The work presents an overview and classification of combined fiber-optic resonant structures, which are essentially a combination of a Fabry-Perot interferometer (FPI) and a fiber Bragg structure. The latter can be represented either as a conventional fiber Bragg grating (FBG), or an addressed fiber Bragg structure (AFBS) of various types. It is shown that such combined resonant structures can serve as sensing elements of various fiber-optic measurement systems, detecting such physical impacts as temperature, pressure, acoustic waves, etc. One of the proposed structures is a fiber optic FabryPerot interferometer based on an open macro cavity at the end face of an optical fiber. Immersing the end face of the optical fiber with macro cavity in liquid leads to the formation of an air bubble where interfacial surfaces act as FabryPerot mirrors, which allows to use the structure for detecting pressure or acoustic waves in liquids. Another considered type of an FPI is composed of layer of borosilicate glass at the end of an optical fiber, acting as a temperature sensor. It is demonstrated that the combination of FPI and fiber Bragg structure can be used as a unified basic element for the various optical fiber sensors, utilizing the Fabry-Perot cavity at the end face of the fiber and the Bragg structure as fundamental sensing elements.
The work presents a concept of a fiber-optic sensing element, which allows to simultaneously measure humidity, pressure and temperature, and is based on a multi-layer Fabry-Perot interferometer (FPI). The basic element of the proposed FPI is a droplet-shape air-filled cavity obtained using the catastrophic fuse effect of a single-mode fiber. The opening of the cavity is covered with a diaphragm composed of a double-layer film, the first (inner) layer of which has an increased thermo-optical coefficient, while the second (outer) layer’s refractive index is sensitive to humidity of the ambient medium. The outer pressure causes the deflection of the diaphragm, which leads to the change of the length of the air-filled cavity. The work also discusses the requirements for the parameters of the materials constituting the sensor, based on the results of the reflected optical spectrum modeling of the sensing structure at various ambient conditions, which would provide the desired measurement performance.
This work considers the issue of creating and promoting student communities focused on attracting young people to modern scientific work and development, in particular in the field of photonics and fiber optics. The materials of the article are based on the experience of implementing the project "Student Community of Photonics and Fiber Optics" and are devoted to management issues in the context of a well-established hierarchical approach to management in classical universities. In the course of the work, tasks were set and ways to solve them were described. Particularly important aspects of the influence on the future career guidance and career aspirations of the youth groups under consideration are reflected.
The article presents further development of radiophotonic sensor systems based on the usage of the addressed (AFBS) and multi-addressed fiber Bragg structures (MAFBS) as sensing elements. Addressed fiber Bragg structure is an emerging type of fiber Bragg grating, which has a spectral response with two or more narrow notches. The presented system comprises two optical filters with linear inclined frequency response, which are used simultaneously to determine the central wavelengths of the AFBSs or MAFBSs. This feature enhances the operating temperature range of the system and allows to simplify its implementation, since the necessity of temperature isolation of the optical filters is eliminated. The algorithm and the mathematical model for the thermal drift compensation of the optical filters are introduced, and their realization is discussed.
The paper discusses mathematical modeling of the optical spectrum of the addressed fiber Bragg structure. The approach uses Lorentz function to describe the spectrum of the transparency window of the addressed structure with two symmetrical π-phase shifts. The approach allowed to define the influence of the width of transparency window on the amplitude and the full width at half maximum of the address frequency spectral component. The obtained relations can be used to estimate the bandwidth of the electric bandpass filters necessary for signal detection, as well as the desired level of optical power of the light source.
The article discusses a possibility of modeling responses of addressed fiber Bragg structures (AFBS) using the normal
Gauss distribution function. The presented study is motivated by the fact that for the reliable operation of the microwave photonic measurement system based on the use of address structures with two identical narrow-band bandwidth responses (2λ-FBG), it is necessary to ensure that a bandwidth of an inclined filter covers the operating range of the AFBS. Since this filter is necessary for estimating a shift of a central wavelength of the AFBS by determining a change in amplitudes of oscillations at a photodetector, which allows measurements of various physical fields without the use of expensive interrogators.
The article discusses a possibility of using fiber-optic sensors based on addressed fiber Bragg structures (AFBS) to assess tire dynamics in real operating conditions. The presented study is motivated by the fact that vehicle dynamics control systems require reliable and cost-efficient sensors for measuring forces acting in the tire contact patch. In this regard, a description of the new technology of multi-sensor measurements for continuous monitoring of tire state is presented. The experimental sensor prototypes use AFBSs with two identical ultra-narrow-band reflection spectra (2λ-FBG). Information from the sensors is implemented as input parameters of algorithms, which allow to estimate key tire-related characteristics, such as pressure in the contact patch, instantaneous angular velocity and effective wheel radius. One of advantages of this technology as compared to analogous devices is the use of an optical photodetector as an interrogator that greatly simplifies the system.
The work presents a concept of microwave-photonic sensor system (MWPSS) for real-time measurement of vehicle tire strain. The proposed MWPSS is based on the addressed fiber Bragg structures (AFBS) with two symmetrical π-phase shifts (2π-FBG). The system utilizes two similar 2π-FBGs with equal bandwidths, the same central Bragg wavelength, and unique address frequency spacing, thus realizing the microwave-photonic measurement methods and providing their address. One of the 2π-FBGs serves for strain measurement, and the second one takes into account the effects of temperature variation. Address frequency analysis of the multiplicative optical response allows to define the central Bragg frequency shift of each sensor and to correlate it with the value of the influencing physical fields (strain and temperature). These values are the input parameters for the estimation system of tire-road contact characteristics and vehicle dynamics control.
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