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.
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.
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.
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.
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