Deformation enduring conveyance of structured light through multimode waveguides and its exploitation for flexible hair-thin endoscopes
The remarkable advancements in our capacity to synthesise structured light have facilitated the generation of any desired optical landscapes and even controlling the spatial distribution of light propagating through optically complex media such as multimode fibres. The availability of precisely defined structured light at the extremity of an exceedingly narrow and flexible cable holds the potential to stimulate a diverse range of highly sought-after applications, encompassing rapid communication, quantum computing, and, notably, imaging. What we lack in reaching these aspirations is the resilience of such light transport to deformations of the waveguide. Although recent theoretical investigations have delineated the attributes of ideal multimode fibres capable of deformation-enduring conveyance of structured light, tangible fibres possessing this indispensable trait to a practical extent remain elusive. Our study takes a deep dive into the precision of commercially available multimode fibres with the highest probability of demonstrating the phenomenon. We identified minuscule imperfections in their refractive index distribution, examined how these affect light transport when the fibre is deformed, and studied their implications for imaging applications. Our investigation has confirmed that these imperfections are indeed responsible for the undesirable alterations introduced into the output structured light fields during bending. Finally, as an alternative to standard graded-index fibres, manufactured by drawing silica-based preforms, we present narrow multimode waveguides in which the refractive-index profile has been established by ion exchange. These waveguides indeed exhibit previously unseen resilience of structured light transport even under severe deformation conditions and aptly fulfil the requirements of imaging applications.