Uncompensated 20 Gb/s Duobinary Polarization Division Multiplexing Transmission over 200 km P. Martelli, P. Boffi*, M. Ferrario, L. Marazzi, P. Parolari, A. Righetti, R. Siano, and Mario Martinelli* CoreCom, Via G. Colombo 81, 20133 Milano, Italy, martelli@corecom.it * also with Politecnico di Milano, Dipartimento di Elettronica e Informazione, Via G. Ponzio 34/5, 20133 Milano Abstract: Transmission of 20 Gb/s optical signal over 200 km of uncompensated SMF fiber is achieved. The duobinary format dispersion robustness has been exploited together with polarization division multiplexing to significantly increase the maximum uncompensated reach. ©2007 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (060.4230) Multiplexing. The duobinary modulation format has excited great interest in recent years due to its chromatic dispersion robustness [1,2]. Experimental results confirms transmission over 200 km of uncompensated standard single mode fiber (SMF) [3] at 10 Gbit/s. Doubling the bit-rate, by increasing the modulation speed, reduces the reach to a fourth. On the other hand polarization division multiplexing (PolDM) is a well known technique for doubling the spectral efficiency [4,5], consisting in transmitting two independent channels with orthogonal states of polarization (SOPs). In this work the duobinary format has been exploited together with PolDM in order to achieve, for the first time at the best of our knowledge, a reach of 200 km at an overall bit-rate of 2x10 Gb/s, without any dispersion compensation. In our experiment a DFB laser at 1548.51 nm has been externally modulated by a commercially available duobinary modulator at a bit-rate of 9.953 Gb/s (STM 64) with a 2 31 -1 PRBS. The modulated optical signal is split in two signals uncorrelated by a 3 km long fiber spoil and equalized in power. These two signals are orthogonally polarized by manually adjustable fiber polarization controllers (PCs) and combined by a micro-optic polarization beam combiner (PBC). A polarization multiplexed signal at an overall bit-rate of about 20 Gb/s is generated and then it is boosted by an erbium-doped fiber amplifier (EDFA) and launched in an uncompensated SMF link. At the receiver the multiplexed channels are optically preamplified by an EDFA and a variable amount of amplified spontaneous emission (ASE) noise, generated by a filtered ASE source, is added in order to modify the optical signal-to-noise ratio (OSNR). The two orthogonally polarized channels are demultiplexed by a fiber polarization beam splitter (PBS) after a manually adjustable fiber PC. A single demultiplexed channel is optically filtered (0.3 nm bandwidth) and received by a commercial 10 Gb/s clock and data recovery (CDR) with an avalanche photo- diode (APD), and finally the bit-error-rate (BER) is evaluated. We have measured the BER as a function of the single demultiplexed channel OSNR, which is detected after polarization demultiplexing by means of an optical spectrum analyzer with resolution of 0.5 nm, while the optical power at the APD input is kept constant to -18 dBm. The experiments have been made for different propagation lengths (0 km, 75 km, 150 km and 200 km). For each propagation length two BER curves are obtained, respectively in case of PolDM transmission and in case of single channel transmission. Figure 1 shows OSNR required for a BER value of 10 -6 versus propagation length. In back-to-back condition there is no penalty in passing from the case of single channel transmission to the case of both polarization multiplexed channels. This is due to the high polarization extinction ratios of both PBC and PBS (about 30 dB), and to the good stability of the SOP at the demultiplexing PBS in back-to-back. The cross-talk between the channels after polarization demultiplexing is thus small and does not induces appreciable penalty with respect to the single channel case. On the contrary in correspondence of the BER measurements for propagation lengths of 75 km and more a penalty is found owing to the cross-talk after polarization demultiplexing. This cross- talk is generated by small SOP fluctuactions, which are enhanced by the fiber propagation, at the demultiplexing PBS. Penalties remain nevertheless not higher than 1 dB both at 75 km and at 150 km. At 200 km OSNR penalty increases to 2 dB and a change in the slope is evident as can be seen in Figure 2, where BER curves versus OSNR are shown. Polarization division multiplexing has been proved an attractive technique to double the transmission rate, while preserving the 10 Gb/s dispersion tolerance of the duobinary format. a2696_1.pdf JTuA140.pdf