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.
This work presents results of test series, performed for earlier on designed and successfully fabricated twisted silica fewmode microstructured optical fibers (MOF) with six GeO2-doped cores. While Part I introduces results of differential mode delay map measurements, Part II is focused on researches of spectral responses, measured for fiber Bragg gratings, recorded in these multi-core MOFs with core graded refractive index profiles and induced twisting 100 revolutions per meter. Specially setup for spectral response measurement for described complicated fiber optic element was developed, that provides selected alignment of matching singlemode optical fiber with particular single core of MOF via free space and reducing of reflection by precision 8 angle cleaving. Comparing analysis of measured spectral responses confirmed written FBGs in 2 of 6 cores, and demonstrated potentiality of fabricated complicated structure, containing multi-core MOF with FBG, for applications in multichannel fiber optic sensors with spatial division multiplexing technique.
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.
This article describes a new method for the Doppler frequency shift (DFS) measurement of a radar microwave signal reflected from a moving object, based on radio photonics technologies. The DFS measurement device has the same structure as the sequential radiophotonic link with filtration and consists of a laser, a block of electro-optical modulators, a fiber Bragg grating (FBG), and a photodetector. The block of electro-optical modulators, in contrast to the known solutions based on a two-port Mach-Zehnder amplitude modulator, is based on two subunits, consisting of connected tandem single-port amplitude and phase modulators (TAPM). The general structure of the TAPM subunits is parallelserial. The microwave signal reflected from the object arrives at the first TAPM, which forms the measurement channel. The second and third TAPMs, connected in series, form a reference channel connected in parallel to the measurement one. The second TAPM receives a reference signal from the locator transmitter at the probing microwave frequency, after which the two-frequency radiation, spaced by twice of the probing frequency, is fed to the third TAPM, which generates from each component of the two-frequency radiation two more with a difference frequency equal to twice the maximum possible DFS. The beats of signals from the measurement and reference channels at the output of the photodetector are three high-frequency (GHz) or low-frequency (MHz) electrical signals, the frequencies and powers of which used for the DFS determination.
This article describes a new approach for the estimation of the direction or of the microwave signal reflected from the object, based on radio photonics technologies. The angle of arrival measurement device has the same structure as the classical fiber-optic communication channel and consists of a laser, a block of electro-optical modulators and a photodetector. The block of electro-optical modulators, in contrast to the known solutions based on a two-port Mach- Zehnder amplitude modulator, is based on two parallel subunits, consisting of tandem single-port amplitude and phase modulators (TAPM). A microwave signal reflected from the object with a time delay, the value of which is determined by the AOA, is sequentially received at the radio frequency inputs of two TAPMs through the receiving antennas connected to them. In this case, the initial components of the laser carrier at the output of the TAPM subunits of both channels are completely suppressed, which significantly distinguishes the proposed solution from the known ones for the better in terms of increasing the measurement accuracy. The beats of the output signals of the TAPMs at the output of the photodetector represent a signal reflected from the object, according to the power of which the AOA can be determined.
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.
This work reports the results of bandwidth measurements and tests, performed for earlier on designed and fabricated pilot lengths of new silica laser-optimized graded-index multimode fibers (LOMFs) with extremely enlarged core diameter up to 100 μm and "typical" "telecommunication" cladding diameter 125 μm. Presented optical fibers are targeted for harsh environment short-range multi-Gigabit onboard cable systems and industrial networks. Proposed LOMF 100/125 differs by specially optimized graded refractive index profile, that provides low differential mode delay (DMD) for selected guided modes. We present some results of tests, performed for fabricated pilot 520 m length of described LOMF 100/125, focused on researches of bandwidth features. They contain DMD map and transfer function measurement as well as 10GBase-LX/SX channels eye-diagram and bit-error-ratio reports with following direct detection of the maximal acceptable optical fiber length for guaranteed 10Gbps channel supporting.
This work presents some results of pulse and spectral responses, performed for laser-excited pilot sample of earlier on designed and fabricated chiral silica few-mode optical fiber (FMF) with induced twisting 66 revolutions per meter (rpm), typical (for telecommunication optical fibers) cladding diameter 125 μm, weakly increased core diameter up to 11 μm and numerical aperture NA=0.22, corresponding to improved height of quasi-step refractive index profile. Described FMF 11/125 provides propagation of 4 and 6 guided modes over “C” and “O”-bands, respectively. We present results of tests, focused on researches of few-mode effects, occurring under laser-excited optical signal propagation over pilot sample optical fiber, and their influence on pulse and spectral responses, including some measurements, performed for fiber Bragg grating, recorded in tested FMF 11/125.
A high-precision symmetric optical vector analyzer (OVA) based on amplitude-phase double sideband (DSB) modulation with suppression of an optical carrier is proposed and numerically simulated. Accurate and stable frequency characteristics are achieved by the formation and minimization of such higher-order components in the probe radiation, the difference frequency of which does not coincide with the difference frequency of the two main measuring components of the first order. In comparison with the known solutions, the processing of spectral information is carried out at the modulation frequency, and not at the doubled frequency; the operating range of OVA has been increased by 2 times in terms of the bandwidth of the tested devices; instead of the standard Pound-Drever-Hall technique used to stabilize the laser wavelength with an additional modulator, the technique of controlling the amplitude and phase of the beating envelope of the main probing components is used; the relative error in the reconstruction of the frequency response of the high-Q optical structure at each point does not exceed 5×10–5. The obtained characteristics are confirmed by numerical modeling of the developed OVA when characterizing the spectral parameters of a narrow-band fiber Bragg grating. The presented method is structurally simple, does not require the use of various generators to perform its main functions and monitoring functions, and can be used to characterize both selective fiber-optic structures and high-Q optical Fabry-Perot resonators, resonators based on whispering modes, ring active and passive resonators, transparency windows of nonlinear crystals, plasmon resonances, etc.
The method for numerical simulation of measuring the concentration of particles of two types in a liquid-based on Rayleigh scattering during their deposition is proposed. The mathematical model takes into account the forces of gravity, hydrostatic lifting force, forces of resistance to motion. Additionally, the contribution of the influence of Brownian liquid motion on the movement of suspended particles was taken into account. The magnitude of the scattering of the light flux as it passes through the volume of liquid with particles suspended in it is modeled. A numerical calculation was carried out according to the developed mathematical model.
The article presents the research results of the modern radar complexes modernization possibility due to the use of methods and elements of microwave photonics in the devices of signals formation for heterodynes and transmitters of radar stations. Application of high potential active phased antenna array with multichannel spatial processing of received signals, based on the use of microwave photonics technology, will significantly improve the basic tactical and technical characteristics of the radar: range resolution, information value, detection range, noise immunity and power consumption. Realization of the listed advantages is reached by introduction of elements, functional knots and components of active phased antenna array subsystems because of electro-optical modulators and demodulators, sources and receivers of optical radiation, optical signal amplifiers, optical delay lines, fiber-optic, and optical passive elements.
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 paper presents the concept of a microwave photonic measuring system based on addressed fiber Bragg structures with a polarization multiplexed response. Mathematical models of response of such structures have been developed and verified by computer and physical modeling. The possibility of constructing transverse load sensors that do not require the introduction of additional sensors for temperature compensation of the Bragg grating wavelength shift is shown. A device for interrogating such sensors is proposed and verified.
In this paper, a two-element sensor of relative humidity of a parallel structure is proposed for consideration, which differs from the existing ones by using address fiber Bragg gratings made in SMF-28 fiber. One of the grids has a quartz shell replaced by polyimide, synthesized using a fiber coating with a full multiplicative response to temperature and deformation caused by humidity. The second grating recorded in a standard fiber and responds to temperature. It is possible to include an additional third grating with a partially etched cladding, which can be used for refract metric measurements of the amount of condensed moisture on the elements of a complete switchgear. All gratings are identical, have the same Bragg wavelength, after manipulating their claddings, but differ in a unique address, which is formed by recording two transparency windows in each of the gratings with different difference frequency space. The transparency windows correspond to phase π-shifts symmetrically located at the same distance from the center of each of the gratings. The structure obtained makes it possible to record information of the measurement conversion at the indicated difference frequencies in the radio range, which significantly increases the speed of measurements of relative humidity and their accuracy by an order of magnitude. In addition to what has been said, it is possible to note the possibility of building a network of these sensors in consecutively arranged switchgear devices, with a different radiofrequency address group being used in each of them.
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 paper discusses approaches to the numerical integration of the second-kind Manakov equation system. Emphasis is placed on the transition from writing equations in dimensional quantities to equations in dimensionless units. A combined explicit-implicit finite-difference integration scheme based on the implicit Crank- Nicolson finite-difference scheme is proposed and substantiated, which allows integrating a non-linear system of equations with a choice of non-linear term at the previous integration step. An algorithm for leveling the disadvantage associated with the definition of the nonlinear term from the previous integration step is proposed. The approach of automatic selection of the integration step, which reduces the total number of integration steps while maintaining the required accuracy of the approximate solution, is substantiated. Examples of the calculation results for some values of the disturbance propagation are given. The limitations imposed by the scheme on the length of the integrable fiber section are described, and approaches are proposed that eliminate these limitations without the need to increase the dimensions of the finite-difference scheme arrays. Requirements for initial boundary conditions were discussed.
New approaches are presented to the process of measuring the chromatic dispersion of a high-speed communication channel based on the classical method for determining the ratio of the radio frequency signal power. The power ratio is recorded at the output of two identical photodetectors, one of which is equipped with a Bragg notch filter at the carrier frequency. New approaches are based on preliminary amplitude modulation of an optical carrier with a radio clock signal frequency with the formation of an additional two-, three or four-frequency radiations to determine the shift between the central wavelengths of the communication channel and the Bragg notch filter, and also the use of methods to increase the signal-to-noise ratio of the measurements. The registration of information at the probing frequency or doubled probing frequency allows obtaining a gain in the signal-to-noise ratio of measurements up to 10-13 dB and performing chromatic dispersion measurements at a signal-to-noise ratio at a clock frequency of 3-5 dB. The penalty for using additional probing frequencies is up to 1 dB.
We proposed recently a new «Smart Grids Plus» concept for digital energy grids design. These grids, in addition to layers of intelligent energy grids and information communication channels, include a layer of diagnostic monitoring based on a passive fiber optic sensor networks. Sensor networks have a hybrid TWDM structure – information exchange channels and integrated fiber optical sensors – core, based on a new technology for address interrogation and multiplexing – special addressable fiber Bragg gratings, combined for arbitrary topologies - point and quasi-distributed. Some examples of diagnostic monitoring nets for temperature control of complete switchgear contacts (point) and bus bars (quasi-distributed) are considered. Their principles of operation are discussed. The main advantages of these sensor networks are using of addressable fiber Bragg gratings simultaneously as sensors and multiplexing elements, and using of PON structure simultaneously as sensor and communication networks.
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.
A new approach to an optical vector analyzer (OVA) realization based on double-side modulation with a suppressed carrier and formation of a reference frequency, different from it, in a fiber Bragg grating with a phase shift is considered. This realization is distinguished by an extended range and high resolution of measurements, which is shown both theoretically and experimentally.
We present an approach to design and optimize a high angular dispersion spectrograph optical scheme. Such an instrument can be built using two volume-phase holographic gratings mounted one after another and simple focusing optics. However, this solution may suffer from high angular and spectral selectivity on the second grating. We propose a design technique combining simple raytracing with diffraction efficiency optimization based on analytical and numerical computations. As an example, we demonstrate a few versions of a compact spectrograph for the 830 to 870 nm near infrared range. Its spectral resolution is up to 0.01 nm, and after the optimization, the efficiency can reach 64.9%, while remaining very uniform for the entire range. Also, by the use of grisms instead of gratings, it is possible to make the optical design axial while keeping almost the same performance and eliminating misalignment issues.
This article deals with problem of carbon brush`s length measurements. There are many applications where regular inspection is not feasible because of a number of factors including, for example, time, labor, cost and disruptions due to down time. Thus, there is a need for a system that can monitor the brush`s length to calculate it`s wear rate, while the component is in operation or without removing of the component from its operational position. We propose a novel method for characterization of carbon brush`s length. This method based on the usage of advantages of the multiplicative response of FBGs and FBG arrays: spectral parameters depend on several aspects, such as grating`s period, refractive index, it`s physical length and so on. We are the first, in our point of view, who proposed to use third parameter for sensing application and prospectively all three parameters for complex measurement: the change of FBG`s length is used to measure length of the brush and it`s wear rate, grating`s central wavelength shift for temperature (due to refractive index change) and mechanical stress (due to grating`s period variations) measurements. The results of modelling and experiments are presented.
One of the promising methods for generating continuous sub-terahertz radiation is the method of photomixing, when the photoconductive material is excited by the emission of two optical continuous lasers with frequencies separated by terahertz frequencies. It should be noted that for effective photomixing the polarization, frequencies and phases of the optical beams introduced into the mixer must be constant, otherwise it is necessary to additionally use a complex system of phase synchronization. In connection with the complexity of such an implementation, variants based on the conversion of single-frequency and broadband laser radiation into two-frequency ones were investigated. The first of these is modulation and consists in the external modulation of single-frequency laser radiation in the electro-optical Mach-Zehnder modulator. The second one is broadband with the allocation of two wavelengths using two rectangular fiber Bragg gratings with a small difference in width. The studies were carried out to create a terahertz photoconductor, the distinguishing feature of which is the use of heterostructures grown on a GaAs substrate in the low-temperature regime as a photoconductive material in the form of a thick gradient layer In(x)Al(1-x)As (x = 0 → 1) with artificially created local deformations and with photoconductive composite InGaAs / InAs / InGaAs quantum wells.
This article describes the design principles of optoelectronic system for instantaneous frequency measurement of microwave signals based on the use of amplitude-phase modulation conversion of single optical carrier into symmetrical dual-frequency ones for additional frequency separation according its wavelength, its modulation by unknown frequency and subsequent “frequency-amplitude” measurement conversion in fiber Bragg grating with Gaussian reflection profile along with the variations of schemes for realization of this method with the corresponding spectrums. Such approach increases the signal-to-noise ratio and allows measuring of several unknown frequencies simultaneously. Distinction and measurement of the instantaneous frequencies made depending on the amplitudes and modulation indexes of the signals received at the photodetector.
In this paper, we consider a number of different methods that form the modern approach to the development of aircraft GTE’s noise suppression systems at service conditions. The herein-presented efficient noise suppression system on the base of fiber optic sensors makes it possible to reduce pulsations at the exhaust nozzle exit and noise levels at the engine outlet section.
Aspects of the paper relate to a wear monitoring system for smart photonic carbon brush. There are many applications in which regular inspection is not feasible because of a number of factors including, for example, time, labor, cost and disruptions due to down time. Thus, there is a need for a system that can monitor the wear of a component while the component is in operation or without having to remove the component from its operational position. We propose a new smart photonic method for characterization of carbon brush wear. It is based on the usage of advantages of the multiplicative response of FBG and LPFG sensors and its double-frequency probing. Additional measuring parameters are the wear rate, the brush temperature, the engine rotation speed, the hangs control, and rotor speed. Sensor is embedded in brush. Firstly the change of sensor length is used to measure wear value and its central wavelength shift for temperature ones. The results of modeling and experiments are presented.
The paper is devoted for presentation of training course for applications and construction principles of poly-harmonic (two-frequency or four-frequency) cw laser systems for characterization of different nonlinear scattering effects in fibers and reflection of devices based on fiber Bragg gratings (FBG) in down-hole telemetric sensor nets, which are widely used in down-hole telemetric systems. In particular, we’ll speak about evaluation of Mandelstam-Brillouin gain contour, Raman scattering contours and FBG reflection spectra characterization. Investigation methods and approaches are based on the unity of resonant structures of generated fiber responses on exciting and probing radiation or external physical fields for all given effects. The main decision is based on poly-harmonic probing of formed resonance responses. Training course united idea is software defined approach for down-hole parameters characterization in spite of measuring conversion principles.
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