Bi-directional optical systems using the Radio-over-Fiber techniques have been discussed, and a new base-station configuration has been proposed. To build high performance fiber-optic systems, some issues should be considered, such as the non-linearit ...
Bi-directional optical systems using the Radio-over-Fiber techniques have been discussed, and a new base-station configuration has been proposed. To build high performance fiber-optic systems, some issues should be considered, such as the non-linearity of the optical modulator, the dispersion induced effects in fiber-optic link, and the configuration of the base-station in bi-directional RoF link. To solve those problems, the dual-parallel modulation technique and the EAM cascaded SOA configuration have been proposed.
The proposed dual-parallel modulation technique is for the linearization of nonlinear transmission characteristics of an EAM. The phase difference between nonlinear signals from two different modulators can be out-of-phase by the proper selection of the modulator bias position, considering the slope variation of each modulator. By using these characteristic, the suppression of IMD3 nonlinear signals from EML and the following dynamic range enhancement could be achieved. However, the proposed dual-parallel modulation scheme is not applicable to the dispersion-induced problems in fiber-optic link. For this reason, the EAM cascaded SOA configuration was newly proposed.
The major features of the SOA in the EAM cascaded SOA configuration are as follows: Firstly, the gain profile in gain saturation region of the SOA can be used for linearization of an optical modulator. Secondly, the negative chirp characteristic of the gain-saturated SOA mitigates the chromatic dispersion-induced effects in fiber-optic link. In third, the optical gain of SOA provides the RF gain to the photo-detected RF signals. Using these useful features, the suppression of the nonlinearity of the optical modulator and the dispersion-induced RF carrier suppression effects was experimentally demonstrated.
Based on these characteristics and the dual-functionality of the EAM which acts as a modulator and photo-detector at the same time, the novel bi-directional RoF structure with a base-station employed the EAM cascaded SOA configuration where the SOA in a base-station provides the optical gain and the negative chirp has been proposed. Thus, the photo-detection efficiency of an EAT can be improved greatly, and the problem of link budget and DICS phenomenon in up-stream link, which is caused by the elimination of the optical source in the BS and the fiber chromatic dispersion respectively, can also be settled.
To verify the proposed idea related to the functionality of the SOA, the experimental demonstration bas been performed. Based on above-mentioned characteristics, the bi-directional SCM data transmission with 6Msps, 16QAM data carried on 2.5GHz RF carrier was successfully demonstrated. Since this proposed the EAM cascaded SOA configuration is very simple and robust, it would be useful for next generation convergence networks.