**3.1. Devices for sensorless modal correction**

Electrostatic membrane deformable mirrors rely on the electrostatic pressure between an ac‐ tuator pad array and a thin metalized membrane [33]. Thus, the more the actuators the bet‐ ter the wavefront resolution that the mirror can control. The use of these deformable mirrors is, then, subjected to the acquisition of the deformation generated by each electrode. On the other hand, this kind of DMs can also be used with the optimization algorithms. The draw‐ back, in this case, is that the higher the number of actuators the longer will take to the algo‐ rithm to converge.

Recently, a new type of deformable mirrors suitable for the direct generation of aberrated wavefronts was designed. The modal membrane deformable mirror, MDM, relies on the use of a graphite layer electrode arrangement (see Fig. 6) for the generation of a continuous dis‐ tribution of the electric field which allows the generation of the low order aberrations (defo‐ cus, astigmatism, coma) and of the spherical aberration.

**Figure 6.** Electrostatic modal membrane deformable mirror, MDM. (a) Layout of the electrodes of the MDM; (b) volt‐ age and electrostatic pressure distribution which generates the astigmatism shape illustrated in the interferogram shown in (c).

The MDM has already been demonstrated to be effective in several fields, as laser focaliza‐ tion [32], image sharpening and Optical Coherence Tomography ( OCT), as it will be dis‐ cussed later.

Another device for the generation of aberrations is the PhotoControlled Deformable Mirror (PCDM), which is schematically represented in Fig. 7. This deformable mirror [35, 36] is composed of an electrostatic membrane while the actuator pad array is replaced by a photo‐ conductive material. Thus, the membrane shape depends on the light pattern projected on the photoconductor. Arbitrary actuator pads can be conveniently achieved by illuminating the photoconductive side of the mirror with a commercially available Digital Light Process‐ ing (DLP) hand-held projector.

**Figure 8.** Generation of the first four Zernike orders with the photocontrolled deformable mirror; the light patterns

Devices and Techniques for Sensorless Adaptive Optics

http://dx.doi.org/10.5772/53550

53

We proved that the image quality can be considerably improved by using these adaptive de‐ vices in an image sharpening setup. For example, the MDM allowed achieving a significant

**Figure 9.** Optimization of an image deteriorated by aberrations; left: initial image; right: image corrected by the MDM

(after 35 measurements).

necessary for their generation are on the left; the obtained corresponding interferograms are on the right.

image sharpening with just about 35 measurements, as illustrated in Fig. 9.

**Figure 7.** Photo-controlled deformable membrane mirror, PCDM. a) Schematic representation of the PCDM and the projection system allowing to achieve arbitrary actuator pads. b) Left: layout of the electrode pattern; right: corre‐ spondingly generated mirror shape; as an example the electrode pattern was chosen to generate astigmatism.

The calculation of the electrode pattern that generates a determined aberration is composed of the following steps:


A few examples of the realized electrode patterns are shown in Fig. 8, together with the cor‐ responding measurements of the aberrated wavefronts.

The MDM has already been demonstrated to be effective in several fields, as laser focaliza‐ tion [32], image sharpening and Optical Coherence Tomography ( OCT), as it will be dis‐

Another device for the generation of aberrations is the PhotoControlled Deformable Mirror (PCDM), which is schematically represented in Fig. 7. This deformable mirror [35, 36] is composed of an electrostatic membrane while the actuator pad array is replaced by a photo‐ conductive material. Thus, the membrane shape depends on the light pattern projected on the photoconductor. Arbitrary actuator pads can be conveniently achieved by illuminating the photoconductive side of the mirror with a commercially available Digital Light Process‐

**Figure 7.** Photo-controlled deformable membrane mirror, PCDM. a) Schematic representation of the PCDM and the projection system allowing to achieve arbitrary actuator pads. b) Left: layout of the electrode pattern; right: corre‐ spondingly generated mirror shape; as an example the electrode pattern was chosen to generate astigmatism.

The calculation of the electrode pattern that generates a determined aberration is composed

**b.** calculation of the membrane shape for each of the 40 × 40 pixels, solving the Poisson

**c.** determination of the pattern by pseudoinversion of the matrix determined at point b.

A few examples of the realized electrode patterns are shown in Fig. 8, together with the cor‐

**a.** division of the projector area into small subsets (i.e. 40 × 40);

equation by the iterative methods;

responding measurements of the aberrated wavefronts.

cussed later.

52 Adaptive Optics Progress

ing (DLP) hand-held projector.

of the following steps:

**Figure 8.** Generation of the first four Zernike orders with the photocontrolled deformable mirror; the light patterns necessary for their generation are on the left; the obtained corresponding interferograms are on the right.

We proved that the image quality can be considerably improved by using these adaptive de‐ vices in an image sharpening setup. For example, the MDM allowed achieving a significant image sharpening with just about 35 measurements, as illustrated in Fig. 9.

**Figure 9.** Optimization of an image deteriorated by aberrations; left: initial image; right: image corrected by the MDM (after 35 measurements).
