Results and Discussions A stable and self-starting mode-locking at the center wavelength around 1550 nm is obtained with the pump power of 70 mW. By the adjustment of the PC, the central wavelength can be tuned continuously in a range from 1545.52 nm to 1554.40 nm, with the spectral widths changed correspondingly. In wavelength-tunable operation, the superior characteristics of CNT-SA ensure stable mode-locking of pulses, and then the broad wavelength spacing caused by the PMF-based BRIF enables the pulses to get through it in a broad spectral range. For one typical single soliton mode-locked state of the fiber laser, the spectrum is centered at 1551.18 nm with a 3-dB spectral width of 2.8 nm. The estimated repetition frequency is 49.8957 MHz from the radio frequency (RF) spectrum. The corresponding pulse width is estimated to be 0.92 ps if a sech²-shape pulse is assumed, with the time-bandwidth product of the pulses being 0.321, which is close to the transform limit. The output power, single pulse energy, and peak power of the output pulses are 1.15 mW, 0.023 nJ, and 25 W, respectively. Owing to the presence of birefringence and the cavity loss, the asynchronous dual-wavelength mode-locked states can be obtained when the pump power is increased to 140 mW and the state of the PC is adjusted. The central wavelengths of the two asynchronous pulse trains are located at 1533.03 nm and 1544.45 nm, respectively, with the 3-dB spectral width being 0.85 nm and 3.02 nm, respectively. The measured RF spectrum shows there are two main peaks at 49.8971 MHz and 49.8957 MHz, which indicates the repetition frequency difference of the two asynchronous pulse trains is 1395 Hz. Under the same pump power, another asynchronous dual-wavelength mode-locked state, with two central wavelengths located at 1545.36 nm and 1555.53 nm, respectively, can be obtained by adjusting the PC carefully. The repetition frequencies of the two asynchronous pulse trains are about 49.8962 MHz and 49.8951 MHz, respectively, with a repetition frequency difference of 1089 Hz. The two asynchronous dual-wavelength mode-locked states are switchable by controlling the PC settings.

Wavelength-tunable and multi-wavelength ultrafast lasers, which can generate pulse trains at different center wavelengths, are applied widely in optical communication, sensing, and spectroscopy. In relevant research, different wavelength selection components, such as Mach-Zehnder interferometer filter, fiber Bragg grating and tunable filter, or birefringence-induced filtering (BRIF) effect are utilized for optical filtering. Compared with the former, the latter does not need to add expensive filter devices in the cavity. Additionally, the latter can also flexibly control the spectral spacing by adjusting the fiber length. Carbon nanotubes (CNTs), which can exhibit broad operation bandwidth at different spectral windows, are hotspots for multi-wavelength pulse generation. In current wavelength-tunable or multi-wavelength passive mode-locking fiber lasers based on CNT saturable absorber (SA), the SA is usually made by CNT/polyimide composite film, which is conducive to the simple and rapid construction of fiber lasers. However, compared with the laser-induced deposition method, this method requires close cooperation with material preparation, without benefiting the parameter controlling. Therefore, a wavelength-tunable and asynchronous dual-wavelength mode-locked Er-doped fiber laser based on CNT-SA obtained by the laser-induced deposition method is proposed. The results provide an approach to realize the wavelength-tunable and multi-wavelength mode-locked fiber laser simultaneously.

The CNT-SA is obtained by the laser-induced deposition method. Firstly, CNTs are soaked in the alcohol solution with surfactant added. Secondly, the solution is transferred into a centrifugal tube after stirring. Thirdly, CNTs are deposited onto the core of a single-mode fiber by the optical power of 5 mW for ~10 min. Finally, the CNT-SA is assembled by fixing two single-mode fibers with an FC/APC ferrule. The wavelength-tunable and asynchronous dual-wavelength mode-locked fiber laser is passively mode-locked by the CNT-SA. A length of ~0.5 m Er-doped fiber is played as the gain medium, which is excited by a 980 nm pump via a wavelength division multiplexer. A polarization-independent isolator is employed to ensure laser unidirectional transmission. A polarization controller (PC) and a length of ~0.5 m polarization-maintaining fiber (PMF) placed in the cavity are utilized for generating the BRIF effect. 30% of the energy is exported by a 30/70 optical coupler.

In this paper, wavelength-tunable and two asynchronous dual-wavelength mode-locking states are achieved in an Er-doped fiber ring laser, based on the birefringence filtering effect generated by PC and PMF, through a CNT-SA prepared by the laser-induced deposition method. By adjusting the pump power and PC, it obtains a stable and self-starting mode-locking at the center wavelength of around 1550 nm with a wavelength-tunable range of 8.88 nm. Moreover, two asynchronous dual-wavelength mode-locking states are obtained with repetition frequencies of around 49.9 MHz, and the repetition frequency differences are 1395 Hz and 1089 Hz, respectively. Multiple asynchronous dual-wavelength mode-locking states are realized with the assistance of the CNT-SA obtained by the laser-induced deposition method. The results are of great significance in fast spectral measurements and multi-scene switching applications.