The main objective of this paper is to create scientifically based principles for design of two-component asymmetric address fiber Bragg structures (λ+λ/π)-AFBS of the combined type. As can be seen from the name of such a structure, it consists of two sequentially installed FBG and FBG with a phase shift, having different wavelengths spaced apart at the address frequency. Additional requirements for the structure of an asymmetric (λ+λ/π)-AFBS are defined as the same bandwidth of both FBGs, which will determine the difference in the power of radiation reflected from them without and with a transparency window. Wave FBG, λ-FBG, is an important part of fiber-optic sensors, which has many advantages, such as compact structure, the ability to organize quasi-distributed measurements, low transmission losses and resistance to electromagnetic interference. In addition, a conventional λ-FBG is practically insensitive to bending and the external refractive index. This makes measuring strain, temperature and other parameters more convenient and accurate. FBG with a phase shift, in particular with a phase shift of π, π-FBG, has attracted widespread attention. π-FBGs not only have the advantages of λ-FBGs, but also have many special sensory characteristics, which are determined by different quantities and methods of generating the phase shift. It should be noted that the presence of narrow-band transparency windows in the π-FBG makes it possible to increase the resolution of sensors compared to the λ-FBG by a number of times equal to the ratio of the transmission bands of the λ-FBG and the transparency window of the π-FBG. Therefore, they have high advanced applied value in various sensor systems. Two-component asymmetric (λ+λ/π)-AFBS of the combined type according to our evaluations should allow us to obtain a synergistic effect from their use in various systems, including intellectual energy systems built according to the Smart Grid Plus concept.
The main objective of this paper is to create scientifically based principles for the development of two-component symmetrical wave address fiber Bragg structures with a phase π shift (2λ/π-AFBS) as new sensitive elements of microwave photonic sensor systems. As can be seen from the model of such a combined structure, it consists of two sequentially installed FBGs with a phase shift. Additional requirements for the structure of a symmetrical 2λ/π-AFBS are defined as the same bandwidth of both FBGs and the transparency windows in them. In asymmetrical 2λ/π-AFBS these values may differ from each other. Phase Nπ-AFBS and wave Nπ-AFBS have their advantages and disadvantages. The corresponding development and elimination of their shortcomings is aimed at the creation of 2λ/π-AFBS with a certain frequency difference between the Bragg frequencies of the first and second FBGs, which is called the main address frequency for transmission, and at which the central wavelengths of the transparency windows are located. The width of the transparency window at the level of maximum reflection of symmetrical FBG components allows the formation of an additional address frequency for reflection, which is one order of magnitude lower in frequency than the main one. In FBG components, transparency windows that are not identical in bandwidth can also be formed, and, accordingly, different additional address frequencies. Two-component symmetric 2λ/π-AFBS of the combined type according to our evaluations should allow us to obtain a synergistic effect from their use in various systems, including intellectual energy systems built according to the Smart Grid Plus concept.
It is well known that the speed limitations inherent in conventional electronics can be overcome using photonic circuits, one of the main building blocks of which is a time differentiator. This device provides the time derivative of the complex envelope of an arbitrary input optical signal. Possible applications include optical pulse shaping, optical computing, information processing systems and ultra-high-speed coding, among other applications. The paper discusses the principles of constructing fractional order differentiators. Using the example of an asymmetric fiber Bragg grating with a π-phase shift operating in reflection, based on addressed fiber Bragg structure of Moiré type, the possibility of implementing a photonic fractional differentiator is shown. We present modeling of the characteristics of the spectral structures of a differentiator, which uses gratings of the same length, but with different modulation depths on both sides of the localization of the π-phase shift, using a new method of homogeneous layers.
Based on the analysis of methods and devices development for all-optical differentiation of signals and addressable fiber Bragg structures as sensitive elements of sensors, the possibility of combining the advantages of the latest versions of their implementation based on symmetric Moiré Bragg gratings has been found. For differentiators, which, as a rule, are considered as separate devices, the issues of their network multiplexing for a single module of photonic processing and computing of multiple signals, as well as the stability of differentiation characteristics with deviations in the temperature regimes of the laser and differentiator with a corresponding mismatch of their central wavelengths, are not touched upon. The advantages of microwave photonic processing of information used for addressable fiber Bragg structures will make it possible to solve the above problems of photonic differentiators when they are combined into a network. This conclusion is proved by the procedure presented in the report for the synthesis of addressable fiber Bragg structures of the Moiré type and the unity of their structure with the structure of photon differentiators.
The article proposes the concept of constructing a comprehensive fiber-optic sensor (CFOS) for simultaneous measurement of relative humidity and air temperature, as well as the temperature of a solar cell. The sensor is represented by a double structure, consisting of a two-stage Fabry-Perot interferometer for measuring relative humidity and air temperature and a two-component wave addressable fiber Bragg structure for measuring the temperature of a solar cell. The sensor is inserted orthogonally to the plane of the solar photovoltaic panel into the technological hole formed in it so that the distance between the addressable fiber Bragg structure(s) and the Fabry-Perot interferometer module allows simultaneous measurement of relative humidity and air temperature, as well as the temperature of the solar panel. A change in relative humidity affects only the refractive index of the outer interferometer, while a change in air temperature affects the refractive index of both interferometers. Changing the temperature of the solar cell only affects the central wavelength of the addressable fiber Bragg structure. By solving a system of equations using the specified parameters, relative humidity and air temperature, as well as the temperature of the solar battery, can be controlled simultaneously. The structure and design of an integrated fiber-optic sensor and the results of the first experiments are presented, which confirmed the possibility of simultaneous measurement of the parameters under consideration that affect the efficiency of solar panels.
Universal problem of spectral characterization for element base of microwave photonic means from broad band electrooptic and optical electronic modulators and receivers to narrow band Bragg and Fabry-Perot filters is very actual. The presented modulating and forming devices contain, in a minimal structure, a special laser diode and two modulators, the first of which is calibrated (master) and has a significant impact on the accuracy of modulation and forming in second (functional), and ensuring the operating modes of both modulators affects the complexity of the devices structure. The last one element is photoreceiver with given characteristics as laser diode ones. The purpose of the work is to present developed devices for various implementation with a minimized structure and simple control, which allow achieving results similar to those obtained using complex multi-stage, two-port, dual, parallel, polarization divided Mach-Zehnder modulators, accompanied with acousto-optic and polarization modulators, all of which have become unavailable under the sanctions pressure. Their advantages can become more significant using microminiaturization technologies – combined microwave and photonic, photonic and quantum integrated circuits (PIC), developing in KNRTU-KAI and UUST, especially under the influence of external climatic factors. These aspects are discussed in the report using the examples of instantaneous frequencies measurements, broadband spectral characteristics of modulators and photoreceivers, quantum key distribution systems with frequency coding. Few words are said about sensor networks.
To eliminate the limitations of known optical thermal anemometers, distributed and point fiber optical flowmeters, we determine the prerequisites to design a microwave photonic dual-FBG differential sensor for flow velocity and direction based on like-FBG gauge – phase shifted fiber Bragg gratings or addressed fiber Bragg structures of wavelength or combined wavelength-phase type. In addition, we offer an economical microwave photonics addressed interrogation scheme that does not require complexed optical spectral analysis. The article evaluates the model of proposed sensor and its characteristics in static and dynamic flow conditions using two different schemes for set-up of similar or different gauges. The aim of the work is to search for scientifically based principles for the development of microwave photonic flow and flow direction fiber optic sensors, with the possibility of increasing the resolution and accuracy of measurements in the region of low flow rates, as well as temperature compensation. To achieve the aim, the typical with similar gauges and non-typical with different gauges liquid flow sensors based on the measurement of differential pressure by optoelectronic methods are considered, and the problem setting for determining the flow direction using microwave photonics approaches is given as one of the options for their development. A universal mathematical model for a microwave photonics approaches is considered and the requirements for choosing an element base other than classical FBGs are determined. The possibilities of using flowmeters with a phase π-shift FBG and addressable fiber Bragg structures as sensors, which at first glance could improve their metrological characteristics, but are not widely used for this, are discussed. The obtained results and directions for further research presented in conclusion.
Seven years ago, we proposed the concept of addressed fiber Bragg structures (AFBS), which simultaneously perform the functions of: a complexed sensitive element based on two FBGs (2λ-AFBS) with different Bragg frequencies or FBG with two π-phase shifts (2π-AFBS), the difference frequency of which is the AFBS address and the value of it is invariant to measured physical fields; a two-frequency laser radiation source, which can operate as in reflection, so as transmission mode respectively to structure above, a self-multiplexed set of sensors, if the difference frequency will be unique for each AFBS, enabling their address multiplexing. In this article, we consider the ontology of AFBS, including the parent structures with 2λ- or 2π-components, successor AFBS with three spectral components and various combinations of difference frequencies: symmetrical and asymmetric, performing the functions of the addressing and converting information signals to the low-frequency region at the same time, along with the functions of rejecting collisions caused by the relative movement of structures relative to each other during measurements. The subjects of interrogation of these structures and their calibration are discussed as well as prospects of AFBS further development based on common tasks born by ontology formalization and decisions of applicability tasks.
Fiber Bragg gratings (FBGs) have attracted considerable attention and have been used to measure various physical parameters. The best known is the use of FBGs to measure structural strain in a direction parallel to the optical fiber. However, it is also possible to use an FBG to measure the load in the direction transverse to the optical fiber. The paper presents scientifically based principles for constructing and results of experimental demonstration of polarization fiber optic sensors for monitoring of transversal load of a new type built on two addressed fiber Bragg structures (AFBS) with a microwave photonics interrogation. AFBS with two phase shifts have two ultra-narrow transmission peaks in the reflection band, which determine its address, expressed in GHz. When a transverse load is applied to AFBS, the transmission peaks will be divided into two due to the difference in the change of the fiber core refraction indices. The distance between the wavelengths of the separated peaks with two polarizations will change when the transversal force changes. The measured dependence between the value of the transverse force and the frequency of the beat-generated microwave signal shows very good linearity. The new sensor has the possibility to increase the resolution and sensitivity of measurements, as well as temperature compensation through the use of various AFBS of wavelength type. The range of measured forces is up to 100 N, the absolute measurement error is 0.1 N, frequency separation sensitivity is 0.1 GHz/N.
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