Roughness Estimation of Multimode Rectangular Optical Waveguide Based on the Scattering Loss Difference Between SMFW and MMFW Structures

Based on the scattering loss difference between the single-mode-fiber-optical-waveguide (SMFW) and multimode-fiber-optical-waveguide (MMFW) structures, a method to estimate the roughness of multimode rectangular optical waveguide (ROW) is proposed. Based on the analysis of the mode coupling characteristics of the SMFW structure and the MMFW structure, the coupling efficiency of higher order mode of the ROW for the MMFW structure is larger than that of the SMFW structure. However, it is reversed for the lower order mode. By further analyzing the mode scattering loss coefficients of ROW induced by roughness, it is found that the total scattering loss for the SMFW structure is less than that for the MMFW structure, which can be used to estimate the roughness. The experimental results show that the roughness is 248.67 nm when the waveguide length is 75 mm and the scattering loss difference is 0.31dB. As the roughness is 275.37 nm by the 3-D optical profiler, this implies that the method is effective. Compared to the conventional methods, the method has many advantages, such as convenience, high efficiency, and ability for large measurement area.