Fabrication of deuterium-loaded fiber Bragg grating and its spectral characteristics in thermal annealing

Abstract

Previous results showed that the non-reversible(hystersis loop) of Bragg wave length shifting in thermal cycling of the Fiber Bragg Grating which is a high germanium doped optical fiber and high pressure hydrogen loaded was due to the diffusion out of the H 2 residue in thermal annealing. In addition, the 0-H absorption peak (1.38nm) causes signal attenuation and stability problem in FBG applications. We demonstrated the fabrication of the D 2 loaded FBG with high stability of Bragg wave length in thermal annealing at temperature up to 250 'C. The spectrum characteristics of the D 2 loaded FBG compare to the H 2 loaded FBG is presented. In general, A 2. B of the D 2 loaded FBG is narrower than H 2 loaded, and 2. ,of the D 2 loaded FBG is more stable than H 2 loaded in thermal annealing. A model base on the UV photoinduced index change in BFG core with D 2 and H 2 loaded to explain the spectrum characteristics between D 2 and H 2 loaded FBG is discussed.Keywords: Fiber Bragg grating, deuterium loading 1.INTRODUCTIONFiber Bragg grating has known to be the most advanced passive component for applications in fiber optic communication and sensor systems because of its low insertion loss, narrow band pass, and the flexibility of manipulating desired spectral characteristics [1][2][3][4][5]. FBG can be easily tailored by induced index change, grating length, chirping, and apodization, so that the desired spectral characteristics, such as filtering wavelength, reflectivity, and band-width can be achieved. In general, a FBG device is used to convert the variation of temperature or strain of the tested environment. The Bragg wavelength shifting then can be measured by interrogating, or by interferometric approach [6][7][8][9]. Thus, the performance of the fiber -optic sensing system is significantly affected by the spectrum stability of the FBG components. Various approaches to fabricate the FBG have been demonstrated [10][11][12][13][14][15][16][17][18][19]. High pressure H 2 loading is one of the well known process to enhance the photo-sensitivity of the high germanium doped fiber by deep UV writing (248 nm). The hydrogen loading process is low cost and reproducible. Previous report showed that the hydrogen loaded FBG has an intrinsic absorption peak at 1.38 um by the 0-H bond [20][21]. This 0-H absorption might cause intrinsic problem to fiber optic communication system where a 1.3 um laser source is usually used. In addition, the residue of H 2 cause the drifting of the effective index change of the fiber, which in turn can affect the reflectivity and Bragg wavelength of the FBG. Although, this drift can be eliminated by cycles of post annealing, but will decrease the reflectivity of the FBG. It is suggested that the hystersis of Bragg wavelength shifting is contribute to the diffusion out of the H2 residue in thermal annealing. In this report, we demonstrate the fabrication of the FBG using high pressure D 2 loading, and shows the improvement of the spectral characteristics of the FBG. 2.EXPERIMENTSSamples of...

Dynamic strain monitoring by wavelength locking between two fiber Bragg gratings fiber sensing system

“…The FBG sensor devices are simple, and have all the advantages normally attributed to fiber sensors, such as electrically passive operation, EMI immunity, high sensitivity, and potential low cost. A transducer of the FBG sensor converts the variation of temperature or strain into Bragg wavelength shift which can be measured by wavelength interrogating, or other sophisticate spectral analysis methods [6][7][8]. Thought, direct wavelength interrogation by a spectrometer is an effective approach to measure the wavelength shift of a perturbed Bragg grating, issues of implementation of fiber sensing system for practical applications, such as robustness of the sensing system, stability of light source, and cost of measuring might affect to the efficiency of the fiber sensing system for real applications.…”