RU2633020C1 - Fiber-optical sensor of vibro-acoustic signals on dopler intra-light (versions) - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: fiber-optic sensor of vibro-acoustic signals on the intra-light Doppler effect contains a radiation source, a sensor and a splitter, the first and the second Bragg gratings and a photodetector. The radiation source has a spectral width exceeding the width of the reflection spectrum of the first Bragg grating. In the first version, the first and the second Bragg gratings are made with a spectral shift of the resonance frequencies with respect to each other. In the second version, the first and the second Bragg gratings have identical parameters for the width of the reflection band and for the resonance frequency of the reflection. Moreover, one of the Bragg gratings is adapted to change the resonance frequency of the reflection.

EFFECT: simplifying the design, increasing the temperature stability of the sensor, increasing the signal-to-noise ratio.

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Description

The invention relates to measuring equipment, mainly to instruments that perform measurements using optical means, in particular to fiber optic sensors (WFD), based on the principle of detecting the frequency shift of optical radiation due to the Doppler effect in a fiber waveguide during dynamic deformation . These sensors can be used as acoustic pressure sensors, acoustic emission sensors, sensors for vibration control systems, etc.

It is known from the prior art that for recording the Doppler frequency shift in the VS, fiber-optic interferometer circuits are used, including interferometer circuits with a heterodyne configuration as the most resistant to external influences due to the fact that their output signal does not depend on the initial phase difference of the interfering rays.

Known VOD based on the optical scheme of a two-arm interferometer with heterodyne detection of Doppler shifts of the optical frequency (patent EP 1400792 A1, IPC G01H 9/00, published March 24, 2004), including a highly coherent laser, two optical splitters, the receiving arm of the interferometer, consisting of a sensitive element (SE) in the form of a curved part of the fiber, and a detector arm, consisting of an acousto-optical modulator (optical frequency shifter by a constant value). The sensor registers the Doppler frequency shift (f _d ) in the CE during its dynamic deformation by a vibroacoustic signal. To select the beat frequency of the signals of the receiving and heterodyne arms of the interferometer in the water, a photodetector and an electronic frequency detector are used.

The disadvantages of the known technical solutions include:

instability of the level (fading) of the output signal due to changes in the state of polarization of interfering waves (polarizing fading) in the optical fibers when external factors are affected by the latter (temperature, bends, hydrostatic pressure, etc.);

the increased level of phase noise associated with the instability of the laser radiation frequency, which is manifested when the lengths of the arms forming the interferometer are unequal, the alignment of which with a high degree of accuracy is a rather complicated technological task;

the need to use a complex expensive acousto-optical modulator in the sensor design, which includes discrete optical and electronic elements;

increased level of crosstalk due to re-reflections between the sensor elements, as a consequence of the high coherence of the radiation source, for example, on discrete elements of the acousto-optical modulator, as well as in the aircraft themselves due to backward (Rayleigh) scattering or reflections (Fresnel) at the points of their connection, for example in places of their welding;

the need to use expensive elements in the design of the sensor, such as an acousto-optic modulator and a highly coherent radiation source;

the need for an electronic frequency detector.

The technical solution described in the article is known (A.K. Batanov, G.Ya Buymistryuk, V.I. Kirillov, V.N. Nikolaev "Methods and means of processing the broadband signal of a fiber-optic acoustic sensor based on the Doppler intraband effect", Transactions XII All-Russian Conference “Applied Technologies in Hydroacoustics and Hydrophysics” 2014, pp. 456-457, Fig. 4), where it is proposed to use a Bragg fiber-optic array, which has an uneven frequency response of a narrow-band filter with a reflection coefficient close to units e and a width of the spectral reflection interval of less than 0.1 nm, while the slopes of the frequency response have a very high slope - more than 10 ⁹ m ^-1 . A copy of the article is attached.

In this technical solution, a highly coherent light source (LD), a sensitive element (SE), a splitter, a Bragg diffraction grating, and a photodetector (PD) are optically interconnected.

This technical solution is selected as the closest analogue (prototype) for the first and second declared options for water.

The prototype has drawbacks, the main one being that it is necessary to precisely correlate the frequency of the optical radiation source and the resonant reflection frequency (transmission) of the Bragg grating and their thermal stabilization under operating conditions, which complicates the design and increases the cost of water supply.

Another disadvantage of the prototype is the need to use a highly coherent laser with a narrow spectral emission line, which also complicates the design and increases the cost of water.

The task to which the claimed variants of the inventions are directed is the creation of a new design of a fiber-optic sensor for vibro-acoustic signals based on the Doppler intraviolet effect (hereinafter - VOD), which allows to provide:

simplification of the design of water

increasing the stability of water operation in various operating conditions,

ensuring the maximum dynamic range of received signals and, as a result,

reduction in the cost of water.

To solve the problem and achieve a new technical result, a group of inventions (two options) is declared, so interconnected that they form a single inventive concept.

The claimed two options for a fiber-optic sensor of vibroacoustic signals based on the Doppler intrafluorescence effect make it possible to eliminate the disadvantages of the prototype and ensure the achievement of a new technical result, namely: to simplify the design, increase the temperature stability of the VOD, and increase the signal-to-noise ratio.

The specified technical result is achieved due to the fact that:

according to option 1, the claimed fiber-optic sensor of vibroacoustic signals based on the intralumina Doppler effect includes optically coupled optical radiation source, a sensitive element, an optical splitter, an optical Bragg diffraction grating and a photodetector. The optical Bragg diffraction grating is connected to one of the outputs of the optical splitter. In contrast to the prototype, the sensor is equipped with a second optical Bragg diffraction grating. The optical radiation source is made of low coherence, the spectral width of which exceeds the width of the reflection spectrum of the first Bragg grating, and is connected in series with an optical splitter, a sensitive element, a second optical Bragg grating and a photodetector. In this case, the first and second optical Bragg gratings are made with a spectral shift of the resonant frequencies relative to each other;

according to option 2, the claimed fiber-optic sensor of vibroacoustic signals based on the intralumina Doppler effect includes optically coupled optical radiation source, a sensing element, an optical splitter, an optical Bragg diffraction grating and a photodetector. The optical Bragg diffraction grating is connected to one of the outputs of the optical splitter. In contrast to the prototype, the sensor is equipped with a second optical Bragg diffraction grating. The optical radiation source is made of low coherence, the spectral width of which exceeds the width of the reflection spectrum of the first Bragg grating, and is connected in series with an optical splitter, a sensitive element, a second optical Bragg grating and a photodetector.

Unlike the first option, in the second version of the fiber-optic sensor, the first and second Bragg optical diffraction gratings are made with identical parameters along the width of the reflection band and the resonant reflection frequency, while one of the gratings is configured to change the resonant reflection frequency.

In the process of patent research, other technical solutions for constructing fiber-optic sensors for vibro-acoustic signals were not identified, in which two Bragg optical diffraction gratings and a low-coherent radiation source would be used to register the shift of the optical spectrum induced by the Doppler effect.

The significance of the differences in the proposed design of water (in the first and second options) from the prototype is determined by the following.

1. The implementation of the optical radiation source of low coherence (broadband), in which the width of the spectrum exceeds the width of the reflection spectrum of the first Bragg grating, eliminates the influence of temperature instability of the frequency of the radiation source on the parameters of the VOD, which ensures its operability in various operating conditions, increases the stability of the sensor to external factors (temperature, bends, hydrostatic pressure, etc.), including in extreme conditions.

2. In the claimed designs of VOD used two optical Bragg diffraction gratings. Moreover, in the first embodiment, Bragg gratings are used, which differ in width of the reflection band and in the resonance frequency of reflection. In the second embodiment, lattices with identical parameters are used, which are made according to a single technological process, which ensures the production of water in the production process with minimal variation in sensitivity.

Moreover, in the design according to the first embodiment, a shift in the resonant frequency of reflection between the Bragg gratings can be carried out during the manufacturing or tuning of the VOD.

The invention is illustrated by drawings, which depict:

in FIG. 1 is a block diagram of the water supply system according to the first and second options;

in FIG. 2, FIG. 3, FIG. 4 and FIG. 5 are graphs explaining the operation of the VOD according to the first and second variants, where, for simplicity of presentation, in this example, optical Bragg diffraction gratings with spectral characteristics in the form of rectangles are used.

Including:

in FIG. Figure 2 shows the spectral characteristic of the optical signal reflected by the first Bragg optical diffraction grating and entering the second Bragg grating, where R ₁ (ν) is the reflection coefficient of the first Bragg grating, ν is the frequency of the optical carrier; δν ₁ is the width of the reflection spectrum; ν ₀₁ is the center frequency (resonance frequency) of the reflected spectrum;

in FIG. Figure 3 shows the spectral transmission characteristic of the second Bragg grating T ₂ (ν), where T ₂ (ν) = 1-R ₂ (ν), and R ₂ (ν) is the reflection coefficient of the second Bragg grating; ν ₀₂ is its central (resonant) reflection frequency; Δν is the shift of the resonance frequency of the second Bragg grating relative to the first grating, where Δν = Δν ₀ ± f _d (t), Δν ₀ is the constant shift between the resonance reflection frequencies of the gratings, which are specified constructively, af _d (t) is the frequency shift determined by Doppler effect;

in FIG. 4 shows the functional dependence of the product R ₁ (ν) * T ₂ (ν) on the value of the frequency gap Δν = ν ₀₂ -ν ₀₁ . From FIG. 4 it follows that each value of Δν corresponds to its own product R ₁ (ν) * T ₂ (ν) and its own area under the graph; changes in Δν due to f _d (t) lead to changes in area;

in FIG. 5 shows the form of the correlation function ψ (Δν), which characterizes the degree of coupling between the transmission spectrum of the second Bragg grating and the spectrum of the signal received at it and which is determined by the expression:

ψ (Δν) = ∫R1 (ν) T2 (ν) dν.

The above positions in the drawings (Figs. 1-5) are valid for the first and second versions of the water supply system. In the drawings are indicated: 1 - low coherent optical radiation source; 2 - optical splitter; 3 - the first optical Bragg diffraction grating; 4 - a second optical Bragg diffraction grating with offset resonance relative to the first optical Bragg grating; 5 - a segment of the aircraft supplying radiation to the sensitive element; 6 - a sensitive element; 7 - photodetector, 8 - radiation spectrum of a low coherent source; 9 - spectrum of the signal supplied to the second Bragg grating; 10 is a transmission spectrum of a second Bragg optical diffraction grating.

In accordance with FIG. 1 fiber-optic sensor of vibroacoustic signals based on the intra-optical Doppler effect (according to the first and second options) consists of a low-coherent optical radiation source 1, optical splitter 2, the first optical Bragg diffraction grating 3, the second optical Bragg 4 diffraction grating with a shifted resonance relative to the first optical diffraction Bragg gratings 3 with a wavelength of reflection, a segment of BC 5 supplying radiation to the sensitive element 6. To the output of the second optical diffraction grating Ki Bragg 4 connected photodetector 7.

As a low coherent optical radiation source 1, a superluminescent diode, or a superluminescent fiber emitter, or another emitter whose emission spectrum width exceeds the width of the reflection spectrum of the first Bragg optical diffraction grating 3 can be used.

As an optical splitter 2, a circulator can be used as a device that allows, on the one hand, to carry out optical isolation between the emitter and other optical elements of the sensor circuit, and on the other hand, to divide optical radiation in a given ratio.

Bragg optical diffraction gratings 3 and 4 can be fiber-optic, integrated-optical or acousto-optical with the same or different spectral parameters.

To ensure the operation of the VOD according to the first option in the field of maximum sensitivity and to ensure the maximum dynamic range of the received signals, the resonance reflection frequencies of the Bragg gratings 3 and 4 should be shifted relative to each other by a certain value Δν ₀ , which depends on the spectral width of the gratings used.

When using Bragg gratings with identical parameters in the sensor according to the second embodiment, the static shift of the resonant frequencies of the gratings relative to each other can be achieved by changing the pitch between the reflecting layers due to their longitudinal deformation. For example, in the case of using fiber optic gratings, this deformation can be carried out using a piezoceramic actuator or by bending a beam onto which the grating is glued. In the case of the use of optical Bragg gratings in the integrated optical design, the reflection wavelength can be controlled using the electric voltage supplied to the external electrodes.

The device operates according to the first and second options as follows.

Optical radiation from a broadband optical radiation source 1, the emission spectrum width of which, as shown in FIG. 2-5, exceeds the width of the reflection spectrum of the first Bragg 3 optical diffraction grating, enters through an optical fiber into the optical splitter 2, after which it passes to the first Bragg grating 3. The optical Bragg 3 grating reflects the radiation received at it at its resonant frequency - ν ₀₁ and the corresponding width of the reflection spectrum δν ₁ in the opposite direction to the optical splitter 2. After passing through the splitter 2, the radiation enters along the segment BC 5 to the sensitive element 6.

When radiation passes through the fiber, forming the sensitive element 6, due to its dynamic deformation, the central radiation frequency ν ₀₁ is displaced by a value proportional to the Doppler shift of the radiation frequency f _d in the sensitive element 6. This radiation is then transmitted to the second Bragg optical diffraction grating 4 having a resonant frequency ν ₀₂ different from the first Bragg 3 grating. The difference between the resonant frequencies of the Bragg gratings 3 and 4, which is defined as Δν = ν ₀₂ -ν ₀₁ , determines the power of the optical signal transmitted through the second Bragg grating 4 and supplied to the photodetector 7, as shown in FIG. 4. The fraction of transmitted radiation is determined by both the static shift between the resonant frequencies of the gratings Δν ₀ and the dynamic shift of the frequency f _d determined by the Doppler effect. The photodetector 7 converts changes in the power of the optical signal into an electrical signal. The shape of the amplitude-frequency characteristics is determined by the type of spectral curves of the Bragg gratings. The most optimal view is a rectangle, as shown in FIG. 5, since in this case the linear section reaches its maximum value.

The total light radiation power (RF), reaching the photodetector 7, is determined by the expression:

Rfp = ∫P (ν) R1 (ν) T2 (ν) dν.

In a particular case, in a fiber-optic implementation of VOD, a superluminescent LED at a radiation wavelength of 1500 nm with a spectral width of approximately 2 nm and with an output power of P ₀ = 100 mW can be used as a radiation source, for example. All other elements may be fiber optic. The output power of the superluminescent LED should be selected based on the fact that the first optical Bragg diffraction grating 3 in the direction of the sensitive element 6 reflects only a small part of the radiation received on it. For example, when using a Bragg grating with a reflection bandwidth of ≤0.1 nm, the radiation power entering the sensing element 6 will be ~ 1 mW.

Claims (2)

1. Fiber-optic sensor of vibroacoustic signals based on the intraband signal Doppler effect, including optically coupled optical radiation source, a sensing element and an optical splitter, a first Bragg optical diffraction grating connected to one of the outputs of the optical splitter, and a photodetector, characterized in that the sensor is equipped with a second Bragg optical diffraction grating, the optical radiation source is made of low coherence, the width of the spectrum of which exceeds the width of the spectrum from the first Bragg optical diffraction grating, and is connected in series with an optical splitter, a sensing element, a second Bragg optical diffraction grating and a photodetector, while the first and second Bragg optical diffraction gratings are made with a spectral shift of the resonant frequencies relative to each other. 2. A fiber-optic sensor of vibroacoustic signals based on the Doppler intraband effect, including an optically coupled optical radiation source, a sensing element and an optical splitter, a first Bragg optical diffraction grating connected to one of the outputs of the optical splitter, and a photodetector, characterized in that the sensor is equipped with a second Bragg optical diffraction grating, the optical radiation source is made of low coherence, the width of the spectrum of which exceeds the width of the spectrum from the first Bragg optical diffraction grating, and is connected in series with an optical splitter, a sensitive element, a second Bragg optical diffraction grating and a photodetector, while the first and second Bragg optical diffraction gratings are made with identical parameters along the width of the reflection band and the resonant reflection frequency, while one of the gratings is configured to change the resonant frequency of reflection.

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