Influence of laser beam quality on modal selection in tapered optical fibers for multipoint optogenetic control of neural activity

Spatial-resolved light delivery in the living mammalian brain is of utmost importance in optogenetics experiments, since it allows for combining cell-type specificity with spatial selective optical control of neural activity. This can be achieved at sub-cellular resolution in first cortical layers with two-photon microscopy, while sub-cortical regions can be accessed by using waveguide-based devices. Tapered and nanostructured optical fibers have been recently proposed as viable devices to dynamically switch light stimuli between different points of deep-brain structures, by exploiting both mode division demultiplexing operated by the fiber taper and excitation of well-defined subsets of guided modes into the fiber [Pisanello et al., Neuron, vol. 82, p.1245 (2014); Pisanello et al., Biomed. Opt. Express, vol. 6, p. 4014 (2015)]. This latter is achieved by selecting the injection angle of a high-quality Gaussian beam, therefore exciting guided modes with defined transversal propagation constants. In this contribution we analyze the effectiveness of this technique as a function of the laser beam profile used to guide light into the fiber, complementing experimental results with ray tracing simulations. Two different commercial laser sources, with sensibly different beam qualities, were used to determine the extent of the injected modal subsets in a 0.22 N.A. optical fiber. This was obtained by measuring the transversal component kt of the wavevector through far-field imaging. We show that even poor quality beams might return satisfactory selection accuracy, provided that care is taken in the design of the coupling strategy.