Research On The Key Technologies Of Mode Control For Mode Division Multiplexing In Optical Fiber Communications

Mode Division Multiplexing (MDM) appears as the most potential approach to increase greatly the capacity of optical fiber communications. Mode control, which includes mode excitation and mode conversion, is the prerequisite and the key technology of the MDM system. Specifically, mode excitation is used to generate the modes of the few mode fiber (FMF) or the multi-mode fiber (MMF) at the transmitter, and mode conversion has been widely used in the mode multiplexer (MUX), the mode demultiplexer (DEMUX), or the core node of data exchange in MDM-based network.The key technologies of mode excitation and higher order mode conversion are studied theoretically, numerically and experimentally in this dissertation. The methods for mode conversion employing spatial spectrum matching (SSM) or mode field radius pre-matching (MFRPM) is proposed based on the principle of mode field matching. More than 20 modes are excited and some higher order modes are converted through an experiment system based on a spatial light modulator (SLM). Then a scheme is proposed for high-precision arbitrary-mode conversion in light of conversion correlation coefficient maximization. The innovations in detail are listed as follows:1. Binary phase modulation method for mode control based on the principle of mode field matchingFirst, a mode conversion method based on spatial spectrum matching (SSM) is proposed to compensate the radial loss of amplitude in the spatial frequency domain. Following the formulation of phase only transfer function and the introduction of waist radius of spatial spectrum, spatial spectrum matching is realized through adjusting the focal lengths of the two lenses according to the spatial spectrum pattern field radii. Some higher order modes conversion are achieved numerically and experimentally, such as LPoi to LP22a, LP02 to LP22a, LP03 to LP22a-Since different lenses are required to convert different modes in SSM method and this physical change brings challenge in the progress of experiment, another mode conversion method taking advantage of the adjustability of the pixels of SLM is proposed based on mode field radius pre-matching (MFRPM). This method employs a phase-only SLM as a phase filter, whose effective area is pre-matched theoretically to the mode field radius of the incident light based on matching coefficient. The influence of the matching degree on the mode excitation and higher order mode conversion is investigated under simulations. In the experiment, more than 20 common modes are excited and some higher order modes are converted among LP11a, LP11b, LP21a, LP21b and LP31b whose radial charges are one. What’s more, mode conversion between modes whose radial and azimuthal direction are both changeful are also experimentally demonstrated among the cases:LP22a to LP21a, LP02 to LP12b, LP02 to LP22b, LP02 to LP22a and LP03 to LP22a.2. Mode conversion combining phase and amplitude modulationA novel mode conversion scheme based on the combination of phase and amplitude modulation (CPAM) is proposed. This method can increase the conversion precision compared to the SSM method and MFRPM method. The pixels of the same phase-only SLM are divided into small grids of 2×2 pixels. The average of the 2×2 pixels in a grid stands for the phase information of the mode conversion transfer function, while the phase difference of neighboring pixels represents the amplitude information of transfer function. It is shown that the gridding phase-only SLM equipped with a following spatial filter which consists of a two-lens system with an aperture, can realize phase and amplitude modulation and compensate the loss of amplitude. Furthermore, both simulation and experimental results demonstrate that the CPAM method can convert more modes with higher precision.3. Arbitrary mode control with high precision based on Simulated Annealing algorithmThe CPAM, SSM and MDEPM methods are all not up to compensating the loss of the amplitude and achieving arbitrary mode conversion. To this end, firstly, an optimal mathematic model is established to maximize the conversion correlation coefficient. Secondly, an arbitrary mode control method with high precision based on Simulated Annealing (SA) algorithm is proposed. The SA algorithm is utilized to calculate the optimal transfer function of the SLM which is a binary phase filter for the mode converter and makes it able to realize arbitrary modes conversion with conversion correlation coefficient higher than 80%. Thirdly, a fast arbitrary mode converter with ultrahigh precision is proposed using multiple phase filter based on improved SA algorithm. Guided by the principle of mode field matching, it is shown that the time consumption is greatly reduced by more than 94% with the leverage of setting the active area of the SLM with local adaptive approach, utilizing multithreading as well as graphics processing units(GPU). On this basis the SA algorithm is improved to calculate a multiple phase filter, and then achieves arbitrary modes conversion with conversion correlation coefficient more than 99%.