Based on the experimental platform of an L-band normal-dispersion mode-locked fiber laser, we report the first observation, to our knowledge, of three novel types of multi-soliton asynchronous pulsation phenomena by virtue of the dispersive Fourier transform technique. The experimental results provide new insights into the complex multi-soliton dynamics under unstable mode-locking conditions. It is confirmed that more than one pulsating solution can coexist in a multi-pulse situation and that each soliton may evolve periodically in different ways. This implies that subsequent experimental and theoretical studies on multi-soliton need to take the differences among pulses into account and retrieve more degrees of freedom.

We propose and demonstrate an ultra-broadband mode converter based on a cascade chirped long-period fiber grating (CLPFG) written in a two-mode fiber. The mode converter can convert fundamental mode (HE even 11 and HE odd 11 modes) into the first order cylindrical vector (CV) modes (TE 01 , TM 01 , HE even 21 and HE odd 21 modes). We design and analyze the mode conversion characteristics of this kind of grating in theory. The simulation results show a 10 dB bandwidth of 170 nm and 20 dB bandwidth of 145 nm can be achieved by optimizing the parameters of the CLPFG. In the terms of experiment, we achieve the broadband mode converter with 10 dB bandwidth of 170 nm from 1472 nm to 1642 nm.

Spatiotemporal mode-locked (STML) fiber lasers have become an excellent platform in nonlinear optics research due to the rich nonlinear evolution process. In order to overcome modal walk-off and realize phase locking of different transverse modes, it is usually crucial to reduce the modal group delay difference in the cavity. In this paper, we use long-period fiber grating (LPFG) to compensate the large modal dispersion and differential modal gain in the cavity, realizing the spatiotemporal mode-locking in step-index fibers cavity. The LPFG inscribed in few-mode fiber could induce strong mode coupling, which has wide operation bandwidth based on dual-resonance coupling mechanism. By using dispersive Fourier transform involved intermodal interference, we show that there is a stable phase difference between the transverse modes constituting the spatiotemporal soliton. These results would be beneficial for the study of spatiotemporal mode-locked fiber lasers.

We demonstrated a wavelength-tunable, ultra-wideband, biconical, long-period fiber grating (BLPFG) mode converter in a two-mode fiber based on fusion taper technology and CO2 laser writing technology. Theoretical and experimental results show that after changing the diameter of the two-mode fiber by fusing and tapering, the dispersion turning point of the fiber is adjusted and wavelength-tunable broadband mode conversion is achieved efficiently. Theoretical simulation shows that the mode conversion bandwidth can cover the O + E + S + C band. In the experiment, we fabricated adiabatic tapers with cladding diameters of 113 μm and 121 μm and wrote gratings on these tapers to achieve dual-resonance coupling, thus realizing mode conversion from LP01 to LP11, with a 15 dB bandwidth of 148.8 nm from 1229.0 nm to 1377.8 nm and of 168.5 nm from 1319.7 nm to 1488.2 nm, respectively. As far as we know, this is the first time that fusion taper technology has been used to adjust the window of the dual-resonant coupling of an optical fiber. This work broadens the scope of application of the dual-resonance effect and proposes a general method for widening the bandwidth of a fiber grating with tunable wavelength.

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