Self- coherent optical OFDM : an interesting alternative to direct or coherent detection

Recently, coherent optical orthogonal frequency division multiplexing (CO-OFDM) has been considered for the next generation 400-Gb/s long haul data transmission. For long haul transmission systems coherent detection is beneficial compared to direct detection as it provides higher sensitivity. Though CO-OFDM offers a virtually unlimited tolerance against linear impairments, phase noise represents major challenge that must be compensated for. Thus, phase noise compensation (PNC) technique must be applied in the digital signal processing (DSP), which is already limited in speed and processing capabilities. Conversely, self coherent optical (SCO-) OFDM is an interesting alternative which does not require any PNC in the DSP. Furthermore, no local oscillator (LO) is required at the receiver. In this paper, an overview of the recently seen self coherent techniques will be given. Generally, self coherent detection is realized by sending an optical carrier along with the data signal like in direct detection systems. At the receiver, however, the optical carrier is extracted from the signal with the use of filters with very narrow bandwidth and used as an LO using heterodyne or homodyne detection. The performance of such a system depends on the filter bandwidth that is used to extract the carrier.