AUTHOR=Wang Xiangdong , Yan Fengping , Guo Hao , Wang Wei , Qin Qi , Yang Dandan , Wang Pengfei , Li Ting , Yu Chenhao , Guan Biao , Kumamoto Kazuo , Suo Yuping , Bai Yan TITLE=Single longitudinal mode narrow linewidth thulium-doped fiber laser based on an eye-shaped dual-ring filter JOURNAL=Frontiers in Physics VOLUME=11 YEAR=2023 URL=https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2023.1188608 DOI=10.3389/fphy.2023.1188608 ISSN=2296-424X ABSTRACT=

A single-longitudinal-mode (SLM) thulium-doped fiber laser, based on an eye-shaped passive dual-ring filter, is designed and constructed. The eye-shaped passive compound cavity consisting of four couplers is used to increase the longitudinal mode spacing, and its performance is numerically analyzed in detail. A homemade uniform fiber Bragg grating serves as a wavelength selection device and a saturable absorber is used to further suppress the intense longitudinal mode competition in the laser cavity, ensuring the single-longitudinal-mode output. The experimental results demonstrate a laser output with a center wavelength of 2,049.85 nm and an optical signal-to-noise ratio of 63 dB. Moreover, the power fluctuation is less than 0.6 dB, and the center wavelength fluctuation is less than 0.03 nm over a continuous measurement period of 60 min, demonstrating an excellent stability. The laser linewidth is measured using an unbalanced Michelson interferometer and β-separation line method, resulting in a linewidth of 11.22 kHz.