As optical fiber has intrinsic dimensional advantage in length direction, it is more and more important in fiber sensor and overlapping with other technologies such as nanotechnology. Ju’s group reported a novel design which utilized fiber technology and nanotechnology. A novel optical fiber having its cladding doped with Au nano-particles was developed by modified chemical vapor deposition process. Absorption peaks of the optical fiber preform and the fiber appearing at 585 and 428 nm respectively, were due to surface plasmon resonance (SPR) of the incorporated Au nano-particles (NPs) in the cladding [
5]. Figure 4 shows the refractive index and the propagation of light through the cladding of the fiber doped with Au NPs. The refractive index difference between the core and cladding was about 0.00125, enabling light signal to propagate into the cladding region not into the core. The cladding width and total diameter of the optical fiber were 2.6 and 124.3 μm, respectively. To confirm formation of Au NPs in the cladding, the optical fiber preforms were examined by transmission electron microscope (TEM) and UV-VIS spectrophotometer. Optical absorption of the optical fibers was also measured to confirm the propagation of light and the existence of Au NPs by the cut-back method using the Optical Spectrum Analyzer. Then, to characterize SPR sensing property, optical absorption of the fiber was measured by putting small drops of the refractive index matching oil with various refractive indices (
n = 1.406–1.436) on the surface of the fiber. The total and detector lengths of the fiber used for the SPR measurement are 50 and 5 cm, respectively, as shown in Fig. 5.