Transmission experiment of multi-gigabit coherent optical OFDM systems over 1000 km SSMF fibre

Introduction: Orthogonal-frequency-division-multiplexing (OFDM) is a special form of multicarrier modulation where a single data stream is transmitted over a number of lower rate orthogonal subcarriers [1]. Such a format has been widely implemented in various digital communication standards. Recently we proposed an equivalent optical multicarrier modulation format, called coherent optical OFDM (CO-OFDM) to combat chromatic dispersion (CD) [2]. It has been shown that CO-OFDM is insensitive to PMD in the fibre and in fact the system margin is improved in high PMD fibre owing to improved polarisation diversity against PDL-induced fading [3]. In this Letter we show the first experimental demonstration of CO-OFDM systems in which 128 OFDM subcarriers with a nominal data-rate of 8 Gbit=s are successfully processed and recovered after 1000 km transmission through SSMF fibre. The experimental work focuses on the proofof-concept study on the feasibility of CO-OFDM signal processing, including OFDM window timing synchronisation, software RF downconversion, OFDM subcarrier and phase recovery, and I=Q detection for optical transmission. Our work serves to substantiate the suitability of the CO-OFDM format for optical transmission without a need for CD compensation.