Lens design strategies for miniature scanning fiber endoscopes

This paper provides guidance for the illumination configuration optical design of scanning fiber endoscopes (SFEs), offering insights into salient optical principles and relevant optical system properties, as well as trade-offs and considerations necessary for achieving the desired image performance in miniature endoscopic systems. The specific design examined is a piezoelectric scanner tube-based cantilevered SFE with a concentrically mounted, cantilevered single mode fiber. The assumption of monochromaticity is made for simplicity. Example designs are provided, simulated in Ansys Zemax OpticStudio. The purpose of this guide is to assist designers who may have limited optical design experience by addressing each phase of the SFE design process, from fiber oscillation modeling to optical element selection. Beginning with the motivation for utilizing SFE and introduction of design principles, the guide then describes appropriate approximations for the object, in this case the light emitted from the tip of the scanning fiber over its scanning range (including deflection, sag, and normal angle). Important optical system properties are described, including optical system length, working distance, field of view, throughput, magnification, common aberrations, resolution, and depth of focus, as well as cost. Example optical designs are then modeled in Ansys Zemax OpticStudio, to illustrate common trade-offs in optical performance. The examples are separated into two main categories, variable magnification systems, and fixed magnification systems. Both variable and fixed magnification designs start from a paraxial pre-design followed by a one utilizing real commercial off the shelf (COTS) lenses, both conventional and gradient index (GRIN) lenses. A final design provides an example of optimizing performance priorities utilizing a hybrid of COTS and custom elements. This guide provides the design steps and starting points needed to create an intuitive and well-performing SFE optical design.