Numerical Calculation of the Light Propagation in Tapered Optical Fibers for Optical Neural Interfaces.

Rosa Mach-Batlle,Ferruccio Pisanello,Massimo De Vittorio,Marco Pisanello,Cristian Ciraci,Filippo Pisano

doi:10.1109/jlt.2021.3118898

Abstract

As implantable optical systems recently enabled new approaches to study the brain with optical radiations, tapered optical fibers emerged as promising implantable waveguides to deliver and collect light from sub-cortical structures of the mouse brain. They rely on a specific feature of multimodal fiber optics: as the waveguide narrows, the number of guided modes decreases and the radiation can gradually couple with the environment. This happens along a taper segment whose length can be tailored to match with the depth of functional structures of the mouse brain, and can extend for a few millimeters. This anatomical requirement results in optical systems which have an active area that is very long compared to the wavelength of the light they guide and their behavior is typically estimated by ray tracing simulations, because finite element methods are too computationally demanding. Here we present a computational technique that exploits the beam-envelope method and the cylindrical symmetry of the fibers to provide an efficient and exact calculation of the electric field along the fibers, which may enable the design of neural interfaces optimized to meet different goals.

Highlights

Optogenetics is a powerful technique that uses light to control the electrical activity of neurons genetically modified to be light sensitive [1, 2]
The design of optical neural interfaces able to deliver light in a controlled manner in deep brain regions is an essential ingredient to achieve an optimal interaction with neurons and exploit the full potential of optogenetics [6]
In order to match the spatial extent of elongated brain regions and achieve an effective mode-division demultiplexing, the length of the tapered region must be of the order of some millimeters, much larger than the wavelength range typically employed in optogenetics (450-600 nm)

Summary

INTRODUCTION

Optogenetics is a powerful technique that uses light to control the electrical activity of neurons genetically modified to be light sensitive [1, 2]. In order to match the spatial extent of elongated brain regions and achieve an effective mode-division demultiplexing, the length of the tapered region must be of the order of some millimeters, much larger than the wavelength range typically employed in optogenetics (450-600 nm) As this makes traditional finite element calculations computationally very demanding, the power that is guided and delivered to the brain is usually estimated using ray tracing simulations [10, 11, 12, 13]. These calculations help predict at what distance from the fiber tip the light is delivered but they do not provide an estimation of the electric and the magnetic field distribution in the targeted region, since they fail to describe interference and diffraction phenomena This fact limits our ability to optimize the delivery and collection of light, and makes it difficult to design new optical neural interfaces with different functionalities. We compare the light delivery depth at different input angles for two different fibers

CALCULATION OF THE INCIDENT FIELD DISTRIBUTION

BEAM ENVELOPE METHOD FOR CALCULATING THE

RESULTS AND DISCUSSION

Results are shown in

DISCLOSURES