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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