The transmission of a pair of phase-conjugated beams is shown to mitigate nonlinear distortion during optical fibre communication, allowing a 400 Gbit s−1 superchannel to be sent over 12,800 km of optical fibre.

Phase-conjugated twin waves for communication beyond the Kerr nonlinearity limit

We present experimental demonstrations using direct-detection and optical-orthogonal frequency division multiplexing (DD-OOFDM) for the compensation of chromatic dispersion in long-haul optical fiber links. Three transmitter designs of varying electrical and optical complexity are used for optical single sideband (OSSB) transmission and the theory behind each design is discussed. The data rates achieved for the three systems are 10, 12, and 20 Gbit/s for fiber distances between 320 and 400 km. A discussion of system overheads is provided together with simulations of the required optical signal-to-noise ratio (OSNR).

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Experimental Demonstrations of Electronic Dispersion Compensation for Long-Haul Transmission Using Direct-Detection Optical OFDM

A modular, compact and widely tunable laser system for the efficient generation of high peak and high average power ultrashort pulses. Modularity is ensured by the implementation of interchangeable amplifier components. System compactness is ensured by employing efficient fiber amplifiers, directly or indirectly pumped by diode lasers. Peak power handling capability of the fiber amplifiers is expanded by using optimized pulse shapes, as well as dispersively broadened pulses. Dispersive broadening is introduced by dispersive pulse stretching in the presence of self-phase modulation and gain, resulting in the formation of high-power parabolic pulses. In addition, dispersive broadening is also introduced by simple fiber delay lines or chirped fiber gratings, resulting in a further increase of the energy handling ability of the fiber amplifiers. The phase of the pulses in the dispersive delay line is controlled to quartic order by the use of fibers with varying amounts of waveguide dispersion or by controlling the chirp of the fiber gratings. After amplification, the dispersively stretched pulses can be re-compressed to nearly their bandwidth limit by the implementation of another set of dispersive delay lines. To ensure a wide tunability of the whole system, Raman-shifting of the compact sources of ultrashort pulses in conjunction with frequency-conversion in nonlinear optical crystals can be implemented, or an Anti-Stokes fiber in conjunction with fiber amplifiers and Raman-shifters are used. A particularly compact implementation of the whole system uses fiber oscillators in conjunction with fiber amplifiers. Additionally, long, distributed, positive dispersion optical amplifiers are used to improve transmission characteristics of an optical communication system. Finally, an optical communication system utilizes a Raman amplifier fiber pumped by a train of Raman-shifted, wavelength-tunable pump pulses, to thereby amplify an optical signal which counterpropogates within the Raman amplifier fiber with respect to the pump pulses.

Modular, high energy, widely-tunable ultrafast fiber source

Polarization mode dispersion in an optical signal transmitted through a waveguide of a communications system is compensated by separating the dispersed signal into components corresponding to principal polarization states. The components are delayed by respective delays differing by a delay increment which is controlled to correspond to the dispersion delay and the delayed components are recombined to provide a dispersion compensated optical output signal. Each of the delays is provided by an chirped Bragg reflector forming part of a delay line, the Bragg reflectors comprising optical fibres with chirped intracore index gratings. Transducers or temperature controllers acting on one of the fibres allows dimensional control of the grating periodicity such that the position of Bragg reflection is variable. Wavelength division multiplexed optical signals are compensated using sampled gratings which allow a common Bragg reflection position for each wavelength.

Polarization mode dispersion compensation

The problem of analytical evaluation of the maximum rate at which information can be reliably transmitted on a nonlinear wavelength division multiplexing fiber-optic channel with a given modulation format and detection strategy is addressed. An approximate solution of the nonlinear Schrodinger equation is adopted to obtain an accurate analytical discrete-time channel model, valid for arbitrary link configurations and modulation formats. By exploiting the concept of mismatched decoding, considering a sub-optimum detection strategy that accounts for intra-channel nonlinearities, the proposed model is employed to derive closed-form expressions of the achievable information rate with various modulation formats. All the analytical results are verified through comparison with numerical simulations in different scenarios.

Achievable Information Rate in Nonlinear WDM Fiber-Optic Systems With Arbitrary Modulation Formats and Dispersion Maps

A transmitter for an optical transmission system transmits an optical sub carrier multiplexed signal comprising number of sub-carriers, onto an optical transmission path, and provides spectral shaping by different magnitudes of the sub-carriers, or different modulation formats for different sub carriers. This spectral shaping can reduce performance degradation by Kerr effect optical non linearities. This can mean higher input powers may be launched. The magnitudes can provide a signal spectrum which is lower near a center of a band of sub carriers than near an edge of the band. Such spectral shaping can be provided in the receiver either to undo the pre emphasis in the transmitter, or to reduce non linearities from components at the receiving side.

Spectral shaping for optical OFDM transmission

In a WDM transmission system carrying amplitude modulated traffic in which significant cross-phase modulation occurs, each of the individual channels is pre-chirped at the transmitter with replicas, or low-pass filtered replicas of the amplitude modulation applied to each of the other channels. Prechirping of each individual channel with a replica of the amplitude modulation applied to that channel may be added in order additionally to provide compensation for self-phase modulation.

Transmission system with cross-phase modulation compensation

In an optical communication system, a train of optical pulses is transmitted such that each pulse carries a set of multiplexed channels. In a receiver, the error rate for each channel is monitored and a subset of channels having the most favorable rate is selected to carry a first data stream. The remaining channels carry a second data stream. Error rate performance is determined on the basis of error detecting codes carried by the channels in the first and second data streams.

Multiplexed transmission of optical signals

We report a novel simple metric for the characterization of nonlinear impairments for both single-channel and wavelength-division-multiplexed dispersion-managed transmission systems. We show that transmission lines having identical nonlinear diffusion bandwidth lead to similar nonlinear degradation.

Simple criterion for the characterization of nonlinear impairments in dispersion-managed optical transmission systems

An optical communications system has a plurality of spans between a transmitter and a receiver. The receiver has optical to electrical conversion circuitry for converting the received optical signal to an electric signal, analogue to digital conversion circuitry and digital signal processing means for analysing the electrical digital signal. The digital signal processing means derives information concerning characteristics of individual spans from the electrical digital signal. This enables parameters such as per-span variations in provisioned power, local dispersion and span loss to be measured. In-service measurements of system characteristics can be used to enable optimisation of the network operation.

Alan Robinson

Papers

Spectral shaping for optical OFDM transmission

Transmission system with cross-phase modulation compensation

Multiplexed transmission of optical signals

Simple criterion for the characterization of nonlinear impairments in dispersion-managed optical transmission systems

Analysis and control of optical communications systems