Transmission of RZ-DQPSK over 6500 km with 0.66 bit/s/Hz spectral efficiency

Transmission of 12.5 Gbit/s RZ-DQPSK signals over 6500 km with different channel spacings is presented. We demonstrate the feasibility of trans-oceanic transmission using DQPSK in DWDM systems with up to 0.66 bit/s/Hz spectral efficiency. Introduction: Differential quadrature phase shift keying (DQPSK) has recently been suggested as a suitable modulation format for optical communication systems (1). With DQPSK, two bits are transmitted for every symbol, thus the symbol rate and the bandwidth of all electronic and electro-optical components can be halved compared to a binary system. This leads to improved dispersion tolerance and allows for closer channel spacing in wavelength division multiplexing (WDM) systems, and very high spectral efficiency have been obtained in recent experiments (2-4). As differential binary phase shift keying (DBPSK), DQPSK also benefits from a 3 dB improved sensitivity when balanced detection is used. Here, we present a transmission experiment over transoceanic distances using optical DQPSK modulation format with a return-to-zero waveform (RZ-DQPSK). In a 64 channel WDM system experiment, we transmit 12.5 Gbit/s RZ-DQPSK signals over a distance of 6500 km. We investigate channel spacings ranging from 133 to 15 GHz, achieving a maximum spectral efficiency of 0.66 bit/s/Hz, and demonstrate that DQPSK is well suited for dense WDM (DWDM) applications. Experiment: The experimental set-up is illustrated in Fig. 1. In the transmitter, we had a total number of 64 continuous wave (CW) lasers spaced 66 GHz, divided into two groups of odd and even channels. The odd numbered lasers were modulated with a 12.5 Gbit/s information rate (or 6.25 Gbit/s symbol rate) RZ-DQPSK signal. First, a Mach Zehnder (MZ) modulator driven with a 2Vπ data signal (Data1) was used to generate a DBPSK signal. Then, a phase modulator (PM) driven with a ½Vπ data signal (Data2) applied a 90o phase modulation to generate a DQPSK signal. Finally, a MZ driven with a Vπ clock signal was used to generate the RZ waveform. The Data1 and Data2 signals were pre-coded so that a pseudo random bit sequence (PRBS) was obtained at the receiver. Depending on the sign of the 45° phase offset in the demodulator, the received pattern was a 2 15 -1 PRBS or an inverted 2 15 -1 PRBS. Even numbered channels were modulated with a 6.25 Gbit/s RZ-DBPSK with a data pattern different from the odd channels. As 6.25 Gbit/s RZ-DBPSK has the same pulse-shape and similar spectrum as 12.5 Gbit/s RZ-DQPSK, the inter-channel cross-talk was not significantly affected by this simplification. The amplifier chain was 465 km long and consisted of 11 fibre spans made of large effective area fiber with a dispersion of 20 ps/nm/km (D+), and inverse dispersion fibre (IDF) with dispersion of -40 ps/nm/km. The average span length was 45 km, and the respective lengths of the D+ and IDF fibres had been adjusted to get a dispersion map with good dispersion and dispersion slope compensation. To reach longer distances, the fibre link was inserted in a re-circulating loop.