The polarization-based collimated beam combiner and the proposed

NOVA fringe tracker (NFT) for the VLTIJeffrey A. Meisner, Sterrewacht Leiden

Walter J. Jaffe, Sterrewacht LeidenRudolf S. Le Poole, Sterrewacht Leiden & TNOSilvania F. Pereira, Technische Universiteit Delft Andreas Quirrenbach, Landessternwarte HeidelbergDavid Raban, TNO Science and IndustryAmir Vosteen, TNO Science and Industry

PRESENTER

Outline of paper Interferometric beam combination and the problem of “photometric crosstalk” The polarization-based collimated beam combiner topology The NOVA Fringe Tracker, designed using that concept Laboratory demonstration and results implementing that topology

Detector

+-

Detector

+ photometriccrosstalk

EA

EB E

1

E2

I2

I1

[ ] = s [ ]E1 E

A

E2 E

B

Basic on-axis beam combiner using partially transmissive reflector (beamsplitter) nominally T=R=50% Subtraction of complementary detected outputs from beamcombiner yields estimate of visibility However unbalanced combiner (T =/= R) with unequal photometric levels (|E

A|2 =/= |E

B|2) leads to photometric

crosstalk in interferometric determination!

|V| cos()

Electric Fields:

[ ] = [ ] [ ]E1

s1A

s1B

EA

E2 s

2A s

2B E

B

Intensities:

+ Interferometry

= [ ] [ ]R T IA

T R IB

If lossless beamsplitter:

+ Interferometry

where R = ½ (1 + ) and T = ½ (1 - ) Or in general:

The photometric asymmetry coefficient

= ( |s1A

| / |s2A

| - |s1B

| / |s2B

| ) / ( |s1A| / |s2A| + |s1B| / |s2B| )

[ ] = [ ] [ ]

Visibility estimator I1 – I

2 inevitably includes a

photometric crosstalk term = (IA – I

B)

where R = ½ (1 + ) and T = ½ (1 - ) Or in general:

I1

|s1A

|2 |s1B

|2 IA

I2 |s

2A|2 |s

2B|2 I

B

Combatting photometric crosstalk - I

Naive approach:Force =0 by making the beamsplitter's T = R = ½but: 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

Combatting photometric crosstalk -II

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 so that the effects of I

A and I

B are cancelled.

Detector

+-

Detector

EA

EB E

1

E2

I2

I1

m IA

m IB

Using photometric pick-offs -II Drawbacks:• Photometric monitoring beams rob power from the interferometric channels• Added photometric corrections I

A and I

B contain detector noise, added

to resulting visibility determination• Want cancellation in both polarizations and at all wavelengths

Even using the optimum m (left graph) there is a substantial increase (right graph) in the noise of the visibility estimate due to (1) and (2)

Noise increase(power)

Optimum m

2

4

6

= .2 .4

1

Combatting photometric crosstalk - IICombatting photometric crosstalk - ICombatting photometric crosstalk - ICombatting photometric crosstalk - ICombatting photometric crosstalk - ICombatting photometric crosstalk -III

OPDModulator

CoherentDetection

Estimate of complex VA

CBD(for instance)

Modulation of the OPD (The most common solution!) ABCD phase stepping, or: Scanning of fringe packet, etc 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 as a function of (t)

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!

The Second Generation Fringe Tracker for the VLTI, planned by ESO to supercede PRIMA

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!)

Last 4 requirements are challenged by concerns arising from photometric crosstalk and the inadequate solutions to it.

1 meter

The NOVA Fringe Tracker (NFT) Result of one of 3 phase A studies to propose to ESO a concept for a second generation fringe tracker for the VLTI

Local Switchyard

VLTI Beams

PolarizationReversers

PolarizationRecombiningStageSpectrometerAnalyzerCameraDetector

VLTI Beams

Wideband interferometry over 1.2 - 2.4 microns simultaneously Spectrally resolved detection over 4 - 7 pixels (reconfigurable) Always at least 2 pixels over K band for dispersion detection Single beam combiner, all wavelengths fixed to same reference plane

Combines up to 4 (6) telescopes pairwise over 4 (6) baselines No spatial filtering (is optional) for highest limiting sensitivity

No fiber injection loss, hassle Wide (effective) visibility fluctuations tolerated

Two-phase interferometric detection with fringe-locking OPD corrections based on Im{V} No need to use ½ the photons for measuring Re{V}, doubles sensitivity. Correction to OPD supplied every frame, no OPD modulation required

Based on the Polarization-Based Collimated Beam Combiner topology Balanced beam combination, photometric crosstalk rejected Can track on low |V| sources, including stars past 1st visibility null Combining AT with UT (20x brighter!): no problem

The NOVA Fringe Tracker (NFT)Main design features

• 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 estimate is 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

The Polarization-Based Collimated Beam Combiner:a solution to the problem of photometric asymmetry

2-phase detection configuration shown

4

3

2

1

PolarizationRecombining

Stage(PRS)

In NFT, Polarization Reversers implemented using (almost) achromatic ½ wave plates. Even channels: at 45o to reverse polarization Odd channels: at 0o, no change in polarization (but compensates for material dispersion) All are rotatable so their roles can be reversed

1

VLTI Beams frompick-off mirrors

LocalSwitchyard

Half wave plates(polarization reversing

in even beams)

M2 mirrors include very short strokeOPD adjustment + piezo, and motors for beam alignment

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!

3

2

1

4

5

6

Constant OPD plane at 45o

2s+1p

3s+2p

4s+3p

5s+4p

s=V p=H

s=V p=H

s=V p=H

s=H p=V

s=H p=V

s=H p=V

1s

1s+(M)p

6s+5p

5s+4p

6p

End-around recombiner (set for M=4 telescopes)

NFT implementation of Polarization Recombining Stage for 6 telescopes, including end-around channelBased on giant prism block with 2 polarizing beamsplitting surfaces per channel

2

1s 2p

45o Rotation

of Coordinate

System

A = 1s + 2p

B = 1s - 2p

PolarizationAnalyzer inrotated system

A = 1s + 2p

B = 1s - 2p

WollastonPrism

CameraLens

DetectorArray

Detectors

Differential Amplifier

+-

IA

IA

IB

IB

Implementation in NFT:

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

NFT Backend Implementation

ArrayDetector

PRS

M4: off-axis paraboloids

M5

Mask with ~.5mm holes at intermediate focus

Spectral Prism (zero-deviation)

Spectral resolution variable by shifting

Wollaston prism

(polarization analyzer)

Channel 1 2 3 4 5 6

blue

red

Possible layout of 2 spectra from each channel on 40 micron pixels of PICNIC detector.

All 6 polarization recombined beams pass through the same optics after diverging from their foci at the mask

(Not a spatial filter)

3

Additional options for the polarization analyzer/detectors (The NFT just uses 2-phase detection --> sin().)But for a visibility measuring interferometer..... Quadrature detection of interference:

A = 1s + 2p

B = 1s - 2p

Special beamsplitting coating:B polarization: T=100%A polarization: R=2/3 T=1/3

Detector0o detection

240o detection

plate at 45o w/r/t A&B

3

3-phase detection (a little less detector noise):

PRS

PolarizationAnalyzer @ 45o

PolarizationRecombined

Beam

p

sSplitter

Polarization

Analyzer @ 45o

¼ waveplate 0o detection

180o detection90o detection270o detection

120o detection

Detectors

Laboratory setup of the polarization-based collimated beam combiner at TU Delft to demonstrate concept

and measure performance achieved

Coherent source supplied by He-Ne laser (polarized at 45o), split into 2 beams (“2 telescopes”) to be polarization recombined and interfered using PBS and 2 photodiode detectors (hidden inside rotating assembly)

He-NeLaser

Mirroron piezoactuator

PolarizationRecombining

PBS

Analyzer:PBS with 2 photodiodesin rotatable assembly

Added “photometric noise” (incoherent) into each beam path from 2 red laser diodes modulated at 250Hz and 1000Hz respectively.

Laserdiodes

(pulsing)

Beamsplitters(non polarizing)to inject laserdiode beams

into beam paths

PolarizationRecombining

PBS

Analyzer& detectors

Scope trace from a single photodetector (unbalanced)showing “photometric noise”from pulsating laser diodes

He-N

eL

aser

LD

Photodiode

Pho

todi

ode

Photodiodepreamps &

differencing circuit

PolarizingBeamsplitters

He-Ne Laser

Piezo

Stack

Laser Diodes (source of “photometric interference”)

Control loopfilter -

integrator

HVAmp

Scan

Lock

Detector assembly rotates about beam axis, nominally at 45o

(for testing)

Hot turbulent air from hair dryer

ATMOSPHERE

CORR

AnalyzerPBS

Detectors

+

PRS

NDFilterT = 3%

LD

1. Experimental Results..... As expected, the “photometric crosstalk” contribution from the pulsing laser diodes, seen in both of the photodetector outputs (left) are rejected when subtracted, leaving only the actual interferometry (small sine wave, right trace) due to scanning of the piezo (visibility was reduced by intentionally misaligning interferometric beams)

A+B(photometry only) in red

A-B(photometry rejected) in white

• Routinely achieve >>100:1 photometric rejection (need to block one interferometric beam to measure!) thus << .01 Rejection stable over time (weeks, if not touched)

OPD residuals from interferometric output (top trace) < .1 radians with hair dryer running (plus induced square wave)

With hair dryer blowing hot turbulent air in 15cm beam path, piezo voltage tracks the induced OPD well. Range ~ 5 wavelengths (=633nm).

Normally no loss of tracking. A fringe jump is easily noticed after turning off hair dryer since the mechanical stability << 100 ms

2. Experimental Results..... In fringe tracking mode, run error signal (~ Im(V) ) from 2-phase interferometric detector into filter-integrator driving the piezo amplifier. With no OPD disturbances, residual noise from interferometry is too small to measure, << .1 radian (= 10nm) 30 Hz square wave added to error signal, causes ~20nm change in equilibrium tracking point. Is cancelled by an equal and opposite interferometric phase detection (below).

3. Experimental Results..... Inserted neutral density filter (T=.03) in one interferometric beam, simulating interference between a VLTI UT (8 meter telescope) and an AT (1.8 meter telescope). Still with laser diode mixed with each beam, pulsing at 250Hz and 1000Hz and hair dryer creating “atmospheric turbulence.” No noticeable change in tracking performance

Demonstration of insensitivity of polarization combined beam to modest optical disturbances which would be impossible if applied to one beam before the PRS Inserted a wine glass in the 15cm space between the PRS and the analyzer (beam passing through both sides of the glass) and wiggled it around. No noticeable effect on tracking, no fringe jump during entire period.

The End

End of presentation

Extra slides follow....

VV

HH

VV

OriginalPolarization

H 2sV 2p

V 3sH 3p

H 4sV 4p

OriginalPolarization

“Polarization Recombined Beams”

Dispersioncompensationfor equal pathlength in glass

Alternative PRS implementations:

s=H p=V

s=H p=V

s=V p=H“Polarization

Recombined Beams”

UsingWollastonPrisms

2