FIELD: measurement technology, optical fiber testing. SUBSTANCE: tested fiber is so connected to auxiliary fiber identical to it that gap between butts of fibers is formed in point of their connection. Radiation is input into auxiliary optical fiber and power level across output of tested optical fiber is measured twice under constant conditions of input of optical radiation into auxiliary optical fiber for different values of gap between butts of fibers. Diameter of mode spot of tested fiber is computed by certain formula with use of measurement results. EFFECT: diminished measurement error. 1 dwg
