Mode field diameter measuring method and mode field diameter measuring device

A mode field diameter measurement device comprising:

• an exciter that excites an LP01 mode and an LP11 mode as test light to an optical fiber to be tested;
• a light intensity measurement unit that measures light intensity output from the optical fiber to be tested while changing an aperture angle of a variable aperture;
• a mode filter that changes an intensity ratio between the LP01 mode and the LP11 mode output from the optical fiber to be tested;
• and
• an arithmetic unit to which the light intensity measured by the light intensity measurement unit is input,
• in which the arithmetic unit
• acquires an electric field spread value by using an integral value of a value related to the light intensity when the aperture angle is changed from zero to a maximum,
• acquires a plurality of the electric field spread value having different intensity ratios of the LP01 mode and the LP11 mode output from the optical fiber to be tested and acquires intensity ratio dependency of the electric field spread value,
• acquires an electric field spread calculated value which is calculated from a mode field diameter of the LP01 mode, a wavelength of the test light, and the intensity ratio and the electric field spread calculated value has a mode field diameter of the LP11 mode as a variable, by setting the electric field spread value in a case where light output from the optical fiber to be tested is only the LP01 mode as the mode field diameter of the LP01 mode, and
• finds the mode field diameter of the LP11 mode in which a square error between the intensity ratio dependency of the electric field spread value and intensity ratio dependency of the electric field spread calculated value with respect to the intensity ratio is minimum.