Fiber-optic Elements Research Articles

Method of formation of relay interference with the use of fiber-optic elements

The work is devoted to the use of fiber-optic elements in order to ensure the isolation of the receiving and transmitting path of the repeater under the stipulation that it is placed on board of the aircraft. The objective of the article is to consider in detail the possibility of formation of a relay interference with the use of fiber-optic elements and ensuring the isolation of the transceiver path. Technical solutions for formation and provision of isolation of the receiving and transmitting path have been proposed. Empirically, the result of a time delay has been obtained, sufficient to endow the spectral components with additional components, ensuring the suppression of the carrier oscillation of at least 10 dB. The presented results form the basis for modernization and industrial production of promising electronic suppression systems, and allow using new techniques for setting relay interference at ready-made active interference stations.

Advances and Challenges of Ultrafast Fiber Lasers in 2–4 µm Mid‐Infrared Spectral Regions

AbstractMid‐infrared (mid‐IR) ultrafast lasers are widely employed in biomedicine, molecular spectroscopy, material processing, and nonlinear optics. With the improvement of fiber gain media and other fiber optical elements, low‐cost, compact, and high‐efficiency fiber lasers open up new opportunities for 2–4 µm pulse generations, which calls for a comprehensive review of their mode‐locking mechanisms, gain media, and fiber laser system performance. This paper, beginning with an overview of pulse‐generation technologies, reviews recent progress on 2–4 µm mid‐IR ultrafast fiber lasers, including Tm3+‐, Ho3+‐doped, Tm3+/Ho3+codoped silicate fiber 2 µm lasers, and Er3+‐, Dy3+‐doped, and Ho3+/Pr3+codoped ZBLAN fiber 2.5–4 µm lasers. Among them, the status of 2–4 µm ultrafast fiber lasers based on 2D material passive mode‐locking is emphatically discussed. Meanwhile, the novel advances on mode‐locking and gain fibers of mid‐IR ultrafast fiber lasers are explored. Furthermore, current and prospective applications of such laser systems are also introduced in detail. This review finally summarizes challenges associated with future development of mid‐IR ultrafast fiber lasers, which provides an outlook on how to achieve more desirable laser performance (e.g., higher average power, higher pulse energy, and longer emission wavelength) that can lead to more practical uses of such lasers.

A Very Fast Single-Step Process for Optical Microfiber Long Period Gratings Fabrication

Optical microfiber long period gratings (MLPGs) with advantages such as low insertion loss, polarization independence, high resonance dip, and simple configuration are broadly involved in different applications, including fiber optics micro-scale components and sensors. However, it is still a grand challenge to fabricate MLPGs by using current long-time multistep processes. Herein, based on the typical arc-discharge fiber heating and pulling technique, a very fast single-step process for MLPGs fabrication is presented. In this process, the applied fiber heating arc-discharge is functionalized by a series of narrow OFF-pulses to simultaneously modulate the being pulled microfibers. Through the developed one-step technique, a low-loss adiabatic optical microfiber with a periodic array of micro-tapers along its waist region is implemented in less than 100 s, which is much faster than current methods. It is shown that the realized single-step MLPGs with the diameter of 20 μm are characterized by high resonance dip (>27 dB), low transmission loss (≤1.2 ± 0.2 dB), narrow FWHM (<20 nm), and high repeatability. The measured external refractive index (RI) sensitivity and temperature dependency of the gratings are 744 nm/RI-unit and 12 pm/°C, respectively. The results of this study could be of interest for the applications of the MLPGs for miniature fiber optics elements and sensors realization.

Development and investigation of the sensitive element of the amplitude fiber-optic temperature sensor based on superimposed chirped Bragg gratings

In this paper we study an original technique for creating an amplitude temperature sensor based on n-pairs of chirped fiber Bragg gratings (CFBGs). The scalable design of the sensor allows to increase the range of the measured signal. The use of the amplitude interrogation technique ensures high speed, elementary nature of the scheme and simple operating conditions. The application of superimposed CFBGs as a sensitive element of an amplitude temperature sensor is investigated. As a result of the work, a method for measuring temperature with a fiber-optic sensitive element based on n-pairs of CFBGs is tested. Two superimposed CFBGs are inscribed in a standard telecommunication single-mode optical fiber using a KrF excimer laser system and Talbot interferometer. One of the superimposed structures is used as a reference element of the sensor and the second – as a sensitive element and is placed in a brass tube, where it is fixed on one side with an adhesive joint. The sensor has an accuracy of 0.48 °C and a sensitivity of 0.20 µW/°C for temperature measurement in the range of 30–70 °C. The results obtained are of great importance in the development of fiber-optic temperature control systems. Implementation of the sensor based on n-pairs of CFBGs allows scaling the power of signal, increasing the ability to detect a slight change in ambient temperature. The created sensor works according to the amplitude polling technique, which is characterized by high accuracy and speed of measurement, minimized scheme, simplified use and easy processing of the detected signal. In addition, the implemented interrogation method corresponds to the trend of developing low-cost fiber-optic systems without losing their main advantages.

Design of optical fiber path for tapered optical fiber array and improvement of light transmission uniformity

Tapered Optical Fiber Array (TOFA) is a type of fiber optic imaging element consisting of several million tapered optical fibers. TOFA is widely used for coupling with CCD/CMOS to achieve high-coupling efficiency and resolution. The inhomogeneity of light transmission is one of the pattern noises in a TOFA, which seriously affects the optical performance and reduces the detection efficiency and recognition accuracy of coupling devices. This is a problem that needs urgent improvement. In this study, the theoretical analysis of the light transmission loss of tapered optical fibers with various structures in TOFA was performed, and the geometric shape for the tapered transition region of a TOFA was designed as the distribution of a linear, parabolic, and cubic function, respectively. Ray-tracing software was used to numerically calculate the light transmission ability of tapered optical fibers in designed three types of TOFAs. It was concluded that the theoretical uniformity of light transmission is the best in the cubic model TOFA. Additionally, the linear, parabolic, and cubic model TOFA samples were manufactured by adjusting the stretching temperature and time, all of which are highly consistent with the designed geometric structure. The relative transmission curve from the axis to the edge of the TOFA was characterized using the fiber optic imaging element transmission detector. It is demonstrated that the light transmission uniformity of experimental linear, parabolic, and cubic model TOFA was 23.82%, 10.73% and 5.44%, respectively, which the experimental cubic model TOFA exhibits the highest uniformity agreeing with the trend of numerical calculation. Therefore, a TOFA with high homogeneity was manufactured by the specified results for the optical fiber path of the tapered transition section, which was created based on theoretical analysis and numerical calculation. The method and results provide a guideline for preparing a TOFA with high performance, large size and high taper ratio, which is of great significance for improving the performance of fiber optic imaging elements and the developing low level-light night vision fields.

Analyzing Transmission Spectra of Fiber-Optic Elements in the Near IR Range to Improve the Security of Quantum Key Distribution Systems

Analyzing Transmission Spectra of Fiber-Optic Elements in the Near IR Range to Improve the Security of Quantum Key Distribution Systems

High‐Resolution Field Measurement of Soil Heat Capacity and Changes in Soil Moisture Using a Dual‐Probe Heat‐Pulse Distributed Temperature Sensing Approach

AbstractIncreasing interest in studying the variability of soil water content and its spatial scale dependency necessitates the development of new techniques to accurately monitor soil water over a wide range of spatial scales. Distributed Temperature Sensing (DTS) techniques offer unprecedented opportunities to measure temperature with a spatial resolution of a few centimeters over several kilometers, which can be used to measure soil moisture. This study is the first of its kind that investigates under field conditions the feasibility of combining the Dual‐Probe Heat‐Pulse (DPHP) technique with the DTS technology to measure soil thermal properties and variation in soil moisture. A field experiment was conducted over a 30 m transect in the Lake Wheeler Field laboratory in Raleigh, NC. Three different DPHP sensors were constructed from combinations of different fiber optic cables and heating elements and were tested to assess their performance and the effect of their construction characteristics on their accuracy. Measurements were taken over different soil moisture conditions and the system performance was compared against independent soil water content sensors. The system was able to track changes in soil water content with a mean RMSE of 0.02 m3 m−3 using the optimal DPHP sensor. The key advantage of the tested system is that it does not need any site‐specific calibrations typically required for other DTS‐based systems. The findings of this study provide some practical information and measures that need to be taken for successful DTS‐DPHP construction and application under field conditions.

Research on the Coupled Modulation Transfer Function of the Discrete Sampling System with Hexagonal Fiber-Optic Imaging Bundles

In this study, we developed a numerical model of the coupled modulation transfer function (coupled-MTF) based on the discrete sampling system from the perspective of optical system imaging quality evaluation for coupled two-dimensional discrete sampling characteristics of the hexagonally aligned fiber-optic imaging bundles and CCD image elements. The results show that when the spatial frequency of the input target signal deviates from the Nyquist frequency by 1%, an increase in the number of fibers leads to a faster convergence of the oscillation of the coupled-MTF, and the coupled-MTF converges to a stable value when the number of fibers reaches 1000 × 1000. The deviation of the spatial frequency of the input target signal from the Nyquist frequency is within 1%, and the oscillatory convergence of the coupled-MTF accelerates with increasing deviation. The coupled-MTF oscillates with the deviation period of the wave peak of the input target signal from the initial position of the fiber center, and the theoretical oscillation spatial period is twice the fiber diameter. This study produces important guidelines for the selection of the number of fibers, input spatial frequency, and initial position deviation of the hexagonally arranged fiber imaging bundles.

Formation mechanism of blemishes in a fiber-optic imaging element.

A rigid fiber-optic imaging element with high fidelity, high resolution, and high contrast is applied in low-level-light night vision and particle detection devices. Any optical fiber in elements is an image transmission unit. However, the independence and integrity of image transmission are disturbed constantly, resulting in blemishes. This paper studies the formation mechanism of the blemishes of rigid fiber-optic imaging elements. The existence of defects with different sizes in or among the optical fibers, and fiber deformation decreasing light transmission ability are simulated theoretically. Then experiments are carried out to verify the simulation results. It is theoretically concluded that a blemish forms when the equivalent sphere radius (RES) of a stone defect is greater than 1.2 µm in a normal fiber, and 1.1 µm in a taper fiber. The RES of a bubble defect is greater than 1.3 µm in the fiber, which can form blemishes. Excessive deformation of fibers behaving as a clad layer with thickness less than 0.3 µm also results in the formation of blemishes. When fiber deformation and the existence of stone are considered, blemishes can occur as long as the size of the stone defect among the fibers is greater than 0.823 µm. The results provide data support and theoretical guidance in solving blemishes and other fixed pattern noises, and significantly improve the engineering and application level of fiber-optic imaging elements.

Comparison of Pulse Wave Signal Monitoring Techniques with Different Fiber-Optic Interferometric Sensing Elements

Pulse wave (PW) measurement is a highly prominent technique, used in biomedical diagnostics. Development of novel PW sensors with increased accuracy and reduced susceptibility to motion artifacts will pave the way to more advanced healthcare technologies. This paper reports on a comparison of performance of fiber optic pulse wave sensors, based on Fabry–Perot interferometer, fiber Bragg grating, optical coherence tomography (OCT) and singlemode-multimode-singlemode intermodal interferometer. Their performance was tested in terms of signal to noise ratio, repeatability of demodulated signals and suitability of demodulated signals for extraction of information about direct and reflected waves. It was revealed that the OCT approach of PW monitoring provided the best demodulated signal quality and was most robust against motion artifacts. Advantages and drawbacks of all compared PW measurement approaches in terms of practical questions, such as multiplexing capabilities and abilities to be interrogated by portable hardware are discussed.

The effect of backside elements on the light emission of a plastic optical fiber assembly

With the recent increase in leisure time and wellbeing trends, the number of individuals who enjoy nighttime leisure activities has rapidly increased. Thus, the demand for luminous night safety clothing ensuring safety and that is also fashionable is increasing. Although higher visibility is important, studies on the influence of the backside elements of optical fiber materials on light emission are limited. The main components of the plastic optical fiber assembly in this study are the plastic optical fiber and backside elements, consisting of retroreflective material, and the backside sheet. The effects of the four types of retroreflective material, which are low, medium, high, and ultra-high luminance types, and the color of the backside sheet, white or black, on the luminance of the plastic optical fiber assembly are evaluated in bright, dusky, and dark environments. The results are analyzed using the statistical analysis software, SPSS 20.0. In the dusky environment, the plastic optical fiber assembly with prism material (80.78 ± 2.17) exhibits the highest luminance. However, there was no difference between the plastic optical fiber assembly with high- (38.52 ± 1.27) and medium-retroreflective material (36.76 ± 0.68). In the dark environment, the plastic optical fiber assembly with the prism material shows the highest luminance (66.83 ± 0.24), and there is no difference between the plastic optical fiber assembly with the low- (24.43 ± 0.42), medium- (24.63 ± 0.36), and high-retroreflective material (25.12 ± 0.42). Also, the luminance of the plastic optical fiber assembly with the white backside sheet is higher than that with the black backside sheet for all retroreflective material in dusky and dark environments. In summary, the optimal constitution of a plastic optical fiber assembly’s backside element is the retroreflective material of a prism type with the white backside color.

2D Propagation Simulation of Variation Parameters of U-shape Fiber Optic

Fiber optic has extraordinary properties and is suitable in sensor applications due to its special potential. Currently, macro bending characteristics of newly developed hetero core fiber optic element are designed and evaluated. This paper presents the preliminary results obtained from the numerical simulation analysis of the bending sensitivity of U-shape fiber optics toward the 2D electromagnetic wave in terms of mesh, curvature radius, core fiber size, and turn number. Fiber optics with core sizes of 4, 9, 50, and 62.5 μm were designed. In addition, the combination of core diameters 50-4-50, 50-9-50, 62.5-4-62.5, and 62.5-9-62.5 μm is evaluated to compare the outcome of transmission power in terms of hetero core structure of fiber optic. Simulation is performed using COMSOL Multiphysics simulation tool. The developed U-shape fiber optic is designed to sense the distortion of reducing power transmission by comparing input and output power. Results show that the selected mesh depends on the size of geometry bending fiber optics, and fine and finer mesh is the best for U-shape fiber optic. Furthermore, the power flow on the fiber decreases with the decreasing curvature radius and increasing turn number. The fiber with a core size combination of 62.5–4–62.5 um has high sensitivity in terms of loss. The attained results possess higher potential in the field of sensor applications, such as displacement, strain, pressure, and monitoring respiration, on human body. This study serves as a basis for further investigation of nanomaterial coating on fiber optics, thereby enhancing its credibility for sensing.

Methods of hot wire creation for fiber-optical thermal anemometer

Subject of Research. The paper presents the study of various methods of optical radiation coupling from the fiber core into its cladding with the aim of creating a fiber-optic anemometer hot wire. The effectiveness of the described methods is evaluated by analyzing the obtained dependencies. Method. The structures of two optical fibers were studied, one of which was used to transmit an information signal, and the other — to create a hot wire. A Bragg grating was formed in the core of the first fiber, and a radiation coupling region was created in the second fiber. These fiber sections were aligned with each other and fixed by a tin-lead alloy. The temperature was measured by monitoring the spectral characteristics of fiber Bragg gratings inscribed by the phase mask method. The areas of radiation coupling were formed either by changing the fiber geometry using custom modes of the arc fusion splicer, or by creating a SMF-MMF-SMF transition. Main Results. Structures are developed which can be used as a hot wire of a fiber-optic anemometer sensitive element. The dependencies of the Bragg resonance wavelength shift on the pump laser power are obtained; the Bragg wavelength shift varies in the range from 0.15 nm to 3 nm as a result of a hot wire creation. The resulting curves are constructed after three experiments for each type of structure, taking into account the standard deviation. They make it possible to judge the effectiveness of various methods of radiation coupling. Comparative analysis of the studied methods is presented. Practical Relevance. The study describes the capabilities of fiber-optic anemometry when creating point and distributed sensors. The results of this work can be used in the areas where the creation of quasi-distributed and point diffusers is necessary, as well as partial or complete coupling of radiation from a fiber core.

A Fiber Optic Interface Coupled to Nanosensors: Applications to Protein Aggregation and Organic Molecule Quantification.

Fluorescent nanosensors hold promise to address analytical challenges in the biopharmaceutical industry. The monitoring of therapeutic protein critical quality attributes such as aggregation is a long-standing challenge requiring low detection limits and multiplexing of different product parameters. However, general approaches for interfacing nanosensors to the biopharmaceutical process remain minimally explored to date. Herein, we design and fabricate a integrated fiber optic nanosensor element, measuring sensitivity, response time, and stability for applications to the rapid process monitoring. The fiber optic-nanosensor interface, or optode, consists of label-free nIR fluorescent single-walled carbon nanotube transducers embedded within a protective yet porous hydrogel attached to the end of the fiber waveguide. The optode platform is shown to be capable of differentiating the aggregation status of human immunoglobulin G, reporting the relative fraction of monomers and dimer aggregates with sizes 5.6 and 9.6 nm, respectively, in under 5 min of analysis time. We introduce a lab-on-fiber design with potential for at-line monitoring with integration of 3D-printed miniaturized sensor tips having high mechanical flexibility. A parallel measurement of fluctuations in laser excitation allows for intensity normalization and significantly lower noise level (3.7 times improved) when using lower quality lasers, improving the cost effectiveness of the platform. As an application, we demonstrate the capability of the fully integrated lab-on-fiber system to rapidly monitor various bioanalytes including serotonin, norepinephrine, adrenaline, and hydrogen peroxide, in addition to proteins and their aggregation states. These results in total constitute an effective form factor for nanosensor-based transducers for applications in industrial process monitoring.

Repetitive Detection of Aromatic Hydrocarbon Contaminants with Bioluminescent Bioreporters Attached on Tapered Optical Fiber Elements.

In this study, we show the repetitive detection of toluene on a tapered optical fiber element (OFE) with an attached layer of Pseudomonas putida TVA8 bioluminescent bioreporters. The bioluminescent cell layer was attached on polished quartz modified with (3-aminopropyl)triethoxysilane (APTES). The repeatability of the preparation of the optical probe and its use was demonstrated with five differently shaped OFEs. The intensity of measured bioluminescence was minimally influenced by the OFE shape, possessing transmittances between 1.41% and 5.00%. OFE probes layered with P. putida TVA8 were used to monitor liquid toluene over a two-week period. It was demonstrated that OFE probes layered with positively induced P. putida TVA8 bioreporters were reliable detectors of toluene. A toluene concentration of 26.5 mg/L was detected after <30 min after immersion of the probe in the toluene solution. Additional experiments also immobilized constitutively bioluminescent cells of E. coli 652T7, on OFEs with polyethyleneimine (PEI). These OFEs were repetitively induced with Lauria-Bertani (LB) nutrient medium. Bioluminescence appeared 15 minutes after immersion of the OFE in LB. A change in pH from 7 to 6 resulted in a decrease in bioluminescence that was not restored following additional nutrient inductions at pH 7. The E. coli 652T7 OFE probe was therefore sensitive to negative influences but could not be repetitively used.

Detection of blemish for fiber-optic imaging elements

The existence of blemishes deteriorates the imaging quality of fiber-optic imaging elements and reduces the yield of finished products in factories. The effective detection of blemishes is a prerequisite for analyzing the causes of blemishes and preventing their generation. An independently developed detection device for fiber-optic imaging elements based on machine vision is implemented to detect blemishes automatically and accurately. The blemish distributions of three typical fiber-optic imaging elements, including a fiber-optic plate (FOP), a fiber-optic inverter (FOI), and a fiber-optic taper (FOT), are compared with each other, and the causes of blemishes in fiber-optic imaging elements are analyzed. The distribution of blemishes in the FOP is random, and the average number (AN) of blemishes in the first zone (central zone) and the second zone (outer zone) with the same area is similar. The AN of blemishes for the FOI prepared by twisting the FOP at high-temperature increases from 272.8 (FOP) to 2125.0, and the AN in the outer zone is 1542.5, higher than that in the central zone. More blemishes are generated by the twisting process, and the distribution of these blemishes is close to the outer zone. A significant amount of chicken wires is found in the outer zone, but the majority of blemishes are still spots. The AN of blemishes for the FOT prepared by stretching the FOP increases from 383.0 (FOP) to 515.0, which is attributed to the increase of the number of blemishes in the first zone from 190.8 to 427.0, but the AN of blemishes in the second zone decreases from 192.3 to 88.0. The stretching process stimulates the formation of blemishes with a distribution that is close to the central ring. These blemishes are randomly distributed inside or at the boundary of the multifiber. Most of the blemishes are still spots.

Sub-shot-noise-limited fiber-optic quantum receiver

We experimentally demonstrate a quantum receiver based on Kennedy scheme for discrimination between two phase-modulated weak coherent states. The receiver is assembled entirely from the standard fiber-optic elements and operates at the conventional telecom wavelength 1.55 microns. The local oscillator and the signal are transmitted through different optical fibers, and the displaced signal is measured with a high-efficiency superconducting nanowire single-photon detector. We show the discrimination error rate two times below the shot-noise limit.

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The actual direction of maritime and inland water transport is alternative energy supply by the optoelectronic means. The study of brushless electromotor with electric energy delivery by the laser ray with using the fiber optic elements and photoelectronic transformers for the robotic digital complex for management and control of small-displacement surveying vessel motion is perspective. The connection between energy and information transmitted in the dynamic electromechanical transformers of the robotic digital complex is examined in the paper. It has been showed that the information and energy methods for processing the same experimental data lead to the identical results, namely, measuring information and energy are transmitted simultaneously through the measuring and information channel. Based on the conducted researches of brushless motor model and patents for inventions obtained by the Admiral Makarov State University of Maritime and Inland Shipping, a four-propellers robotic digital complex for managing and controlling the small-displacement self-propelled electric propulsion surveying vessel motion. The robotic small - displacement surveying vessels in conjunction with the base automatic surveying posts, combined by the atmospheric radio-optical and terrestrial fiber-optic communication channels, can be the basis of terrestrial segment of the digital measuring and information network of the Russian inland waterways.

Polarization properties of nematic liquid crystal cell with tapered optical fiber

Abstract In the paper, an extended analysis of the polarization properties of a liquid crystal cell with a biconically tapered single-mode telecommunication optical fiber was presented. These properties are a result of a sample geometry and used LC materials. They were analyzed by using two theoretical models based on the matrix decomposition methods, i.e., polar and singular-value one. By measuring Mueller matrices, information about losses, depolarization, dichroism and birefringence was obtained. In the experiment two types of tested samples filled with well-known 6CHBT and E7 liquid crystals were prepared and all optical parameters were shown as the voltage dependence. The tested samples have dichroic properties and for both models calculated PDL is similar and it increases from 2.6 to 6.6 dB for E7 and from 0.4 to 2.7 dB for 6CHBT with voltage changes within the range of 40 – 190 V. Optical losses simultaneously decrease from 30 dB to 27 dB and from 36 dB to 28 dB, respectively. The birefringence properties cannot be directly comparable due to differences between both applied models but voltage fluctuations of these parameters are not significant. These results confirm expected dichroic properties of designed device and complete knowledge about its working principles. Moreover, presented analysis validates usefulness of the singular-value decomposition model applied to dichroic optical fiber elements.

A multi-point voltage sensing system based on PZT and FBG

Optical fibers and fiber elements are being used in voltage/current sensing equipment for control and protection of smart grid in power system. In this paper, multi-point sensing system designs and their application scheme in power grid are presented. Since three-phase voltage is the basic voltage signal in power grid, a three-phase voltage sensing system based on fiber Bragg grating (FBG) and lead zirconate titanate (PZT) tubes is proposed. This voltage system sensing for 0.5 kV–5 kV (peak to peak) over frequencies from 50 Hz to 20 kHz, instead of only at 50 Hz or 60 Hz, was firstly conducted. Basic performance was then tested. System responses to 50 Hz voltage, sinusoidal voltage and triangular voltage at 50 Hz/1 kHz have been acquired and analyzed. In addition, a three-phase high-voltage test and a single-phase grounding test with three-phase transformers are performed and analyzed. The results demonstrate the system’s potential for distributed sensing and sensor network establishment with optical power ground wires (OPGWs) in smart grid.