A research team from Shanghai Institute of Optics and Fine Mechanics (SIOM) of the Chinese Academy of Sciences (CAS) simulated and experimented the suppression of Stimulated Brillouin Scattering and Brillouin gain spectrum characterization in an acoustic anti-guided delivery fiber for high power narrow linewidth laser. The results were published in Optics Express on Jan. 16, 2023.

High-power narrow linewidth lasers have attracted increasing attention owing to their high coherence and high beam quality. Unfortunately, a series of physical factors such as nonlinear optical effects, can limit the light intensity and power. In particular, stimulated Brillouin scattering (SBS) is a critical factor. SBS involves the transfer of optical and acoustic waveguides to Stokes scattered light through electrostriction. This generates a large amount of Stokes scattered light in the laser, and the excited scattered light strengthens the acoustic waveguide. Hence, the interaction produces strong scattering. Therefore, the study of the delivery fiber that can effectively suppress SBS effect from the optical fiber material itself is an urgent problem to be solved.

The research team optimized the radial refractive index profile to change the optical and acoustic field distributions and theoretically analyzed the BGS from single-doped and co-doped fibers with different acoustic refractive profiles. The results indicated the usage of Al₂O₃ anti-guides acoustic waves and reduces the spatial overlap between the optical and acoustic fields. Because of all of the dopants increase the acoustic index except for Al, which decreases it.

The fiber preform was prepared by chemical vapor deposition and the fiber was made by wire drawing. The test system were built to test the Brillouin threshold and BGS. Finally, the experimental results demonstrated that the SBS threshold of an Al/Ge co-doped fiber was 9.4 dB higher than that of a conventional single-mode fiber G.652D and that the Brillouin gain coefficient was 1.041×10^-11 m/W，the BFS and Brillouin bandwidth of the main peak were 16.22 GHz and 17.72 MHz, respectively.

Thus, the acoustic waveguide modes play an essential role in SBS suppression, and the simulated and measured BGS values are consistent. These results indicate that fibers with a higher SBS threshold can be applied in the field of high-power lasers after further optimizing the production process.

Fig.1 Radial refractive index profile of the preform. (Image by SIOM)

Fig.2 The simulated and measured BGS of the Al/Ge co-doped fiber. (Image by SIOM)

Article website
https://doi.org/10.1364/OE.470550