Flexible transponder design aided by agile binary bit encoder

In order to accommodate variable bit rates and achieve flexible transmission distances, a binary bit encoding scheme is incorporated in the dual-polarization (DP) quadrature phase-shift keying (QPSK) transmitter to manipulate the signal polarization multiplexing schemes into polarization-multiplexed (PolMux), polarization-switched (PolSw), and polarization-alternated (PolAl) schemes, such that the transmission bit rate could vary among 4B, 3B, and 2B, where B stands for the baud rate of the DP-QPSK signal. With the versatile polarization schemes, the trade-off between the transmission performance and system bit rate could be flexibly adjusted depending on the system requirements and conditions. The performance of these modulation formats has been evaluated experimentally at 32 Gbaud in a dispersion-managed fiber (DMF) link. At a 0.5 dB system margin, the transmission reach of PolSw and PolAl QPSK increased to ~6400 and ~8500 km, respectively, compared to the ~4500 km transmission distance using the PolMux QPSK signal. This improvement is attributed to the lower bit rate of PolSw QPSK (96 Gbits/s) and PolAl QPSK (64 Gbits/s) and to the superior characteristics of the PolSw and PolAl schemes. For example, PolAl QPSK performs as good as the PolMux BPSK format to show similar receiver sensitivity. At the end, a flexible transponder platform with full software-defined optics features is discussed using the proposed flexible transmitter configuration and other techniques, such as optical multi-tone generation and adaptive forward error correction, to enable the transponder to carry standard 10G/40G/100G data.