RU2377497C1 - Facility for measuring deformations on base of quasi-distributed fibre-optical sencors on bragg grids - Google Patents

Abstract

FIELD: measuring technology.

SUBSTANCE: facility consists of wide-band super-luminescent diode (SLD), radiation of which comes to the first pole of four-pole splitter via convertible spectral filter and via its fourth pole to the fist pole of three-band splitter, the second pole of which is connected with a calibrating cuvette, while the third pole is connected with the third photo-receiving device (PRGD). Output of the calibrating cuvette containing acetylene, is connected with an input of the second PRGD. Radiation from the third pole of the four-pole splitter comes to Bragg sensors, while radiation reflected from them comes to the input of the first PRD (photo-receiving device) via the third and the second poles. Outputs of PRD are connected with a computer via and analogue-digital converter; the computer determines spectre of acetylene attenuation by signals from the second and the third PRGD; and by the most contrast spikes it calibrates modulation of SLD spectre by means of which it calibrates signals from Bragg sensors picking-up the first PRD.

EFFECT: increased sampling frequency of Bragg sensors, also facilitation of increased number of sensors in facility.

7 cl, 10 dwg

Description

The invention relates to measuring equipment in terms of creating an information-measuring system for recording a signal from a set of fiber-optic sensors based on Bragg gratings. The invention can be used to increase the speed and accuracy of strain measurements by a device based on quasi-distributed fiber-optic sensors on Bragg gratings.

Until recently, the creation of information-measuring systems for deformation control was an overly expensive solution, including in terms of the costs of maintaining and operating such systems. The fact is that traditional measuring transducers (sensors) used in such IMSs, as a rule, require a power supply and an own transmission line of the measurement information signal, as well as a line for supplying control signals. In addition, the operating conditions of the sensors are quite severely limited by environmental parameters, by the effects of aggressive environments, high voltage and electromagnetic interference.

The advent of distributed and quasi-distributed fiber-optic sensors based on Bragg gratings radically changed this situation and made it possible to create an IIS for monitoring infrastructure facilities with acceptable characteristics for practice. This type of sensors does not require power supply, the sensitive area of the sensor is essentially combined with fiber-optic transmission lines of measurement information. The degree of influence of environmental conditions, electromagnetic interference, high voltage, aggressive media on fiber-optic sensors is much lower than on sensors using electrical conversion of measurement information. In addition, in terms of their metrological characteristics, fiber-optic sensors are superior to traditional analogues using other construction principles.

However, with all of the above advantages, quasi-distributed information-measuring systems based on Bragg sensors are inferior in speed (polling rate) to systems based on strain gauges. At the same time, high polling frequencies of sensors are especially necessary for studying the dynamics of deformation during fast processes (destruction during an explosion, collisions with fast moving objects) in order to optimize the design of the object. In addition, systems with high polling frequencies are necessary for studying the deformations of aircraft structures in order to determine the service life and maximum permissible loads. Therefore, the task of developing an IMS based on Bragg sensors is relevant and technically relevant.

There are many devices based on quasi-distributed fiber-optic sensors on Bragg gratings for measuring strains, and therefore it is advisable to consider only the closest analogue.

It is known (see Fig. 1) a measuring device based on quasistributed fiber-optic sensors on Bragg gratings (US 6573489 B1, published 03.06.2003), selected as the closest analogue and containing a continuous broadband radiation source in the form of a superluminescent diode, optically connected with a tunable spectral filter, which is a Fabry-Perot interferometer, the output of which is optically connected to the first pole of a 2 × 2 four-pole splitter and through its fourth pole to the calibration input hydrogen cyanide suites, the output of which is optically connected to the input of the second photodetector, and through the third pole the radiation source is optically connected to the quasi-distributed fiber-optic sensors on the Bragg gratings so that the radiation reflected from them through the third and second poles of the four-pole splitter enters the input the first photodetector, the outputs of the first and second photodetectors are connected respectively to the first and second inputs of the analog-to-digital converter (A CPU), the output of which is connected to the input of the computing device.

The principle of operation of such a device is based on measuring the shift of the resonant wavelengths of radiation reflected from a set of Bragg sensors. That is, each Bragg sensor reflects radiation in a narrow spectral range in the vicinity of the resonant wavelength, the value of which is linearly related to the optical length of the Bragg grating. Spectrum measurement is performed by scanning the spectrum with a tunable filter. One of the photodetectors registers the spectrum of reflected signals from sensors (FPU1), and the second channel serves to calibrate the signal according to the wavelength scale (FPU2).

The disadvantage of this device is that in the known device to ensure a small error in measuring the resonant wavelengths of the sensors in the entire working spectral range, it is proposed to use all the absorption peaks of hydrogen cyanide, including those with low contrast, which reduces the potential dynamic range of the system, and therefore, to reduce performance, and to reduce the number of sensors that can be used in the device.

The limiting sampling frequency of a photodetector at one wavelength tuning point can be estimated from the relation, the premise for which is the position that the contrast (or depth) of the absorption maximum should be greater than the modulation contrast of the envelope of the spectrum of the superluminescent diode taking into account noise:

where k _cv = 0.5 is the coefficient of power transfer to the channel containing the calibration cell with acetylene and the second photodetector FPU2 (hereinafter referred to as the calibration channel) on a wavelength scale,

T _to - transmittance of the calibration channel,

P _East - the power of the radiation source,

P _{UE equivalent} = 20 pW Hz ^1/2 - specific power equivalent to the noise of a photodetector,

η is the transmittance of the Fabry-Perot interferometer,

m ₂ = 0.1 - the maximum absorption of hydrogen cyanide,

m ₁ = 0.02 is the relative amplitude of modulation of the envelope of the radiation spectrum of the SLD,

γ is a coefficient that takes into account how many times the absorption maximum must be greater than the maximum of the signal caused by noise.

From expression (1), taking into account the number of points in one measurement cycle, we obtain:

where δλ = 1 nm is the resolution of the wavelength tuning of the scanning Fabry-Perot interferometer,

Δλ _slave = 40 nm - working spectral range,

Δλ _slave / δλ - the ratio that determines the number of points in one cycle.

Here, the measurement cycle is understood as the registration of the signal of the FPU2 photodetector at all points of tuning of the Fabry-Perot scanning interferometer.

In accordance with expression (2) for typical parameters (T _k = 0.8, P _source = 1 mW, η = 2 · 10 ^-3 , m ₁ = 0.02, m ₂ = 0.1, γ = 2) the limiting sampling frequency limited by the FPU noise in the calibration channel is 36 Hz.

The technical result to which the invention is directed is to increase the frequency of the interrogation of quasi-distributed Bragg sensors, as well as to increase the number of sensors that can be placed in the device.

The specified technical result is achieved due to the fact that in the device for measuring strains based on quasistributed fiber-optic sensors on Bragg gratings, containing a continuous broadband radiation source, optically connected to a tunable spectral filter, the output of which is optically connected to the first pole of a 2 × 2 four-pole splitter and through its fourth pole - with the input of the calibration cell, the output of which is optically connected to the input of the second photodetector, and through h the third pole, the radiation source is optically connected to the quasi-distributed fiber-optic sensors on the Bragg gratings in such a way that the radiation reflected from them through the third and second poles of the four-pole splitter is fed to the input of the first photodetector, the outputs of the first and second photodetectors are connected to the first and second the inputs of the analog-to-digital Converter (ADC), the output of which is connected to the input of the computing device, additionally introduced a third photodetector at a triple and a 1 × 2 three-pole splitter, the radiation source is capable of generating radiation with a spectrum at the output, the envelope of which is modulated in amplitude, and the calibration cell contains acetylene, and the fourth pole of the four-pole splitter is optically connected to the input of the calibration cell through the first and second poles of the three-pole splitter and through its third pole - with the input of the third photodetector, the output of which is connected to the third input of the ADC, and the computing device is made with zmozhnostyu calibration signal from the output of the third photodetector for two contrasting most absorption peaks of acetylene and subsequent calibration of the signal output from the first photodetector by quasi-distributed fiber optic sensors in the Bragg gratings using a calibrated signal obtained from the output of the third photodetector.

In this case, it is possible that the second photodetector was located in the output window of the calibration cell.

To obtain the optimal signal-to-noise ratio, it is most expedient that the four-pole splitter be made with the possibility of dividing the power of radiation coming in through its first pole into the fourth and third poles with a ratio of 25% / 75% and with the possibility of directing 75% of the radiation power coming in through the third pole to the second pole.

In addition, it is advisable that the three-pole splitter was made with the possibility of dividing the power of the radiation entering through its first pole into the second and third poles with a ratio of 50% / 50%.

As a radiation source with a radiation spectrum whose envelope is modulated in amplitude, a superluminescent diode (SLD) can be used, the crystal faces of which have a reflection coefficient providing a predetermined modulation depth in amplitude of the envelope of the radiation spectrum of a superluminescent diode.

As the two most contrasting absorption peaks of acetylene, it is most advisable to use two absorption peaks corresponding to wavelengths of 1519.137 nm and 1530.371 nm, which are fundamental constants.

At the same time, a computer, including a computer, can be used as a computing device.

The invention is further described in more detail using the drawings.

Figure 1 shows the structural diagram of the device is the closest analogue.

Figure 2 shows the structural diagram of the proposed device.

Figure 3 shows the emission spectrum at the output of the SLD.

Figure 4 shows the spectrum of radiation reflected from the Bragg sensors.

Figure 5 shows on an enlarged scale part of the radiation spectrum at the output of the SLD.

Figure 6 shows the radiation spectrum at the outlet of the cell with acetylene.

7 shows the determination of the safety factor γ.

On Fig shows the transmission spectrum of acetylene.

Figure 9 shows the calibration of the envelope of the radiation spectrum at the output of the SLD.

Figure 10 illustrates the determination of the position of adjacent peaks of the envelope of the radiation spectrum.

Figure 2 conditionally shows a structural diagram of the proposed device.

As a radiation source for the implementation of the present invention, it is necessary to use a continuous broadband radiation source, at the output of which radiation is formed, the spectral envelope of which is modulated in amplitude. This is necessary in order to enable calibration of the radiation from the radiation source. Moreover, such radiation sources are known from the prior art, for example, a radiation source disclosed in US Pat. No. 5,663,822, publ. 09/02/1997, using a source of optical white noise and a Fabry-Perot resonator, which filters radiation by wavelengths to form a spectrum of resonant wavelengths, which are divided into M channels, in each of which modulation is performed, after which the signals are summed into one optical signal. However, such a radiation source has a complex structure and large dimensions.

It seems most appropriate to use a superluminescent diode SLD, indicated in FIG. 2, item 1, as such a radiation source. The crystal faces of this diode, in fact, are a Fabry-Perot resonator and have a certain reflection coefficient, the magnitude of which determines the amplitude of the modulation depth of the envelope of the radiation spectrum in amplitude. In this case, the modulation depth is described by the following expression [see SuperlumDiodes Ltd., 2004, Shidlovsky, V.E. Superluminescent Diodes. Short overview of device operation principles and performance parameters, http: /www.superlumdiodes.com/pdf/sld_overview.pdf; p.4]:

where G = exp ((g-α) · L) is the total gain, R ₁ , R ₂ are the reflection coefficients of the faces of the SLD. With an SLD power of 10 mW, the gain G is about 1000. Then, for the reflection coefficient of the first face R ₁ = 0.001 and the reflection coefficient of the second face R ₂ = 0.001, we obtain the modulation depth m = 0.2.

Such modulation of the SLD spectrum can be used as reference points for calibration on a wavelength scale. This allows you to increase the working spectral range, because the width of the SLD spectrum at half maximum is about 50 nm (see figure 3).

The tunable PF filter installed in series with the SLD, indicated by pos. 2 in FIG. 2 and made in the form of a Fabry-Perot scanning interferometer, performs discrete tuning according to the wavelength of the radiation emerging from the PF. As a result, there is a certain set of points on the wavelength scale corresponding to these wavelengths.

The output of the tunable spectral filter PF is optically connected to the first, input, pole of the 2 × 2 four-pole splitter, designated 3 in FIG. 2, and through its fourth, output, pole to the first, input, pole of the 1 × 2 three-pole splitter, indicated item 5. The second, output, pole of the three-pole splitter 5 is optically connected to the input of the calibration cell KK, which is indicated in Fig. 2 by pos. 6 and in the proposed device, in contrast to the prototype, contains acetylene. The output of the calibration cell КК is optically connected to the input of the second photodetector of the FPU, indicated by pos. 8. The third, output, pole of the three-pole splitter, indicated by pos. 5, is optically connected to the input of the third photodetector (FPU), indicated by pos. 9. Through the third, input-output pole of the 2 × 2 four-pole splitter (element 3 in FIG. 2), the SLD (element 1 in FIG. 2) is optically connected to the N quasi-distributed fiber optic sensors D1 ... DC on the Bragg gratings, collectively indicated in FIG. 2 pos.4, so that the radiation reflected from them through the third and second, output, poles of the four-pole splitter 3 is fed to the input of the first photodetector FPU, designated pos.7. The outputs of the first, second, and third photodetector FPUs, labeled 7, 8, and 9, are connected respectively to the first, second, and third inputs of the analog-to-digital converter of the ADC, labeled 10, the output of which is connected to the computer input, labeled 11 .

The four-pole splitter, indicated by pos. 3, divides the power of radiation coming in through its first pole into the fourth and third poles with a ratio of 25% / 75% and directs 75% of the radiation power coming in through the third pole from the Bragg sensors, collectively indicated by pos. 4, exit through the second pole.

In this case, the three-pole splitter, indicated by pos. 5, divides the power of the radiation coming through its first pole into the second and third poles with a ratio of 50% / 50%.

The device operates as follows.

The radiation from the SLD (element 1 in FIG. 2) passes through a tunable filter (element 2 in FIG. 2) and enters the first pole of a 2 × 2 optical four-pole splitter (25% / 75%), indicated by pos. 3. Through the third pole, splitter 3 is connected to a set of N sensors, collectively indicated in FIG. 2, position 4, based on Bragg fiber optic arrays D1 ... DN, to which 75% of the radiation entering through the first pole of the four-pole splitter indicated by pos. 3. Reflected from the sensors, designated together by pos. 4, the radiation through the same third pole enters the four-pole splitter, labeled by pos. 3, and through its second, output, pole enters the first photodetector of the FPU, indicated by pos. 7, which records the signal, reflected from the Bragg sensors indicated in pos. 4. This signal is illustrated in the graph in FIG. 4.

At the same time, 25% of the radiation entering through the first pole of the four-pole splitter, indicated by pos. 3, leaves through its fourth pole and goes to the first, input, pole of the 1 × 2 three-pole splitter (50% / 50%), represented in Fig. 2 by 5. Through the second, output, pole of this three-pole splitter, 50% of the radiation enters the calibration channel containing a KK calibration cuvette with acetylene having more contrast absorption maxima (transmission minima) than the hydrogen cyanide used in the prototype, and the second photodetector FPU device, indicated in Fig. 2, item 8, which registers a signal at the output of the cell with acetylene (Fig. 6).

At the same time, through the third, output, pole of the three-pole splitter (element 5 in FIG. 2), 50% of the radiation is supplied to the third photodetector FPU, indicated in FIG. 2 by pos. 9, which registers the signal transmitted from the superluminescent diode 1 (in FIG. 5, a fragment of FIG. 3) is shown enlarged.

The absorption spectrum of acetylene (see Fig. 8) can be obtained by dividing the signal from the FPU 8 (Fig.6) by the signal from the FPU 9 (Fig.5). The safety factor is entered here.

which shows how much more contrast the peak of absorption of acetylene should be, so that it can be distinguished against the background of modulation of the envelope of the emission spectrum of the SLD (element 1 in figure 2) and noise (see Fig. 7, which shows an enlarged section of the graph shown in Fig. .6, to clarify the value of the safety factor γ). We take the value γ = 2. By dividing the signals, the influence of modulation of the envelope of the radiation spectrum, i.e. contrast increases, therefore, the threshold value of the permissible noise level increases, which is proportional to the polling frequency of the Bragg sensors in the system. This increases the speed of the system.

Using the two most contrasting absorption peaks (minimum transmittance) of acetylene (Fig. 8), which correspond to the wavelengths of 1519.137 and 1530.371 nm and are fundamental constants, it is possible to calibrate the modulated envelope of the SLD emission spectrum on the wavelength scale. Calibration consists in determining the period of amplitude modulation of the envelope of the radiation spectrum of the SLD (element 1 in figure 2) and determining with its help the wavelengths of all the maxima of the envelope of the radiation spectrum (Figs. 9, 10). Using the obtained values, the signal received from the Bragg sensors 4 is further calibrated (Fig. 4). Thus, the calibration of the signal from the Bragg 4 sensors is carried out not by the absorption peaks of acetylene, but by the calibrated envelope of the emission spectrum of the SLD.

Because as a result of the proposed calibration, we can take into account the effect of modulation of the radiation spectrum of the source, then we can assume m ₁ = 0. Then expression (1) for the limiting polling frequency is converted to the form:

Given the fact that k _sv1 = k _sv2 = 0.75, we can approximately assume that k _sv1 · k _sv2 ≈k _sv = 0.5 and, accordingly, the frequency will be: f ₂ = 780 Hz under the initial initial conditions m ₂ = 0.5 , γ = 2, δλ = 1 pm, Δλ _slave = 40 nm. Thus, in the proposed system, the polling frequency increases by about 20 times.

The signals from the photodetector FPU devices (elements 7, 8, and 9 in FIG. 2) are respectively supplied to the first, second, and third inputs of the analog-to-digital converter of the ADC (element 10 in FIG. 2), after which they are fed to a computer (element 11 in FIG. .2), where three arrays of numbers I1 _j , I2 _j and I3 _{j are} recorded, with each j element in these arrays corresponding to the signal intensity measured by the above FPU devices at the corresponding central wavelength of the Fabry-Perot interferometer 2. That is, in as a result, there is a set of intensities I1 _j , I2 _j and I3 _j and the set of corresponding wavelengths λ _j . Moreover, the values of wavelengths λ _{j are} determined with an error caused by the nonlinearity of the piezoelectric drive of the tunable filter in the form of a Fabry-Perot interferometer (element 2 in FIG. 2).

To minimize the error caused by the nonlinearity of the tunable filter (element 2 in figure 2), the following signal processing algorithm is used:

1) the signal I2 _{j is} divided by I3 _j , as a result of which an array I4 _{j is formed} , the elements of which correspond to the absorption spectrum of acetylene, i.e. each element of the array corresponds to a point on the absorption spectrum of acetylene;

2) in the resulting array I4 _j , two minima are selected (i.e., two elements having minimum values) corresponding to the two most contrasting absorption peaks, the distance between which is known with high accuracy on the wavelength scale;

3) two elements I3 _a and I3 _{b of the} array I3 _j corresponding (by number) to the determined absorption peaks are determined (see Fig. 9);

4) the number of maxima between these elements of the array and the number of elements in the array from element I3 _a to the first maximum and from the last maximum to element I3 _{b are} determined (see Fig. 10);

5) find the period of the envelope of the SLD emission spectrum (element 1 in FIG. 2) by dividing the distance on the wavelength scale found in clause 2 (between I4 _a and I4 _b ) by the number of periods found in clause 4 (between I3 _a and I3 _b );

6) all maxima of the array I3 _j are calibrated using the period calculated in clause 5; as a result, we obtain the array I3 _j ';

7) we find the elements of the array I1 _j corresponding to the found calibrated maxima of the array I3 _j ';

8) determine the number of elements of the array I1 _j located between the elements defined in clause 7;

9) we assign to each element of the array I1 _{j a} new calibrated value of the wavelength, as a result we get the array I1 _j ';

10) we find the maxima in the array I1 _j '.

The obtained values of the maxima of the array I1 _j 'correspond to the values of the signal reflected from the Bragg sensors, jointly designated pos.4, taking into account the error caused by the nonlinearity of the tunable filter, indicated by pos.2 in Fig.2.

Thus, the proposed device for measuring deformations based on quasi-distributed fiber-optic sensors on Bragg gratings provides an increase in the frequency of polling of quasi-distributed Bragg sensors, as well as an increase in the number of sensors that can be placed in the device.

Claims (7)

1. A device for measuring strains based on quasi-distributed fiber optic sensors on Bragg gratings, comprising a continuous broadband radiation source optically connected to a tunable spectral filter, the output of which is optically connected to the first pole of a 2x2 four-pole splitter and through its fourth pole to the input of the calibration cell the output of which is optically connected to the input of the second photodetector, and through the third pole the radiation source is optically connected to the quasir distributed fiber-optic sensors on the Bragg gratings in such a way that the radiation reflected from them through the third and second poles of the four-pole splitter is fed to the input of the first photodetector, the outputs of the first and second photodetector devices are connected, respectively, to the first and second inputs of the analog-to-digital converter ( ADC), the output of which is connected to the input of a computing device, characterized in that a third photodetector and a three-pole p a 1 × 2 splitter, the radiation source is configured to generate radiation with a spectrum at the output, the envelope of which is modulated in amplitude, and the calibration cell contains acetylene, and the fourth pole of the four-pole splitter is optically connected to the input of the calibration cell through and through the first and second poles of the three-pole splitter the third pole - with the input of the third photodetector, the output of which is connected to the third input of the ADC, and the computing device is configured to calibrate the signal from the output of the third photodetector according to the two most contrasting absorption peaks of acetylene, and subsequent calibration of the signal from the output of the first photodetector from quasi-distributed fiber-optic sensors on the Bragg gratings using the received calibrated signal from the output of the third photodetector. 2. The device according to claim 1, characterized in that the second photodetector is located in the output window of the calibration cell. 3. The device according to claim 1, characterized in that the four-pole splitter is configured to split the power of radiation entering through its first pole into the fourth and third poles with a ratio of 25% / 75% and with the possibility of directing 75% of the radiation power entering through the third pole to the second pole. 4. The device according to claim 3, characterized in that the three-pole splitter is configured to split the power of the radiation coming through its first pole into the second and third poles with a ratio of 50% / 50%. 5. The device according to any one of claims 1 to 4, characterized in that as a radiation source with a radiation spectrum, the envelope of which is modulated in amplitude, a superluminescent diode is used, the crystal faces of which have a reflection coefficient that provides a given modulation depth in amplitude of the envelope of the radiation spectrum superluminescent diode. 6. The device according to any one of claims 1 to 4, characterized in that the absorption peaks corresponding to the wavelengths of 1519.137 and 1530.371 nm, which are fundamental constants, are used as the two most contrasting absorption peaks of acetylene. 7. The device according to any one of claims 1 to 4, characterized in that a computer is used as a computing device.

Images