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Novel Semi-Transparent Optical Position Sensors for high-precision alignment monitoring applications

Orlando, Florida. Novel Semi-Transparent Optical Position Sensors for high-precision alignment monitoring applications. Sandra Horvat , F.Bauer, V.Danielyan, H.Kroha Max-Planck-Institute for Physics, Munich,Germany 13.06.2002. Introduction. ATLAS detector @CERN. ALMY System:

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Novel Semi-Transparent Optical Position Sensors for high-precision alignment monitoring applications

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  1. Orlando, Florida Novel Semi-TransparentOptical Position Sensorsfor high-precisionalignment monitoring applications Sandra Horvat, F.Bauer, V.Danielyan, H.Kroha Max-Planck-Institute for Physics, Munich,Germany 13.06.2002.

  2. Introduction ATLAS detector @CERN • ALMY System: • high-precision alignment monitoring • designed forhigh energy physics experiments • laser beam along the alignment line • ALMY sensors measure its position • Requirements: • high position resolution • high transmittance • insensitivity to high magnetic fields • long term illumination • radiation hardness 23 m End-Cap Octant Sandra Horvat 1/13 MPI for Physics,Munich

  3. ALMY Optical Monitoring System • Multi-point measurement of the laser beam position : • collimated laser beams (690 nm and 780 nm) • single mode fibers (SMF), gaussian beam profile • semi-transparent silicon sensors for 2D position measurement, from the inducedphoto current distribution Sandra Horvat 2/13 MPI for Physics,Munich

  4. ALMY Optical Position Sensors 64x64 diodes, 312.5 mm pitch, 20x20 mm2 • Transparent active surface: • chemical vapour deposition of a-Si:H layer(0.3-1.0 mm) from the plasma phase • 2 layers of ITO electrodes(50-100 nm) • glass substrate(0.5-1.0 mm) • anti-reflective coating • Custom designed readout electronics: • analog signal multiplexed, amplified (I-V converter), digitized, • transmitted to PC for online analysis • readout speed: 10 ms • up to 30 sensors serially to PC port • only commercial components • laser controler 6 cm 6 cm Sandra Horvat 3/13 MPI for Physics,Munich

  5. Setup for the Sensor Tests • Scan of the sensor surface with a laser beam, • using stepping motors of 1 mm positioning accuracy • position resolution • photo current response • laser beam deflection • transmittance • long term illumination ~200 sensors Sandra Horvat 4/13 MPI for Physics,Munich

  6. Transmittance transmittance @780 nm: 80-90% • Minimized reflectivity: • optimized thicknesses of a-Si:H and ITO layers • anti-reflective coating on the back side of the glass • up to 10 sensors along the 780 nm beam transmittance @690 nm: 70-75% Sandra Horvat 5/13 MPI for Physics,Munich

  7. Photo Current Response Finite wedge angle of the glass substrate can cause the interference patterns, due to the reflected light. • Efficiently suppresed! • anti-reflective coating • laser diodes with short coherence length Saturation (strip current 1mA): 1 mW/cm2 @690 nm 10 mW/cm2 @780 nm Sensitivity: 0.1 A/W @690 nm 0.01 A/W @780 nm • uniform distribution • over the whole surface • laser beam profile • remains undistorted Sandra Horvat 6/13 MPI for Physics,Munich

  8. Position Resolution • Typical production batch: • S/N>1000 • local resolution: 1 mm • overall resolution: 5 mm • uniform distribution 5.012mm 5.746mm Sandra Horvat 7/13 MPI for Physics,Munich

  9. Laser Beam Deflection Glass wafers: • specially polished glass wafers • parallel surfaces • remain undeformed during the antireflecive coating • deflection <5mrad Sensors: Glass quality is unchanged during sensor production for T of up to 2000. uniform distribution, deflection angle < 5mrad Sandra Horvat 8/13 MPI for Physics,Munich

  10. Long Term Illumination (Staebler-Wronski effect) Photo current response after 1500 h of illumination with 690 nm @ 1mW/cm2 • photo current response in a-Si:H deteriorates under illumination • degradation in position resolution • local sensitivity minimum increases with time • dependance on the wavelenghth and the beam intensity Sandra Horvat 9/13 MPI for Physics,Munich

  11. Long Term Illumination Laser Beam, below saturation • degradation at a constant rate • depends on the absorbed light, rather than illumination intensity • 500 h correspond to a tolerableposition measurement error of5 mm • non-additive effect under an additional daylight illumination 1% 690 nm @ 1 mW/cm2 Laser Beam & daylight equivalent effect 3.5% 780nm @ 10 mW/cm2 1.5-2.5% 780 nm @ 10 mW/cm2 Sandra Horvat 10/13 MPI for Physics,Munich

  12. Radiation Hardness: 1014 neutrons/cm2,1 MeV eq. Naked silicon cells: before and after irradiation (no degradation) I-V characteristics dark current bias voltage: 1V Sandra Horvat 11/13 MPI for Physics,Munich

  13. Radiation Hardness: 1013 neutrons/cm2 Sensor module with electronics: Change in the transmittance: <1% Change in the photo current: <2% No degradation! Additionally: No degradation after g-irradiation (10 MRad). Sandra Horvat 12/13 MPI for Physics,Munich

  14. Summary ALMY sensors: transparent a-Si:H position sensors application in optical alignment monitoring systems • Sensor performance : • dynamic range 20x20 mm2 • position resolution 5 mm • high quality parallel glass substrate • transmittance 80-90% @780 nm • uniformity over the whole active surface • radiation hard (1013 n/cm2, 10 g MRad) • insensitive to high magnetic fields (Hall mobility <10-2 cm/Vs) • negligible degradationunder illumination with laser beam intensities below saturation (10 mW/cm2 @780 nm) optimized and verified in extensive tests • larger-scale production under preparation Sandra Horvat 13/13 MPI for Physics,Munich

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