Fiber Bragg Structures Research Articles

Ontology of Addressed Fiber Bragg Structures as a New Type of Sensor Elements

Ontology of Addressed Fiber Bragg Structures as a New Type of Sensor Elements

Fiber-optic temperature sensor based on glass Fabry – Perot interferometer

In this work, analytical expressions are obtained that allow one to determine the shift of the central wavelength of a four-component addressable fiber Bragg structure (FAFBS). The computer simulation of the use of CHAVBS in the problems of measuring the temperature of the windings of power transformers has been carried out. Simulation showed that more accurate temperature measurement results are obtained when the circuit operates in reflection, i.e. at an additional address frequency. With the use of FAFBS, there is no need to install additional frequency filters. It is not required to use address component recovery schemes compared to the three-component AFBS option. This fact is a significant difference between the developed systems based on AFBS and systems based on two- and three-component AFBS.

Comparative Analysis of the Methods for Fiber Bragg Structures Spectrum Modeling

The work is dedicated to a comparative analysis of the following methods for fiber Bragg grating (FBG) spectral response modeling. The Layer Sweep (LS) method, which is similar to the common layer peeling algorithm, is based on the reflectance and transmittance determination for the plane waves propagating through layered structures, which results in the solution of a system of linear equations for the transmittance and reflectance of each layer using the sweep method. Another considered method is based on the determination of transfer matrices (TM) for the FBG as a whole. Firstly, a homogeneous FBG was modeled using both methods, and the resulting reflectance spectra were compared to the one obtained via a specialized commercial software package. Secondly, modeling results of a π-phase-shifted FBG were presented and discussed. For both FBG models, the influence of the partition interval of the LS method on the simulated spectrum was studied. Based on the analysis of the simulation data, additional required modeling conditions for phase-shifted FBGs were established, which enhanced the modeling performance of the LS method.

Overview of Addressed Fiber Bragg Structures’ Development

An addressed fiber Bragg structure (AFBS) is a special type of fiber Bragg grating simultaneously performing the functions of a two-frequency radiation shaper and a sensitive element. An AFBS forms a two-frequency optical spectral response at its output, the difference frequency of which is invariant to measured physical fields and is referred to as the address frequency of the AFBS. Each of the AFBSs in the system has its own address frequency; therefore, a number of such structures can be interrogated simultaneously enabling the addressed multiplexing. In this article, we provide an overview of the theory and technology of AFBS, including the structures with three or more spectral components with various combinations of difference frequencies, both symmetrical and asymmetric. The subjects of interrogation of AFBSs, their fabrication and calibration are discussed as well. We also consider a wide range of applications in which AFBS can be used, covering such areas as oil and gas production, power engineering, transport, medicine, etc. In addition, the prospects for the further development of AFBS are proposed that mitigate the shortcomings of the current AFBSs’ state of the art and open up new possibilities of their application.

Addressed Combined Fiber-Optic Sensors as Key Element of Multisensor Greenhouse Gas Monitoring Systems.

The design and usage of the addressed combined fiber-optic sensors (ACFOSs) and the multisensory control systems of the greenhouse gas concentration on their basis are investigated herein. The main development trend of the combined fiber-optic sensors (CFOSs), which consists of the fiber Bragg grating (FBG) and the Fabry–Perot resonator (FPR), which are successively formed at the optical fiber end, is highlighted. The use of the addressed fiber Bragg structures (AFBSs) instead of the FBG in the CFOSs not only leads to the significant cheapening of the sensor system due to microwave photonics interrogating methods, but also increasing its metrological characteristics. The structural scheme of the multisensory gas concentration monitoring system is suggested. The suggested scheme allows detecting four types of greenhouse gases (CO2, NO2, CH4 and Ox) depending on the material and thickness of the polymer film, which is the FPR sensitive element. The usage of the Karhunen–Loève transform (KLT), which allows separating each component contribution to the reflected spectrum according to its efficiency, is proposed. In the future, this allows determining the gas concentration at the AFBS address frequencies. The estimations show that the ACFOS design in the multisensory system allows measuring the environment temperature in the range of −60…+300 °C with an accuracy of 0.1–0.01 °C, and the gas concentration in the range of 10…90% with an accuracy of 0.1–0.5%.

Addressed Fiber Bragg Structures in Load-Sensing Wheel Hub Bearings.

The work presents an approach to instrument the load-sensing bearings for automotive applications for estimation of the loads acting on the wheels. The system comprises fiber-optic sensors based on addressed fiber Bragg structures (AFBS) with two symmetrical phase shifts. A mathematical model for load–deformation relation is presented, and the AFBS interrogation principle is described. The simulation includes (i) modeling of vehicle dynamics in a split-mu braking test, during which the longitudinal wheel loads are obtained, (ii) the subsequent estimation of bearing outer ring deformation using a beam model with simply supported boundary conditions, (iii) the conversion of strain into central frequency shift of AFBS, and (iv) modeling of the beating signal at the photodetector. The simulation results show that the estimation error of the longitudinal wheel force from the strain data acquired from a single measurement point was 5.44% with a root-mean-square error of 113.64 N. A prototype load-sensing bearing was instrumented with a single AFBS sensor and mounted in a front right wheel hub of an experimental vehicle. The experimental setup demonstrated comparable results with the simulation during the braking test. The proposed system with load-sensing bearings is aimed at estimation of the loads acting on the wheels, which serve as input parameters for active safety systems, such as automatic braking, adaptive cruise control, or fully automated driving, in order to enhance their effectiveness and the safety of the vehicle.

Multi-Addressed Fiber Bragg Structures for Microwave-Photonic Sensor Systems.

The new theory and technique of Multi-Addressed Fiber Bragg Structure (MAFBS) usage in Microwave Photonics Sensor Systems (MPSS) is presented. This theory is the logical evolution of the theory of Addressed Fiber Bragg Structure (AFBS) usage as sensors in MPSS. The mathematical model of additive response from a single MAFBS is presented. The MAFBS is a special type of Fiber Bragg Gratings (FBG), the reflection spectrum of which has three (or more) narrow notches. The frequencies of narrow notches are located in the infrared range of electromagnetic spectrum, while differences between them are located in the microwave frequency range. All cross-differences between optical frequencies of single MAFBS are called the address frequencies set. When the additive optical response from a single MAFBS, passed through an optic filter with an oblique amplitude–frequency characteristic, is received on a photodetector, the complex electrical signal, which consists of all cross-frequency beatings of all optical frequencies, which are included in this optical signal, is taken at its output. This complex electrical signal at the photodetector’s output contains enough information to determine the central frequency shift of the MAFBS. The method of address frequencies analysis with the microwave-photonic measuring conversion method, which allows us to define the central frequency shift of a single MAFBS, is discussed in the work.

Helical Bragg gratings filter for orbital angular momentum wave

Effective filtration of orbital angular momentum (OAM) modes is essential to achieve multiple-channel data transmission in an OAM fiber for telecommunication applications. The nature of helical-shaped OAM wave propagation possessing many topological charges implies the need for uniquely designed index-modulated filters to conform to its twisted propagating direction in the fiber. Here, different from previous structures in demultiplexing OAM modes, a helical-shaped fiber grating structure concept is proposed as a pathway toward efficient demultiplexing of OAM modes in a fiber. A comparison of our proposed structure with the conventional fiber Bragg filter in transmitting an OAM wave shows a stronger reflection of the higher order OAM mode. We suggest that the proposed fiber Bragg structure has strong potential to be used for OAM fiber telecommunication technology.

Многоадресные волоконные брэгговские структуры в радиофотонных сенсорных системах

The article describes the transition concept from addressable fiber Bragg structures and microwave-photonics sensor systems based on them to multicast fiber Bragg structures. The difference between multicast structures and address structures is that in the fiber Bragg structure formes three or more super narrow-band frequency components, spaced from each other by the microwave frequency. The central frequencies shift of multicast Bragg structures is determined by the result of processing the signal of optical frequencies beats at the photodetector, which parameters judge the applied physical fields. We see the solved problem of uniquely determining the central (Bragg) frequency shift of the multicast fiber Bragg structure, with a unique set of address frequencies.

Combined Brillouin sensor system for simultaneous local and distributed temperature and strain measurements for downhole telemetry

The work proposes and experimentally demonstrates the feasibility of creating a combined sensor system for simultaneous local and distributed temperature and strain measurements using the Brillouin optical frequency distributed analysis. The system uses a quasi-distributed approach based on two-element fiber Bragg structures (TEFBS), which makes it possible to carry out simultaneously distributed and local measurements of temperature. The proposed highly integrated circuit uses a conventional narrow-band optical source corresponding to the spectral range of temperature measurements, and a common receiving module on the same sensor fiber. For distributed detection, a single-mode optical fiber is used, and for the local detection, two TEFBS pass-through and reflective types are used.

Distributed Raman sensor system with point spots for downhole telemetry

The article explores the use of two-element fiber Bragg structures as point spots for a distributed downhole telemetry, namely, to calibrate Raman systems and to refine their readings for distributed temperature, and additionally get values of pressure at the end of probe fiber. For this purpose, a structural scheme of a distributed-pointed sensor system was proposed that combines a distributed temperature sensor based on incoherent optical frequency reflectometry using Raman scattering with a point thermomanometers based on two-element fiber Bragg structures for observing wells at a wellhead and at the end. Thus, according to the results of the research, the following principles are proposed: the principles of building combined fiber optic systems for the downhole thermometry to solve the problems of compensating for an influence of temperature in point manometry and to clarify the readings of Raman distributed sensors at their key points.

Modelling and record technologies of address fibre Bragg structures based on two identical ultra-narrow gratings with different central wavelengths

Address fiber Bragg structures (AFBS) make it possible to effectively solve the problem of sensors interrogation and multiplexing in multi-sensor networks with microwave photonic processing of information. Based on the method of inverse Fourier transform, a mathematical model of the optical fiber refractive index profile was constructed to form 2λ-FBG AFBS with two FBG with identical spectral responses at separated wavelengths. As the initial parameters for the construction of the mathematical model, the desired spectral profile of 2λ-FBG AFBS was specified, including the reflection coefficient and the width of the transmission band of its two identical ultra-narrow-band gratings and the separation between them. On the basis of the study of the mathematical model, the possibility of selecting the necessary values of the refractive index and the laws of its modulation is shown, allowing the spectral profile of 2λ-FBG AFBS to be formed so that they can be used as a sensitive element, transforming information from optical range to radiofrequency one. The analysis of the formation and recording methods for 2λ-FBG AFBS was carried out. To implement given structures, the technology, using of an ultraviolet argon laser, the classic phase masks with sequential recording of several arrays with precise movement and strain of the fiber were chosen. Further paper deals with issues of interrogation of the developed structures and principles of measurement system construction on their basis.

Modelling and record technologies of address fiber Bragg structures based on gratings with two symmetrical pi-phase shifts

Address fiber Bragg structures (AFBS) make it possible to effectively solve the problems of interrogation and multiplexing of sensors in multi-sensor networks with microwave photonic processing of information. Based on a complex method of transmission matrices and the coupling of directional modes, a mathematical model was constructed to determine the spectral profile of a fiber Bragg grating with two discrete symmetric phase π shifts (2π-FBG). Based on the study of the mathematical model, the possibility of selecting the necessary parameters of 2π-FBG AFBS is shown, which allow forming its spectral profile in such a way that the specified structure can be used as a sensitive element of the sensor and provides the necessary linear displacement in the optical range and preserves the required frequency separation – address – between discrete symmetric phase shifts location in the radio frequency range. The analysis of the formation and recording methods for 2π-FBG AFBS was carried out. To implement given structures, the technology, using of an ultraviolet argon laser, the classic phase masks with sequential recording of several arrays with precise movement of the fiber were chosen. Further paper deals with issues of interrogation of the developed structures in few- and multi-sensor implementations.

Fiber-optic sensors for complex monitoring of traction motors

The paper presents the results of solving a scientific and technical problem related to the development of the fundamentals of building fiber-optic measuring systems for integrated monitoring of the state of a traction motor based on fiber Bragg structures that allow to measure temperature, wear and vibration of a brush-collector unit. Mathematical models of the process of measuring these parameters were developed and tested, a solution was proposed for constructing devices for interrogating such sensors.

Addressed fiber Bragg structures in quasi-distributed microwave-photonic sensor systems

The article describes the theory and technique of addressed fiber Bragg structures and a new class of microwave-photonic sensory systems based thereon, the distinctive feature of which is that the fiber Bragg structure forms two ultra-narrowband frequency components separated by a unique address frequency spacing. The offset of the central frequencies of the Bragg structures is determined via processing a beat signal of the address frequencies on the photodetector, with its parameters making it possible to evaluate the physical fields applied. We formulate and solve a problem of unambiguously determining the central (Bragg) frequency shift of the addressed fiber Bragg structures with unique address frequencies and the same Bragg frequency. These are then combined into a single multi-sensor system with multiplexed response reception on a single photodetector.

Fiber-Optic Acceleration Sensor on Duplex Fiber Bragg Structures

We propose an assessment of the possibility to use narrow-band fiber Bragg structures as the basis of a measuring system for an acceleration sensor (accelerometer). The assessment of applicability is carried out on the basis of mathematical and computer simulation and analytical assessments of the model's working capacity. We propose to solve the problem by transferring a signal from the optical (THz) to the radio frequency (GHz) range of measurements due to the theory and technology of radiophoton methods. The result is a significant increase in the signal-to-noise ratio. We suggest to use the measuring system based on two narrow-band fiber Bragg gratings with the following two properties: central frequencies are in the optical range, and a difference of central frequencies is in the radio frequency range of the spectrum. The optical signal obtained in the optoelectronic circuit is fed to a photodetector. An analysis of the resulting electrical signal allows to make a conclusion on the magnitude and direction of the force acting along the axis of the sensor sensitivity. The magnitude of the effective inertia force is directly related to the acceleration acting on the measuring system. We propose an approach to determine the magnitude of the effective force. The approach does not require measurement of the intermediate radio frequency, and determines the dependence of the effective force on the magnitude of the dual-frequency beats modulation factor. We propose to implement the design of the sensor in such a way as to minimize the effect of temperature deviations on the measuring system as a whole.

Tunable acoustic bursts for customized tapered fiber Bragg structures

The inscription of customized fiber Bragg structures under acoustic excitation is proposed and demonstrated. By setting the proper acoustic excitation function, the desired spectrum of the grating can be achieved. In this paper, the effect of applying a burst acoustic wave during the process of Bragg grating inscription using the direct writing method through a phase mask is investigated. Among all the characteristics that can be changed due to a periodic defect insertion in the uniform Bragg structure, the control of phase is used to demonstrate this assertion. The results of numerical simulations are shown to be in excellent agreement with experiments. It can be further extended to generate multiple phase shifts in optical fiber and/or waveguides, if the defects are inserted at different positions, as is shown.

Compound Phase-Shifted Fiber Bragg Structures as Intrinsic Magnetic Field Sensors

We report on the simulation and characterisation of compound phase-shifted fiber Bragg structures for use as novel in-fiber magneto-optical point sensors. Through simulation we show that the Faraday rotation spectra of phase-shifted gratings can be tailored by tuning the ratio of substructure lengths. A design process for tailoring the magneto-optical spectrum is illustrated, and it is shown that a general optimum structure exists for producing a region of total reflection incorporating magneto-optical Faraday rotation that is enhanced both in strength and spectral width. A practical optical fiber system that exploits the proposed distributed feedback (DFB) structures to enable novel all-fiber sensors for the dual measurement of magnetic field strength and temperature is described in detail, and the sensor response is simulated. The study is supported by laboratory fabrication of the proposed fiber DFB structures which demonstrates the principle of enhancement in terms of tailored group delay spectra and highlights practical issues for sensor packaging.