CN111707301A - Demodulation system and method of fiber Bragg grating - Google Patents
Demodulation system and method of fiber Bragg grating Download PDFInfo
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Abstract
The invention provides a demodulation system and a demodulation method of fiber Bragg gratings, wherein the system comprises the following steps: the light source is used for emitting broadband light and entering the broadband light through a first port of the first optical circulator, a second port of the first optical circulator outputs the broadband light to the reference grating, the reference grating reflects light with a reference wavelength, and light with other wavelengths enters the fiber Bragg grating; the fiber Bragg grating is used for receiving and reflecting the light of the reference grating, and the reflected light with the sensing wavelength and the reflected light with the reference wavelength enter the third port from the second port of the first optical circulator and are output to the interferometer; the interferometer receives the light beam output by the third port of the first optical circulator and divides the light beam into two paths, wherein one path carries a disturbance source to enable the arm length difference of the two paths to generate random change; the device also comprises a second circulator, wherein one path of the light is measured by a reference grating filter to obtain light with a sensing wavelength, and the other path of the light is measured by the reference grating filter to obtain light with a reference wavelength. And an unequal-arm-length interferometer with a phase disturbance source is adopted to carry out spatial sampling on echo signals, so that the demodulation speed is increased.
Description
Technical Field
The invention relates to the technical field of demodulation of fiber Bragg gratings, in particular to a demodulation system and a demodulation method of a fiber Bragg grating.
Background
The optical fiber sensor is generally applied to the fields of temperature sensing, curvature sensing and the like due to the advantages of high sensitivity, strong anti-interference capability, variable fine shape of the optical fiber and the like, wherein the optical fiber Bragg grating is most widely applied to the optical fiber sensor due to the advantages of mature manufacturing process, capability of processing a plurality of sensing points on one optical fiber and the like.
The technology of the existing fiber Bragg grating is mature, the fiber Bragg grating is characterized in that after the traditional communication fiber is subjected to doping, hydrogen loading and other processing, the grating with the periodically changed refractive index is engraved in the fiber by utilizing the photoetching technology along the light propagation direction, each interface with the changed refractive index can reflect a small part of light energy, when the interface interval (grating period) is consistent with the wavelength of certain light, the energy of the wavelength is strongly reflected, and other wavelengths normally pass through and are not reflected. When the grating is affected by bending, temperature change and the like to cause the period of the grating to change, the wavelength reflected by the grating changes, and the demodulator can measure the bending or temperature change suffered by a certain grating by measuring the wavelength change of the reflected light. Gratings are very sensitive to changes in external conditions, and typically, a bend in the order of a microradian can cause a change in the wavelength of light.
At present, the fiber Bragg grating process is mature, and the main research focus of the fiber Bragg grating process lies in a demodulation method. The demodulation method which is mature in engineering is low in speed, the general rate of return is below hundred Hz, most products are in Hz level, and the demodulation speed is related to the number of gratings and the demodulation precision, so that the fiber Bragg grating is widely applied to the fields of long-term deformation measurement of buildings and long-term environment monitoring which have low requirements on the measurement speed. The optical fiber sensor has the advantages of interference resistance, high precision, random deformation and the like, is very suitable for being applied to the fields of robots, wearability and the like, but is limited by the problems that the current demodulation speed is low, the demodulation instrument is high in cost and the like, and cannot be applied in a large scale.
There are two general types of fiber bragg grating demodulation methods, one is laser wavelength scanning in the spectral range, and the other is demodulation by a spectrometer. The wavelength scanning type demodulator generally adopts a laser with variable wavelength or a wavelength identifier with variable cavity length, the device uses PZT or MEMS device to drive the distance change of the front and back reflecting surfaces of the resonant cavity, also uses electro-optic or thermo-optic effect to change the refractive index of the resonant cavity medium so as to change the optical length, the light wavelength in the light path is periodically scanned, the reflected light of the fiber Bragg grating is converted into a pulse train on a time sequence, when the wavelength of the reflected light of a certain grating is changed, the pulse returns to the time change so as to obtain the sensing quantity, the method needs to carry out full spectrum range scanning in each measurement, so the measuring speed is slower. The second method is to use a spectrometer to simultaneously collect all spectral information in the spectral range, and although the demodulation speed is high, the spectrometer is expensive, and the method is generally used in a laboratory. Specifically, the following is described.
A demodulation method using spectral scanning. One type of spectral scanning is to change the cavity length of a laser, the laser repeatedly excites the medium in a cavity to generate laser by the reflection of the front and back surfaces of the cavity, the wavelength of the laser is determined by the optical length of the cavity, the emitted laser wavelength is changed when the length of the cavity is changed, the emitted laser wavelength is periodically scanned in the spectral range when the length of the cavity is periodically changed, the scanning period of the cavity is fixed, so the period of the emitted laser wavelength is fixed, the light energy is not reflected when the wavelength is inconsistent with the grating period, the light energy is reflected when the wavelength is consistent with the grating period, and the reflected light is different because the periods of a plurality of gratings are different, so the distribution of the gratings on the spectrum is changed into a pulse string on a time sequence. When a certain grating is curved, the pulses reflected by the grating change in time, whereby a curved curvature is obtained. The disadvantages are that: (1) the whole spectrum needs to be scanned in each measurement, the measurement speed is low, the measurement frequency of the current commercialized demodulator is generally below hundred Hz, and the joint angle sampling frequency/return rate is often required to be above KHz or even higher in the robot field/wearable field; (2) because of the use of variable precision optics, such demodulators are costly, unsuitable for miniaturization and difficult to adapt to all operating conditions.
The laser emits wide spectrum light covering the whole wave band, the light returning to the grating wavelength after being reflected by each grating is displayed as discrete spectral peaks on the spectrum, and the whole spectrum is read at one time through the spectrometer and the change of the spectral peaks is analyzed. The method can read the whole spectrum at one time, and is faster than the spectrum scanning method. The disadvantages are that: (1) the spectrometer is very expensive and the solution is generally only used in the laboratory. (2) Conventional spectrometers typically have spectral resolutions that are not as high as the resolution of the one-technique approach.
Therefore, a demodulation method of the fiber bragg grating with high measurement speed and low cost is lacked in the prior art.
The above background disclosure is only for the purpose of assisting understanding of the concept and technical solution of the present invention and does not necessarily belong to the prior art of the present patent application, and should not be used for evaluating the novelty and inventive step of the present application in the case that there is no clear evidence that the above content is disclosed at the filing date of the present patent application.
Disclosure of Invention
The invention provides a demodulation system and a demodulation method of Fiber Bragg Gratings (FBGs) to solve the existing problems.
In order to solve the above problems, the technical solution adopted by the present invention is as follows:
a demodulation system for fiber bragg gratings comprising: the device comprises a light source, a first optical circulator, a reference grating, a fiber Bragg grating, an interferometer and a disturbance source; the light source is used for emitting broadband light and entering the broadband light through a first port of the first optical circulator, a second port of the first optical circulator outputs the broadband light to the reference grating, the reference grating reflects light with a reference wavelength, and light with other wavelengths enters the fiber Bragg grating; the fiber Bragg grating is used for receiving and reflecting the light of the reference grating, and the reflected light with the sensing wavelength and the reflected light with the reference wavelength enter a third port from the second port of the first optical circulator and are output to the interferometer; and the interferometer receives the light beam output by the third port of the first optical circulator and divides the light beam into two paths, wherein one path carries a disturbance source to enable the arm length difference of the two paths to generate random change, and the two paths of arm length difference respectively detect the light with the sensing wavelength and the light with the reference wavelength.
Preferably, the device further comprises a second circulator, a reference grating filter, a first detector and a second detector; the interference mixed signal enters from a first port of the second circulator, a second port outputs to the reference grating filter, the reflection wavelength of the reference grating filter is consistent with the reference wavelength, the light of the reference wavelength is reflected, and the light of the sensing wavelength enters the first detector to be detected; light of the reference wavelength enters from the second port of the second circulator, and a third port outputs to the second detector to be detected.
Preferably, the disturbance source is an ambient disturbance dither, a vibration element, a phase modulator or a tactile signal of the fiber bragg grating.
The invention also provides a demodulation method of the fiber bragg grating, which demodulates the fiber bragg grating by adopting any one of the demodulation systems of the fiber bragg grating, and specifically comprises the following steps: s1: selecting a disturbance source and a sampling rate according to the grating number of the fiber Bragg grating; s2: adopting a compressed sensing algorithm, taking the light intensity value output by each interferometer as a primary random sampling result s of a time domain signal under disturbance of a disturbance source, and taking the arm length difference value of two paths of the interferometers as the position of random sampling
S3: according to the position of random sampling
Constructing a recovery matrix A from the randomly sampled locations
Multiplying the frequency domain orthogonal basis psi; s4: and recovering the frequency domain signal f according to the recovery matrix A and the random sampling result s, and completing the demodulation of the fiber Bragg grating.
Preferably, the arm length difference of the two paths of the interferometer is obtained according to the light intensity of the light with the sensing wavelength, the light intensity of the light with the reference wavelength and the reference wavelength.
Preferably, the arm length difference of the two paths of the interferometer is calculated according to the light intensity of the light with the sensing wavelength, the light intensity of the light with the reference wavelength and the reference wavelength by the following formula:
wherein, P1And P2Respectively the light intensity of two paths of the interferometer, lambda is the reference wavelength, L1And L2Respectively the lengths of two paths of the interferometer, n is the refractive index,
is phase noise.
Preferably, the frequency domain orthogonal basis ψ is a discrete cosine transform orthogonal basis.
Preferably, the reflection wavelength of each of the fiber bragg gratings is obtained by iteration of an optimization algorithm.
Preferably, the optimization algorithm is a convex optimization algorithm or an orthogonal matching pursuit algorithm.
Preferably, the stopping condition of the iteration is the number of the gratings of the fiber bragg grating and the wavelength range of each grating; and dividing the spectrum into spectrum regions according to the number of the gratings, and selecting the wavelength with the maximum likelihood from each spectrum region.
The invention has the beneficial effects that: the demodulation system and the demodulation method of the fiber Bragg grating are provided, the MZ interferometer with the unequal arm length and a phase disturbance source is adopted to carry out spatial sampling on echo signals, the scheme that a spectrum needs to be scanned or a spectrometer and other precise light splitting devices are adopted in the traditional demodulation method is avoided, and the demodulation speed is improved.
Further, the system of the present invention avoids the use of large-scale MZ interferometer arrays, further reducing system complexity and cost.
Drawings
Fig. 1 is a schematic diagram of a demodulation system of a fiber bragg grating according to an embodiment of the present invention.
Fig. 2 is a diagram illustrating the conversion of wavelength information into light intensity information according to an embodiment of the present invention.
Fig. 3 is a schematic diagram of random sampling in an embodiment of the present invention.
Fig. 4 is a schematic diagram of a demodulation method of a fiber bragg grating in an embodiment of the present invention.
FIG. 5 is a diagram illustrating a compressed sensing algorithm according to an embodiment of the present invention.
Detailed Description
In order to make the technical problems, technical solutions and advantageous effects to be solved by the embodiments of the present invention more clearly apparent, the present invention is further described in detail below with reference to the accompanying drawings and the embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
It will be understood that when an element is referred to as being "secured to" or "disposed on" another element, it can be directly on the other element or be indirectly on the other element. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or be indirectly connected to the other element. In addition, the connection may be for either a fixing function or a circuit connection function.
It is to be understood that the terms "length," "width," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," and the like are used in an orientation or positional relationship indicated in the drawings for convenience in describing the embodiments of the present invention and to simplify the description, and are not intended to indicate or imply that the referenced device or element must have a particular orientation, be constructed in a particular orientation, and be in any way limiting of the present invention.
Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the embodiments of the present invention, "a plurality" means two or more unless specifically limited otherwise.
Abbreviations and key term definitions:
MZ interferometers, also known as mach-zehnder interferometers, are configured to divide a light beam of a same light source into two paths (each path is also referred to as an arm), and then combine the two paths to generate an interference phenomenon, wherein the optical path difference (arm length difference) of the two paths determines whether the light intensity is increased or decreased after interference. The interferometer described below is a MZ interferometer.
2. The linewidth is the spectral distribution width of the laser light, and the laser light emitted by a typical laser does not have only one wavelength, but has a spectral distribution, for example, a 940nm laser diode typically emits light from 930nm to 950 nm.
3. The coherence length is the optical path difference of stable coherence generated by two beams of light, generally, the narrower the line width of the laser, the larger the coherence length, and the relationship between the coherence length and the line width is approximately:
wherein, the left side of equal sign is the line width, L is the arm length, and n is the medium refractive index.
In the prior art, a demodulation method using spectral scanning is used. One type of spectral scanning is to change the cavity length of a laser, the laser repeatedly excites the medium in a cavity to generate laser by the reflection of the front and back surfaces of the cavity, the wavelength of the laser is determined by the optical length of the cavity, the emitted laser wavelength is changed when the length of the cavity is changed, the emitted laser wavelength is periodically scanned in the spectral range when the length of the cavity is periodically changed, the scanning period of the cavity is fixed, so the period of the emitted laser wavelength is fixed, the light energy is not reflected when the wavelength is inconsistent with the grating period, the light energy is reflected when the wavelength is consistent with the grating period, and the reflected light is different because the periods of a plurality of gratings are different, so the distribution of the gratings on the spectrum is changed into a pulse string on a time sequence. When a certain grating is curved, the pulses reflected by the grating change in time, whereby a curved curvature is obtained. The whole spectrum needs to be scanned in each measurement, the measurement speed is low, the measurement frequency of the current commercialized demodulator is generally below hundred Hz, and the joint angle sampling frequency/return rate is often required to be above KHz or even higher in the robot field/wearable field; because of the use of variable precision optics, such demodulators are costly, unsuitable for miniaturization and difficult to adapt to all operating conditions.
In another demodulation method, a laser emits a wide spectrum light covering the whole waveband, the light returning to the grating wavelength after being reflected by each grating is displayed as a discrete spectrum peak on the spectrum, and the whole spectrum is read at one time by a spectrometer and the change of the spectrum peak is analyzed. The method can read the whole spectrum at one time, and is faster than the spectrum scanning method. However, spectrometers are very expensive and the solution is generally only used in laboratories. Conventional spectrometers typically have spectral resolutions that are not as high as the resolution of the one-technique approach.
In another demodulation method, an edge filter is constructed, in which a slope of one edge of the filter is low and linearity is good, so that a spectrum frequency domain signal is converted into a strong or weak signal of optical energy. Because the edge filter can only output light intensity with one variable, most of the demodulation schemes can only demodulate one grating, and some schemes adopt a method of transmitting short-time pulse, and correspond to gratings at different positions with different echo times, but the method needs narrow-pulse laser and a photoelectric receiving device with high sampling rate, and the gratings need to be separated by a long distance, so that the method is not convenient to apply in some wearable products (such as data gloves); the edge filter may be affected by temperature, deformation and the like to cause unstable frequency domain response; edge filters constructed coherently are susceptible to phase noise, which may originate from the laser itself or from external influences on the optical path.
As shown in fig. 1, the present invention provides a demodulation system of a fiber bragg grating, including: the device comprises a light source 1, a first optical circulator 2, a reference grating 3, a fiber Bragg grating 4, an interferometer 5 and a disturbance source 6;
the device comprises a light source 1, a reference grating 3, a fiber Bragg grating 4 and a first optical circulator 2, wherein the light source 1 is used for emitting broadband light and enters through a first port 7 of the first optical circulator 2, a second port 8 of the first optical circulator outputs the broadband light to the reference grating 3, the reference grating 3 reflects light with a reference wavelength, and light with other wavelengths enters the fiber Bragg grating 4; the light transmitted by the reference grating 3 passes through the fiber Bragg grating 4 and is reflected, and the reflected light with the sensing wavelength and the reflected light with the reference wavelength enter the third port 9 from the second port 8 of the first optical circulator 2 and are output to the interferometer 5;
and the interferometer 5 receives the light beam output by the third port 9 of the first optical circulator 2 and divides the light beam into two paths, wherein one path carries the disturbance source 6 to enable the arm length difference of the two paths to generate random change, and the two paths of light with the reference wavelength and the light with the reference wavelength are respectively detected.
In one embodiment of the present invention, the system further comprises a second circulator 10, a reference grating filter 11, a first detector 12 and a second detector 13;
the interference mixed signal enters from a first port 14 of the second circulator 10, a second port 15 outputs to a reference grating filter 11, the reflection wavelength of the reference grating filter 11 is consistent with the reference wavelength, light of the reference wavelength is reflected, and light of the sensing wavelength enters the first detector 12 to be detected; light of a reference wavelength enters from the second port 15 of the second circulator 10 and a third port 16 is output to a second detector 13 to be detected.
By adopting the MZ interferometer with unequal arm length and a phase disturbance source to carry out spatial sampling on echo signals, the traditional demodulation method avoids the need of scanning a spectrum or adopting schemes such as a precise light splitting device such as a spectrometer and the like, and improves the demodulation speed. Meanwhile, the use of a large-scale MZ interferometer array is avoided, and the complexity and the cost of the system are further reduced.
In one embodiment of the invention, the disturbance source is an ambient disturbance dither, a vibration element, a phase modulator or a tactile signal of the fiber bragg grating.
1. The method does not need to carry out any restriction on the optical fiber, can generate optical path jitter of about 0 to dozens of lambda-Hz, has the defects of low and unstable speed and has the advantages of simplest structure, and only needs to expose the optical fiber of the double arms of the MZ interferometer, and the scheme is suitable for carrying out long-term slow change monitoring on environment, buildings and the like or is applied to the field without the requirement of measuring speed;
2. the vibration element mechanically stretches the optical fiber, can generate vibration of hundreds to thousands of lambda-Hz, and has larger amplitude, and the vibration element can be arranged on one arm of the MZ interferometer;
one arm of the MZ interferometer passes through a phase modulator, and the scheme utilizes the electro-optic/thermo-optic phase modulator in communication to generate high-speed optical path jitter from K lambda-Hz to M lambda-Hz, and has the defects of higher cost and higher voltage;
4. the interference comes from the sensing signal, the simplest method is to make one arm of the MZ interferometer close to the fiber bragg grating of the sensing area, when the physical structure of the grating is affected by the bending/temperature change of the measured material, the same interference will affect the physical length of one arm of the interferometer, the method has the advantages that the measured value is generated when the external signal to be measured changes, and the measured value is more accurate when the signal to be measured changes. Compared with the scheme 1, the interference source in the scheme 1 is an external noise signal which is completely random, the interference source in the scheme is a signal to be detected, the larger the signal to be detected is changed, the larger the interference effect is, and the more accurate the measurement is.
The invention introduces the compressed sensing into the demodulation method of the fiber Bragg grating. Compressive sensing (Compressive sensing), also known as Compressive sampling, Sparse sampling, Compressive sensing. The method is used as a new sampling theory, obtains discrete samples of signals by random sampling through developing the sparsity of the signals under the condition that the sampling rate is far less than the Nyquist sampling rate, and then reconstructs the signals perfectly through a nonlinear reconstruction algorithm. The optical fiber Bragg grating is suitable for adopting compression sensing demodulation due to the fact that the degree of freedom is large, the number and the frequency spectrum range are determined, and the number of sampling channels is small.
The demodulation system of the invention firstly converts the wavelength information into the light intensity information, secondly carries out random sampling and determines the sampling position. It mainly comprises the following components
1. As shown in fig. 2, laser beams of a certain wavelength are respectively combined by optical fibers with unequal lengths (MZ interference), and coherence is generated, and the intensity of the coherent beam is a function of the wavelength and the optical path difference between two arms:
where E is the energy, c is the speed of light, and λ is the wavelength of light.
2. As shown in fig. 3, an MZ interferometer with an unequal arm length having a phase disturbance source is used to spatially sample the echo signal, and an arm length difference is obtained from the wavelength of light having a reference wavelength.
The sampling number is the number of coherent cavities and receivers, the number of the light Bragg gratings is often large, hundreds of coherent devices are possibly needed for conventional sampling, the structure is too complex, and only the number of the coherent devices slightly larger than the detection freedom degree is needed by adopting a compressed sensing method.
As shown in fig. 4, the present invention further provides a demodulation method of a fiber bragg grating, which demodulates the fiber bragg grating by using any one of the demodulation systems of the fiber bragg grating, and specifically includes the following steps:
s1: selecting a disturbance source and a sampling rate according to the grating number of the fiber Bragg grating;
s2: adopting a compressed sensing algorithm, taking the light intensity value output by each interferometer as a primary random sampling result s of a time domain signal under disturbance of a disturbance source, and taking the arm length difference value of two paths of the interferometers as the position of random sampling
S3: according to the position of random sampling
Constructing a recovery matrix A from the randomly sampled locations
Multiplying the frequency domain orthogonal basis psi;
s4: and recovering the frequency domain signal f according to the recovery matrix A and the random sampling result s, and completing the demodulation of the fiber Bragg grating.
The invention utilizes the interference source to change the random arm length difference of the MZ interferometer, and when the random phase is larger or faster, the MZ interferometer can be equivalent to a plurality of sampling channels by utilizing high-speed sampling. The modern communication device can widely achieve the sampling frequency above megahertz, and if the sampling frequency is F and the measurement return rate required by the fiber grating is F, the number of channels of the equivalent coherent device is N ═ F/F, and since F can be above MHz and F is in the kHz level, hundreds of channels of the equivalent coherent device can be detected and captured, and hundreds of degrees of freedom can be detected and captured.
In an embodiment of the invention, the arm length difference of the two paths of the interferometer is obtained according to the light intensity of the light with the sensing wavelength, the light intensity of the light with the reference wavelength and the reference wavelength.
Specifically, the arm length difference of the two paths of the interferometer is calculated by the following formula:
wherein, P1And P2The light intensities of the two paths of the interferometer are respectively, lambda is the entering reference wavelength, L1And L2Respectively the lengths of two paths of the interferometer, n is the refractive index,
is phase noise.
Since the relationship of the arm length difference to the wavelength is a cosine relationship in the present invention, the frequency domain orthogonal basis ψ is a discrete cosine transform orthogonal basis.
Fig. 5 is a schematic diagram of a compressed sensing algorithm according to the present invention.
And iterating through an optimization algorithm to obtain the reflection wavelength of each grating in the fiber Bragg grating. The iteration stopping conditions comprise the number of the gratings of the fiber Bragg grating and the wavelength range of each grating; and dividing the spectrum into spectrum regions according to the number of the gratings, and selecting the wavelength with the maximum likelihood from each spectrum region.
In step S3, the affine space a × f ═ S is a convex set, and the optimization algorithm is a convex optimization algorithm or an orthogonal matching pursuit algorithm. The iteration stop condition is the frequency number and the position range in the prior condition. The principle of the compressed sensing algorithm on signals is that frequency leakage is uniformly distributed in a frequency spectrum through random sampling instead of being fixed at a frequency doubling position, so that actual signals are highlighted, and the principle of mathematics is that correlation among vectors is reduced through random distribution coefficients, and a full-rank matrix is constructed in limited sampling.
Specifically, regarding the convex optimization algorithm: the object of convex optimization is a convex function on a convex set, the convex set refers to each point on a set, the connecting line of the points is also in the set, and the convex function is simple and has a judgment method that f is satisfied in a certain definition domain ((x)1+x2)/2)≤(f(x1)+f(x2) A convex function in a convex set, and the extreme point is a minimum point, so the main flow of convex optimization is to judge that the inequality constraint condition is a convex set (domain is a convex set), judge that the equality constraint condition is a convex function (constraint relation is a convex function), and then obtain the extreme point by using the conventional method such as least squareThe return light wavelengths of the gratings occupy respective wave bands and are not covered by each other, so that only one solution needs to be solved, and the rest solutions are meaningless physically.
According to the above description, the domain where the solutions are located is a convex set (each solution is in a certain region on the spectrum, and the region has no "vacuole"), and a × f is a convex function (the function is an affine function, the affine functions are all convex functions, and the second-order partial derivative thereof is zero), and then the solution is the optimal solution in convex optimization, and the problem is the convex optimization problem.
The above explanations are all to explain why the present invention can only solve one minimum value, and the practical algorithm is not related to the above description, and only needs to solve the minimum value of 1 norm satisfying a × f ═ s by using least squares, which is generally a method of solving a normal equation set a of a × f ═ sTAf＝ATAnd (5) solving the s.
The orthogonal matching and tracking algorithm is an iterative algorithm for solving the least square solution of an over-determined equation set and is improved by the matching and tracking algorithm. The essence of the matching algorithm is to find the best matching solution from the dictionary, and the process of finding is iterative, which is well suited because the number and approximate distribution area of the solutions are known in the application. The flow and principle of the matching tracking algorithm are as follows:
(1) first initialize residual e0＝s
(2) Finding out the base with maximum inner product (correlation degree) from dictionary
The principle of this step is that the observed signal s is formed by linearly combining the individual base lines in ψ, so that the correlation maximum base is most likely in the original signal
(3) New residual is the orthogonal projection of old residual minus old residual in maximum base space
The principle of this step is that the projection of the old residual onto the largest basis represents the "content" of the basis component present in the residual, if that "The residual content is unchanged or even increased to indicate that the component is not contained, the three steps are iterated until the residual error is stable, namely the residual content is stable, the iteration is ended, and the stability criterion can be judged by the inflection point (derivative) of the residual error
(4) Since the number of solutions (bases) is known in this application, the bases of the number of previous "number of solutions" can be selected in step (2) for iterative computation
In the above algorithm, the residual error in step (3) is obtained by subtracting the projection of the old residual error from the base, and it is known that since the solution s is composed of a plurality of bases, the residual error is not in the same direction as any one of the bases, the manner of solving the residual error in this step is equivalent to solving the third edge of two edges of a triangle, and then reconstructing a new triangle by using the third edge and the edge in another new direction, an inaccurate but vivid comparison is that the behavior is equivalent to not continuously drawing a spiral in a planar space, so that the number of iterations is large and the time is long, and thus the improved algorithm is an orthogonal matching tracking algorithm, which has the following steps and principles:
(1) first initialize residual e0＝s
(2) Finding the base with the largest inner product with the residual error, and forming a matrix A from left to right by taking the base as a column vector, for example, finding the vector in the first iteration
Then
Found in the second iteration
Then
(3) Iterate r times, residual is er＝er-1-A(ATA)-1ATer-1The principle of this step is to use the historical information of the constructed A matrix in the previous iteration, wherein A (A)TA)-1ATIs a radical of formula AProjected vector of the spanned space, a readily understandable metaphor if
Is the x-axis, A contains only
Said "space formed by the basis sheets contained in A" is then the x-axis, A (A)TA)-1ATFor projection operator on x-axis, if A includes
The x-axis and the y-axis, the "space defined by the base included in A" is the xy-plane, A (A)TA)-1ATThe operator is the xy plane projection, and so on. Therefore, the meaning of the formula started in the step is that the projection of the last residual error minus the last residual error in the space formed by stretching the bases used in history is turned towards the direction of the base once a new base is found, and the solution, namely the direction of the initial residual error, is formed by the directions of the first bases which can be found, so that the residual error of the last iteration is finally turned to the solution direction, and the method avoids the problem that the solution time is wasted because the residual error is circled in the space all the time due to the uncertain direction in the matching and tracking algorithm
(4) This step is consistent with the algorithm described previously.
The invention provides a fiber Bragg grating demodulation method based on compressed sensing, which solves the problems of low demodulation speed and high demodulation instrument cost of the traditional fiber Bragg grating.
The invention firstly provides a scheme that a MZ interferometer with unequal arm length and a phase disturbance source is adopted to carry out spatial sampling on echo signals, thereby avoiding the need of scanning a spectrum or adopting a precise light splitting device such as a spectrometer and the like in the traditional demodulation method and improving the demodulation speed; the compressed sensing idea is introduced for the first time, the requirement on the number of MZ interferometers is reduced through sampling randomization, and the cost of the interferometer array is greatly reduced.
An embodiment of the present application further provides a control apparatus, including a processor and a storage medium for storing a computer program; wherein a processor is adapted to perform at least the method as described above when executing the computer program.
Embodiments of the present application also provide a storage medium for storing a computer program, which when executed performs at least the method described above.
Embodiments of the present application further provide a processor, where the processor executes a computer program to perform at least the method described above.
The storage medium may be implemented by any type of volatile or non-volatile storage device, or combination thereof. Among them, the nonvolatile Memory may be a Read Only Memory (ROM), a Programmable Read Only Memory (PROM), an erasable Programmable Read-Only Memory (EPROM), an electrically erasable Programmable Read-Only Memory (EEPROM), a magnetic random Access Memory (FRAM), a Flash Memory (Flash Memory), a magnetic surface Memory, an optical Disc, or a Compact Disc Read-Only Memory (CD-ROM); the magnetic surface storage may be disk storage or tape storage. Volatile Memory can be Random Access Memory (RAM), which acts as external cache Memory. By way of illustration and not limitation, many forms of RAM are available, such as Static Random Access Memory (SRAM), Synchronous Static Random Access Memory (SSRAM), Dynamic Random Access Memory (DRAM), Synchronous Dynamic Random Access Memory (SDRAM), Double data rate Synchronous Dynamic Random Access Memory (DDRSDRAM, Double DataRateSync Synchronous Random Access Memory), Enhanced Synchronous Dynamic Random Access Memory (ESDRAM, Enhanced Synchronous Dynamic Random Access Memory), Synchronous link Dynamic Random Access Memory (DRAM, Synchronous Dynamic Random Access Memory), Direct Memory (Direct Memory Access Memory, Random Access Memory). The storage media described in connection with the embodiments of the invention are intended to comprise, without being limited to, these and any other suitable types of memory.
In the several embodiments provided in the present application, it should be understood that the disclosed system and method may be implemented in other ways. The above-described device embodiments are merely illustrative, for example, the division of the unit is only a logical functional division, and there may be other division ways in actual implementation, such as: multiple units or components may be combined, or may be integrated into another system, or some features may be omitted, or not implemented. In addition, the coupling, direct coupling or communication connection between the components shown or discussed may be through some interfaces, and the indirect coupling or communication connection between the devices or units may be electrical, mechanical or other forms.
The units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units, that is, may be located in one place, or may be distributed on a plurality of network units; some or all of the units can be selected according to actual needs to achieve the purpose of the solution of the embodiment.
In addition, all the functional units in the embodiments of the present invention may be integrated into one processing unit, or each unit may be separately regarded as one unit, or two or more units may be integrated into one unit; the integrated unit can be realized in a form of hardware, or in a form of hardware plus a software functional unit.
Those of ordinary skill in the art will understand that: all or part of the steps for implementing the method embodiments may be implemented by hardware related to program instructions, and the program may be stored in a computer readable storage medium, and when executed, the program performs the steps including the method embodiments; and the aforementioned storage medium includes: a mobile storage device, a Read-Only Memory (ROM), a Random Access Memory (RAM), a magnetic disk or an optical disk, and other various media capable of storing program codes.
Alternatively, the integrated unit of the present invention may be stored in a computer-readable storage medium if it is implemented in the form of a software functional module and sold or used as a separate product. Based on such understanding, the technical solutions of the embodiments of the present invention may be essentially implemented or a part contributing to the prior art may be embodied in the form of a software product, which is stored in a storage medium and includes several instructions for causing a computer device (which may be a personal computer, a server, or a network device) to execute all or part of the methods described in the embodiments of the present invention. And the aforementioned storage medium includes: a removable storage device, a ROM, a RAM, a magnetic or optical disk, or various other media that can store program code.
The methods disclosed in the several method embodiments provided in the present application may be combined arbitrarily without conflict to obtain new method embodiments.
Features disclosed in several of the product embodiments provided in the present application may be combined in any combination to yield new product embodiments without conflict.
The features disclosed in the several method or apparatus embodiments provided in the present application may be combined arbitrarily, without conflict, to arrive at new method embodiments or apparatus embodiments.
The foregoing is a more detailed description of the invention in connection with specific preferred embodiments and it is not intended that the invention be limited to these specific details. For those skilled in the art to which the invention pertains, several equivalent substitutions or obvious modifications can be made without departing from the spirit of the invention, and all the properties or uses are considered to be within the scope of the invention.
Claims (10)
1. A demodulation system for fiber bragg gratings comprising: the device comprises a light source, a first optical circulator, a reference grating, a fiber Bragg grating, an interferometer and a disturbance source;
the light source is used for emitting broadband light and entering the broadband light through a first port of the first optical circulator, a second port of the first optical circulator outputs the broadband light to the reference grating, the reference grating reflects light with a reference wavelength, and light with other wavelengths enters the fiber Bragg grating;
the fiber Bragg grating is used for receiving and reflecting the light of the reference grating, and the reflected light with the sensing wavelength and the reflected light with the reference wavelength enter a third port from the second port of the first optical circulator and are output to the interferometer;
and the interferometer receives the light beam output by the third port of the first optical circulator and divides the light beam into two paths, wherein one path carries a disturbance source to enable the arm length difference of the two paths to generate random change, and the two paths of arm length difference respectively detect the light with the sensing wavelength and the light with the reference wavelength.
2. The fiber bragg grating demodulation system of claim 1 further comprising a second circulator, a reference grating filter, a first detector and a second detector;
the interference mixed signal enters from a first port of the second circulator, a second port outputs to the reference grating filter, the reflection wavelength of the reference grating filter is consistent with the reference wavelength, the light of the reference wavelength is reflected, and the light of the sensing wavelength enters the first detector to be detected; light of the reference wavelength enters from the second port of the second circulator, and a third port outputs to the second detector to be detected.
3. The fiber bragg grating demodulation system of claim 1 wherein said disturbance source is an ambient disturbance dither, a vibration element, a phase modulator, or a tactile signal of said fiber bragg grating.
4. A method for demodulating a fiber bragg grating, characterized in that the demodulation system of a fiber bragg grating according to any one of claims 1 to 3 is used to demodulate the fiber bragg grating, and specifically comprises the following steps:
s1: selecting a disturbance source and a sampling rate according to the grating number of the fiber Bragg grating;
s2: adopting a compressed sensing algorithm, taking the light intensity value output by each interferometer as a primary random sampling result s of a time domain signal under disturbance of a disturbance source, and taking the arm length difference value of two paths of the interferometers as the position of random sampling
S3: according to the position of random sampling
Constructing a recovery matrix A from the randomly sampled locations
Multiplying the frequency domain orthogonal basis psi;
s4: and recovering the frequency domain signal f according to the recovery matrix A and the random sampling result s, and completing the demodulation of the fiber Bragg grating.
5. The method for demodulating a fiber bragg grating according to claim 4, wherein the arm length difference of the two paths of the interferometer is obtained according to the light intensity of the light with the sensing wavelength, the light intensity of the light with the reference wavelength, and the reference wavelength.
6. The demodulation method of fiber bragg grating as claimed in claim 5, wherein the arm length difference of the two paths of the interferometer is calculated according to the light intensity of the light with the sensing wavelength, the light intensity of the light with the reference wavelength and the reference wavelength by the following formula:
wherein, P1And P2Respectively the light intensity of two paths of the interferometer, lambda is the reference wavelength, L1And L2Respectively the lengths of two paths of the interferometer, n is the refractive index,
is phase noise.
7. The method for demodulating a fiber bragg grating according to claim 4, wherein the frequency domain orthogonal basis ψ is a discrete cosine transform orthogonal basis.
8. The method for demodulating a fiber bragg grating as claimed in claim 4, wherein the reflection wavelength of each of the fiber bragg gratings is iteratively obtained by an optimization algorithm.
9. The method for demodulating a fiber bragg grating according to claim 8, wherein the optimization algorithm is a convex optimization algorithm or an orthogonal matching pursuit algorithm.
10. The method for demodulating a fiber bragg grating according to claim 8, wherein the stop condition of the iteration is the number of gratings of the fiber bragg grating, the wavelength range of each of the gratings; and dividing the spectrum into spectrum regions according to the number of the gratings, and selecting the wavelength with the maximum likelihood from each spectrum region.
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