CN105115436A

CN105115436A - sensing device and method for monitoring stress and temperature

Info

Publication number: CN105115436A
Application number: CN201510471900.9A
Authority: CN
Inventors: 姚晓天
Original assignee: BEIJING GAOGUANG TECHNOLOGY Co Ltd; General Photonics Optoelectronic Technology (beijing) Co Ltd; SUZHOU OPTORING TECHNOLOGY Co Ltd
Current assignee: Yueshen Innovation Co ltd
Priority date: 2015-08-04
Filing date: 2015-08-04
Publication date: 2015-12-02
Anticipated expiration: 2035-08-04
Also published as: CN105115436B

Abstract

The invention relates to a sensing device and a method for monitoring stress and temperature. The device includes a sensing substrate, an optical birefringent medium, a light source, a linear optical polarizer, an interferometer and a processor. The device and the method obtain a temperature change of the optical birefringent medium according an interval change between polarization crosstalk peaks, and obtain stress and/or strain of the optical birefringent medium according to peak values of the polarization crosstalk peaks. Thus monitoring of stress, strain and temperature can be performed on an object to be measured by utilization of the same optical birefringent medium, and the strain, the stress and the temperature change are not influenced mutually.

Description

Sensing device and method for monitoring stress and temperature

Technical Field

The invention relates to the field of monitoring, in particular to a sensing device and a method for monitoring stress, strain and temperature.

Background

Temperature, stress and strain are very important physical quantities in the application of optical fiber sensing technology, and in the field of safety monitoring of large bridges, tunnels, dams and the like, parameters such as temperature, stress and strain need to be monitored. The stress refers to an external force or an internally generated force to which the object is subjected, and the strain refers to deformation, such as expansion, bending, or displacement, etc., due to the stress to which the object is subjected. The existing optical fiber sensing technology measures temperature, stress and strain respectively by using a separate measurement system, for example, a fiber grating measurement system measures a measured object by points, each point measures temperature or measures stress or strain, the system cost is high, and overall distributed measurement cannot be realized. The existing polarization crosstalk principle distributed optical fiber sensing measurement technology only analyzes stress (such as external pressure) but does not have an analysis method for strain and temperature, and particularly how to convert the strain of a measured object into the stress of a polarization crosstalk detection medium, so that a crosstalk measurement value for generating reaction strain has no specific scheme or measure.

Disclosure of Invention

Based on this, this scheme provides one kind and can monitor method and device of strain, stress and temperature through the mode of polarization crosstalk measurement.

A method of monitoring stress and temperature, comprising the steps of: linearly polarized light with a coupled broadband enters an optical birefringent medium, the linearly polarized light is transmitted along two orthogonal polarization modes of the optical birefringent medium, and the optical birefringent medium outputs an optical output signal; directing said optical output signal through a linear optical polarizer, said polarizer mixing two orthogonal polarization modes of said optical output signal with each other; guiding linearly polarized light generated by the optical polarizer to enter an interferometer so as to obtain interference between two orthogonal polarization modes to generate a polarization crosstalk peak; and obtaining the temperature change of the optical birefringent medium according to the change of the distance between the polarization crosstalk peaks, and obtaining the stress and/or strain of the optical birefringent medium according to the peak value of the polarization crosstalk peaks.

In one embodiment, the optically birefringent medium is provided with a predetermined polarization crosstalk.

In one embodiment, the optically birefringent medium comprises a polarization maintaining fiber.

In one embodiment, the method further comprises the following steps: an optical output signal output by the optical birefringent medium enters an optical retarder, so that delay is generated between two orthogonal polarization modes.

In one embodiment, the optical birefringent medium is attached to an object to be measured for monitoring stress, strain and temperature of the object to be measured.

In one embodiment, the optical birefringent medium is disposed on a sensing substrate, and the sensing substrate is attached to an object to be measured and used for monitoring stress, strain and temperature of the object to be measured.

In one embodiment, the optical birefringent medium is distributed in one-dimensional space or two-dimensional space on the sensing substrate to monitor the spatial distribution of stress, strain and temperature of the object to be measured.

In one embodiment, a predetermined polarization crosstalk is applied to the optically birefringent medium through the sensing substrate.

A sensing device for monitoring temperature changes, strain and/or stress of an object, comprising: the optical birefringent medium is used for sensing the temperature change, the stress and/or the strain of the object to be detected; the light source is used for generating broadband linearly polarized light, the linearly polarized light is transmitted along two orthogonal polarization modes of the optical birefringent medium, and the optical birefringent medium outputs an optical output signal; a linear optical polarizer for receiving said optical output signal and mixing two orthogonal polarization modes of said optical output signal with each other; an interferometer for receiving light transmitted through the linear optical polarizer and obtaining interference between two orthogonal polarization modes to generate a polarization crosstalk peak; and the processor is used for obtaining the temperature change of the optical birefringent medium according to the change of the spacing between the polarization crosstalk peaks and obtaining the stress and/or strain of the optical birefringent medium according to the peak values of the polarization crosstalk peaks.

In one embodiment, it further comprises: and the optical retarder is used for receiving an optical output signal output by the optical birefringent medium, delaying two orthogonal polarization modes and transmitting the delayed two orthogonal polarization modes to the linear optical polarizer.

In one embodiment, the sensing device further includes a sensing substrate connected to the object to be measured, and the optical birefringent medium is disposed on the sensing substrate in a line array manner or an area array manner.

In one embodiment, a protrusion or a groove is disposed on the sensing substrate, and the protrusion or the groove is in contact with the optical birefringent medium, so that the optical birefringent medium has a predetermined polarization crosstalk.

In one embodiment, the sensing substrate is provided with a plurality of through holes, the optical birefringent medium passes through the through holes, and the optical birefringent medium is distributed on two sides of the sensing substrate, so that the optical birefringent medium has a preset polarization crosstalk.

In one embodiment, the sensing substrate is provided with a plurality of through hole pairs, the optical birefringent medium passes through the through hole pairs, and the optical birefringent medium is distributed on one side of the sensing substrate, so that the optical birefringent medium has preset polarization crosstalk.

In one embodiment, the sensing substrate is provided with a zigzag channel, and the optical birefringent medium is arranged in the zigzag channel, so that the optical birefringent medium has a preset polarization crosstalk.

In one embodiment, the direction of the polarization axis of the optically birefringent medium is 45 ° to the normal direction of the sensing substrate.

The method and the device can simultaneously obtain stress, strain and temperature by utilizing the peak value and the distance of the polarization crosstalk peak generated in the optical birefringent medium, and can monitor the stress, the strain and the temperature change of the object to be detected by utilizing the same optical birefringent medium.

Drawings

FIG. 1 is a schematic structural diagram of a sensing device;

FIG. 2A is a schematic diagram of a polarization crosstalk analyzer, FIG. 2B is an orientation of an optical linear optical polarizer with respect to an optical axis of a polarization-maintaining fiber, and FIG. 2C illustrates the case where stress is present at multiple locations along the polarization-maintaining fiber to produce crosstalk between two orthogonal polarization modes of the polarization-maintaining fiber;

FIG. 3 shows an exemplary apparatus for measuring polarization crosstalk of an optically birefringent medium (e.g., polarization maintaining fiber) by adding an optical delay device between the fiber under test and an optical interferometer, and further illustrates the operation of the apparatus;

FIG. 4 illustrates a schematic diagram of a configuration for measuring an optical birefringent medium polarization crosstalk analyzer based on the application of a birefringent dispersion compensation function;

fig. 5 shows an example of a polarization crosstalk curve of a polarization-maintaining fiber ring. The cross-talk envelope amplitudes and widths of the output and input interfaces of the fiber loop are shown at both ends, and the birefringence dispersion compensation before (solid line) and after (dashed line) the middle region of the fiber loop is shown;

fig. 6A demonstrates the measurement of the envelope width induced by stress at various different locations of the crosstalk peaks for the polarization maintaining fiber sample in the system of fig. 4, and fig. 6B shows the measured values for crosstalk D for the input connector for exemplary six different polarization maintaining fiber lengths.

FIGS. 7A and 7B illustrate cross-talk curves when stress is applied to a polarization maintaining fiber per 0.5 meter length on a 1-dimensional polarization maintaining fiber sensor;

FIG. 8A illustrates birefringence measurement curves as a function of temperature, FIG. 8B shows the associated retardation function for the polarization crosstalk curve in a polarization-maintaining fiber, and FIG. 8C further illustrates an expanded view of positions 48 and 49;

FIG. 9A is a diagram illustrating a one-bit sensor substrate according to one embodiment, FIG. 9B is a diagram illustrating a function of a corresponding polarization crosstalk peak according to the embodiment of FIG. 9A, FIG. 9C is a diagram illustrating a one-dimensional sensor substrate according to another embodiment, and FIG. 9D is a diagram illustrating a function of a corresponding polarization crosstalk peak according to the embodiment of FIG. 9C;

FIG. 10A is a schematic diagram of a two-dimensional sensing substrate, FIG. 10B is a functional diagram of a corresponding polarization crosstalk peak, and FIG. 10C is a position schematic diagram of a corresponding stress and temperature variation, according to an embodiment;

FIG. 11A is a schematic top view and FIG. 11B is a schematic side view of a one-dimensional sensor substrate according to another embodiment;

FIG. 12A is a schematic top view and FIG. 12B is a schematic side view of a one-dimensional sensor substrate according to another embodiment;

FIG. 13 is a functional plot of the corresponding polarization crosstalk peaks for the structures of FIGS. 11 and 12;

FIG. 14A is a schematic structural diagram of a two-dimensional sensor substrate according to another embodiment, FIG. 14B is a functional diagram of a corresponding polarization crosstalk peak, and FIG. 14C is a schematic positional diagram of a corresponding stress and temperature variation;

FIG. 15 is a schematic structural diagram of a sensing substrate with one-dimensionally distributed Z-shaped polarization maintaining fibers;

FIG. 16 is a schematic structural diagram of an embodiment of a sensing substrate with two-dimensionally distributed Z-shaped polarization maintaining fibers;

FIG. 17 is a schematic structural diagram of another embodiment of a sensing substrate with two-dimensional individual Z-shaped polarization maintaining fibers;

FIG. 18 is a block flow diagram of a method of measuring stress, strain and temperature.

Detailed Description

A sensing device is disclosed that can be used to measure both stress (or strain) and temperature of an object to be measured simultaneously (or separately). Referring now to fig. 1, a sensing device 10 includes a sensing substrate 11, an optically birefringent medium 12, a polarization crosstalk analyzer 13, and a processor 14. The sensing substrate 11 may be made of a flexible or elastic material, and may be connected to an object to be measured to measure the stress (or strain) and the temperature of the object to be measured. The optical birefringent medium 12 may be provided on the sensing substrate 11, and in the present embodiment, the optical birefringent medium 12 may be a polarization maintaining fiber, but it may also be other types of optical birefringent media, such as birefringent crystals, e.g., yttrium vanadate, quartz crystals, etc., glass with prestress, etc. While the polarization maintaining fiber is disposed on the sensing substrate 11, the sensing substrate 11 may apply a predetermined polarization crosstalk to the polarization maintaining fiber, and the polarization crosstalk may be preset by applying a predetermined regular stress or bending to the polarization maintaining fiber by the sensing substrate 11. In this embodiment, the polarization crosstalk analyzer 13 may include a light source, an optical retarder, a linear optical polarizer, an interferometer. The light source is configured to generate broadband linearly polarized light that propagates along two orthogonal polarization modes of optically birefringent medium 12, and optically birefringent medium 12 outputs an optical output signal. The output signal is received by an optical retarder that imparts a delay between two orthogonal polarization modes and is transmitted to a linear optical polarizer. The linear optical polarizer mixes the two orthogonal polarization modes of the optical output signal with each other. The interferometer receives light transmitted through the linear optical polarizer and obtains interference between two orthogonal polarization modes to generate a polarization crosstalk peak. And the processor 14 obtains the temperature change of the optical birefringent medium according to the change of the distance between the polarization crosstalk peaks, and obtains the stress value of the optical birefringent medium according to the peak value of the polarization crosstalk peaks, so that the temperature and the stress (or strain) value of the object to be measured can be monitored simultaneously (or respectively). Fig. 1 also schematically shows that two weights 20 are disposed on the sensing substrate 11, and the two weights 20 can simulate the magnitude of the stress applied to the object to be measured. Of course, the sensing device 10 may not include the sensing substrate 11, and the optical birefringent medium may be directly disposed on the object to be measured.

The following describes a specific measurement method of polarization crosstalk. In polarization maintaining fiber, when the incident light is strictly aligned with the fast axis or the slow axis of the polarization maintaining fiber, optical coupling between two polarization modes of the polarization maintaining fiber is generated due to internal defects of the polarization maintaining fiber or external pressure. Mode coupling between the slow and fast axes of polarization maintaining fibers is known as polarization crosstalk. One way to describe polarization crosstalk is the ratio of the light intensities of the two polarization modes transmitted in the slow and fast axes, respectively. In a specific application, it is necessary to determine the position of occurrence of polarization crosstalk and the degree of polarization crosstalk in the polarization-maintaining optical fiber.

Interference of light waves traveling along the slow and fast axes of the polarization maintaining fiber produces a real interference signal and a ghost interference signal at the location where the polarization maintaining fiber is coupled, the ghost interference signal being caused by multiple coupling of light waves between the plurality of crosstalk points. When there are multiple strong crosstalk points in the polarization maintaining fiber, ghost peak signals will be strong, resulting in erroneous determination of crosstalk position and intensity.

Please refer now to fig. 2A-2C. In fig. 2A, broadband light 101 emitted from a broadband light source is input into a polarization maintaining fiber at position a 110. Light 101 has a polarization component that is aligned with the slow axis of the polarization-maintaining fiber. Pressure at location B causes polarization coupling between the two orthogonal polarization states along the fast and slow axes of the polarization maintaining fiber, producing a polarization component aligned with the fast axis. Because the two polarization components are transmitted at different speeds in the fiber, at the output 111 of the fiber (position C), the two polarization components experience different delays:

Δz＝n_sz-n_fz＝Δnz(1)

wherein n is_sAnd n_fThe refractive indices along the slow and fast axes, respectively, the difference Δ n between the two indices being the birefringence index, z being the distance from the coupling point B to the output point C. If the polarization axis of the optical polarizer 120 placed after the fiber output 111 is at 45 to the slow axis of the polarization maintaining fiber (see fig. 2B), half of the light power of each of the two polarization components passes through the polarizer and has the same linear polarization state, which is oriented in the same direction as the polarization axis of the optical polarizer 120.

Thus, when the interferometer is used to receive light output from the optical polarizer 120, the optical polarizer 120 acts to optically interfere the received light, which contains polarization components in two polarization modes, respectively, and such optical interference can be used for polarization crosstalk measurement.

Here, a michelson interferometer is used as an example of the interferometer. The beam splitter 130 is operative to receive light output from the optical polariser 120 and to split the received light into a first light beam for transmission along a first optical path 142 to the fixed mirror 140 and a second light beam for transmission along a second optical path 143 to a movable mirror 141. An actuator is used to control the position of the movable mirror 141 to adjust the optical path length difference of the second optical path 143 with respect to the first optical path 142. Two mirrors 140, 144 transmit the two beams of light back to the beam splitter 130 along first and second paths, respectively. The returning beams from the two mirrors 140 and 141 spatially overlap each other at the beam splitter 130 to produce optical interference output light 132, which includes an interference signal having periodic interference peaks, which results from a shift in the position of mirror 141. Since the distance between two adjacent interference peaks generated by moving the mirror 141 is Δ nz, the position of the coupling point in the polarization maintaining fiber is as follows from equation (1): z is Δ z/Δn. Thus, the interference points can be located using the interferogram, while the coupling ratio can be calculated from the interference peaks.

Fig. 2C illustrates a case where multiple coupling points may be included in the polarization-maintaining fiber. In this case, the measurement process will be more complicated. It is assumed that (n +1) coupling points (x) exist in the polarization-maintaining optical fiber₀x₁x₂··x_n) The linearly polarized wave packet 112 input along the slow axis is split into 2n wavelet packets traveling along the slow axis and 2n wavelet packets traveling along the fast axis at the output 113 of the polarization maintaining fiber. Thus, at the ith coupling point, two wave packet sequences P_siAnd P_fiThe polarization is respectively along the slow axis and the fast axis, and the transmission light path of the polarization comprises 2i wave packets, and the optical path of the wave packets can be expressed as follows:

wherein P is_siJ and P_fiJ are respectively represented in the sequence P_siAnd P_fiThe jth wave packet. The optical path length of the wave packet sequence after the (i +1) th coupling point can be calculated by:

based on equation (3), the optical length of the wave packet at the output end of the polarization-maintaining fiber can be obtained by the following equation:

wave packet sequence P_snAnd P_fnCorresponding intensity of_snAnd I_fnCan be calculated from the following formula,

wherein c is_nIs at the coupling point x_nAnd can be used as a parameter for crosstalk definition: crosstalk ═ abs (10 logC)_n)。

After passing through the 45-degree optical polarizer 120, the two wave packet sequences Psn and Psn originally transmitted along the slow axis and the fast axis of the polarization fiber are mixed into one wave packet sequence and polarized along the transmission direction of the optical polarizer 120. The optical length P and the corresponding light intensity of the wave packet sequence transmitted along the polarization direction of the optical polarizer 120 can be calculated by the following formula:

any two pulses in the wave packet sequence P (see formula 7) can generate an interference signal while the second optical path mirror 141 moves to change its position, and the position of the interference fringe is determined by the delay difference between the two pulses. The total number of the N coupling points is 2N (2N-1) peaks, wherein N peaks indicate the actual coupling points, and the rest are ghost peaks. These ghost peaks not only produce spurious coupling signals but may also form part of the true interference peaks produced by the true coupling points, and therefore these ghost peaks will reduce the accuracy of measuring the crosstalk distribution and amplitude.

As can be seen from equations (7) and (8), the wave packet sequence contains two groups, one group being represented by the upper half of equation (7) as P_sn-1Along the slow axis in the polarization-maintaining optical fiber; the other set is represented by P in the lower half of equation (7)_fn-1Along the fast axis in the polarization maintaining fiber. At P_sn-1The position of the interferogram of any two pulses in the group and the length (x) of the last segment of the polarization-maintaining fiber_n-x_n-1) Independently, they all have a delay difference of less than (x)_n-1-x₀) And delta n. The position of the interference pattern of any two pulses in the Pfn-1 group is also related to the length (x) of the last section of the polarization maintaining fiber_n-x_n-1) Independently, they all have a delay difference of less than (x)_n-1-x₀) And delta n. For the interference between the upper and lower halves in the wave packet P, respectively from P_sn-1And P_fn-1Any wave in the group (b) has a delay difference of (x)_n-1-x₀)△n+(P_sn-1，_j-Pfn-1，_k). If the length x of the last section of polarization maintaining fiber_n-x_n-1Longer than the total length of the polarization maintaining fiber from 0 to n-1, the interference peak at the position is divided into two groups, one group is formed by the position P_sn-1Or P_fn-1Interference generated between any two wave packets is formed; another group is respectively composed of P_sn-1A wave packet sum P of the sequence_fn-1One wave packet of the sequence interferes. A section of polarization maintaining fiber link with a high Extinction Ratio (ER) typically shows a small coupling coefficient c1, c2 ….. ci for the polarization maintaining fiber, so there is a relatively high power in the pulse P1 of equation (7). If the interference term is neglected more than 3 orders due to coupling more than two times, and there are only n interference signals in the second set of interference groups, the delay difference between the first optical path 142 of the reference arm and the second optical path of the change arm in the corresponding interferometer (fig. 2A) is:

where the exact corresponding polarization maintaining fiber is from 0 to n-1 coupling points.

To reduce ghost peaks, an optical retarder may be inserted between the polarization-maintaining fiber and the optical polarizer 220 to selectively introduce an additional optical retardation into one of the two polarization modes of light transmitted by the polarization-maintaining fiber. Fig. 3 shows a typical apparatus for measuring polarization crosstalk in an optical polarization medium (e.g., polarization maintaining fiber), which incorporates an optical delay device between the polarization maintaining fiber under test and the interferometer, and the operation of the apparatus is described in detail below. After passing through the polarization maintaining fiber 202 to be measured, the incident light 201 is divided into two orthogonal wave packet sequences, and is polarized along the slow axis and the fast axis respectively. Retarder 210 adds a delay L between two orthogonal wave packet sequences, where the delay L in air should be longer than Δ n × L, where Δ n is the birefringence and L is the polarization maintaining fiber length. In this example, an additional retardation L is applied to light polarized in the slow axis direction by the polarization maintaining fiber. After passing through the 45 ° optical polarizer 220, the two wave packet sequences with the additional retardation L are mixed together and have the same polarization state, which is determined by the optical polarizer 220. Interferometer 230 after optical polarizer 220 is used to generate a series of interference signals delayed between Δ n × 1 and (L- Δ n × 1). These interference signals correspond only to real signals caused by the polarization coupling generated at the coupling position, and ghost peaks are suppressed or eliminated. The processor 240 is configured to receive the output signal of the interferometer 230 and process the output signal to measure the position and strength of the polarization maintaining fiber coupling point.

Consider the situation where there are three coupling points x along the polarization maintaining fiber₁、x₂、x₃And the polarization direction of the light input into the polarization maintaining fiber is along the slow axis of the polarization maintaining fiber, and no fast axis component exists. At each coupling point, light is coupled not only from the slow axis polarization mode into the fast axis polarization mode, but also from the fast axis polarization mode into the slow axis polarization mode. As a result of this coupling, the sequence of wave packets output by the polarization maintaining fiber contains wave packets that are coupled multiple times.

After passing through the 45 polarizer, the wave packets in the slow and fast axis directions will be mixed with each other. If the mixed light is input to the interferometer, a series of interference peaks can be obtained when the retardation of one arm of the interferometer is changed. The generated interference peak represents a real coupling point and a ghost peak, which is different from the real coupling point and generates an error in identifying the real coupling point. Ghost peaks may also be superimposed on the true peaks, reducing the crosstalk measurement accuracy.

To suppress the number and amplitude of unwanted ghost peaks, an optical retarder 210, see FIG. 3, may be inserted at the output of the polarization-maintaining fiber and at the input of the polarization-maintaining fiber. The retardation device has polarization selectivity and can add additional retardation between the fast axis and the slow axis. Thus, the wave packet sequences of the fast and slow axes are separated in time after passing through the analyzer. If the same delay is given between the movable arm and the fixed arm of the interferometer, interference signals of zero order, second order and higher order cannot be generated when the delayer scans; therefore, most of ghost peaks disappear in the measurement. Thus, the device shown in FIG. 3 has higher measurement accuracy, greater dynamic range and higher sensitivity than the devices based on other interferometric methods shown in FIG. 2.

The polarization-selective optical retarder 210 of fig. 3 may have a variety of composition configurations, and the device of fig. 3 may be selected according to the needs of different applications. The light transmitted in the two polarization modes of the polarization-maintaining fiber is divided into two independent optical signals transmitted along two independent optical paths by adopting a polarization beam splitter, an adjustable optical delay mechanism can add adjustable optical delay to the two independent optical signals before beam combination, and the two independent optical signals are combined into one optical signal through a linear optical polarizer behind an interferometer so as to be further processed. These devices can be configured as fixed optical retardation devices that produce an ideal optical retardation Δ L (> nx 1) or variable retardation that is controllable at the ideal optical retardation Δ L described above. With the proper delay, ghost peaks can be suppressed, see fig. 3.

Spatially resolved polarization crosstalk measurement of Polarization Maintaining (PM) fibers has a variety of applications, such as distributed stress sensing, fiber optic gyroscope coil detection, polarization maintaining fiber birefringence and beat length measurement, polarization crosstalk position identification, and polarization maintaining fiber quality inspection. A scanning michelson white light interferometer may be used to measure such distributed polarization crosstalk. However, as the length of the Fiber Under Test (FUT) increases, the measured crosstalk peak will be broadened due to birefringence dispersion, so that the spatial resolution and measurement accuracy will be reduced for polarization maintaining fibers over a certain length, e.g., several hundred meters.

The techniques provided herein can be used to improve the resolution and measurement accuracy of distributed polarization crosstalk measurements. In some implementations, the broadening of the polarization crosstalk peak caused by birefringence dispersion can be recovered by simply multiplying the measurement data with a compensation function. The birefringence dispersion variation can be found by finding the width of the crosstalk envelope at a known distance. The technology can effectively improve the spatial resolution and amplitude precision of the long polarization maintaining fiber space resolution polarization crosstalk measurement.

The following section provides specific details of mathematically compensating for birefringence dispersion, which can improve the spatial resolution and measurement accuracy in measuring polarization crosstalk. The effect of birefringence dispersion on the polarization crosstalk measurement can be mathematically compensated. A white light interferometer based on a distributed polarization crosstalk analyzer is described herein. The device is used for measuring an initial space resolution polarization crosstalk peak along the polarization-maintaining optical fiber, and the spectral width of the crosstalk peak is used as a position function so as to obtain birefringence dispersion delta D. In addition, multiplication of the compensation function and the raw measured crosstalk data can clear dispersion caused by crosstalk peak broadening. The experiment is carried out by adopting a polarization maintaining optical fiber ring with the length of 1.05km, and the experiment proves that the method can effectively improve the spatial resolution and the crosstalk measurement precision and can be easily incorporated into analysis software. The described techniques can be used for a variety of applications, such as obtaining accurate polarization crosstalk values for polarization-maintaining coils that are more than a few hundred meters long, and can also be used for alien-triggered crosstalk and such crosstalk measurements.

Figure 4 shows an apparatus for measuring a polarization maintaining fiber loop. The device can be integrated into a distributed polarization crosstalk analyzer. Light 101 from a polarized broadband light source 301 is coupled into one principal polarization axis of an optically birefringent medium (e.g., polarization maintaining fiber) 110. Such polarized light sources 301 may be implemented in a variety of configurations, such as a combination of a broadband light source and a polarizer. In the example of fig. 4, the polarized broadband light source 301 employs a polarized superluminescent diode light Source (SLED) of short coherence length. Light 101 is directed in the slow axis direction at point a, which is an input fiber connector for connecting polarization maintaining fiber ring 110. The polarization maintaining fiber loop 110 terminates at the output connector C and the polarizer 120 is at an angle, e.g., 45, to the two principal polarization axes. Referring to FIG. 2B, the polarizer 120 receives a portion of the output light of the polarization maintaining fiber ring 110 and mixes the two orthogonal polarizations thereof together.

A polarization maintaining fiber ring is a birefringent medium that provides two orthogonal polarization modes along the fast and slow axes of the polarization maintaining fiber. At input point a, the input polarization of light 101 is aligned with one of the polarization axes (e.g., the slow axis) of the polarization-maintaining fiber. The optical output signal exiting the fiber birefringent medium 110 is directed to interferometer 230, resulting in optical interference between the two orthogonal polarization modes. Interferometer 230 generates an interference signal 312. Photodetector 150 converts signal 312 into a detector signal with optical interference information. The data acquisition device or data acquisition card (DAQ)330 converts the detector signals into data; a processor 340 (e.g., a microprocessor or computer) is configured to receive the data and process the resulting optical interference information to obtain an envelope spectrum function of the polarization crosstalk between the two orthogonal polarization modes. Notably, the processor 340 is programmed with a compensation function that reduces spectral broadening of the envelope spectral function caused by optical birefringence dispersion in the birefringent medium, the compensation function being based on measurements of the optically birefringent medium 110 to the envelope spectral function.

As shown in fig. 4, interferometer 230 is a fiber-based interferometer that includes a fiber coupler 310 having four fiber ports: port 1 acts as an interferometer input port, receiving light from polarizer 120; port 2 is the interferometer output port, outputting signal 312; port 3 is for connection to a first optical path of interferometer 230; port 4 is for connection to a second optical path of interferometer 230. The fiber coupler 310 splits the beam from the polarizer 120 into two beams, the first beam propagating toward port 3 and the first optical path, and the second beam propagating toward port 4 and the second optical path. The first optical path comprises an optical fiber terminating at a first faraday mirror 321; the faraday mirror 321 has the function of rotating the polarization of the light beam by 45 °, so that a 90 ° rotation is produced in the polarization of the reflected light. Similarly, the second optical path includes an optical fiber that terminates at a second faraday mirror 322, the reflected light polarization being rotated by 90 °. Subsequently, the reflected light beams of the first and second optical paths are mixed at the fiber coupler 310, causing interference due to an optical path difference therebetween. This is a michelson interferometer. Variable delay 323 is used to control the relative delay between the two paths. In fig. 3, a variable delay element 323 is placed in the first optical path to regulate the relative delay, and the processor will further operate as a control device in response to a delay control signal 342 from the processor 340. In operation, variable delay element 323 scans to operate interferometer 230 as a scanning michelson interferometer.

In the example of fig. 4, at point B of the polarization-maintaining fiber ring, polarization crosstalk is caused by external interference factors, and some light is coupled from the initial polarization of polarization-maintaining fiber ring 110 along the slow axis to the fast axis with a coupling coefficient h ═ I1/I2, h representing the intensity or power ratio between the two polarizations, where I1 and I2 are the powers of the fast and slow axes, respectively. In the example of fig. 4, at point B of fiber ring 110, external interference causes a polarization crosstalk, and some of the originally polarized light along the slow axis is coupled to the fast axis with a coupling coefficient h ═ I1/I2, I1 and I2 being the power of the fast and slow axes, respectively. Because the light polarization along the fast axis is faster than it propagates along the slow axis, at the output point C of the fiber optic ring 110, the fast light component leads the slow light component by Δ nZ, where Δ n is the group birefringence of the polarization maintaining fiber ring 110 and Z is the length between the crosstalk point B and the fiber end C. Optical polarizer 120, placed at the output end of the fiber at a 45 angle to the slow axis, produces interference on the two components of the same polarization direction on scanning michelson interferometer 230. Scanning the relative optical paths, an interference peak appears when the polarizers spatially coincide, and disappears when they are separated by more than one coherence length of the light source 301. The position B is a crosstalk occurrence point and can be calculated by the formula Z ═ Δ Z/Δn, and the crosstalk amplitude h can be obtained from the interference signal amplitude. Fig. 4 shows a series of signals at three locations A, B, C illustrating polarization components along the slow and fast axes.

The envelope of the crosstalk peak (interference peak) is affected by the spectral distribution of the light source 301 and the birefringence dispersion Δ D of the polarization maintaining fiber 110. If SLED301 has a gaussian spectral shape, the crosstalk envelope (coherence) γ birefringence dispersion function Δ D and the distance Z of the crosstalk points are derived:

wherein,

d＝(ΔnZ-d)(11)

ρ＝2πc(Δλ/λ₀)²ΔDZ＝αΔDZ(12)

ΔD＝dτ/dλ＝-[ω²/2πc](d²Δβ/dω²)₀(13)

in the above equation, d is the path imbalance for a scanning Michelson interferometer, ρ is the cumulative birefringence dispersion along the polarization maintaining fiber, c is the vacuum velocity, and Δ λ and λ₀Respectively, the spectral width and the central wavelength of the light source, Δ β is the propagation constant difference of the two polarization eigenmodes, and w0 is the 1/e width of the interference envelope when the dispersion ρ is 0. This width is also the coherence length of the light source. The parameter d can be adjusted by changing the path difference d of the delay line in the interferometer according to equation (11). When the path imbalance d compensates for the optical path difference Δ nZ between the two polarization modes, an interference signal occurs. When the optical path imbalance d compensates for the optical path difference Δ nZ, an interference signal appears. Equations (11) and (13) show that the magnitude and waveform of the measured crosstalk envelope is a function of Δ D and Z. The degrading effect of birefringence dispersion Δ D in crosstalk measurement is a reduction in the crosstalk envelope amplitude and linewidth broadening.

It is apparent that the effect of birefringence dispersion can be removed by directly multiplying the crosstalk measurement by the dispersion compensation function K (ρ):

thus, by multiplying equation (14) and equation (10), the original crosstalk envelope can be fully recovered:

to obtain the compensation function, the birefringence dispersion Δ D or ρ is first obtained. Equation (10) fits the relationship between envelope broadening and birefringence dispersion:

W/W_o＝(1+ρ²)^1/2＝(1+(αΔD)²Z²)^1/2(16)

thus, by measuring the width of the crosstalk envelope at the input (Z ═ L) and output (Z ═ 0) ends of the fiber, the birefringence dispersion Δ D can be easily calculated. In practical application, in order to improve the precision of the Delta D, the width of crosstalk envelope curves of the polarization maintaining optical fiber at a plurality of positions needs to be measured, and the Delta D is obtained through a curve fitting formula (16).

FIG. 5 is a polarization crosstalk curve for the polarization maintaining fiber coil from FIG. 4, which reflects the effect of birefringence dispersion on the measured crosstalk peaks and how to compensate for these effects. The peaks at the far left and right correspond to the crosstalk caused by the output and input connectors a and C. The small peak between the two is the stress induced crosstalk during the fiber winding process. The solid line inserted to the right shows that birefringent dispersion has two adverse effects: (1) envelope broadening (2) occurs at the crosstalk connector a with a reduction in the crosstalk peak amplitude. The dashed lines indicate that the envelope and amplitude of the crosstalk is repaired after dispersion compensation. In particular, the spectral peak width at the input connector is 34.1um with dispersion compensation, which is very close to the left peak width of 32.4um caused by the output connector C with zero dispersion (Z ═ 0).

Fig. 6 shows the measured spectral width as a function of the distance Z, various experiments were performed by using the system shown in fig. 4. Measurements of multiple polarization crosstalk points are made at different locations along the fiber under test. The measurements clearly show that the square of the spectral width is proportional to the distance Z, due to the effect of the birefringence dispersion. This characteristic corresponds to equation (16). Under such test conditions, the spatial resolution of polarization crosstalk measurements over distances of 200 meters is reduced due to the broadening of the lines by the birefringence dispersion.

The birefringence dispersion Delta D of the polarization maintaining optical fiber can be accurately obtained by least square fitting of the data of the formula (16) and is 0.0014 ps/(kmnm). And (5) carrying the fitting value Delta D into a formula (14) to complete a dispersion compensation function. The dispersion compensation function is multiplied by the original measured crosstalk data to obtain improved crosstalk data, and the dependence of polarization crosstalk on birefringence dispersion Delta D is eliminated.

Fig. 6A shows the envelope linewidths of the crosstalk peaks caused by pressure at different locations along the polarization maintaining fiber. The squares in fig. 6A indicate the spectral width after dispersion compensation, and the dots indicate the spectral width without dispersion compensation. Fig. 6B is a cross-talk measurement for an input connector and six different lengths of polarization-maintaining fiber (5m,205m,405m,605m,805m, 1005 m). The crosstalk of the input connector is fixed and 5 segments of 200 meters of fiber are spliced sequentially to the tail end of the input connector to increase dispersion. The polarization crosstalk amplitude decreases with fiber length due to birefringence dispersion and is repaired after compensation.

Therefore, the dispersion compensation technique can effectively mitigate the reduction of crosstalk amplitude and the line broadening caused by dispersion. Similarly, based on the use of a broadband light source (e.g., white light) in the interferometer by a polarization crosstalk analyzer, this compensation technique can effectively improve the spatial resolution and measurement accuracy of the crosstalk amplitude.

For the sensor device arrangement shown in fig. 4, a polarized superluminescent diode (SLED) has a short coherence length (e.g., about 25 μm) and is coupled to the slow axis of the polarization maintaining Fiber Under Test (FUT) (point a). Fig. 4 illustrates that at another location point B, polarization crosstalk is induced by external interference, which causes some light initially polarized in the slow axis to be polarized with a coupling coefficient parameter h ═ I₁/I₂Coupled to the fast axis, I₁And I₂Representing the light intensity in the fast and slow axes, respectively. Since light polarized along the fast axis propagates faster than light truly polarized along the slow axis, it is faster at the fiber output endThe light component of (A) is advanced by (delta Z) which is an optical path difference from the slow light component, delta n is a refractive index of the optical fiber group, and Z is a length of the optical fiber between the crosstalk generation point (B) and the output point (C). The polarizer is placed at the output end of the optical fiber. A polarizer at 45 ° to the slow axis is placed at the end of the fiber. The polarization components of the slow and fast axes are projected into the same direction of the polarizer, producing a two-component interference pattern on the scanning michelson interferometer. Scanning a relative optical path, and if the two polarization components are overlapped in space, generating an interference peak; when separated by more than one light source (e.g., SLED) coherence length, the interference peak disappears. The position of the crosstalk point B and the output point C can be calculated by the formula Z ═ Δ Z/Δn. If there are multiple polarization crosstalk points beyond the B position shown in fig. 4, a second order interference peak occurs because light coupled into the fast axis at a crosstalk point will be coupled back into the slow axis at a subsequent crosstalk point. Such second order coupling can cause crosstalk ghost peaks and cause white light interference clutter. The sensor of fig. 4 is a ghost-free distributed polarization crosstalk analyzer that uses a differential group delay (delay device) inside the device to remove interfering ghost peaks in the second-order coupling, enabling accurate identification and measurement of large amounts of unambiguous polarization crosstalk.

Based on the above-described characteristics, the polarization-maintaining fiber can be embedded in the sensor substrate as a stress sensing element, and the polarization crosstalk of the polarization-maintaining fiber can be caused by the strain field. The cross-talk change can be viewed as an indicator of the change in external pressure/strain applied to the polarization-maintaining fiber. Polarization crosstalk is more sensitive to lateral pressure exerted on the polarization-maintaining fiber than it is to axial strain or pressure. Thus, the sensor of fig. 4 can measure transverse strain. In many applications the belt measurements are typically related to axial strain/pressure, such as structural sensing. The techniques and apparatus described below convert axial strain/pressure to transverse pressure, enabling the apparatus of FIG. 4 and other sensor devices based on sensing mechanisms to detect and monitor axial strain/pressure distributions, thereby measuring axial mechanical parameters. For example, both lateral pressure and axial strain may be measured with the sensor strip/sheet disclosed in this document.

FIGS. 7A and 7B illustrate the measurement of polarization crosstalk curves for 280 m polarization maintaining fiber wound into a roll as a function of polarization crosstalk analyzer internal interferometer retardation Δ Z. As shown in fig. 8A, the leftmost and rightmost peaks are observed to correspond to crosstalk contributions from the input and output connectors, respectively, with a slight axial misalignment between the polarization of the light and the polarization-maintaining fiber axis. FIG. 8B shows that equidistant periodic crosstalk peaks were induced by applying stress to each 0.5 meter polarization maintaining fiber.

Further investigation of the measurements and of fig. 7A, 7B, the measurements show that cross-talk peaks are induced at different locations of local stress or strain changes (response per 0.5 meter of applied stress). The amplitude of each crosstalk peak represents the amount by which stress is generated therein or applied to the corresponding location. Thus, this amplitude information can be used to measure stress or strain in the polarization maintaining fiber, as discussed in detail in the previous sections of this patent document. It is noted that the spacing between two adjacent crosstalk peaks is a different indicator of the crosstalk peak and, as explained below, may be used to measure the local temperature. The difference in information in peak separation including amplitude and polarization crosstalk as disclosed in this document is the basis for dual measurements of temperature and stress/strain by using the same sensor and by using the same probe light received from the sensor.

The crosstalk peak amplitude and the crosstalk peak interval have a significant property that the crosstalk curves in fig. 7A and 7B are two orthogonal quantities, which are independent of each other and can be obtained separately. Thus, the change in local stress or strain results in a change in the amplitude of the crosstalk peak, which does not affect the measured crosstalk peak separation indicative of local temperature. In contrast, changes in local temperature at locations along the polarization maintaining fiber may result in changes in the crosstalk peak pitch that do not affect the crosstalk peak amplitude values of the crosstalk peaks that represent local stress/strain levels.

The following describes a method of temperature measurement of the interval between crosstalk peaks. In a length of polarization maintaining fiber, the set of birefringence Δ n is a linear function of temperature and can be expressed as:

Δn＝γ(T₀-T)

therefore, the local temperature change is reflected in the polarization crosstalk of the change in birefringence caused by the temperature change.

Referring to FIGS. 8A, 8B and 8C, FIG. 8A illustrates birefringence measurement curves as a function of temperature. When an is changed at a local position, the position of the crosstalk peak or the crosstalk peak will be changed accordingly. The variation between the crosstalk peak intervals caused by the position variation of the peak, as shown in fig. 8B, shows the relative delay functions at 80 c (dotted line) and 40 c (solid line) in the polarization maintaining fiber. Fig. 8C further illustrates an expanded view of positions 48 and 49, and the 50 th crosstalk peak at 80 ℃ (dashed line) and 40 ℃ (solid line). Thus, the variation based on the crosstalk peak pitch can be determined by measuring the local pitch variation and the local temperature variation. If the local stress or strain can also be changed simultaneously, the magnitude of the cross talk peak at the corresponding position will also be changed. Thus, measuring the peak separation and detecting the peak amplitude of the crosstalk peak allows simultaneous stress and temperature measurements.

Based on a temperature measurement technique using the separation between the cross-talk peaks of probe light received by the interferometer apparatus of FIG. 4, the change in local separation can be used to determine the change in local temperature at the location corresponding to the peak corresponding to the local separation.

The following specifically describes the manner in which the polarization maintaining fiber is disposed on the sensor substrate. The polarization maintaining optical fiber is arranged on the sensing substrate in a linear array mode or an area array mode, and the sensing device distributed in the linear array mode is a one-dimensional (1D) sensor strip which can measure 1D stress and temperature along a specific direction; the sensing device distributed in an area array is a two-dimensional (2D) sensor panel or sheet that can measure stress and temperature across the surface of the structure.

Referring to fig. 9, the polarization maintaining fibers in fig. 9 are distributed on the sensing substrate in a linear array manner. The polarization maintaining fiber 110 is placed in a long groove formed in the sensing substrate 11, and the slow optical axis or the fast optical axis of the polarization maintaining fiber 11 forms an angle of 45 degrees with the normal line of the long surface. Polarization axis orientation of polarization maintaining fiber 110It is ensured that polarization maintaining fiber polarization crosstalk is sensitive to variations in applied pressure. Typically, the slow and fast axes are set at 45 degrees to the pressure direction, which is most sensitive to pressure. Referring to fig. 9A, a sensing substrate 11 is formed with a groove extending along a length thereof. The sensing substrate 11 may be made of a deformable or resilient material so that the sensing substrate 11 may deform with the target device to which it is attached. For example, the sensing substrate 11 may be plastic or a material having a certain desired elasticity, such as nylon and acetal resin materials. During the process of laying the polarization maintaining fiber in the proper direction of the polarization axis (slow axis or fast axis) to the ribbon, the slow axis can optionally be identified by an appropriate method, such as inspecting the polarization maintaining fiber 110 using an optical magnification device and observing the slow and fast axes of the polarization maintaining fiber 110 before laying it into the groove in the correct fiber orientation. FIG. 9B shows the corresponding polarization crosstalk peaks at positions Z1 and Z2 resulting from pressure applied to the sensor shown in FIG. 9A. As shown in fig. 9C, the polarization maintaining fiber 110 is disposed on the sensing substrate 11, and the sensing substrate 11 is provided with an elongated through groove on which the polarization maintaining fiber 110 is placed. The slow or fast optical axis of the polarization maintaining fiber 110 is at 45 degrees to the normal of the sensing substrate 11. The polarization axis orientation of the polarization maintaining fiber 110 ensures that polarization maintaining fiber polarization crosstalk is sensitive to changes in applied pressure. Typically, the slow and fast axes are set at 45 degrees to the pressure direction, which is most sensitive to pressure. The sensing substrate 11 may be made of a deformable or resilient material so that the sensing substrate 11 may deform with the target device to which it is attached. For example, the sensing substrate 11 may be plastic or a material having a certain desired elasticity, such as nylon and acetal resin materials. During the process of laying the polarization maintaining fiber in the proper direction of the polarization axis (slow or fast), the slow axis can optionally be identified by an appropriate method, such as inspecting the polarization maintaining fiber 110 using an optical magnification device and observing the slow and fast axes of the polarization maintaining fiber before laying the fiber in the correct fiber orientation into the groove. This method of providing a groove on the substrate to ensure that the fast and slow axes of the polarization maintaining fiber are at 45 ° to the pressure direction can also be used in the one-dimensional fiber lay scheme of fig. 9. Here, the sensing substrate 11 is provided with projections or recesses 301 periodically along the transverse through-grooves,these protrusions or recesses 301 are fixed locations where stress or strain is applied to cause the expected cross-talk peaks at the corresponding locations. The cross-talk peaks may vary with the local spacing of the temperature variations due to local temperature variations caused by fire or other heat sources. FIG. 9D shows in position Z₁And Z₂The corresponding polarization crosstalk peak resulting from the pressure applied to the sensor shown in fig. 9C. Fig. 9D shows, in addition to the corresponding polarization crosstalk peaks caused by pressure, that when the local temperature changes due to heat or other influences at a particular location, the spacing between the corresponding crosstalk peaks also changes. A change in stress on the polarization maintaining fiber at one location will only cause a change in the cross-talk peak along the vertical axis and will not affect the change in the local cross-talk peak pitch for temperature measurement.

The polarization maintaining fibers in fig. 10 are distributed in an area array on the sensing substrate. The sensing substrate 11 is provided with through grooves distributed two-dimensionally, and the polarization maintaining fibers 110 are disposed in the through grooves. The sensing substrate 11 is also provided with a plurality of protrusions or grooves 301, which generate a predetermined number of polarization crosstalk peaks each time the optical fiber passes through the protrusions or grooves 301. These pre-loaded polarization crosstalk peaks act as position markers, as shown in FIG. 10B, where the polarization crosstalk is a function of position Z. The peak of polarization crosstalk induced by external pressure or stress is shown in fig. 10B, also presented on the XY plot of fig. 10C. As shown in fig. 10A, at Z₁、Z₂And Z₃With external stress applied at the location, the corresponding polarization crosstalk peak is shown in fig. 10B. The pitch of the corresponding cross-talk peaks changes at the corresponding temperature change occurrence location.

The polarization maintaining fibers in fig. 11 are distributed in a linear array on the sensing substrate. Referring to fig. 11A and 11B, fig. 11A is a top view of the sensing substrate 11, and fig. 11B is a side view of the sensing substrate 11. The sensing substrate 11 is provided with a plurality of through holes 302, the polarization maintaining fiber 110 passes through the through holes 302 to generate a predetermined polarization crosstalk peak, and the polarization maintaining fiber 110 is sequentially disposed on two sides of the sensing substrate 11, i.e. the polarization maintaining fiber 110 passes through a first through hole 302 from a first side of the sensing substrate 11 to a second side, and then returns to the first side through another through hole 302 over a distance (10 cm in some applications). The passage of the fiber through each via 302 may result in a predetermined amount of polarization crosstalk at the location of the via 302, at which point fiber bending may occur. In this embodiment, optical fiber guide grooves may be added to both sides of the sensor substrate 11 to hide the polarization maintaining optical fiber, and the fast/slow axis and the pressure direction of the polarization maintaining optical fiber may be ensured to be about 45 degrees, and the polarization maintaining optical fiber in the grooves may be covered with an adhesive or an adhesive tape. An important feature of this sensor design, polarization maintaining fiber, in this configuration is that the sensor is sensitive to localized axial strain applied to the sensor strip, as shown in fig. 11A and 11B, since the sensor can convert axial strain to transverse stress to produce polarization crosstalk. Likewise, when axial compressive tension is applied to portions of the ribbon, reduced polarization crosstalk can occur where the fiber bends. In the present embodiment, the intervals between the through holes 302 are equidistant; of course these spacings can be set as a practical matter.

Referring now to fig. 12, in the present embodiment, the vias are in the form of via pairs 303, and the vias 303 are opened on the sensing substrate 11. Fig. 12A and 12B schematically show a top view and a side view of a sensor substrate. Polarization maintaining fiber 110 passes up and down through each pair of two holes to initiate crosstalk, as shown in fig. 12B. The pitch of the vias of the via pair 303 is small, while the distance between the pair of vias 303 is much larger, so that the main part of the polarization maintaining fiber is on one side of the sensing substrate 11 and only a small part of the polarization maintaining fiber is on the other side. Bending of the polarization maintaining fiber 110 occurs where the polarization maintaining fiber 110 passes through the pair of through holes 303, which can create internal polarization crosstalk. Different intervals may be selected for different applications. A layer of tape or adhesive may be attached to the surface of the sensor substrate 11 to cover the polarization maintaining fiber and protect it from potential damage.

The sensing substrates shown in fig. 11 and 12 have similar structures, and both have similar Z-functions. Referring to FIG. 13 (the cross-talk peaks of FIG. 13 would be unequally spaced relative to FIG. 12), when both are at Z₁And Z₂With stress acting, there is a large bias in the figureThe peak value of the polarization crosstalk, and when the local heating is carried out, the direct distance of the peak value of the polarization crosstalk also changes.

The polarization maintaining fibers 110 in fig. 14 are distributed on the sensing substrate 11 in an area array, and through holes 302 are opened on the path of the polarization maintaining fibers 110. As shown in fig. 14A, the sensing substrate 11 is provided with through holes 302 for generating predetermined polarization crosstalk marks, which can measure lateral pressure or horizontal strain on the sensing substrate 11. The structure of fig. 14A employs a via design (solid lines indicate that the optical fibers are on the upper surface of the substrate and dashed lines indicate that the optical fibers are disposed on the lower surface of the substrate through vias) such that passing the polarization maintaining optical fibers through the small holes causes a predetermined crosstalk peak and converts the local transverse strain to axial stress. In FIG. 14A, X (Z)₁Process) and Y (Z)₂At) may cause polarization crosstalk to occur at the location where the local strain is applied. As shown in fig. 14A, when Z ═ Z₃The stress may also cause cross talk. Fig. 14B and 14C are pressure measurement methods. The data can be presented in a three-dimensional graph, where X and Y represent the location of pressure/strain and the vertical axis represents the crosstalk/pressure values. And when the distance between the crosstalk peaks is larger than the threshold, whether the local part is heated can be judged. Of course, in this embodiment, the through-hole 302 may be replaced with a pair of through-holes, and the polarization maintaining fibers may be provided in an area array on the sensor substrate.

Referring to fig. 15, in the present embodiment, the polarization maintaining fibers 110 are distributed on the sensing substrate 11 in a linear array. A zigzag path is formed on the sensing substrate 11, and the polarizing fibers 110 are embedded or placed in the zigzag path. The sensor substrate material may be nylon, polyoxymethylene resin or other material having certain elastic or mechanical properties such as young's modulus. The bending of the polarization maintaining fiber 110 at the corners of the zigzag path can cause polarization crosstalk, resulting in a series of polarization crosstalk peaks having a certain amplitude. When an axial tensile strain is applied to the sensing substrate 11, tension is created along the polarization maintaining fiber 110 and increases the stress on the polarization maintaining fiber at the fiber bend, resulting in an increase in polarization crosstalk. Likewise, when an axial compressive strain is applied to a portion of the sensing substrate 11, a reduced polarization crosstalk occurs at the bend of the optical fiber. In the present embodiment, the grooves in the sensor substrate 11 are engraved zigzag to form zigzag paths, and the direction is changed once every certain pitch. A thin layer of silicone adhesive or other type of coating may be applied along the groove direction to guide the direction of the empty optical fiber. The polarization maintaining fibers were embedded in the grooves immediately after the first layer of silicone was applied. Subsequently, a second layer of glue covers the optical fibers, and the adhesive tape can be used to further ensure good adhesion between the optical fibers and the sensor strip. Of particular note are the corners where internal cross-talk peaks occur and change significantly under local strain so that the fiber does not come out due to perturbations at the corners. When the glue is cured, the previous tape is removed and another layer of tape is protected over the entire sensor surface. When the polarization maintaining fiber 110 is laid into the groove, the slow or fast axis of the polarization maintaining fiber 110 is at an angle of 45 degrees to the surface normal of the sensor strip to provide maximum measurement sensitivity. Similarly, as temperature changes, the spacing between the polarization crosstalk peaks also changes.

Similarly, when the polarization maintaining fibers are distributed on the sensor substrate in an area array, the paths of the polarization maintaining fibers can be arranged in a zigzag shape. Referring to fig. 16, the path of the polarization maintaining fiber 110 disposed in the sensing substrate 11 is zigzag, and the changing direction of the path of the polarization maintaining fiber 110 is at the level of the sensing substrate 11. In this embodiment, the sensing substrate 11 is provided with a corresponding Z-shaped groove, and the polarization maintaining fiber 110 is guided along the groove along with the silica gel. The embedded optical fibers may be covered with a protective layer of adhesive tape. Such a sensor substrate 11 is sensitive to local strain variations and can be used to detect or identify X, Y directional local strain field variations and temperature variations. With continued reference to fig. 17, the polarization maintaining fiber 110 has Z-shaped fiber paths on both sides of the sensing substrate 11, and the polarization maintaining fiber 110 paths on the same positions on both sides are substantially vertical. In the illustration, the sensing substrate 11 is engraved with long triangular grooves on both sides, and the directions are perpendicular to each other. The polarization maintaining fiber 110 is adhered to the other end of the sensing substrate 11 from one end along the groove and then adhered to the other side of the panel in reverse. The polarization maintaining fiber 110 is embedded in the groove to be protected. For example, each large triangle has a width of 8cm and a height of 24 cm. Because the triangular grooves on the two sides are perpendicular to each other, the 2D sensor plate is sensitive to changes of pressure or strain and can identify the direction of applied pressure. Of course, the zigzag distribution of the polarization maintaining fiber is not limited to the above distribution, and those skilled in the art should understand that the distribution may be other distributions, and in short, the polarization maintaining fiber has a predetermined polarization crosstalk when disposed on the sensing substrate.

Also disclosed herein is a method of measuring stress, strain and temperature, referring to fig. 18, the method comprising the steps of:

s110: linearly polarized light with a coupled broadband enters the optical birefringent medium, the linearly polarized light is transmitted along two orthogonal polarization modes of the optical birefringent medium, and the optical birefringent medium outputs an optical output signal. In this embodiment, the optically birefringent medium may be disposed directly on the object to be measured or disposed on the sensing substrate. The optical birefringent medium may also be provided with a predetermined polarization crosstalk, and the optical birefringent medium may comprise a polarization maintaining fiber, or a birefringent crystal such as yttrium vanadate, a quartz crystal, or a glass with a pre-stress, etc. When the optical birefringent medium is arranged on the sensing substrate, the optical birefringent medium can be distributed in a one-dimensional space or a two-dimensional space on the sensing substrate, preset polarization crosstalk is applied to the optical birefringent medium through the sensing substrate, and then the sensing substrate is attached to an object to be detected, so that the stress and the temperature of the object to be detected are monitored.

S130: an optical output signal output by the optical birefringent medium enters the optical retarder, so that delay is generated between two orthogonal polarization modes.

S150: light passing through the optical retarder is directed through a linear optical polarizer that mixes the two orthogonal polarization modes of the light passing through the optical retarder with each other.

S170: linearly polarized light generated by a linear optical polarizer is guided into an interferometer so as to obtain interference between two orthogonal polarization modes to generate a polarization crosstalk peak.

S190: and obtaining the temperature of the optical birefringent medium according to the distance between the polarization crosstalk peaks, and obtaining the stress value of the optical birefringent medium according to the peak value of the polarization crosstalk peaks.

It should be noted that the sensing substrate of the present invention is listed for facilitating the layout of the birefringent medium and the object to be measured during the measurement process, and in some applications, it may also be possible to apply the birefringent medium (such as polarization maintaining fiber) directly onto the object to be measured to sense the stress, strain and temperature of the object to be measured without passing through the sensing substrate.

One of the effects of presetting a polarization crosstalk point through the sensing substrate is that the birefringent medium can be bent to enable the birefringent medium to sense the stress change of the sensing substrate in the axial direction (namely along the plane direction of the sensing substrate); the second function is to judge the change of the crosstalk peak distance caused by the temperature change through the known preset crosstalk point position, thereby judging the change of the temperature. In application, instead of setting a preset polarization crosstalk point on the sensing substrate, a known crosstalk peak position can be obtained by using a crosstalk point generated by the self structure of the object to be measured, such as a corner, a gap and the like on a building structure, so as to judge the change of the crosstalk peak distance caused by the temperature change; instead of using the known position of the crosstalk point, when the change of the stress or strain of the measured object introduces a crosstalk peak, and when the temperature changes (for example, the stress or strain caused by the deformation of the measured object due to the temperature change), the change of the temperature can be found while judging the stress or strain through the change of the distance between the crosstalk peaks.

The technical features of the embodiments described above may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments described above are not described, but should be considered as being within the scope of the present specification as long as there is no contradiction between the combinations of the technical features.

The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.

Claims

1. A method of monitoring stress and temperature, comprising the steps of:

linearly polarized light with a coupled broadband enters an optical birefringent medium, the linearly polarized light is transmitted along two orthogonal polarization modes of the optical birefringent medium, and the optical birefringent medium outputs an optical output signal;

directing said optical output signal through a linear optical polarizer, said polarizer mixing two orthogonal polarization modes of said optical output signal with each other;

guiding linearly polarized light generated by the optical polarizer to enter an interferometer so as to obtain interference between two orthogonal polarization modes to generate a polarization crosstalk peak;

and obtaining the temperature change of the optical birefringent medium according to the change of the distance between the polarization crosstalk peaks, and obtaining the stress and/or strain of the optical birefringent medium according to the peak value of the polarization crosstalk peaks.

2. The method according to claim 1, wherein the optically birefringent medium is provided with a predetermined polarization crosstalk.

3. The method of claim 1, wherein the optically birefringent medium comprises a polarization maintaining fiber.

4. The method of claim 1, further comprising the step of:

an optical output signal output by the optical birefringent medium enters an optical retarder, so that delay is generated between two orthogonal polarization modes.

5. The method of claim 1, wherein the optically birefringent medium is attached to an object under test for monitoring stress, strain and temperature of the object under test.

6. The method of claim 1, wherein the optically birefringent medium is disposed on a sensing substrate attached to an object to be measured for monitoring stress, strain and temperature of the object to be measured.

7. The method of claim 6, wherein the optically birefringent medium is spatially distributed in one or two dimensions on the sensing substrate to monitor the spatial distribution of stress, strain and temperature of the object to be measured.

8. The method according to claim 6 or 7, characterized in that a predetermined polarization crosstalk is applied to the optically birefringent medium through the sensing substrate.

9. A sensing device for monitoring temperature changes, strain and/or stress of an object, comprising:

the optical birefringent medium is used for sensing the temperature change, the stress and/or the strain of the object to be detected;

the light source is used for generating broadband linearly polarized light, the linearly polarized light is transmitted along two orthogonal polarization modes of the optical birefringent medium, and the optical birefringent medium outputs an optical output signal;

a linear optical polarizer for receiving said optical output signal and mixing two orthogonal polarization modes of said optical output signal with each other;

an interferometer for receiving light transmitted through the linear optical polarizer and obtaining interference between two orthogonal polarization modes to generate a polarization crosstalk peak;

and the processor is used for obtaining the temperature change of the optical birefringent medium according to the change of the spacing between the polarization crosstalk peaks and obtaining the stress and/or strain of the optical birefringent medium according to the peak values of the polarization crosstalk peaks.

10. The apparatus of claim 9, wherein the optically birefringent medium comprises a polarization maintaining fiber.

11. The apparatus of claim 9, further comprising:

and the optical retarder is used for receiving an optical output signal output by the optical birefringent medium, delaying two orthogonal polarization modes and transmitting the delayed two orthogonal polarization modes to the linear optical polarizer.

12. The apparatus of claim 9, wherein the sensing apparatus further comprises a sensing substrate for connecting with an object to be measured, and the optical birefringent medium is disposed on the sensing substrate in a line array manner or an area array manner.

13. The apparatus of claim 12, wherein the sensing substrate has a protrusion or groove disposed thereon, and the protrusion or groove is in contact with the optically birefringent medium such that the optically birefringent medium has a predetermined polarization crosstalk.

14. The device of claim 12, wherein the sensing substrate has a plurality of through holes, the optical birefringent medium passes through the through holes, and the optical birefringent medium is distributed on two sides of the sensing substrate, so that the optical birefringent medium has a predetermined polarization crosstalk.

15. The device of claim 12, wherein the sensing substrate has a plurality of via pairs, the optical birefringent medium passes through the via pairs, and the optical birefringent medium is distributed on one side of the sensing substrate, such that the optical birefringent medium has a predetermined polarization crosstalk.

16. The apparatus of claim 12, wherein the sensing substrate has a zigzag channel formed therein, and the optical birefringent medium is disposed in the zigzag channel such that the optical birefringent medium has a predetermined polarization crosstalk.

17. The apparatus of claim 12, wherein the direction of the polarization axis of the optically birefringent medium is 45 ° to the normal direction of the sensing substrate.