Period doubling is a universal bifurcation of central importance in all disciplines of nonlinear science, which generally signals the existence of chaotic dynamics in the vicinity of the system parameters. Although observed in diverse ultrafast laser configurations, there is still no consensus on its physical origin. Real‐time spectral characterization techniques have recently allowed revisiting pulsating dynamics, from period‐2 to long‐period pulsations. Following a contextual review, this article presents a variety of bifurcation sequences entailing the spectral pulsations of dissipative solitons. These experiments, using ultrafast fiber lasers operated in both chromatic dispersion regimes, are confronted with numerical simulations to demonstrate that self‐phase modulation represents a general mechanism triggering period‐2 bifurcations. In addition, by ramping up the pump power, original sequences of period‐doubling bifurcations intertwined with more complex bifurcations are presented, where entrainment phenomena are unveiled. These findings provide a more general understanding of the period‐doubling bifurcation in ultrafast laser systems while highlighting their potentially intricate combinations with complex bifurcations, which may be exclusively observable from the spectral domain.

The discovery of different types of nanomaterials including the one-dimensional and two-dimensional materials used as saturable absorbers (SAs) in the applications of ultrafast lasers in recent years increases the ultrafast laser design flexibility and boosts the laser performances. A major research avenue is to explore the potential of nanomaterials for further enhancing the performances of ultrafast lasers in terms of pulse power. To this aim, in this study, using a hot-injection method and drop-coating technology, a fiber-based lead sulfide quantum dot (PbS QD) is synthesized, and its potential as a SA for the generation of higher-power pulses is demonstrated in an erbium-doped fiber laser (EDFL). Experimental results show that the optical damage threshold of the SA is greater than 152.6 mJ/cm ² , and the modulation depth is up to 29.5%. The implementation of the PbS QD as a SA placed in an EDFL enables the laser to yield 2.84 ps ultrashort pulses with an average output power of 59.4 mW at a repetition rate of 6.97 MHz. To the best of our knowledge, it is the highest average output power obtained in ultrafast fiber lasers mode-locked by zero-dimensional QD materials. The results suggest the great potential of PbS QDs in the application that requires the generation of high-power pulses in ultrafast lasers.

Gallium sulfide (GaS), with a hexagonal structure, has received extensive attention due to its graphene-like structure and derived optical properties. Here, high-quality GaS was obtained via chemical vapor synthesis and then prepared as a saturable absorber by the stamp-assisted localization-transfer technique onto fiber end face. The stability of the material and the laser damage threshold are maintained due to the optimized thickness and the cavity integration form. The potential of the GaS for nonlinear optics is explored by constructing a GaS-based Erbium-doped mode-locked fiber laser. Stable femtosecond (~448 fs) mode-locking operation of the single pulse train is achieved, and the robust mode-locked operation (>30 days) was recorded. Experimental results show the potential of GaS for multi-functional ultrafast high-power lasers and promote continuous research on graphene-like materials in nonlinear optics and photonics.

Magnetic-based polydimethylsiloxane microspheres (MPMs) are fabricated and its applications for measurement of magnetic field are investigated. The MPMs are prepared by first incorporating the uniformly sized Mn₃O₄ nanocrystals into polydimethylsiloxane (PDMS) matrix, and then using a self-assembling method to obtain a chain of microspheres on the fiber taper. The MPMs are regarded as optical resonators, demonstrating whispering gallery modes (WGM) with a high Q factor in the 1310 nm band. Experimental results show that the resonance wavelengths of the WGM spectra depend on the strength of the magnetic field since the change in the applied filed cause the mechanical deformation of the MPMs, leading to a shift in the resonant wavelength of the resonator. Using as magnetic field sensors, the performances of the MPMs in terms of sensitivities and measurement ranges have been optimized. Thus, the sensitivity of 101.9 pm/mT and the measurement range up to 8 mT are achieved with a MPM (~270 μm in diameter). Besides, the performance of the MPM after two-week storage is analyzed, showing the excellent long-term stability.

We demonstrated a simple approach to fabricate an all-fiber polarizer by directly inscribing a long-period grating in a two-mode fiber using femtosecond laser pulses. Femtosecond laser processing features high spatial resolution, enabling the inscription of gratings with local asymmetric refractive index modulation, which facilitates polarization high-order mode conversion with varying efficiency. By connecting the grating to a single-mode fiber pigtail, the component exhibited a maximum polarization-dependent loss of 32.5 dB and a 10 dB bandwidth of approximately 30 nm. Also, gratings integrated into two-mode fiber exhibit lower sensitivity to temperature and environmental refractive index, making them more resistant to external disturbances than single-mode fiber devices. The operational wavelength of the polarizer is flexibly adjustable by varying the grating period, enabling tuning across different wavebands for ultrafast laser applications. By employing the grating-based polarizer as a polarization-selective element in a nonlinear polarization rotation fiber laser, femtosecond-level fundamental frequency pulses and harmonic mode-locked pulses were achieved by adjusting the cavity polarization state and pump power. The two states of ultrafast pulses, with stable output, exhibited a signal-to-noise ratio exceeding 70 dB. The femtosecond laser inscribed all-fiber polarizers in this experiment are expected to play a key role in future integrated ultrafast laser systems due to their compact structure and the simplicity of direct fabrication.