Technical articles

Defocus generation with MMDM and PDM

What is the maximum defocus correction I can get with your mirror? Is it possible to generate both negative and positive defocus? Which mirror, PDM or MMDM, do you advice me to use for correction of Z2,0? — Here, at Flexible Optical, we receive quite often these questions. This short article explains the aspects of the generation/correction of the defocus aberration with MMDM and PDM and considers the factors limiting the amplitude of the correction with various types of mirrors.

The actuator stroke (see Fig.1) is the most important factor defining the maximum aplitude of the correctable defocus. The maximum stroke of the pdm actuator is 8 µm, one side, at 400V driving voltage (maximum allowed voltage), and 6µm at 300 V (recommended maximum voltage). MMDM, depending on the aperture size, has maximum stroke from 9 to 30 µm. Thus with 37-channel 15 mm MMDM, one can obtain a bigger defocus range than with the 37-channel 30mm PDM. But considering the fact that the active aperture, or the beam size on the deformable mirror, is usually less than the full aperture (about 2/3 of the full aperture), the active defocus range does not coincide with the maximum center deflection; moreover, the reference surface is flat for the PDM and concave (defocus) for the MMDM, and the range for MMDM and PDM becomes comparable. 

Figure 1. Scematical drawing illustrating differneces in defocus generation with MMDM and MMDM (click here for larger version).


MMDMs with larger aperture have bigger maximum deflection (up to 15 um). But if you aberration is mainly defocus, we recommend you to consider our 30- and 50-mm PDMs with 18 actuators — due to the absence of the actuators inside the active aperture, they have defocus range corresponding to  about +-12 µm deflection of the central point. Figure 2 shows interferograms obtained with 18-channel 30-mm PDM for maximum deflection and maximum defocus for 0.1 µm rms error in the reflected wavefront.

Figure 2. Interferograms showing maximum deflection and defocus for 30-mm 18-ch PDM
in300 out300 defoc_4_7mic defoc-5_8mic
full aperutre, 300V applied to the internal ring of actuators, 0V to the external one

full aperutre, 300V applied to externalring of actuators, 0V to the internal one

20mm aperture, defocus +4.7 µm

20mm aperture, defocus -5.8 µm