Fig. 1: Optical framework for applications of ultrathin fibers with shaped pattern write-in in a freely moving animal.

A Optical system for automatic fiber bundle detection and optimized fiber coupling from an acousto-optically modulated (AOM) diode-pumped solid-state (DPSS) laser. Feedforward control from the ferrule angle allows the galvo scanner to track one of the selected fibers (f1, f2, f3, and f4) in a freely moving animal. B A 30 µm fiber next to a 230 µm fiber. Scale bar: 200 µm. The inset depicts a neuron to illustrate the size relationship. C Minimal bending radius for fibers with different outer/core diameter ratios: 230/200 µm (yellow), 125/100 µm (green), 65/50 µm (blue), and 30/24 µm (red). Scale bar: 2 mm. D Implantation schematics. The fibers (yellow), matrix connector (gray), electrode ribbon cables (orange/red), and electrode holder (green) were all cemented (pink) to the bone. E Coupling from 60 µm fiber (large filled yellow circle) on the laser side to two or four 30 µm fibers (small filled yellow circle) on the animal side. F Topological relation between implanted fibers and fiber-matrix connector. The through-hole was 75 μm in diameter, which allowed us to insert four 30 µm fibers. G Simplified 3D schematic illustration of the circular fiber-matrix connector plates, with a triangular cutout to illustrate the cross-section through the 380 µm-thick plate, created in silicon using microsystems processing (four 4 × 4 arrays are shown for improved visibility; the actual connector had four 6 × 6 arrays). H Photograph of fiber-matrix connector. Fibers were reverse-illuminated to visualize their location in the fiber-matrix connector. Scale bar: 1 mm. I Left: Fiber hole alignment for a 30/24 µm outer/core diameter fiber (top) and a 65/50 µm outer/core diameter fiber (bottom). Scale bar: 50 µm. Right: Quantification of the offset between two connector plates for two different hole sizes for 30 µm-diameter fiber (24 µm core) and 65 µm-diameter fiber (50 µm core). J Connector transmission efficiency and crosstalk between neighboring fibers in the connector. K Coated 2 mm fiber in milk. Scale bar: 1 mm. Note that, among the different emission directions tested (white, blue, and red arrows), the radial side-emitted component (red arrow) is larger than the axial-forward component (blue arrow). Source data are provided as a Source Data file.