THORLABS DMH40-F01, Operation Manual

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© 2021 Thorlabs
2 Operating Principle
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2.6 Relaxing the Mirror
In the previous, chapters some essential properties of piezoelectric materials - hysteresis and
creep (drift) - were described. These properties have a non-negligible impact on the control of
the mirror surface by applying voltages. The resulting surface shape depends not only on the
present voltage pattern, but also on the "history" of previous changes.
Example: Assume a starting condition where the mirror was flat at 150 V:
1. The segment voltage has been changed from 150 V to 300 V (maximum convexity), and
then back to 150 V.
In this case, the hysteresis' impact is that after applying 150 V the mirror remains some-
what convex. The creep effect, on the other hand, makes the surface continue to move in
the direction of the last adjustment: the mirror surface becomes more and more flat. If we
wait long enough, it will become concave.
2. The segment voltage has been changed from 150V to 0V (maximum concavity) and then
back to 150V.
This is the opposite case: The mirror remains slightly concave due to hysteresis, and con-
tinues to flatten over time due to creep.
The effects described above are more significant, the larger the change in the applied voltage.
The Thorlabs Deformable Mirror software provides a Relax feature to eliminate or at least to
minimize the above impacts. It will carry out 3 steps:
1. Remember the actual voltage pattern
2. Damped oscillation of the applied voltage symmetrically with respect to the original voltage.
This means that to a segment with an initial voltage of 150V a "sequence" of 300 V - 0 V -
285 V - 15 V -... 165 V - 145 V - 150V will be applied during relax. This maximum relax
amplitude will be the less the closer the initial segment voltage was to the limit (0 V or 300
V).
3. Restore the voltage pattern that was present prior to Relax
This way, the mirror is moved in both directions with decreasing amplitude so that hysteresis
and creep (drift) effects will be minimized.
When should the Relax be done?
Relax is recommended to prevent recently-made large voltage changes from impacting the next
experiment, e.g.:
· after starting up the software
· after applying voltage patterns and their subsequent restoration
· prior to a new experiment
· prior to starting closed loop control using a wave front sensor
How to Relax segments
To relax the mirror segment, click the 'Relax' button in the tool bar.
An appropriate tool tip appears when hovering the mouse over the the the 'Relax' button.
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