Characteristics of Brillouin scattering in semiconductor-doped optical fibers

Specialty optical fibers by doping semiconductor materials in the fiber core have attracted significant attention owing to unique performance diversification and potential in various fields of fiber communication and fiber sensing. In this study, we investigated the Brillouin scattering of semiconductor-doped optical fiber fabricated by incorporating semiconductor material of lead sulfide (PbS) into the fiber core. Employing a Brillouin optical time domain analysis (BOTDA) system, the distance-resolved Brillouin scattering spectrum (BGS) and Brillouin frequency shift (BFS) of a spool of homemade PbS semiconductor-doped fiber was characterized. The results show that the BFS along the fiber distance at room temperature turns out to be non-uniform, spanning from 9.384 GHz to 9.476 GHz, which mainly results from PbS doping conditions at different fiber locations during the fiber drawing process. Besides, the BGS of PbS semiconductor-doped fiber exhibits a Lorentzian profile with the BGS bandwidth of about 44 MHz, which is around 4 times broader than that of standard single-mode fiber (SSMF). Additionally, temperature-dependent BFS measurement was conducted. It reveals that the slope of the BFS versus the increasing temperature is uneven, primarily due to the discrepancy of the refractive index and longitudinal acoustic velocity of the fiber core material under non-uniform PbS semiconductor doping conditions. The trend of temperature coefficient aligns closely with that of BFS, indicating a strong correlation between the Brillouin scattering characteristics and doping condition. This work not only provides important guidance for optimizing the design and fabrication of PbS semiconductor-doped fibers but also promising applications in distributed fiber sensing.