Fig. 1From: All-fiber device for single-photon detectiona Schematic of conventional free space coupling. Lower panel: key-shaped substrate with SPD [23, 33]. b Schematic of the conventional grating coupling from the fiber to the on-chip waveguide. Lower panel: rectangular silicon substrate with SPD [34, 35]. c Schematic of the single-mode fiber-based direct coupling. Lower panel: fiber-based SPDBack to article page