Content area
Full Text
A 40 Gbit/s transmission experiment on an installed link designed for dense wavelength division multiplexing (DWDM) transmission at 10 Gbit/s, using the alternate-phase return-to-zero (APRZ) modulation format is presented. The experiment confirms the superior nonlinear tolerance of APRZ, even with typical DWDM filtering.
Introduction: Adding 40 Gbit=s channels to existing 10 Gbit=sdense wavelength division multiplexing (DWDM) systems is a cost-effective means to upgrade capacity, but poses challenges in terms of nonlinear impairments [1], specifically, intra-channel cross-phase modulation (IXPM), causing timing jitter, and intra-channel four-wave mixing (IFWM), causing ghost pulses and amplitude jitter. IXPM can be reduced by proper dispersion pre-compensation, while IFWM can be reduced with phase-modulation techniques [2-4]. One such technique is the alternate-phase return-to-zero (APRZ) modulation format [4],in which the optical phase between neighbouring bits differs by a value Df. APRZ's nonlinear tolerance has been studied theoretically, numerically, and experimentally [4-8]. In this Letter we present the first experiment in which APRZ transmission at 40 Gbit=sisdemonstratedonaninstalledDWDMsystem designed for transmission at 10Gbit=s.
Field experiment setup: The APRZ transmitter, depicted in Fig 1a,is implemented as a pulse generator (an actively modelocked ring laser) producing 2.7 ps pulses, followed by an MZM for data modulation and by a phase modulator driven by a 20 GHz sinusoidal signal. The resulting sine-APRZ [6] is shown in Fig. 1b. The data signal is a PRBS of length 231 ^ 1, obtained by time multiplexing four 10 Gbit=s PRBS sequences of length 231 ^ 1, properly delayed. The amplitude, Df, and delay, Dt, of the phase modulation is controlled by means of a variable attenuator and a tunable delay line Dt ¼ 0 through the experiment. The experiment is run in two steps. In a first step the signal from the APRZ transmitter is launched directly into the transmission link. In a second phase, a wavelength multiplexer (MUX) is placed between the transmitter and the link. The MUX acts as a Gaussian filter of order 1.5, with 3 dB passband of 75 GHz, as shown in Fig. 1c. The experiment is run over an installed six-span link between the cities of Stockholm and Hudiksvall, in Sweden. The link, depicted in Fig. 2, being designed to operate at 10 Gbit=s, is dispersion under-compensated, specifically by 800 ps=nm per way....