Transmission of 25-Gb/s RZ-DQPSK signals with 25-GHz channel spacing over 1000 km of SMF-28 fiber

We report transmission of nine 25-Gb/s return-to-zero differential quadrature phase-shift keyed (RZ-DQPSK) dense wavelength-division-multiplexing signals with 25-GHz channel spacing over 1000 km of single-mode fiber (SMF-28) in a recirculating loop. The loop uses all erbium-doped fiber amplifiers (EDFAs) and has an amplifier spacing of 100 km with an average loss of 25 dB between EDFAs and a maximum span loss of up to 30 dB. All channels were copolarized launched. No precompensation or postcompensation was employed. To the best of our knowledge, this is the first transmission test of multichannel RZ-DQPSK signals operating at 25 Gb/s with a spectral efficiency of 0.8 b/s/Hz. The transmission distance is limited by amplified spontaneous emission noise due to the high span losses. Nevertheless, our result indicates that upgrading the capacity of long-haul terrestrial systems using RZ-DQPSK modulation format should be feasible.