Optical phase conjugation in fiber-optic transmission systems

S.L. Jansen

Research output: ThesisPhd Thesis 1 (Research TU/e / Graduation TU/e)

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As the data rate of long-haul transmission links is increased, the design and realization of the transmission link becomes more difficult. As a result, more sophisticated methods are required to improve the transmission quality. The robustness of a transmission link can be increased and its structure greatly simplified by the use of mid-link optical phase conjugation (OPC). OPC is a promising technology to compensate for deterministic, phase related impairments (i.e. the Kerr effect and chromatic dispersion) in long-haul transmission systems. This thesis assesses the regenerative capabilities of OPC for the compensation of distortions that occur in modern transmission systems. The focus of the research is on transmission systems where OPC is employed to compensate for both chromatic dispersion and nonlinear impairments. The dispersion map (i.e. the dispersion as a function of the transmission distance) of such a transmission system is completely different from that of a conventional transmission system. The accumulated dispersion along the link of an OPCbased transmission system is significantly higher than that of a conventional transmission system. We investigated the influence of the dispersion map of OPC on nonlinear impairments using the non return-to-zero amplitude-shift-keying (NRZ-ASK) modulation format. As a result, it is shown that the peak powers that occur in the OPC transmission system are at a 10-Gbit/s/channel data rate significantly higher than the peak powers that occur in a conventional transmission system. The higher peak powers in the OPC based transmission system lead to an increased self-phase modulation (SPM) penalty. Through phase conjugation most of the SPM impairments are compensated for. However, when multiple wavelength division multiplexed (WDM) channels at narrow channel spacing are used for transmission, cross-phase modulation (XPM) is the dominating transmission impairment. Although XPM is principally a deterministic distortion, it must be treated as non-deterministic due to the dispersion of the transmission link. With simulations and experiments we show that because of this, the XPM compensation through OPC is marginal. At a 40 Gbit/s data rate, the peak powers that occur in the OPC-based transmission system are similar to those that occur in the conventional transmission system. In 40 Gbit/s WDM transmission systems the influence of XPM is relatively low. These transmission systems are rather limited by intra-channel nonlinear impairments such as SPM, intrachannel XPM (IXPM) and intrachannel FWM (IFWM). We show experimentally that in this case, the performance of the OPC transmission system is better than that of the conventional transmission system. When OPC is used to compensate for the chromatic dispersion, the OPC must be placed in the middle of the transmission link. This technique is often referred to as "midlink OPC". However, in some transmission links it is not possible to place the OPC exactly in the middle. Therefore, several configurations with a transmission length of 700 km to 900 km were assessed where the OPC was placed 100 km from the middle of the transmission link. In this experiment practically no bit-error ratio (BER) degradation was observed in the off-center configuration. Recently, strong interest has been shown in phase-shift keying modulation (PSK) formats such as differential phase-shift-keying (DPSK). DPSK’s main advantages over ASK are that it is more robust to narrowband optical filtering and has a 3 dB higher sensitivity in combination with balanced detection. However, unlike ASK signals PSK signals can be distorted by nonlinear phase noise (NPN). For long-haul transmission systems, the impact of NPN is so severe that the performance of DPSK is in some cases even worse than that of ASK. The impact of nonlinear phase noise is studied for 10.7-Gbit/s DPSK in an 800-km transmission link. In this experiment it is shown that impairments due to nonlinear phase noise can be significantly reduced using optical phase conjugation. The dependence of the location of the OPC within the transmission link is assessed as well. Allowing a penalty of 1 decade in BER from the optimum, the OPC-unit can be varied over a wide range, from nearly 1/3 to 2/3 of the transmission link. The combination of mid-link OPC is assessed with 21.4-Gbit/s return-to-zero differential quadrature phase-shift keying (RZ-DQPSK) in an ultra long-haul transmission experiment. Error-free transmission after FEC is realized over 10,200 km for all 22 WDM channels. In this experiment, a single OPC-unit is used in the middle of the link to compensate for an accumulated chromatic dispersion of over 160,000 ps/nm. Along the transmission line, the dispersion accumulates in this experiment to more than 80,000 ps/nm. This is significantly higher than the maximum accumulated dispersion in the conventional transmission system (approximately 3,000 ps/nm). The high accumulated dispersion results in an extreme overlap of the pulses along the transmission line. With this experiment we show that despite the high dispersion, the feasible transmission distance of the OPC based transmission system is 44% greater than that obtained in the conventional transmission system. By doubling the data rate and keeping the 50-GHz channel spacing, a 0.8-bit/s/Hz spectral efficient WDM transmission system is realized. At 42.8-Gbit/s RZ-DQPSK, transmission over 5,000 km was realized with mid-link OPC. Compared to the feasible transmission distance obtained at 21.4-Gbit/s, the feasible transmission distance is reduced by about 50%. This reduction of transmission distance with 50% corresponds to the 3-dB OSNR penalty that is present between 21.4-Gbit/s and 42.8-Gbit/s RZ-DQPSK in the back-toback configuration. For the conventional transmission system, a greater reduction in the feasible transmission distance (factor of 2.4) is measured due to increased penalties that result from a combination of self phase modulation and nonlinear phase noise. Comparing the feasible transmission distance of the OPC to the conventional transmission system an improvement of 60% is observed in this experiment.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Electrical Engineering
  • Khoe, G.D. (Djan), Promotor
  • Koonen, A.M.J. (Ton), Promotor
  • de Waardt, Hugo, Copromotor
Award date26 Jun 2006
Place of PublicationEindhoven
Print ISBNs90-386-1803-4
Publication statusPublished - 2006


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