Modes and numerical aperture of a fiber optic cable

[fisico30]

Hello Forum,
the numerical aperture NA of a fiber optics is given by the square root of the squared core index minus the squared cladding index.
The NA indicates the angular ability to collect light from the outside of the fiber, i.e. the max semi-angle over which light will enter the fiber and suffer TIR and be guided.

If NA is large, does it automatically mean that the fiber will be multimode?
Does a single mode fiber automatically have a small NA?

In the formula for NA there is no mention about the diameter of the core. I would think that the larger the core the larger the NA...single mode fibers have small size core...

thanks
fisico30

 

[anorlunda]

In fiber optics, NA is expressed in terms of the range of angles rather than the diameter. Remember that the index of refraction in fibers varies non linearly with radius.