photon counting arrays for ao wavefront sensors

SPIE 2005 - San Diego - J. Vallerga

John Vallerga, Jason McPhate, Anton Tremsin and Oswald Siegmund

Space Sciences Laboratory, University of California, Berkeley

Bettina Mikulec and Allan Clark

University of Geneva

Photon counting arrays for AO wavefront sensors


Future WFS detector requirements

• High optical QE for fainter guide stars

• Lots of pixels - eventually 512 x 512– More accuators

– More complex LGS images (parallax, gated, etc)

– Off null / open loop operation

• Very low (or zero!) readout noise

• kHz frame rates


Advantages of multi-pixel sampling of Shack-Hartmann spots

Non-linearity of 2 x 2 binning

Quad cell (2x2) algorithm for Gaussian input

-1

0

1

-1 0 1 Centroid true position

Calculated position

Sigma = 0.2

Sigma = 0.4

Sigma = 0.6

Sigma = 0.8

Sigma = 1.0


Advantages of multi-pixel sampling of Shack-Hartmann spots

Linear response off-nullInsensitive to input widthMore sensitive to readout noise

4 x 4 6 x 64x4 COG non-linearity for Gaussian input

-1

-0.5

0

0.5

1

-1 -0.5 0 0.5 1

Centroid true position

Calculated position(center of gravity )

Sigma = 0.4Sigma = 0.8Sigma = 1.2

6x6 COG non-linearity for Gaussian input

-1

-0.5

0

0.5

1

-1 -0.5 0 0.5 1

Centroid true position

Calculated position(center of gravity)

Sigma = 0.4Sigma = 0.8Sigma = 1.2


Centroid in presence of noise:

8 x 8Noiseless35% QE

10 photons

- - -

100 photons

1000 photons

8 x 82.5 e- rms90% QE

6 x 62.5 e- rms90% QE

4 x 42.5 e- rms90% QE


Photon Counting

QADC

V v

EventsEvents

Charge integrating

Threshold

EventsCount(x,y,t)


Avalanche Photodiodes (APDs, Geiger mode)

•Single photon causes breakdown in over-voltaged diode

•QE potential of silicon

•Arrays in CMOS becoming available

But

•Appreciable deadtime

•Low filling factor•High dark counts, crosstalk and afterpulsing


APD arrays

Edoardo CharbonEcole Polytechnique Federale de Lausanne

32 x 32


L3CCD (e2V Technologies)

•Integrates charge

•Multiplies charge in special readout register

•Adjust gain such that e < 1e-

But

•Multiplication noise doubles photon noise variance

•Single readout limiting frame rate


Imaging, Photon Counting DetectorsCharge distribution on stripsCharge CloudMCP stackTube Window withphotocathodeγ

Photocathode converts photon to electron

MCP(s) amplify electron by 104 to 108

Rear field accelerates electrons to anode

Patterned anode measures charge centroid


MCP Detectors at SSL Berkeley COS FUV for Hubble (200 x 10 mm windowless)

25 mm Optical Tube

GALEX 68 mm NUV Tube (in orbit)


GaAsP Photocathodes

Hayashida et al. Beaune 2005 NIM


Wavefront Sensor Event Rates

• 5000 centroids

• Kilohertz feedback rates (atmospheric timescale)

• 1000 detected events per spot for sub-pixel centroiding

5000 x 1000 x 1000 = 5 Gigahertz counting rate!

• Requires integrating detector


Our concept

• An optical imaging tube using:– GaAsP photocathode– Microchannel plate to

amplify a single photoelectron by 104

– ASIC to count these events per pixel


Medipix2 ASIC Readout

Each pixel has amp, discriminator, gate & counter.

256 x 256 with 55 µm pixels (buttable to 512 x 512).

Counts integrated at pixel. No charge transfer!

Developed at CERN for Medipix collaboration (xray)

Input

Preamp

Disc.

Disc. logic Mux. 13 bit

counter –ShiftRegister

Clock out

Shutter

Lower Thresh.

Disc.

Mux.

Previous Pixel

Mask bit

Analog Digital

Upper Thresh.

Next Pixel

Mask bit

Polarity

~ 500 transistors/pixel


Vacuum Tube Design


Technology advantage

High QE CCDs

Number of pixels CCDs, Medipix

Readout noise APD, Medipix, L3CCD

Frame rate Medipix, CCD

Gating Medipix


Assumed performance parameters

CCDMedipix-

MCP

Binning 2 x 2 6 x 6 8 x 8 8 x 8

QE (%) 90 90 90 35

Readout noise 2.5 e- 2.5 e- 2.5 e- 0

Seeing width (pxls FWHM) 0.75 2.25 3 3

Diffract. width (pxls FWHM) 0.5 1.5 2 2

SPIE 2005 - San Diego - J. Vallerga€

(σ Δφ2 )ph =

π 2

2ln(2)(Nph )•NTND

⎛

⎝ ⎜

⎞

⎠ ⎟

2

•NT

2 + NW2

2NT2 + NW

2

⎛

⎝ ⎜

⎞

⎠ ⎟

2

(σ Δφ2 )det =

π 3

32(ln(2))2•σ det

(Nph )

⎛

⎝ ⎜

⎞

⎠ ⎟

2

•NT

2 + NW2

ND

⎛

⎝ ⎜

⎞

⎠ ⎟

2

(σ Δφ2 )tot = (σ Δφ

2 )det +(σ Δφ2 )ph

Gaussian weighted center of gravity algorithm:

From Fusco et al SPIE 5490. 1155, 2004


Centroid error vs. input fluence

Centroid estimator error vs. technique

0.100

1.000

10.000

100.000

1 10 100 1000

Input number of photons

Centroid Error (rms, radians)

CCD Quad cellCCD 8x8 weightedCCD 6x6 weightedMedipix 8x8 weighted


Summary

• Noiseless detectors outperform CCDs at low fluence

• “Crossover” point at 90 photons for 8x8 binning using best performance values

• Higher if weighting/correlation schemes not used

MCP/Medipix Status

• First tube in Fall 2005

• GaAs tube in 1st half of 2006


Acknowledgements

• Univ. of Barcelona

• University of Cagliari

• CEA

• CERN

• University of Freiburg

• University of Glasgow

• Czech Academy of Sciences

• Mid-Sweden University

• University of Napoli

• NIKHEF

• University of Pisa

• University of Auvergne

• Medical Research Council

• Czech Technical University

• ESRF

• University of Erlangen-Nurnberg

Thanks to the Medipix Collaboration:

This work was funded by an AODP grant managed by NOAO and funded by NSF


UV photon counting movie

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.


First test detector• Demountable detector

• Simple lab vacuum, no photocathode

• Windowless – UV sensitive


Sub-pixel spatial linearity

LampPinhole

Detector


Spot size vs gain

Pinhole grid mask

(0.5 x 0.5 mm)

Gain: 20,000

Rear Field: 1600V

Threshold: 3 ke-

Gap: 500µm


Avg. movement of 700 spots

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

0 5 10 15 20 25

Lamp Position (mm)

Centroid Position (µm)

Delta X

Delta Y

1 pixel


Position error (550 events/spot)

0

5

10

15

20

25

30

35

40

45

50

-20 -15 -10 -5 0 5 10 15 20

Centroid difference (microns)

Number of centroids

rms = 2.0 µm


Flat Field

1200 cts/bin - 500Mcps

MCP deadspots

Hexagonal multifiber boundaries


Flat Field (cont)

Histogram of Ratio consistent with counting

statistics (2% rms)Ratio Flat1/Flat2


Readout Architecture33

28 b

it P

ixel

Col

umn

0

3328

bit

Pix

el C

olum

n 25

5

3328

bit

Pix

el C

olum

n 1

256 bit fast shift register

32 bit CMOS output LVDS out

• Pixel values are digital (13 bit)

• Bits are shifted into fast shift register

• Choice of serial or 32 bit parallel output

• Maximum designed bandwidth is 100MHz

• Corresponds to 266µs frame readout


“Built-in” Electronic Shutter

• Enables/Disables counter

• Timing accuracy to 10 ns

• Uniform across Medipix

• Multiple cycles per frame

• No lifetime issues

• External input - can be phased to laser

photon counting arrays for ao wavefront sensors

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