Ultrashort lasers at wavelengths of approximately 980 nm not only can serve as pump sources for ytterbium- or erbium-doped lasers and amplifiers but also can generate blue light at 490 nm through frequency doubling. Blue light has broad applications in biological imaging, medical diagnostics, and underwater communications. Whereas extensive research and discussions have been conducted domestically and internationally regarding the theory, experiments, and technology of 980-nm fiber lasers, studies focusing on their startup dynamics are few. Therefore, our investigation on the startup dynamics of 980-nm narrow-spectral-band fiber lasers will contribute significantly to the design and optimization of 980-nm fiber lasers.

A narrow-spectral-band fiber laser with a linear cavity structure was mode locked using a semiconductor saturable absorber mirror (SESAM). By adjusting the pump power, we achieved single-, double-, and triple-pulse mode-locking states. A high-speed oscilloscope was used to measure the temporal evolution of the self-starting pulse, and a detailed analysis was conducted.

A narrow-spectral-band fiber mode-locked laser with a center wavelength of 977 nm and a bandwidth of 0.08 nm [Fig. 2(a)] was successfully constructed, with a repetition frequency of 30.62 MHz [Fig. 2(b)]. By adjusting the pump power, we achieved single-, double-, and triple-pulse mode-locking states. The multipulse state of this narrow-spectral-band laser exhibits a progressive startup process that differs from the simultaneous generation and synchronous evolution of multiple pulses reported for broad-spectral-band lasers. During the double-pulse startup of the narrow-spectral-band laser, the leading pulse first undergoes beating and transient single-pulse stages [Fig. 4(b)]. When the leading pulse enters the transient-bound-state stage, a new pulse is generated and gradually strengthens until its intensity matches that of the leading pulse, thus ultimately forming a stable double-pulse mode-locking state [Figs. 4(c) and (d)]. In the triple-pulse startup, the leading pulse similarly undergoes beating and transient single-pulse stages, with a new pulse generated during the beating process. The two pulses gradually reach equal intensities and enter the transient-bound-state stage, during which a third pulse is progressively formed and strengthened. When all three pulses reach the same intensity level, stable triple-pulse mode locking is established [Fig. 5(d)]. This is likely due to the delayed pulse splitting caused by the weakening of nonlinear effects arising from the limited spectral width. Broad-spectral-band pulses have a narrow pulse width in the Fourier transform limit and a high peak power. After the main pulse is formed upon initiation, under nonsteady-state conditions, they are disturbed and rapidly segmented into subpulses, thus exhibiting a synchronized formation process during multipulse mode locking. By contrast, narrow-spectral-band pulses have a wider pulse width in the Fourier transform limit and a lower peak power. Upon initiation, the main pulse is formed first, and as nonlinear effects accumulate, the main pulse transfers energy, thus resulting in the formation of new pulses and a progressive self-starting process.

In this study, a fiber mode-locked laser with a central wavelength of 977 nm and a width of 0.08 nm was realized using an SESAM, which achieved a maximum output power of 7.51 mW. Adjusting the pump power enabled single-, double-, and triple-pulse mode locking to be realized. The temporal evolution of the self-starting pulse was measured using a high-speed oscilloscope. Unlike a previously reported startup process for broad-spectral-band lasers, a progressive multipulse startup process, instead of a synchronous one, was observed in this study. This is likely due to the delayed pulse splitting caused by the weakening of nonlinear effects arising from the limited spectral width. Our study facilitates the design and optimization of optical fiber lasers in the 980 nm band.