AU2020103491A4

AU2020103491A4 - A twin array Michelson fiber optic white light interferometry strain gauge

Info

Publication number: AU2020103491A4
Application number: AU2020103491A
Authority: AU
Inventors: Jun Yang; Yonggui YUAN
Original assignee: Harbin Engineering University
Current assignee: Harbin Engineering University
Priority date: 2020-11-17
Filing date: 2020-11-17
Publication date: 2021-01-28
Anticipated expiration: 2028-11-17

Abstract

The present invention proposes a twin array Michelson fiber optic white light interferometry strain gauge, consisting of a broad spectrum light source, a light detector, a coupler, a single mode fiber, an optical attenuator, an optical delay line and a sensing array consisting of twin fiber sensors. In which, the broad spectrum light emitted by the broad spectrum light source is split by the coupler, one beam enters one arm via the optical attenuator, and the other beam enters the another arm via the optical delay line, the light signals transmitted back from the two arms enter the light detector via the coupler to be detected and analysed. The characteristics of the invention are: simultaneous measurement of strain and temperature, while simplifying the complexity of the system, reducing testing costs, ensuring real-time testing and improving the reliability of measurement. The structure is simple and easy to implement. 2/3 2 _______2__/_3 77 702 2 5 7 77 4 FIG. 3 701 701 70 701 702 702 4 FIG. 4 8 7 I(T 7 78 FIG. 5

Description

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DESCRIPTION TITLE OF INVENTION

A twin array Michelson fiber optic white light interferometry strain gauge

TECHNICAL FIELD

[0001] The invention relates to a measurement device, particularly relates to a fiber optic

interferometry strain gauge based on the principle of space division multiplexing (SDM).

BACKGROUND ART

[0002] Fiber optic interferometers that employ low coherence, broad spectrum light sources are

commonly referred to as white light fiber interferometers. Compared with other fiber optic

interferometers, in addition to high sensitivity, intrinsically safe, anti-electromagnetic

interference and other advantages, the most important feature is that the absolute measurement

on measures to be tested such as pressure, strain, and temperature. As such, white light

interferometric fiber optic interferometer is widely used in physical, mechanical, environmental,

chemical and biomedical measurement.

[0003] In recent years, the white light interferometric sensing technology has been greatly

developed, one of the hotspot is the development of fiber optic sensors and measurement systems

based on SDM and are used in measuring physical quantities such as strain, temperature, pressure, and others. The background to the development of multiplexing technology is largely due to the fact that in practical measurement and test applications, the sensing of a single location or a single physical quantity is far from satisfying the requirements of people to acquire the whole thing or the system state. Often, online or real-time measurements of the distribution of multiple or multi-point position distribution of physical quantities are required, such as during the non-destructive testing and monitoring of large structures or critical areas (hydroelectric power stations, dams, bridges, etc.) to determine their safety conditions, it needs to implant sensors in critical areas to build a monitoring network, then extract on information such as the internal displacement, strain, and temperature. As such, the number of sensors are usually dozens or hundreds of, if the test system is connected with only a single point sensor, the cost of the test will undoubtedly increase significantly and the reliability of the system will be reduced.

Therefore, the use of multiplexing technology, the interrogation of measurement information

from several sensors with the same demodulation system greatly simplifies system complexity

and ensures measurement accuracy and reliability. In the same time, the use of multiplexing

technology lowers the manufacturing costs of single point sensors, which greatly reduce the cost

of testing, improves the quality-price ratio, and makes the fiber optic sensors more advantageous

than the traditional sensors.

[0004] The multiplexing technologies that have already been developed are: time division

multiplexing (TDM), frequency division multiplexing (FDM or FMCW), wavelength division

multiplexing (WDM), code division multiplexing (CDM) and SDM. In which, TDM, FDM and

SDM are applied to white light interferometric sensing systems.

[0005] Santos, et al. (Santos, J.L., Jackson, D.A., Coherence sensing of time-addressed optical

fiber sensors illuminated by a multimode laser diode Appl. Opti, 30, 5068-5077, 1991) has

developed TDM, it is a multiplexing technique for address by using the delay effect of optical

fibers on the light waves. This method is complex, with limited multiplexing numbers, small

measurement range and low accuracy.

[0006] Liu, et al. (Liu, T., Fernando, G.F., A frequency division multiplexed low-finesse optical fiber Fabry-Perot sensor system for strain and displacement measurements, Review of Scientific Instruments, 71(3), 1275-1278, 2000) developed the FDM technology, which directly measures the multiplied results of optical spectrum output by multiple Fabry-Perot interferometers with different cavity lengths using a spectral analyzer. This method is limited by the cavity lengths and cavity length differences, and the number of interferometers to be multiplexed is limited to a few.

[0007] Compared to other types of fiber optic sensors, sensing systems based on SDM are a unique feature of white light interferometric sensing technology. In white light interferometric sensing systems, where different sensor lengths are typically required, there is only a single white light interferometric signal for each sensor in the multiplexed array over its coherent length. Scan the time or spatial optical path through a discrete reference interferometer, this can distinguish between multiple sensors and to demodulate and interrogate the physical quantities to be measured, making it easy to implement multiplexing without the need for more complex time or frequency multiplexing techniques. This multiplexing technique is also known as SDM because of the discrete nature of the interference signals of each sensor in the optical path scan space.

[0008] Yuan, et al. (Yuan, L.B., Zhou, L.M., Jin, W., Quasi-distributed strain sensing with white light interferometry: a novel approach, Optics Letters, 25, 1074-1076, 2000) has published the SDM, where the multiple sensors can be easily demodulated and interrogated using a discrete reference interferometer for continuous optical path scanning in both time and space, providing a simple structure and an accurate method for multiplexing.

[0009] In 2006, the applicant disclosed a multiplexing fiber optic interferometer and its nesting construction method (Chinese Patent No. CA1963399A), and invented an all-fiber interferometry fiber and its implementation method that can construct a sensor array and a network, which solves the problem of small number of multiplexing and complex structure of fiber optic interferometers. In the low coherence twisted Sagnac-like fiber optic deformation sensing device (Chinese Patent No. 101074867A) disclosed by the applicant in 2007, although the interferometer structure can theoretically solve the problem of damage resistance, its structure is too complex, especially the optical signal travels through too many paths, the intensity is weak, and the signal-to-noise ratio is low, which is not conducive to deployment and use in actual measurement.

[0010] For both physical quantities, strain and temperature, the response of a fiber optic sensor is

intrinsic, i.e., changes in ambient temperature as well as external stresses cause the sensor output.

In multiplexing fiber optic sensing arrays and networks, especially for the measurement and

application of smart structures, strain sensors, whether point or large scale, encounter

temperature compensation problems. The temperature compensation problem, therefore, is a

very important and extremely difficult problem for strain sensing measurements and other fiber

optic sensing measurements.

SUMMARY OF INVENTION

[0011] It is an object of the present invention to propose a twin array Michelson fiber optic white

light interferometry strain gauge based on the principle of white light interference and SDM, to

realize the measurement of absolute optical path of each sensor's end face.

[0012] The invention is realized by the following technical solutions:

[0013] A twin array Michelson fiber optic white light interferometry strain gauge, consisting of a

broad spectrum light source 1, a light detector 2, a 3dB fiber optic coupler 3, a single-mode connection fiber optic 4, an optical attenuator 5, an optical delay line 6 and twinfiber sensors 7.

The fiber optic sensors 7 are connected at the beginning and end to form two sensing arrays 8,

one for temperature measurement and the other for simultaneous temperature and strain

measurements; the corresponding sensors in the sensing array are two identical twin sensors. The

broad spectrum light emitted by the broad spectrum light source 1 is split by the 3dB fiber 2x2

coupler 3, one beam enters the array 8 consisting of an array of fiber sensors 7 via the optical

attenuator 5 as the reference optical path; the other beam enters the array 8 consisting of another

array of fiber sensors 7 via the optical delay line 6 as the measurement optical path. The light

signals transmitted back from the two sensing arrays enter the light detector 2 via the 3dB fiber

2x2 coupler 3, the measurement optical path and the reference optical path are located close to

each other in the environment to be measured, and the reference optical path is isolated from the

environment to be measured. The measurement optical path senses both the strain and the

ambient temperature, while the reference optical path is only used to sense the temperature, i.e.,

the temperature has the same effect on the reference optical path and the measurement optical

path.

[0014] The invention can also include:

[0015] The fiber optic sensor 7 is constructed by a length of fiber optic of any length with

vertically reflecting ends of arbitrary reflectivity at both ends, it adds the ceramic ferrule (701) at

both ends of the section of the single-mode optical fiber of any length intercepted according to

the actual measurement needs, the end faces are polished to get a fiber with end faces that are

perpendicular to the direction of light transmission with a reflection rate of more than 1% of any

length; the fiber optic sensor 7 is connected to the sensor of optical fiber through the ceramic

sleeve 702. Several such identical fiber optic sensors 7 are head-to-tail connected to form a serial

fiber optic sensor array; the two sensor arrays are connected to the reference optical path and the

measurement optical path of the interferometer to form a twin sensor array interferometer.

[0016] The twin fiber optic sensor array is a serial array of two exactly same arras of fiber sensors connected head-to-tail.

[0017] The optical delay line 6 described consists of a pyramid prism with a concave angle of 900 and a linear scanning stage.

[0018] The broad-spectrum light sources 1, the optical detector 2, the 3dB fiber 2x2 coupler 3, the single-mode connection optical fiber 4, the optical attenuator 5, the optical delay line 6, and the array 8 of twin fiber sensors 7, are all operating in the single-mode state.

[0019] The measurement and reference optical paths are both connected to the same fiber optic sensor array, while the optical delay line 6 is connected in the measurement path and the optical attenuator 5 is connected in the reference optical path.

[0020] The present invention embeds the twin fiber optic sensor array in the reference and measurement optical paths of the fiber interferometers, so that the white light interference strips are obtained by matching the optical paths accumulated by the reference optical wave travelling multiple round trips in the optical delay line, and this can achieve the construction of the multiplexing fiber interferometer which connects the twin sensor array.

[0021] The basic principle of the invented method is the principle of interference and SDM with low coherence and broad spectrum light (white light). The twin array fiber optic interferometer is also developed from the simplest structured fiber optic interferometer as shown in FIG. 1. The Michelson interferometer as shown in FIG. 1, the broad spectrum light emitted by the light source 1 is split into two beams by the 3dB single-mode fiber 2x2 coupler 3, a beam of light enters into the single-mode connection fiber 4 as a measurement arm and is reflected by the end surface mirror 6. The reflected light arrives at the photodetector 2 via the connection fiber 4 and the coupler 3, and this light beam is the measurement signal light. Another light enters the single mode connection fiber 5, which acts as a reference arm, and is reflected by the continuously variable optical delay line 7 and also arrives at the photodetector 2 via the connection fiber 5 and the coupler 3, and is called the reference signal light. The measurement signal light and the reference signal light are coherent-superposed on the surface of the detector, since the coherence length of a broad spectrum low-coherent light source is very short, ranging from a few micrometers to tens of micrometers, only when the optical path difference between the reference and measurement signal light is smaller than the coherence length of the light source, coherence superposition will occur and a white interferogram will be output. The intensity of the interfering strips can be expressed as follows:

I=I]+I2+2I- I - 12(x). cos(k. x+) (1)

In the formula, Ii and 12 are the signal intensity of the reference light beam and the measurement

light beam, k is the wavenumber, x is the optical path difference of the two interference signals,

(p is the initial phase, and y(x) is the autocorrelation function of the light source.

[0022] The white light interferometric strips are characterized by a central strip with one main

maximum, which corresponds to the position of zero optical path difference, i.e., when the

reference beam and the measurement beam have equal optical path, then the reference beam and

the measurement beam have a matching optical path relationship. The central interferometric

strip can be obtained by changing the delay amount of the optical delay line to vary the optical

path of the reference signal. The central strip position provides a reliable absolute position

reference for the measurement, when the optical path of the measurement beam changes due to

the influence of the external physical quantity to be measured, the absolute value of the

measured quantity can be obtained simply by the white interferometric strip position change

obtained by scanning the reference arm optical path.

[0023] Interference strips in a fiber interferometer based on the white light interference principle

only occur within a few micrometers to tens of micrometers of the optical path match. Using this characteristics, multiplexing of sensing interferometer can be achieved without using complex time or frequency multiplexing techniques. As shown in FIG. 2, if the reference optical path is in series with a number of the same sensors to form a sensor array, the corresponding expansion of the scanning range of the measurement optical path can achieve the serial use of a number of sensors, but the expansion of the scanning range faces a lot of difficulties. Hence, as shown in FIG. 3, the measurement optical path is also connected in series with the same number of fiber optic sensors to form a twin fiber optic sensor array, which avoids the difficulty of expanding the scanning range of the optical path and only requires the addition of an optical delay line on one side of the optical path with a small scanning range. This technique is called SDM due to the discrete nature of the corresponding interference signals of each sensor in the optical scan space.

[0024] The basic idea of the construction of white light fiber interferometer based on SDM is that the optical paths introduced by different measurement optical paths or sensors can be matched one to one by scanning the optical paths of the reference beam, so that the resulting white light interferometric strips are independent of each other in the optical path scanning space and they do not interfere with each other. The splitting of the measurement beam can be achieved by adding reflective surfaces to the optical fiber, and the matching of the reference beam can be achieved directly by using a continuously variable optical delay line to scan the optical path.

[0025] The strain measurement of fiber optic sensors is inevitably affected by changes in ambient temperature because the response of the sensor to strain and temperature is intrinsic. Therefore, in the measurement and application of smart structures, one of the most difficult problems for strain sensors, whether point or large scale, is how to eliminate the influence of ambient temperature, i.e., temperature compensation problems. The invention uses the twin sensor array to eliminate the temperature influences during the measuring process, the basic idea is that the measurement arm and the reference arm are placed close to each other in the environment to be measured, and the reference arm is isolated from the environment to be measured. The measurement arm senses both the strain and the ambient temperature, and the reference arm senses only the temperature, i.e., the temperature has the same effect on the reference arm and the measurement arm. In this way, although the length changes of the fiber optic sensors in the measurement arm is related to both temperature and strain, the length of the fiber optic sensor in the reference arm affected by temperature is the same as that of the measurement arm, so that when the measurement light interferes with the reference light, the effect of temperature is eliminated, i.e., the change of the peak white light interference is only related to the magnitude of the strain, and the corresponding strain value can be calculated from the value of the peak offset to realize the measurement.

[0026] The advantages and features of the invention are:

[0027] (1) The twin fiber optic sensor array fiber optic interferometer constructed by the present invention can realize simultaneous measurement of strain and temperature, and the sensors do not affect each other during the measurement, which simplifies the complexity of the system, reduces the testing cost, ensures the real-time testing system, and improves the reliability of the measurement.

[0028] (2) The invention uses a few of devices, has a simple combination and is easy to achieve.

[0029] (3) The fiber optic materials and devices used in the present invention are standard fiber optic communication components, which are inexpensive, readily available, and conducive to promotion.

BRIEF DESCRIPTION OF DRAWINGS

[0030] FIG. 1 is a schematic diagram of the simplest structure of a fiber optic interferometer using a white light light source.

[0031] FIG. 2 is a schematic diagram of the structure of a white light source interferometer with

multiple fiber optic sensors in series with a reference light path.

[0032] FIG. 3 is a schematic diagram of the structure of a white light light source interferometer

with a twin array.

[0033] FIG. 4 is a schematic diagram of the structure of afiber optic sensor.

[0034] FIG. 5 is a schematic diagram of the structure of a twinfiber optic sensor array embedded

in the object to be measured.

[0035] FIG. 6 is a diagram of the white light interference signal of the twin fiber optic sensor

array interferometer.

DESCRIPTION OF EMBODIMENTS

[0036] The invention is further described below in connection with embodiments and drawings,

but should not be construed as limiting the scope of protection of the invention.

[0037] Embodiment: Scheme for a multiplexing fiber optic interferometer constructed using twin

array fiber optic sensors, in conjunction with FIG. 3, it can be seen in the figure that the multiplexing twin fiber optic interferometer consists of a broad spectrum light source 1, a light detector 2, a 3dB fiber optic 2x2 coupler 3, a single-mode connection fiber optic 4, an optical attenuator 5, an optical delay line 6 and arrays 8 consisting of twin fiber sensors 7.

[0038] The fiber sensor 7 is constructed of a length offiber optic of any length with vertically

reflecting ends of arbitrary reflectivity at both ends, the classical structure is shown in FIG. 4.

Ceramic ferrule (701) is added to both ends of the section of the single-mode optical fiber of any

length intercepted according to the actual measurement needs, the end faces are polished to get a

fiber with end faces that are perpendicular to the direction of light transmission with a reflection

rate of more than 1%. The fiber optic sensor 7 is connected to the sensor of optical fiber through

the ceramic sleeve 702, the ceramic sleeve can protect the sensors end faces. Several such

identical fiber optic sensors 7 are head-to-tail connected to form a serial fiber optic sensor array;

the two sensor arrays are connected to the reference optical path and the measurement optical

path of the interferometer to form a twin sensor array interferometer.

[0039] As shown in FIG. 3, the measurement arm and the reference arm are connected to the

same fiber optic sensor array, and the optical delay line 6 is connected to the measurement arm to

match the optical path to the measurement beam. When the optical distance difference between

the reference arm and the measurement arm is within the coherence length of the light source, a

white light interference strip will be generated, which will be at the center of the interference

pattern and will have the maximum amplitude. For the measurements, one arm of the twin array

(the measurement arm) was buried in the structure to be measured, and in order to investigate the

variation of this arm, the other arm (the reference arm) was placed in a tube and placed near the

first arm, as shown in FIG. 5. Due to the close proximity of the two arms, they are considered to

be at the same temperature. The optical path delay line is adjusted so that the light returned by

the corresponding fiber optic sensors in the two twin sensor arrays interferes, resulting in an

interference strip as shown in FIG. 6. When the strain in the measurement arm and the ambient

temperature change, the change in the length of the fiber optic sensor in the measurement arm is

temperature and strain dependent, while the change in the length of the fiber optic sensor in the reference arm can be considered as temperature dependent only, which will inevitably lead to a drift in the interference peak. The measurement can be made by calculating the corresponding strain or temperature change by comparing the offset of the interference slit for each pair of fiber optic sensors.

Claims

1. A twin array Michelson fiber optic white light interferometry strain gauge, consisting of a

broad spectrum light source (1), a light detector (2), a 3dB fiber optic coupler (3), a single-mode

connection fiber optic (4), an optical attenuator (5), an optical delay line (6) and twin fiber

sensors (7). The characteristics are: the fiber optic sensors (7) are connected at the beginning and

end to form two sensing arrays (8), one for temperature measurement and the other for

simultaneous temperature and strain measurement; the corresponding sensors in the sensing

arrays are two identical twin sensors. The broad spectrum light emitted by the broad spectrum

light source (1) is split by the 3dB fiber 2x2 coupler, one beam enters the array (8) consisting of

an array of fiber sensors (7) via the optical attenuator (5) as the reference optical path; the other

beam enters the array (8) consisting of another array of fiber sensors (7) via the optical delay line

(6) as the measurement optical path. The light signals transmitted back from the two sensing

arrays enter the light detector (2) via the 3dB fiber 2x2 coupler (3), the measurement optical path

and the reference optical path are located close to each other in the environment to be measured,

and the reference optical path is isolated from the environment to be measured. The

measurement optical path senses both the strain and the ambient temperature, while the reference

optical path is only used to sense the temperature, i.e., the temperature has the same effect on the

reference optical path and the measurement optical path.

2. As claimed in claim 1, a twin array Michelson fiber optic white light interferometry strain

gauge, its characteristics are: the fiber optic sensor (7) is a section of single-mode optical fiber of

any length intercepted according to the actual measurement needs, the section of the single-mode

optical fiber of any length intercepted according to the actual measurement needs is equipped

with ceramic ferrule (701) at both ends, the end faces are polished to get fiber end faces that are

perpendicular to the direction of light transmission with a reflection rate of more than 1%; the

fiber optic sensor (7) is connected to the sensor of optical fiber through the ceramic sleeve (702).

3. As claimed in claim 2, a twin array Michelson fiber optic white light interferometry strain

gauge, its characteristic is: the optical delay line (6) described herein consists of a pyramid prism

with a concave angle of 900 and a linear scanning stage.

4. As claimed in claim 3, a twin array Michelson fiber optic white light interferometry strain

gauge, its characteristic is: the broad-spectrum light sources (1), the optical detector (2), the 3dB

fiber 2x2 coupler (3), the single-mode connection optical fiber (4), the optical attenuator (5), the

optical delay line (6), and the array (8) of twin fiber sensors (7), are all operating in the single

mode state.