Design and evaluation of a 0.5 mm 3D printed optical system for multiphoton microendoscopy

Early detection of cancer is key to improving patient survival outcomes. Multiphoton microscopy (MPM) is a promising modality for imaging early cancers. For minimally invasive screening in vivo, implementation in small and flexible microendoscope systems is desired. We are developing a 1.0 mm diameter flexible side-firing helically scanning multiphoton endoscopic system for imaging the epithelial layer of small tissue lumens. The system is configured for simultaneous four-channel image acquisition in 2-photon excited fluorescence (2PEF), 3-photon excited fluorescence (3PEF) and second and third-harmonic generation (SHG & THG). Multiphoton signal is generated using 1400 nm femtosecond pulsed light delivered via the core of a dual clad fiber (DCF), and its first cladding is utilized to collect the visible to near-infrared wavelength multiphoton signal. The distal end of the endoscope consists of a 0.5 mm diameter 3D printed objective lens system that both illuminates tissue and collects the return signal. Implementing MPM in side-firing endoscope configurations is challenging due to the high numerical aperture (HNA) and short working distance (WD) requirements. In addition, the cylindrical shape of the outer sheath imparts additional aberration that must be appropriately compensated for. Additive manufacturing (3D printing) allows for the design of more complex monolithic multi-element lens systems suitable for this application. We have designed an objective lens system for 2-photon polymerization (2PP) printing process, designed to perform multiphoton imaging. The imaging lens system features a custom fold prism with an aspheric surface on its exit face, and an additional biconic surface to compensate for the sheath induced aberration. The system also includes a fiber centering ferrule that correctly centers the DCF and positions it axially, while minimizing assembly errors. Prototyping began with a simpler low numerical aperture (LNA) system with a longer working distance. This allowed for optimization of design, printing parameters, and assembly techniques. Insights from the LNA design and testing were then used in the design of an HNA optical system. We have modelled the performance of these lens systems in Zemax OpticStudio and developed an experimental setup that allows for the measurement of performance both with and without the outer sheath present.