Microscopy is the workhorse of contemporary life science research, enabling morpho­logical and chemical inspection of living tissue with ever-increasing spatial and temporal resolution. Even though modern micro­scopes are genuine marvels of engineering, minute deviations from ideal imaging conditions will still lead to optical aber­rations that rapidly degrade imaging quality. A mismatch between the refractive indices of the sample and its immer­sion medium, deviations in the thickness of sample holders or cover glasses, the effects of aging on the instrument – such deviations can manifest themselves in the form of spherical aber­ration and focusing errors. Also, particularly for deep tissue imaging, an essential tool in neuro­biology research, an inhomo­geneous refractive index of the sample and its complex surface shape can lead to addi­tional higher order aber­rations.

Adaptive optics (AO), an image correction technique first used in astro­nomical telescopes for compen­sating the effects of atmospheric turbulence, is the state-of-the-art method to dynamically correct for sample and system-induced aber­rations in a micro­scopy system. A typical AO system features an active, shape­shifting optical element that can reproduce the inverse of the wavefront error present in the system. Commonly taking the form of either a deformable mirror or a liquid crystal spatial light modulator, the limi­tations of this element define the quality of achievable aber­ration correction and thus the widespread appli­cability of AO microscopy. Now, researchers from the University of Freiburg, Germany, have made a signi­ficant advance in AO microscopy through the demons­tration of a new AO module comprising two deformable phase plates (DPPs).

In contrast to defor­mable mirrors, the DPP system is a wavefront modulator operating in trans­mission, enabling direct AO inte­gration with existing microscopes. In this AO configuration, similar to hi-fidelity loud­speakers with separate woofer and tweeter units, one of the optical modu­lators is optimized for low-spatial frequency aber­rations, while the second is used for high-frequency correction. A major challenge for an AO system with multiple phase modulators is how to place them on opti­cally equivalent (conjugate) positions, often requiring multiple addi­tional optical components to relay the image until it reaches the detector. Therefore, configuring even two modu­lators in an AO system is very challenging.

Since the DPPs are <1 milli­meter in thickness, cascading two or more modu­lators within acceptable proximity becomes substantially more practical. The Freiburg team also developed a new method to optimally control multiple phase modulators regardless of their individual specifications, potentially enabling cascading of many more devices for increased range and fidelity. To demons­trate its performance, the team integrated their new AO system into a custom-built fluores­cence microscope, where sample-induced aber­rations are iteratively estimated without a wavefront sensor. Imaging experiments on synthetic samples demons­trated that the new AO system not only doubles the aber­ration correction range, but also greatly improves correc­tion quality. The work demons­trates that more advanced aber­ration correction schemes, such as multi-conjugate adaptive optics, can be implemented as easily and with new and more advanced control methods. (Source: SPIE)

Reference: P. Rajaeipour et al.: Cascading optofluidic phase modulators for performance enhancement in refractive adaptive optics, Adv. Phot. 2, 066005 (2020); DOI: 10.1117/1.AP.2.6.066005

Link: Dept. of Microsystems Engineering IMTEK, University of Freiburg, Freiburg, Germany