tracker is needed to cophase 4 (or in the future 6) VLTI telescopes in order that the science instrument can coherently integrate (thus obtaining a good SNR on dim targets) and also to obtain visibility phase either with respect to an off-axis reference star or with respect to the target itself at the fringe-tracker's wavelength.

Therefore the ability to perform interferometry on a faint target is dependent on the fringe-tracker's ability to lock onto its phase or the phase of a reference star found nearby.

Thus the three most important performance goals driving the design of the NFT are:

• Sensitivity

• Sensitivity

• Sensitivity

Achievement of highest limiting sensitivity (dimmest star on which reasonably reliable tracking can be performed) through: High throughput: - Use of free space optical paths rather than integrated optics - No need for spatial filtering (fiber injection losses) but still use pinholes for spatial mode reduction in order to maximize performance (as demonstrated by simulations)- Minimal number of non-cemented optical surfaces (AR coated)Resulting throughput calculated: 85%, guaranteed: 80% Minimize wavefront degradation before beam-overlap: - 3 reflecting and 3 transmitting surfaces including VLTI pickoff and PBS surfaceResult: calculated V=.977 at 2 m Minimal spectral resolution (but variable) of 4 or 5 spectral channels over 1.2 – 2.4 m, just in order to gauge dispersion and fringe-jumps In original configuration, use of only one of the two quadrature phases for interference! Full quadrature detection wastes half of the light measuring the real part of visibility which supplies no first-order information regarding OPD offset.(However doing this over a very wide band requires a more elaborate dispersion compensator than otherwise required). Partial quadrature detection available with 2011 hardware additions.

Likewise no optical power wasted on photometric monitoring which supplies no first-order information regarding OPD offset.-Variable photometric monitoring available with 2011 hardware additions. Slower detector readout rate, when possible (but ESO requests 1-2 kHz update rates).


PRS

M4 x1

M5 x1

Spectral prism (optional)Wollaston prismCamera lensDetector

fixed

Align, coarse OPD + piezo

/2 plates

Minimal configuration to verify components and performance on 1 baseline

fixed

align

align

pinhole

PBS

NFTUpgradability

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Polarization Reversers(rotatable)

PRS M4's

M5's

Mask


End-around combiner

Configuration for up to 4 telescopes.....

NFTUpgradability


Local Switchyard

Polarization Reversers

PRS M4's

M5's

Mask


End-around combiner

NFTUpgradability

All 6 channels now installed(unless you want even more!)


Further advantages of the technological choices adopted by the NFT: Not acutely affected by input polarization mismatches, no SNR loss at all due to birefringence between vertical and horizontal polarizations, since each pairwise beam combination only uses one or the other. Beam overlap is performed very early in the optical chain, after which OPD shifts, vibrations, or poor optical surface quality has no effect on interferometric visibility or the measured phase of interference. A 4 or 6 beam combining topology can be used at full efficiency with only N telescopes if N is even. Modest SNR loss if N is odd.

Delay-tracking control loop and simulation of NFT

Control loop not part of NFT proposal per se, but used in NFT simulations to demonstrate suitability of hardware.

• A basic linear fringe-tracking algorithm proposed, identical to what the analog electronics in the experimental setup did: Delay line motion = gain * Im{V} (too simple!).

• A complicated algorithm was developed (but this had not been a requirement of the original ESO call!) to examine the detector outputs in 2 or more spectral channels in order to detect fringe jumps and order correction jumps. Also generates a judgement of “verified tracking intervals” needed in order for a science instrument to (properly) employ the fringe-tracker's data to the fullest. This was successfully simulated.

• Specifically in response to ESO, a method was developed for the detection of loss of tracking (actually several parallel indicators have been identified!), and successfully simulated.

Using a frame time of 5ms, tracking is successful at a K band photon flux of only 70,000 per second with a typical tracking rate over 80%. Periods when the tracker was locked onto a side fringe (three are shown) are detected by the tracking algorithm after typically 1/2 second, leading to a correction and invalidation of the intervals where itmay not have been correctly tracking (“validated” intervals are marked in green). Well less than 1% of “validated” intervals are found to actually have been off of the intended fringe peak, causing an apparent visibility reduction in the science instrument of less than 1%. OPD is measured in femtoseconds. The left and right plots represent twodifferent detector noise realizations when tracking the same atmospheric OPD and incident wavefronts.

Tracking statistics predicted for 1ms detector frame rate, tracking on H & K band light

1% criterion

The problem of “photometric crosstalk” due to a beam-combiner lacking photometric symmetry. This is a key issue addressed by the particular topology of the NFT.

An ideal 2 x 2 on-axis beam combiner:

Detector

+-

Detector

EA

EB E

1

E2

I2

I1

|V| cos()

Ideal 50/50 Beamsplitter

The problem of “photometric crosstalk” due to a beam-combiner lacking photometric symmetry. This is a key issue addressed by the particular topology of the NFT.

An actual beam combiner:

Detector

+-

Detector

Photometriccrosstalk

EA

EB E

1

E2

I2

I1

|V| cos()+ (I

A – I

B)

where:

= ½ (R - T)

Ideal 50/50 Beamsplitter

Visibility estimator I1 – I

2 inevitably includes a

photometric crosstalk term = (IA – I

B)

Naive approach to prevent photometric crosstalk:Force =0 by making the beamsplitter's T = R = ½

But: It will not generally be constant over wavelength It will almost always be different between the polarizations Moreover this is difficult to do in the first place!

For instance, the VINCI (fiber) beam combiner, regularly adjusted for maximum fringe contrast, had a very unpredictable (almost never near zero!) as plotted over 3 years:

+.6

.4

.20

-.2-.4-.6

Cancelling photometric crosstalk term using photometric pick-offs:Split off a fraction m of the incoming optical powers, and add them in the correct proportions to the interferometric outputs.

+-

A

I2

I1

m IA

m IB

Drawbacks:• Photometric monitoring beams rob power from the interferometric channels• Added photometric corrections m I

A and m I

B

contain their own detector noise, added to resulting visibility determination• Want cancellation in both polarizations and at all wavelengths: generally impossible!

OPDModulator

CoherentDetection

Estimate of complex VA

C B D(for instance)

Combating photometric crosstalk by chopping the phase (modulation of the OPD): This is the most common solution! For instance: ABCD phase stepping, or scanning of fringe packet Then the visibility appears as an AC signal on the photodetector. Just ignore the DC (and low frequencies) then. So measure |E

1+ E

2 exp(j )|2 + (I

A – I

B) as a function of (t), and the

photometric crosstalk term can be eliminated (if it stays constant!).

Drawbacks: Requires more than one detector readout to measure a visibility Since the photometrys and OPD are changing due to the atmosphere, these fluctuating components leak into the result. In order to reduce that effect, a faster readout speed is required, reducing sensitivity in a NIR instrument.

Solution employed by the NFT: precise 50/50 beamsplitting through use of the “Polarization-Based Collimated Beam Combiner” topology!

• Combines beams pairwise.• Each telescope's light is split by polarization, to be combined with 2 other telescopes.• Each combination produces 2 (or more) interferometric outputs based on balanced combination: visibility estimates are immune to photometric crosstalk!

Requires 3 essential stages:

Beam 1

Beam 2

Beam 3

Beam 4

PolarizationReversers(even channels only)

PolarizationRecombinaton

Stage

PolarizationAnalyzer

@ 45o

+

-

1

2 3

PolarizationRecombined

Beam

VisibilityEstimate

DetectorsH

Detectors

H

V

V

p

ps

s 2-phase detection configuration shown

VV

HH

VV

OriginalPolarization

3s 4p

2s 3p

1s 2p

H 2sV 2p

V 3sH 3p

H 4sV 4p

OriginalPolarization

PolarizingBeam Splitters

Polarization Recombining Stage (PRS) Consists of multiple Polarizing Beamsplitters (PBS) s polarization from telescope M is paired with p polarization of telescope M+1 into same spatial mode but a different polarization (thus a different mode) Thus the “polarization recombined beam” can proceed through various (non polarized) optical elements and both waves are affected identically Only when they finally reach the polarization analyzer are the two waves actually interfered and directed onto 2 (or more) photodetectors

TelescopeBeams

PolarizationRecombinedBeams

2

Important point:After the PRS, the “polarization recombined beams” no longer need to be treated according to interferometric standards. OPD variations/instability, wavefront degradation do not affect the visibility or rejection of photometric crosstalk!

1s 2p

45o Rotation

of Coordinate

System

A = 1s + 2p

B = 1s - 2p

Polarization Analyzer in rotated system

Differential Amplifier

+-

IA

Telescope 1

Telescope 2

Detector A

Detector B

Polarization Analyzer produces output beams implementing a balanced beamcombiner. When their powers are detected and differenced, photometric crosstalk is suppressed!

3

Detectors

IB

Beam from the polarization recombination stage (PRS) has the two telescopes' light in H and V polarizations

Now tilt your head by 45o!

Analyze the polarizations in the A and B directions to get the opposite interference phases with equal photometric contributions!

Immunity to photometric crosstalk demonstrated in experiment conducted at TU Delft. Interference from He-Ne laser (left) detected using Polarization-Based Collimated Beam Combiner” topology, with added incoherent light (“photometric noise”) injected into each “telescope” beam from independently pulsating laser diodes. Cancellation of photometric noise by ratio >> 100:1 (stable over weeks) with intended interference signal retained.

Pulsating laser diodes

He-Nelaser

Piezo

Polarization analyzer (PBS) at 45o and detectors

“Telescope beam 1”

“Telescope beam 2”

Injected “Photometric noise”

Coherent “starlight” split toward both “telescope beams”

Each detector output (left) contains an equal combination of the two photometric signals, but an opposite amount of the visibility signal!

A+B(photometry only) in red

A-B(photometry rejected) in white

Shown: actual recorded scope data of detectors (left) and sum and difference (right) while scanning the OPD. The “photometric noise” was much (~ 10x) stronger than the “starlight,” but is completely rejected in the difference signal. In locked mode (with feedback to the piezo, not shown) the same photometric rejection is present.

Modification of response by inserting an additional optical element:

1s 2p

45o Rotation

of Coordinate

System


+-

IA

IB

Original system: IA – I

B = Re{V}

1s 2p

45o Rotation

of Coordinate

System


+-

IA

IB

Detection of quadrature phase: IA – I

B = Im{V}

1s 2p

45o Rotation

of Coordinate

System


+-

IA

IB

Photometric detection IA = I

1 I

B = I

2

/4 plate at 0o

/2 plate at 22.5o

Modification of NFT beam combiner in order to allow for (partial) detection of quadrature visibility component and/or photometry

A = 1s + 2p

B = 1s - 2p

WollastonPrism

CameraLens

DetectorArray

IA

IB

Basic implementationIA – I

B = Re{V}

Augmented system:I'

A – I'

B = Im{V}

I''A = I

1

I''B = I

2

/4 plate with wedge

etc.

I'A

IA I

A

Proposed layout on PICNIC detector:

IB I

A etc.

IB I

B

/2 plate with wedge

I'B I'

A

Slightly deflected beams with quadrature or photometric information detected on adjacent pixels, for instance

So, adding it all up: The VLTI and its second-generation instruments (particularly MATISSE) requires an updated fringe-tracker in order to achieve its potential. The NOVA consortium submitted a concept which meets all of the requirements and achieves a high sensitivity (limiting magnitude). The proposal relies on standard optical components and techniques, is easily understood and modelled. The unique feature of the beam combiner has been tested in the laboratory and (unsurprisingly) works. A stripped-down version for only one baseline or otherwise simplified could be easily and relatively cheaply built and tested on the sky, for further verification. So, where does ESO go from here?

?The end

WRITE HERE!

Modulation of the OPD -II Drawbacks:• Requires more than one detector readout to measure a visibility• Since the photometry (and OPD!) is changing due to the atmosphere, this fluctuating component leaks into the result.• In order to reduce that effect, a faster readout speed is required, reducing sensitivity in a NIR instrument.

Example: the incoherent power spectrum (top) and coherently integrated power spectrum (bottom) of 4 consecutive VINCI observations (14 Aug. 2001) of eps sco at different framerates:

590 Hz: too slow. Spectrum is broadened by atmospheric OPD

3384 Hz: too fast. Detector noise enhanced due to short exposure

Just right! (For this VERY bright star!)

Noisier

MoreWhiteNoiseLF

Noise

Good!

TrackingOK

CheckingWiggle

Drift Upward

Drift Down

Finddirectionof last

confirmedtracking

LostWiggle Set limit

to drift

At limit of drift,Reverse drift, increase limit inother direction

Wiggle above 3 level, on fringe

Wiggle above 3 level, on fringe

Induced “telescope failures” simulated at these times, causing tracking failure

Sprial search pattern invoked following detection of tracking failrure

Green indicates intervals which have been “validated” as acceptable for use of data produced by science instrument.

See NFT main document or update document for explanation.

A laboratory setup of the concept of the Polarization-Based Collimated Beam Combiner was carried out in order to verify and demonstrate its performance goals.Many thanks to the TU Delft applied optics department and especially to the optics technician Thim Zuidwijk for facilitating.

He-

Ne

Las

er

LD

ApertureMask

Photodiode

Photodio

de

Photodio

de

Photodiode

Photodiode preamps,differencing circuit,

PolarizingBeamsplitters

He-Ne Laser

Polarization at ~ 45o

Piezo

Stack

LD

Laser Diodes (source of “photometric interference”)

Control loopfilter -

integrator

HVAmp

Scan

Lock

PolarizationAnalyzer &Detectors

Detector assembly rotates about beam axis, nominally at 45o

+

(for testing)

Detector Read-out

Electronics

PolarizingBeamsplitters

Piezo

Stack

Real-timeComputer

HVAmp

To next combiner

DetectorArray

Beam from Telescope 1

Beam from Telescope 2

etc.

1s + 1p

2s + 2p 1s + 2p

2s

PolarizationAnalyzer at 45o1s

Actual application, using starlight!

And/or VLTI delay line, stroke = 120m

Polarization Reverser

Vertical polarizationin VLTI tunnels

The signal from just a single photodetector (without balancing from the other one) showing the combined waveform of the two laser diodes (incoherent source), one at about 1KHz, the other at 4KHz. These added “photometric fluctuations” are both highly rejected by the differential amplifier when the angle of the analyzer is correctly set to 45

o and the relative gains are set

appropriately.

He-

Ne

Las

er

LD

ApertureMask

Photodiode

Photodio

de

Photodiode


Piezo

Stack

LD


integrator

HVAmp

Lock


+

(for testing)

VPD

= sin()

Hot turbulent air from hair dryer

ATMOSPHERE

CORR

See movie 1basic demo

He-

Ne

Las

er

LD

ApertureMask

Photodiode

Photodio

de

Photodiode


Piezo

Stack

LD


integrator

HVAmp

Lock


+

(for testing)

VPD

= sin()

ATMOSPHERE

See movie 2dim beam

NeutralDensityFilterx .03

Added!

He-

Ne

Las

er

LD

Photodiode

Photodio

de

Photodiode


Piezo

Stack

LD


integrator

HVAmp

Lock


+

(for testing)

VPD

= sin()

ATMOSPHERE

See movie 3wine glass

PRS

Optical disturbancefollowing PRS doesnot degrade visibilityor photometric symmetry

Documents

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