Fibre-optical microlenses fabricated by two-photon direct laser writing for optical endoscopy

Abstract

A magical door to explore the microscopic world has been opened by optical microscopes which have become an essential tool in fluorescence imaging and live-cell imaging. Differently, optical endoscopy can image samples that reside deep below the surface of the hollow or solid tissues and organs. Surprisingly, the long history of the optical endoscopes can be traced back to ancient Greece and Rome. Nowadays, modern optical endoscopy introduced with fibre-optics has become increasingly significant in the applications of the portable handheld imaging devices, endoscopy imaging inside the freely moving animal subjects, observing the symptoms of some special diseases and behaviours of animals for a long term, and accomplishing medical diagnosis or endoscopic microsurgery minimally invasively, owing to its high performance, compact size and flexibility. Obtaining high resolution and miniaturisation for fibre-optical endoscopy remains a challenge. The fibre-optical microlens for optical endoscopy is essential for improving the imaging resolution of and miniaturising the fibre-optical endoscopy probe. However, there is a trade-off among resolution, dimensions and complexity in the fibre-optical microlens for optical endoscopy. High resolution, miniaturisation and low complexity cannot be simultaneously achieved because the current lens systems are restricted in shape and dimensions, owing to the limitations in fabricating complex surfaces with sub-millimetre dimensions. Recently, two-photon direct laser writing (DLW) has presented great potential for fabricating various micro-optics for different applications because of the fineness and smoothness of the fabricated microstructures. The photoresist polymerised by two-photon DLW acts as an optical material (e.g., lens materials); thus, the structures fabricated by DLW can be used as optical components. Micro-optics fabricated by two-photon DLW on the facet of the fibre makes these optical components accessible for fibre-optical applications. The aim of this thesis is to demonstrate novel fibre-optical microlenses fabricated by two-photon DLW for optical endoscopy. First, a high-resolution miniaturised singlet free-form focusing fibre-optical microlens, has been demonstrated by fabricating on the fibre facet using two-photon DLW. A series of free-form focusing fibre-optical microlenses with numerical apertures (NAs) of 0.3, 0.6, and 0.9 on the fibre facet are designed for fibre-optical endoscopy with only one aspherical surface. These free-form focusing fibre-optical microlenses are aberration-free at working wavelengths of 561, 590, and 630 nm. Two-photon DLW of the free-form focusing fibre-optical microlenses on the fibre facet has been demonstrated. The diameter of each free-form focusing fibre-optical microlens is only 30 um. The micro-optics structure, including a microlens and a holder, has an outer diameter of 40 um, as well as a length of 80-90 um. The diameter is 10-20 times smaller than that of a GRIN microlens. The miniaturisation of the lens system has neared the limit for fibre-optical endoscopy. Therefore, the size of the lens system is no longer the primary limitation for the miniaturisation of the fibre-optical endoscope if a free-form focusing microlens is applied. The 0.9 NA free-form focusing microlens provides a resolution of 0.85 um at a wavelength of 561 nm. If it is applied in the confocal imaging modality of fibre-optical endoscopy, a resolution of approximately 0.60 um is expected. Second, high-resolution confocal fibre-optical endoscopy imaging with the free-form focusing microlens printed on the facet of an optical fibre by two-photon DLW has been demonstrated. The imaging process of confocal fluorescent endoscopy imaging using the free-form focusing fibre-optical microlens fabricated on the fibre facet is explained with the image formation. Confocal endoscopy imaging of a fluorescent three-bar structure and fluorescent beads has been achieved. The imaging resolution has reached 0.81 um. Third, the miniaturised free-form wide-field fibre-optical microlens fabricated on the facet of a fibre with two-photon DLW has been demonstrated. The imaging formation of the free-form wide-field fibre-optical microlens is conducted. The large spacing distance of the fibre cores mainly limits the resolution of the system. The free-form wide-field fibre-optical microlens can be fabricated on glass and the fibre facet. For miniaturisation, the fabricated structure including the holder has a diameter of 80 um and a height of 54 um. The diameter is 5-10 times smaller than that of a GRIN microlens. Last, wide-field optical endoscopy imaging using the free-form wide-field fibre-optical microlens and beam splitter fabricated by two-photon DLW has been demonstrated. Wide-field fibre-optical endoscopy imaging using the wide-field microlens and the multi-core fibre is achieved with a magnification of 1/88×, and an effective imaging area on the multi-core fibre facet including approximately 300 pixels. Two types of miniaturised beam splitter are fabricated on the fibre facet at the detection end by two-photon DLW for integration of the detection system. For confocal fibre-optical endoscopy imaging, the circular beam splitter and collective microlens are fabricated on the fibre facet. The whole fabricated structure has a diameter of 50 um and a length of 120 um. For wide-field fibre-optical endoscopy imaging, the square beam splitter is fabricated on the fibre facet. The square beam splitter has a side length of 100 um and a height of 100 um. With miniaturised imaging and detection components, the ultra-light wide-field optical endoscopy imaging system using the free-form wide-field fibre-optical microlens and beam splitter has been achieved. In conclusion, this study has demonstrated free-form focusing and wide-field fibre-optical microlenses fabricated by two-photon DLW on the fibre facet for confocal and wide-field fibre-optical endoscopy. The miniaturisation of the optical endoscopy probe has been developed significantly because of the ultra-compact microlens on the fibre. In addition, at the detection end of the fibre, the beam splitter can be integrated with a micrometre scale. The knowledge in this thesis has provided huge potential in many relevant fields in in vivo imaging and other biomedical applications, especially in brain and neuron imaging.