Analysis And Research On Radio Over Fiber Links With Wide Dynamic Range

With the development of the global information society, the optical communication gets more and more popular because it can meet the requirement for higher capacity and data rate. The Radio over Fiber(ROF) technology arouses much more attention in recent years, which combines the large bandwidth of the optical fiber and the flexibility of wireless communication technologies. The paper aims at the optical generation of millimeter wave signals and the ROF links with wide dynamic range. In ROF links, optical subcarrier modulation techniques are commonly used, such as optical double sideband(ODSB) modulation and optical single sideband(OSSB) modulation. In order to avoid the effect of the fiber dispersion, the OSSB modulation is often chosen for long-distance transmission.The ways to fulfill the electro-optical modulation are various, and the Mach-Zehnder modulator(MZM) gets the most popularity because of its simple structure and low cost. In this paper, a dual-drive MZM(DDMZM) is adopted for OSSB format signals. However, its transfer curve is a sine curve and the nonlinearity caused by harmonics and inter-modulation distortion is severe in the instance of one optical carrier carrying two RF signals. In order to solve the problem, many kinds of modified modulators are proposed, such as the ring-assisted MZM and the dual parallel MZMs.In this thesis, two modified modulators are proposed and analyzed in details. One is the dual parallel Mach-Zehnder modulator incorporating MMI couplers(MMI-DPMZM). In this structure, the Y splitters are replaced by the MMI couplers, while the single-driver MZMs are replaced by the dual-driver MZMs. This structure can prohibit most of the third inter-modulation distortion by changing the optical power ratio and the RF power ratio. Therefore, the high linearity and OSSB format signals can be obtained. Analysis shows that, compared to the DDMZM, the SFDR is improved by 16.4d B and the link loss only increases 0.42 d B. To further strengthen the SFDR, another structure, the series parallel Mach-Zehnder modulator incorporating MMI couplers(MMI-SPMZM), is proposed which changes the relationship between the controllable parameters. Analysis shows that, compared to the DDMZM, the SFDR is improved by 17.9d B and the link loss only increases 0.42 d B. Compared to the MMI-DPMZM, the SFDR gets improved by 1.5d B, while the link loss is constant. In addition, we also make simulations with these structures and all the simulation results are consistent with the theoretical derivation.