Ultra-fast pulse fiber lasers are extensively employed in fields such as fiber optic communication, medicine, and precision material processing due to their compact structure and high beam quality. Lock mode technology is an effective method for achieving ultra-short pulses. Actively mode-locked lasers introduce active modulation devices into the laser cavity and adopt external modulation signals to change the optical signal characteristics, achieving a laser mode-locked pulse output. They feature flexibility, controllability, and stable output pulses. In recent years, graphene has been extensively studied due to its excellent electro-optical properties. Research has shown that an external electric field can alter the Fermi level of graphene to achieve light absorption modulation. Therefore, graphene-based electro-optic modulators have the potential to achieve actively mode-locked lasers. We present the construction of an efficient and high-speed graphene all-fiber mode-locked device, which achieves high-speed adjustment of graphene's optical performance with low modulation power consumption and high modulation efficiency.
The device is composed of graphene, single-mode optical fiber, polydimethylsiloxane (PDMS), and silver (Ag) film, forming a capacitive device (GCD) structure. By selecting glass as the substrate and leveraging magnetron sputtering technology to deposit 50 nm silver on the glass as the bottom electrode, silver has sound conductivity, which is beneficial for reducing the device resistance. The insulation layer is a spin-coated 200 nm PDMS layer, and the thinner insulation layer can effectively reduce the capacitance value of the device. Meanwhile, hydrofluoric acid (HF) with a concentration of 20% is utilized to corrode standard single-mode optical fibers to 15 µm with a corrosion length of 5 mm, and the corroded single-mode fiber is transferred to PDMS. The device is placed in a UV ozone cleaning machine (multi-frequency, CCI UV250-MC) for 10 minutes to improve the hydrophilicity of the insulation layer PDMS. There is graphene dispersion with a selected concentration of 0.1 mg/mL (Nanjing Xianfeng Nanomaterial Technology Co., Ltd., XFZ20 dispersion). Graphene solution is dropped onto the optical fiber, dried, and then inkjet printed with a silver electrode layer on the device using a microelectronic printer (Power Supply Technology Co., Ltd., MP1100). Finally, the prepared device is connected to the circuit board using silver wire. The GCD device is connected between the isolator and polarization controller by fusion, and a spectrum analyzer, digital oscilloscope, power meter, autocorrelator, and spectrum analyzer are adopted to record the locked pulse signal, including spectrum, pulse width, repetition rate, and output power.
The GCD device is connected to the fiber laser system and a pump power of 80 mW is employed for actively mode-locked experiments. At a pump power of 80 mW, the AC signal amplitude increases from 0 V to 5 V, and the average output power decreases from 1.328 mW to 1.130 mW. After calculation, the insertion loss of the device increases from 1.54 dB to 2.46 dB (Fig. 4). Subsequently, a periodic AC signal (12.2 MHz) that is consistent with the resonant frequency of the laser cavity is applied to the graphene device. Under low voltage amplitude, the control of the graphene Fermi level is limited, resulting in a limited range of dynamic changes in the absorption of graphene devices. Therefore, unstable mode-locked pulse signals are observed. When the voltage amplitude increases to 5 V, the most stable mode-locked pulse signal is observed to achieve active mode locking of the laser. The narrowest pulse width of the mode locking signal is 298 ps (Fig. 5). Meanwhile, by increasing the modulation frequency to twice the resonant frequency of the laser cavity (24.4 MHz), the optical signal inside the cavity undergoes frequency doubling oscillation under the graphene device control, leading to harmonic mode-locked operation. The mode-locked pulse signal is slightly unstable, possibly due to the insufficient response speed of graphene to support the migration time of charge carriers at higher modulation rates, resulting in modulation depth changes in the device. This is also the reason for the wider pulse width of 315 ps, corresponding to a frequency of 24.4 MHz, which achieves active repetition frequency control in mode-locked lasers.
We introduce an actively mode-locked fiber laser based on a graphene all-fiber structure. It combines single-mode optical fibers that have undergone lateral corrosion treatment with graphene capacitor structures and utilize the evanescent wave coupling effect of optical fibers to interact with graphene, achieving efficient modulation. Meanwhile, by utilizing the inherent high carrier mobility of graphene, high-speed adjustment of optical performance can be achieved with lower modulation power consumption. The experimental results show that under a modulation signal of ±5 V, the fiber laser obtains controllable repetitive mode-locked pulses at a fixed pump power of 80 mW, with frequencies of 12.2 MHz and 24.4 MHz respectively. The corresponding mode-locked pulse widths are 298 ps and 315 ps respectively, and the laser center wavelength is 1558 nm. Meanwhile, by changing the amplitude of the AC signal (0-5 V), the average output power of the laser can be adjusted within the range of 1.328 mW to 1.130 mW. The research results provide references for achieving low-power and integrated actively mode-locked lasers, and have practical significance for developing efficient and integrated actively mode-locked laser systems.
