A complete optical orthogonal frequency division multiplexing(OOFDM) system is presented in this paper,which is employed to achieve long distance transmission with high efficiency.The OOFDM signal in 4-quadrature amplitude modulation(QAM) with the code rate of 10 Gb/s is transmitted 50 km in the fiber,and the constellation diagram and error rate obtained through direct detection show good performance.Two detection fashions that use avalanche photodiode(APD) and traditional photodiode(PIN) respectively is co...