Chromatic dispersion (CD) in single-mode optical fiber distorts pulses and is a big obstacle against the upgrading of long-haul, dynamically routed wavelength division multiplexed (WDM) transmission systems at 10 Gbit/s and beyond. High-performance adaptive dispersion compensators are required as well as reliable low-cost hardware for the detection of residual CD. Targeting high-capacity metro systems, full-band 1.6-Tb/s (40x40Gbit/s) adaptive CD compensation is demonstrated in this experiment, using CSRZ-ASK and NRZ-DPSK modulation formats. A multichannel dispersion compensator, tunable in the range -700 to -1500 ps/nm, is automatically controlled by arrival time detection in one of the 40 transmitted WDM channels. Dispersion and its slope are tuned simultaneously by a thermal gradient of the grating-based compensator to match the parameters of standard single-mode fiber (SSMF) with lengths between 44 and 94 km.
Two 2x10 Gbit/s quaternary intensity modulation signals (4-IM) can be generated using quadrature amplitude modulation (QAM), with unequal modulation amplitudes in two orthogonal quadratures. Two 10 Gbit/s NRZ ASK signals and a QPSK modulator allow to generate 4-IM with the same bandwidth as an NRZ-ASK signal. Measured sensitivity at a BER of 10-9 and chromatic dispersion (CD) tolerance are -21.2 dBm and ~ +130 ps/nm, respectively. Two duobinary 10 Gbit/s data streams and a QPSK modulator allow to generate a 9-point QAM signal, with the same bandwidth as a duobinary signal. A stub filter with a frequency response dip at 5 GHz was used to generate the duobinary signals. Detected as a 4-IM, this scheme features a sensitivity and a CD tolerance of -21 dBm and ~ +140 ps/nm, respectively. A polarization and phase insensitive direct detection receiver with a single photodiode has been used to detect all generated QAM signals as 4-IM signals.
The capacity limit of a thermally controlled fiber Bragg grating-based chromatic dispersion compensator, which was initially designed for 10 Gb/s operation, was investigated in a 40 Gb/s system. A CS-RZ DQPSK polarization division multiplex (PolDM) system was used as a testbed. An equivalent quasi error-free 5.94 Tb/s capacity was demonstrated when dispersion of up to 73.8 km of SSMF was compensated. The dispersion slope compensation was satisfactory for C-band operation. Additionally, it was found that the compensator introduced band-pass filtering behaviour, which reduced the compensator bandwidth as the dispersion setting was increased. It was also found that even after 41.5 km, there was around 2 dB penalty introduced to DQPSK system while 5 dB penalty to DQPDK-PolDM, referring to BER of 10-5.