Due to the rapid development of fiber laser technology, tunable fiber lasers have become an important development direction of fiber lasers. Their flexible wavelength tuning characteristics are highly valuable in optical communication, optical sensing, and spectral synthesis. However, to achieve a periodic and stable wavelength tuning operation, tunable fiber lasers often use electrically driven piezoelectric ceramics, heaters, fiber gratings or tunable sweeping filters, and other sweeping devices, which results in a complex structure and reduced output performance of the fiber laser, significantly limiting its development and practical applications.

In recent years, a new type of tunable fiber laser based on the self-sweeping effect has gained the interest of researchers. Compared with general tunable fiber lasers, the self-sweeping fiber laser can achieve spontaneous, stable, and periodic wavelength tunability without using complex tuning elements or electrically driven drives. The phenomenon of the self-sweeping effect was first reported in 1962 when a periodic shift of wavelength was observed in a ruby laser. Half a century later, based on the excellent waveguide medium of optical fiber, researchers in Russia realized the first ytterbium-doped (Yb-doped) self-sweeping fiber laser. Thereafter, based on the typical Fabry-Perot linear cavity structure, researchers observed the self-scanning effect in different bands using different gain media. These bands have greatly facilitated the application of self-sweeping fiber lasers in various research fields and self-sweeping fiber lasers have become one of the research hotspots. In this study, we design a linear-cavity-controlled Yb-doped self-sweeping fiber laser based on a circulator, which is easy to implement, inexpensive, and flexible in controlling the self-sweeping characteristics, such as self-sweeping range and self-sweeping rate, potentially expanding the practical applications of self-sweeping fiber lasers.

In this study, a Yb-doped self-sweeping fiber laser based on a circulator is built based on the previous Fabry-Perot linear cavity structure using a circulator instead of a flat-cut port of the fiber as the reflecting end of the resonant cavity. The resonant cavity of the laser consists of a fiber loop mirror and a circulator. The 3-port and 1-port of the circulator are fused together to form a ring.The laser enters from the 2-port, passes through the 3-port, and returns to the Yb-doped fiber for amplification from the 1-port. The flat-cut port of the fiber can provide 3.5% energy of the Fresnel reflected light, whereas the remaining 96.5% energy of the laser is transmitted out of the cavity, resulting in large losses and a high self-sweeping threshold. Although the circulator has some insertion loss, it can significantly reduce the self-sweeping threshold compared with the fiber flat-cut port. Additionally, since the self-sweeping range and self-sweeping rate are dependent on the output power, the output power of the laser can be changed to regulate the self-sweeping range and self-sweeping rate by adjusting the intracavity loss. By introducing a mechanically variable optical attenuator in the circulator, the self-sweep range and self-sweep speed of the laser can be adjusted flexibly by changing the intracavity loss.

When the laser is operated with self-sweeping mode, its self-sweeping threshold is only 22.5 mW, the output slope efficiency is 10.65%, and a self-sweeping range of 1065.4217-1071.4225 nm (approximately 6 nm) and a self-sweeping rate of 0.56-8.83 nm/s are obtained, which can be observed in the range of 12.08-115.2 kHz (Fig. 4). The optical signal-to-noise ratios (OSNRs) of the laser output spectra are all greater than 40 dB, with a maximum value of 53.18 dB, indicating a good output performance (Fig. 5). A summary of the characteristics of the self-sweeping law reveals that the self-sweeping rate and average pulse repetition frequency are consistent as a function of the output power, i.e., both increase with the increase of output power and are linearly proportional to the square of the output power. After the introduction of a mechanically variable optical attenuator, the experimental results obtained by adjusting the optical attenuator to change the intracavity loss show that with the increase of intracavity loss, the self-sweeping band range gradually drifts toward the short wavelength direction, the self-sweeping rate gradually becomes slower, and the self-sweeping range gradually becomes narrower. The sweeping range in the whole process can cover 1056.5773-1067.4093 nm, approximately 10.83 nm (Fig. 6). Therefore, using a variable optical attenuator to continuously change the resonant cavity loss is an effective technical method for achieving flexible output control of the self-sweeping fiber laser.

In this study, a linear cavity Yb-doped self-sweeping fiber laser that produces a normal self-sweeping effect is realized using a circulator as the reflecting end of the resonant cavity rather than a flat-cut port of the fiber. The cavity structure is simple, easy to implement, and inexpensive. The self-sweeping threshold of the laser is only 22.5 mW, the output slope efficiency is 10.65%, and a self-sweeping range of 1065.4217-1071.4225 nm (approximately 6 nm) and a self-sweeping rate of 0.56-8.83 nm/s with an average pulse repetition frequency of 12.08-115.2 kHz are obtained. The OSNRs of the laser output spectra are all greater than 40 dB, with a maximum of 53.18 dB, which indicates good output performance. When a mechanically variable optical attenuator is introduced into the circulator, the self-sweeping characteristics, such as self-sweeping range and self-sweeping rate, can be adjusted by adjusting the variable optical attenuator to change the cavity loss and extend the sweeping range to 10.83 nm. Our experiments provide a simple, effective, stable, and low-cost method for self-sweeping modulation, potentially facilitating the practical application of self-sweeping fiber lasers in spectral measurement, analysis, and other laser applications.