Input-Output Phenomena

If a ray is incident at angle θ, it will ideally emerge from a fiber at angle θ. In practice, however, the azimuthal angle on emergence varies so rapidly with θ, the length and diameter of the fiber, etc. that the emerging ray spreads to fill an annulus of a cone twice angle θ, as shown in Figure 2-5. The exit end of a fiber will act as a prism if it is not cut perpendicular to the fiber axis. A bias cut will tip the exit cone as shown in Figure 2-6. Thus,  where β is the axis of the deflected ray and α is the cut angle to the normal of the fiber. The preservation of angle θ on exit is only an approximation. Diffraction at the ends, bending, striae, and surface roughness will cause decollimation or opening of the annulus. The striae and roughness cause progressive decollimation; diffraction and bending may be regarded as end finish factors (Figure 2-7). The effect is most apparent in systems in which collimated transmission is emphasized.

 

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