THORLABS DMH40-F01, Operation Manual

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© 2021 Thorlabs 11
DMH40
Creep Consequences
Consider the creep effect particularly when performing adaptive correction using a feedback
loop control: As long as the control loop is closed, slow drift effects of the DMH40 are corrected
automatically.
As soon as the loop control is stopped, the last voltage pattern remains applied to the mirror
segments, but the physical mirror shape will start a slow, continuous creep which leads to a
wavefront drift.
Note
Even if a voltage pattern that produced a defined wavefront is reproduced exactly, the original
defined wavefront will not be reproduced exactly.
The Thorlabs Deformable Mirror software is not able to compensate for the creep effect. Creep-
ing can be eliminated only within a closed loop control system. In other words, piezoelectric de-
formable mirrors cannot be used to exactly set a deterministic wavefront distortion.
Shift of Hysteresis Curve
Finally, the creep overlays the hysteresis, so that the hysteresis curve will be shifted, depending
on the voltage applied to all segments before measuring the hysteresis curve. The diagram be-
low illustrates that.
Shift of Hysteresis Curve
The creep effect is present regardless of whether the DMH40 is switched on or off. After the
DMH40 was switched-off for a long period, the hysteresis was measured. The creep let the mir-
ror’s concave shape at 0 V increase, which leads to a shift of the hysteresis curve downwards
(blue curve).
Then a segment voltage of 300 V was applied for several hours. The hysteresis (red curve) is
shifted by about 2.1 µm towards convex direction, equivalent to about 8% of the spherical
range.
However, both hysteresis amplitudes are equal, they are just shifted.