Transmitting solitons forever
Nonlinear optical mirrors combined with dispersion management can control dispersion so well that, theoretically, solitons could be transmitted over unlimited distances of single-mode fiber. Sonia Boscolo, Sergei Turitsyn, and Keith Blow at Aston University (Birmingham, England) numerically demonstrated that dispersion management and in-line nonlinear optical mirrors (NOLMs) can achieve all-optical passive regeneration of a soliton data stream at 40 Gbit/s.1
The researchers used a transmission line made of a number of blocks, each composed of 32.5-km single-mode fiber, 6.8 km dispersion-compensating fiber, a fiber amplifier, and fixed Gaussian filter. Every five blocks, a NOLM was placed in the line. The NOLM incorporated a 50:50 splitter and a loop of dispersion-shifted fiber that provided zero dispersion. They placed an EDFA near the splitter. They required that the output power be the same as the input power.
"The NOLM is an interferometer with two possible output ports," said Blow. "Normal operation has the light being reflected back down the input fiber. If a p phase shift occurs in the interferometer then a 'fringe shift' occurs, which in this case corresponds to the light being transmitted onward in the system." The phase shift comes from self-phase modulation in the fiber nonlinearity, which is different for the two pulses in the NOLM because they have different intensities as a result of the offset amplifer in the NOLM. "The power is chosen so that the peak of the pulse is fully transmitted but the wings of the pulse are not," said Blow.
"Fiber NOLMs have an ultrafast response (subpicosecond), which most saturable absorbers do not," Blow adds. "They also have a different switching characteristic which enables us to stabilize the peak power of the pulse."
This approach overcomes problems with timing jitter and soliton interactions.
- S. Boscolo, S. K. Turitsyn, K. J. Blow, Electron. Lett. 37(2), 112 (Jan.18, 2001).