阅读说明：本技术 一种超低温下光纤光栅应变灵敏度的测量装置和方法 (Device and method for measuring strain sensitivity of fiber bragg grating at ultralow temperature ) 是由 高红春 唐才杰 李保勇 易小龙 蓝天 崔留住 卞贺明 王甫 薛渊泽 梁宏光 于 2021-06-09 设计创作，主要内容包括：本发明公开了一种超低温下光纤光栅应变灵敏度的测量装置和方法,包括光纤、光纤光栅、毛细管、石英基底、超低温粘接胶、光纤光栅温度传感器；光纤的一部分刻写有光纤光栅,光纤光栅位于毛细管内部,室温条件下光纤光栅受到预加载拉应力,光纤光栅两端的光纤、毛细管通过超低温粘接胶固定在石英基底上,光纤光栅温度传感器靠近光纤光栅粘贴固定。本发明公开的将预加载拉应力的光纤光栅粘贴于与光纤同材料、热膨胀系数较小的石英基底的测量方法,为光纤光栅提供了稳定的应变输入,克服了传统测量方法中测试工装在超低温下不稳定、材料力学/热学参数不准确等因素对超低温下光纤光栅应变灵敏度测量的影响。(The invention discloses a device and a method for measuring the strain sensitivity of a fiber grating at ultralow temperature, wherein the device comprises an optical fiber, the fiber grating, a capillary tube, a quartz substrate, ultralow-temperature adhesive glue and a fiber grating temperature sensor; the fiber grating is engraved on one part of the optical fiber, the fiber grating is positioned in the capillary, the fiber grating is subjected to pre-loading tensile stress at room temperature, the optical fiber and the capillary at two ends of the fiber grating are fixed on the quartz substrate through ultralow-temperature adhesive, and the fiber grating temperature sensor is adhered and fixed close to the fiber grating. The invention discloses a measuring method for sticking a fiber grating with pre-loaded tensile stress on a quartz substrate which is made of the same material as an optical fiber and has a smaller thermal expansion coefficient, which provides stable strain input for the fiber grating and overcomes the influence of factors such as instability of a testing tool at ultralow temperature, inaccurate material mechanics/thermal parameters and the like on the measurement of the strain sensitivity of the fiber grating at the ultralow temperature in the traditional measuring method.)

1. The utility model provides a measuring device of fiber grating strain sensitivity under ultralow temperature which characterized in that: the device comprises an optical fiber (1), an optical fiber grating (2), a capillary tube (3), a quartz substrate (4), an adhesive (5) and an optical fiber grating temperature sensor (6); the fiber grating (2) is partially engraved on the optical fiber (1), the fiber grating (2) is arranged in the capillary tube (3), the fiber grating (2) is subjected to pre-loaded tensile stress at room temperature, the optical fiber (1) and the capillary tube (3) at two ends of the fiber grating (2) are fixed on the quartz substrate (4) through the adhesive (5) adaptive to the use temperature range of the measuring device, and the fiber grating temperature sensor (6) is fixed on the quartz substrate (4) in a sticking manner.

2. The measurement device of claim 1, wherein: the inner diameter of the capillary tube (3) is larger than the outer diameter of the fiber grating and is less than or equal to 1.5 times of the outer diameter of the fiber grating (2), and air is filled between the inner surface of the capillary tube (3) and the outer surface of the fiber grating (2).

3. The measurement device of claim 1, wherein: the capillary tube (3) is made of a material with the elastic modulus not lower than 1 GPa.

4. The measurement device of claim 1, wherein: the fiber grating (2) is subjected to pre-loaded tensile stress under the normal temperature condition, and the value of the tensile stress is measured by utilizing strain calibration data of the fiber grating (2) under the room temperature condition.

5. The measurement device of claim 1, wherein: the distance between the fiber grating temperature sensor (6) and the fiber grating (2) is not more than 5 mm.

6. The measurement device of claim 1, wherein: the ultralow temperature is not higher than-100 ℃.

7. A method for measuring the strain sensitivity of fiber bragg grating at ultra-low temperature is characterized by comprising the following steps:

during the manufacture of the measuring device of claim 1, the initial wavelength λ of the fiber grating temperature sensor at room temperature is recorded_T0Initial wavelength lambda of the fiber grating_ε0,T0Wavelength lambda of optical fiber grating after pre-strain_ε,T0；

Placing the measuring device of claim 1 under the ultralow temperature condition T to be measured, and recording the wavelength λ of the fiber grating after the temperature is stabilized_ε,TAnd wavelength lambda of fiber grating temperature sensor_T；

The fiber grating is subjected to strain calibration at room temperature to obtain the strain sensitivity coefficient K at room temperature_ε,T0；

According to the initial wavelength lambda of the fiber grating temperature sensor at room temperature_T0Wavelength lambda at ultra-low temperature T_TInitial wavelength lambda of fiber grating at room temperature_ε0,T0Wavelength lambda of optical fiber grating after pre-strain_ε,T0Wavelength lambda of fiber grating at ultralow temperature T_ε,TAnd the strain sensitivity coefficient K at room temperature_ε,T0Determining the strain sensitivity K of the fiber grating at ultralow temperature T_ε,T。

8. The measurement method according to claim 7, characterized in that: strain sensitivity K of fiber bragg grating under ultralow temperature T_ε,TIs shown as

9. The measurement method according to claim 7, characterized in that: the measuring device is obtained by the following steps:

a) inserting the fiber bragg grating (2) into the capillary tube (3), wherein the grating region is positioned in the center of the capillary tube (3);

b) recording the initial wavelength lambda of the fiber grating (2) at room temperature by using a fiber grating demodulator_ε0,T0Initial wavelength lambda of a fiber grating temperature sensor (6)_T0；

c) Fixing one end of an optical fiber (1) on an optical platform, fixing the other end of the optical fiber (1) on a micro-displacement platform, adjusting the micro-displacement platform, and applying pre-strain to the fiber bragg grating (2);

d) part of optical fibers and capillary tubes (3) at two ends of the fiber grating (2) are adhered and fixed on the quartz substrate (4) by using adhesive glue (5), and the thickness of the adhesive glue (5) just covers the optical fibers and the capillary tubes (3);

e) after the adhesive (5) is cured, recording the wavelength lambda of the optical fiber grating (2) applying the pre-strain_ε,T0；

f) And taking the two ends of the optical fiber (1) down from the optical platform and the micro-displacement platform, and pasting the optical fiber grating temperature sensor close to the optical fiber grating.

Technical Field

The invention belongs to the technical field of optical fiber sensing, and particularly relates to a device and a method for measuring strain sensitivity of an optical fiber grating at ultralow temperature.

Background

The fiber grating sensor is successfully applied to structural health monitoring of missiles, rockets, space vehicles and the like due to the advantages of small size, light weight, electromagnetic interference resistance and the like, but the fiber grating sensor mainly works in a normal temperature range at present, and the fiber grating ultra-low temperature strain testing technology is still in a research stage. The low-temperature strain test of the structures such as the low-temperature storage tank, the pipeline and the like of the rocket still adopts the low-temperature strain gauge, so that the price is low and the installation is convenient; however, the measurement error is easily caused by factors such as zero drift and sensitivity drift caused by temperature change, and electromagnetic interference.

The fiber grating strain sensor is cross-sensitive to temperature and strain, and can simultaneously measure by using a stress-free packaged fiber grating temperature sensor to compensate zero drift of the fiber grating strain sensor. Strain sensitivity is a key parameter of a fiber grating strain sensor, and scholars at home and abroad carry out extensive research on the characteristics of the fiber grating strain sensor at ultralow temperature. Research results show that the strain sensitivity of the fiber bragg grating at the liquid nitrogen temperature changes by 0.1-13% relative to the room temperature value. The existing testing method is difficult to obtain accurate strain input under the ultralow temperature condition under the influence of the stability of the tested tool at the ultralow temperature, the accuracy of material mechanics/thermal parameters and the like, so that the low-temperature strain sensitivity of the fiber bragg grating has no uniform standard. How to obtain the strain input of the fiber bragg grating under the ultralow temperature condition is a key problem of measuring the strain sensitivity of the fiber bragg grating.

The optical fiber is made of quartz, and the thermal expansion coefficient of quartz is lower, about-0.7 x 10 within-196 deg.C to +20 deg.C^-6/℃～+0.55×10^-6/° c, the peak value of the thermal strain is less than or equal to 35 μ ∈. The quartz substrate and the fiber grating belong to the same material, and the strain exerted by the quartz substrate on the fiber grating is smaller than the thermal strain peak-to-peak value of the quartz material, and can be ignored relative to larger pre-loading strain (about 5000 mu epsilon). The method overcomes the influence of factors such as instability of the test tool at ultralow temperature, inaccurate material mechanics/thermal parameters and the like on the measurement of the strain sensitivity of the fiber bragg grating.

Disclosure of Invention

The technical problem solved by the invention is as follows: the device and the method for measuring the strain sensitivity of the fiber bragg grating at the ultralow temperature overcome the defects of the prior art, and overcome the problem that the strain input of the fiber bragg grating at the ultralow temperature is inaccurate in the existing measuring method.

The technical solution of the invention is as follows: a measuring device for fiber bragg grating strain sensitivity at ultralow temperature comprises an optical fiber, a fiber bragg grating, a capillary tube, a quartz substrate, adhesive glue and a fiber bragg grating temperature sensor; the fiber grating is partially engraved on the optical fiber, the fiber grating is arranged in the capillary, the fiber grating is subjected to pre-loaded tensile stress at room temperature, the optical fiber and the capillary at two ends of the fiber grating are fixed on the quartz substrate through bonding glue which is adaptive to the use temperature range of the measuring device, and the fiber grating temperature sensor is fixed on the quartz substrate in a sticking way.

Furthermore, the inner diameter of the capillary is larger than the outer diameter of the fiber grating and is less than or equal to 1.5 times of the outer diameter of the fiber grating, and air is filled between the inner surface of the capillary and the outer surface of the fiber grating.

Furthermore, the capillary is made of a material with the elastic modulus not lower than 1 GPa.

Furthermore, the fiber grating is subjected to a pre-loaded tensile stress under the normal temperature condition, and the value of the tensile stress is measured by using strain calibration data of the fiber grating under the room temperature condition.

Further, the distance between the fiber grating temperature sensor and the fiber grating is not more than 5 mm.

Further, the ultralow temperature is not higher than-100 ℃.

A method for measuring the strain sensitivity of fiber bragg grating at ultra-low temperature comprises the following steps:

in the manufacturing process of the measuring device, the initial wavelength lambda of the fiber grating temperature sensor at room temperature is recorded_T0Initial wavelength lambda of the fiber grating_ε0,T0Wavelength lambda of optical fiber grating after pre-strain_ε,T0；

The measuring device is placed under the ultralow temperature condition T to be measured, and the wavelength lambda of the fiber grating is recorded after the temperature is stable_ε,TAnd wavelength lambda of fiber grating temperature sensor_T；

The fiber grating is subjected to strain calibration at room temperature to obtain the strain sensitivity coefficient K at room temperature_ε,T0；

According to the initial wavelength lambda of the fiber grating temperature sensor at room temperature_T0Wavelength lambda at ultra-low temperature T_TInitial wavelength lambda of fiber grating at room temperature_ε0,T0Wavelength lambda of optical fiber grating after pre-strain_ε,T0Wavelength lambda of fiber grating at ultralow temperature T_ε,TAnd the strain sensitivity coefficient K at room temperature_ε,T0Determining the strain sensitivity K of the fiber grating at ultralow temperature T_ε,T。

Further, the strain sensitivity K of the fiber grating at ultralow temperature T_ε,TIs shown as

Further, the measuring device is obtained by the following steps:

a) inserting the fiber grating into a capillary tube, wherein a gate region is positioned in the center of the capillary tube;

b) recording initial wavelength lambda of fiber grating at room temperature by using fiber grating demodulator_ε0,T0Initial wavelength λ of fiber grating temperature sensor_T0；

c) Fixing one end of the optical fiber on an optical platform, fixing the other end of the optical fiber on a micro-displacement platform, adjusting the micro-displacement platform, and applying pre-strain to the fiber bragg grating;

d) adhering and fixing part of optical fibers and capillary tubes at two ends of the fiber grating on the quartz substrate by using adhesive glue, wherein the thickness of the adhesive glue just covers the optical fibers and the capillary tubes;

e) after the adhesive is cured, recording the wavelength lambda of the optical fiber grating subjected to the pre-strain_ε,T0；

f) And taking the two ends of the optical fiber from the optical platform and the micro-displacement platform, and pasting the optical fiber grating temperature sensor close to the optical fiber grating.

Compared with the prior art, the invention has the advantages that:

(1) the invention provides a device and a method for measuring the strain sensitivity of an optical fiber grating at an ultralow temperature, which are used for preloading tensile stress of the optical fiber grating, pasting the optical fiber grating on a quartz substrate which is made of the same material as an optical fiber and has a smaller thermal expansion coefficient, providing stable strain input for the optical fiber grating, overcoming the influences of factors such as instability of a test tool, inaccuracy of material mechanics/thermal parameters and the like of the existing test method at the ultralow temperature, and realizing the measurement of the strain sensitivity of the optical fiber grating at the ultralow temperature.

(2) The invention discloses a device and a method for measuring the strain sensitivity of an optical fiber grating at an ultralow temperature, which utilize an optical fiber grating temperature sensor to carry out temperature compensation on the zero drift of the optical fiber grating, thereby realizing the accurate measurement of the strain sensitivity of the optical fiber grating at the ultralow temperature.

Drawings

Fig. 1 is a schematic structural diagram of a device according to a specific embodiment of the device and the method for measuring the strain sensitivity of the fiber bragg grating at the ultra-low temperature of the present invention.

Detailed Description

The device and method for measuring the strain sensitivity of the fiber bragg grating at the ultra-low temperature disclosed by the invention are further described in detail with reference to the accompanying drawings and specific embodiments.

As shown in FIG. 1, the device and method for measuring the strain sensitivity of fiber bragg grating at ultra-low temperature disclosed by the invention comprises an optical fiber 1, a fiber bragg grating 2, a capillary tube 3, a quartz substrate 4, ultra-low temperature adhesive 5 and a fiber bragg grating temperature sensor 6; a part of the optical fiber 1 is engraved with an optical fiber grating 2; the fiber grating 2 is positioned inside the capillary 3; the optical fiber and the capillary 3 at two ends of the fiber grating 2 are fixed on the quartz substrate 4 through the ultralow temperature adhesive 5; the fiber grating temperature sensor 6 is close to the fiber grating 2 and is fixedly adhered to the quartz substrate 4.

The capillary tube 3 is made of a material having a high elastic modulus, such as a polyimide tube. The inner diameter of the capillary 3 is not more than 1.5 times the outer diameter of the fiber grating 2, for example, for a fiber grating having an outer diameter of 125 μm, the inner diameter of the capillary 3 may be 126 to 187 μm. Air is filled between the inner surface of the capillary 3 and the outer surface of the fiber grating 2, so that the chirp problem of the fiber grating 2 is avoided.

The quartz substrate 4 is made of the same material as the fiber grating 2 in small thermal expansion coefficient, and has the same thermal strain under the condition from room temperature to ultralow temperature, so that the thermal strain difference between the substrate material and the fiber grating is reduced, and the strain input of the fiber grating is stabilized.

The ultra-low temperature adhesive 5 adopts low temperature resistant epoxy glue, such as DW-1, DW-3 and the like, and has good ultra-low temperature environment adaptability.

A method for measuring the strain sensitivity of fiber bragg grating at ultralow temperature has the following measurement theory:

the relative change of the center wavelength λ of the fiber grating 2 can be expressed as

Δλ/λ＝K_εΔε+K_TΔT (1)

Wherein, Delta lambda is the variation of wavelength, Delta epsilon is the strain applied to the fiber grating by the substrate structure, Delta T is the variation of environment temperature, and K_εAnd K_TRespectively, the strain sensitivity and the temperature sensitivity of the fiber grating 2, respectively

Wherein Δ λ_εAnd Δ λ_TRespectively representing the wavelength variations, P, of the fiber grating 2 due to the strain Deltaε and the temperature variation DeltaT_e、α_nAnd alpha_ΛRespectively representing the elastic-optical coefficient, the thermo-optical coefficient and the thermal expansion coefficient of the optical fiber, and n represents the effective refractive index of the fiber core of the optical fiber. Temperature sensitivity K of the fiber grating 2_TThe zero drift of the fiber grating 2 can be temperature compensated by using a fiber grating temperature sensor 6 with different wavelengths, so that the measurement accuracy of the strain sensitivity of the fiber grating is improved.

At an initial temperature T₀(e.g., room temperature 20 ℃ C.), initial strain ε₀(e.g., 0. mu. epsilon.) the initial wavelength of the fiber grating 2 is λ_ε0,T0The initial wavelength of the fiber grating temperature sensor 6 is lambda_T0. Applying a pre-strain delta epsilon to the fiber grating 2_T0(e.g., 5000. mu. epsilon.) and the wavelength of the fiber grating 2 is lambda_ε,T0. Preset strain delta epsilon_T0Is shown as

Wherein K_ε,T0For the strain sensitivity of the fiber grating 2 at room temperature, the calibration method refers to GB13992-2010-T metal paste type resistance strain gauge. The fiber grating 2 is subjected to a pre-loaded tensile stress under a normal temperature condition, and the value of the tensile stress is measured by using strain calibration data of the fiber grating 2 under the room temperature condition.

The quartz substrate 4 adhered with the fiber grating 2 and the fiber grating temperature sensor 6 is placed under the condition of ultralow temperature (such as liquid nitrogen temperature-196 ℃), and the wavelength of the fiber grating 2 is lambda_ε,TThe wavelength of the fiber grating temperature sensor 6 is lambda_T. Strain sensitivity K of the fiber grating 2 at ultra-low temperature T_ε,TIs shown as

Wherein λ_ε0,TRepresents the zero point wavelength, Delta epsilon, of the fiber grating 2 under the ultralow temperature condition_TRepresenting the strain to which the fiber grating 2 is subjected under ultra-low temperature conditions, the strain applied to the fiber grating 2 by the quartz substrate is fixed, i.e. delta epsilon_T＝Δε_T0。

Zero wavelength lambda of fiber grating 2 at ultralow temperature_ε0,TCan be obtained from the wavelength of the fiber grating temperature sensor,

therefore, the strain sensitivity K of the fiber grating 2 at the ultra-low temperature T_εTIs shown as

In summary, the strain sensitivity of the fiber grating 2 at ultra-low temperature can be measured by the apparatus and method of the present invention.

The invention discloses a method for measuring the strain sensitivity of a fiber grating at an ultralow temperature, which comprises the following specific operation steps:

a) the fiber bragg grating 2 is subjected to strain calibration at room temperature to obtain a strain sensitivity coefficient K_ε,T0；

b) Inserting the fiber bragg grating 2 into the capillary 3, wherein the grid region is located in the center of the capillary;

c) recording the initial wavelength lambda of the fiber grating 2 at room temperature_ε0,T0Initial wavelength λ of the fiber grating temperature sensor 6_T0；

d) Fixing one end of an optical fiber 1 on an optical platform, fixing the other end of the optical fiber 1 on a high-precision micro-displacement platform, adjusting the high-precision micro-displacement platform, and applying pre-strain to the fiber bragg grating 2;

e) part of optical fibers and capillary tubes 3 at two ends of the fiber grating 2 are adhered and fixed on a quartz substrate 4 by using ultralow-temperature adhesive glue, and the thickness of the adhesive glue just covers the optical fibers and the capillary tubes 3;

f) adhesive for bonding at ultralow temperatureAfter curing is complete, the wavelength λ of the prestrained fiber grating 2 is recorded_ε,T0；

g) Taking the two ends of the optical fiber 1 down from the optical platform and the high-precision micro-displacement platform, and pasting the fiber grating temperature sensor 6 close to the fiber grating 2;

h) placing the quartz substrate 4 adhered with the fiber grating 2 and the fiber grating temperature sensor 6 in an ultralow temperature condition (such as a heat insulation barrel filled with liquid nitrogen), and recording the wavelength lambda of the fiber grating 2 at the ultralow temperature_ε,TAnd wavelength lambda of the fiber grating temperature sensor 6_T；

i) The strain sensitivity K of the fiber grating 2 under the ultralow temperature condition is calculated by using the formula (7)_ε,T；

j) And repeating the steps to carry out test verification.

The detailed description of the invention is not part of the common general knowledge of a person skilled in the art.