Real-time observation of soliton build-up dynamics in a mode-locked laser
- May. 31, 2019
Original article: Revealing the behavior of soliton buildup in a mode-locked laser
Mode locking is a technique that locks a large number of longitudinal modes together to produce ultrafast pulses. Mode-locked lasers have enabled some of the most precise measurements, e.g., metrology with frequency combs. Passively mode-locked fiber lasers have widespread applications in the fields of fiber telecommunication, optical sensing, metrology, and microscopy, because of their compact and low-cost configuration as well as excellent features of high stability and low noise. Mode-locked fiber laser is also an ideal platform for researchers to explore new optical nonlinear phenomena.
Starting dynamics of mode-locked lasers have been investigated experimentally and theoretically since 1990s, but the spectral dynamics during the build-up of pulse lasers has not been measured directly. The transient non-repetitive processes cannot be measured by conventional technologies due to the speed limitation of electronic devices. Real-time spectroscopy based on an emerging time-stretch dispersive Fourier transform (TS-DFT) technique can map the spectral information of optical waves into the time domain, thus opening several fascinating explorations of nonlinear dynamics in mode-locked lasers. The TS-DFT technique provides a powerful way for real-time, single-shot measurements of ultrafast phenomena. By using such technique, Herink et al. observed the internal dynamics of soliton molecules (Science 356, 50, 2017) and resolved the build-up of solitons in mode-locked lasers (Nat. Photon. 10, 321, 2016). They demonstrated the process from transient to stable bound states. However, the entire build-up dynamics of solitons in mode-locked lasers have not been observed directly so far.
Recently, researchers at Zhejiang University demonstrate the first observation of the entire build-up process of solitons in a mode-locked laser, revealing two possible ways to generate the solitons. One way includes the dynamics of raised relaxation oscillation, quasi mode-locking stage, spectral beating behavior, and finally the stable single-soliton mode-locking. The other way contains, however, an extra transient bound-state stage before the final single-pulse mode-locking operation. Figure 1 exhibits the conceptual representation of different stages during the entire build-up process of solitons in a mode-locked laser. The plots in the top and down rows demonstrate the entire build-up process of solitons in temporal and spectral domains, respectively.
Figure 1. Conceptual representation of the entire build-up process of solitons in a mode-locked laser, successively undergoing the raised relaxation oscillation, quasi mode-locking stage, spectral beating dynamics, transient bound-state stage and stable mode-locking. Top and down rows: mode-locking in temporal and spectral domains, respectively.
These findings provide new perspectives into the ultrafast transient dynamics and bring real-time insights into laser design and applications. The real-time spectroscopy technique is expected to provide new insight into a wider class of phenomena in complex nonlinear systems.