R. W. Tkach

Four-photon mixing and high-speed WDM systems

The trend toward higher bit rates in lightwave communications has increased interest in dispersion-shifted fiber to minimize dispersion penalties. At the same time optical amplifiers have increased interest in wavelength multiplexing. These two methods of increasing system capacity, if used together, can result in severe degradation due to fiber nonlinearity. This paper discusses the impact of the principal nonlinearity, four-photon mixing, and describes strategies that allow simultaneous use of high bit rates and WDM. >

Reduction of four-wave mixing crosstalk in WDM systems using unequally spaced channels

Crosstalk due to four-wave mixing (FWM) is the dominant nonlinear effect in long-haul multichannel optical communication systems employing dispersion-shifted fiber. A technique to design the channel frequency allocation in order to minimize the crosstalk due to FWM is presented. It is shown that suitable unequal channel separations can be found for which no four-wave mixing product term is superimposed on any of the transmitted channels. This is obtained at the expense of some expansion of the system bandwidth. Simulations are presented to show the effectiveness of this technique in a 10-channel, 10-Gb/s per channel, system.<<ETX>>

WDM systems with unequally spaced channels

Crosstalk due to four-wave mixing (FWM) is the dominant nonlinear effect in long-haul multichannel optical communication systems employing dispersion-shifted fiber. A method is discussed to find non-uniform channel separations for which no four-wave mixing product is superimposed on any of the transmitted channels, therefore suppressing FWM crosstalk. The residual crosstalk, due to channel power depletion only, is analytically evaluated for intensity-modulated repeaterless wavelength-division-multiplexed (WDM) systems and compared to experimental results. The theory includes the effect of the channel depletion on the amplitude of each phase-matched FWM wave. The probability of error is evaluated including the statistics of the pattern dependent channel depletion. The BER curve computed for an 8-channel WDM system is found to be in good agreement with experimental results. In the experiment, repeaterless transmission of eight 10 Gb/s WDM channels over 137 km (11 Tb/s-km) of dispersion-shifted fiber was demonstrated and error-free operation was achieved over a wide range of input powers using unequally spaced channels. The same system with equally spaced channels could not achieve a probability of error lower than 10/sup -6/. The use of unequal channel spacing allowed fiber input power to be increased by as much as 7 dB, which could be translated into a fivefold increase of the bit rate per channel (and therefore of the system capacity), or to an increase in the system length of about 30 km. >

Equalization in amplified WDM lightwave transmission systems

The authors introduce simple techniques to solve gain equalization problems in amplified wavelength-multiplexed lightwave transmission systems. By adjusting the channel powers at the terminals in a prescribed way, either the output powers or the output signal-to-noise ratios of all the channels can be equalized. All adjustments can be made using information provided by system telemetry. No new equipment, upgrades, or adjustments are required at intermediate amplifier sites.<<ETX>>

Fast-link control protection of surviving channels in multiwavelength optical networks

Link control, a new technique for protecting surviving channels on a per-link basis from fast power transients resulting from network reconfigurations or line failures, is demonstrated in a 560-km eight-amplifier wavelength division multiplexed link carrying seven 2.5-Gb/s channels plus the link control channel.

1-Tb/s transmission experiment

A 1-Tb/s aggregate capacity (50 channels each at 20 Gb/s) was transmitted through 55 km of nonzero-dispersion fiber. Fifty channels were generated by polarization multiplexing 25 wavelengths.

8*10 Gb/s transmission through 280 km of dispersion-managed fiber

Eight WDM channels each operating at 10 Gb/s were transmitted through 280 km of dispersion-shifted fiber concatenated with small amounts of conventional fiber. No degradations from optical nonlinearities were observed.<<ETX>>

High-Capacity Optical Transmission Systems

The 25 years since the founding of the Journal of Lightwave Technology have seen more than three orders of magnitude increase in the capacity of optical transmission systems. This dramatic increase was brought about by the deployment of WDM and advances in high-speed transmission technologies.

What is the actual capacity of single-mode fibers in amplified lightwave systems?

The combination of amplifier noise and optical nonlinearities limits the maximum capacity of single-mode fibers in amplified lightwave systems. Using fundamental considerations, it is shown that the capacity of single-mode fibers in transcontinental amplified lightwave systems is not terabits/second but less than 100 Gb/s with ideal amplifiers, and significantly less in realistic systems.<<ETX>>

Effect of modulation statistics on Raman crosstalk in WDM systems

Stimulated Raman scattering crosstalk in intensity-modulated WDM systems is analyzed, including modulation statistics. The actual penalty is found to be lower than that expected from a worst-case analysis. For a large number of channels, the depletion due to stimulated Raman scattering becomes half of the value found with a worst-case analysis and assumes a deterministic behavior.<<ETX>>