Long-range multicore optical fiber displacement sensor.

Multicore fiber (MCF) sensors is an emerging research field. MCFs provide new alternatives for the development of innovative sensing devices whose performance cannot be achieved with conventional optical fibers. In general, MCFs have diameters similar to that of a standard single mode fiber (SMF) and have multiple individual cores sharing a common cladding. So far, a number of sensors based on MCF have been reported. Our group, for example, has reported different sensors based on single MCF interferometers that consist of a piece of MCF spliced to SMF [1,2]. One key issue of such sensors is the difficulty of discriminating the target parameter (measurand) from unwanted external perturbations that may be present during the use of the sensor. To address this issue, here, we introduce an innovative architecture that consists of two identical (twin) MCF interferometers in series that are interrogated in reflection mode, see Fig. 1. Our architecture has important advantages. As there are two MCF interferometers, one can be used as a reference or filter and the other as a sensor [3]. The signal of our twin MCF interferometers is the multiplication of individual reflection spectra. Consequently, their reflection exhibits a single narrow peak whose position in wavelength is easy to track. We have observed that periodic bending (vibrations) in one them resulted in prominent shifts in the reflection spectrum.

Humidity sensing and temperature response performance of polymer gel cold-spliced optical fiber Fabry-Perot interferometer

In this article, we have proposed and experimentally demonstrated an optical fiber refractive index (RI) sensing interrogation system realized by extrinsic Fabry–Perot interferometer (EFPI) structure-based microwave photonic filter (MPF) technology. EFPI is considered as a sensing element, as well as a spectrum slicer of the MPF. The shift of the central frequency of the MPF’s passband caused by the change of the RI in the Fabry–Perot (FP) cavity can be monitored to demodulate the RI variation. In our experiment, four EFPI sensors with different FP cavity lengths have been applied for the RI sensing test. The experimental results show that the sensitivity can be tailored by adjusting the length of the FP cavity, and the longer the length of the FP cavity, the higher the sensitivity can be achieved. When the length of the FP cavity is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$960 ~\mu \text{m}$ </tex-math></inline-formula> , a high sensitivity of 1.2 GHz/RIU has been obtained. The proposed sensing system can transfer the changes of the interference pattern in the optical domain to the central frequency shift of the MPF’s passband in the electronic domain and thus avoid the limitation of measurement range induced by the periodic characteristics of the FP interference spectrum. The proposed sensing system also has the advantages of high sensitivity, the feasibility of multiplexing, temperature insensitivity, good repeatability and stability, simple fabrication, low cost, and so on.

In this letter, we report on coupling optics for connecting single-core single-mode fibers with multicore single-mode fibers. After a brief discussion of the options for such coupling systems, we describe our approach of using bulk optics to fabricate low-loss and low-crosstalk devices for both 7- and 19-core multicore fibers with the free-space design approach. This enables the use of the same coupling device with a variety of multicore fibers whose structural parameters differ from one sample to another. We present the results of experimental evaluations of these devices, discuss various causes of insertion loss, and analyze the coupling loss in detail.

A highly sensitive fiber-optic extrinsic Fabry-Perot interferometer (EFPI) is designed and demonstrated experimentally. The EFPI is composed of two sections of single mode fiber (SMF), a segment of silica capillary tube (SCT), an air cavity, and cured polymer which is filled in the SCT. The sensing part of structure of proposed EFPI is the air cavity, and the cured polymer has a modulating effect on the air cavity. Due to the high thermal expansion coefficient (TEC) of the polymer, the sensor exhibits a high sensitivity to temperature. Experimental results show that the temperature sensitivity of the proposed sensor reaches −2.88 nm/°C with a perfect linearity of 99.88%. Therefore, the proposed sensor with high-sensitivity, ease of fabrication and low cost is favorable to be used in high-accuracy temperature measurement.

The paper focuses on analysis of inductive and optical displacement sensors and their use in industrial applications. Differential inductive displacement sensors, usually in the form of linear variable differential transformer LVDT, are widely used in hydraulic applications because they have excellent accuracy, robust construction and are able to measure large displacements (in hundreds of millimeters range). However, they are large, heavy and offer slow measurements. For fast measurements, in millisecond range, optical sensors like time-of-flight ranging sensor could be a better choice, and present paper investigates their performances and compares them in this regard with inductive sensors.

The factors limiting the resolution of displacement sensors based on the extrinsic Fabry-Perot interferometer were studied. An analytical model giving the dependency of extrinsic Fabry-Perot interferometric (EFPI) resolution on the parameters of an optical setup and a sensor interrogator was developed. The proposed model enables one to either estimate the limit of possible resolution achievable with a given setup, or derive the requirements for optical elements and/or a sensor interrogator necessary for attaining the desired sensor resolution. An experiment supporting the analytical derivations was performed, demonstrating a large dynamic measurement range (with cavity length from tens of microns to 5mm), a high baseline resolution (from 14pm), and good agreement with the model.

By splicing and tapering at the fusion point of one-core single-mode fiber and three- or four-core single-mode fiber, an effective bitapered fiber coupling technique is implemented. Based on the beam propagation method, the bitapered coupling characteristics between the one-core fiber and the multicore single-mode fiber are simulated and analyzed. The theoretical prediction is confirmed by the experimental results, and the difference between the simulation and the experimental results is also discussed.

A compact, highly sensitive optical fiber displacement and curvature radius sensor is presented. The device consists of an adiabatic bi-conical fused fiber taper spliced to a single-mode fiber (SMF) segment with a flat face end. The bi-conical taper structure acts as a modal coupling device between core and cladding modes for the SMF segment. When the bi-conical taper is bent by an axial displacement, the symmetrical bi-conical shape of the tapered structure is stressed, causing a change in the refractive index profile which becomes asymmetric. As a result, the taper adiabaticity is lost, and interference between modes appears. As the bending increases, a small change in the fringe visibility and a wavelength shift on the periodical reflection spectrum of the in-fiber interferometer is produced. The displacement sensitivity and the spectral periodicity of the device can be adjusted by the proper selection of the SMF length. Sensitivities from around 1.93 to 3.4 nm/mm were obtained for SMF length between 7.5 and 12.5 cm. Both sensor interrogations, wavelength shift and visibility contrast, can be used to measure displacement and curvature radius magnitudes.

We are studying a friction force control mechanism using levitation mechanism. We usually measure levitation height of the levitation mechanisms by the use of a displacement sensor. An inchworm using the levitation mechanisms was proposed, and realized micron order precise displacement over large horizontal working range. An optical fiber displacement sensor used in this research can measure the vertical levitation height but cannot follow the horizontal displacement. Therefore, it is difficult for the displacement sensor to measure the levitation height since the levitation mechanism moves horizontally even though the levitation height is in the vertical direction. We estimate the levitation height by measuring the capacitance between the levitation mechanism and the substrate, which realizes sensorless levitation estimation. The circuit which detects the electric capacitance is a series RC circuit. We apply sine wave AC voltage to the series RC circuit and measure the terminal voltage of the resistance. The estimation method proposed can extend the working range. This paper describes the comparison of the levitation height estimated by the method proposed and one measured by the optical fiber displacement sensor.

A modified design of the extrinsic Fabry-Perot interferometric (EFPI) optical fiber sensor to allow complete characterization of multicomponent strain fields is proposed. The novel EFPI includes a sensor head with two input fibers such that the respective interference pattern from the end face of the output multimode fiber are in quadrature. Any strain field possessing a component along the line joining the axes of the two fibers causes the initial phase difference to be modulated. The measured change in phase difference is employed to determine this transverse component of the strain field. Absolute, rather than differential measurements of strain are possible using the absolute EFPI sensing system. Applications of the modified sensor-head to smart materials and structures are discussed.

Space-division multiplexing (SDM) systems based on few-mode multi-core fibers (FM-MCFs) utilize both spatial channels (fiber cores) and modes (optical modes per core) to maximize transmission capacity. Unlike laboratory FM-MCFs or field-deployed single-mode multi-core fibers (SM-MCFs), SDM transmissions over field-deployed FM-MCFs in outdoor settings have not been reported. Therefore, concerns remain that environmental interference and cabling stress could worsen inter-core and intra-core modal crosstalk and impact the performance of SDM systems over FM-MCFs. In this paper, we demonstrate successful bidirectional SDM transmission over a 5-km, field-deployed seven ring-core fiber (7-RCF) with a cladding diameter of 178 μm. Our measurements show no significant differences in attenuation and mode coupling compared to pre-cabling conditions, confirming the fiber’s resilience to environmental disturbances and adaptability to cable deployment. Using the field-deployed 7-RCF, bi-directional SDM transmission is implemented, achieving spectral efficiency (SE) of 2×201.6 bit/(s Hz) which sets a new record in field-deployed fiber cables that is a tenfold increase over previous systems. Furthermore, these results were achieved using a small-scale 4×4 multiple-input multiple-output (MIMO) scheme with a time-domain equalization (TDE) tap number not exceeding 15. These results demonstrate the substantial potential of using SDM techniques to significantly enhance SE and expand capacity in practical fiber-optic transmission applications.

An extrinsic Fabry-Perot interferometric (EFPI) fiber optic sensor is presented for measurement of strain at high ambient temperatures. The sensor is fabricated using a femto-second (fs) laser. The EFPI sensor is fabricated by micromachining a cavity on the tip of a standard single-mode fiber and is then self-enclosed by fusion splicing another piece of singlemode fiber. The fs-laser based fabrication makes the sensor thermally stable to sustain temperatures as high as 800 &deg;C. The sensor is relatively insensitive towards the temperature as compared to its response towards the applied strain. The sensor can be embedded in Carbon fiber/Bismaleimide (BMI) composite laminates for strain monitoring at high ambient temperatures.

The special optical fibers, with large mode field, are developed for fabricating high power optical fibers passive components. If a conventional fiber optical laser is used as optical source, the coupling efficiency between special optical fibers and conventional single mode fibers (SMFs) need to be improved. We investigate the specialty optical fibers property and its effect on the coupling efficiency using an electric arc fusion splicer. Improper splicing parameters can lead to coupling efficiency sharply decreased. Several splicing methods are utilized, and all the results are proposed. Comparing these results, we find a set of splicing parameters for reducing the scattering loss at the splice point. It was demonstrated that properly splicing parameters can make coupling efficiency greatly increased; furthermore, a balance between splicing strength and coupling loss was achieved.

We experience Internet traffic growth of 100 times every 10 years. However, the capacity of existing standard single-mode fiber is approaching its fundamental limit regardless of significant realization of transmission technologies which allow for high spectral efficiencies. Space division multiplexing (SDM) based on multicore fibers (MCFs) has emerged as a solution to the problem of saturation of the capacity of optical transmission systems. This article presents the recent progress on the MCFs for future large capacity long-distance transmission systems. In MCFs, there is a tradeoff relationship between low crosstalk and high multiplicity, therefore the maximum number of cores and the core arrangement have to be carefully determined based on the required crosstalk level and core size. The state-of-the-art of fabricated MCFs and the transmission experiments using MCFs are reviewed. The current maximum capacity-distance product in MCF transmission is 368.2 (184.1+184.1) Pb/s/fiber km with the relative spatial efficiency of 4.7 compared with a standard single-mode fiber. In order to increase the spatial efficiency as well as the capacity-distance product further in MCFs, the possibility of heterogeneous MCFs and few-mode MCFs is also presented.