This study presents an assembly-free ball lens structure at the tip of tapered multimode optical fiber to enhance the light collection efficiency for pH measurements. A 35 µm diameter ball lens was fabricated at the sensor tip. In addition, a thin layer of fluorescence dye was mixed with sol-gel that formed at the fiber tip for pH sensing. The simulation result demonstrates the light propagation on the ball lens tip. The experiment results reveal that the proposed sensor has a rapid response time (< 3 seconds), high sensitivity, and pinpoint accuracy (±1.0%) in the pH range of 6.0-8.0.
Hydration reactions of cement-based materials are of great significance to their properties and durability. Various technologies have been investigated to study the hydration processes regarding reaction heat, chemical changes, or microstructures. As a non-destructive chemical analysis technique, Raman spectroscopy provides detailed information about chemical structures, which takes advantage of tracking and monitoring the chemical change during the hydration reaction. In this study, a novel in situ fiber optic Raman probe was utilized to continuously monitor the long-term hydration process of cement clinker stages, from early to late hydration stages and from fresh to hardened state of paste samples. With the remarkable capability of this technique for dry or moist, crystalline or amorphous samples, the hydration process of tricalcium silicate (C3S) pastes with different water-to-solid (w/s) ratios can be monitored from the beginning of the hydration reaction. The main hydration products, especially C-S-H and silicate (CH), have been successfully identified, and there in situ quantitative changes have been continuously monitored. The effect of the w/s ratio on the hydration process of C3S slurries is also discussed. Moreover, the X-Ray Powder Refraction (XRD) results strongly correlate with the Raman spectra of the hydration products, demonstrating the technique's reliability. By comparing with the existing in situ fiber optic Raman spectroscopy technique, the proposed sensor performs a significantly better signal-to-noise ratio (SNR), providing essential aid for future use in the construction field for monitoring and assessing the health and performance of concrete structures.
This study presents a novel in situ high-temperature fiber optic Raman probe that enables the study of the physical properties and structure of molten samples at temperatures up to 1400 °C. To demonstrate the functionality of the high-temperature fiber optic Raman probe, different composition mold fluxes were evaluated in this report. The Raman spectra at flux molten temperature were successfully collected and analyzed. A deconvolution algorithm was employed to identify peaks in the spectra associated with the molecular structure of the components in each sample. The experimental results demonstrate that the composition-dependent Raman signal shift can be detected at high temperatures, indicating that molten materials analysis using a high-temperature Raman system shows significant promise. This flexible and reliable high-temperature Raman measurement method has great potential for various applications, such as materials development, composition and structure monitoring during high-temperature processing, chemical identification, and process monitoring in industrial production.
Food quality and safety have been critical issues in the world. There is an urgent need for a fast, simple, selective, and inexpensive food detection method for the identification of the degree of food spoilage. As a molecular analysis tool, Raman spectroscopy has the advantages of high selectivity, accurate analysis, simple operation, and low sample consumption. This paper reports a novel remote fiber optic Raman sensor for real-time application in food spoilage detection. Eight volatile organic compounds (VOC) liquids that typically generated by corrupted food were under-tested. The proposed sensor successfully captures the back-scattered Raman spectra for all testing samples with various dilution levels. Multiple machine learning algorithms are also applied to further analyze the correlation between Raman spectra and molecules in spoiled foods by diluting chemical samples. As a result of combining with Raman spectroscopy and machine learning algorithm, the remote fiber optic Raman probe allows qualitative measurements of VOC samples at 100-fold dilution. In comparison with surface-enhanced Raman scattering (SERS), the remote fiber optic Raman sensor allows for direct Raman spectroscopy detection without sample and SERS substrate preparation, which opens a new chapter on the nondestructive and sensitive detection of food analytes.
This paper reports a novel Extrinsic Fabry-Pérot Interferometer (EFPI) sensor platform based on ~50 μm-diameter porous silica microspheres attached to the ends of single-mode optical fibers. The glass spheres, with 45% internal void volumes, act as geometrically well-defined Fabry-Pérot (FP) cavities that produce interferograms that only depend on the index of refraction of guest molecule types and loadings. The primary advantage of the sensor is that the silica micropores inside the glass spheres present inherent surface hydroxyl groups, which can be chemically modified using a wide selection of silanization reactions. Silanized silica microspheres provide a novel and broad sensor platform where myriad silane coupling agents act as bridges connecting organic and inorganic materials. Commercially available silanization reagents are diverse and afford silica pores with selectivity for sensing chemicals and biochemicals. When guest molecules are adsorbed in the pores of the microspheres, a proportional change in the light path length can be calculated and measured. A gas sample generator consisting of vapor generators, analyte permeation tubes, and flow controllers were configured to characterize the sensor response to various volatile organic compounds. An optical interrogator with a 1 Hz scan frequency and 80 nm wavelength range was employed for full spectral scanning and data acquisition. Experimental results demonstrate shifts of the interferogram when an EFPI glass microsphere is exposed to different vapors and vapor concentrations. Future work will compare EFPI results of guest molecule adsorptions by unaltered versus silanized porous glass microspheres.
A miniaturized fiber-optic Raman probe for Raman spectroscopy, which can eliminate the high backscattering Raman signal from the long-fused silica fiber that is used for the biochemical application, is presented. Its main purpose is to provide a technique for the detection of very small substances and separate Raman backscattering signal of optical fiber. After a brief introduction of the traditional fiber Raman technology, the experimental operation of the design optimization of the miniaturized fiber-optic Raman probe was discussed. We successfully used the home build fiber taper device to combine seven multi-mode fibers as one fiber taper with approximately 30µm as the probe diameter for Raman spectral analysis. By comparing the traditional fiber-optic Raman sensor and the miniaturized fiber-optic Raman probe with experiments on a variety of materials, the correlation of the Raman signal has been demonstrated. We observed that the miniaturized fiber-optic Raman probe not only effectively removes the backscattering Raman signal of the fiber itself, but also provides a comparable signal-noise ratio, which provides an argument for this research.
The continuous casting process for steel production utilizes specially designed oxyfluoride glasses (mold fluxes) to lubricate the mold and control the steel solidification process. The composition of the flux controls important properties, such as viscosity, basicity, and crystallization rate, which in turn influences the quality of the as-cast product. However, these fluxes also interact with the steel during casting, causing chemistry shifts that must be anticipated in the design of the flux.
Today, the in-service chemistry of the flux must be determined by taking flux samples from the mold during casting and then processing the samples off-line to determine chemistry and other physical properties, such as viscosity. Raman spectroscopy provides an alternative method for flux analysis, with the possibility of performing direct on-line analysis during casting. Raman spectroscopy has the unique ability to identify specific molecules through well-resolved vibrational bands that provide fingerprint signatures of the structure of the molecules. Specific peaks in the Raman spectra can be correlated with flux chemistry and viscosity.
The work reported here aims to assess the structure and chemical composition of flux samples at high temperatures using fiber-optic Raman spectroscopy. Results from Raman spectral analyses captured the 1300 °C for a range of flux chemistries are presented. The experimental results demonstrate that the composition-dependent Raman signal shift can be detected at high temperatures and that on-line flux analysis using a high-temperature Raman system shows significant promise.
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