Narrow-linewidth SLM Q-switched laser pulses have wide applications in precision spectroscopy, differential laser radar, and coherent detection. After series of trials on SLM Q-switched pulses with ring-cavity fiber lasers due to the higher energy storage, injection seeding technique may be considered as the most promising way.

However, to date, no enough study that shows the pulses generated with the injection seeding scheme are truly SLM has been reported and meanwhile the linewidth is still too wide.

A study team led by Prof. MAO Qinghe at Anhui Institute of Optics and Fine Mechanics (AIOFM), Hefei Institutes of Physical Science of the Chinese Academy of Sciences obtains highly stable mode-hopping-free single-longitudinal-mode (SLM) Q-switched laser pulses with the linewidth of only 1.5 MHz, which may be the narrowest linewidth for a Q-switched fiber laser so far.

In their study, they demonstrate a Q-switched ring-cavity fiber laser with the injection seeding technique, and obtain the narrow-linewidth SLM Q-switched laser pulses. Besides, the repetition rate and the pulse width can also be adjusted from 1 to 100 kHz, and 50 to 400 ns, respectively.

In fact, the group has devoted years of efforts to obtaining such narrow-linewidth Q-switched pulses with fiber lasers.

At the very first, they designed and developed a low-noise continuous-wave SLM fiber laser, which has the linewidth less than 10 kHz, the relative intensity noise of -110 dB/Hz and the frequency noise of 45 dB 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 1.5 MHz 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 0.2 mJ while the linewidth is still less than 2 MHz.

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 results were published in the journal of IEEE Photonics Journal with the title of "Mode-Hopping-Free Single-Longitudinal-Mode Actively Q-Switched Ring Cavity Fiber Laser With an Injection Seeding Technique." 

Schematic diagram of SLM Q-switched fiber laser. (Image by YAO Bo) 

SLM characteristics and linewidth of Q-switched fiber laser. (Image by YAO Bo)