Spectrum efficiency improvement of directly detected OFDM based on balance receiver

A novel optical single-sideband directly detected orthogonal frequency division multiplexing (DD-OFDM) system based on balance receiver consisting of two PINs is proposed and demonstrated. By introducing phase shift of π to one arm of the proposed receiver, inter-modulation distortion (IMD) near the optical carrier can be eliminated efficiently. The simulation results show that our system improves spectral efficiency to about 1.54 bps/Hz with 4-quadrature amplitude modulation (4QAM) mapping, and the optical carrier and signal power ratio (CSPR) has negligible impact on the OSNR performance, which can be improved by 3 dB with negligible error vector magnitude (EVM) penalty. Additionally, the optimum CSPR can be controlled by adjusting them respectively. 5 Gbit/s data with 4QAM mapping and 6.25% cyclic prefix (CP) inserted is successfully transmitted over 320 km of standard single mode fiber (SSMF) and our system exhibits a negligible penalty. Higher spectral efficiency can be achieved with more complex modulation formats and polarization multiplexing.