Abstract
The Bragg wavelength of a polymer optical fiber Bragg grating can be permanently shifted by utilizing the thermal annealing method. In all the reported fiber annealing cases, the authors were able to tune the Bragg wavelength only to shorter wavelengths, since the polymer fiber shrinks in length during the annealing process. This article demonstrates a novel thermal annealing methodology for permanently tuning polymer optical fiber Bragg gratings to any desirable spectral position, including longer wavelengths. Stretching the polymer optical fiber during the annealing process, the period of Bragg grating, which is directly related with the Bragg wavelength, can become permanently longer. The methodology presented in this article can be used to multiplex polymer optical fiber Bragg gratings at any desirable spectral position utilizing only one phase-mask for their photo-inscription, reducing thus their fabrication cost in an industrial setting.
Highlights
Polymer optical fibers (POFs) exhibit different mechanical and optical properties compared with silica optical fibers [1]
POF Bragg grating (POFBG) made of hydrophilic polymers, such as poly(methyl methacrylate) (PMMA), can be used for humidity monitoring [6]
The results show that fiber stretching is an important parameter to consider during the annealing process, since it determines the direction and degree of Bragg wavelength shifting
Summary
Polymer optical fibers (POFs) exhibit different mechanical and optical properties compared with silica optical fibers [1]. POFs have higher flexibility in bending, higher fracture toughness, higher failure strain, and lower Young modulus [2] The latter characteristic causes POF Bragg grating (POFBG) sensors to have higher sensitivity in pressure [3], stress [4], and acoustic-wave detection [5]. A number of POFBG annealing experiments have been reported since 2011, for multiplexing and for other purposes such as strain, stress, pressure and humidity sensitivity enhancement of POFBG sensors [10, 13], longer operational range in temperature detection applications [14], and hysteresis effect reduction of sensors under mechanical cycling loading [15]. A novel annealing methodology is demonstrated, which utilizes the fiber axial strain or stress during the annealing process to tune the Bragg wavelength at any desirable spectral position over a spectral range that extends above the recorded Bragg wavelength. The results show that fiber stretching is an important parameter to consider during the annealing process, since it determines the direction and degree of Bragg wavelength shifting
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