CN109724648B - Device and method for synchronously measuring temperature and strain based on orthogonal polarization dual-wavelength laser multi-longitudinal-mode self-mixing effect - Google Patents

Abstract

The invention belongs to the technical field of optical detection, and in particular relates to a device and method for synchronously measuring temperature and strain based on the multi-longitudinal mode self-mixing effect of orthogonally polarized dual-wavelength lasers. The measuring device includes a laser light source that emits two orthogonal polarized lights of different wavelengths, a sensing unit, a vibrating target, a polarization selective optical switch, a light splitting element and a signal processing unit. The measurement method is as follows: the laser light source emits two orthogonally polarized lasers with different wavelengths, the vibrating target vibrates, and the outgoing laser passes through the polarization selective optical switch at different times to switch the two output orthogonal polarization lasers to the vibrating target, and feed back to the laser light source. A self-mixing signal is formed in the resonator, and different compensation distances are obtained under the polarization state 1 laser and the polarization state 2 laser, respectively. The signal processing unit is used to obtain the ambient temperature value and strain value of the sensing fiber at the same time. This measurement method can realize the temperature Simultaneous measurement of strain and strain.

Description

Simultaneous measurement of temperature and Apparatus and method for straining

technical field

The invention belongs to the technical field of optical detection, and in particular relates to a device and method for synchronously measuring temperature and strain based on the multi-longitudinal mode self-mixing effect of orthogonally polarized dual-wavelength lasers.

Background technique

At present, optical measurement methods have been maturely used in temperature measurement and strain measurement due to their advantages of non-contact measurement, high measurement sensitivity and high measurement accuracy. In the on-line analysis and testing of smart materials and structures, simultaneous on-line measurement of multiple parameters is the current development trend of structural health state testing. Simultaneous measurement of static strain and temperature is particularly important in error correction of thermal strain.

In the field of temperature measurement technology, the current temperature measurement methods are mainly divided into two categories: contact type and non-contact type. Contact temperature measurement methods mainly include thermistor temperature measurement method, fiber grating temperature measurement method, etc.; non-contact temperature measurement methods mainly include infrared temperature measurement method, traditional interference optical temperature measurement method, etc. Based on the advantages of the thermistor's small size and good mechanical properties, the thermistor temperature measurement method is mainly suitable for measuring point temperature, surface temperature and rapidly changing temperature, but it has poor reproducibility and poor interchangeability. Shortcomings. The fiber grating temperature measurement method is suitable for the occasions that need to measure the surface temperature with high precision, but due to its material characteristics, its temperature measurement range is small, and it needs to be combined with an expensive spectrometer or complex demodulation technology, and the application cost is high . When the infrared temperature measurement method is applied to measure the real temperature, it is greatly affected by the emissivity of the measured object, and the measurement accuracy is low, which does not meet the needs of high-precision temperature measurement. The traditional interferometric optical temperature measurement method generally adopts the Mach-Zehnder scheme. The signal light and the reference light are in different optical paths, which are greatly affected by the environment, and the structure is relatively complex and debugging is difficult.

In the field of strain measurement technology, traditional strain measurement methods mainly use resistance strain gauges (resistance strain gauges) to achieve measurement. Generally, this method can only measure the surface strain of the component, and it is difficult to display its internal strain, and has the disadvantages of large size of the measuring instrument, low measurement sensitivity, small dynamic range, and it is not easy to be embedded in the composite material. The methods of measuring strain using optics mainly include photoelasticity measurement, holographic interferometry, moiré method, fiber grating method, traditional optical interferometry, etc. Among them, photoelasticity measurement method, holographic interferometry, moiré method and other methods have stress models. Complexity, limited measurement materials, cumbersome processing process, and large amount of data to be processed; the fiber grating method needs to be connected to a spectrometer to observe the specific position of the grating reflection wavelength under different strains. The measurement cost is high and it is easily affected by the environment. However, traditional optical interferometry (such as Michelson, Mach-Zehnder, etc.) needs to obtain the strain size by collecting the interference signal between the sensing arm and the reference arm. Since the signal light and the reference light are in different optical paths, the The environment has a great influence, the structure is complex and the debugging is difficult; the Fabry-Parot type strain sensor uses the interference effect of light in the air cavity to sense the strain, but the air cavity is easily disturbed by the environment and the optical path is limited. Not suitable for high sensitivity strain measurements.

Simultaneous measurement of temperature and strain parameters The main optical measurement schemes currently used are the temperature and strain dual-sensor measurement scheme and the single-sensor temperature-strain dual-parameter measurement scheme. The dual-sensor measurement scheme obviously has the problems of bulky system, high cost and cross-sensitivity to temperature and strain. At present, the mainstream solutions for the dual-parameter measurement of temperature and strain with a single sensor mainly include the double-envelope FBG scheme based on fiber Bragg grating, the FBG fundamental frequency domain second harmonic scheme, the FBG scheme with different cladding sizes, the FBG and long period grating scheme, and the single Two-wavelength polarization interferometry scheme on optical fiber, etc. However, the above schemes are all based on the simultaneous change of the sensing signal wavelength with temperature and strain, and then separate and measure them. It requires the combination of expensive spectrometers or wavelength meters and complex wavelength demodulation schemes to achieve simultaneous measurement of temperature and strain dual parameters.

SUMMARY OF THE INVENTION

The purpose of the present invention is to solve the above problems. Based on the laser multi-longitudinal mode self-mixing effect, the present invention provides a device for synchronously measuring temperature and strain based on the orthogonal polarization dual-wavelength laser multi-longitudinal mode self-mixing effect, which effectively overcomes the problems of the existing methods, and the method.

The device for synchronously measuring temperature and strain based on the multi-longitudinal mode self-mixing effect of orthogonally polarized dual-wavelength lasers provided by the present invention has the following technical solutions:

A device for synchronously measuring temperature and strain based on the multi-longitudinal mode self-mixing effect of orthogonally polarized dual-wavelength lasers, including a laser light source, a sensing unit, a vibrating target, a polarization selective optical switch, a light splitting element and a signal processing unit,

The laser light emitted by the laser light source is orthogonally polarized light with two different wavelengths,

The laser light source, the sensing unit, and the vibrating target are arranged in sequence, and the polarization selective optical switch and the light splitting element are sequentially arranged on the optical path between the laser light source and the sensing unit,

The sensing unit includes a sensing fiber, the sensing fiber is placed in an environment where the temperature and strain are to be measured, and the laser light emitted by the laser light source enters the sensing fiber through the light splitting element,

The bottom of the vibration target is fixed on the slider, the slider is arranged on the slide rail and can move horizontally along the slide rail, the vibration target receives the laser light emitted by the laser light source and is attached to the vibration surface of the vibration target. The reflective structure is fed back into the resonator of the laser light source to form a self-mixing signal,

The polarization selective optical switch receives the laser light emitted by the laser light source, and switches the laser light to output orthogonal polarization state 1 laser and polarization state 2 laser. The wavelength of the polarization state 1 laser is λ ₁ , and the The laser wavelength of polarization state 2 is λ ₂ ,

The light splitting element is a coupler for splitting the self-mixing signal to the detector, and the detector converts the received optical signal into an electrical signal,

The signal processing unit receives the electrical signal for analysis and processing, and simultaneously obtains changes in temperature and strain of the environment where the sensing fiber is located.

In certain embodiments, the laser light source is an orthogonally polarized dual-wavelength laser, and the orthogonally polarized dual-wavelength laser is selected from the group consisting of orthogonally polarized Zeeman dual-frequency lasers, orthogonally polarized Nd:YAP dual-wavelength lasers, orthogonally polarized Any of the He-Ne dual-frequency lasers;

Or the laser light source consists of two multi-longitudinal mode lasers with different wavelengths, a set of polarizers and a polarization-maintaining coupler.

In some embodiments, the vibration target is a speaker driven by a signal generator or a piezoelectric ceramic.

In some embodiments, the polarization selective optical switch is a liquid crystal achromatic polarization selective optical switch, a magneto-optical wavelength selective optical switch or a prismatic wavelength selective optical switch.

In some embodiments, the sensing fiber is a single-mode fiber, a multi-mode fiber, a birefringent fiber, or a photonic crystal fiber.

In some embodiments, an attenuator and a collimator are also included, the attenuator is located between the light splitting element and the sensing unit, the collimator is located between the sensing unit and the vibrating target, so One end of the sensing fiber is connected to the collimator.

In some embodiments, the signal processing unit is a computer or an oscilloscope.

The present invention also provides a method for simultaneously measuring temperature and strain based on the above-mentioned device for synchronously measuring temperature and strain based on the multi-longitudinal mode self-mixing effect of orthogonally polarized dual-wavelength lasers, comprising the following steps:

(1) The laser light source emits laser light, the laser light emitted by the laser light source is orthogonally polarized light of two different wavelengths, the vibration target is set to a vibration state, and the polarization selective optical switch is adjusted so that the wavelength is λ ₁ The polarization state 1 laser passes through, observe the multi-longitudinal mode self-mixing signal on the signal processing unit, move the slider, keep the waveform of the signal processing unit in the same phase or the phase delay is an integer multiple of 2π, and record the movement of the slider. the first compensation distance δL _c1 ;

(2) Adjust the polarization selective optical switch so that the polarization state 2 laser with the wavelength of λ ₂ passes through, observe the multi-longitudinal mode self-mixing signal on the signal processing unit, and move the slider to keep the waveform of the signal processing unit. The same phase or phase delay is an integer multiple of 2π, and the second compensation distance δL _c2 of the slider movement is recorded;

(3) According to the simultaneous equations of the first compensation distance δL _c1 obtained in step (1) and the second compensation distance δL _c2 obtained in step (2), the temperature change value of the environment where the sensing fiber is located ( ΔT) and change in strain (Δε):

In the formula, Λ _1,T , Λ _1,ε are the temperature sensitivity coefficient and strain sensitivity coefficient of the sensing unit corresponding to the polarization state 1 laser, Λ _2,T , Λ _2,ε are the polarization state 2 laser corresponding to The temperature sensitivity coefficient of the sensing unit and the strain sensitivity coefficient of the sensing unit, k _0j is the wave number of the j mode in vacuum, and j refers to the jth longitudinal mode mode in the laser.

The device and method for synchronously measuring temperature and strain based on the multi-longitudinal mode self-mixing effect of orthogonally polarized dual-wavelength lasers provided by the present invention are based on the output signal waveform change caused by the multi-longitudinal mode laser self-mixing feedback signal with the change of temperature and strain, and By adjusting the cavity length of the external cavity, the self-mixing signal waveform is tracked and the strain and temperature changes of the position to be measured are measured in real time. The present invention has the following advantages:

1. The structure is simple, and the temperature and strain information can be obtained in real time only by observing the intensity change waveform of the output signal and tracking and compensating through the outer cavity;

2. The temperature and strain sensing units are in the same position, which is a better intrinsic measurement scheme, reducing the interference of the measurement system by other sensitive factors.

Description of drawings

1 is a schematic plan view of a device for synchronously measuring temperature and strain based on the multi-longitudinal mode self-mixing effect of orthogonally polarized dual-wavelength lasers provided in Embodiment 1 of the present invention;

2 is a schematic plan view of a device for synchronously measuring temperature and strain based on the multi-longitudinal mode self-mixing effect of orthogonally polarized dual-wavelength lasers provided in Embodiment 2 of the present invention;

3 is a phase diagram of temperature and strain in Example 2 of the present invention;

FIG. 4 is a schematic diagram of a simulation result of Embodiment 2 of the present invention.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

Example 1

As shown in FIG. 1 , a device for synchronously measuring temperature and strain based on the self-mixing effect of an orthogonally polarized dual-wavelength multi-longitudinal mode laser includes an orthogonally polarized dual-wavelength laser 1 , a sensing unit 2 , a vibration target 3 , and a polarization selective optical switch 4 , splitting element 5 , attenuator 6 and signal processing unit 7 . The sensing unit 2 includes a sensing fiber, the sensing fiber is placed in the environment of the temperature and strain to be measured, the pigtail of the orthogonal polarization dual-wavelength laser 1 is connected to one end of the sensing fiber, and the orthogonal polarization dual-wavelength laser 1 is to be The laser light source is measured, and the outgoing laser light passes through the sensing fiber, and shoots onto the vibrating target 3 from one end of the sensing fiber. The vibrating target 3 receives the laser light emitted by the orthogonally polarized dual-wavelength laser 1 and feeds it back into the resonator of the orthogonally polarized dual-wavelength laser 1 through the reflection structure 31 on the vibrating target 3 to form a self-mixing signal. The bottom of the vibrating target 3 is fixed on the slider 32 , the slider 32 is arranged on the slide rail 33 and can move horizontally along the slide rail 33 , and the slide rail 33 and the outgoing laser are on the same straight line. A polarization selective optical switch 4 and a light splitting element 5 are sequentially arranged on the optical path between the orthogonally polarized dual-wavelength laser 1 and the sensing unit 2 . The polarization selective optical switch 4 receives the laser light emitted by the orthogonal polarization dual-wavelength laser 1, and switches the laser light to two output orthogonal polarization state 1 laser and polarization state 2 laser, and the polarization state 1 laser wavelength is λ ₁ , the wavelength of the polarization state 2 laser is λ ₂ . The optical splitting element 5 is a coupler, which is used to split the self-mixing signal to the detector 8, the detector 8 converts the received optical signal into an electrical signal, and the signal processing unit 7 receives the electrical signal for analysis and processing , and simultaneously obtain the temperature and strain changes of the environment where the sensing fiber is located.

In the above device, the orthogonal polarization dual-wavelength laser 1 is an orthogonal polarization Zeeman dual-frequency laser, an orthogonal polarization Nd:YAP dual-wavelength laser or an orthogonal polarization He-Ne dual-frequency laser; the vibration target 3 is driven by a signal generator. Speaker or piezoelectric ceramics; polarization selective optical switch 4 is liquid crystal achromatic polarization selective optical switch, magneto-optical wavelength selective optical switch or prismatic wavelength selective optical switch; sensing optical fiber is single-mode optical fiber, multi-mode optical fiber , birefringent fibers or photonic crystal fibers. The above-mentioned device also includes an attenuator 6 and a collimator 9, the attenuator 6 is located between the light splitting element 5 and the sensing unit 2, the collimator 9 is located between the sensing unit and the vibration target, and the other side of the sensing fiber is One end is connected to the collimator 9, and the collimator 9 ensures that the laser is emitted in parallel to the surface of the target object; the signal processing unit 7 is a computer or an oscilloscope.

The method of utilizing the above-mentioned device to measure temperature and strain simultaneously is as follows: the orthogonal polarization dual-wavelength laser 1 is used as the laser light source to be measured, the vibrating target 3 vibrates, and the outgoing laser passes through the polarization selective optical switch 4, and at time t ₁ , the polarization selective optical switch is regulated and the output is turned on. Polarization state 1 laser (wavelength λ ₁ ), at time t ₂ , adjust the polarization selective optical switch to switch the output polarization state 2 laser (wavelength λ ₂ ) to the vibrating target, and the outgoing laser is reflected by the reflective structure 31 attached to the vibrating surface of the vibrating target Then, it is fed back to the resonator of the orthogonally polarized dual-wavelength laser 1 to form a self-mixing signal, and the slider 32 is moved along the slide rail 33 under the polarization state 1 laser and the polarization state 2 laser, respectively, to obtain the compensation distance. The compensation distance makes each polarization In order to change the distance between the vibrating target and the orthogonally polarized dual-wavelength laser, the waveforms in the state of the same phase are kept in the same phase or the phase delay is an integer multiple of 2π, so as to obtain a multi-longitudinal mode self-mixing signal with indistinguishable waveforms. At the same time, the attenuator 5 is used to adjust the feedback light. The intensity of the laser self-mixing signal under different laser external cavity lengths under the laser of polarization state 1 and the laser of polarization state 2 is collected by the detector 8, and the electrical signal of the received laser self-mixing signal is analyzed by the signal processing unit 7, and Different compensation distances in polarization state 1 and polarization state 2 can obtain the change values of temperature and strain in the environment where the sensing fiber is located. The specific steps are as follows:

For the laser self-mixing signal of a multi-longitudinal mode laser, the different longitudinal modes of the laser only interfere with its own mode, and the final laser self-mixing signal is the superposition of the laser self-mixing signal intensity formed by the respective longitudinal modes. According to the relevant interference mixing theoretical model, Without considering the influence of speckle, the self-mixing signal strength of multi-longitudinal mode laser is:

In formula (1), op _tj is the total optical path of the external cavity of the j mode, β is the total number of starting modes in the multi-longitudinal mode laser, j is the j-th longitudinal mode mode in the laser, and I ₀ is the initial total light intensity , ΔI _j is the amplitude of the j-mode laser light intensity change, ω ₀ is the angular frequency of the laser, c is the speed of light in vacuum, n _g is the refractive index of the laser cavity medium group, L ₀ is the laser cavity cavity length, cc In the complex conjugate calculation of the previous formula, the refractive index changes corresponding to different longitudinal modes of the laser in the same material can be ignored;

When the optical path or phase of the sensing unit changes, there are:

op _tj =op ₀ +δop _s +δop _c =op ₀ +δ(n _s L _s )+δ(n _c L _c ) (2)

In formula (2), op ₀ is the initial optical path of the laser external cavity, δop _s is the optical path change of the sensing unit caused by temperature strain, δop _c is the compensation optical path, n _c is the refractive index of the air in the external cavity, and n _s is the sensing element. Unit refractive index, L _s is the total geometric length of the actual path of laser transmission in the sensing unit, L _c is the compensation length, in formula (3) φ _0j is the initial phase of the j-mode laser in one round trip of the external cavity, δφ _sj is the phase change of the sensing unit caused by temperature change and strain, and δφ _cj is the compensation phase change. When measuring temperature change and strain, δφ _sj = -δφ _cj .

When each mode waveform maintains the same phase or the phase delay is an integer multiple of 2π, there is no waveform separation in the superposition of laser self-mixing signals of different longitudinal mode modes, namely:

op _tj = 2mn _g L ₀ (4)

In formula (4), m is the external cavity mode series of the laser, which is a positive integer. Therefore, the laser has a series of special position points, so that the superimposed laser self-mixing signal does not produce waveform separation. From formula (2), it can be seen that when When the ambient temperature strain of the sensing fiber changes, the optical path or phase of the light in the sensing fiber will change, resulting in a change in the op _tj of each mode, so that the m value is no longer an integer, and the superimposed laser self-mixing The signal waveform will be separated. At this time, by adjusting the sliding device, the position of the external feedback object is changed to compensate the optical path or phase change, so that the superimposed laser self-mixing signal waveform becomes a complete waveform again, and then the position of the external feedback object is measured. Compensate the phase change, and then obtain the optical path change δop _s of the sensing unit caused by the change of ambient temperature strain. After the light wave passes through the sensing unit of length L, the phase delay of the outgoing light wave can be expressed as

φ=2πnL/λ ₀ =βL (5)

In the formula, β is the propagation constant of the light wave in the sensing unit, λ ₀ is the propagation wavelength of the light wave in the sensing medium, n is the refractive index of the sensing medium, then the change of the phase delay of the light wave under the action of external factors can be written in the following form

Δφ=βΔL+LΔβ (6)

In the formula, the first term represents the change of the phase delay caused by the change of the length, and the second term represents the change of the phase delay caused by the change of the refractive index. The phase delay changes. From (5) and (6), it can be deduced that the change of the phase delay of the sensing unit caused by the change of ambient temperature and strain is as follows:

δΦ _s =Λ _i,T ΔT+Λ _i,ε Δε (7)

When the measurement of the compensation outer cavity is vacuum or air, the refractive index is 1, that is

δΦ _s =Λ _i,T ΔT+Λ _i,ε Δε=δΦ _c =-k _0j δL _c (8)

Λ _{i, T} is called the temperature sensitivity coefficient, Λ _{i, ε} is the strain sensitivity coefficient of the sensing unit, taking dual-wavelength orthogonally polarized light as an example: when i=1, Λ _{1, T} , Λ _{1, ε} are polarization state 1 The temperature sensitivity coefficient of the sensing unit and the strain sensitivity coefficient of the sensing unit corresponding to the laser, δΦ _s1 is the phase change caused by the change of temperature strain in the laser sensing area of polarization state 1, when i=2, Λ _2,T , Λ _{2, ε} is the temperature sensitivity coefficient of the sensing unit and the strain sensitivity coefficient of the sensing unit corresponding to the polarization state 2 laser, δΦ _s2 is the phase change caused by the temperature strain change of the polarization state 2 laser sensing area, and the matrix form is:

Among them, Λ is the sensitivity matrix. During the measurement process, by measuring the compensation length of the external cavity under the same temperature and strain under different polarization modes (wavelength) light sources, the strain and temperature at the sensing point can be obtained from equations (8) and (9). The change value of , that is

Among them, δL _c1 is the compensation distance when the laser in polarization state 1 passes through the polarization selective optical switch, and the slider moves relative to the initial position, δL _c2 is the compensation distance between the laser in polarization state 2 and the polarization selective optical switch, and the slider moves relative to the initial position,

Therefore, by means of time-division measurement (time t ₁ and time t ₂ ), by measuring the compensation distance δL _{c2 under the laser with polarization state 1 at time t 1 and measuring the compensation distance δL c2 under the} laser with polarization state 2 at time t ₂ , and obtain the change values of temperature and strain of the environment where the sensing fiber is located.

Example 2

As shown in Figure 2, the device for synchronously measuring temperature and strain based on the multi-longitudinal mode self-mixing effect of orthogonally polarized dual-wavelength lasers includes two multi-longitudinal mode lasers 11, 12 with different wavelengths, polarizers 101, 102, polarization maintaining Coupler 10 , sensing unit 2 , vibrating target 3 , polarization selective optical switch 4 , light splitting element 5 , attenuator 6 and signal processing unit 7 . The sensing unit 2 includes a sensing fiber, the sensing fiber is placed in the environment of the temperature and strain to be measured, and the pigtails of the two multi-longitudinal mode lasers 11 and 12 with different wavelengths are connected to a polarizer 101 and 102 respectively. Lasers with different polarization states are generated, and are connected to one end of the sensing fiber through a polarization-maintaining coupler. Two multi-longitudinal mode lasers 11 and 12 with different wavelengths are the laser light sources to be measured. One end shoots onto the vibrating target 3 . The vibrating target 3 receives the laser light emitted by the orthogonally polarized dual-wavelength laser light source 1 and feeds it back into the resonators of the two multi-longitudinal mode lasers 11 and 12 with different wavelengths through the reflection structure 31 on the vibrating target 3 to form a self-mixing signal. The bottom of the vibrating target 3 is fixed on the slider 32 , the slider 32 is arranged on the slide rail 33 and can move horizontally along the slide rail 33 , and the slide rail 33 and the outgoing laser are on the same straight line. Polarizers 101 and 102 , polarization maintaining coupler 10 , polarization selective optical switch 4 and beam splitting element 5 attenuator 6 are sequentially arranged on the optical path between two multi-longitudinal mode lasers 11 and 12 of different wavelengths and the sensing unit 2 . The polarization selective optical switch 4 receives the laser light emitted by the two multi-longitudinal mode lasers 11 and 12 with different wavelengths, and switches the laser light to output orthogonal polarization state 1 laser and polarization state 2 laser, and the wavelength of the polarization state 1 laser is λ ₁ , the laser wavelength of polarization state 2 is λ ₂ . The optical splitting element 5 is a coupler for splitting the self-mixing signal to the detector 8, the detector 8 converts the received optical signal into an electrical signal, and the signal processing unit 7 receives the electrical signal for analysis and processing, At the same time, the change values of temperature and strain of the environment where the sensing fiber is located are obtained.

In the above device, the laser light source 1 is composed of two different wavelength multi-longitudinal mode lasers 11, 12, a set of polarizers 101, 102 and a polarization-maintaining coupler 10; the vibration target 3 is a signal The speaker or piezoelectric ceramics driven by the generator; the polarization selective optical switch 4 is a liquid crystal achromatic polarization selective optical switch, a magneto-optical wavelength selective optical switch or a prismatic wavelength selective optical switch; the sensing optical fiber is the sensing optical fiber and the single-mode optical fiber , multimode fiber, birefringent fiber or photonic crystal fiber. The above-mentioned device also includes an attenuator 6 and a collimator 9, the attenuator 6 is located between the light splitting element 5 and the sensing unit 2, the collimator 9 is located between the sensing unit and the vibration target, and the other side of the sensing fiber is One end is connected with the collimator 9, and the collimator 9 ensures that the laser is emitted in parallel to the surface of the target object; the signal processing unit 7 is a computer or an oscilloscope.

The method for simultaneously measuring temperature and strain by using the above-mentioned device is as follows: two multi-longitudinal mode lasers 11 and 12 with different wavelengths are used as the laser light source to be measured, the vibrating target 3 vibrates, and the outgoing laser light passes through the polarization selective optical switch 4 at the time _t1 . Adjust the polarization selective light switch to output the polarization state 1 laser (wavelength λ ₁ ), at time t ₂ , adjust the polarization selective optical switch to switch the output polarization state 2 laser (wavelength λ ₂ ) to the vibrating target, and the outgoing laser is attached to the vibrating surface of the vibrating target After reflection by some reflective structures 31, two multi-longitudinal mode lasers 11 and 12 with different wavelengths form a self-mixing signal in the resonator cavity, and the slider 32 is moved along the slide rail 33 under the polarization state 1 laser and the polarization state 2 laser, respectively. The compensation distance is obtained. The compensation distance keeps the waveforms in each polarization state in the same phase or the phase delay is an integer multiple of 2π, so as to change the distance between the vibration target and the orthogonally polarized dual-wavelength laser light source, and obtain multi-longitudinal modes with indistinguishable waveforms. At the same time, the intensity of the feedback light is adjusted by the attenuator 5, the laser self-mixing signal under different laser external cavity lengths under the polarization state 1 laser and the polarization state 2 laser is collected by the detector 8, and the signal processing unit 7 is used to receive the signal. The electrical signal of the laser self-mixing signal is analyzed, and the different compensation distances in polarization state 1 and polarization state 2 are obtained. At the same time, the changes in temperature and strain of the environment where the sensing fiber is located are obtained. The specific steps are as follows:

For the laser self-mixing signal of a multi-longitudinal mode laser, the different longitudinal modes of the laser only interfere with its own mode, and the final laser self-mixing signal is the superposition of the laser self-mixing signal intensity formed by the respective longitudinal modes. According to the relevant interference mixing theoretical model, Without considering the effect of speckle, the self-mixing signal strength of multi-longitudinal mode laser is:

In formula (1), op _tj is the total optical path of the external cavity of the j mode, β is the total number of starting modes in the multi-longitudinal mode laser, j is the j-th longitudinal mode mode in the laser, and I ₀ is the initial total light intensity , ΔI _j is the amplitude of the j-mode laser light intensity change, ω ₀ is the angular frequency of the laser, c is the speed of light in vacuum, n _g is the refractive index of the laser cavity medium group, L ₀ is the laser cavity cavity length, cc In the complex conjugate calculation of the previous formula, the refractive index changes corresponding to different longitudinal modes of the laser in the same material can be ignored;

When the optical path or phase of the sensing unit changes, there are:

op _tj =op ₀ +δop _s +δop _c =op ₀ +δ(n _s L _s )+δ(n _c L _c ) (2)

In formula (2), op ₀ is the initial optical path of the laser external cavity, δop _s is the optical path change of the sensing unit caused by temperature strain, δop _c is the compensation optical path, n _c is the refractive index of the air in the external cavity, and n _s is the sensing element. Unit refractive index, L _s is the total geometric length of the actual path of laser transmission in the sensing unit, L _c is the compensation length, in formula (3) φ _0j is the initial phase of the j-mode laser in one round trip of the external cavity, δφ _sj is the phase change of the sensing unit caused by temperature change and strain, and δφ _cj is the compensation phase change. When measuring temperature change and strain, δφ _sj = -δφ _cj .

When each mode waveform maintains the same phase or the phase delay is an integer multiple of 2π, there is no waveform separation in the superposition of laser self-mixing signals of different longitudinal mode modes, namely:

op _tj = 2mn _g L ₀ (4)

In formula (4), m is the external cavity mode series of the laser, which is a positive integer. Therefore, the laser has a series of special position points, so that the superimposed laser self-mixing signal does not produce waveform separation. From formula (2), it can be seen that when When the ambient temperature strain of the sensing fiber changes, the optical path or phase of the light in the sensing fiber will change, resulting in a change in the op _tj of each mode, so that the m value is no longer an integer, and the superimposed laser self-mixing The signal waveform will be separated. At this time, by adjusting the sliding device, the position of the external feedback object is changed to compensate the optical path or phase change, so that the superimposed laser self-mixing signal waveform becomes a complete waveform again, and then the position of the external feedback object is measured. Compensate the phase change, and then obtain the change δop _s of the optical path of the sensing unit caused by the change of ambient temperature strain. After the light wave passes through the sensing unit of length L, the phase delay of the outgoing light wave can be expressed as

φ=2πnL/λ ₀ =βL (5)

In the formula, β is the propagation constant of the light wave in the sensing unit, λ ₀ is the propagation wavelength of the light wave in the sensing medium, n is the refractive index of the sensing medium, then the change of the phase delay of the light wave under the action of external factors can be written in the following form

Δφ=βΔL+LΔβ (6)

In the formula, the first term represents the change of the phase delay caused by the change of the length, and the second term represents the change of the phase delay caused by the change of the refractive index. The phase delay changes. From (5) and (6), it can be deduced that the change of the phase delay of the sensing unit caused by the change of ambient temperature and strain is as follows:

δΦ _s =Λ _i,T ΔT+Λ _i,ε Δε (7)

When the measurement of the compensation outer cavity is vacuum or air, the refractive index is 1, that is

δΦ _s =Λ _i,T ΔT+Λ _i,ε Δε=δΦ _c =-k _0j δL _c (8)

Λ _{i, T} is called the temperature sensitivity coefficient, Λ _{i, ε} is the strain sensitivity coefficient of the sensing unit, taking dual-wavelength orthogonally polarized light as an example: when i=1, Λ _{1, T} , Λ _{1, ε} are polarization state 1 The temperature sensitivity coefficient of the sensing unit and the strain sensitivity coefficient of the sensing unit corresponding to the laser, δΦ _s1 is the phase change caused by the change of temperature strain in the laser sensing area of polarization state 1, when i=2, Λ _2,T , Λ _{2, ε} is the temperature sensitivity coefficient of the sensing unit and the strain sensitivity coefficient of the sensing unit corresponding to the polarization state 2 laser, δΦ _s2 is the phase change caused by the temperature strain change of the polarization state 2 laser sensing area, and the matrix form is:

Among them, Λ is the sensitivity matrix. During the measurement process, by measuring the compensation length of the external cavity under the same temperature and strain under different polarization modes (wavelength) light sources, the strain and temperature at the sensing point can be obtained from equations (8) and (9). The change value of , that is

Among them, δL _c1 is the compensation distance when the laser in polarization state 1 passes through the polarization selective optical switch, and the slider moves relative to the initial position, δL _c2 is the compensation distance between the laser in polarization state 2 and the polarization selective optical switch, and the slider moves relative to the initial position,

Therefore, by means of time-division measurement (time t ₁ and time t ₂ ), by measuring the compensation distance δL _{c2 under the laser with polarization state 1 at time t 1 and measuring the compensation distance δL c2 under the} laser with polarization state 2 at time t ₂ , and obtain the change values of temperature and strain of the environment where the sensing fiber is located.

Figure 3 shows the effect of a 190mm long bow-tie fiber as a sensing unit on the self-mixing interference phase of two wavelengths of orthogonally polarized lasers (wavelengths of 633nm and 670nm) when the temperature varies from 5°C to 100°C. , and the effect of strain from 25 μm to 500 μm on the self-mixing interference phase of the above-mentioned two wavelengths orthogonally polarized lasers. Λ _1,T =1.156rad/m°C, Λ _1,ε =0.0677rad/μm is the temperature sensitivity coefficient and strain sensitivity coefficient corresponding to the polarization state 1 laser (λ ₁ =633nm); Λ _2,T =1.229rad/ m°C, Λ _2,ε =0.0611rad/μm is the temperature sensitivity coefficient and strain sensitivity coefficient corresponding to the polarization state 2 laser (λ ₂ =670 nm). It can be seen from Figure 3 that the effects of temperature and strain on the two wavelengths of orthogonally polarized laser light are different, which meets the above theoretical analysis requirements and can realize the above technical solution.

Based on the above technical solution, an experimental device is established. The experimental device uses two multi-longitudinal mode lasers with different wavelengths as light sources, and uses simulation software to simulate. For simplicity, we only consider dual-mode orthogonal polarization dual-wavelength multi-longitudinal lasers with the same amplitude. The intensity superposition waveform of the mode laser self-mixing signal. In the simulation, the dual wavelengths are 633nm and 670nm, the initial outer cavity distance is 325mm, the total length of the outer cavity fiber is 20m, and the length of the sensing unit fiber is 190mm. The simulation diagram is shown in Figure 3. It can be seen from Figure 4 that when the temperature difference and strain are both 0, the initial optical path of the laser external cavity is 29325mm, which is an integer multiple of n _g L ₀ , m=29325, and the laser self-mixing signal waveform does not separate at this time. . When the temperature of the sensing unit increases by 5°C and the strain size is 25 μm, the phase of the sensing unit changes slightly, and the overlapping laser self-mixing signal waveforms are separated. Laser), 0.50mm (for 633nm wavelength laser) At this time, the laser external cavity phase becomes an integer multiple of φ _g again, m=29325, and the superimposed laser self-mixing signal waveform disappears separately. Substitute the compensation external cavity length of 0.44mm (for 670nm wavelength laser) and 0.50mm (for 633nm wavelength laser) into formulas (10) and (11), and calculate ΔT=5°C, Δε=25μm.

To sum up, by using the time-sharing measurement method, by measuring the compensation distance, the changes of the temperature and strain of the corresponding sensing unit are finally obtained, and the simultaneous measurement of the strain and temperature of the laser sensing unit is realized.

The above are only preferred feasible embodiments of the present invention, and are not intended to limit the present invention, and the present invention is not limited to the above examples. Those skilled in the art, within the essential scope of the present invention, can Or replacement, should also belong to the protection scope of the present invention.

Claims (8)

1. a device for synchronously measuring temperature and strain based on orthogonal polarization dual-wavelength laser multi-longitudinal mode self-mixing effect, it is characterized in that, comprises laser light source, sensing unit, vibration target, polarization selective optical switch, spectroscopic element and signal processing unit, The laser light emitted by the laser light source is orthogonally polarized light with two different wavelengths, The laser light source, the sensing unit, and the vibrating target are arranged in sequence, and the polarization selective optical switch and the light splitting element are sequentially arranged on the optical path between the laser light source and the sensing unit, The sensing unit includes a sensing fiber, the sensing fiber is placed in an environment where the temperature and strain are to be measured, and the laser light emitted by the laser light source enters the sensing fiber through the light splitting element, The bottom of the vibration target is fixed on the slider, the slider is arranged on the slide rail and can move horizontally along the slide rail, the vibration target receives the laser light emitted by the laser light source and is attached to the vibration surface of the vibration target. The reflective structure is fed back into the resonator of the laser light source to form a self-mixing signal, The polarization selective optical switch receives the laser light emitted by the laser light source, and switches the laser light to output orthogonal polarization state 1 laser and polarization state 2 laser. The wavelength of the polarization state 1 laser is λ ₁ , and the The laser wavelength of polarization state 2 is λ ₂ , The light splitting element is a coupler for splitting the self-mixing signal to the detector, and the detector converts the received optical signal into an electrical signal, The signal processing unit receives the electrical signal for analysis and processing, and simultaneously obtains changes in temperature and strain of the environment where the sensing fiber is located. 2. The device for synchronously measuring temperature and strain based on the multi-longitudinal mode self-mixing effect of orthogonally polarized dual-wavelength lasers according to claim 1, wherein the laser light source is an orthogonally polarized dual-wavelength laser, and the orthogonal The polarization dual-wavelength laser is selected from any one of the orthogonal polarization Zeeman dual-frequency laser, the orthogonal polarization Nd:YAP dual-wavelength laser, and the orthogonal polarization He-Ne dual-frequency laser; Or the laser light source consists of two multi-longitudinal mode lasers with different wavelengths, a set of polarizers and a polarization-maintaining coupler. 3 . The device for synchronously measuring temperature and strain based on the multi-longitudinal mode self-mixing effect of orthogonally polarized dual-wavelength lasers according to claim 1 , wherein the vibration target is a speaker or a piezoelectric ceramic driven by a signal generator. 4. The device for synchronously measuring temperature and strain based on orthogonal polarization dual-wavelength laser multi-longitudinal mode self-mixing effect according to claim 1, wherein the polarization selective optical switch is a liquid crystal achromatic polarization selective optical switch, a magnetic Optical wavelength selective optical switch or prismatic wavelength selective optical switch. 5. The device for synchronously measuring temperature and strain based on the multi-longitudinal mode self-mixing effect of orthogonally polarized dual-wavelength lasers according to claim 1, wherein the sensing fiber is a single-mode fiber, a multi-mode fiber, a birefringent fiber Fiber or photonic crystal fiber. 6 . The device for synchronously measuring temperature and strain based on the multi-longitudinal mode self-mixing effect of orthogonally polarized dual-wavelength lasers according to claim 1 , further comprising an attenuator and a collimator, wherein the attenuator is located in the Between the spectroscopic element and the sensing unit, the collimator is located between the sensing unit and the vibration target, and one end of the sensing fiber is connected to the collimator. 7 . The device for synchronously measuring temperature and strain based on the multi-longitudinal mode self-mixing effect of orthogonally polarized dual-wavelength lasers according to claim 1 , wherein the signal processing unit is a computer or an oscilloscope. 8 . 8. The method for simultaneously measuring temperature and strain according to the device for synchronously measuring temperature and strain based on the multi-longitudinal mode self-mixing effect of orthogonally polarized dual-wavelength lasers described in any one of claims 1-7, characterized in that, comprising the following steps: step: (1) The laser light source emits laser light, the laser light emitted by the laser light source is orthogonally polarized light of two different wavelengths, the vibration target is set to a vibration state, and the polarization selective optical switch is adjusted so that the wavelength is λ ₁ The polarization state 1 laser passes through, observe the multi-longitudinal mode self-mixing signal on the signal processing unit, move the slider, keep the waveform of the signal processing unit in the same phase or the phase delay is an integer multiple of 2π, and record the movement of the slider. the first compensation distance δL _c1 ; (2) Adjust the polarization selective optical switch so that the polarization state 2 laser with the wavelength of λ ₂ passes through, observe the multi-longitudinal mode self-mixing signal on the signal processing unit, and move the slider to keep the waveform of the signal processing unit. The same phase or phase delay is an integer multiple of 2π, and the second compensation distance δL _c2 of the slider movement is recorded; (3) According to the simultaneous equations of the first compensation distance δL _c1 obtained in step (1) and the second compensation distance δL _c2 obtained in step (2), the temperature change value of the environment where the sensing fiber is located ( ΔT) and change in strain (Δε):

In the formula, Λ _1,T , Λ _1,ε are the temperature sensitivity coefficient and strain sensitivity coefficient of the sensing unit corresponding to the polarization state 1 laser, Λ _2,T , Λ _2,ε are the polarization state 2 laser corresponding to The temperature sensitivity coefficient of the sensing unit and the strain sensitivity coefficient of the sensing unit, k _0j is the wave number of the j mode in vacuum, and j refers to the jth longitudinal mode mode in the laser.

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