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We discuss coherent optical orthogonal frequency division multiplexing (CO-OFDM) as a suitable modulation technique for long-haul transmission systems. Several design and implementation aspects of a CO-OFDM system are reviewed, but we especially focus on phase noise compensation. As conventional CO-OFDM transmission systems are very sensitive to laser phase noise a novel method to compensate for phase noise is introduced. With the help of this phase noise compensation method we show continuously detectable OFDM transmission at 25.8 Gb/s data rate (20 Gb/s after coding) over 4160-km SSMF without dispersion compensation. [ABSTRACT FROM PUBLISHER]

We discuss high spectral efficient long-haul transmission using multilevel modulation formats. In particular, we investigate long-haul transmission with a 1.6-b/s/Hz spectral efficiency using polarization-multiplexed return-to-zero differential quadrature phase-shift keying. In an experimental demonstration, we show 40 times 85.6-Gb/s transmission with a 50-GHz channel spacing over 1700 km (18 times 94.5 km) of standard-single-mode fiber [ABSTRACT FROM PUBLISHER]

In this paper, we review the recent progress in transmission experiments by employing optical phase conjugation (OPC) for the compensation of chromatic dispersion and nonlinear impairments. OPC is realized with difference frequency generation (DFG) in a periodically poled lithium-niobate (PPLN) waveguide, for transparent wavelength-division multiplexed (WDM) operation with high conversion efficiency. We discuss extensively the principle behind optical phase conjugation and the realization of a polarization independent OPC subsystem. Using OPC for chromatic dispersion compensation WDM 40-Gb/s long-haul transmission is described. As well, transmission employing both mixed data rates and mixed modulation formats is discussed. No significant nonlinear impairments are observed from the nonperiodic dispersion map used in these experiments. The compensation of intrachannel nonlinear impairments by OPC is described for WDM carrier-suppressed return-to-zero (CSRZ) transmission. In this experiment, a 50% increase in transmission reach is obtained by adding an OPC unit to a transmission line using dispersion compensating fiber (DCF) for dispersion compensation. Furthermore, the compensation of impairments due to nonlinear phase noise is reviewed. An in-depth analysis is conducted on what performance improvement is to be expected for various OPC configurations and a proof-of-principle experiment is described showing over 4-dB improvement in Q-factor due to compensation of nonlinear impairments resulting from nonlinear phase noise. Finally, an ultralong-haul WDM transmission of 22times20-Gb/s return-to-zero differential quadrature phase-shift keying (RZ-DQPSK) is discussed showing that OPC can compensate for chromatic dispersion, as well as self-phase modulation (SPM) induced nonlinear impairments, such as nonlinear phase noise. Compared to a "conventional" transmission link using DCF for dispersion compensation, a 44% increase in transmission reach is obtained when OPC is emplo- - yed. In this experiment, we show the feasibility of using only one polarization-independent PPLN subsystem to compensate for an accumulated chromatic dispersion of over 160 000 ps/nm [ABSTRACT FROM PUBLISHER]

In this paper, the performance of midlink spectral inversion (MLSI) is compared with the performance of "conventional" dispersion compensation fiber (DCF)-based transmission for two data formats: 43-Gb/s ON-OFF keying nonreturn-to-zero (OOK-NRZ) and 43-Gb/s duobinary. In the MLSI-based system, a polarization-diverse subsystem was used for spectral inversion employing magnesium-oxide-doped periodically poled lithium niobate (PPLN) waveguide technology. The transmission link consists of 8 × 100 km standard single-mode fiber (SSMF) using erbium-doped fiber amplifiers (EDFAs) for amplification. Compared to the DCF-based system, it is seen that the MLSI-based configuration enhances the dispersion tolerance for both the NRZ and the duobinary modulation formats. It is concluded that the combination of the MLSI and the duobinary modulation format yields a highly dispersion-tolerant stable 43-Gb/s transmission system. [ABSTRACT FROM PUBLISHER]

Nonlinear phase noise (Gordon-Mollenauer phase noise) can limit the transmission distance for phase-shift-keyed modulation formats. In this paper, the compensation of nonlinear phase noise by a midlink optical phase conjugation (OPC) is studied. A proof-of-principle experiment is presented showing an over 4-dB improvement in Q factor when OPC is employed in a differential phase-shift-keying (DPSK) system. Also, an ultra long-haul OPC-based differential quadrature phase-shift-keying (DQPSK) transmission experiment is studied to show the impact of self-phase modulation (SPM)-induced impairments, including nonlinear phase noise, in a transmission line. OPC results in a 44% increase in transmission distance when compared to a "conventional" transmission system using dispersion compensating fiber (DCF) for chromatic dispersion compensation. [ABSTRACT FROM PUBLISHER]

A compact integrated all-optical wavelength converter based on an asymmetric Sagnac loop is demonstrated. We show that a 40-Gb/s nonreturn-to-zero data signal can be converted over the entire C-band employing an asymmetric Sagnac loop. Compared to the back-to-back configuration, a receiver sensitivity power penalty of less then 2.1 dB is measured for both up- and down-conversion. [ABSTRACT FROM PUBLISHER]