1. A method of assembling a waveguide neural interface device, comprising the steps of: a) providing a substrate comprising a first proximal substrate branch and a second proximal substrate branch extending proximally from at least a distal substrate portion; b) mounting at least one waveguide to the substrate, wherein the waveguide is configured to carry light along a light path from a proximal waveguide portion supported at the first proximal substrate branch to a distal waveguide portion including at least one distal light directing element supported at the distal substrate portion, the light directing element being configured to emit light carried by the waveguide away from a longitudinal axis of a distal portion of the light path; c) supporting at least one electrode site on the distal substrate portion, wherein the at least one electrode site is electrically connected to an interconnect supported on the second proximal substrate branch; d) providing a circuit board comprising a first circuit board face delineated from a second circuit board face by a fold line; e) mounting the first and second proximal substrate branches supporting the respective proximal waveguide portion and the interconnect to the first circuit board face; f) mounting a light source to the second circuit board face; and g) folding the circuit board along the fold line to thereby move the respective first and second circuit board faces into close proximity with each other so that the light source is positioned to shine light into the proximal waveguide portion at the first proximal substrate branch so that the shone light travels along the light path to then be emitted from the waveguide adjacent to the at least one electrode site by the light directing element at the distal substrate portion.