Brillouin Gain Spectrum And Transmission Characteristics In Liquid-filled Optical Fiber

Stimulated Brillouin scattering (SBS) is widely applied to the fields of fibersensing, all-light switch, slow light, filtering and so on. In the process of research, theBrillouin gain spectrum (BGS), which is the key factor in the measurement, is themain physical quantity to be tested, once it is proposed. The existing technique ofBGS shaping focuses on pump modulation. As a positive nonlinear medium, silicaoptical fiber has the highest frequency of usage to introduce SBS. While, the range ofbandwidth and frequency shift of its intrinsic gain spectrum is awfully small, leadingto the unavailability to obtain the adjustable bandwidth of BGS even if the pumpmodulation is applied. For the characteristic of SBS has a close relationship with theproperty of medium, the substantial measure to the performance of SBS lies in theconvertible material of optical fiber. In this paper, the conception of SBS inliquid-filled optical fiber is put forward to solve the restriction of carrying medium.In addition to the application of pump modulation, different liquids to fill can makethe BGS shaping more flexible and make the adjustment range more wide as well inthis way. Moreover, liquids with high refractive index are always selected to be thefiber core. However, as to some liquids with high nonlinear but low refractive index,they cannot be utilized effectively. Therefore, liquid-filled simplified hollow-corephotonic crystal fiber (HC-PCF) is introduced based on the unavailable of lowrefractive index liquids, to enlarge of regime of fiber carriers to be filled with allkinds of liquid.In this paper, the liquid to fill consists of CS2with high gain factor and CCl4with broad Brillouin bandwidth. The characteristic of SBS in thus liquid-filledoptical fiber is calculated. By means of adjusting the mixing ratio of these twoliquids, flat-top BGS with bandwidth of50MHz~2GHz is obtained under themodulation of pump modulation. In experiment, the intrinsic BGS of2.53GHzfrequency shift and34MHz bandwidth are obtained in CS2-core fiber. When thepump is modulated to2or3spectral lines, the bandwidths of broadened and flat-top BGS are65MHz and91MHz, respectively.Then, with the help of MATLAB, the transmission characteristic of liquid-filledoptical fiber is analyzed by the multi-physics coupling analysis software ofCOMSOL. As an example, the simplified HC-PCF is filled by different liquids suchas water, ethanol and FC-40. The characteristics of confinement loss and dispersionof such a liquid-filled optical fiber are calculated theoretically. Finally, a transmissionwindow with low loss and low dispersion in the range of600nm~1000nm is obtained,in order to lay the foundation for the application of SBS.