Reflective Properties of a Polymer Micro-Transducer for an Optical Fiber Refractive Index Sensor

Abstract

A polymer microtip manufactured at the end of a multi-mode optical fiber by using the photopolymerization process offers good reflective properties, therefore, it is applicable as an optical fiber sensor micro-transducer. The reflective properties of this microelement depend on the monomer mixture used, optical fiber type, and light source initiating polymerization. Experimental results have shown that a proper selection of these parameters has allowed the design of a new class of sensing structure which is sensitive to the refractive index (RI) changes of a liquid medium surrounding the microtip. An optical backscatter reflectometer was applied to test a group of micro-transducers. They were manufactured from two monomer mixtures on three different types of multi-mode optical fibers. They were polymerized by means of three optical light sources. Selected micro-transducers with optimal geometries were immersed in reference liquids with a known RI within the range of 1.3–1.7. For a few sensors, the linear dependences of return loss and RI have been found. The highest sensitivity was of around 208 dB/RIU with dynamic 32 dB within the range of 1.35–1.48. Sensing characteristics have minima close to RI of a polymer microelement, therefore, changing its RI can give the possibility to tune sensing properties of this type of sensor.

Polymer Microtip on a Multimode Optical Fiber as a Threshold Volatile Organic Compounds Sensor

“…Both presented microtips have similar geometry and hemispherical shape. Surfaces are smooth as it was expected using LED sources for photopolymerization in the experiment [ 27 , 34 , 35 , 36 ]. Geometry of the presented UV microtip [ Figure 3 a] is as follows: a base diameter of 96 ± 1 μm and a high one of 24 ± 1 μm.…”

“…Manufacturing of an optical fiber microtip based on the photopolymerization process was previously described many times [ 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 33 , 34 , 35 , 36 ]. In Figure 1 , the sketch (a) and the image (b) of the manufacturing set-up were presented.…”

“…Previous studies have shown the possibility to fabricate the microtips on various optical fibers (i.e., SMFs [ 22 , 23 , 25 , 26 ], photonic crystal [ 24 ] and multi-mode fiber (MMF) made with silica [ 27 ] and plastic [ 25 ]). The most important applications of such micro-optical element are in scanning optical microscopes (SOM) [ 28 ], optical fibers interconnections [ 29 , 30 , 31 ], optical fiber couplers/splitters [ 32 ], VCESEL illumination profile controllers [ 33 ], and sensors [ 34 , 35 , 36 ]. In the first application, the SMF was applied to achieve a submicron resolution of the designed optical fiber-based SOM [ 28 ].…”

“…The concept of using such a microelement as a RI sensors transducer was validated by the measurement of return loss of optical backscattered signal while the microtip was immersed in various reference liquids. The highest sensitivity of 208 dB/RIU, with a dynamic range of 32 dB, for the RI range within 1.35–1.48 was achieved [ 36 ]. These studies have suggested new potential applications for polymer microtips.…”

“…Attracted VOC particles cover the surface of the microtip and form liquid droplets as the result of condensation which changes the RI at the interface between the microtip and the external material. This change can be measured by an optical backscatter reflectometer [ 36 ]. To test the proposed sensor, the following volatile materials were used: trimethyl phosphate (TMP), 1,4-thioxane (THX), acetone, toluene, and ammonia aqueous.…”