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.