Intrinsic propagation dynamics of bright and dark pulses in long-haul fiber systems: a controlled study

Optical solitons are crucial for long-haul fiber-optic communication due to their ability to balance dispersion and nonlinear effects. Both bright and dark solitons have been widely studied. However, a systematic and controlled comparison of their intrinsic propagation characteristics in long-haul systems remains limited. This paper presents a structured and controlled comparison of bright and dark pulses over a 600 km fiber-optic link using a circulating loop technique under identical system configuration. These pulses exhibit soliton-like propagation within the optical fiber by satisfying the soliton formation condition. To ensure a fair evaluation, any advanced generation or system-level optimization techniques are deliberately excluded. The key performance metrics, including optical signal-to-noise ratio (OSNR), Q-factor, bit error rate (BER), timing jitter, and pulse broadening, are analyzed over transmission distances. The results demonstrate that both pulses degrade with increasing transmission distance. However, dark pulses exhibit comparatively improved performance trends under the considered system conditions. Specifically, they achieve higher optical signal-to-noise ratio (OSNR), improved Q-factor, and lower bit error rate (BER). In addition, dark pulses show limited timing instability compared to bright pulses. This behavior is attributed to their intrinsic propagation characteristics, due to operating in the normal dispersion regime and the presence of a stable continuous-wave background, which enhances robustness against noise and nonlinear effects. Although bright pulses are well established in practical deployment, the potential of dark pulse in system-level performance and scalability requires further investigation. Therefore, this study provides a fundamental insight of bright pulses and dark pulses, while suggesting dark pulses as promising carriers for future long-haul optical communication systems.