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