In this work, an experimental study of non-stationary pulse dynamics consisting of explosion events and chaotic multi-pulsing from an all-normal dispersion Yb-fiber laser operating in noise-like pulse (NLP) state have been reported. During explosion in NLP state, which resembled soliton explosion, dispersive Fourier transform measurements revealed large, intermittent amplitude fluctuations in the real-time spectrum indicative of rogue waves. The fluctuations either appeared randomly at the lasing wavelength within the Yb-gain spectra or at the frequency downshifted Raman Stokes wavelength. It was also observed that such fluctuation at the Raman wavelength could even exceed the amplitude of the spectral components at the main lasing wavelength. With suitable adjustments to polarization controllers, the laser operation could be switched to chaotic multi-pulsing state where the acquisition of a large number of consecutive roundtrips revealed inter-pulse motion with unique trajectories occurring over millisecond time scales.
In this paper, numerical simulations of an all-normal dispersion ring cavity mode-locked fiber laser have been reported, revealing the existence of rogue waves in the chaotic transition regime between a stable single-pulse state and a multi-pulse state. The chaotic states manifest as a result of multi-pulsing instability induced by the intra-cavity spectral filtering effect and were studied by gradually decreasing the filter bandwidth from a stable or quasi-stable state to a stable multi-pulsing state. For a specific set of cavity parameters and a range of Gaussian-shaped filter bandwidths, stable dissipative solitons characterized by a cat-ear-shaped spectrum were obtained. Reducing the filter bandwidth below the stable range first produced non-stationary quasi-stable states containing multiple soliton explosions and then eventually a stable multi-pulsing state with individual dissipative solitons. The histograms of spectral intensities in the quasi-stable states exhibited long-tailed distributions containing rogue waves. Rogue waves were also observed during the build-up of the dissipative soliton from white Gaussian noise even though the pulse finally evolved to a stable state. By modifying the cavity parameters, noise-like pulses (NLPs) were obtained which are by nature a quasi-stable state and exhibited rogue waves in the spectral intensity histogram. In the NLP state of operation, the reduction of filter bandwidth below a certain range produced multiple dissipative solitons with stable waveform. Additionally, the influence of different filter shapes on the state transition dynamics was also explored. It was found that the range of filter bandwidths for which chaotic states exist varies for different filter shapes depending on their spectral confinement.
We present a numerical model for mode-locked lasers and ultrafast nonlinear amplifiers in which the saturated gain profiles along active fibers are judiciously computed using experimental pump powers as input. This eliminates the need for approximating the gain profile by using a small-signal gain coefficient and saturation energy to simulate pulse propagation in active fibers. Our model shows good agreement with experiments involving mode-locked cavities at 1 µm with a silica glass host doped with ytterbium ions. Accurate results are also obtained for continuous-wave and mode-locked laser cavities around 2.8 µm, which uses ZBLAN fiber doped with erbium ions. In the case of Er:ZBLAN fiber, we use the model to show regions of stable mode-locking delivering a single pulse as output and how the spectral width changes with variation in doping concentration and fiber lengths. Our model enables accurate numerical modeling of mode-locked fiber lasers and ultrafast amplifiers, and can be useful in guiding the design of new architectures, understanding complex intracavity laser dynamics, and optimizing device output.
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