In this work, a high-sensitivity distributed lateral pressure sensor based on Brillouin Optical Time-Domain Reflectometry (BOTDR) technique is proposed to enhance the pressure sensitivity using a double-coated Single-Mode Fiber (SMF). Firstly, a pressure-induced strain model is developed to analyze the enhancement mechanism of pressure sensitivity by Brillouin Frequency Shift (BFS). With increasing of outer coating radius, or with decreasing of the Young’s modulus and Poisson’s ratio of outer coating, the lateral pressure sensitivity increases. By using a double-coated SMF whose outer coating radius is 1500 μm, a high sensitivity of -3.51 MHz/MPa in the range of 0-30 MPa is achieved, which is about five times higher than that of single-coated SMF. At a spatial resolution of 1.5 m, the maximum measurement error of the proposed pressure sensing system is less than 0.09 MPa.
We proposed a novel fiber-optic distributed refractive index (RI) sensing method by optical frequency domain reflectometry (OFDR), which is qualified for most scenes such as solution diffusion process and multi-analytes detection that require distributed RI sensing ability. We use a section of multi-core fiber (MCF) as the sensing fiber, which consists of one central core and six outer surrounding cores. The MCF is etched by hydrofluoric acid until the outer cores are exposed to the air, thus the effective index of outer cores will be affected by the RI of the surrounding samples, which will lead the wavelength shift of the Rayleigh backscattering (RBS) spectrum. In order to realize distributed RI sensing, we introduce the OFDR technique, which has excellent ability of spatial localization. Then, by demodulating the wavelength shift of the local RBS spectrum, the distributed RI sensing is realized. The sensing length of 19 cm and the spatial resolution of 5.3 mm are achieved in experiment. The RI sensing range is as wide as 1.33~1.44 RIU, and the maximum sensitivity of 47 nm/RIU is obtained around 1.44 RIU. In addition, the central core can be used for temperature compensation because it keeps isolated to the surrounding sample. The experimental results indicate that the temperature sensitivity reaches 9.8 pm/°C during 25~80°C. By temperature compensation, the RI measurement error induced by temperature disturbance can be eliminated. Finally, we successfully detect the diffusion process of glycerol in water using the proposed fiber-optic distributed RI sensor.
Brillouin-based optical fiber sensors have been developed over the past few years and played a significant role for distributed temperature and strain measurements, which include many properties such as high measurement accuracy, large measurement range and environmental suitability. Among these sensors, Brillouin optical fiber sensors via optical chirp chain (OCC) become an ideal choice for ultrafast distributed measurement to distinguish quick-changing events in practical applications. This paper begins with the introduction of the concept and the generation schemes of OCC, the spectra distortion characteristic of OCC Brillouin signal are analyzed. The efforts towards such OCC based Brillouin optical fiber sensing for ultrafast measurement are reviewed here as well, with the OCC based Brillouin optical time domain analysis and the OCC based Brillouin optical time domain reflectometry, which give distinguished performance for dynamic measurement, long distance measurement and one end access measurement. Meanwhile, the consequent future challenges of OCC based distributed sensing are discussed and presented, such as as high spatial resolution and high accuracy measurement.
We proposed a multiplexing method of optical fiber Fabry-Perot interferometer (FPI) for multi-point refractive index (RI) sensing application. This method is based on the frequency modulated continuous wave (FMCW) interferometry, in which a series of FPIs are connected by fiber couplers as like the bus-structure. Due to the spatial difference among FPIs, the reflection signals from different FPIs can be separated in spatial domain by Fourier transform. Applying inverse Fourier transform, the interference spectra of every FPIs can be demodulated independently in wavelength domain. Three FPIs are multiplexed for verifying the multi-point RI sensing ability of the proposed method. The experimental results show that the proposed multiplexing method performs well in multi-point RI sensing applications. In the RI range of 1.3334~1.3410, the RI sensitivities of the multiplexed FPIs are as high as 1200 nm/RIU, and the RI measurement accuracies are as high as 5×10-6 RIU. In addition, the sensing system can demodulate the interference spectrum from the reflected signal as low as 5 pW. When the light source power is 3.2 mW, the theoretical maximum multiplexing number is 8000. This work provides an efficient solution for multi-point RI sensing.
A distributed optical fiber pressure sensor based on Brillouin scattering technique is proposed and experimentally demonstrated, where double-layer polymer coatings are used on the single-mode fibers (SMF) to improve the Brillouin frequency shift (BFS) pressure sensitivity. The single-coated and double-coated fibers are designed to demonstrate their BFS pressure sensitivity, where the outer coating radius of double-coated fibers are 450 μm, 1000 μm and 1500 μm, respectively. Experimental results show that the BFS pressure sensitivity are -0.74 MHz/MPa, -1.61 MHz/MPa, -2.59 MHz/MPa and -3.51 MHz/MPa in the pressure range of 0-30 MPa for above four kinds of fibers. According to the experimental results, it can be concluded that the BFS pressure sensitivity can be improved with increasing of outer coating radius or decreasing of outer coating Young's modulus and Poisson's ratio. The maximum BFS pressure sensitivity is measured about 5 times higher than single-coated SMF, it is of great significance for SMF to be used in practical distributed pressure measurement.
We propose a novel dual-channel Surface Plasmon Resonance (SPR) fiber sensors based on the incident angle adjusting method. By grinding fiber tip to form wedge-shape with different angles, we can easier adjust the incident angle in fiber, and then the SPR wavebands will change corresponding. The simulation and experiment demonstrate that the SPR wavebands will red-shift with increasing of the fiber grinding angle. Based on this, we cascade two fiber tips whose grinding angles are 5° and 15° respectively. Under the tests of the refractive index (RI) range from 1.333 to 1.385, the SPR wavebands are 576~683nm and 677~955nm respectively. Therefore, we can demodulate SPR signal by wavelength division multiplexing (WDM) technology. Sequentially, we can detect two analytes simultaneously. This dual-channel SPR fiber sensor has important significance in the fields of multichannel liquid refractive indices and temperature selfreference measurements.
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