What is the numerical aperture of a fiber?

The numerical aperture (NA) of a fiber is a number that defines its light gathering capability. A larger NA corresponds to a larger acceptance angle, which results in the ability to collect more light. The NA of a multi-mode fiber can be calculated from the index of refraction values for the fibers core (n_core) and cladding (n_clad).

NA = √ (n_core² - n_clad²)

However, for single-mode fibers, the Gaussian nature of the beam going through the fiber needs to be considered and this equation will not be as accurate.