中国激光, 2021, 48 (5): 0501013, 网络出版: 2021-03-03   

21 MHz~100 kHz重复频率亚皮秒NALM锁模光纤激光器 下载: 1455次特邀研究论文

Sub-Picosecond NALM Mode-Locked Fiber Laser with 21 MHz-100 kHz Repetition Rate
石宇航 1,2程昭晨 1,2,*彭志刚 1,2游雨 1,2夏童 1,2王璞 1,2,*
作者单位
1 北京工业大学材料制造学部激光工程研究院, 北京 100124
2 北京工业大学北京激光应用技术研究中心, 北京 100124
摘要
超短脉冲光纤激光器在工业、医学、科研等许多领域有着重要的应用。报道了基于全保偏非线性放大环形镜(NALM, nonlinear amplifying loop mirror)锁模的掺镱光纤激光器,通过调整腔内无源光纤的长度和位置,实现了21 MHz~100 kHz重复频率下的锁模。在21.16 MHz重复频率下实现了3 dB光谱带宽为9.1 nm、脉宽为5.3 ps的单脉冲锁模输出,经压缩后脉宽为352 fs。当重复频率为5.92 MHz时,获得了3 dB光谱带宽最宽为30 nm和压缩脉宽最窄为177 fs的锁模脉冲输出。受限于光纤长度,当最低重复频率为100 kHz时,从振荡器直接输出的锁模脉冲的单脉冲能量为104 nJ,脉宽为300 ps,经压缩后脉宽为1.053 ps。在所有重复频率下,锁模脉冲都具有宽光谱、可压缩至亚皮秒量级等特性,并且不是耗散孤子共振或者类噪声脉冲。其中,当重复频率为388 kHz时,脉宽为62.7 ps、单脉冲能量为20.8 nJ的NALM锁模种子源经过单级光纤放大器后,单脉冲能量可以直接放大到3 μJ,最终脉宽可以被压缩至537 fs,整个激光器系统不含脉冲选择器件和额外的多级光纤放大级,结构十分紧凑。
Abstract

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.

石宇航, 程昭晨, 彭志刚, 游雨, 夏童, 王璞. 21 MHz~100 kHz重复频率亚皮秒NALM锁模光纤激光器[J]. 中国激光, 2021, 48(5): 0501013. Yuhang Shi, Zhaochen Cheng, Zhigang Peng, Yu You, Tong Xia, Pu Wang. Sub-Picosecond NALM Mode-Locked Fiber Laser with 21 MHz-100 kHz Repetition Rate[J]. Chinese Journal of Lasers, 2021, 48(5): 0501013.

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