Comparison of DPSK and OOK modulation format in 160 Gbit=s transmission system S. Ferber, R. Ludwig, C. Boerner, A. Wietfeld, B. Schmauss, J. Berger, C. Schubert, G. Unterboersch and H.G. Weber Single-polarisation 160 Gbit=s OTDM transmission over 3  80 km dispersion-managed SLA fibre using DPSK and OOK modulation is reported. The influence of the fibre input power is experimentally and theoretically investigated. A significant advantage for DPSK is revealed. Introduction: Several transmission experiments have recently been reported using differential phase-shift keying (DPSK)-modulation format and in particular return-to-zero (RZ)-DPSK with balanced detection at channel data rates up to 160 Gbit=s [1–3]. Compared with on–off keying (OOK), RZ-DPSK modulation format has several advantages. First, a 3 dB improvement in receiver sensitivity due to the balanced detection scheme is expected [4]. This gives an increased system margin, which can be used to reduce the input power to the fibre spans or to increase the length of the fibre link [1–3]. Moreover, as the RZ-DPSK modulation format has lower peak powers and constant pulse amplitude, it reduces fibre nonlinearity induced effects such as self-phase modulation (SPM) and cross-phase modulation (XPM) [5]. However, the DPSK modulation format is more sensitive to phase disturbances caused by fibre nonlinearities [6]. In this Letter, we experimentally compare the power dependence of OOK and DPSK modulation formats. The bit error rate (BER) against the input power to each fibre span in a single-channel, single- polarisation 160 Gbit=s transmission over a 3  80 km Ultrawave fibre link was measured. A conventional setup without Raman amplification and without FEC was used. Also a comparison with numerical results is given. Experiment: The optical time division multiplexing (OTDM) trans- mission system is shown in Fig. 1. The 160 Gbit=s data transmitter comprised a 40 GHz optical pulse source, a modulator driven by a pattern generator and a fibre delay line multiplexer providing a single- polarisation 160 Gbit=s pseudorandom bit sequence (PRBS) signal (word length 2 7  1). For OOK the LiNbO 3 dual-drive Mach–Zehnder type modulator was operated in push–pull mode using the conven- tional operation scheme. For phase modulation, the bias was changed to the null of the transmission curve and the driving voltage to twice the p-voltage [4]. Owing to the pseudo-statistical nature of PRBS, a separate DPSK-encoder was not needed. The 160 Gbit=s data signal (wavelength 1554 nm, pulse width 1.4 ps, transform limited sech 2 pulses) was transmitted over a 240 km fibre link. Fig. 1 Experimental setup The fibre link comprised three spans of 80 km Ultrawave fibre provided by OFS-Denmark consisting of 57 km super large area (SLA) fibre (0.187 dB=km, D ¼ 20 ps=km=nm) and 23 km inverse dispersion fibre (IDF) (0.234 dB=km, D ¼44 ps=km=nm). The actual span losses were 19.5, 17.5 and 21.4 dB in the experiment due to adapters and non-perfect splices. For the measurements, the average input power P in into each span was varied (1, ... , þ18 dBm). The pulse width at the output of the transmission link varied between 1.8 and 2.0 ps due to PMD. Fig. 2a shows the optical spectrum of the 160 Gbit=s DPSK-modulated signal after the third fibre span for different fibre span input powers. Fig. 2 a Spectrum of 160 Gbit=s DPSK signal after last span b After demultiplexing and decoding at both DLI-arms at 40 Gbit=s Fig. 3 Samples of 160 Gbit=s DPSK BER-curves for different fibre input powers and received DPSK eye-diagrams at 40 Gbit=s The 160 Gbit=s receiver consisted of a 160 to 40 Gbit=s optical demultiplexer and a PLL-based optical clock recovery, each using a single, bidirectionally operated electroabsorption modulator [7, 8]. The 40 Gbit=s receiver comprised optical preamplification including an optical filter (2.6 nm), the detector and a 40 Gbit=s BER tester. For DPSK a delay line interferometer (DLI) was used as demodulator fol- lowed by a monolithically integrated, balanced photodetector provided by u 2 t Photonics. The spectrum of the demultiplexed and decoded DPSK-signal at both outputs of the DLI are shown in Fig. 2b. For OOK a fast u 2 t photodiode was used as detector directly after the amplifiers. Fig. 3 shows BER measurements against the received power at 160 Gbit=s (as indicated in Fig. 1). For clarity, only three sample BER curves for DPSK modulation are shown at low, medium and high input power to each span. The insets show corresponding eye diagrams. At input powers >15 dBm the signal is disturbed by intra- channel four-wave mixing. This results in a strong performance varia- tion (indicated in Fig. 3 by the hatched area) depending on the relative phase of the 160 Gbit=s pulses. The receiver sensitivity at a BER ¼ 10 9 was 26.7 dBm for DPSK modulation and 22.6 dBm for OOK modulation. Whereas 3 dB improvement for DPSK is caused by the balanced detection scheme, we attribute the remaining difference to the slightly different experimental conditions. ELECTRONICS LETTERS 2nd October 2003 Vol. 39 No. 20