With the advancements in laser technology, pulse lasers have been widely used in optical fiber communication, medical surgery, power transmission, material processing, and other fields. Pulse lasers can be classified into bright- and dark-pulsed lasers. A bright pulse refers to a significant increase in light intensity under a stable continuous light background, which is highly sensitive to noise and loss generated during the transmission of the optical waveguide and results in its power gradually decreasing during the transmission process. By contrast, a dark pulse exhibits a significant decrease in light intensity against a stable and continuous light background. Compared with bright pulses, dark pulses have unique advantages such as better stability in the presence of noise and reduced loss in optical fibers, and they are less influenced by stimulated Raman scattering within the pulse. These advantages enable dark pulses to have broad prospects in the fields of signal processing and long-distance communication. This study demonstrated that a fiber laser based on a Ti₃C₂T_x optical device has the advantages of a lower dark-pulsed pumping threshold power and higher opto-optical conversion efficiency. It can realize the switching of bright and dark pulses as well as dark-pulsed pairs, thereby enriching the photon application of Ti₃C₂T_x in fiber lasers.

The nonlinear optical device investigated in this study was composed of a quartz substrate, HF-etched single-mode fiber (SMF), and Ti₃C₂T_x material. Ti₃C₂T_x powder of 10 mg was dissolved in 10 mL of N-methylpyrrolidone solvent and stirred with a magnetic mixer. Ultrasonicated treatment was then conducted in an ice bath for 120 min. Finally, the supernatant was centrifugated for 30 min at 3500 r/min, and 1 mg/mL of Ti₃C₂T_x dispersion was obtained. A section of 30 cm standard single-mode fiber SMF (core diameter: 9 μm; cladding diameter: 125 μm) was examined, on which a wire stripper was used to strip the coating layer and regulate the corrosion length to 5 mm. The treated SMF was then fixed on a quartz substrate and placed in a fume hood for corrosion, and a 40% HF solution was added to the corrosion area for 103 min. The diameter of the SMF corrosion area prepared by the HF corrosion method was 15 μm. Finally, Ti₃C₂T_x was attached to the surface of the etched optical fiber via light deposition, which allowed the evanescent wave to fully interact with Ti₃C₂T_x. The produced nonlinear optical device was connected to an optical fiber mode-locked laser cavity, and relevant information related to the pulse signal, including the spectrum, pulse repetition rate, pulse width, and output power, was obtained using a power meter, fast photodiode, digital oscilloscope, and optical fiber spectrometer.

When the pump power reaches 18 mW, a continuous wave (CW) output appears in the laser cavity. When the pump power is continuously increased to 65.27 mW, the laser outputs a bright-pulse sequence, where the period is 74 ns, the central wavelength is 1562.75 nm, the spectral bandwidth of 3 dB is 0.56 nm, the repetition frequency is 13.5 MHz, and the radio frequency (RF) signal-to-noise ratio (SNR) output of the bright pulse is approximately 56 dB. These indicate that the laser has good stability. When the pump power is increased to 110.2 mW and the polarization state is changed by carefully adjusting the polarization controller, a dual-wavelength dark-pulsed output is obtained with central wavelengths of 1562.34 and 1563.11 nm. The pulse duration is 3.8 ns, repetition rate is 13.5 MHz, and SNR is 49 dB. The fluctuation amplitude of the pulse train is small, and the operation of the dark pulse is relatively stable. When the pump power is increased to 163.62 mW, a stable pair of dark and bright pulses is generated by adjusting the polarization controller to alter the polarization state. The average output power of the laser increases linearly, and the corresponding light-to-light conversion efficiency is 6.55%.

This study presented a bright- and dark-pulsed switching mode-locked fiber laser based on Ti₃C₂T_x, which uses HF to etch SMF to generate evanescent waves. Its material properties were analyzed by Raman spectroscopy, and nonlinear absorption was examined using balanced synchronous dual-detector technology and a self-made femtosecond pulse source. When the threshold power and repetition frequency are 110.2 mW and 13.5 MHz, respectively, a dual-wavelength dark pulse output with central wavelengths of 1562.34 and 1563.11 nm, pulse duration of 3.8 ns, and SNR of 49 dB is obtained. When the pump power is changed and the polarization state is adjusted, the light and dark pulses and dark-pulsed pairs can be switched. Experimental results show that Ti₃C₂T_x has significant application potential in fiber lasers and significantly promotes the development of nonlinear optics.