Objective The repetition rate of a mode-locked fiber laser pulse is one of the most important defining parameters. On-demand repetition rates vary, serving a wide range of applications. For example, lasers with high pulse repetition rates (between tens of MHz and a few GHz) are not only used to generate optical frequency combs or coherent stacked pulses, but also offer high-precision wavelength calibration for astronomical spectrographs. In contrast, lasers with low pulse repetition rates (below 1 MHz) are highly valued in industrial laser materials processing to keep the characteristics of cooling machining. The mode-locked lasers with high repetition rates can easily be generated by shortening the laser cavity length. However, when it comes to the lasers with low repetition rates, the things get more complex and costly because these lasers require an additional acousto-optic modulator (AOM) or electro-optic modulator (EOM) to pick pulses from a laser source with high repetition rates. Compared with the traditional SESAM (semiconductor saturable absorption mirror) mode-locked fiber lasers, the mode-locked fiber lasers with nonlinear amplifying loop mirrors (NALM) as artificial saturable absorbers demonstrate the advantages of fast relaxing time, high damage threshold, life expectancy, all-fiber structure, and ultralow repetition rates over a relative broadband. These pulses can be generated by elongating the cavity length. After compression, these pulses have a sub-picosecond duration. In this study, we report a sub-picosecond NALM mode-locked fiber laser at different repetition rates varying from 21 MHz to 100 kHz by adjusting the length of the passive fiber at the proper position in the oscillator. All pulses with different repetition rates can be compressed to a sub-picosecond level.

Methods We constructed the NALM mode-locked fiber laser based on an all-fiber structure and an all-polarization-maintaining design. To realize different repetition rates in the mode-locked laser pulse output, we gradually elongated the oscillator cavity length via inserting two additional passive fiber segments into two different locations in the cavity (Fig. 1). In case of pulse repletion rates below 1 MHz, the length and position of the two passive fiber segments were carefully designed to avoid accumulating excessive nonlinear effects, such as stimulated Raman scattering, which undermined the stability of mode-locked pulses and decreased the output pulse energy. In particular, the first passive fiber segment was spliced after the gain fiber, whereas the second one was spliced after the output coupler. Finally, we demonstrated a 20 μm/130 μm Yb-doped double cladding fiber amplifier with an ultralow repetition rate NALM mode-locked fiber laser as seed instead of any pulse-picking device.

Results and Discussions First, we obtained a self-starting mode-locked pulse train with a repetition rate of 21.16 MHz at the main and NALM pump powers of 90 mW and 126 mW. The output pulse was centered at 1030 nm with a 3-dB bandwidth at 9.1 nm, and the pulse duration was compressed from 5.3 ps to 352 fs by a pair of 1379 lp/mm gratings (Fig. 2). Second, we obtained mode-locked pulse trains with repetition rates of 5.92 MHz, 1.28 MHz, 457 kHz, 280 kHz, 181 kHz, and 100 kHz by adjusting the cavity length through the addition of two passive fiber segments. The cavity lengths were 33.8, 156, 438, 714, 1106, and 2000 m, respectively (Table 1 and Fig. 3). While the pulse repetition rate decreased from 21 MHz to 100 kHz, the pulse energy and duration increased by two orders of magnitude, namely, from 1 nJ to 104 nJ and 5.3 ps to 300 ps, respectively. The broadest 3-dB spectrum at 30 nm was demonstrated at a repetition rate of 5.92 MHz, which corresponded to the shortest compressed pulse duration of 177 fs. Importantly, all the pulses were compressed to a sub-picosecond level. Especially for the ultralow repetition rate pulses, it was a good choice for ultralow repetition rate laser system to be front-end seeded without any pulse-picking devices. Finally, a 388-kHz, 62.7-ps, 20.8-nJ NALM mode-locked seed was amplified to 3 μJ after a single 20/130-μm Yb-doped double cladding fiber amplifier stage (Fig. 5), which was further compressed to 537 fs.

Conclusions We report on a mode-locked ytterbium-doped fiber laser with a nonlinear amplifying loop mirror in an all-polarization-maintaining designed cavity. The repetition rates of the laser vary between 21 MHz and 100 kHz by adjusting the length of passive fiber at the proper position of the oscillator. The 5.3-ps mode-locked pulse with a 3-dB bandwidth at 9.1 nm is first obtained at a repetition rate of 21.16 MHz, which is then compressed to 352 fs. The broadest 3-dB bandwidth at 30 nm and the shortest compressed pulse duration of 177 fs are demonstrated at a repetition rate of 5.92 MHz. Limited by the length of available passive fiber, we obtain a pulse with a maximum energy up to 104 nJ and duration of 300 ps directly from the oscillator at the lowest repetition rate of 100 kHz. The pulse is subsequently compressed to 1.053 ps. All the output mode-locked pulses at different repetition rates with a broad spectral bandwidth are characterized and compared with the traditional low repetition rate mode-locked fiber lasers. They are compressible to sub-picosecond, which is different from the dissipative soliton resonance and noise-like pulses. A 388-kHz, 62.7-ps, 20.8-nJ NALM mode-locked laser seed is amplified to 3 μJ after a single-fiber amplifier stage, which could be further compressed to 537 fs, and this whole laser system is very compact without any pulse-picking components or multistage fiber amplifiers.