The aim of this work was to design a miniature size optical fiber sensors` transducer which is sensitive to refractive index changes. A polymer microtip manufactured by photopolymerization at the end of a multi-mode optical fiber was used as a transducer. Proper geometry parameters of the microptips allow to use these elements as transducers for reflective types of the refractive index sensors. Two various multi-mode optical fibers were used in the experiments, i.e. a gradient-index with a 62.5 μm core diameter and a step-index with a 105 μm core diameter. They were selected to test influence of light propagation properties on the optical fiber transducer reflective parameters. Optical Backscatter Reflectometer was applied to validate mentioned above transducers’ parameters. The principle of sensors’ work was based on measurement of an amplitude of back scattered signal when the refractive index of liquid around the microtip was changed. Experimental tests were made when the selected microtip was immersed into liquids with various refractive indices in the range of 1.30 – 1.70. Minimal amplitude of the reflected signal occurred when the refractive index of the liquid was equal to the refractive index of the microtip. For both tested optical fiber types, changes of the liquid refractive index caused proportional changes of the signal reflected from the microtip. Maximal measured sensitivity of such optical fiber transducer of 180 dB/RIU with a dynamic range of 32 dB was achieved. Therefore, it is possible to use these micro-elements as transducers for the refractive index optical fiber sensor.
Photopolymerization process is present in the optical fiber technology since its beginning. Organic coatings preventing degradation of an optical fiber was the most important implementation of this process which ensured its practical industry application in telecommunication. However, this process can be implemented to design specialty transducers related to both optical and chemical sensors, as well. Simple optical transducer can be a microtip being an extinction of the optical fiber core as a polymer microelement. A chemical transducer can be a thin polymer layer manufactured on the tapered part of optical fiber. The second type of transducer can be applied to chromatography measurements as a solid phase microextraction fiber. In the paper, technology of the tapered optical fiber and photopolymer sensing layer formations were described. In the first step a standard heat and pull technique was used to taper an optical fiber which, then, was cleaved in two symmetric pieces. Light was launched to such a single element and output optical power from tapered part of the fiber was measured to optimize the photopolymerization process. Placing this element in a photopolymerizable monomer mixture and using a specially selected holder allow for manufacturing a polymer sorption layer applicable as a solid phase microextraction fiber. Extended description of technology of this chemical transducer type and preliminary experimental results displaying its feasibility were presented.
The main aim of this paper is to present a geometry shaping of a polymer microtip manufactured on the end of different multi-mode optical fibers by using UV radiation. Used fabrication process is based on a photopolymerization where the microtip grows on one end of the optical fiber when light is launched to its other end. Two different types of the photopolymerizable mixtures and two typical multi-mode optical fibers with different core sizes and refractive index profiles were used for tested microtips. In order to assess their geometries scanning electron microscopy images were used. These images have showed that the influence on the microtip geometry has optical power and spectral characteristics of the UV light. The sensor’s transducer properties were evaluated on the basis of backscatter measurements when a refractive index of a surrounded medium of the microtip changes. Dynamic ranges of tested optical fibers with selected microtips have reached 25-28 dB for the gradient-index multi-mode optical fiber while for the step-index one were around 14 dB. Future research will be focused on increase of dynamic range for the sensor’s application and will be tested as micro-lens.
Photopolymerizable microelements manufactured on the end face of optical fibers as microtips or microbridges between two optical fibers have drawn our attention as very attractive elements for optical fiber sensors’ transducers. In our first approach we have studied feasibility of a photopolymerizable microtip as a refractive index transducer formed on the end face of a standard multimode fiber (MMF). In the paper we have shown an optimization process in which we have taken into account two important parameters, i.e., optical power and exposition time. Depending on those parameters we have got different shapes and sizes of the growing microtips what were than tested on a scanning electron microscope. In the experimental step each of manufactured microtip was immersed in liquids with known refractive indices (1.4 - 1.6) and amplitudes of the optical backscattered signals were measured by OBR4600. The minimal reflected signal was comparable to the refractive index of a microtip. This conclusion resulted from measurement of the optical fiber without photopolymer elements for which minimal signal was of 1.48, what corresponded with the core of a standard MMF. When the refractive index of external liquid was higher or lower than the mikrotip’s refractive index, then, the reflection signal increased. The linear range of reflected signal can be used to monitor refractive index changes. Described in the paper photopolymerizable microtip can be useful for optical fiber transducers due to its small size and low costs. Moreover, by modification of a photopolymer mixture we can tune refractive index of a microtip and its minimal reflected signal, as well. The aim of further research will be to manufacture a transducer for the optical fiber sensor with higher sensitivity and to prepare numerical simulations of reflection.
This paper presents the technology of performing an effective glued connection between optical fibers made from silica (SOF) and polymer (POF) and a pair of polymer optical fibers (POF-POF). This study has been undertaken in order to establish the influence of cleaving for quality of fiber preparation (its cutting in particular), type of glue, as well as joint spot protection. The prototype of a hot cleaver of POF, made in Institute of Applied Physics MUT, was minimalized and adapted to a single use of blade. Matching geometry of connected structures was optimized by adjusting optical fibers to each other. The result of this research was to define particular distance between fibers. It turned out that the optimized distance amounts to 30 μm. Experiment showed that a joint made of optical glue has given loss of less than 0.2 dB. The next step was to involve protection of the mechanical joint. It turned out that glass capillary complies with the requirements. In order to confirm the effectiveness of the chosen glue connection, measurements of technical parameters on patch cords with MMF – POF and POF – POF connections were made. It was stated that SOF – POF connections can work within the range of -40°C + 60°C workable for humidity simulation without loss change. However, connections POF – POF are unstable with respect to temperature change. Modal characteristics of near- field were also observed.