We present what we believe is the first conjugate adaptive optics (AO) extension that can be retrofitted into a commercial microscope by being positioned between the camera port and the image sensor. The extension features a deformable phase plate (DPP), a refractive wavefront modulator, and indirect wavefront sensing to form a completely in-line architecture. This allows the axial position of the DPP to be optimized by maximizing an image quality metric, which is a cumbersome task with deformable mirrors as the correction element. We demonstrate the performance of the system on a Zeiss AxioVert 200M microscope equipped with a 20× 0.75 NA air objective. To simulate sample-induced complex aberrations, transparent custom-made arbitrary phase plates were introduced between the sample and the objective. We demonstrate that the extension can provide high-quality full-field correction even for large aberrations, when the DPP is placed at the conjugate plane of the phase plates. We also demonstrate that both the DPP position and its surface profile can be optimized blindly, which can pave the way for plug-and-play conjugate-AO systems.
A simulation tool has been developed for the evaluation of Deformable Phase Plate (DPP) in optical systems. The tool, in the form of a dynamic-link library (DLL), predicts the behavior of a DPP by incorporating its empirically measured characteristics.
Microscopy through inhomogeneous media distorts the wavefront of the transmitted light and results in the loss of resolution and contrast. This wavefront distortion may be compensated by the use of adaptive optics (AO). The state-of-the-art AO systems for commercial microscopes generally use deformable mirrors for wavefront modulation, which are difficult to integrate as they require the use of folded imaging paths. Herein, we present a compact and miniaturized AO ’add-on’ that fits between an objective and the turret as a simpler ’plug-and-play’ alternative. The AO add-on features a deformable phase plate - a transmissive, optofluidic wavefront modulator that is capable of wavefront correction up to 7th radial Zernike modes within a clear aperture of 10 mm, and open-loop control based-on sensorless wavefront estimation.
Diffraction-limited imaging in microscopy is only possible if different layers within the objective’s working distance all have an uniform refractive index. However, in many practical imaging problems the samples are inhomogeneous in optical density, and refraction through them introduces field-dependent wavefront aberrations. On the image plane these are manifested in varying degrees of resolution and contrast degradation across the field-of-view. In pupil adaptive optics (AO), where a wavefront modulator is accommodated at the objective’s pupil-plane, a single correction profile is applied for all fields, and the correctable field-of-view (FoV) is limited by the isoplanatic patch. In the alternative configuration of conjugate adaptive optics, where the corrective element and the dominant aberrating layer within the sample sit at optically conjugate planes, effective correction across the entire FoV is possible in principle. This configuration, however, is relatively difficult to implement with deformable mirrors for wavefront modulation, since several folded optical paths have to be constructed. In this work, we present the design and evaluation of a completely in-line conjugate-AO system based on a refractive wavefront modulator (DELTA7 from Phaseform GmbH, Germany). We evaluate the performance of the system on a Zeiss Axiovert inverted microscope by imaging fluorescent beads through custom-fabricated phase plates.
We discuss the implementation and performance of an adaptive optics (AO) system that uses two cascaded deformable phase plates (DPPs), which are transparent optofluidic phase modulators, mimicking the common woofer/tweeter-type astronomical AO systems. One of the DPPs has 25 electrodes forming a keystone pattern best suited for the correction of low-order and radially symmetric modes; the second device has 37 hexagonally packed electrodes better suited for high-order correction. We also present simulation results and experimental validation for a new open-loop control strategy enabling simultaneous control of both DPPs, which ensures optimum correction for both large-amplitude low-order, and complex combinations of low- and high-order aberrations. The resulting system can reproduce Zernike modes up to the sixth radial order with stroke and fidelity up to twice better than what is attainable with either of the DPPs individually. The performance of the new AO configuration is also verified in a custom-developed fluorescence microscope with sensorless aberration correction.
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