Experimental measurement of the number of modes for a multimode optical fiber

For a multimode optical fiber, the number of modes (N_m) can be calculated by analytic theory when the fiber is straight, twist-free, and strain-free. In practice, however, the fiber is subject to distortions that modify its mode characteristics. In this Letter, we present an experimental method to interrogate the mode properties of a multimode optical fiber. We experimentally measured the transmission matrix of a multimode optical fiber and performed singular value decomposition. We proved, both theoretically and experimentally, that the rank of the transmission matrix is equal to N _m. We expect that the suggested method will contribute to the fields of the biomedical optics and optical communications where optical fiber is widely used.