NEC Corporation has developed a signal-reshaping large-scale integrated-circuit (LSI) that facilitates a 40 Gbps optical fiber transmission system.

The new LSI enhances the quality and length of 40-Gbps-based wavelength division multiplexing (WDM) long-haul transmission as compared with conventional systems, due to its ability to compensate for signal distortion resulting from optical fiber impairments. The new integrated-circuit is based on indium phosphide (InP) compound semiconductor technology.

In its research, NEC has proved that the maximum length of error-free transmission with a data rate of 40 Gbps can be at least doubled without replacing the existing fiber infrastructure, bringing 40 Gbps transmission systems closer to economic feasibility. The new LSI can compensate for signal distortion caused by polarization mode dispersion (PMD), which is a kind of signal distortion evident in transmissions of 40 Gbps or higher in optical fiber communication. PMD is the type of dispersion that occurs in singlemode fiber due to a lack of perfect symmetry in the fiber and from external pressures on the cable. Light travels over singlemode fiber in two polarization states. Over long distances, PMD causes each one to arrive at the receiving end at a different time.
The LSI is also expected to contribute to a size reduction in WDM system hardware.

40-Gbps-based WDM systems will soon be in commercial use to meet the strong demand for higher data rates per wavelength. In order to achieve this goal it was necessary to realize error-free long distance transmission, which is exceedingly difficult to achieve, because 40 Gbps signals are significantly more susceptible to distortion by PMD, than lower line rates are. Although PMD can be alleviated to some extent by using state-of-the-art optical fiber, from a practical and economical point of view, it is important to realize 40 Gbps transmission systems using optical fiber networks that are already in use for 10 Gbps systems.

While reshaping signals that are distorted during long distance transmission is common in optical fiber transmission systems, coping with PMD-related distortion is challenging as the resultant waveform is often too complex to reshape and partly because the PMD distortion is an instantaneous and random event in nature and can always occur due to mechanical vibration of the fibers. In addition, even if an optical device that can cope with PMD issues were to become available, it would increase both the cost and size of the WDM system.