Green picosecond pulse laser has a wide range of applications in coherent anti-Stokes Raman scattering imaging, material micromachining, and other fields. Especially in biomedical imaging, green picosecond pulses with different repetition frequencies are directly related to sample damage, penetration depth, and signal quality. Therefore, it is meaningful to study the efficient generation of green pulse lasers with different repetition frequencies.

The nonlinear amplification loop mirror (NALM) mode-locked resonant cavity is used as the seed pulse. After a fiber pre-amplifier, an acousto-optic modulator (AOM) is used to change the repetition frequency outside the optical resonant cavity. Then, a master oscillator power amplifier (MOPA) scheme is used to obtain a high-power infrared laser with different repetition frequencies. After that, a green pulsed laser with different repetition frequencies is generated by second harmonic technology (SHG) in a frequency doubling quasi-phase-matched (QPM) periodically poled lithium niobate (PPLN) crystal.

An all-polarization-maintaining ytterbium-doped fiber laser with an adjustable repetition frequency is built in the experiment, which can output a picosecond pulse laser with an adjustable repetition frequency of 5-20 MHz. The frequency doubling of MgO-doped periodically poled lithium niobate (PPMgLN) crystal is studied. At the repetition frequencies of 20, 10, and 5 MHz, the corresponding maximum conversion efficiencies are 39.2%, 35.3%, and 31.0%, respectively, corresponding to output powers of 744, 600, and 496 mW, respectively, as shown in Fig. 2(d). The output spectra [Figs. 2(b), (e)], pulse widths [Figs. 2(c), (f)], and temperature phase matching curve [Fig. 3(a)] are compared. Finally, the stability and beam profile of the frequency doubling laser are tested. The relative jitter of the output power for the green laser is as low as 0.74% within four hours, as shown in Fig. 3(c).

In this paper, an efficient frequency doubling scheme based on an all-polarization-maintaining ytterbium-doped fiber laser with adjustable repetition frequency is verified, and the effects of repetition frequency, fundamental frequency optical power, and crystal temperature on frequency doubling efficiency are tested. A green picosecond pulse laser with different repetition frequencies is obtained by using PPMgLN crystal. The highest conversion efficiency is 39.2%, and the corresponding highest average power of the green laser is 744 mW. The measured root mean square (RMS) of frequency doubling optical power fluctuation is 0.74% within four hours. The proposed scheme has a simple structure and high stability. It provides a useful reference for the efficient generation of green pulsed lasers from high-power Yb-doped fiber lasers with different repetition frequencies.