far-field spot and light intensity. The experimental results reveal that the influence of the shape

of light spot changed with the op- tical coupling condition on the value of NA is not distinct

a lot. The strong diffraction due to the very small fiber core can affect the

measurement of NA remarkably. The first sub-maximum of the diffraction peak needs to be con-

sidered in the calculation in both methods. NA is measured when the center diffraction spot is

bright. The distance between the fiber and the observation plane is suitable in order to

decrease the experiment errors. Too long or too short distance can make the error increase

when using the far-field spot method.