An active reflective component can change its focal length by physically deforming its reflecting surface. Such elements exist at small apertures, but have yet to be fully realized at larger apertures. This paper presents the design and initial results of a large-aperture active mirror constructed of a composite material called carbon fiber reinforced polymer (CFRP). The active CFRP mirror uses a novel actuation method to change radius of curvature, where actuators press against two annular rings placed on the mirror’s back. This method enables the radius of curvature to increase from 2000mm to 2010mm. Closed-loop control maintains good optical performance of 1.05 waves peak-to-valley (with respect to a HeNe laser) when the active CFRP mirror is used in conjunction with a commercial deformable mirror.
Implementation of AO in high performance microscopes is very dependent on the type of microscopy and the
nature of the studied specimen. In this communication, we present the comparison between different
implementations of AO sectioning microscopes. The analysis of the benefits and drawbacks of the correction
strategies is also presented. Finally, we presented some results obtained by using the discussed strategies of
The imaging depth of two-photon excitation fluorescence microscopy is partly limited by the inhomogeneity of the refractive index in biological specimens. This inhomogeneity results in a distortion of the wavefront of the excitation light. This wavefront distortion results in image resolution degradation and lower signal level. Using an adaptive optics system consisting of a Shack-Hartmann wavefront sensor and a deformable mirror, wavefront distortion can be measured and corrected. With adaptive optics compensation, we demonstrate that the resolution and signal level can be better preserved at greater imaging depth in a variety of ex-vivo tissue specimens including mouse tongue muscle, heart muscle, and brain. However, for these highly scattering tissues, we find signal degradation due to scattering to be a more dominant factor than aberration.
Several quantitative phase imaging techniques, such as digital holography, Hilbert-phase microscopy, and phase-shifting
interferometry have applications in biological and medical imaging. Quantitative phase imaging measures
the changes in the wavefront of the incident light due to refractive index variations throughout a 3-D specimen. We
have developed a multimodal microscope which combines optical quadrature microscopy (OQM) and a Shack-
Hartmann wavefront sensor for applications in biological imaging. OQM is an interferometric imaging modality
that noninvasively measures the amplitude and phase of a signal beam that travels through a transparent specimen.
The phase is obtained from interferograms with four different delayed reference wavefronts. The phase is then
transformed into a quantitative image of optical path length difference. The Shack-Hartmann wavefront sensor
measures the gradient of the wavefront at various points across a beam. A microlens array focuses the local
wavefront onto a specific region of the CCD camera. The intensity is given by the maximum amplitude in the
region and the phase is determined based on the exact pixel position within the region.
We compare the amplitude and quantitative phase information of poly-methyl-meth-acrylate (PMMA) beads in oil
and one-cell and two-cell mouse embryos with micrometer resolution using OQM and the Shack-Hartmann. Each
pixel in OQM provides a phase measurement, whereas multiple pixels are used in Shack-Hartmann to determine the
tilt. Therefore, the simple Shack-Hartmann system is limited by its resolution and field-of-view. Real-time imaging
in Shack-Hartmann requires spatial averaging which smoothes the edges of the PMMA beads. The OQM has a
greater field-of-view with good resolution; however, it is a complex system requiring multiple optical components
and four cameras which may introduce additional artifacts in processing quantitative images. The OQM and Shack-
Hartmann has certain advantages depending on the application. A combination of these two systems may provide
improved quantitative phase information than either one alone.cHJl