Optical fiber vibration sensors are an appropriate alternative for piezoelectric devices, which are electromagnetic sensitive to the external conditions. Most of the vibration sensors demonstrated in previous publications resist to different interferometers or Bragg’s gratings. Such sensors require a long time of stabilization of an optical signal, because they are vulnerable to undesirable disturbance. In majority, time response of an optical sensor should be instantaneous, therefore we have proposed an in- line vibration sensing passive element based on a tapered fiber. Micrometer sized optical fiber tapers are attractive for many optical areas due to changes process of boundary conditions. Such phenomena allow for a sensitive detection of the modulation phase. Our experiment shows that a singlemode, adiabatic tapered fiber enables detecting an acoustic vibration. In this study, we report on Mach- Zehnder (MZ) interferometer as a vibration sensor which was composed of two 50/50 couplers at 1550 nm. In the reference arm we used a 4 meter singlemode optical fiber (SMF28), while in the arm under test we placed tapered optical fibers attached to a metal plate, put directly on speaker. Researches carried out on different tapered fibers which diameter of a taper waist was in the range from 5 μm to 25 μm, and each taper was characterized by optical losses less than 0,5 dB. The measured phase changes were over a frequency from 100 Hz to 1 kHz and an amplitude in the range from 100 mVpp to 1 Vpp. Although on account of a limited space we have showed only the results for 100 Hz. Nevertheless, experimental results show that this sensing system has a wide frequency response range from a few hertz to one of kilohertz, however for some conditions, a standard optical fiber showed better result.
Propagation of a light beam in optical fiber tapers depends on few parameters like diameter of a taper waist region, adiabatic or non-adiabatic shape of a whole, as well as boundary condition connected with refractive index surrounding the taper. As external materials many mixtures can be used allowing to manufacture different applications [1-3]. In this work we applied liquid crystals’ medium due to its optical properties - anisotropic as a cladding for the manufactured taper. Properties of liquid crystals can be modified by electric or magnetic fields, as well as by temperature change. These features make liquid crystal fibers very important for optical applications. In this work we have built an optical cell which consists of a liquid crystals’ mixture of 1550C1 . The applied taper can be described by the following parameters: diameters of 10+/- 0.5 μm, losses lower than 0.5 dB in visible range, manufactured on FOTET. The tapered fiber was put in a liquid-crystal cell made of two tin oxide coated glass plates separated by spacers. The value of the used electric field was contained in the range of 0V - 160V. This experiment demonstrates ability of changing optical transmission in a tapered fiber immersed in a liquid-crystal medium. The range of optical transmission observation was between 500-700 nm which was connected with an LC transmission. Also, were performed measurements of influence of temperature in the range of 20-80°C on the proposed device. For temperature higher than 50°C, increased optical power propagating in the tapered fiber was observed.
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.
In this paper, we present the LPFG fabrication using an optical filament splicer GPX- 3400 (Vytran) with tapering option where adiabatic, periodical tapers were fabricated by applying tensile stress to the fiber. The maximum range of the furnace movement is a few dozen millimeters from home position, therefore it is possible to apply the point-by-point method without introduction of additional, unnecessary stress. We used an “Omega”- shape filament to achieve homogenous structure of the tapers. Under this work, we have done the LPFGs on a commercial singlemode fiber SMF28 with the cladding’s diameter of 125 μm and the singlemode fiber Nufern 1550B-HP-80 with the cladding’s diameter up to 80 µm and polarization maintaining fiber PM1550-XP. A series of LPFG devices was fabricated with different periods and a total length of fiber grating depending on number of tapers. The analysis of attenuation bands was carried out by observing transmission spectrum in the range of 1200 nm to 1700 nm, whereas the fiber with LPFG was illuminated by a broadband light source. We obtained the optimum parameters of LPFG fabrication, separately for each types of the optical fibers.
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.