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Piezo and retroflector calibration

Piezo and retroflector calibration. Understanding the details of interferometer performance. Starting point. We have a d istance m eter i nterferometer ( DMI ) design We want to measure end point movements to the nanometre level Nanometre resolution is routine in interferometry –.

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Piezo and retroflector calibration

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  1. Piezo and retroflector calibration Understanding the details of interferometer performance

  2. Starting point • We have a distance meter interferometer (DMI) design • We want to measure end point movements to the nanometre level • Nanometre resolution is routine in interferometry – The devil is in the details

  3. DMI layout • Reminder of the interferometer schematic Launch head

  4. Launch head prototype • Made from Titanium • close match: thermal expansion of BK7 glass • beam-splitter cube and right angle prism (reflector) are both made of BK7 To reflector at far end of DMI

  5. End point reflector • For many experiments it can be simpler to use a mirror • Needs to be realigned after any adjustment to the launch head • Beam can be made to return along the same line using iris in the path • An alternative is a retro-reflector • Will not require adjustment, provided launch beam hits it "roughly" in the middle • Beam returns along parallel line but with a walk-off

  6. Retroreflectors • Three flat surfaces • Mutually perpendicular • Incoming beam (x,y,z) • after 3 reflections • exits along (-x,-y,-z)

  7. What does calibration mean? • We want to understand • how well a perceived change in length matches a real change in length • we need to predict expected interferometer performance • Controlled experiment • would induce deliberate changes in end point reflector • compare these with recorded "apparent" length changes from measurement • Prerequisite: Calibration • understanding the end point reflector • understanding the "induced movements"

  8. Some thought is required... but not too much thought... • you can easily go round in circles • Basic steps: • induce mirror motions along interferometer axis with piezo voltage changes • measure change in interferometer phase at fixed frequency • this will set resolution limits on all other experiments

  9. Advanced steps • Repeat experiment using retroreflector • Move retroreflector in tranverse axes too • Record effects on apparent length changes as function of transverse position • Compare results to mathematical model • Report on your findings and document the project • Influence the future design of our interferometers

  10. Lab work • We will • explain laser safety and take you through departmental training • Show you laboratory equipment including: • stable laser and amplifier • piezo and piezo driving equipment • interferometer head and optics • how to handle fibres • how to read out signals using photodiodes and • data acquisition systems

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