Modal power analysis for two-mode fibers illuminated by an offset beam based on near field pattern measurement

A novel method for determining mode power distribution in a two-mode fiber illuminated by an offset beam by measuring the near field pattern on the test fiber endface is developed. This method utilizes the different field symmetries of the fundamental and the second order modes. Coupling efficiencies from a quasi-Gaussian beam to the fundamental and the second order modes in a two mode fiber were measured by the new technique for various offset launching conditions, and were in good agreement with the theoretical ones.