Purity characterization of mixed mode groups in a directional mode convertor using OFDR method

• Applying optical frequency domain reflectometry to characterize mode purity in directional mode convertors. • Transforming mode identification into a spatial imaging and frequency-domain Fresnel peak separation problem. • Realizing precise purity measurement of 99.15% ± 0.35% for the LP 11 mode through Fresnel reflection peak power integration. We characterize mode groups and its purity in an all-fiber directional mode convertor with 0.49 dB insertion loss using optical frequency domain reflectometry. The target LP 11 mode is observed among the remaining LP 01 mode and other high order modes in frequency domain due to their different group velocities and round-trip time delay. LP 11 mode is identified at a 12.56 m few-mode fiber end in experiment, with an averaged purity of 99.15% ± 0.35% obtained by power proportion of its own Fresnel peak over the spectrum. Our demonstration promises a reliable and practical approach for mode purity characterization in mode-division multiplexing optical fiber networks.