Patent ID: 7633608

Claim:
A measurement method for measuring non-circularity of a core part of an optical fiber base material having the core part and a clad part, the method comprising: immersing the optical fiber base material in liquid having a refractive index substantially equal to that of the clad part of the optical fiber base material; irradiating parallel light from a side face of the optical fiber base material to measure an intensity distribution of transmitted light; measuring a width between two change points, at which two bright spaces corresponding to the clad part are changed to a dark space and which correspond to two boundaries between the clad part and the core part, on the intensity distribution to obtain a relative value for a core diameter; rotating the optical fiber base material to further obtain the relative value for the core diameter at plural points for a circumferential direction; and obtaining non-circularity of the core part based on the obtained plurality of relative values for the core diameter.