Objective Owing to the wide application of Qswitched random fiber lasers (RFLs) in numerous fields, such as medical and information encryption, the pulse characteristics and evolution of RFLs should be investigated comprehensively. Hence, the pulse characteristics and evolution of actively Qswitched random fiber lasers based on a random-phase-shift fiber Bragg grating (RPS-FBG) are investigated in this study. The pulse characteristics of the actively Qswitched RFLs with ring and linear cavities are compared. Results show that in the ring-cavity RFL, different pulse states such as multi-period, splitting, and chaotic pulses are obtained by adjusting the pump power and modulation frequency of the electro-optic modulator (EOM). In the linear-cavity RFL, pulse splitting and multi-period phenomena are similarly observed, which is consistent with the evolution of the ring-cavity RFLs. Moreover, external laser injection does not change the pulse evolution but only reduces the number of splitting pulses and generates a weak dark pulse. Methods Actively Qswitched RFLs with ring (Fig. 1) and linear cavities (Fig. 9) are investigated experimentally. The effects of pump power, EOM frequency, and external laser injection on the pulse characteristics of the RFLs and their evolution are analyzed. A photodetector (bandwidth of 200 MHz) and an oscilloscope (sampling rate of 2 GSa/s) are used to measure the pulse train of the RFLs, and an optical spectrum analyzer is used to characterize the RFL spectra. Results and Discussions For the ring-cavity RFL, when the modulation signal is fixed, the output pulse is varied from a multi-period state to a single-period state and then to a pulse-splitting state with a chaotic state among different pulse-splitting states as the pump power increases (Fig. 3), while its spectrum remains relatively constant (Fig. 4). For example, the P-1/2 state is achieved at a pump power of 50 mW, whereas the P-1/1 state is obtained at a pump power of 80 mW. At pump powers of 140, 200, 240, and 270 mW, the pulse state evolves sequentially into the P-2/1, P-3/1, P-4/1, and P-5/1 states, respectively. When the pump power is fixed, the pulse varies from a pulse-splitting state to a multi-period state as the modulation frequency increases (Figs. 5 and 6), whereas its spectrum remains relatively constant (Fig. 7). For example, P-5/1, P-3/1, P-2/1, P-3/2, P-3/2, and P-1/1 pulse states are obtained at modulation frequencies of 1, 2, 3, 4, 5, and 6 kHz, respectively (Fig. 5); P-1/2, P-1/3, and P-1/4 pulse states are obtained at 19, 23, and 31 kHz, respectively; and P-1/4, P-1/3, P-1/2, and P-1/1 pulse states are obtained at 36, 48, 56, and 62 kHz, respectively (Fig. 6). In other words, the pulse characteristics and evolution of the multi-period, splitting, and chaotic pulses in the ring-cavity RFL vary with the pump power and modulation frequency, as shown in Fig. 8. For the linear-cavity RFL, the pulse varies from a pulse-splitting state to a multi-period state as the modulation frequency increases (Fig. 10), whereas it varies from a multi-period state to a pulse-splitting state as the pump power increases (Fig. 11). P-22/1, P-6/1, P-3/1, and P-1/2 pulse states are obtained at modulation frequencies of 0.7, 2.0, 3.0, and 10.0 kHz, respectively, under a fixed pump power of 73 mW. P-1/2, P-3/1 and P-4/1 pulse states are obtained at pump powers of 73.0, 120.5, and 172.3 mW, respectively, under a fixed modulation frequency of 10 kHz. When an external laser with different wavelengths is injected, the number of pulses in one cycle increases with the pump power (Fig. 12), and its spectrum shows two lasing wavelengths (Fig. 13). In summary, the pulse evolution of the linear-cavity RFL is similar to that of the ring-cavity RFL. Regardless of the presence or absence of laser injection, as the pump power increases, the output pulses of the RFL transform from a multi-period state to a single-period state and then to a pulse-splitting state. Light injection does not fundamentally change the pulse evolution but only reduces the number of splitting pulses. Conclusions The experimental results indicate that the pulse parameters can be controlled by adjusting the pump power and modulation frequency under multi-period, splitting, and chaotic pulse states in both actively Qswitched RFLs with ring and linear cavities. Moreover, the pulse evolution with pump power and modulation frequency is highly consistent. External laser injection reduces the number of splitting pulses and generates weak dark pulses. In summary, the pulse characteristics of actively Qswitched RFLs based on an RPS-FBG are determined by the pump power, modulation frequency, and external injection. These RFLs are promising for applications in biological simulations and encrypted communication.
