Shrinking the volumetric footprint of gas sensors is desirable as it allows for nonintrusive, nonperturbing gas mixture analysis and access to tight enclosures. Micro-resonators are a perfect candidate for these sensors as their size parameter (~micron) is minimal, and the typical surface propagating whispering gallery modes can interact with an analyte without disrupting the environment. The large, quality factor (Q) of these resonant cavity modes enables long interaction lengths on the order of 100s of centimeters between the optical field and analyte. Thus, the presence of a gas different than the nominal environment will result in a shift of the resonant properties, including the resonant wavelength, amplitude, and quality factor, that can be detected in real-time. To illustrate this effect, we utilized a spherical micro resonator on the end of a piece of optical fiber, formed using standard ball lens fabrication, and excited the resonant modes using a tapered optical fiber connected to tunable Infrared laser. The resonator was fixed in contact with the tapered region of fiber, and the assembly was placed inside an in-house, optically coupled, vacuum-tight vessel for gas testing. We compared the spectral response of air, pure CO2, and pure N2 gas, observing spectral shifting and broadening of the cavity resonances. In addition, the effect of vessel temperature on resonance peak position due to the thermo-optic effect was investigated and quantified. Lastly, a feedback arm was added to the setup to reduce signal noise and automated data analysis was implemented to improve data clarity.
We demonstrate an all-fiber super-continuum (SC) laser based near infrared (1160nm to 2350nm) spectroscopy system that is capable of measuring protein (gluten) levels in wheat flour, at a stand-off distance. We show that reflectance spectrum between 1160nm and 2350nm can be used to measure protein levels in wheat flour. The measured protein concentration with the partial least square regression shows a good linear correlation (R square >0.95) to the protein level measured by the Dumas method with standard error variance down to 0.5 percent. Our system could be used for non-destructive, real-time determination of the protein level of wheat flour at a stand-off distance in industrial settings such as food factories or flour milling plants.
All-fiber integrated super-continuum (SC) sources are described based on a platform architecture that can operate in the visible, near-infrared, short-wave infrared, mid-wave infrared and long-wave infrared, with demonstrated SC wavelengths ranging from 0.47 to 12 μm. Modulation instability initiated SC generation leads to a simple SC source with no moving parts and that uses o_-the-shelf components from the mature telecommunications and fiber optics industry. The resulting light sources are basically a cascade of fibers pumped by fiber-pigtailed laser diodes and some drive and control electronics; thus, the SC sources have the potential to be cost-effective, compact, robust and reliable. Starting from fused silica fibers, the SC spectrum can be extended to shorter or longer wavelengths by cascading fibers with appropriate dispersion and/or transparency. As one example, we demonstrate a long-wave infrared SC source that generates a continuous spectrum from approximately 1.57 to 12 μm using a fiber cascade comprising fused silica fiber followed by ZBLAN fluoride fiber followed by sulfide fiber and, finally, a high-numerical-aperture selenide fiber. The time-averaged output power is as high as 417 mW at 33% duty cycle, and we observe a near-diffraction-limit, single spatial-mode beam across the entire spectral range. A prototype is described that is based on a three-layer architecture with a form factor of 16.7 × 10 × 5.7 and that plugs into a standard wall plug. This SC prototype has been used in a number of field tests as the active illuminator for stand-off FTIR system over distances of 5 to 25 m, thus enabling identification of targets or samples based on their chemical signature. Further optimization of the SC source will also be described to increase the output power and to reduce the form factor.
We demonstrate infrared spectroscopy systems that are capable of predicting acrylamide level in powders of potato fries based on all-fiber high power supercontinuum (SC) lasers in both the short-wave and mid-wave infrared spectral range. Two SC lasers used in this study cover wavelength range of 670nm to 2500nm and 1600nm to 11000nm, respectively. We use the spectroscopy system to measure 32 French fry samples with different acrylamide concentrations calibrated by gas chromatography-mass spectroscopy (GCMS). Our predicted acrylamide concentrations show a good linear correlation to the measured acrylamide concentration obtained through GCMS, and the partial least square regression analysis shows standard error down to 145ppb. Based on our results, our system could provide a non-destructive alternative method for determining the acrylamide in food samples at a stand-off distance, which could be important for near-line or in-line quality control purposes.