Research On Principal Mode In Spatial Mode Multiplexed Optical Communication System

With the rapid development of society informatization and economic globalization, there appear a large number of new communication services, such as video on demand, video conference, cloud storage and internet protocol television (IPTV). These services are bringing about a crucial challenge for the rate and bandwidth of communication network. Due to the continued rapid growth of internet traffic demand over the past two decades, wavelength division multiplexing (WDM) has been an important technical scheme of data networks. Recent research suggests that, however, the growth rate of WDM transmission capacity is coming down and rapidly approaching the Shannon limits of single-mode-fiber (SMF) system. In order to deal with such contradiction, therefore, new exploration on extending transmission capacity and distance has become urgent.In current optical transmission systems, the widespread use of many other physical dimensions, such as time, frequency, polarization and multimode modulation, has close to the limits while spatial dimension remains to be technical research. After the introduction of spatial mode dimension, mode division multiplexing (MDM) in few-mode fiber (FMF) is considered to be an important solution to the further improvement of transmission capacity. Nevertheless, the eigenmodes in fiber have different group velocities and the resulting mode dispersion (MD) will limit the bandwidth and transmission distance. To lower the impact caused by MD and reduce the complexity of signal processing in the receiving end, it is need to find a transmission status of modes to minimize MD, at which the output field and frequency are first-order-independent. Through expanding the principal states of polarization (PSP) model in SMF, we found this mode status which is known as principal mode (PM) in FMF/multimode fiber (MMF).The main works in this paper are shown as follows:(1) Owing to the higher complexity and unavoidable strong mode mixing in MMF-based MDM system, we use a FMF as transmission media in PM research. For the propagation of dual-polarization twofold degenerate modes, we choose a FMF to guide four lowest-order modes and design its physical parameters.(2) In order to give the complete description of spatial-mode and polarization-mode coupling in the FMF, we improve a fiber model. The fiber is divided into segments, each with mismatch and rotation with respect to the previous section. Splice mismatch, rotation angle and segment length satisfy Gaussian random process which conforms to the actual situation.(3) Due to the lower complexity of system implementation and mode conversion, dual-LP11mode propagation has attracted considerable attention, and successful dual-LP11mode, dual-polarization transmissions over FMFs have already been performed experimentally. In such a scenario, recently, propagation of the LP21mode has been experimentally verified to be bending-effect-immune. Therefore, the comparison of dual-LP11mode and dual-LP21mode transmission is required, to determine which is more suitable for a twofold linear polarization mode-based MDM transmission system. In this paper, the polarization-dependent principal mode-related properties of the twofold degenerate LP11/21modes propagation are compared in terms of differential group delay (DGD) and power distribution. Through the calculation, we obtain that dual-LP21mode propagation is more suitable for MDM transmission system in term of DGD. By exciting the light pulse into input polarization-dependent PMs, the first-order MD and polarization mode dispersion (PMD) can be avoided.