Development of fiber-based siglemode-multimode fiber with micro convex lens displacement sensor

Abstract

Various sensitive industrial applications require micro-displacement detection in order to realize a precise movement control. This micrometer displacement can be detected using a fiber-based displacement sensor that offers micro displacement detection and is immuned against electromagnetic radiation. This kind of sensor, however, has limitations on its sensing range and sensitivity. In order to comprehend the limitations, a new configuration of fiber-based displacement sensor with improved sensing range and sensitivity is designed and presented in this thesis. The developed displacement sensor works according to the Fabry-Perot Interferometry (FPI) principle. In general, the proposed displacement sensor consists of two parts; an optical semireflecting fiber mirror attached with micro-convex lens as a sensor head and a highly reflective coated gold mirror. These two components are arranged in parallel to form a Fabry-Perot cavity. In this work, the new sensor configuration is realized by fusion splicing a segment of 9/125 µm single mode fiber (SMF) to one end of 10 mm long section of 62.5/125 multimode fiber (MMF). The other end of the MMF is ultraviolet (UV) cured with a liquid composition of Norland Optical Adhesive 61 (NOA) that forms a micro-convex lens at the sensor head. Physical characterization of the fabricated SMF-MMF with NOA micro-convex lens (SMFMMF- Lens) sensor shows that this sensor has a reflectivity of 6.8% with 210 µm focal length, f(h). These outcomes attribute to an increase of reflected optical power and also an improvement on the sensing range. In order to sense the displacement, 200 nm thickness of sputtered gold mirror is attached to the movable object for characterization process. The SMF-MMF-Lens sensor performances are analyzed in terms of intensity and fringe response analysis. For this purpose, a broad light source ranging from 1530 nm to 1565 nm wavelength is injected into the sensor and the reflected light is captured using an optical spectrum analyzer (OSA). The intensity response showed that this SMF-MMF-Lens sensor managed to sense displacement within 10 µm to 520 µm sensing range with sensitivity of 566.4 µW/µm. Employing OSA with 1 nm resolution results in the SMF-MMF-Lens sensor having resolution of about 1.77 pm/W. Within the tested range, 10 µm to 310 µm displacement range exhibits a good linear response which corresponds to 3/2 of the lens focal length. For the fringe response analysis, it is identified that the SMF-MMF-Lens sensor was able to detect displacement of 10 µm to 520 µm sensing range with the sensitivity of 0.0284 fringes/ µm. The entire sensing range for fringe analysis is linear. For comparison purposes, conventional sensors with SMF and SMF-MMF configurations are fabricated for sensor performance analysis. The sensitivity of SMF-MMF-Lens sensor improved at about 77.72% and 9.7% in comparison to the conventional SMF-MMF sensor for its intensity and fringes response analysis, respectively.