CN119043388A - Light intensity stepping frequency modulated full-weak grating array demodulation device and method - Google Patents
Light intensity stepping frequency modulated full-weak grating array demodulation device and method Download PDFInfo
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Abstract
The present disclosure relates to the technical field of optical fiber sensing, and provides an isotactic weak grating array demodulation device and method for modulating light intensity step frequency, which realizes multi-wavelength scanning of an isotactic weak optical grating array by step-adjusting laser wavelength. And at each wavelength, the demodulation capability of the reflected signals of the fiber grating is enhanced by carrying out line width broadening and intensity modulation on laser. After the reflected signal is processed by an amplitude-frequency phase-frequency response unit, a frequency response function under different modulation frequencies is generated, and time domain response data is obtained through inverse Fourier transform. Finally, spectrum splicing is carried out on the time domain response data with different wavelengths, so that full-range accurate testing of the fiber bragg grating array is realized. The method synchronously improves the detection range, the spatial resolution and the signal to noise ratio of the weak fiber grating array, so that the method can be applied to the field of large-range and high-definition monitoring, and is suitable for high-precision demodulation of the positions and the wavelengths of all gratings in the long-distance, large-capacity and high-density full-identical weak fiber grating array.
Description
Technical Field
The present disclosure relates to the field of optical fiber sensing technology, and is especially one kind of light intensity stepped frequency modulated full weak grating array demodulation device and method.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
At present, the fiber bragg grating sensing technology has wide application in quasi-distributed measurement of physical quantities such as temperature, strain and the like by virtue of the advantages of electromagnetic interference resistance, high response speed, long-term stable operation in a severe environment and the like. However, most of the existing fiber grating demodulation systems adopt high-reflectivity fiber gratings, and the signal intensity and the sensitivity are high, but the grating multiplexing number of each channel is limited due to the limitation of the spectral bandwidth, and is usually about 10 to 20. In addition, these systems often suffer from poor mechanical reliability of the connection point when connecting or welding optical fibers, especially in long-distance, large-scale monitoring, which is difficult to compare favorably with distributed optical fiber sensing technology.
Along with the development of the large-scale preparation and online writing technology of the weak gratings, the multiplexing capability of the isotactic weak grating array is obviously improved, but the returned useful signals are weak due to the extremely low reflectivity of the weak gratings, so that signal crosstalk and shadow effect are easy to generate, and the accurate demodulation of wavelengths and the accurate positioning of positions become a great challenge. The existing time division multiplexing technology based on pulse light and demodulation technology based on light frequency domain reflection respectively face the problem that pulse width is inversely proportional to spatial resolution and detection distance, and the problem of nonlinear tuning of a light source caused by the increase of frequency scanning range limit the measurement distance, resolution and signal-to-noise ratio of a system. The existing time division multiplexing technology based on pulse light faces the technical problem that the pulse width is inversely proportional to the spatial resolution and the detection distance when the fiber bragg grating sensor is demodulated. In particular, a narrow pulse can increase spatial resolution, but due to the limitation of pulse width, its detection distance decreases correspondingly and the signal-to-noise ratio decreases. At the same time, the narrow pulses require a higher bandwidth photodetector, resulting in increased system noise and increased cost. These factors together limit the application of time division multiplexing techniques in long distance and high accuracy monitoring. Demodulation techniques based on Optical Frequency Domain Reflection (OFDR), while excellent in terms of dynamic range and signal-to-noise ratio, have a measurement distance and spatial resolution that are severely limited by the frequency sweep range of the light source. As the frequency scanning range increases, the problem of nonlinear tuning of the light source becomes more prominent, which causes different beat frequencies of the scattered signal and the reference signal at the same position at different time points, thereby affecting the demodulation accuracy and the signal-to-noise ratio of the system. In addition, complex signal processing algorithms increase the complexity and cost of the system, limiting the practical application effectiveness of OFDR techniques.
Although there have been some patents and studies attempting to optimize fiber grating demodulation systems, such as improving signal processing capability and system integration design by pulsed wavelength scanning lasers and OFDR techniques, the core difficulties in demodulation of weak grating signals have not been effectively solved, particularly while improving the dynamic range and spatial resolution of the system, while maintaining low cost and high real-time performance.
Disclosure of Invention
In order to solve the above problems, the present disclosure provides an isotactic weak grating array demodulation device and method for modulating the light intensity step frequency, which synchronously improves the detection range, spatial resolution and signal to noise ratio of the weak optical fiber grating array, so that the demodulation device and method can be applied in the field of large-scale and high-definition monitoring, and is suitable for high-precision demodulation of each grating position and wavelength in the long-distance, large-capacity and high-density isotactic weak optical fiber grating array.
In order to achieve the above purpose, the present disclosure adopts the following technical scheme:
one or more embodiments provide an isotactic weak grating array demodulation device of light intensity stepping frequency modulation, which comprises a narrow-band wavelength tunable laser, an input laser modulation unit and a reflected light processing unit;
The narrow-band wavelength tunable laser is used for sequentially emitting laser with different wavelengths, and converting the wavelength of the laser output by the next measurement according to a set step length after the one-time measurement is realized;
The input laser modulating unit is used for carrying out line width broadening on laser and carrying out intensity modulation on the basis of modulating signals with different modulating frequencies to obtain input laser of the full-weak grating array;
the reflected light processing unit comprises a frequency-phase-frequency response unit which is used for filtering and converting the light reflected by the full weak grating array to obtain an amplitude-frequency response value and a phase-frequency response value under the corresponding modulation frequency.
The demodulation method of the full-identical weak grating array demodulation device based on the light intensity stepping frequency modulation comprises the following steps:
Sequentially emitting laser with different wavelengths;
performing linewidth broadening on the laser of each wavelength to form low-coherence laser, performing intensity modulation on the low-coherence laser according to the modulation signals of the step frequency to obtain input lasers corresponding to different modulation signals, and sequentially inputting the input lasers into an isotactic weak fiber grating array;
detecting the reflected light of the full-weak fiber grating array, and processing the detection signal based on an amplitude-frequency phase-frequency response unit to obtain an amplitude-frequency phase-frequency response value under the modulation frequency;
Combining amplitude-frequency phase-frequency response values corresponding to all modulation frequencies under the same wavelength to form a frequency response function, and obtaining time domain response data containing the position information and the reflection intensity information of all gratings in the full weak fiber grating array under the corresponding wavelength through inverse Fourier transform;
And performing spectrum splicing and comparison on the reflection intensities of the weak gratings at the same position in the corresponding time domain response data under different wavelengths to obtain a test result of the isotactic weak fiber grating array.
Compared with the prior art, the beneficial effects of the present disclosure are:
(1) In the method, the narrow-band wavelength tunable laser scans laser light in different wavelengths in the isotactic weak grating array by adjusting the laser wavelength in a stepping way, and each grating in the isotactic weak grating array can be effectively excited by combining line width broadening and intensity modulation of the laser light by the input laser modulation unit. The reflected light processing unit processes the returned reflected light signals, and amplitude-frequency response and phase-frequency response data are obtained through the frequency-phase-frequency response unit, so that accurate demodulation of the grating array is realized. The device keeps lower system complexity and cost while improving the system resolution and dynamic range, and realizes the high-precision demodulation of the weak grating array.
In addition, the adopted light intensity modulation is step frequency modulation, and the retention time of each modulation frequency is not lower than the round trip propagation time of the detection light in the isotactic weak fiber grating array, so that enough time is ensured, the linearity of the frequency modulation and the uniformity of step frequency intervals are ensured, and the accuracy of the conversion from frequency response to time domain response is ensured.
(2) The demodulation method realizes multi-wavelength scanning of the isotactic weak fiber grating array by stepwise adjusting the laser wavelength. And at each wavelength, the demodulation capability of the reflected signals of the fiber grating is enhanced by carrying out line width broadening and intensity modulation on laser. After the reflected signal is processed by an amplitude-frequency phase-frequency response unit, a frequency response function under different modulation frequencies is generated, and time domain response data is obtained through inverse Fourier transform. Finally, spectrum splicing is carried out on the time domain response data with different wavelengths, so that full-range accurate testing of the fiber bragg grating array is realized.
(3) The maximum measurement distance of the device in the disclosure is only related to the size of the stepping frequency, the measurement distance can be improved by reducing the size of the stepping frequency, the minimum spatial resolution is related to the bandwidth size of frequency modulation, the minimum spatial resolution index can be improved by increasing the modulation frequency range, the two indexes are realized without mutual restriction factors, and the high-precision demodulation of the positions and the wavelengths of all gratings in the long-distance, large-capacity and high-density isotactic weak fiber grating array can be synchronously realized by combining the wavelength scanning technology.
The advantages of the present disclosure, as well as those of additional aspects, will be described in detail in the following detailed description of embodiments.
Drawings
The accompanying drawings, which are included to provide a further understanding of the disclosure, illustrate and explain the exemplary embodiments of the disclosure and together with the description serve to explain and do not limit the disclosure.
Fig. 1 is a schematic diagram of a demodulation apparatus of an isotactic weak grating array modulated by light intensity stepping frequency according to an embodiment 1 of the present disclosure;
fig. 2 is a wavelength tuning schematic of the output of the narrow-band wavelength tunable laser 1 of embodiment 1 of the present disclosure;
Fig. 3 is a schematic diagram of the line width broadening of laser light driven by the optical fiber phase modulator 2 via the broadband noise source 3 according to embodiment 1 of the present disclosure;
FIG. 4 is a schematic diagram of the modulation of the stepping frequency of the laser intensity according to embodiment 1 of the disclosure;
fig. 5 is a schematic diagram of the structure of the amplitude-frequency phase-frequency response unit 8 in embodiment 1 of the present disclosure;
FIG. 6 is a schematic diagram of the conversion from frequency response to time domain response at a laser wavelength according to embodiment 1 of the present disclosure;
fig. 7 is a schematic view of a wavelength splice fit of a weak fiber bragg grating at a certain position in embodiment 1 of the present disclosure;
Fig. 8 is a flow chart of a demodulation method of an isotactic weak grating array with light intensity stepping frequency modulation according to embodiment 1 of the present disclosure;
FIG. 9 is a graph of actual demodulated isotactic weak grating array local grating reflection intensity of embodiment 1 of the present disclosure;
FIG. 10 is a plot of center wavelength of weak grating reflection at a location in an fully demodulated fully weak grating array of example 1 of the present disclosure;
wherein: 1, a narrow-band wavelength tunable laser, 2, an optical fiber phase modulator, 3, a broadband noise source, 4, an optical fiber intensity modulator, 5, a direct digital frequency synthesizer I, 6, an optical fiber circulator, 7, a photoelectric detector, 8, a amplitude-frequency phase-frequency response unit, 9, a signal processing and control circuit, 10 and an industrial control computer;
804. a second direct digital frequency synthesizer 805, a first mixer 806, a first low pass filter 807, a third direct digital frequency synthesizer 808, a phase-locked loop 809, a sinusoidal signal buffer unit 810, a phase shifter 811, a second mixer 812, a third mixer 813, a second low pass filter 814, a third low pass filter 815, an analog-to-digital converter 816, an analog-to-digital converter 817, and an FPGA unit;
101. A first optical output interface, 102, a first electrical signal input interface, 201, a first optical input interface, 202, a second electrical signal input interface, 203, a second optical output interface, 301, a modulation port, 401, a second optical input interface, 402, a third electrical signal input interface, 403, a third optical output interface, 501, a second electrical signal output interface, 502, a fourth electrical signal input interface, 601, a third optical input interface, 602, a first optical bidirectional transmission port, 603, a fourth electrical signal output port, 701, a fourth optical input interface, 702, a detection signal output port, 801, a fifth electrical signal input interface, 802, a sixth electrical signal input interface, 803, a fifth electrical signal output interface, 901, a seventh electrical signal input interface, 902, a first electrical signal communication interface, 903, a first control signal interface, 904, a second control signal interface, 905, a third control signal interface.
Detailed Description
The disclosure is further described below with reference to the drawings and examples.
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments in accordance with the present disclosure. As used herein, the singular is also intended to include the plural unless the context clearly indicates otherwise, and furthermore, it is to be understood that the terms "comprises" and/or "comprising" when used in this specification are taken to specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof. It should be noted that, without conflict, the various embodiments and features of the embodiments in the present disclosure may be combined with each other. The embodiments will be described in detail below with reference to the accompanying drawings.
Example 1
In one or more embodiments, as shown in fig. 1 to 10, an isotactic weak grating array demodulation device for modulating light intensity step frequency comprises a narrow-band wavelength tunable laser 1, an input laser modulation unit and a reflected light processing unit;
The narrow-band wavelength tunable laser 1 is used for sequentially emitting laser with different wavelengths, and converting the wavelength of the laser output by the next measurement according to a set step length after one measurement is realized;
The input laser modulating unit is used for carrying out line width broadening on laser and carrying out intensity modulation on the basis of modulating signals with different modulating frequencies to obtain input laser of the full-weak grating array;
specifically, the modulation frequency of the laser light under each wavelength is respectively converted by a set frequency step, and the laser light is measured for a plurality of times to obtain the input laser light of the isotactic weak grating array under different intensities;
The reflected light processing unit comprises an amplitude-frequency phase-frequency response unit 8, which is used for filtering and converting the light reflected by the full weak grating array to obtain an amplitude-frequency response value H AFR(fk) and a phase-frequency response value H PFR(fk under the corresponding modulation frequency;
In this embodiment, the narrow-band wavelength tunable laser 1 scans laser light with different wavelengths in the isotactic weak grating array by adjusting the laser wavelength in a stepping manner, and combines line width broadening and intensity modulation of the laser light by the input laser modulation unit, so that each grating in the isotactic weak grating array can be effectively excited. The reflected light processing unit processes the returned reflected light signals, and amplitude-frequency response and phase-frequency response data are obtained through the frequency-phase-frequency response unit, so that accurate demodulation of the grating array is realized. The device improves the spatial resolution and the dynamic range of the system (the larger the dynamic range is, the longer the measurable distance is), simultaneously keeps lower system complexity and cost, and realizes the high-precision demodulation of the weak grating array. In addition, the distinguishing capability of different grating positions is enhanced by a stepping frequency modulation mode, and the problems of signal crosstalk and shadow effect in the traditional method are avoided.
In some embodiments, a narrow band wavelength tunable laser 1 is used to produce a continuous laser of several tens milliwatts in power with a wavelength that varies at equally spaced intervals by Δλ, with a wavelength variation ranging from λ min to λ max, where λ max＝λmin +mΔλ. The total variation of wavelengths is M times in a variation period, which is about several nanometers, and the middle wavelength is the same as the center wavelength lambda c of the isotactic weak fiber array.
In some embodiments, the input laser modulation unit includes a fiber phase modulator 2, a broadband noise source 3, a fiber intensity modulator 4, and a direct digital frequency synthesizer one 5;
the optical fiber phase modulator 2 is connected with the output end of the narrow-band wavelength tunable laser 1 and is used for carrying out phase modulation on input continuous laser;
The broadband noise source 3 is connected with a modulation port of the optical fiber phase modulator 2 to generate broadband random weak noise, and excites the optical fiber phase modulator 2 to perform phase modulation on input continuous laser, so that the line width of the laser is widened, the coherence of light waves is reduced, and the coherent crosstalk of light among gratings is reduced;
the optical fiber intensity modulator 4 is connected with the output end of the optical fiber phase modulator 2 and modulates the light intensity of the low-coherence laser under the drive of the stepping modulation frequency output by the direct digital frequency synthesizer I5;
a direct digital frequency synthesizer I5 for generating a radio frequency modulation signal with frequency step change, and connecting with a modulation port of the optical fiber intensity modulator 4 to provide a modulation signal;
Specifically, the frequency range of the rf modulation signal of the direct digital frequency synthesizer one 5 is F start to F end,fend＝fstart +nΔf, Δf is the step length of the frequency modulation, F start =Δf can be taken, so F end = (n+1) Δf, and the modulation bandwidth is Δf=nΔf, which can be several hundred MHz to several tens GHz.
The time t keep of each modulation frequency is not lower than the round trip propagation time of the laser light in the length L of the isotactic weak fiber grating array:
tpropagate＝L*ngroup/2c
Where c is the speed of light in vacuum and n group is the average of the group refractive indices of the incident and back-reflected light.
Specifically, at a certain modulation frequency f k =kΔf, k=1, 2,3,. The.i., N, n+1; the optical power output by the optical fiber intensity modulator 4 is:
Where P 0 is the optical power entering the optical fiber intensity modulator 4, m is the modulation depth, Is the initial phase of the modulated signal.
In the scheme, the continuous laser modulated by intensity is used as the detection light, so that the average power of signals reflected by the full-length weak fiber grating array is improved to the maximum extent, the grating reflectivity in the full-length weak fiber grating array can be reduced to 0.001%, the shadow effect is effectively overcome, the signal to noise ratio is improved, and meanwhile, the continuous laser is subjected to line width broadening, the coherence of the continuous laser is reduced, the crosstalk between adjacent gratings is reduced, and the grating capacity in the full-length weak reflection grating array is further improved.
Further, the device also comprises an optical fiber circulator 6 and a photoelectric detector 7;
the optical fiber circulator 6 is connected with the optical fiber intensity modulator 4, continuous laser modulated by light intensity is incident to an isotactic weak fiber grating array through the optical fiber circulator 6, and reflected light is returned to the optical fiber circulator 6 in sequence after being reflected by each weak fiber grating;
At this time, at a certain modulation frequency f k, the returned optical power is:
wherein z is the position of the reflection point of the full weak optical fiber array, alpha is the attenuation coefficient of the transmission and reflection of the incident light in the optical fiber, Is the phase of the light introduced by the modulation during transmission.
The photoelectric detector is connected with the optical fiber circulator and is used for detecting the light intensity of returned light reflected by the full weak optical fiber grating array, converting the light intensity into current, ignoring a direct current item irrelevant to a certain modulation frequency f k, and the signal of the alternating current part is as follows:
Wherein V k is the signal amplitude after photoelectric conversion and is in direct proportion to mP 0eα·z.
In some embodiments, the reflected light processing unit includes an amplitude-frequency phase-frequency response unit 8, a signal processing and control circuit 9, and an industrial control computer 10 connected in sequence;
The amplitude-frequency phase-frequency response unit 8 processes the detection signal returned by the isotactic weak fiber grating array into two paths of intersecting signals, and then adopts an I/Q demodulation algorithm to obtain an amplitude-frequency response value H AFR(fk) which is proportional to V k and is at the modulation frequency f k Proportional phase frequency response value H PFR(fk);
The signal processing and controlling circuit 9 is configured to perform fusion processing on the amplitude frequency response value H AFR(fk) and the phase frequency response value H PFR(fk) to obtain time domain response data H x (t) of the position information and the intensity information of each grating in the full-weak fiber grating array;
the industrial control computer 10 performs processing such as display and fitting on the data processed by the signal processing and control circuit 9.
Further, the amplitude-frequency phase-frequency response unit 8, as shown in fig. 5, includes a direct digital frequency synthesizer two 804, a mixer one 805, a low-pass filter one 806, a direct digital frequency synthesizer three 807, a phase-locked loop 808, a sinusoidal signal buffer unit 809, a phase shifter 810, a mixer two 811, a mixer three 812, a low-pass filter two 813, a low-pass filter three 814, an analog-to-digital converter one 815, an analog-to-digital converter two 816, and an FPGA unit 817;
A direct digital frequency synthesizer II 804 generates a local oscillation signal U (f l, t) with a step change signal frequency which keeps a fixed difference with the modulation frequency;
Specifically, the local oscillator signal generated by the direct digital frequency synthesizer two 804 is:
Wherein, For the initial phase of the local oscillation signal, the frequency of the local oscillation signal is f l, the frequency difference f i between the frequency of the local oscillation signal and the frequency of f k is always kept fixed in the modulation process, namely, f l＝fi+fk＝fi +kΔf is also changed in a stepping way, and V l represents the voltage amplitude of the local oscillation signal.
A mixer one 805 for mixing the detected signal U (f k, t) of the ac portion with the local oscillator signal U (f l, t);
The low-pass filter one 806 filters the signal mixed by the mixer one 805, performs electrical heterodyne detection to obtain an intermediate frequency signal, and the obtained intermediate frequency signal may be expressed as:
in the embodiment, an electrical heterodyne detection technology is adopted, the detection signal and the local vibration signal are mixed and filtered and then become intermediate-frequency signals which are easy to collect and process, the frequency is about several kHz, the detection bandwidth is narrow, the noise interference is effectively reduced, and the signal-to-noise ratio and the dynamic range are improved;
A direct digital frequency synthesizer III 807 for generating a reference signal having a frequency f ref equal to the frequency difference f i, forming a stable sinusoidal reference signal by a phase-locked loop 808 Phase-shifted by phase shifter 810 to form a stable cosine reference signal
A mixer two 811 for mixing the intermediate frequency signal U (f i, t) with the sinusoidal reference signal U sin(fref, t);
low-pass filter two 813 filters the signal mixed by mixer two 811 to obtain:
An analog-to-digital converter I815, which converts the data filtered by the low-pass filter II 813 into a digital signal X k-D;
A mixer three 812, which mixes the intermediate frequency signal U (f i, t) with the sinusoidal reference signal U sin(fref, t);
Low pass filter three 814 filters the data mixed by mixer three 812 to obtain:
An analog-to-digital converter II 816 for converting the data filtered by the low-pass filter III 814 into a digital signal Y k-D;
FPGA unit 817, the signals of X k-D and Y k-D are subjected to I/Q demodulation algorithm to obtain amplitude-frequency response value H AFR(fk) which is proportional to V k and is at modulation frequency f k Proportional phase frequency response value H PFR(fk).
In the above embodiment, the local oscillation signal generated by the direct digital frequency synthesizer two 804 is mixed with the detected signal to obtain the intermediate frequency signal. Amplitude and phase frequency information can be extracted from the detected signal by processing with a multistage low-pass filter and mixer. Finally, FPGA 817 processes the analog-to-digital converted signal by an I/Q demodulation algorithm to obtain amplitude-frequency response and phase-frequency response values at different modulation frequencies. These values can reflect the spectral characteristics and the position distribution of an isotactic weak grating array. And by adopting an electrical heterodyne detection technology, the signal-to-noise ratio and the measurement accuracy of the system are effectively improved. The introduction of the FPGA unit 817 enables the real-time processing of large-scale data, and the demodulation efficiency and the real-time performance of the system are obviously improved. In addition, by reasonable frequency selection and signal processing, the system is able to accurately measure and resolve the reflection characteristics of the grating.
The signal processing and controlling circuit is connected with the output end of the amplitude-frequency phase-frequency response unit 8, receives the amplitude-frequency response value H AFR(fk) and the phase-frequency response value H PFR(fk under each step modulation frequency, forms a frequency response function H (f) in the whole frequency modulation period, and obtains time domain response data H x (t) containing the position information and the intensity information of each grating in the isotactic weak fiber grating array under a certain wavelength lambda x of the narrow-band wavelength tunable laser through inverse Fourier transform IFFT.
Further, the signal processing and control circuit 9 is configured to provide the trigger signal under a unified clock to the narrow band wavelength tunable laser 1, the direct digital frequency synthesizer 15 and the amplitude-frequency phase-frequency response unit 8, so as to reduce phase jitter.
And the industrial control computer 10 is connected with the signal processing and control circuit 9 and is used for receiving the time domain response data output by the signal processing and control circuit 9 and performing processing such as display, fitting and the like.
When the wavelength is tuned from lambda min to lambda min +Mdelta lambda, M+1 groups of time domain response data are obtained, wherein the positions of the weak fiber gratings are the same, but the reflection intensities are different, and the wavelength values of the weak fiber gratings at all positions are obtained through splicing fitting of the reflection intensities of different wavelengths at the same position.
According to the characteristics of a linear motion invariant system, the spatial resolution of the fully-weak grating array demodulation device with light intensity stepping frequency modulation in the embodiment is that the maximum measurable distance is L max＝c/2ngroup Deltaf.
The maximum measurement distance is only related to the size of the stepping frequency, the measurement distance can be improved by reducing the size of the stepping frequency, the minimum spatial resolution is only related to the bandwidth size of frequency modulation, the minimum spatial resolution index can be improved by increasing the modulation frequency range, and the realization of the two indexes does not have mutual restriction factors.
The following description will be made with reference to specific embodiments;
The demodulation device comprises a narrow-band wavelength tunable laser 1, an optical fiber phase modulator 2, a broadband noise source 3, an optical fiber intensity modulator 4, a direct digital frequency synthesizer 5, an optical fiber circulator 6, a photoelectric detector 7, an amplitude-frequency phase-frequency response unit 8, a signal processing and control circuit 9 and an industrial control computer 10.
Assuming that the center wavelength of the grating in the isotactic weak fiber grating array is 1550nm, the interval between the fiber gratings is 10cm, the array length is approximately 10km, and the reflectivity of each weak fiber grating is 0.001%.
The narrow-band wavelength tunable laser 1 is connected to the first control signal interface 903 of the signal processing and control circuit 9 through the first electrical signal input interface 102, and repeatedly generates continuous laser light with a wavelength of 1549nm to 1551nm under the control of the signal processing and control circuit 9, the optical power is about 20dBm, the wavelength is changed at intervals of 10pm, and the wavelength is changed 200 times in one tuning period. As shown in fig. 2, a schematic diagram of the wavelength tuning of the output of a narrow band wavelength tunable laser over time is shown.
The optical fiber phase modulator 2 is connected with the first optical output interface 101 of the narrow-band wavelength tunable laser 1 through the first optical input interface 201, and is used for carrying out phase modulation on input continuous laser, the working wavelength range of the optical fiber phase modulator 2 is 1525nm to 1605nm, the output laser wavelength range of the narrow-band wavelength tunable laser 1 is covered, the bandwidth is 10GHz, and the output laser of the tunable laser 1 can be processed;
The optical fiber phase modulator 2 changes the coherence of the continuous laser light by phase modulating the laser light so that the laser light has different phase characteristics during propagation. After phase modulation, the linewidth of the laser is widened, which is helpful to excite each grating in the full weak grating array and reduce the interference of the reflected light signal. By means of phase modulation, the influence of laser coherence on the demodulation precision of the system can be effectively reduced, and the detection sensitivity of the weak grating is improved. The method can also avoid phase noise generated in the optical fiber transmission process, thereby improving the stability of the system.
The broadband noise source 3 is connected with the second electric signal input interface 202 of the optical fiber phase modulator 2 through the first electric signal output interface 201, outputs a broadband random noise signal with the power of about-10 dBm, widens the line width of the input continuous laser to about 1GHz, and ensures that the coherence length of the laser is less than twice of the grating interval, thereby reducing the coherence crosstalk of light among gratings. As shown in fig. 3, a schematic diagram of the line width broadening of laser light by driving the fiber phase modulator via a broadband noise source is shown.
The random weak noise generated by the broadband noise source 3 is applied to the optical fiber phase modulator through the modulation port 301, so that the phase of the laser is caused to randomly change within a certain range, and the linewidth of the laser is widened. The laser with the widened line width has lower coherence, which is beneficial to demodulating the signal of the full weak grating array and can reduce the crosstalk between reflected light.
By adding a broadband noise source, the coherence of laser is further reduced, the detection capability of the system to the weak grating is improved, and the problem of signal crosstalk is effectively solved. Meanwhile, broadband noise can also smooth laser spectrum lines, and demodulation accuracy is improved.
The optical fiber intensity modulator 4 is connected with the second optical output interface 203 of the optical fiber phase modulator 2 through the second optical input interface 401, and is used for modulating the intensity of the input continuous laser, the working wavelength range of the optical fiber intensity modulator 4 is 1525nm to 1605nm, and the frequency range of the input radio frequency modulation signal is 0Hz to 15GHz.
The optical fiber intensity modulator 4 adjusts the light intensity of the laser light to generate corresponding responses at different modulation frequencies. The direct digital frequency synthesizer 5 is responsible for generating a precise step modulation frequency signal to ensure that the reflected light generated by the laser in the fully weak grating array can be stably detected. The time kept by each modulation frequency is matched with the propagation time of the laser in the grating array, so that the effective acquisition of signals is ensured. The modulation mode can accurately control the light intensity and the modulation frequency of the laser, improves the measurement accuracy and the reliability of the system, particularly can keep stable signal response in long-distance optical fiber transmission, and effectively avoids measurement errors caused by mismatching of the modulation frequency.
The direct digital frequency synthesizer I5 is connected with the second control signal interface 904 of the signal processing and control circuit 9 through the fourth electric signal input interface 502 to generate a modulation signal of 10kHz to 2GHz+10kHz, the power of the modulation signal is about 10dBm, the step frequency is 10kHz, the frequency holding time under each modulation frequency is not less than 1ms, and the second electric signal output interface 501 is connected with the third electric signal input interface 402 of the optical fiber intensity modulator 4 to enable the optical fiber intensity modulator 4 to modulate the light intensity of continuous laser by adopting the modulation signal. As shown in fig. 4, a schematic diagram of the laser intensity stepping frequency modulation is shown.
The optical fiber circulator 6 is connected with the third optical output interface 403 of the optical fiber intensity modulator 4 through the third optical input interface 601, continuous laser light modulated by light intensity is used as detection light to be input into the full-weak optical fiber grating array through the first optical bidirectional transmission port 602, then optical signals reflected by the weak optical fiber gratings are transmitted to the fourth optical signal output port 603 through the first optical bidirectional transmission port 602, and the isolation among the ports of the optical fiber circulator 6 is more than 50dB.
The photodetector 7 is connected to the fourth optical signal output port 603 of the optical fiber circulator 6 through the fourth optical input interface 701, converts the received optical signal into an electrical signal, and has a bandwidth of about 20GHz, and optionally amplifies the output photocurrent while detecting the optical signal by using an avalanche photodiode of indium gallium arsenide.
The use of the optical fiber circulator 6 improves the transmission efficiency and the signal recovery rate of the optical signal, reduces the loss of the optical signal, and is beneficial to enhancing the signal-to-noise ratio of the system. The high sensitivity of the photodetector 7 ensures accurate conversion of the weak reflected signal, and improves the performance of the whole demodulation system.
The amplitude-frequency phase-frequency response unit 8, as shown in fig. 5, includes a fifth electric signal input interface 801, a sixth electric signal input interface 802, a fifth electric signal output interface 803, a second direct digital frequency synthesizer 804, a first mixer 805, a first low-pass filter 806, a third direct digital frequency synthesizer 807, a phase-locked loop 808, a sinusoidal signal buffer 809, a phase shifter 810, a second mixer 811, a third mixer 812, a second low-pass filter 813, a third low-pass filter 814, a first analog-to-digital converter 815, a second analog-to-digital converter 816, and an FPGA unit 817.
The amplitude-frequency phase-frequency response unit 8 is connected with a fourth electric signal output port of the photoelectric detector 7 through a fifth electric signal input interface 801, and the processing procedure is as follows:
The electric signal output from the photodetector 7 is input to the first mixer 805 through the detection signal output port 702;
generating a modulating signal A with the frequency range of 5kHz to 2GHz+5kHz and the stepping frequency of 10kHz by a direct digital frequency synthesizer II 804;
the modulation signal A and the electric signal output by the photoelectric detector 7 are mixed in a mixer I805;
Filtering by a first low-pass filter 806 to obtain an intermediate frequency signal B with a main frequency of 5 kHz;
a reference signal with the frequency of 5kHz is generated by a direct digital frequency synthesizer III 807, phase-locked and shaped into a standard sine signal C by a phase-locked loop 808, input into a sine signal buffer unit 809, phase-shifted by a phase shifter 810 and changed into a standard cosine signal D;
The cosine signal D and the intermediate frequency signal B are mixed in a mixer II 811, the obtained signal is filtered in a low-pass filter II 813, and the obtained signal is converted into a digital signal E1 by an analog-to-digital converter I815;
the sinusoidal signal C is mixed with the intermediate frequency signal B in the mixer three 812, the resulting signal is filtered in the low pass filter three 814, converted to a digital signal E2 by the analog-to-digital converter two 816,
The two sets of digital signals E1 and E2 are then transmitted to the FPGA unit 817 to perform an I/Q demodulation algorithm, so as to obtain amplitude-frequency and phase-frequency response values containing weak grating position and intensity information at a certain modulation frequency of the direct digital frequency synthesizer-5, and output the amplitude-frequency and phase-frequency response values through the fifth electrical signal output interface 803.
The signal processing and controlling circuit 9 sequentially receives and stores the amplitude frequency and phase frequency response values outputted by the fifth electric signal output interface 803 of the amplitude frequency and phase frequency response unit 8 through the seventh electric signal input interface 901, and when the modulation signals of each frequency generated by the direct digital frequency synthesizer one 5 return the amplitude frequency and phase frequency response values at a certain wavelength, the modulation signals form a frequency response function, and performs inverse fourier transform IFFT, so that the position information and the reflection intensity information of each weak fiber grating at the wavelength can be obtained.
Fig. 6 is a schematic diagram showing the conversion from frequency response to time domain response at a certain laser wavelength in the present embodiment. The control signal is then output by the first control signal interface 903 of the signal processing and control circuit 9 to cause the narrow band wavelength tunable laser 1 to switch outputting the continuous laser light of the next wavelength, and the above-described process is repeatedly performed. A third control signal interface 905 of the signal processing and control circuit 9 is connected to the sixth electrical signal input interface 802, transmitting the control signal to the amplitude-frequency phase-frequency response unit;
The industrial control computer 10 is connected with the first data communication interface 902 of the signal processing and control circuit 9 through the second data communication interface 1001, sequentially receives time domain data containing position information and reflection intensity information of each weak fiber grating under each laser wavelength returned by the first control signal interface 903 of the signal processing and control circuit 9, and obtains wavelength information of each weak fiber grating at each position through splicing fitting of reflection intensities of the same position and different laser wavelengths, and displays the wavelength information, as shown in fig. 7, as a schematic diagram of splicing fitting of wavelength of each weak fiber grating at a certain position in this embodiment.
Fig. 8 is a schematic flow chart of a demodulation method of an isotactic weak grating array with light intensity stepping frequency modulation provided by the embodiment, and fig. 9 and fig. 10 are a reflected light intensity diagram of a local grating of the isotactic weak grating array and a fitting diagram of a central wavelength of a weak grating at a certain position, which are actually demodulated by the invention. Finally, through the implementation of the device and the method, the spatial resolution is 5cm, the maximum measurement distance is 10km, the grating wavelength resolution is 10pm, and the device and the method are suitable for high-precision demodulation of the positions and the wavelengths of all gratings in a long-distance, large-capacity and high-density full-identical weak fiber grating array.
Example 2
Based on embodiment 1, the demodulation method of the isotactic weak grating array demodulation device based on light intensity stepping frequency modulation in the embodiment comprises the following steps:
step 1, emitting laser with different wavelengths;
Specifically, the narrow-band wavelength tunable laser 1 is controlled to output continuous laser light of a set wavelength converted in steps according to the wavelength;
Step 2, performing linewidth broadening on the laser of each wavelength to form low-coherence laser, and performing intensity modulation on the low-coherence laser according to the modulation signals of the step frequency to obtain input laser corresponding to different modulation signals, and inputting the input laser into an isotactic weak fiber grating array;
Step 3, detecting the reflected light of the identical weak fiber grating array, and processing the detection signal based on an amplitude-frequency phase-frequency response unit to obtain an amplitude-frequency phase-frequency response value under the modulation frequency, wherein the amplitude-frequency phase-frequency response value comprises an amplitude-frequency response value H AFR(fk) and a phase-frequency response value H PFR(fk);
Step 4, combining amplitude-frequency phase-frequency response values under each modulation frequency into a frequency response function H (f), and obtaining time domain response data H x (t) containing the position information and the reflection intensity information of each grating in the isotactic weak fiber grating array when the wavelength is lambda x through inverse Fourier transform;
Step 5, spectrum splicing and comparison are carried out on the weak grating reflection intensities at the same position in the time domain response data corresponding to the starting wavelength lambda min to the ending wavelength lambda max, and a test result of the identical weak fiber grating array is obtained;
The laser device realized in the steps can convert the wavelength to output laser with different wavelengths, the wavelength is converted according to the set step length, the measurement of the isotactic weak fiber grating array under the laser with different wavelengths is realized, the input laser obtained by stepping different modulation signals is used as the isotactic weak fiber grating array for measurement aiming at the laser signal under each step length, the modulation of the laser signal in the whole wavelength and the whole frequency range can be realized, and the high-precision demodulation of each grating position and wavelength in the isotactic weak fiber grating array can be realized.
The following description will be made with specific implementation steps;
S1, controlling a narrow-band wavelength tunable laser 1 to emit laser light at a set wavelength, and setting the narrow-band wavelength tunable laser 1 to output continuous laser light with a central wavelength of lambda x and an initial wavelength of lambda min;
s2, performing line width broadening on the continuous laser in the S1 by using an optical fiber phase modulator 2 driven by a broadband noise source 3 to form low-coherence laser;
S3, generating a modulation signal with the frequency of f k by a direct digital frequency synthesizer I5 to drive an optical fiber intensity modulator 5, and carrying out intensity modulation on the low-coherence laser in S2, wherein the initial modulation frequency is f start;
s4, enabling the laser modulated by the S3 to enter an isotactic weak fiber grating array, detecting reflected light through a photoelectric detector 7, and inputting the reflected light into a amplitude-frequency phase-frequency response unit 8 to obtain an amplitude-frequency response value H AFR(fk) and a phase-frequency response value H PFR(fk) under the modulation frequency;
S5, the signal processing and control circuit 9 stores the data generated in the step S4, and simultaneously triggers the direct digital frequency synthesizer I5 to increase the frequency of the generated modulation signal by delta f;
s6, repeating the steps S3 to S5 until the amplitude frequency response H AFR(fend) and the phase frequency response data H PFR(fend) of the maximum modulation frequency f end are returned, and setting the modulation frequency of the direct digital frequency synthesizer I to f start;
S7, combining amplitude-frequency phase-frequency response values under each modulation frequency stored by the signal processing and control circuit 9 into a frequency response function H (f), and obtaining time domain response data H x (t) containing the position information and the reflection intensity information of each grating in the full-weak fiber grating array when the wavelength is lambda x through inverse Fourier transform, and simultaneously increasing the output center wavelength of the narrow-band wavelength tunable laser by delta lambda;
S8, repeating the steps S1 to S7 until time domain response data h max (t) of the laser tuning termination wavelength lambda max is returned, and setting the laser output wavelength of the narrow-band wavelength tunable laser to lambda min;
s9, spectrum splicing is carried out on the reflection intensity of the weak grating at the same position in the time domain response data corresponding to the starting wavelength lambda min to the ending wavelength lambda max, so that the wavelength of the identical weak fiber grating is demodulated;
S10, repeating the steps S1 to S9, and judging the position and the size of the full weak fiber grating array which generates strain or temperature change by comparing the wavelength changes of the weak fiber gratings at the front and rear positions twice;
The foregoing description of the preferred embodiments of the present disclosure is provided only and not intended to limit the disclosure so that various modifications and changes may be made to the present disclosure by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present disclosure should be included in the protection scope of the present disclosure.
While the specific embodiments of the present disclosure have been described above with reference to the drawings, it should be understood that the present disclosure is not limited to the embodiments, and that various modifications and changes can be made by one skilled in the art without inventive effort on the basis of the technical solutions of the present disclosure while remaining within the scope of the present disclosure.
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