Within this work, we present a novel approach for Structural Health Monitoring (SHM) of composite materials during the autoclave curing process. For this research, samples of Carbon Fiber Reinforced Polymer (CFRP) have been manufactured with optical fibers embedded, both in the middle of the stack and on its surface. The strain development of the CFRP during the curing process has been monitored employing the Optical Frequency Domain Reflectometry (OFDR) technique. This technique allows the conditions of the element to be tracked not only during the operational lifetime of the composite, as in most of the research in this field, but also throughout the manufacturing process. The reported measurements demonstrate the potential of distributed optical fiber sensors for monitoring CFRP structures while their manufacturing, providing a unique opportunity for early detection of damages that may result in critical failures (e.g. delamination and debonding) and for improving the quality of the manufacturing processes and the final strength of the product.
Sensors based on fiber optics are irreplaceable wherever immunity to strong electro-magnetic fields or safe operation in explosive atmospheres is needed. Furthermore, it is often essential to be able to monitor high temperatures of over 500°C in such environments (e.g. in cooling systems or equipment monitoring in power plants). In order to meet this demand, we have designed and manufactured a fiber optic sensor with which temperatures up to 900°C can be measured. The sensor utilizes multi-core fibers which are recognized as the dedicated medium for telecommunication or shape sensing, but as we show may be also deployed advantageously in new types of fiber optic temperature sensors. The sensor presented in this paper is based on a dual-core microstructured fiber Michelson interferometer. The fiber is characterized by strongly coupled cores, hence it acts as an all-fiber coupler, but with an outer diameter significantly wider than a standard fused biconical taper coupler, which significantly increases the coupling region’s mechanical reliability. Owing to the proposed interferometer imbalance, effective operation and high-sensitivity can be achieved. The presented sensor is designed to be used at high temperatures as a result of the developed low temperature chemical process of metal (copper or gold) coating. The hermetic metal coating can be applied directly to the silica cladding of the fiber or the fiber component. This operation significantly reduces the degradation of sensors due to hydrolysis in uncontrolled atmospheres and high temperatures.
Nowadays technology allows to create highly effective Intruder Detection Systems (IDS), that are able to detect the presence of an intruder within a defined area. In such systems the best performance can be achieved by combining different detection techniques in one system. One group of devices that can be applied in an IDS, are devices based on Fiber Optic Sensors (FOS). The FOS benefits from numerous advantages of optical fibers like: small size, light weight or high sensitivity. In this work we present a novel Microstructured Optical Fiber (MOF) characterized by increased strain sensitivity dedicated to distributed acoustic sensing for intelligent intruder detection systems. By designing the MOF with large air holes in close proximity to a fiber core, we increased the effective refractive index sensitivity to longitudinal strain. The presented fiber can be easily integrated in a floor system in order to detect any movement in the investigated area. We believe that sensors, based on the presented MOF, due to its numerous advantages, can find application in intelligent IDS.
Monitoring the geometry of an moving element is a crucial task for example in robotics. The robots equipped with fiber bend sensor integrated in their arms can be a promising solution for medicine, physiotherapy and also for application in computer games. We report an all-fiber intensity bend sensor, which is based on microstructured multicore optical fiber. It allows to perform a measurement of the bending radius as well as the bending orientation. The reported solution has a special airhole structure which makes the sensor only bend-sensitive. Our solution is an intensity based sensor, which measures power transmitted along the fiber, influenced by bend. The sensor is based on a multicore fiber with the special air-hole structure that allows detection of bending orientation in range of 360°. Each core in the multicore fiber is sensitive to bend in specified direction. The principle behind sensor operation is to differentiate the confinement loss of fundamental mode propagating in each core. Thanks to received power differences one can distinguish not only bend direction but also its amplitude. Multicore fiber is designed to utilize most common light sources that operate at 1.55 μm thus ensuring high stability of operation. The sensitivity of the proposed solution is equal 29,4 dB/cm and the accuracy of bend direction for the fiber end point is up to 5 degrees for 15 cm fiber length. Such sensitivity allows to perform end point detection with millimeter precision.
We present research on optical fiber sensors based on microstructured multi-core fiber. Elaborated sensor can be advantageously used in hard-to-reach areas by taking advantage of the fact, that optical fibers can play both the role of sensing elements and they can realize signal delivery. By using the sensor, it is possible to increase the level of the safety in the explosive endangered areas, e.g. in mine-like objects. As a base for the strain remote sensor we use dual-core fibers. The multi-core fibers possess a characteristic parameter called crosstalk, which is a measure of the amount of signal which can pass to the adjacent core. The strain-sensitive area is made by creating the tapered section, in which the level of crosstalk is changed. Due to this fact, we present broadened conception of fiber optic sensor designing. Strain measurement is realized thanks to the fact, that depending on the strain applied, the power distribution between the cores of dual-core fibers changes. Principle of operation allows realization of measurements both in wavelength and power domain.
Our research results concerning a space-dedicated C-band optical amplifier for application in telecommunication satellites are presented in this article. The device is based on a 7-core microstructured fiber where independent access to each core is granted by an all fiber fan-in/ fan-out coupler. The amplifier properties are described as well as its performance after irradiation to a maximal dose of 100 kRad. Also the difference between two kinds of fiber material compositions is discussed with regard to radiation resistance.
A shape sensor exploiting Brillouin scattering measurements in multicore fibers is presented. Based on previous reports1, the shape sensor’s principle of operation is firstly described. The presented idea is realized through Brillouin Frequency Shift (BFS) measurements in the time domain along the entire multicore fiber. The BFS value is related to the strain value in each core and the differential inter-core strains lead to the bend radius and orientation. Authors present an experimental demonstration of the shape sensor using a 7-core microstructured optical fiber.
In this paper we propose and evaluate performance of the higher order mode filter for 850 nm multi-mode fiber transmission. First the operation principles of the filters are presented and then experimental validation of manufactured optical components is made in an optical transmission system. Excellent operation in the 850 nm transmission experiments up to 54 Gbit/s is shown.
This paper focuses on the utilization of crosstalk phenomenon to construct an innovative strain sensor. In our experiments, we take advantage of special fiber design and technology of fiber post-processing in order to receive strain sensing areas. We present results, which indicate possibility of achieving strain sensitivity at level of several mε/nm with negligible temperature cross-sensitivity at the same time. Furthermore after coating the sensor with the developed copper and gold coatings, it can be easily applied in extremely high temperature (e.g. 500 – 800 ⁰C) and/or aggressive media applications.
Multi-core fibers are recognized as the medium designed to be used in telecommunication for space division multiplexing. At the same time, they can be advantageously used in sensor technology. The most crucial parameter for multi-core fibers is crosstalk, as its presence at a high level is found to be highly undesirable in telecommunication applications. However, this phenomenon can be used advantageously in the construction of new types of fiber optic sensor.
For the strain sensor, we used a dual-core microstructured fiber. In the research presented, we take advantage of the technology of fiber post-processing, namely fiber tapering. This treatment, which enables changes in the conditions for interference between supermodes, makes the fiber sensitive to elongation. In the un-tapered section, supermodes do not interfere efficiently (crosstalk <-50 dB), whereas in the tapered section the crosstalk increases significantly (crosstalk = 0 dB meaning all the power from one core can be transferred to the neighboring core), creating a strain sensitive area. The distribution of power between the cores of a multi-core fiber at the output of the sample depends on the elongation of the sample. The strain value can be read off both in the domain of power and wavelength. Research results show that sensor performance can be adjusted by changing the taper length and ratio. The results presented are promising for the construction of a temperature independent strain sensor, whose strain sensitivity (17nm/mε) is far better than optical fiber sensors based on Fiber Bragg Gratings. Meanwhile, the temperature sensitivity is negligible assuring no cross-sensitivity.
We present the novel 7-core and 19-core hole-assisted fibers designed to satisfy the most demanding requirements of the ITU-T G.657.B3 recommendation for bend-insensitive fibers. The fibers are compatible with standard single-mode fibers with regard to modal properties, dispersion characteristics, and transmission loss. The fibers presented exhibit no crosstalk and it is possible to use them together with other multiplexing methods like CWDM or DWDM. Dedicated fanin/ fan-outs have been created in order to make immediate use in industry possible. The hole-assisted 19-core fiber with single-mode cores is being presented for the very first time.
The use of optical elements and other photonic components makes it possible to overcome telecommunication satellite’s bottleneck problems such as size and weight reduction. Despite the unquestionable potential of such elements, nowadays they are not widely used in systems operating in space. This is due to many factors, including the fact that space radiation has disruptive influence on optical fibre. Namely it introduces additional radiation induced attenuation (RIA) that significantly lowers efficiency of optical fibre based systems. However, there is a possibility to produce radiation-hardened (rad-hard) components. One of them is seven core erbium-doped active fibre (MC-EDF) for fibre amplifiers in satellites that we have been developing. In this paper we present a detailed description of seven core structure design as well as experimental results. We report that average gain of 20 dB in C-band with noise figure of 5.8 dB was obtained. We also confirmed that low crosstalk value for a multicore fibre amplifier based on our fibre can be achieved.
The authors designed and fabricated optical power splitters, which make an alternative solution to existing commercial products. The proposed solutions use multicore microstructured optical fiber designed for new generation telecommunication networks made in Spatial Division Multiplexing (SDM) system. The splitters presented in this paper aim to have low loss and to be compatible with existing elements of optical networks, and in the same time to eliminate disadvantages of existing splitters. Two designs presented in this paper are made in all-fiber technology in order to ensure high environmental stability. The authors present detailed description and experimental results for both optical power splitters’ designs.
In presented work, we examined the structures of dual-core fibers paying special attention to the possibility of using them for sensing. In the hole-assisted fiber structure, the character of propagation in the cores was changed fluently, by post-processing the fiber, i.e. tapering with collapsing the holes. Fiber post-processing changed the conditions for supermodes interference and thus the different scale of power transfer between cores was observed. In the paper we investigated the influence of the taper parameters (taper waist, length and ratio) on the properties of the fiber. We have also studied the behaviour of the transmitted signal, while putting post-processed segment of fiber into different external conditions. Presented research shows a great potential of using modified hole-assisted fibers as sensing elements.
In this work we present a novel highly Ge doped microstructured fiber design dedicated for fiber Bragg grating (FBG) inscription and longitudinal strain sensing. Three series of the reported fiber differentiated by air-hole diameters were drawn and presented. After numerical analysis of the propagation conditions (with effective refractive index, loss and mode area calculated) in the real structures, the fibers were subjected to femtosecond FBG inscription. We show the resulting typical FBG spectra, as well as measure the longitudinal strain sensitivity of the fabricated samples and its dependence on the microstructure geometry. An increase of approx. 4% in the Bragg wavelength strain sensitivity was noticed for an increase of the large air-hole diameter of approx. 10%.
Multicore optical fibers gained a lot of interest in the different fields of fiber sensing. This is mainly due to the possibility of carrying out simultaneous measurements on each core and simplicity of such fiber fabrication. A disadvantage of multicore fibers is related to the complex introducing and detecting light from the individual cores. In this paper we discuss two classes of multicore fiber sensors and optical fiber elements for addressing individual cores in this kind of fibers.
Constantly refined technology of manufacturing increasingly complex photonic crystal fibers (PCF) leads to new optical fiber sensor concepts. The ways of enhancing the influence of external factors (such as hydrostatic pressure, temperature, acceleration) on the fiber propagating conditions are commonly investigated in literature. On the other hand longitudinal strain analysis, due to the calculation difficulties caused by the three dimensional computation, are somehow neglected. In this paper we show results of such a 3D numerical simulation and report methods of tuning the fiber strain sensitivity by changing the fiber microstructure and core doping level. Furthermore our approach allows to control whether the modes’ effective refractive index is increasing or decreasing with strain, with the possibility of achieving zero strain sensitivity with specific fiber geometries. The presented numerical analysis is compared with experimental results of the fabricated fibers characterization. Basing on the aforementioned methodology we propose a novel dual-core fiber design with significantly increased sensitivity to longitudinal strain for optical fiber sensor applications. Furthermore the reported fiber satisfies all conditions necessary for commercial applications like good mode matching with standard single-mode fiber, low confinement loss and ease of manufacturing with the stack-and-draw technique. Such fiber may serve as an integrated Mach-Zehnder interferometer when highly coherent source is used. With the optimization of single mode transmission to 850 nm, we propose a VCSEL source to be used in order to achieve a low-cost, reliable and compact strain sensing transducer.
Microstructured optical fibers (MOF) sometimes also referred to as photonic crystal fibers (PCF) have been a subject of extensive research for over a decade. This is mainly due to the fact that by changing the microstructure geometry (e.g. distribution and size of the air-holes) fiber properties can be significantly modified to better fit specific applications. In this manuscript we present a novel fiber design with three large air-holes neighboring the core and report on how the air-hole diameter influences the effective refractive index strain sensitivity. As direct measurement of the effective refractive index change may be complex and challenging, we propose to use fiber Bragg gratings (FBG) in our sensing set up. The Bragg wavelength is a function of the effective refractive index, hence the external strain changes can be monitored through the Bragg wavelength shift with a simple optical spectrometer. Furthermore we also include an analysis of the fibers temperature sensitivity.
Photonic crystal fibers (PCF), sometimes also referred to as microstructured fibers (MSF), have been a subject of
extensive research for over a decade. This is mainly due to the fact that by changing the geometry and distribution of the
air holes the fiber properties can be significantly modified and tailored to specific applications. In this paper we present
the results of a numerical analysis of the influence of the air-hole distribution on the sensitivity of the propagated modes’
effective refractive index to externally applied longitudinal strain. We propose an optimal strain sensitive fiber design,
with a number of fibers drawn and experimentally evaluated to confirm the theoretical results. Furthermore as the direct
measurement of the effective refractive index change may be complex and challenging in field environment, we propose
to use fiber Bragg gratings (FBG) in our sensing set-up. As the Bragg wavelength is a function of the effective refractive
index, the external strain changes can be monitored through the Bragg wavelength shift with a simple optical
spectrometer. Moreover, since the PCF is also optimized for low-loss splicing with standard single mode fiber, our novel
sensor head can be used with standard off-the-shelf components in complex multiplexed sensing arrays, with the
measured signal transmitted to and from the sensor head by standard telecom fibers, which significantly reduces costs.
Phase sensitivities of temperature, longitudinal strain or pressure, are very important fiber features in sensing and
telecommunication applications. The most common ways to modify such sensitivities are to change the material
properties (by adjusting the core doping level) or employ microstructured fibers (which properties strongly depend on the
cross-section geometry). We decided to investigate strain sensitivity influenced by effective mode field area and mode
field diameter as clear consequence of fiber cross-section geometry.
In this paper we present the results of a three dimensional numerical analysis of the correlation between the fiber mode
field diameter and its longitudinal strain sensitivity. Both conventional and microstructured (commercially available and
custom designed) fibers are investigated. Furthermore we compare the theoretical results with experimental data. To
measure fiber sensitivity we developed a dedicated all-fiber Mach-Zehnder interferometer which enables the
measurement of strain induced phase changes in various fiber types (including conventional and microstructured fibers).
As a conclusion of our work we present relationship between strain sensitivity and MFD .
In this work we would like to present the results of low loss coupling of a novel soft glass fiber for super continuum generation with standard single mode fiber by a filament splicing method. For our experiment we used an all solid soft glass microstructured fiber (MSF) made from a composition of F2 lead-silicate glass and NC21 borosilicate glass. The structure and material properties of the fiber were optimized to achieve all normal dispersion (ND) flattened around 1560 nm, which offers two general advantages for supercontinuum generation. The ND supercontinuum avoids soliton dynamics, hence it is less sensitive to pump laser shot noise and has larger degree of coherence than supercontinuum in the anomalous dispersion range. Furthermore flattening around 1560 nm indicates optimal supercontinuum pump wavelength, which is readily available from erbium doped femtosecond fiber lasers. Using Vytran splicing station (GPX3400) we were able to achieve repeatable splice loss between a standard fused-silica single mode fiber (SMF28) and the low-melting-temperature soft glass MSF as low as 2.12 dB @1310 nm and 1.94 dB @ 1550 nm. The developed very low loss splicing technology together with the above mentioned all solid soft glass MSF advantages give very promising perspectives for commercial applications.
We report on a concept of the all-fiber intensity bend sensor, which is based on photonic crystal fibers with asymmetric air-hole structure. The cross-section of these fibers consists of two regions with different air-filling factors on opposite sides of the fiber core. The design enables the measurement of the bending radius as well as the bending orientation. Moreover, the mode field area of our fibers is adjusted to that of standard single mode fibers (SMFs), which allows easy fabrication of the sensor with the use of widely available light sources and detectors pigtailed with SMFs.
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