Tapered fibers for optogenetics: Gaining spatial resolution in deep brain regions by exploiting angle-selective light injection systems

This work describes a method to inject light into tapered optical fibers (TFs) in order to achieve site-selective and wide-volume optical control of neural activity with a single optical setup. The method relies on a Galvanometric - Resonant mirrors scan head, employed to inject light into the fiber to: (i) select a specific subset of guided modes obtaining localized illumination or (ii) to scan between the different guided modes at high-rate to obtain full NA-like light injection on time scales >0.2 ms. This is shown by analysing the light emission profiles from the TF with different voltage biases applied to the Galvanometric and the Resonant mirrors, and comparing them with a standard Full NA light-injection. With the aim to find the experimental parameters to obtain efficient Full NA - like emission profile, the Emission Length and the First Emission Diameter of TFs were extracted and compared with an ideal wide-volume emitter. Both Galvanometric and Resonant scanners methods shown a good agreement with the reference values of the standard Full NA injection, allowing neuroscientists to switch from site-selective to wide-volume illumination using a single optical setup.