Wavelength-tunable and multi-wavelength mode-locked fiber lasers are essential in fields such as communication technology, fiber sensing, and ultrafast photonics. While wavelength tuning is commonly achieved by incorporating additional tunable filters, most of these filters function solely as filters and require separate mode-locking techniques. Recently, the saturable absorber based on graded index multimode fiber (GIMF) has gained attention due to its advantages of easy fabrication, high damage threshold, and strong anti-interference capabilities. It can serve both as a mode-locked device and a filter. In this study, we propose a method featuring a simple structure, high loss threshold, and low insertion loss to achieve wavelength-tunable laser output and dual-wavelength switchable mode-locked pulse output of erbium-doped fiber lasers.

We propose a graded index multimode fiber-single mode fiber-graded index multimode fiber (GSG) structure, which functions both as a saturable absorber for mode-locking and a filter for wavelength tuning. The GIMF length is fixed at 11.2 cm to align with the self-imaging period, and the GIMF 1 and GIMF 2 have identical parameters to create a symmetrical structure. We theoretically calculate the length range of the single-mode fiber that meets the requirements and conduct comparative experiments within this range. Experimental results show that when the length of the single-mode fiber is between 2.7 cm and 5 cm, the GSG structure functions effectively as a mode-locked device and filter with wavelength tuning capability. We test the GSG structure with a fiber laser and use a digital oscilloscope, optical spectrum analyzer, autocorrelator, radio frequency analyzer, and power meter to measure output pulse characteristics including spectrum, pulse width, repetition rate, and output power.

Testing the GSG structure with a fiber laser reveals several key observations. At a pump power of 15 mW, we observe continuous wave output on the oscilloscope. As the pump power increases to 24 mW, we see Q-switched mode-locking. At 46 mW, we achieve a stable mode-locked state, with no pulse splitting or harmonics detected up to the maximum pump power of 289 mW. When the pump power is set to 87 mW, the center wavelength of the pulse is 1558.6 nm, with a pulse width of 0.6 ps and a 3 dB bandwidth of 5.95 nm (Fig. 5). By gradually adjusting the polarization controller, we achieve mode-locked pulse output with tunable wavelengths. The central wavelength of the mode-locked pulse can be stably tuned across the range of 1550.9 nm to 1558.9 nm (Fig. 6). At a pump power of 110 mW, we observe both mode-locked pulse and continuous wave outputs. By further adjusting the polarization controller, we obtain dual-wavelength mode-locked pulse switching at central wavelengths of 1531.6 nm and 1556.2 nm (Fig. 7). In the stable mode-locked state, adjusting the polarization controller alters the transmission characteristics of the GSG structure. These changes affect the wavelength-dependent gain and loss equilibrium points within the cavity and the polarization hole burning effect. Additionally, variations in the distribution of gain peaks in the erbium-doped fiber contribute to wavelength tuning and switching.

We present an all-fiber structure based on GIMF-SMF-GIMF that combines the saturable absorption characteristics of GIMF with the Mach-Zehnder (MZ) filtering effect of the GSG structure to achieve single-wavelength tunable and dual-wavelength switchable mode-locked pulse outputs in erbium-doped fiber lasers. With a polarization controller adjustment, we obtain a mode-locked pulse with a repetition rate of 8.5 MHz, a pulse width of 0.6 ps, a signal-to-noise ratio of 64 dB at 87 mW pump power, and a wavelength tuning range of 1550.9?1558.9 nm. At 110 mW pump power, we achieve mode-locked pulse switching and dual-wavelength mode-locked laser output. The GIMF-SMF-GIMF all-fiber structure, with its simplicity, high loss threshold, and low insertion loss, provides a valuable reference for designing compact, multifunctional fiber laser mode-locked devices.