Narrow-linewidth single-longitudinal-mode (SLM) Q-switched laser pulses have wide applications in precision spectroscopy, differential laser radar, and coherent detection. Many methods have been tried for the SLM Q-switched pulses with ring-cavity fiber lasers because of the higher energy storage, and the injection seeding technique may be the most promising way. However, to date, no enough evidence shows that the pulses generated with the injection seeding scheme are truly SLM and the linewidth is still too wide.
Recently, the research group led by Prof. MAO Qinghe in Anhui Institute of Optics and Fine Mechanics (AIOFM), Hefei Institutes of Physical Science, demonstrates a Q-switched ring-cavity fiber laser based on the injection seeding technique, and obtains highly stable mode-hopping-free SLM Q-switched pulses with the linewidth of only 1.5MHz, which may be the narrowest linewidth for a Q-switched fiber laser so far. Meanwhile, the repetition rate and the pulse width can also be adjusted from 1 to 100 kHz, and 50 to 400 ns, respectively.
This group has concentrated in obtaining such narrow-linewidth Q-switched pulses with fiber lasers for many years of untiring effort. They first designed and developed a low-noise continuous-wave SLM fiber laser, which has the linewidth less than 10 kHz, the relative intensity noise of -110dB/Hz and the frequency noise of 45dB Hz2/Hz, respectively. Then they stabilized the frequency of the SLM fiber laser to the gas molecular absorption lines with home-made low-pressure gas cell based on the hollow-core photonic crystal fiber. With the frequency-stabilized SLM fiber laser as the injection seed, after having eliminated various adverse effects, for instance, the frequency shift of the fiber pigtailed acousto-optic modulator, finally they have successfully achieved the 1.5MHz linewidth SLM Q-switching operation for ring-cavity fiber laser based on the injection seeding technique. By using optical fiber amplifiers, they have also enhanced the SLM Q-switched pulse energy to 0.2mJ while the linewidth is still less than 2MHz. This kind of SLM Q-switched pulsed fiber laser may find wide applications in many fields, such as fine structure analysis of materials, precision spectroscopy, high-quality laser radar, gravitational wave detection, etc.
This work is in part supported by the National Key Basic Research Program of China, the Natural Science Foundation of China, the Strategic Priority Research Program of the Chinese Academy of Sciences, and the Key Research Program of the Chinese Academy of Sciences. The main result of this research was published in the journal of IEEE Photonics Journal entitled Mode-Hopping-Free Single-Longitudinal-Mode Actively Q-Switched Ring Cavity Fiber Laser With an Injection Seeding Technique.( The article links to http://ieeexplore.ieee.org/document/7822927/)
SLM characteristics and linewidth of Q-switched fiber laser. (Image by YAO Bo)