A Fiber-optic Sensing Technique for Sub-micrometer Distance Measurement between the Surgical Tools and Retina

Many retina diseases require very often a delicate surgical intervention, which is performed by different minimally invasive instruments and tools. They must be miniature but still robust and capable to accurately determine the separation to the retina target in real-time. Because of that they have to be instrumented with a sensing technique, which may interactively read the actual distance toward the target without any risk of injuring the retina. In this paper, we present a fiber-optic sensing technique suitable for integration into surgical instruments. The technique is composed of low- and high-coherence interferometry that allows sub-micrometer absolute distance measurement between the surgical tools and retina without any scanning unit in the reference arm. The technique is demonstrated utilizing the experimental setup based on a $2\times 2$ fiber-optic coupler simultaneously powered by an SLD and VCSEL as low- and high-coherence light sources, respectively. The sensing arm was directed toward the eye model, made of the inner silicon layer and outer polylactide (PLA) 3D printed eyeball. The distance between the sensing fiber tip and target has been measured using both low- and high-coherence interferograms over a rather large dynamic range of about 2 mm with the accuracy of 655 nm that corresponds with $\lambda/2$ of the high-coherence light source.