We demonstrated an inexpensive, simple and ultra-sensitive refractive index (RI) sensor based on a long tapered tip optical fiber combined with a straightforward image analysis method. The tapering length was optimized through beam propagation simulations and trapezoidal tip fibers were fabricated using a single-step chemical etch process. A simple measurement setup was built that consists of a single wavelength light source (λc= 660 nm), a cuvette, an objective lens, and a camera. The sensitivity of the fibers was measured using saline solutions with different concentrations. The light rays exiting the fiber tip along the tapered section form a circular interference pattern on the camera, whose size in the central part very strongly depends on the surrounding refractive index. By analyzing the areal changes in the center of the fringe patterns for different saline solutions, we obtained an unprecedented sensitivity value of 24160 dB/RIU (refractive index unit), which is the highest value reported so far among intensity-modulated fiber refractometers. We also performed beam propagation simulations to predict the behavior of the tapered tip fiber sensor. The experimental results are consistent with the simulations. This sensor is ultra-sensitive, simple, easy-to-fabricate, and low-cost, which makes it a promising tool for on-site measurements and point-of-care applications such as DNA tests based on loop-mediated isothermal amplification.
An in-fiber Lyot interferometer for temperature measurement is presented. The sensor utilizes high temperature-dependence of the birefringence in Panda polarization maintaining fibers to achieve high resolution in temperature measurements. Temperature variation modulates the phase difference between the polarization modes propagating in different modes of the Panda fiber. The Lyot interferometer produces a spectrum which varies with the phase difference. Therefore, by monitoring this spectrum a high resolution of 0.003°C was achieved. A fiber Bragg grating is added to the setup to expand its dynamic range. This sensor does not need complicated fabrication process and can be implemented in many applications.
We demonstrate a label free fiber optic sensor for detection of mercury ions in aquatic solutions. This sensor utilizes aptamers as bio-recognition element which traps mercury ions and cause a refractive index change in the vicinity of the sensor. Refractive index variations lead to a change in the transmission spectrum that can be used to calculate the concentration of mercury ions in that solution. The concentration of 1 nM mercury ions was detected which is below the specific amount determined by the US environmental protection agency as the maximum authorized contaminant level of Hg2+ ions in drinking water.
In this paper, a new type of dynamic chemical etching is used to fabricate different fiber tips with different cone angles. It was done by controlling surface level of hydrofluoric acid relative to the fiber, with changing volume of the acid in the container using a syringe pump. Using this method the cone angle of the tip is effectively controlled and angles between 1° and 30° was obtained.
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