SPECTRALLY EFFICIENT MULTICARRIER SYSTEMS FOR FIBER-OPTIC TRANSMISSION

Abstract

The purpose of this research is to provide a comprehensive study of spectrally efficient multicarrier systems for fiber-optic transmission. Multicarrier optical systems partition a high-data rate digital signal in a wavelength channel into multiple subcarriers. The data rate on each subcarrier can be sufficiently low and thus, the tolerance to transmission impairments can be significantly improved. Although different modulation and detection techniques are used, all the multicarrier systems investigated in this dissertation achieve a high spectral efficiency of 1 Baud/s/Hz. Orthogonal frequency-division multiplexing (OFDM) and Nyquist wavelength-division multiplexing (Nyquist-WDM) are the two basic approaches to achieve the 1 Baud/s/Hz spectral efficiency. OFDM allows spectral overlap of adjacent subcarriers and crosstalk elimination by integration at the receiver; Nyquist-WDM limits the spectral spreading of each subcarrier channel to the symbol rate per subcarrier to avoid spectral overlap. In terms of detection method, both direct detection and coherent detection can be applied in multicarrier systems. This dissertation focuses on the use of three high spectral efficiency optical multicarrier systems. In the theoretical and experimental investigation of a 11.1Gb/s FFT-based OFDM system, a simple dual-drive Mach-Zehnder modulator (MZM) was employed in the transmitter and direct detection in the receiver, which provided an OFDM system implementation with reduced complexity. The data was transmitted through 675km uncompensated standard single-mode fiber (SMF). Next, a 22.2Gb/s digital subcarrier multiplexing based (DSCM-based) OFDM system was used in conjunction with 10 subcarrier channels using QPSK modulation. In this system, an IQ modulator was utilized in the transmitter and coherent detection in the receiver. By using coherent detection, the receiver was able to dynamically select the desired subcarrier channels for detection without changing the system configuration. The present research also explored a 22.2Gb/s 10 subcarrier Nyquist-WDM system with coherent detection, compared its system performance with OFDM systems, and subsequently examined the impact of filter roll-off factor. Finally, a systemic comparison of the three proposed multicarrier systems was performed in terms of their transmission performance and system flexibility. The design tradeoffs were analyzed for different applications and summarized principles for the modern multicarrier fiber-optic system design.