Dynamic distortion in resonant galvanometric optical scanners

Quantification of optical lensing by cellular structures in the living human eye

Cells and other microscopic phase objects can be visualized in the living retina, non-invasively, using non-confocal light detection schemes in adaptive optics scanning light ophthalmoscopes (AOSLOs). There is not yet widespread agreement regarding the origin of image contrast, nor the best way to render multichannel images. Here, we present data to support the interpretation that variations in the intensity of non-confocal images approximate a direct linear mapping of the prismatic deflection of the scanned beam. We advance a simple geometric framework in which local 2D image gradients are used to estimate the spherocylindrical refractive power for each element of the tissue. This framework combines all available information from the non-confocal image channels simultaneously, reducing noise and directional bias. We show that image derivatives can be computed with a scalable, separable gradient operator that minimizes directional errors; this further mitigates noise and directional bias as compared with previous filtering approaches. Strategies to render the output of split-detector gradient operations have been recently described for the visualization of immune cells, blood flow, and photoreceptors; our framework encompasses these methods as rendering astigmatic refractive power. In addition to astigmatic power, we advocate the use of the mean spherical equivalent power, which appears to minimize artifacts even for highly directional micro-structures such as immune cell processes. We highlight examples of positive, negative, and astigmatic power that match expectations according to the known refractive indices and geometries of the relevant structures (for example, a blood vessel filled with plasma acts as a negatively powered cylindrical lens). The examples highlight the benefits of the proposed scheme for the visualization of diverse phase objects including rod and cone inner segments, immune cells near the inner limiting membrane, flowing blood cells, the intravascular cell-free layer, and anatomical details of the vessel wall.

Suppression of Subpixel Jitter in Resonant Scanning Systems With Phase-locked Sampling

Resonant scanning is critical to high speed and in vivo imaging in many applications of laser scanning microscopy. However, resonant scanning suffers from well-known image artifacts due to scanner jitter, limiting adoption of high-speed imaging technologies. Here, we introduce a real-time, inexpensive and all electrical method to suppress jitter more than an order of magnitude below the diffraction limit that can be applied to most existing microscope systems with no software changes. By phase-locking imaging to the resonant scanner period, we demonstrate an 86% reduction in pixel jitter, a 15% improvement in point spread function with resonant scanning and show that this approach enables two widely used models of resonant scanners to achieve comparable accuracy to galvanometer scanners running two orders of magnitude slower. Finally, we demonstrate the versatility of this method by retrofitting a commercial two photon microscope and show that this approach enables significant quantitative and qualitative improvements in biological imaging.

Spatial and directional contrast dependence in Lissajous-scanning projection systems

Scanning-based image formation fundamentally differs from its classical lens-based counterpart. Therefore, established classical performance evaluation methods fail to determine the theoretical limitations of scanning-based optical systems. We developed a simulation framework and a novel performance evaluation process to evaluate the achievable contrast in scanning systems. Applying these tools, we conducted a study determining the resolution limits of different Lissajous scanning approaches. For the first time, we identify and quantify spatial and directional dependencies of the optical contrast and demonstrate their significant impact on the perceived image quality. We prove that the observed effects are more pronounced for Lissajous systems with high ratios of the two scanning frequencies. The presented method and results can lay the foundation for a more sophisticated application-specific design of next-generation scanning systems.

Evolution of adaptive optics retinal imaging [Invited]

This review describes the progress that has been achieved since adaptive optics (AO) was incorporated into the ophthalmoscope a quarter of a century ago, transforming our ability to image the retina at a cellular spatial scale inside the living eye. The review starts with a comprehensive tabulation of AO papers in the field and then describes the technological advances that have occurred, notably through combining AO with other imaging modalities including confocal, fluorescence, phase contrast, and optical coherence tomography. These advances have made possible many scientific discoveries from the first maps of the topography of the trichromatic cone mosaic to exquisitely sensitive measures of optical and structural changes in photoreceptors in response to light. The future evolution of this technology is poised to offer an increasing array of tools to measure and monitor in vivo retinal structure and function with improved resolution and control.

Dynamic wavefront distortion in resonant scanners

Dynamic mirror deformation can substantially degrade the performance of optical instruments using resonant scanners. Here, we evaluate two scanners with resonant frequencies >12kHz with low dynamic distortion. First, we tested an existing galvanometric motor with a novel, to the best of our knowledge, mirror substrate material, silicon carbide, which resonates at 13.8 kHz. This material is stiffer than conventional optical glasses and has lower manufacturing toxicity than beryllium, the stiffest material currently used for this application. Then, we tested a biaxial microelectromechanical (MEMS) scanner with the resonant axis operating at 29.4 kHz. Dynamic deformation measurements show that wavefront aberrations in the galvanometric scanner are dominated by linear oblique astigmatism (90%), while wavefront aberrations in the MEMS scanner are dominated by horizontal coma (30%) and oblique trefoil (27%). In both scanners, distortion amplitude increases linearly with deflection angle, yielding diffraction-limited performance over half of the maximum possible deflection for wavelengths longer than 450 nm and over the full deflection range for wavelengths above 850 nm. Diffraction-limited performance for shorter wavelengths or over larger fractions of the deflection range can be achieved by reducing the beam diameter at the mirror surface. The small dynamic distortion of the MEMS scanner offers a promising alternative to galvanometric resonant scanners with desirable but currently unattainably high resonant frequencies.

Towards distortion-free imaging of the eye

The high power of the eye and optical components used to image it result in "static" distortion, remaining constant across acquired retinal images. In addition, raster-based systems sample points or lines of the image over time, suffering from "dynamic" distortion due to the constant motion of the eye. We recently described an algorithm which corrects for the latter problem but is entirely blind to the former. Here, we describe a new procedure termed "DIOS" (Dewarp Image by Oblique Shift) to remove static distortion of arbitrary type. Much like the dynamic correction method, it relies on locating the same tissue in multiple frames acquired as the eye moves through different gaze positions. Here, the resultant maps of pixel displacement are used to form a sparse system of simultaneous linear equations whose solution gives the common warp seen by all frames. We show that the method successfully handles torsional movement of the eye. We also show that the output of the previously described dynamic correction procedure may be used as input for this new procedure, recovering an image of the tissue that is, in principle, a faithful replica free of any type of distortion. The method could be extended beyond ocular imaging, to any kind of imaging system in which the image can move or be made to move across the detector.

Correction of resonant optical scanner dynamic aberrations using nodal aberration theory

The rapid oscillation of galvanometric resonant optical scanners introduces linear astigmatism that degrades transverse resolution, and in confocal systems, also reduces signal [V. Akondi et al., Optica 7, 1506, 2020]. Here, we demonstrate correction of this aberration by tilting reflective or refractive optical elements for a single vergence or a vergence range, with and without the use of an adaptive wavefront corrector such as a deformable mirror. The approach, based on nodal aberration theory, can generate any desired third order aberration that results from tilting or decentering optical surfaces.

In-vivo sub-diffraction adaptive optics imaging of photoreceptors in the human eye with annular pupil illumination and sub-Airy detection

Adaptive optics scanning light ophthalmoscopy (AOSLO) allows non-invasive visualization of the living human eye at the microscopic scale; but even with correction of the ocular wavefront aberrations over a large pupil, the smallest cells in the photoreceptor mosaic cannot always be resolved. Here, we synergistically combine annular pupil illumination with sub-Airy disk confocal detection to demonstrate a 33% improvement in transverse resolution (from 2.36 to 1.58 μm) and a 13% axial resolution enhancement (from 37 to 32 μm), an important step towards the study of the complete photoreceptor mosaic in heath and disease. Interestingly, annular pupil illumination also enhanced the visualization of the photoreceptor mosaic in non-confocal detection schemes such as split detection AOSLO, providing a strategy for enhanced multimodal imaging of the cone and rod photoreceptor mosaic.