Technical articles

Direct control of Zernike modes using piezo and membrane DM

Direct control of Zernike modes is beneficial in feedforward control algorithms, and in optimization-based adaptive optics,  such as laser intracavity AO,   adaptive image sharpness control,  smart PSF control in microscopy,  optimal imaging through turbulence with stochastic optimization,  etc.  In all these cases the dynamic control of Zernike modes provides for very quick quasi-optimal convergence to the maximum value of the  target function.

Here we show a  15 mm 37-channel OKO Micromachined Membrane Deformable Mirror (MMDM) and 30 mm 37-channel Piezoelectric Deformable Mirror (PDM) operated by Zernike polynomials in feedforward control mode. No wavefront sensor is used during the mirror operation. Both mirrors are controlled by our free MiZer software.

PDM has been  pre-calibrated with a Shack-Hartmann wavefront sensor,  to form  initially flat shape  plus any linear combination of 20 Zernike polynomials in a clear aperture of 20 mm. We have applied a special approach to reduce the hysteresis inherent to piezoelectric actuators. It consists in the application of fading alternate voltage to the actuator. We call it "degauss" for its similarity with degaussing of magnetized objects. As shown in our recent paper, it reduces the hysteresis error by a factor of 5. The video below shows direct interferometric patterns of the piezo DM surface, registered in a Twyman-Green interferometer,   with respect to a flat reference.

For our Chinese customers, the video is available here

MMDM has been  pre-calibrated with a Shack-Hartmann wavefront sensor,  to form  initially concave spherical shape  plus any linear combination of 20 Zernike polynomials in a clear aperture of 10 mm.  The video below shows direct interferometric patterns of the MMDM surface, registered in a Twyman-Green interferometer,   with respect to a flat reference  in the first part of the video,  and a concave spherical reference with 5 m  ROC, in the second part of the video. 

For our Chinese customers, the videos are available here

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