We performed numerical simulations based on the generalized nonlinear Schrödinger equation to investigate the coherence of supercontinuum (SC) generated by multi-pulse pumping with varying peak powers in all-normal dispersion (ANDi) fibers. The study explores and explains the nonlinear dynamics responsible for spectral coherence degradation at high peak powers. The results indicate that the spectral coherence of multi-pulse pumped SC is determined by the quality of the spectrum at the moment when pulses begin to overlap in the time domain. High peak powers cause noise to rapidly amplify through the coupling of stimulated Raman scattering (SRS) and four-wave mixing (FWM) during the Stage I evolution, while also accelerating pulse overlap, leading to insufficient coherent photon generation. To mitigate spectral coherence degradation in multi-pulse pumped SC generation, we propose two methods: introducing an initial chirp to the pulse pairs and employing multi-wavelength pulse pumping. Both approaches aim to introduce a frequency difference during Stage I evolution, which accelerates the generation of coherent photons between pulses, ensuring spectral coherence is maintained at the moment of pulse overlap.
A scheme for generating a width-tunable optical pulse is proposed and experimentally demonstrated, which is based on the Mach–Zehnder modulator’s traveling wave modulation characteristics in the Sagnac loop. In the proposed scheme, the clockwise light is modulated, and the counterclockwise light remains unmodulated and acts as a continuous-wave light. By adjusting the polarization controller and the bias voltage of the modulator, the generation of tunable-width pulses can be achieved. Experimental results show that the full-width at half-maximum of pulses (20 GHz) can be tuned continuously from 10.97 to 25.06 ps, with corresponding duty cycles of 21.94% and 50.12%, which is consistent with the simulated results. An 80 to 10 Gbit/s optical time division multiplexing (OTDM) signal after 100 km transmission demultiplexing experiment has also been demonstrated with the time window provided by the proposed scheme. The experimental results demonstrate the applicability of this scheme for high-speed OTDM signal demultiplexing after long-distance transmission. Furthermore, by adding an additional photodetector and an electric bandpass filter to the proposed scheme, both demultiplexing and clock extraction can be achieved simultaneously.
To overcome the limitation of low spectral broadening efficiency in the normal group-velocity dispersion (GVD) regime, utilizing a multi-pulse pump source induces nonlinear effects between pulses, leading to the generation of new frequency components at extended wavelength, thus expanding the spectral range. In the process of single-pulse pumped supercontinuum generating, the evolution of non-frequency shift components of pulse tail plays a crucial role. In the case of multi-pulse pumping, the overlap of pulses makes the interaction between non-frequency shift and frequency shift components more complex. In this work, a numerical model for multi-pulse pump supercontinuum generation based on the generalized nonlinear Schrödinger equation (GNLSE) is established. The fourth-order Runge-Kutta in the interaction picture method (RK4IP) is employed to analyze the evolution of inter-pulse non-frequency shift components of multiple pulses during their transmission in the normal GVD regime. The results demonstrate that as the transmission distance increases, the non-frequency shift components at the edges of the pulse group exhibit an asymmetric evolution trend; the ones between the pulses undergo a transition from asymmetric to symmetric evolution, and this transition is significantly accelerated when the time interval between incident pulses shortens. The frequency components at the front and rear edges of the pulse group are primarily influenced by Cross-Phase Modulation (XPM) and Stimulated Raman Scattering (SRS), but the asymmetric evolution is mainly caused by SRS. While third-order dispersion (TOD) can lead to asymmetrical spectral broadening, its impact on the tail non-frequency shift components is relatively minor.
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