Passively synchronized mode-locked fiber lasers for coherent anti-Stokes Raman imaging

Towards retrieving the temporal profile of unequal pulse bound states from autocorrelation measurements

Optical autocorrelators are typically employed to retrieve the temporal information of bound-state lasers; however, they suffer from limitations when multiple sub-pulses coexist with varying intensities and pulse widths. To this end, this study investigates the impact of differences in pulse temporal intensity and width between two unequal pulses in a bound-state laser on the corresponding autocorrelation trace. Maps of autocorrelation trace in terms of the temporal profile of unequal pulse bound state are created to evaluate their internal relationship. Specifically, the autocorrelation peak intensity ratio exhibits an approximatively Gamma cumulative distribution function relationship with respect to either individual variation in the temporal intensity or width ratio. Additionally, we examine the impact of oscillating tail intensity on the autocorrelation trace. These results reveal that variations in pulse temporal characteristics complicates the retrieval of accurate temporal information. To address this, we provide a checklist based on careful examination of the autocorrelation trace to retrieve the temporal profile of bound states with unequal pulses. This work provides guidance for approximately retrieving the temporal information of a bound-state laser with variations in sub-pulse shapes from the autocorrelation trace and underscores the necessity for more sophisticated analytical methods to disentangle the complexities arising from non-uniform pulse parameters.

Temporal soliton dynamics of synchronised ultrafast fibre lasers

Synchronised ultrafast soliton lasers have attracted great research interest in recent decades. However, there is a lack of comprehensive understanding regarding the buildup mechanism of synchronised pulses. Here, we report a dynamic analysis of independent and synchronised solitons buildup mechanisms in synchronised ultrafast soliton lasers. The laser comprises an erbium-doped fibre cavity and a thulium-doped fibre cavity bridged with a common arm. Pulses operating at two different wavelengths formed in the cavities are synchronised by cross-phase modulation-induced soliton correlation in the common fibre arm. We find that the whole buildup process of the thulium-doped fibre laser successively undergoes five different stages: continuous wave, relaxation oscillation, quasi-mode-locking, continuous wave mode-locking and synchronised mode-locking. It is found that the starting time of the synchronised solitons is mainly determined by the meeting time of dual-color solitons. Our results will further deepen the understanding of dual-color synchronised lasers and enrich the study of complex nonlinear system dynamics.

Synchronized triple-wavelength fiber lasers at 1, 1.55, and 1.9 µm

Synchronized lasers working at different wavelengths are of great significance for numerous applications, such as high-energy femtosecond pulse emission, Raman microscopy, and precise timing distribution. Here, we report synchronized triple-wavelength fiber lasers working at 1, 1.55, and 1.9 µm, respectively, by combining the coupling and injection configurations. The laser system consists of three fiber resonators gained by ytterbium-doped fiber, erbium-doped fiber, and thulium-doped fiber, respectively. Ultrafast optical pulses formed in these resonators are obtained by passive mode-locking with the use of a carbon-nanotube saturable absorber. A maximum cavity mismatch of ∼1.4 mm is reached by the synchronized triple-wavelength fiber lasers in the synchronization regime by finely tuning the variable optical delay lines incorporated in the fiber cavities. In addition, we investigate the synchronization characteristics of a non-polarization-maintaining fiber laser in an injection configuration. Our results provide a new, to the best of our knowledge, perspective on multi-color synchronized ultrafast lasers with broad spectral coverage, high compactness, and a tunable repetition rate.

Generation of broadband parabolic pulses based on a pre-chirper free, core-pumped nonlinear fiber amplifier for coherent anti-Stokes Raman imaging

We report the generation of parabolic pulses with broadband spectrum from a core-pumped Yb-doped fiber amplifier seeded by a dispersion managed fiber oscillator. The net cavity dispersion of Yb-doped oscillator was continuously changed from 0.074 to -0.170 ps², which enabled us to achieve dissipative soliton, stretched pulse and soliton mode-locking operations. Spectral evolution processes in the core-pumped nonlinear fiber amplifier seeded by various input solitons were investigated experimentally and theoretically. Our finding indicates that cavity dispersion of oscillator can be used to engineer the input pulse parameter for amplifier, thus forming a pre-chirper free fiber amplification structure. In the experiment, we obtained 410-mW parabolic pulses with spectral bandwidth up to 56 nm. In combination with a passively synchronized frequency-doubled Er-doped fiber laser, we have demonstrated coherent anti-Stokes Raman imaging. The compact dual-color fiber laser source may facilitate practical applications of nonlinear biomedical imaging beyond the laboratory environment.

Carbon nanotube-synchronized dual-color fiber laser for coherent anti-Stokes Raman scattering microscopy

In this Letter, we present a passively synchronized dual-color fiber laser at 1.03 and 1.53 µm for coherent anti-Stokes Raman scattering (CARS) microscopy. This fiber laser consists of both Yb- and Er-doped laser cavities, combined by an ∼1.2m common branch with a single-walled carbon-nanotube-based saturable absorber. Stable passively synchronized mode-locked state is obtained within a total cavity length mismatch of 50 µm. We demonstrate the capability of the synchronized dual-color fiber laser for CARS spectroscopy in both low- and high-wavenumber regions, with a resolution of 6.6cm^-1. Furthermore, a CARS image in the field of view 150×60µm² of polystyrene at the Raman shift of 3041cm^-1 has also been achieved. The results show the feasibility of our passively synchronized dual-color fiber laser to be employed as a stable and low-cost ultrafast laser source for CARS.