Guifang Li

Electronic post-compensation of WDM transmission impairments using coherent detection and digital signal processing.

A universal post-compensation scheme for fiber impairments in wavelength-division multiplexing (WDM) systems is proposed based on coherent detection and digital signal processing (DSP). Transmission of 10 x 10 Gbit/s binary-phase-shift-keying (BPSK) signals at a channel spacing of 20 GHz over 800 km dispersion shifted fiber (DSF) has been demonstrated numerically.

Recent advances in coherent optical communication

Recent progress in coherent optical communication, a field revived by advances in digital signal processing (DSP), is reviewed. DSP-based phase and polarization management techniques make coherent detection robust and practical. With coherent detection, the complex field of the received signal is fully recovered, allowing compensation of linear impairments including chromatic dispersion and polarization-mode dispersion using digital filters. In addition, fiber nonlinearities can also be compensated by using backward propagation in the digital domain.

Space-division multiplexing: the next frontier in optical communication

Space-division multiplexing (SDM) uses multiplicity of space channels to increase capacity for optical communication. It is applicable for optical communication in both free space and guided waves. This paper focuses on SDM for fiber-optic communication using few-mode fibers or multimode fibers, in particular on the critical challenge of mode crosstalk. Multiple-input–multiple-output (MIMO) equalization methods developed for wireless communication can be applied as an electronic method to equalize mode crosstalk. Optical approaches, including differential modal group delay management, strong mode coupling, and multicore fibers, are necessary to bring the computational complexity for MIMO mode crosstalk equalization to practical levels. Progress in passive devices, such as (de)multiplexers, and active devices, such as amplifiers and switches, which are considered straightforward challenges in comparison with mode crosstalk, are reviewed. Finally, we present the prospects for SDM in optical transmission and networking.

Mode-division multiplexed transmission with inline few-mode fiber amplifier

We demonstrate mode-division multiplexed WDM transmission over 50-km of few-mode fiber using the fibers LP01 and two degenerate LP11 modes. A few-mode EDFA is used to boost the power of the output signal before a few-mode coherent receiver. A 6×6 time-domain MIMO equalizer is used to recover the transmitted data. We also experimentally characterize the 50-km few-mode fiber and the few-mode EDFA.

Coherent optical communication using polarization multiple-input-multiple-output

Polarization-division multiplexed (PDM) optical signals can potentially be demultiplexed by coherent detection and digital signal processing without using optical dynamic polarization control at the receiver. In this paper, we show that optical communications using PDM is analogous to wireless communications using multiple-input-multiple-output (MIMO) antennae and thus algorithms for channel estimation in wireless MIMO can be ready applied to optical polarization MIMO (PMIMO). Combined with frequency offset and phase estimation algorithms, simulations show that PDM quadrature phase-shift keying signals can be coherently detected by the proposed scheme using commercial semiconductor lasers while no optical phase locking and polarization control are required. This analogy further suggests the potential application of space-time coding in wireless communications to optical polarization MIMO systems and relates the problem of polarization-mode dispersion in fiber transmission to the multi-path propagation in wireless communications.

Ultrahigh-speed reconfigurable logic gates based on four-wave mixing in a semiconductor optical amplifier

We have demonstrated simple reconfigurable all-optical logic operations based on four-wave mixing in semiconductor optical amplifier and encoding information in the polarization of the input signals. Experimental implementation of six logic functions (including XOR, XNOR, AND, NOR, etc.) operating at 10 Gb/s were realized by simply adjusting two polarization controllers in the setup

Multimode fiber amplifier with tunable modal gain using a reconfigurable multimode pump

We propose a method for controlling mode-dependent gain in a multimode Erbium-doped fiber amplifier by tuning the mode content of a multimode pump. The resulting device is suitable for mode-division multiplexed transmission.

Long distance transmission in few-mode fibers

Using multimode fibers for long-haul transmission is proposed and demonstrated experimentally. In particular few-mode fibers (FMFs) are demonstrated as a good compromise since they are sufficiently resistant to mode coupling compared to standard multimode fibers but they still can have large core diameters compared to single-mode fibers. As a result these fibers can have significantly less nonlinearity and at the same time they can have the same performance as single-mode fibers in terms of dispersion and loss. In the absence of mode coupling it is possible to use these fibers in the single-mode operation where all the data is carried in only one of the spatial modes throughout the fiber. It is shown experimentally that the single-mode operation is achieved simply by splicing single-mode fibers to both ends of a 35-km-long dual-mode fiber at 1310 nm. After 35 km of transmission, no modal dispersion or excess loss was observed. Finally the same fiber is placed in a recirculating loop and 3 WDM channels each carrying 6 Gb/s BPSK data were transmitted through 1050 km of the few-mode fiber without modal dispersion.

Phase-and-amplitude regeneration of differential phase-shift keyed signals using a phase-sensitive amplifier

DPSK phase-and-amplitude regeneration with a NOLM-based phase-sensitive amplifier is demonstrated experimentally. For a highly degraded input signal, maximum differential phase errors were reduced from 82 degrees to 41 degrees , while the SNR was improved by more than 5-dB. Differential phase Q-factor improvement was better than 6-dB. The PSA was operated free of excess noise due to stimulated Brillouin scattering by using a binary phase modulated pulse train as the pump. The impact of pump fluctuations on regeneration performance is clarified. The regenerated signal was characterized by measurement of the constellation diagram by linear optical sampling, giving the first directly measured evidence of DPSK phase regeneration.

Phase Regeneration of NRZ-DPSK Signals Based on Symmetric-Pump Phase-Sensitive Amplification

Symmetric-pump phase-sensitive amplification (SP-PSA) is investigated experimentally. Symmetric pump waves are derived using carrier-suppressed return-to-zero modulation. The SP-PSA is used for phase regeneration of a phase-noise degraded nonreturn-to-zero differential phase-shift keying signal, significantly improving signal quality