Dynamic performance of microelectromechanical systems deformable mirrors for use in an active/adaptive two-photon microscope.

Active optics such as deformable mirrors can be used to control both focal depth and aberrations during scanning laser microscopy. If the focal depth can be changed dynamically during scanning, then imaging of oblique surfaces becomes possible. If aberrations can be corrected dynamically during scanning, an image can be optimized throughout the field of view. Here, we characterize the speed and dynamic precision of a Boston Micromachines Corporation Multi-DM 140 element aberration correction mirror and a Revibro Optics 4-zone focus control mirror to assess suitability for use in an active and adaptive two-photon microscope. Tests for the multi-DM include both step response and sinusoidal frequency sweeps of specific Zernike modes (defocus, spherical aberration, coma, astigmatism, and trefoil). We find wavefront error settling times for mode amplitude steps as large as 400 nm to be less than 52???s, with 3 dB frequencies ranging from 6.5 to 10 kHz. The Revibro Optics mirror was tested for step response only, with wavefront error settling time less than 80???s for defocus steps up to 3000 nm, and less than 45???s for spherical aberration steps up to 600 nm. These response speeds are sufficient for intrascan correction at scan rates typical of two-photon microscopy.

Four-zone varifocus mirrors with adaptive control of primary and higher-order spherical aberration.

Electrostatically actuated deformable mirrors with four concentric annular electrodes can exert independent control over defocus as well as primary, secondary, and tertiary spherical aberration. In this paper we use both numerical modeling and physical measurements to characterize recently developed deformable mirrors with respect to the amount of spherical aberration each can impart, and the dependence of that aberration control on the amount of defocus the mirror is providing. We find that a four-zone, 4 mm diameter mirror can generate surface shapes with arbitrary primary, secondary, and tertiary spherical aberration over ranges of ±0.4, ±0.2, and ±0.15 µm, respectively, referred to a non-normalized Zernike polynomial basis. We demonstrate the utility of this mirror for aberration-compensated focusing of a high NA optical system.