Single- and multi-layer micro-scale diffractive lens fabrication for fiber imaging probes with versatile depth-of-field

Hair-thin optical fiber endoscopes have opened up new paradigms for advanced imaging applications in vivo. In certain applications, such as optical coherence tomography (OCT), light-shaping structures may be required on fiber facets to generate needle-like Bessel beams with large depth-of-field, while in others shorter depths of field with high lateral resolutions are preferable. In this paper, we demonstrate a novel method to fabricate light-shaping structures on optical fibres, achieved via bonding encapsulated planar diffractive lenses onto fiber facets. Diffractive metallic structures have the advantages of being simple to design, fabricate and transfer, and our encapsulation approach is scalable to multi-layer stacks. As a demonstration, we design and transfer a Fresnel zone plate and a diffractive axicon onto fiber facets, and show that the latter device generates a needle-like Bessel beam with 350 mu m focal depth. We also evaluate the imaging performance of both devices and show that the axicon fiber is able to maintain focussed images of a USAF resolution target over a 150 mu m distance. Finally, we fabricate a two-layer stack of Fresnel zone plates on a fiber and characterise the modified beam profile and demonstrate good imaging performance. We anticipate our fabrication approach could enable multi-functional complex optical structures (e.g. using plasmonics, polarization control) to be integrated onto fibers for ultra-thin advanced imaging and sensing.