STED Simulation and Analysis
development to completion of the commercial product. Today’s optical design solutions provide great tools for engineers to simulate and optimize optical system performance. One of these tools that is widely used in the photonics industry is Zemax OpticStudio®
. However, these tools are limited by what manufactur-
ers are willing to share about the design and content of their components. Many engineers are strongly motivated to use standard off-the-shelf catalog parts as much as pos- sible in their system design rather than using customized parts. In practice, however, seldom do catalog parts allow full simulation capabilities. Intellectual property protection, among other reasons, leads to catalog optical components and modules that cannot be simulated reliably and effec- tively, making it difficult for engineers to integrate and ana- lyze their performance in their system design. Tis lack of analysis capabilities can result in unpleasant surprises when the purchased parts do not operate as expected or as defined in specifications. Zemax introduced a partial solution to overcome this
Figure 1: Schematic setup of STED microscope system showing the excita- tion (532 nm) and depletion (640 nm) lasers used to create the donut-shaped beam.
laser damage threshold, surface deviation and micro-rough- ness, and mechanical properties. STED microscopy is a technique for measuring submi-
cron structures, which requires the system to be highly precise and accurate. Otherwise, overall performance may be severely affected. Te precision accuracy is where diffractive optics, with all the aforementioned advantages, fit in. Optical system design tools are used worldwide to shorten
development time, to decrease the number of prototyping iterations, and to reduce expenses all the way from prototype
problem with their “Black Box models.” Te Black Box allows users to perform a ray tracing analysis on a specific opti- cal device while preventing them from seeing the internal structure of the device. Unfortunately, the Black Box models are limited to geometrical ray tracing and to Huygens point spread function (PSF) methods, and can be used only in sequential mode. In their article “How to design a confocal fluorescent microscope in OpticStudio” [1], Zemax used a custom-developed system in a non-sequential mode, but this option is not feasible when using catalog parts protected by Black Boxes. In STED microscopy, the two key parts that are most
challenging to model are the focusing objective and the vortex lens. Occasionally, a Black Box file for the focusing objective and a VL simulation for the physical optics [2] are available. However, it is unlikely that both will work in a sequential mode. There are several known methods to model a VL in
Zemax. The custom delay-locked loop (DLL) phase sur- face is one, grid phase sag is another, and stereolithogra- phy (STL) sag CAD files are the last (hybrid sequential or non-sequential mode). The most effective method for geo- metrical optics ray tracing is the cus- tom DLL for phase surface developed by Holo/Or Ltd. With this method, aperture size is automatically adjusted and has variable parameters for topo- logical charge “m” and sign to define the phase direction clockwise or coun- terclockwise. Figure 3 summarizes the challenges
of designing a STED optical system and the proposed solution for each. Te simulation presented in this
Figure 2: Diffractive surface of a 16-level VL DOE measured with an optical profilometer. 2021 May •
www.microscopy-today.com
paper assumes a basic level of knowledge and understanding of lasers, optical ele- ments, and their parameters. In addition, entry-level practice experience doing optical design with Zemax OpticStudio®
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