In optics, soliton is a term used to describe a phenomenon in passive mode-locked lasers where both the spectral and temporal structure of the laser pulse disappears abruptly but is bound to resume its original shape after several round trips. Up to date, experimental observation of this phenomenon has been scarce despite it being widely explored theoretically. So far, the fact that the time scale of the soliton explosion is from several hundred nanoseconds to microseconds, and it occurs randomly, are the main reasons for the limited experimental observations and use of conventional spectrometers. Recent technological progress has led to the utilization of fiber lasers by using the time-stretch dispersive Fourier transform in measuring single-shot spectrum in real time. Unfortunately, recent results have shown the existence of successive soliton explosions in a fiber laser due to intracavity nonlinear effect. Therefore, further work is necessary as the latter can lead to new opportunities in complex ultrafast nonlinear phenomena physics.
Recently, Masayuki Suzuki and Hiroto Kuroda at Aichi-Medical University in collaboration with Ozdal Boyraz and Bahram Jalali at University of California and Hossein Asghari at Loyola Marymount University and also Paul Trinh at Silicon Lightwave, Inc. (California) carried out experimental observations of periodical spectral changing by soliton explosions in a passively mode-locked Yb fiber laser via nonlinear polarization evolution using time-stretch dispersive Fourier transform. Specifically, they demonstrated the first ever observation of periodic spectrum changing via soliton explosion. Their work is currently published in the research journal, Optics Letters.
Briefly, the research method used by the scientintists entailed observation of a soliton explosion which was limited to an all normal dispersion configuration fiber laser that was operating in transition regime between stable mode-locking and noise-like pulse generation. Additionally, the soliton explosion utilized were those appearing in particular transitions varying from a narrowband to a broadband mode-locking operation. All in all, the researchers here utilized time stretch to capture 7220 consecutive single-shot spectra over a 100 μs time window in real time.
The authors observed that by taking simultaneous measurements of spectrum and pulse energy at three different output points in the laser cavity, the soliton explosion’s dynamics were related to an accumulated residual third-order dispersion in the oscillator.
In summary, the study by Masayuki Suzuki and colleagues reported on soliton explosions in the stretched-pulse configuration of a nonlinear polarization evolution Yb fiber laser with time-stretch dispersive Fourier transform. Generally, their study entailed observing solitons explosions in the unstable transition regime between two different mode-locking states. They observed that the strong spectral broadening originated from the self-phase modulation that was induced by propagating the long length of single mode fiber, which was spliced after Yb-doped fiber. Altogether, their results present novel insights into understanding the pulse behavior in an unstable mode-locking condition.
Masayuki Suzuki, Ozdal Boyraz, Hossein Asghari, Paul Trinh, Hiroto Kuroda, Bahram Jalali. Spectral periodicity in soliton explosions on a broadband mode-locked Yb fiber laser using time-stretch spectroscopy. Volume 43, Number 8 / 2018 / Optics Letters.Go To Optics Letters