Abstract

By recirculating soliton pulses many times around a closed 42-km fiber loop with loss exactly compensated by Raman gain, we have successfully demonstrated transmission without electronic regeneration over distances in excess of 4000 km.¹ The loop is closed on itself with an all-fiber version of a Mach-Zehnder interferometer so that pump light at 1500 nm (~300 mW from a cw color center laser, power precisely controlled by feedback) is efficiently coupled into the loop, while at the same time ~95% of the signal light (a train of 55-ps pulses from a mode-locked color center laser operating at 1600 nm) continues to recirculate around the loop. The trip is terminated when the pump is turned off. A sample of the signal train leaving the loop is detected by an ultrafast diode (response time, 9 ps), whose output is sent to a microwave spectrum analyzer. We then infer the pulse shape in time from the measured pulse envelope spectrum. Up to ~4000 km, we see almost no change in effective pulse width. At ~4800 km, ${\tau }_{\text{eff}}{=}_{\backslash }\overline{2{\tau }_0}$, but this apparent broadening is due to jitter in pulse arrival times. We have also been able to study interaction between closely spaced soliton pairs over thousands of kilometers. Our experiments have strong and positive implications for the development of an all-optical long-distance transmission system.

© 1988 Optical Society of America