Multilevel modulation formats for robust long-haul high capacity transmission

Abstract

In the past couple of years we have witnessed a rapid growth in the required bandwidth due to a variation of Internet applications. This has trigged a significant interest in the development of optical transponders with fast data rates and high spectral efficiencies. The recent developments in DSP speeds brought optical coherent detection back into the picture, which has opened a whole new world of robust multilevel optical modulation formats capable of matching the increase in required transmission capacity. Recently, polarization-multiplexed (POLMUX) differential phase shift keying (DPSK) with coherent detection has been proposed for realizing ultra long-halu transmission with a data rate of 43 Gb/s. In the first part of this thesis we compare the suitability of 43 Gb/s POLMUX-RZ-DPSK and POLMUX-RZ differential quadrature phase shift keying (DQPSK) modulation for ultra long-haul optical transmission. We experimentally demonstrate that the higher robustness against nonlinear impairments of 43 Gb/s POLMUX-RZ-DPSK allows for a v 40% increase in feasible transmission distance compared to POLMUX-RZ-DQPSK. This extra reach of 43 Gb/s POLMUX-RZ-DPSK has enabled us to demonstrate a transoceanic transmission over more than 7000 km of field deployed fiber. After the data rate of 40 Gb/s, 100 Gb/s seemed to be the next logical step for the optical communication community. The transmission of 100 Gb/s channels has been successfully demonstrated using several modulation formats such as VSB and DQPSK with direct detection. However, the most cumbersome issues with such solutions were the susceptibility to linear optical effects and the low SE of the signal. In 2007 the first 111 Gb/s long-haul transmission using POLMUX-RZ-DQPSK with a SE of 2 b/s/Hz together with coherent detection has been demonstrated. Due to the high SE of the signal, the ability to compensate almost all kinds of linear effects and the enhanced sensitivity of a coherent detection receiver this solution was considered very favorably. In the second part of this thesis we investigate on the tolerance of the 111 Gb/s POLMUX-RZ-DQPSK signal to the different transmission impairments. The compatibility of the 111 Gb/s channels with existing networks has been investigated both using lab experiments and field trials. We demonstrated transmission of 111 Gb/s channels over field deployed fiber with co-propagating 10.7 Gb/s OOK and 43 Gb/s DPSK channels. We showed that by carefully choosing the launch power levels and the neighboring channels configuration, a 111 Gb/s channel can fit very well in currently existing transmission links. Furthermore, We studied the tolerance of the 111 Gb/s signal to the different nonlinear transmission impairments in different link structures. We analyze the effect of dispersion maps for different fiber types, and derived some rules for the reduction of performance difference between dispersion managed and non-dispersion managed links. Furthermore, we confirm that the super large effective area (SLA) fiber family can provide around 3 dB advantage in the tolerance to nonlinear effects compared to standard single mode fiber (SSMF). Despite of the high OSNR requirements of POLMUX- 16 level quadrature amplitude modulation (16QAM) and its low tolerance to nonlinear transmission effects it can achieve a SE as high as 4 b/s/Hz. Consequently, POLMUX-16QAM can enable the transmission of 200 Gb/s channels on the standard 50 GHz channel grid. In the last part of this thesis we demonstrate the generation and detection of a 224 Gb/s POLMUX-RZ-16QAM signal with around 4 dB of penalty in comparison to the theoretical limits. Furthermore, we report the transmission of eleven 224 Gb/s POLMUX-RZ-16QAM channels over 670 km of SSMF with a channel spacing of 50 GHz and a SE of 4.2 b/s/Hz. A long-haul transmission for the eleven 224 Gb/s channels over 1500 km has also been realized through employing advanced fiber types.