Generation and Transmission of Optical Solitons at Ultrahigh Bit Rates

Licentiate thesis, 1998

This thesis deals with the generation and transmission of optical solitons in ultrahigh-speed fiber-optic communication systems. For optical soliton pulses the fiber dispersion and nonlinearity can under certain circumstances cancel each other, thus allowing the solitons to remain undistorted over long distances. This makes the solitons very attractive for the realization of future ultrahigh-speed long-distance optical communication systems. One important aspect in this context is the generation of jitter and chirp-free short optical pulses at high bit rates and in the 1550 nm region, where optical fibers have the lowest loss. For this purpose actively mode-locked erbium fiber ring lasers (AML-EFRLs) are considered to be one of the most promising candidates. In this work we have constructed a dispersion-managed and polarization-maintaining 10 GHz AML-EFRL, producing stable and virtually chirp-free 3-8 ps wide soliton pulses at 1560 nm. The laser can also produce nearly chirp free 15 ps wide Gaussian pulses. The pulses have a timing jitter below 0.2 ps and the highest reported super-mode noise suppression of 90 dB. This strikingly high super-mode noise suppression is believed to be a result of the dispersion-management in the laser cavity. The wavelength is tunable within 1530-1570 nm. We also experimentally show that the stable soliton operation regime and the pulse-width tunability of the AML-EFRL is extended by increasing the laser output coupling. Although optical solitons ideally remain undistorted over arbitrarily long distances, in reality the fiber loss and the soliton interactions put limits on soliton-based transmission links. We have numerically studied an interaction-limited 40 Gbit/s soliton system and shown that the effects of the interactions can be significantly reduced simply by using electrical lowpass filters with optimized bandwidths at the receiver. The interaction word length and pulse-width dependence have also been studied.