| Large capacity terabit optical transmission system has become the development direction of next-generation optical communications because of data traffic carrying of fiber-optic backbone network.Therefore, it is necessary to research on a variety of multiplexing technologies enhancing channel capacity and spectrum efficiency. Among these techniques, Nyquist-WDM allows for the optical signal arranging in wavelength without interval between channel, and higher order modulation format can carry a plurality of information. Moreover,Multi-carrier light source may become the most economical and competitive sources for terabit optical transmission multiplexing system,because it can provide a plurality of sub-carriers simultaneously only utilizing a laser. Orbital Angular Momentum (OAM) multiplexing technology based on special phase enables multiple OAM modes information transfer in the new same fiber, which makes fiber optic transmission capacity and spectrum efficiency improved once again.Problems of the new multiplexing technologies in terabit optical transmission system are studied theoretically, numerically and experimentally in this dissertation. These include the generation of high-quality multi-carrier light sources, higher-order modulation optical signals, Nyquist-WDM optical signals, and optical OAM modes, and the theory for optical OAM modes in ring fibers. The innovations are summarized as follows:1. A modified scheme is proposed to improve TNR of the multi-carrier by inserting an AMZI in RFS loop.Transfer function of AMZI is derived in detail, and then the power of the seed light and ASE noise after AMZI is obtained. The results show that the power of the optical signal does not change, and the power of ASE noise reduces to half, so tone to noise of the optical signal enhances 3dB. Based on these reasons, AMZI can be inserted in the re-circulating frequency shifting (RFS) to improve tone to noise ratio (TNR) of the multi-carrier. The output multi-carrier of 20 and 50 tones are simulated using MATLAB, which verified the effectiveness of the improved scheme.We set up the multi-carrier generation system, and obtained the output multi-carrier experimentally based on the traditional and modified scheme respectively. In addition, we carried out experiments focused on the scheme of AMZI placed in and outside RFS, and observed optical pulse shape of the multi-carrier by optical spectrum analyzer (OSA) with resolution of up to 0.16pm, which allows us to be more careful observation of the actual shape of the subcarrier details. The experimental results also demonstrated that the modified scheme were improved more than the traditional scheme in terms of TNR performance.2. Nyquist-WDM-QPSK optical signal is generated experimentally by designing filter carefully to QPSK optical signal based on Waveshaper.Through the analysis of the filter bandwidth and channel bandwidth specification of the programmable optical signal processor (Waveshaper),the transfer function of Waveshaper is derived. Then we designed the transfer function carefully to filter the QPSK optical signal shaping based on Waveshaper. At last, we set up experimental system, and realized the Nyquist-WDM-QPSK signal carrying roll-off factor. The scheme provides a theoretical reference for the generation of Nyquist-WDM signal for high-order modulation format.3. An analytical and numerical formalism for studying optical OAM modes in step-index ring fibers is proposed.Starting from Maxwell’s equations and basic optical-waveguide theory, we analyze the vector optical fields in step-index ring fibers. Via theoretical analysis and numerical calculation, we solve for the eigenmodes in the ring fiber - OAM modes characterized by Bessel functions. We present a mode-analysis theory for the step-index ring fiber,which is applicable to a wide variety of parameter configuratiuons. We numerically obtain the dispersion curves of OAM modes, analyze the effects of the ring radii and the material refractive indices on the effective refractive index of the OAM modes, and identify the fiber structural parameters suitable for OAM-based multiplexing. For comparsion, we also utilize the expertized optical engineering software Comsol to calculate the OAM mode properties and find good agreement with our formalism. Our results show that the effective refractive index difference between the originally-degenerate OAM modes increases with the material refractive-index difference between the annular layer and the cladding layers. Also, the ratio of the inner and outer radii also significantly affects the effective refractive index difference of the OAM modes. |
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