Investigation on multi-core fiber and few-mode fiber and their applications on optical fiber sensing

Abstract

Few-mode fiber (FMF) and multi-core fiber (MCF) are two typical specialty optical fibers in the space-division multiplexing (SDM) system, in which modes and cores act as independent channels to carry information in order to achieve capacity improvement. In addition to the SDM communication system, FMF and MCF also show excellent performance in the optical fiber sensing community. The properties of mode and cores can be utilized for the measurement of many parameters. This thesis focuses on investigating the FMF and MCF, and further exploring their sensing applications. Firstly, based on the basic parameters of FMF, mode calculation is presented in detail, and the electric field distribution of the modes, including the fundamental mode and high-order modes, is simulated. According to mode properties, a novel technique named spatially and spectrally resolved (S²) imaging is introduced and employed for mode characterization in FMF. Based on the proposed signal processing method, as a result, the differential mode group delay between modes, intensity as well as phase distribution of modes and mode coupling in FMF are obtained. Besides, the S² imaging system is modified and accurate mode characterization is achieved, then an advanced algorithm is also proposed for detailed mode-pairs classification. For FMF-based sensing, a miniatured modal interferometer based on mode interference in FMF is explored and also demonstrated for temperature measurement. As a result, dual-sensitivity performance is achieved, which can be utilized for multi-parameters measurement. Similar to modes in FMF, the cores in MCF can be also utilized in some novel sensing applications. Thus, in this thesis, a torsion sensor based on inter core-mode coupling in seven-core fiber (SCF) is proposed. The torsion sensor is designed by tapering a SCF and splicing single-mode fibers (SMFs) on both ends. As a result, different sensitivities are achieved with different pre-twist angles on tapered SCF, and the highest can reach 1 nm/°, which stands out in current torsion sensors. Besides, this torsion sensor is also demonstrated to achieve twist direction discrimination with stable performance. The mode coupling dynamics and optical anisotropy are theoretically analyzed in twisted SCF to discuss the sensitivity performance. This work demonstrates the potential sensing application of MCF. Finally, another novel sensing application of FMF and MCF, the non-invasive vital signs monitoring, is investigated. The monitors are based on the interference of modes and cores, which can help to design and fabricate in-line interferometers for stable as well as accurate vital signs monitoring. The twin-core fiber (TCF) sensor is firstly designed by splicing SMFs on both ends with optimized offset distances. The fabricated sensor is embedded as a smart mattress, and both heartbeat rate (HR) and respiration rate (RR) are obtained successfully in a non-invasive way. In addition, the SCF and two kinds of FMFs, including two-mode fiber and four-mode fiber, are fabricated as in-line interferometers for contactless vital signs monitoring. FMF-based interferometers can realize simultaneous HR and RR monitoring while the SCF can monitor the RR accurately. Besides, traditional Mach-Zehnder interferometers are leveraged for remote activities monitoring and myocardial contractility assessment. In summary, based on the properties of modes and cores, FMF and MCF are demonstrated with excellent performance in temperature sensing, torsion sensing and non-invasive vital signs monitoring.