RU171551U1 - DISTRIBUTED FIBER OPTICAL MEASURING SYSTEM WITH BRAGG SENSORS - Google Patents

Abstract

The utility model relates to fiber-optic measuring equipment and can be used in aviation and space technology, chemical, petrochemical and biomedical industries, transport, construction. A distributed fiber-optic system with Bragg sensors contains a radiation source, a circulator, an optical fiber, sensors based on Bragg gratings, a spectral analyzer built, for example, based on controlled unbalanced Mach-Zehnder interferometers, a control and processing unit including a photodetector and microcontroller, a photodetector through the amplifier is connected to the signal input of a synchronous detector, the reference input of which is connected to a sinusoidal voltage generator, and the output of the synchronous detector is es series-connected low-pass filter and polarity detector connected to the digital inputs of the microcontroller. In this case, the digital output of which is connected to the digital input of the digital-to-analog converter, and the analog output of the digital-to-analog converter is connected to the first input of the adder, the second input of the adder is connected to a sinusoidal voltage generator, and the output of the adder is connected to the control electrodes of the spectral analyzer through a matching circuit. The technical result is a decrease in the measurement error of physical parameters. 6 ill.

Description

The inventive utility model relates to fiber-optic measuring equipment and can be used in aviation and space technology, chemical, petrochemical and biomedical industries, transport, construction.

One of the key problems that determine the practical application of Bragg sensors is the development of methods and equipment that allow high-precision measurement of small changes in the positions of the maximum reflection of sensors.

As one of the analogues, a distributed fiber-optic measuring system for processing the signals of Bragg fiber-optic sensors using a spectral analyzer based on a tunable Fabry-Perot interferometer [1], [2] (see Fig. 1 of Appendix 1) can be considered.

The specified distributed fiber-optic measuring system for processing signals from fiber-optic Bragg sensors contains a radiation source 1, a circulator 2, an optical fiber and Bragg sensors 3, a spectral analyzer based on a tunable Fabry-Perot interferometer 4 and a photodetector 5 installed in the control and processing unit. The radiation source 1 is connected to the input port of the circulator 2, which directs light energy through the bidirectional port of the circulator 2 to the Bragg sensors 3 via a fiber optic cable. The radiation reflected from the Bragg sensors 3 via a fiber-optic cable and the bidirectional port of the circulator 2 goes to the output port of the circulator 2 and then to the input of the tunable Fabry-Perot interferometer 4. In this processing device, the radiation reflected from the Bragg sensors is passed through the Fabry-Perot filter which passes only a narrow band of the spectrum in the reflected broadband signal. The position of the maximum transmittance of the filter depends on the distance between the mirrors of the interferometer. Fixing one of the mirrors of the interferometer to piezoceramics (PC) allows you to control the spectral transmission of the filter by changing the relative position of the mirrors when applying voltage to the PC. Since such a filter is controllable, this allows one to obtain the spectrum of the reflected signal and, thereby, to measure the position of the maximum in its spectrum in units of electric voltage in the control and processing unit.

The disadvantage of this measurement system is the need for mechanical movements of one of the mirrors of the interferometer, which does not allow working at high frequencies, as well as its susceptibility to mechanical vibrations.

A distributed fiber-optic measuring system is known in which a spectral analyzer is made on the basis of a Bragg tunable filter [2], [3] (see Fig. 2 of Appendix 1).

In this distributed fiber-optic measuring system, the radiation source 1 is connected to the input port of the first circulator 2, and its bidirectional port is connected by a fiber-optic cable to the Bragg sensors 3. The output port of the first circulator 2 is connected to the input port of the second circulator 4, and the bidirectional port of the second the circulator 4 is connected to a tunable filter on the Bragg grating 5. The output port of the second circulator 4 is connected to a photodetector 6, and the photodetector is connected to a scanning device 7 installed in the unit ION and handling. The radiation reflected from the Bragg sensors 3 through the fiber-optic cable and the bidirectional port of the first circulator 2 goes to the output port of the first circulator 2 and then, through the input port of the second circulator 4 and the bidirectional port of the second circulator 4, to the Bragg tunable filter 5. Reflected from last, the radiation through the bidirectional port of the second circulator 4 goes to its output port and then to the photodetector 6. The scanning device 7 changes the period of the Bragg grating 5 and, as a result, the wavelength. The control and processing unit fixes at what wavelength from the photodetector 6 there will be a maximum signal.

The disadvantage of this measurement system is its low noise immunity.

A distributed fiber-optic measuring system is known in which a spectral analyzer is based on a Mach-Zehnder interferometer with unbalanced arms [4] (see Fig. 1 of Appendix 2).

The specified measuring system contains a radiation source 1 connected to the input port of the circulator 2, which directs light energy through the bidirectional port of the circulator 2 to the Bragg sensor 3 via a fiber optic cable. The output port of the circulator 2 is connected to the input of the Mach-Zehnder fiber interferometer with unbalanced arms 4, and its output is connected to the photodetector 7 with an amplifier. The output from the amplifier is connected to a phase-shifting device 5 and a system for registering light pulses 6 installed in the control and signal processing unit.

, где:The system operates as follows. Radiation reflected from the Bragg sensor 3 is passed through an uneven Mach-Zehnder 4 interferometer with a controlled path difference.

where:

π is 3.14;

n is the refractive index;

λ is the wavelength;

d is the difference in the lengths of the arms of the interferometer.

With a change in the phase difference of the interferometer, the position of the maximum transmittance changes, which is described by the dependence (1 + cosϕ). When the maximum of the spectrum of the reflected signal coincides as a result of external action on the Bragg sensor with the maximum transmission of the Mach-Zehnder interferometer, a signal jump will be observed at the photodetector 7. The signal optimized by the amplifier from the photodetector 7 is used to determine the wavelength at which the signal jumped, by the system of registration of light pulses 6, installed in the control unit and signal processing.

The disadvantages of this measurement system are the shallow transfer characteristic of the unequal Mach-Zehnder interferometer in the region ϕ = 0 and the low noise immunity of the system when determining the information jump of the signal.

The closest analogue (prototype) of the claimed utility model is “FIBER OPTICAL MEASURING SYSTEM (OPTIONS)” [5], containing a broadband radiation source, an optical splitter for several channels, a circulator, an optical receiver, an optical fiber sensor, a control and processing unit and a tunable element (see Fig. 2a, Fig. 2b of Appendix 2).

The tunable element according to the first embodiment of the device is made on the basis of an electro-optical modulator constructed according to the scheme of an unbalanced Mach-Zehnder interferometer. The tunable element according to the second embodiment comprises a circulator and an electro-optical tunable filter. Tunable elements are made on the basis of an electro-optical crystal such as lithium niobate or lithium tantalate.

In FIG. Appendix 2a shows a diagram of a fiber-optic measuring system with a spectrometer based on an unbalanced Mach-Zehnder interferometer. In FIG. 2b of Appendix 2 presents a diagram of a fiber-optic measuring system with a spectrometer based on an electro-optical tunable filter on a Bragg grating.

The fiber-optic measuring system according to option 1 (Fig. 2a of Appendix 2) contains a broadband radiation source 1, an optical splitter for several channels 2, a circulator 3, an optical receiver 5, an optical fiber sensor 4, a control and processing unit 6, a tunable element 7, with the output of the broadband radiation source 1 is connected to an optical coupler 2, one of the outputs of which is connected to the first input of the circulator 3, the second output of which is connected to the fiber optic sensor 4, and the third output is connected to the tunable m element 7, which in turn is connected to an optical receiver 5 connected to the control and processing unit 6, and the control outputs from the processor unit 6 are connected to the tunable element 7, while the tunable element 7 is based on the electro-optical modulator 8, constructed according to the scheme unbalanced Mach-Zehnder interferometer.

The fiber-optic measuring system according to option 2 (Fig. 2b of Appendix 2) is characterized in that the tunable element comprises a circulator 9 and an electro-optical tunable filter 10, the second input of the circulator 9 being connected to the input of the electro-optical tunable filter 10.

The device according to option 1 and 2 works as follows. A broadband radiation source 1 is connected to a coupler 2, which directs light energy through different channels, each channel is connected to a circulator 3, where the radiation enters the first input and exits at the second output connected to one or more Fabry-Perot sensors and / or Bragg gratings 4. The radiation, reflected from the sensors and changing its spectrum, goes to the second output of the circulator 3, goes to the third output and gets to the tunable element 7, which is option 1 unbalanced interferometer Mach-Zehnder 8, made on an electro-optical crystal such as lithium niobate, and according to option 2, consists of a circulator 9 and an electro-optical tunable filter 10. When a modulating voltage 7 is supplied from the control and processing unit 6 to a tunable element 7, for example, the phase difference increases proportionally between the arms of the interferometer 8 and, thus, the spectrum is scanned. The radiation passing along the shoulders of the electro-optical modulator 8 interferes at its output and enters the optical receiver 5, the electric signal from which is received by the control and signal processing unit 6.

In option 2, the radiation from the output of the circulator 3 enters the tunable element 7, which consists of a circulator 9 and an electro-optical tunable filter based on the Bragg grating 10. The radiation, reaching the first input of the circulator 9, passes to its second input and enters the electro-optical tunable filter 10 , reflecting from which, again passes through the circulator 7 and the output at the third output. When a voltage is applied from the control and processing unit, for example, in a sawtooth shape, the reflection spectrum of the filter 9 changes, and thus, the spectrum of the radiation reflected from the sensors 4 is scanned.

The disadvantage of this measurement system is its low noise immunity, which does not allow high accuracy to measure small changes in the positions of the reflection maxima of the Bragg gratings when determining the information wavelength.

The objective of the claimed utility model is to reduce the measurement error of physical parameters.

The technical result aimed at achieving the task is achieved by increasing the accuracy of measuring the wavelengths at which the reflection maxima of the Bragg sensors are located, and lies in the fact that in the known distributed fiber optic system containing a radiation source, circulators, optical fiber, sensors on Bragg gratings, a spectral analyzer based on controlled unbalanced interferometers, for example, Mach-Zehnder, a control unit including a photodetector and microcontroller, ph the receiver is connected through an amplifier to the signal input of a synchronous detector, the reference input of which is connected to a sinusoidal voltage generator, and the output of the synchronous detector is connected through a series-connected low-pass filter and a polarity detector to the digital inputs of the microcontroller, the digital output of which is connected to the digital input of the digital-to-analog converter, while the output of the digital-to-analog converter is connected to the first input of the adder, the second input of the adder is connected to the generator voltage, and the output of the adder through the matching circuit is connected to the control electrodes of the spectral analyzer.

In FIG. 1 is a functional diagram of the inventive distributed fiber optic measuring system with Bragg sensors.

Distributed fiber optic measuring system contains:

- broadband radiation source 1;

- circulator 2;

- fiber optic Bragg sensors 3;

- control and processing unit 4;

- spectrum analyzer 5;

- photodetector 6;

- amplifier 7;

- synchronous detector 8;

- sinusoidal voltage generator 9;

- low-pass filter 10;

- polarity detector 11;

- microcontroller 12;

- digital-to-analog converter (DAC) 13;

- adder 14;

- approval scheme 15.

The output of the spectral analyzer 5 is connected in series with the photodetector 6, amplifier 7, and with the signal input of the synchronous detector 8, the sinusoidal voltage generator 9 is connected to the reference input of the synchronous detector 8, the output of which is connected to the digital inputs of the microcontroller 12 through a low-pass filter 10 and the polarity detector 11, digital the output of the microcontroller 12 is connected to the digital input of the digital-to-analog converter 13, the analog output of the digital-to-analog converter is connected to one of the inputs of the adder 14, the second input of the adder 14 is connected to the sinusoidal voltage generator 9, the output of the adder 14 via a matching circuit 15 connected to the control electrodes of the spectral analyzer 5, made, for example, according to the scheme of unbalanced Mach-Zehnder interferometers.

The system operates as follows. A broadband radiation source 1, connected to the input port of the circulator 2, directs light energy through the bi-directional port of the circulator 2 to the Bragg sensors 3 via a fiber optic cable. The radiation reflected from the Bragg sensors 3 through the fiber-optic cable and the bidirectional port of the circulator 2 goes to the output port of the circulator 2 and enters the control and processing unit 4 to the input of the spectral analyzer 5, based on, for example, unbalanced Mach-Zehnder interferometers. The control electrodes of unbalanced Mach-Zehnder interferometers from the sinusoidal voltage generator 9 are supplied via the adder 14 and the matching circuit 15 with a voltage that changes the difference in the travel of the arms of the interferometers according to a sinusoidal law, as shown in FIG. 2, where:

F - radiation intensity;

λ is the wavelength.

This leads to the fact that the position of the maximum transmittance on the wavelength scale of Mach-Zehnder interferometers also varies according to a sinusoidal law. At the same time, the microcontroller 12 at its digital outputs programmatically sets up sequentially increasing code combinations falling on the digital input of the digital-to-analog converter (DAC) 13, as a result of which the voltage at the output of the DAC 13 increases stepwise-linearly. The magnitude of the step is determined by the bit depth of the DAC. Thus, at the inputs of the adder 14, as well as on the control electrodes of the spectral analyzer 5, there is simultaneously a sinusoidal and linearly increasing voltage, which leads to a corresponding change in the difference in the travel of the arms of the interferometers. As a result of this, the signal at the photodetector 6 will have the form as shown in FIG. 2. That is, in the upstream portion of the reflection spectrum of the Bragg sensor, the signal at the input to the synchronous detector 8 will be in phase with the voltage from the generator 9, and in the downstream portion of the reflection spectrum of the Bragg sensors, the signal at the entrance to the synchronous detector 8 will differ in phase with voltage generator 9 180 degrees. At the maximum point of the reflection spectrum of the Bragg sensor, the signal at the input to the synchronous detector 8 will have a significantly lower amplitude and doubled frequency. At the output of the synchronous detector 8, after the low-pass filter 10, there will be a positive constant voltage on the upstream portion of the reflection spectrum of the Bragg sensor, close to zero of the peak point of the spectrum, and a negative DC voltage on the downstream portion of the reflection spectrum of the Bragg sensor. The polarity detector 11 will set the logic unit to the first digital output if the operating point of the spectrometer is on the upstream portion of the reflection spectrum of the Bragg grating. If the operating point of the spectrometer is on the downstream portion of the reflection spectrum of the Bragg sensor, the polarity detector 11 will set the logic unit to the second digital output. At the digital output of the polarity detector 11, there will be a logical zero if the wavelength of the spectrometer is at the maximum point of the reflection spectrum of the Bragg sensor.

These digital signals will be received by the microcontroller 12, and the information wavelength of the reflected radiation of the corresponding Bragg sensor will be determined from the code at the input of the DAC 13 at this point in time.

The inventive utility model due to the high sensitivity of the synchronous detector, as well as its ability to emit a signal against the background of extraordinary powerful noise, will make it possible to measure with small accuracy small changes in the positions of the maximum reflection of the Bragg sensor.

The broadband radiation source may be a standard superluminescent light emitting diode operating at a central wavelength of 1.55 μm.

All system components, including the circulator and optical receiver, are standard for telecommunication applications; control and processing unit can be performed using standard analog and microprocessor technology.

The controllable unbalanced Mach-Zehnder electro-optical interferometer, presented as a possible basis for a spectral analyzer, can be performed on an electro-optical crystal such as lithium niobate or lithium tantalate by the standard method for producing integrated-optical circuits.

The technical result is achieved by increasing the accuracy of measuring the wavelengths at which the reflection maxima of the Bragg sensors are located. The refractive index of the spectral analyzer is modulated, and subsequent synchronous detection makes it possible to isolate the signal against the background of extremely powerful noise, which makes it possible to create a compact spectrometer that does not have elements distributed in the space and mechanically moving parts, thereby increasing the reliability of the system and increasing the measurement speed and simplify the device.

Information sources

1. Kersey A.D. Berkoff T.A. Morey W.W. Multiplexed fiber Bragg grating strain sensor-system with a fiber Fabry-Perot wavelength filter // Optics Letters. - 1993. - Vol. 18, No. 16. - P. 1370-1372.

2. Kulchin Yu.H. Distributed fiber optic measuring systems. - M .: Fizmatlit, 2001 .-- 272 p.

3. Jackson D.A. Ribeiro A.V. L. Simple multiplexing scheme for a fiberoptic grating sensor network // Optics Letters. - 1993. - Vol. 18, No. 14. - P. 1192-1194.

4. Kersey A.D. A review of recent developments in fiber optic sensor technology // Optical fiber technology. - 1996. - Vol. 2, No. 3. - P. 291-317.

5. Patent 2520963, Russian Federation, IPC G01B 9/02, G01D 5/26. Fiber-optic measuring system (options) / Yaceev V.A .; Applicant and patent holder Optical Measuring Systems Limited Liability Company. - No. 2012135565/28; declared 08/20/12; publ. 02/27/14, bull. No. 6. - 1 p.: Ill.

Claims (1)

A distributed fiber-optic measuring system with Bragg sensors, containing a radiation source, a circulator, an optical fiber, sensors based on Bragg gratings, a spectral analyzer constructed, for example, based on controlled unbalanced Mach-Zehnder interferometers, a control and processing unit including a photodetector and microcontroller, characterized in that the photodetector is connected through an amplifier to the signal input of a synchronous detector, the reference input of which is connected to a sinusoidal voltage generator the output of the synchronous detector through a series-connected low-pass filter and a polarity detector is connected to the digital inputs of the microcontroller, the digital output of which is connected to the digital input of the digital-to-analog converter, while the analog output of the digital-to-analog converter is connected to the first input of the adder, the second input of the adder is connected to a sinusoidal voltage generator and the adder output through the matching circuit is connected to the control electrodes of the spectral analyzer.

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