Advanced techniques for high-performance optical transmission and sensing

© 2018 Dr. Jian FangIn this thesis, we explore the advanced techniques for both optical transmission and optical fibre sensing. For the optical transmission, we design a low differential modal delay (DMD) few-mode fibre with distributed long period gratings. With the help of random strong mode coupling, we verify that our scheme produces a significant DMD reduction, controllable bandwidth and environmental robustness over a long distance, leading to a great potential of high-capacity optical transmission. We next explore the application of few-mode fibre in optical fibre sensing area. A novel multi-parameter sensing technique will be demonstrated based on elliptical-core few-mode fibre. We also provide a general method for discriminative sensing with more than two modes. Experimental results show an advantageous performance for the discrimination between the temperature and strain, providing a feasible way of separating multiple parameters simultaneously. Besides the few-mode fibres technologies, in this thesis, we will introduce two advanced techniques for high-performance distributed fibre sensing. We will first propose an ultra-fast Brillouin fibre sensing technique. By utilising the dual-polarisation orthogonal-frequency division multiplexing and coherent detection, our scheme could monitor the distributed fibre status with only single-shot measurement. The ultimate sensing speed is only limited by the fibre length, which will significantly boost the sensing speed. After that, a complex-domain Brillouin fibre sensing technique is demonstrated, by detecting the complex Brillouin spectrum directly and estimating the Brillouin frequency on the complex domain. With the complex nonlinear regression method, rigorous closed-form expressions are derived for the Brillouin frequency uncertainty. Our technique will improve the sensing accuracy by a factor of square root of 2 and the SNR by a 3dB, which is confirmed by both simulations and experiments