Fig. 6: Light paths in different optical fibres for the analysis of the divergence angles (or equivalently, the acceptance angles).

Red paths represent the light reflected from the upper core/cladding interface of the optical fibre, and green paths represent the light reflected from the lower core/cladding interface of the optical fibre. a, b In the bare multimode optical fibre, the light can be reflected from the upper (a) or lower (b) core/cladding interface. c,d In the optical fibre capped with a spherical microlens, the light can be reflected from the upper (c) or lower (d) core/cladding interface. e–h In the optical fibre capped with a conical microlens, the light has various paths. In one case, the light experiences no reflection in the conical surface after being reflected from the upper (e) or lower (f) core/cladding interface. In the other case, the light experiences one hop (i.e., reflection) in the conical surface after being reflected from the upper (g) or lower (h) core/cladding interface