Results and Discussions The light absorption intensity of the all-fiber capacitive device based on the GO/PS composite increases with the driving voltage [Fig. 5 (a)], which indicates that the absorption characteristics of the device can be effectively adjusted by the electrical field. The feasibility of the experiment is also verified. To further observe the difference in the optical transmission efficiency of the device under positive and negative voltages, the variation in the optical transmission capability of the device with the driving voltage under a low pump power (35 mW) is tested [Fig. 5 (b)]. The results show that the device achieves higher modulation efficiency under the positive voltage due to the properties of n-type semiconductors caused by slight doping. The nonlinear optical absorption characteristics of the GO in the modulator can be adjusted by the driving voltage, and the intracavity loss can thereby be modified. On this basis, the variation of the insertion loss of the GO/PS modulator is studied when the driving voltage increases from -60 V to 60 V. As the positive bias voltage reaches up to 20 V, the insertion loss of the device is reduced from 2.30 dB to 0.86 dB [Fig. 7 (b)]. Then, the device is applied to the all-fiber mode-locked ring laser system (Fig. 6). The results show that the laser achieves the switching among three operating modes: continuous wave, Q-switched mode locking and continuous wave mode locking under a certain range of the modulation voltage (a range from 0 V to 20 V). This all-fiber electro-optic modulator also reduces the pulse width of the mode-locked signal to 20 ps. Radio-frequency (RF) spectral measurements are conducted to characterize the stability of the fiber laser. The RF spectra of the mode-locked laser are measured [Fig. 9(d)]. The signal-to-noise ratio (SNR) of the RF signal can reach up to around 60 dB at 21.40 MHz. To further verify the stability of the laser, the average output power of the laser in 180 minutes is measured (Fig. 10). According to the results, the all-fiber mode-locked ring laser based on the GO/PS electro-optic modulator can work steadily for a long time.

Short-pulse fiber lasers have attracted great interest in numerous fields, including fiber communication, laser medical treatment, bioengineering, and material processing, owing to their compact structure, high efficiency, and robust stability. Physically, mode locking is an effective method to obtain ultrashort pulses, and it can be divided into active mode locking and passive mode locking. Passive mode locking based on saturable absorbers (SAs) has been widely studied. Among the SA materials, two-dimensional (2D) materials, such as graphene, black phosphorus, and molybdenum disulfide (MoS₂), are promising candidates due to their distinctive nonlinear saturable absorption ability, ultrafast recovery rate, and low cost. Plenty of passively mode-locked fiber lasers based on 2D materials have been developed to achieve different operating modes, such as continuous wave, Q-switched mode locking, and continuous wave mode locking by changing the pump power. However, due to the relatively fixed optical absorption characteristics of 2D materials, switchable operating modes of a mode-locked laser system are difficult to achieve without external modulation. The present study reports a novel kind of electro-optic modulator that is composed of graphene oxide (GO) and polystyrene (PS) microspheres and exhibits adjustable absorption characteristics under the action of an external electric field. The designed modulator, with a high optical transmission capability, enables electrically modulated fiber lasers to be switched among various operating modes, including continuous wave, Q-switched mode locking, and continuous wave mode locking. The proposed basic strategy and findings are expected to facilitate the design of new switchable ultrashort-pulse lasers based on electro-optic modulators.

The GO/PS all-fiber capacitive device is mainly prepared on a quartz substrate by the microelectronic printing process, and the main preparation process is presented in Fig. 1. Improving the modulation efficiency of the device requires a straightforward, low-cost, and effective approach in which PS microspheres can enhance the interaction area between the laser and the material by creating a local field to restrict the divergence of the evanescent light. The modulation characteristics of the modulator can be studied by measuring the optical characteristic curve of the device. The results show that the saturated absorption by the GO in the modulator can easily be achieved at 1550 nm when the driving voltage is applied, indicating enhanced optical transmission efficiency. Finally, an all-fiber mode-locked ring laser system is studied and constructed. The operating mode of the laser can be actively switched by integrating the device with the laser system and applying different electrical fields.

This study presents the preparation, characterization, and analysis of a novel all-fiber capacitive device based on the GO/PS composite, which can serve as a saturable absorber in all-fiber mode-locked ring laser systems. Due to the variation of chemical potential and internal carrier concentration of the GO, the optical absorption characteristics of the GO can be adjusted by an external electrical field. The effective interaction area between the GO and the evanescent light is strictly limited by the device specification. For this reason, an effective method of combining the GO with PS microspheres is adopted. According to the principle of optical waveguides, the evanescent light leaked from the fiber is confined to the GO/PS microspheres. A local field is thereby created, ultimately enhancing the effective utilization of the evanescent light. The mode switching of the laser is successfully implemented by changing the driving voltage, and the pulse width of the mode-locked pulse signal is reduced to 20 ps. In addition, the insertion loss of the device is lowered from 2.30 dB to 0.86 dB and the average output power of the laser is increased from 1.09 mW to 1.52 mW by adjusting the amplitude of the applied voltage. The proposed all-fiber capacitive device is expected to further promote the development of switchable pulsed fiber lasers due to its compatibility and high modulation efficiency.