Due to their notable advantages, such as high beam quality, high stability, high efficiency, and low cost, passively mode-locking fiber lasers exhibit extensive application potential in various fields, including optical communication, precision machining, and fiber sensing. The saturable absorber, a crucial optical element, plays a key role in determining mode-locking performance. However, despite their widespread application in mode-locking technologies, semiconductor saturable absorber mirrors, carbon nanotubes, and two-dimensional nanomaterials are hindered by limitations such as low damage thresholds, suboptimal stability, and complex fabrication processes. Therefore, the development of superior saturable absorbers is essential for enhancing fiber laser performance. In this paper, we propose a passively mode-locking fiber laser based on the multi-modal interference effect, leveraging its saturable absorption and tunable filtering properties to achieve the generation of different pulse types.

A tunable pulse-type mode-locking fiber laser is constructed in this paper. A graded-index multi-modal fiber (GIMF) is incorporated into the laser cavity, serving as both a high-damage-threshold saturable absorber and an optical filter. Due to the presence of principal modes (PMs) in the multi-mode fiber, the group delay induced by these modes exhibits a linear relationship with the fiber length and is polarization-dependent. In addition, the bandwidth of the spectral filter formed by the GIMF is influenced by the group delay between these PMs. By adjusting the polarization state of the polarization controller to introduce different group delay values, the filter’s bandwidth can be modified, enabling control over the output pulse type. Numerical simulations based on the Ginzburg?Landau equation are conducted to analyze the influence of different filter parameters on laser pulse transmission characteristics by adjusting β2a.

By appropriately tuning the polarization controller, a conventional soliton mode-locked fiber laser is established at a pump power of 251 mW (Fig. 4). The center wavelength is 1572.1 nm, with a 3 dB bandwidth of 4.6 nm. The full width at half maximum (FWHM) of the pulse is approximately 0.902 ps, with a fundamental repetition frequency of 5.43 MHz and a signal-to-noise ratio of 66.2 dB. The output spectrum, radio frequency spectrum, and power stability of the soliton pulses are monitored over a 24 h period (Fig. 5). When the pump power is increased to 334 mW and 230 mW respectively, and the polarization state is adjusted, the fiber laser generates stretched pulses and self-similar pulses (Fig. 6). The center wavelength of the stretched pulses redshifts to 1586.7 nm, with a 3 dB bandwidth expanded to 6.2 nm and an FWHM of approximately 1.69 ps. The self-similar pulse exhibits a center wavelength of 1591.6 nm, a 3 dB bandwidth of 6.5 nm, and an FWHM of approximately 3.10 ps. By leveraging the linear relationship between the group delay of the PMs in the GIMF and its length, additional pulse types are achieved by adjusting the polarization states of the fiber laser. Specifically, Lorentz pulse and triangular pulse are obtained at pump powers of 255 mW and 278 mW, respectively (Fig. 7). The Lorentz pulse exhibits peak wavelengths at 1573.1 nm and 1589.6 nm, with an FWHM of approximately 1.42 ps. The triangular pulse has a center wavelength of 1571.1 nm, a 3 dB bandwidth of 1.3 nm, and an FWHM of approximately 2.11 ps. In addition, the spectral center wavelength fluctuations of the four pulse types are continuously monitored in 24 h (Fig. 8), confirming the laser system’s stability. Numerical simulations based on the Ginzburg?Landau equation demonstrate that by adjusting the β2a value, pulses of various shapes can be achieved. The fitting curve R2 values all exceed 0.995. In addition, simulations indicate that triangular pulses can be produced when net dispersion within the cavity is in the range of -0.009 ps2 to 0.005 ps2.

The proposed fiber laser successfully generates multiple pulse types, including conventional soliton pulses, stretched pulses, self-similar pulses, Lorentz pulses, and triangular pulses, by utilizing the multi-modal interference effect. This is achieved through polarization state adjustments within the resonant cavity, allowing fine control over the bandwidth and group delay in the SMF?GIMF?SMF structure. The findings of this paper provide valuable insights for the development of compact and versatile mode-locking fiber laser devices.