Ultrafast fiber lasers have a large spectral bandwidth, very high peak power, and ultrashort pulse duration, which makes them more widely employed in many fields such as optical communications, medicine, and sensing. Ultrashort pulses are mainly generated by both Q-switching and mode-locking, where Q-switching is a technique that modulates the quality factor of the laser cavity to form a pulse under the high-quality factor in the laser cavity. Meanwhile, the energy is released in the form of laser light, leading to microsecond to nanosecond pulses. On the other hand, mode-locked pulses are formed by inducing a fixed phase relationship between the oscillating modes of the laser cavity, and their repetition frequency is determined by the round-trip time of the light in the cavity, which is typically at the mHz level. The passive mode-locking technique commonly adopted for intracavity soliton generation is a comprehensive balance of dispersion, nonlinearity, gain, and loss in the cavity. Actually, despite the growing number of studies on the implementation of Q-switched and mode-locked techniques in various lasers, there is a lack of concrete experiments on the implementation of Q-switched and mode-locked continuous switching in these systems, and even less is known about the state evolution and switching transient dynamics of the two types of pulses. Peng et al. reported the establishment process of dissipative solitons in mode-locked fiber lasers, measured the corresponding spectral dynamics using the time-stretch dispersive Fourier transform (TS-DFT) technique, and found that mode-locking was accompanied by multi-pulse generation due to modulation instability before mode-locking. Liu et al. discovered a new mode of soliton formation, or the evolutionary formation dynamics of mode-locked pulses that transition from the Q-switched phase, after observing the entire establishment process of soliton molecules in mode-locked lasers. Although all the above studies involve both Q-switched and mode-locked pulse states, they are invariably unmanipulated transient kinetic processes observed at specific pump powers. Till now, the pulse dynamics of the continuous switching between Q-switching and mode-locking have not been investigated in detail, and the continuous switching of such pulse states can be manipulated by modulating the pump. The Q-switched and mode-locked continuously switchable fiber lasers based on the nonlinear polarization rotation effect can periodically switch between Q-switched and mode-locked states under the effect of pump intensity modulation, and the switching dynamics are investigated by the real-time Fourier transform spectral detection technique.

The fiber laser consists of a 976 nm semiconductor laser (LD), a signal generator (SG; RIGOL, DG1022U), a wavelength division multiplexer (WDM), an erbium-doped fiber (EDF), a 70∶30 optocoupler (OC), a polarization-dependent grating (PDG), two polarization controllers (PCs), and a polarization-independent isolator (PI-ISO). The gain fiber (EDF) is 2 m long and is connected to the pump light source via 980/1550 nm wavelength division multiplexing (WDM), and the rest of the fiber components of the cavity are single-mode fibers (SMF-28E) with a dispersion of -22.8 ps²/km. The full cavity length is 6.17 m and corresponds to a repetition frequency of 33.529 MHz, and the net dispersion in the cavity is 0.0272 ps². The LD characteristics, such as output waveform and power, are controlled by the SG, and a PI-ISO is utilized to ensure the directionality of the light inside the cavity. The PDG, which is inscribed on a polarization-maintaining fiber (PMF) using a CO₂ laser, has a high polarization-dependent loss (PDL) and low insertion loss and is integrated with two PCs, which are equivalent to saturable absorbers for enabling nonlinear polarization rotation (NPR) mode locking. Meanwhile, a 70∶30 fiber coupler is employed to extract 30% of the optical power from the cavity for testing, and the output pulse is split into two beams by a 50∶50 OC, where one beam is connected to a spectrometer (OSA; YOKOGAEA, and AQ6370D) for spectroscopic measurements, and the other is stretched and broadened by an 8 km dispersion-compensated fiber (DCF) through an 18 GHz high-speed photodetector. Additionally, the optical signals are converted to electrical signals by an 18 GHz high-speed photodetector (PD; HSPD4018), and then connected to an oscilloscope (OSC; LeCroy, SDA 11000) with a bandwidth of 11 GHz to record the real-time spectra.

First, the pump modulation frequency is set to 2 kHz, and the Q-switched and mode-locked states are periodically switched over a time scale of 1 ms. During a modulation cycle, the first thing that happens at low levels is the buildup of the Q-switched laser from noise. With the switching between low and high levels, the laser first enters into chirp oscillations and subsequently achieves stable mode-locking after an unstable mode-locked phase. With the switching from low to high levels, the Q-switched phase ends at a cycle number of about 6500, the energy of the ultrashort pulses in the cavity rises with the relaxation oscillations, and the pulse spectrum is subsequently broadened under the effect of self-phase modulation. However, the critical pulse energy required to achieve stable mode-locking is subsequently reduced due to the gain bandwidth limitation. In the next stage of the evolution (after 7880 cycles), mode-locking begins. As the modulation level switches to a low level, the decrease in pump power reduces the laser cavity energy, with a gradual decrease in soliton energy, while the spectrum begins to shrink toward shorter wavelengths. Until after 250 cycles, the soliton in the cavity is annihilated. After 710 cycles, the energy density level in the cavity is restored and the Q-switched pulse is re-established after 18.5 µs. Without changing other experimental conditions, the pump modulation frequency is adjusted to 5 and 10 kHz respectively. Meanwhile, more frequent intracavity energy fluctuations at 5 kHz than those at 2 kHz cause the chaotic state before mode-locking to produce multi-soliton competition. Figs. 5(c) and (d) compare the pulses of the two states at 5 kHz and 10 kHz modulation frequencies. The period and shape of the pulses of the two states basically remain stable at different modulation frequencies. This indicates that the modulation frequency change has little effect on the Q-switched and mode-locked pulses under the steady state. Without changing other experimental conditions, the pump modulation frequencies are adjusted to 5 and 10 kHz respectively.

In summary, an ultrafast laser capable of continuously switching between Q-switched and mode-locked states is proposed and investigated, and its state-switching dynamics under the effect of modulated pumping are investigated by the TS-DFT technique. At a low level, the laser outputs Q-switched pulses, and a stable mode-locked pulse can be established in the cavity when the modulation level is switched to a high level. A rise in the pump modulation frequency accelerates the establishment time of both pulses, but exceeding a certain upper-frequency limit causes the annihilation of both states. This continuous switching mechanism of laser output pulses breaks through the limitation of single and uncontrollable laser output pulse types in the past and provides a new idea for the design and optimization of ultrafast fiber lasers.