Transcript 1. Initial calibration talk 30. June 08

Piezo and retroflector
calibration
Understanding the details of
interferometer performance
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
DMI layout
• Reminder of the interferometer schematic
Launch head
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
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
Retroreflectors
• Three flat surfaces
• Mutually perpendicular
• Incoming beam (x,y,z)
• after 3 reflections
• exits along (-x,-y,-z)
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"
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
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
Lab work
• We will
– explain laser safety and take you through
departmental training
– Show you laboratory equipment including:
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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