56 Tbit / s , 1 . 6 bit / s / Hz , 40 Gbaud RZ-DQPSK polarization division multiplex transmission over 273 km of fiber

We report on 16×2×2×40 Gbit/s RZ-DQPSK transmission over a 273 km fiber link with a BER < 10. Due to polarization division multiplex each WDM channel carries 160 Gbit/s although the symbol rate is only 40 Gbaud. Polarizations are demultiplexed automatically by a LiNbO3 polarization transformer. Introduction The powerful tool of optical code division multiplex together with alternate WDM channel polarizations allows for a record 1.6 bit/s/Hz transmission [1] throughout the whole C band. Competing conventional techniques are more PMD and chromatic dispersion tolerant and support larger amplifier spacings. Polarization division multiplex [2-4] and DQPSK transmission [3-9] each can double fiber capacity by their increased spectral efficiencies. Both techniques have been combined to transmit 4x10 Gbit/s per WDM channel [3, 4]. Here we report for the first time to our knowledge 4x40 Gbit/s per WDM channel transmission with automatic polarization control. Transmission setup Fig. 1 shows the RZ-DQPSK polarization division multiplex 16×2×2×40 Gbit/s per WDM channel transmission setup, similar to [9]. 16 WDM signals (192.2 ... 193.7 THz) with about 100 GHz channel spacing are combined with equal polarizations and modulated together. The electrical part of the transmitter features a 16:1 multiplexer which processes 16 2.5 Gbit/s mutually delayed 2-1 PRBS data streams to form a 2-1 PRBS at 40 Gbit/s, and modulator drivers for a dual-drive DPSK modulator. (D)QPSK is generated in a subsequent all-fiber temperaturestabilized Mach-Zehnder interferometer with a differential delay τ of about 3-symbol durations (~75 ps) and active phase control by means of a piezo fiber stretcher in one of the arms. At one interferometer output, a 193.0 THz optical bandpass filter (BPF), a 12-GHz photoreceiver, and a subsequent RF diode detector are used to measure the RF power carried by the optical DQPSK signal. When the two optical signals are superimposed in quadrature, there is no interference and hence no RF power, except for the clock frequency that is outside the photoreceiver bandwidth. A quadrature control loop based on a 10 kHz lock-in detection scheme stabilizes the interferometer phase by minimizing the RF power. The depth of the 10 kHz phase modulation is only ~0.01 rad (rms). The laser frequencies are fine-tuned to points of a 1/(2τ ) ≈ 6.7 GHz raster so that each WDM channel contains a proper DQPSK signal. The channel spacing is roughly an odd multiple of the raster point spacing. This means that each WDM channel had at least one neighbor where in-phase and quadrature data streams are combined with opposite polarities, hence a different optical pattern. After a later differential interferometric demodulation in the receiver this means that in-phase and quadrature data streams are exchanged. In the transmitter a dual-drive modulator driven at half the clock rate carves 8-ps pulses and thereby completes the RZ-DQPSK signal generation. Finally, the DQPSK signal is split, differentially delayed by 112 symbol durations (~2.8 ns) and recombined with orthogonal polarizations (PolDM). Since this particular polarization multiplexer was available, interleaving of orthogonally polarized pulses in the time domain was not tested. Anyway , pulse interleaving is not necessarily advantageous [10]. The optical signals are transmitted over 4 fiber spans (81+69+60+63 km) with a total length of 273 km, consisting of 153 km of SSMF and 120 km of NZDSF. DCF with a total dispersion of –3150 ps/nm is inserted between inline EDFAs. Fiber and DCF launch powers are +0.5 ... +5 dBm and –3 ... –1 dBm per WDM channel, respectively. EDFA input powers are –20 ... –15 dBm per WDM channel.