Characteristic Analysis And Development Of Novel Few Mode Fiber And Related Devices

The study of few-mode fiber, FMF, has significant theoretical and practical value for the next generation of ultra high-speed large-capacity clog-free optical network,optical fiber sensing and fiber laser.Multi-channel and low nonlinearity in FMFs are involved in optical communication to develop the system’s capability and resolve communication errors caused by nonlinearity, and this has provided an important guarantee for the next generation of optical communication system. This thesis focuses on the basic theoretical and experimental research of multilayer-core FMF, MC-FMF. The main research direction is presented to the structure design and fabrication of MC-FMF which is applied to the construction of novel optical devices.The theoretical analysis and experiment results further perfect our understanding of the relationship between the fiber configurations and characteristics. Most of the works are at the forefront of this field.In this paper, the main research results and innovation points are as follows:1. A novel homemade MC-FMF is proposed, and studied the influence rule of fiber geometry parameter on characteristics in depth. By adjusting the structural parameters, the mode number can be controlled, and large-mode-area and low-bending-loss can be achieved simultaneously. A mathematical model of four-layer core FMF is built, and its characteristics are calculated and analyzed by using a finite element method. A dual-mode fiber with large-mode-area and low-bending-loss is fabricated by using optimized structural parameters. The mode area of the dual-mode fiber is 280μm2, which is four times as much as the single-mode fiber’s, and its bending loss is 0.42dB/turn when its bending radius is 4.5cm. The relationship between misalignment loss of the collimator and mode-area of fiber pigtail is presented. Benefit from the large-mode-area property of MC-FMF, a collimator is successfully demonstrated based on MC-FMF. The misalignment loss of a multi-layer fiber based collimator is experimentally characterized, and the results show the value of redundancy increases by 10%-20% compare to conventionally single-mode fiber collimator. This work is likely to overcome the difficulty of spatial light coupling by using MC-FMF.2. A strict dual-mode elliptical multilayer-core fiber (EMCF) is proposed. The influences of structural parameters of EMCF on birefringence, polarization and nonlinearity are systematically discussed. The polarization maintaining characteristics are found in EMCFs. A formula is derived to accurately estimate the bending losses for non-circular symmetrical optical fibers. This model fully considers the effect of fiber structure on fiber refractive index profiles and factors that might be associated with bending loss, and the calculation results essentially confirm the reason of the bending loss differences in bending direction. Based on this formula, the fiber bending performance in all directions is indicated. An EMCF sample is fabricated. At typical bending radius and bending direction, two degenerate fundamental modes of EMCF can be divided, leading to a single-mode single-polarization operation.3. The optical devices based on the pattern interference using FMFs is proposed. According to modal interferometry, the coupling principles for these devices is introduced. The scheme, which two sections of non-circular-symmetrical EMCFs are spliced, is proposed. Two passive devices, including Mach-Zehner interferometer (MZI) as well as Michelson interferometer (MI), are proposed with FMF. With MZI and MI, we theoretically evaluate the performance for pressure and liquid-level sensing. Moreover, sensing systems based on the proposed interferometers for temperature, pressure, refractive index and liquid-level are carried out. Based on FMF, a laser sensing system is proposed. In order to improve the resolution of interferometer type sensor, we demonstrate a FMF-MZI based ring laser, which can be exploited as a probe for liquid-level sensing. The output wavelength of ring laser is modulated by liquid-level, achieving a high extinction ratio of 43dB with 3dB bandwidth of 0.05nm. The sensitivity of the proposed liquid-level sensor is about 50.6 pm/mm.4. A few-mode fiber-Bragg-grating (FM-FBG) is proposed and fabricated. The coupling principle is theoretically analyzed and the coupling mode equations are solved. The effects of modal power ratio, modal refractive index difference and coupling coefficients on the resonant intensity, bandwidth and wavelength interval of FM-FBG are calculated. Switchable dual-wavelength laser combined with FM-FBG is fabricated. Based on the spectral hole burning (SHB) effect and the fine adjustment of the gain and loss at each wavelength, the laser can be switched between dual-wavelength and single-wavelength region easily. By using the relevant character between guided modes and operating wavelength, the generation of the fundamental mode laser and mixed mode laser is realized, which is used as mode laser.