In this work, we report the influence of ambient refractive index changes on the output power of a multimode fiber with long period grating (LPG) written on it. It is seen experimentally that the cladding mode profiles of such a LPG sensor is very sensitive to the change in refractive index of the surrounding medium. In order to study the influence of ambient refractive index, we have utilized LPG with different periods (100 micrometer, 250 micrometer and 500 micrometer) and multimode fibers with different core diameter (200 micrometer and 400 micrometer). The variation in the output power as a function of different period and fiber dimensions is also investigated. In addition to the many advantages of optical sensors, the unique features such as low insertion loss, low back reflection and easy fabrication of LPG sensors make the present measurement system simple, reliable and accurate for measuring refractive index of the medium surrounding the cladding.
This article describes the design, development and characterization of an evanescent wave fiber optic sensor (EWFS) for the detection of toxic ammonia gas. Sol-gel technology is employed for immobilizing a reversible ammonia sensitive dye on the middle unclad region of a multimode fiber. Ammonia gas permeating into the immobilized dye, the
color of the dye changes reversibly from yellow to blue with increasing concentration of ammonia gas. The concentration of ammonia gas can be determined by measuring the absorption at a given wavelength. The response of the sensor to different concentrations of ammonia, recovery time, reusability, etc of the developed sensor is also discussed. The techniques described here is also relevant to the detection of a wide range of other gases.
We report a novel technique to enhance the thermal lens signal intensity by the use of silver nanosol. Thermal lens signal measurements of the laser dye rhodamine 6G dissolved in double distilled water has been carried out using the 532 nm excitation from a Diode pumped solid state (DPSS) laser, both in the presence and absence of silver sol for various concentrations. It has been observed that there is a very noticeable increase in signal intensity when silver nanosol is added to the dye solution. Though the enhancement varies with concentration, it is generally greater than 56 percent. The results presented in this work are of great importance to configuration and optimization of extremely sensitive thermal lens instruments, which is the trend in the development of analytical instruments. We have also carried out the fluorescence quantum yield (Qf) measurements of the dye using the dual beam thermal lens technique for two different pump powers and observed that at higher pump power there is only about 10% reduction in the quantum yield at low concentration in comparison with about 50% reduction at low pump power. At higher concentrations, the percentage reductions are almost identical in both cases.
This article describes the design, development and characterization of a fiber optic sensor based on evanescent wave absorption in Long Period Grating (LPG) on multimode optical fibers, for the detection of toxic ammonia gas. The sensing length of the sensing element is only 10mm, but the dynamic range is as good as that of the conventional evanescent wave sensors. Sol-gel technology is employed for immobilizing a reversible ammonia sensitive dye on the
grating region of a multimode fiber. Ammonia gas permeating into the immobilized dye, the color of the dye changes reversibly from yellow to blue with increasing concentration of ammonia gas. The concentration of ammonia gas can be determined by measuring the absorption at a given wavelength. The dynamic range of the sensor is from 0.027 to 2.04 mM/Ltr.
In the present study, we report the design and development of a cost-effective, simple, sensitive LED - based fiber optic sensor for detecting trace amounts of chloride ions in water. A multimode plastic clad silica (PCS) fiber with the cladding removed and long period grating (LPG) written on its middle region acts as the sensing element. Exposure of the sensing region to water samples containing spectroscopic reagents for the detection of chloride ions causes evanescent wave absorption, which increases with increase in concentration. The limit of detection of the sensor is found to be a few parts per billion and the operational range covers more than five orders of magnitude.