Generation of 131 fs mode-locked pulses from 2.8 μm Er:ZBLAN fiber laser

Mid-infrared ultrafast laser has been generally produced from optical parametric oscillator or amplifier, which requires a powerful pump source and precise time synchronization. Mode-locked fluoride fiber laser is a new way to generate mid-infrared ultrafast laser, which possesses the advantages of compactness, robustness, and low cost, and is considered as a promising substitute for optical parametric oscillator or amplifier. Pulse duration is a key parameter for ultrafast laser. Mid-infrared pulses with femtosecond duration are desired for many applications such as supercontinuum generation, pump-probe measurement, and soliton self-frequency shift. The pulse duration of Er:ZBLAN fiber mode-locked laser reported so far is beyond 200 fs limited by water molecule absorption. S. Antipov et al adoped Ho:ZBLAN fiber with an operation wavelength of 2.9 μm to reduce the water molecule absorption, however, the pulse duration was only decreased to 180 fs.

The research group, led by Prof. Guoqiang Xie from Shanghai Jiao Tong University, reported their latest work in Chinese Optics Letters, Vol. 18, Issue 3, 2020 (Hongan Gu, Zhipeng Qin, Guoqiang Xie, Ting Hai, Peng Yuan, Jingui Ma, Liejia Qian. Generation of 131 fs mode-locked pulses from 2.8 μm Er:ZBLAN fiber laser[J]. Chinese Optics Letters, 2020, 18(3): 031402) on a mode-locked fluoride fiber laser with pulse duration as short as 131 fs, which is the shortest mode-locked pulse from mid-infrared fluoride fiber lasers.

Compared with the previously reported mode-locked Er:ZBLAN fiber lasers, this work makes a significant progress on pulse shortening. Because fluoride fiber has an anomalous dispersion in mid-infrared, mode-locked fluoride fiber laser usually works in the soliton regime due to the balance between nonlinearity and dispersion. It is the key to find the optimal balance point between nonlinearity and dispersion for shortening pulse. By optimizing the gain fiber length to realize the optimal balance between nonlinearity and dispersion, the researchers achieved 131 fs mode-locked pulses. Numerical simulation is consistent with the experimental results, which further confirms the reliability of the method. Moreover, the method can be extended to other rare-earth-doped (Tm3+、Dy3+、Ho3+、Er3+) fluoride fiber lasers in the spectral range of 2-4 μm.

The work makes an important step for the performance improvement of mode-locked fluoride fiber lasers, and will push forward to their applications in chirped pulse amplification, mid-infrared supercontinuum generation, mid-infrared frequency comb, pump-probe measurement, frequency down conversion, and medical field.

Experimental setup of 131 fs mode-locked Er:ZBLAN fiber laser at 2.8 μm