Figure 1 (a): Conventional optical transmission technology

For ultra-high-speed transmission of 40 Gbps per wavelength with conventional on/off modulation, waveform distortion (CD) from the optical fiber becomes significant, and optical dispersion compensators on each span, in each optical inline amplifier , are necessary to cancel this distortion. The amount of waveform distortion due to CD varies with distance, so suitable values must be designed and configured for each span by pre-measurement. Configuring the dispersion compensators to eliminate waveform distortion also takes time.

 Figure 1 (b): Digital coherent transmission technology with new DSP function

A known digital signal is introduced in the main digital signal by the DSP in the optical transmitter beforehand, which then transmits the optical signal. A large amount of waveform distortion is added to the optical signal by the inherent transmission characteristics of the optical fiber. At the optical receiver, the optical signal including waveform distortion is converted to an electrical signal and digitized by the receiver DSP. The receiver DSP extracts the known digital signal from the received digital signal, rapidly obtaining a measure of the waveform distortion. The distortion added to the main digital signal is the same as to the known digital signal, so by having the receiver DSP apply digital filtering to eliminate the measured waveform distortion from the received digital signal, the distortion added by transmission over optical fiber can be eliminated, recovering the original main digital signal.