AU2020103626A4

AU2020103626A4 - An optical path autocorrelator used for distributed fiber strain sensing measurement

Info

Publication number: AU2020103626A4
Application number: AU2020103626A
Authority: AU
Inventors: Jun Yang; Yonggui YUAN
Original assignee: Harbin Engineering University
Current assignee: Harbin Engineering University
Priority date: 2020-11-24
Filing date: 2020-11-24
Publication date: 2021-02-04
Anticipated expiration: 2028-11-24

Abstract

The present invention proposes an optical path autocorrelator used for distributed fiber strain sensing measurement. It comprises a broad spectrum light source (1), a circular multi-beam generator (2), an optical path autocorrelation detection unit (3), a transmission fiber (4), and a fiber sensor array (5), connected in turns from the beginning to the end. The circular multi-beam generator (2) comprises a 2x2 fiber coupler (21), a first three-port optical circulator (22), a fiber collimator (23), and a movable optical mirror (24). The optical path autocorrelation detection unit (3) comprises a second three-port optical circulator (31), an optical detector (32), and an interference signal detecting and processing unit (33). The invention can realize the real-time monitoring and measuring of physical quantities such as multi-point strain or deformation, also reduces power loss during sensor multiplexing and increases system stability. 3/3 - - 2" "23 2 2224 4 3 31 5 Broad g c light source a b ------------------------------------------------------------------ ------ ----- -------- 1 21 etector 51 Signal processmng FIG. 5 501 501 501 501 3004 502 502 FIG. 6

Description

3/3 - - 2" "23 2

2224 4 3 31 5

Broad g c light source a b ------------------------------------------------------------------ ------ -------------

1 21 etector

Signal 51 processmng

FIG. 5

501 501 501 501 3004

502 502 FIG. 6

DESCRIPTION TITLE OF INVENTION

An optical path autocorrelator used for distributed fiber strain sensing measurement

TECHNICAL FIELD

[0001] The invention belongs to the fiber technology field, particularly relates to an optical path

autocorrelator used for distributed fiber strain sensing measurement.

BACKGROUND ART

[0002] Fiber interferometers driven by low coherence, broad spectrum light sources, such as

narrow spectrum light-emitting diode (LED), amplified spontaneous emission (ASE), or

superluminescent diode (SLD), are often referred to as white light fiber interferometers. The

classic fiber white light interferometer is shown in FIG. 1, a Michelson interferometer is built

with an optical fiber, and using either a broad spectrum light source, LED or ASE as the light

source. It achieves the measurement of the physical quantity to be measured by detecting white

light interference strips with a detector. The working principle is as below, the broad spectrum

light emitted from the low coherent broad spectrum light source 11 goes into the single-mode

fiber, it is then split into two beams by the 3dB optical fiber 2x2 coupler 13, one beam goes into

the single-mode fiber 14 acting as the measurement arm, it is reflected by the optical reflective

surface 15 of the back end and returns along the original path. It arrives at the photodetector 12

through the single-mode fiber 14 and the coupler 13, this is the measurement signal light.

Another light split by the coupler 13 of the light emitted by the light source 11 enters the single

mode connection fiber 16 acting as the reference arm, the grin lens 17, and returns back to the

photodetector 12 along the original path after being reflected by the moving mirror 18, this light

is referred as the reference signal light. The reference and measurement signal light coherent

superpose on the surface of the detector, due to the extremely short coherence length of the broad

spectrum light source, about several micrometers to several tens of micrometers, the coherent

superposition will only occur when the optical path difference between the reference signal light

and the measurement signal light is less than the coherent length of the light source, and a white

light interference pattern is emitted, as shown in FIG. 2.

[0003] As shown in FIG. 2, the white light interference pattern is characterized by a primary

maximum known as the central strip. It corresponds to the zero optical path difference, that is, it

corresponds to when the optical paths of the reference beam and the measurement beam are

equal, and it is called that the reference beam and the measurement beam has an optical path

matching relationship. When the optical path of the measurement arm changes, the optical path

of the reference signal can be changed by adjusting the delay amount of the fiber delay line, a

central interferometric strip can be obtained. The position of the central strip provides a reliable

absolute reference position for the measurement, when the change in the external quantity to be

measured causes a change in the optical path of the measured light beam, we can obtain the

absolute value of the physical quantity of quantity to be measured from the change in the

position of the central interferometric strip by the optical path scanning of the reference arm.

Compared to other fiber interferometers, in addition to the advantages of high sensitivity,

intrinsic safety, and resistance to electromagnetic field interference, the most important feature is

the ability to make absolute measurements of pressure, strain, temperature, and others quantities

awaiting measurements. Therefore, white light interferometric fiber interferometers are widely

used for the measurement of physical, mechanical, environmental, chemical, and biomedical

quantities.

[0004] To solve the multiplexing problems of sensors, in 1995 Wayne V Sorin and Douglas M.

Baney of H-P, Inc. disclosed a multiplexing method for a white light interference sensor based on an optical path autocorrelator (U.S. Patent: Patent No. 5557400). As shown in FIG. 3, it is based on the Michelson interferometer structure, using the optical path difference formed by the optical signal between the fixed arm and the variable scanning arm of the Michelson interferometer, and the reflected light signal optical path difference between the front and back of thefiber sensor, match the them to achieve the optical path autocorrelation to obtain the white light interference signal of the sensor. Then, multiplexing of optic sensors is accomplished by matching each sensor one by one in multiple head-to-end serial fiber sensor arrays by changing the sizes of optical path difference between the scanning arm and the fixed arm.

[0005] Except for the multiplexing problems of sensors, the applicant published a low coherence twisted Sagnac-like fiber deformation sensing device in 2007 and 2008 (Chinese patent application no. 200710072350.9) and the space division multiplexing Mach-Zehnder cascade fiber interferometer and its measurement method (Chinese patent application no. 200810136824.6) are primarily used to solve the problem of damage resistance in the deployment of fiber sensor arrays. The applicant published afiber Mach-Zehnder and a Michelson interferometer array combined measurer (Chinese patent application no. 200810136819.5), and a twin array Michelson fiber optic white light interferometry strain gauge (Chinese patent application no. 200810136820.8), which are designed to solve the problem of temperature interfering measurement and simultaneous measurement of temperature and strain in white light fiber interferometer multiplexing. The applicant published a simplified multiplexing white light interferometric fiber sensing demodulation device (Chinese patent application no. 200810136826.5) and a distributed fiber white light interferometric sensor array based on a tunable Fabry-Perot resonant cavity (Chinese patent application no. 200810136833.5), which introduces circular cavity and F-P cavity optical path autocorrelators to simplify the topology of multiplexed interferometers, constructing common optical paths, and improving temperature stability. The applicant also published a dual reference length low coherence fiber circular network sensing demodulator (Chinese patent application no. 200810136821.2) that introduces a 4x4 fiber coupler optical path autocorrelator, intended to solve the problem of simultaneous measurement of multiple reference sensors.

[0006] However, in the aforementioned space division multiplexing (SDM) based interferometer

structures, most of the optical path autocorrelators uses structures such as Michelson

interferometers, Mach-Zehnder interferometers, and Fabry-Perot interferometers. At least one

NxM fiber coupler (e.g., 2x2, 3x3 or 4x4 fiber coupler) is present in the optical path

autocorrelator in order to form signal beams (at least two or more) with a certain optical path

difference that matches the optical path differences of the reflected signal from the front and

back of the fiber sensor. Due to the spectroscopic and symmetrical characteristics of the NxM

fiber coupler, the above optical path autocorrelator invariably suffers from two problems: one is

the large power attenuation of the light source and the low efficiency of the light source, only a

small portion of the signal light generated by the light source reaches the sensor array and is

received by the detector to form an autocorrelation peak. In the optical path structure of the

W.VSorin disclosure shown in FIG. 3, only about 1/4 of the light source power is involved in the

optical autocorrelation process, the rest is attenuated by the coupler. Secondly, due to the

symmetrical nature of the optical path topology, the light source and the detector reciprocate in

the optical path topology, and theoretically at least the same value of light signal is returned to

the light source as that received by the detector. Although the type of light source used is broad

spectrum, it is not very sensitive to feedback compared to a laser light source. However,

excessive signal power feedback, especially for sources with large spontaneous radiation gain

such as SLDs and ASEs, can cause resonance of the light source. In less severe cases, it results in

a reduction in the power of the light signal from which the light source occurs; in more severe

cases, large fluctuations in the power of the interfering signal during white light interference can

reduce the measurement accuracy of the optical self-coherence peak; in extreme cases, it can be

detrimental to the light source and cause irreversible damage to the light source.

SUMMARY OF INVENTION

[0007] The purpose of the present invention is to provide an optical path autocorrelator for

distributed fiber strain sensing measurement that can realize real-time monitoring and

measurement of physical quantities such as multipoint strain or deformation, solve the problems of excessive power loss, low utilization of the light source, and the degradation of measurement accuracy due to light feedback from a light source when multiple sensors are multiplexed in one fiber, and increase the stability of the system.

[0008] The purpose of the invention is achieved by the followings:

[0009] It comprises a broad spectrum light source, a circular multi-beam generator, an optical

path autocorrelation detection unit, a transmission fiber, and a fiber sensor array, connected in

turns from the beginning to the end. The circular multi-beam generator comprises a 2x2 fiber

coupler, a three-port optical circulator, a fiber collimator, and a movable optical mirror. The

optical path autocorrelation detection unit comprises a three-port optical circulator, an optical

detector, and an interference signal detecting and processing unit.

[0010] The 2x2 fiber coupler has two optical input signals a and g, and two optical output signals

b and c. The spectral ratio between the two optical output signals b and c can be adjusted

between 1% and 99% as needed.

[0011] The three-port optical circulator has an optical input d, an optical output e and an optical

reflection port f. The optical signal input from the optical input d is only output from the optical

output e, and the optical signal input from the optical output e is only output from the optical

reflection port f.

[0012] The circular multi-beam generator described, the light signal has a single-transmission

characteristic, the beam emitted from the light source does not return to the light source. The

optical output signal end c and the optical input signal end g of the 2x2 fiber coupler are

connected to the optical input end d and the optical reflection port f of the first three-port optical circulator, respectively, and the optical output signal end e of the three-port optical circulator is connected to the fiber collimator. A portion of the optical signal entering the circular multi-beam generator via the optical input signal end a of the 2x2 fiber coupler is output directly from b, and another portion is reflected by a movable optical mirror after being ejected from the fiber collimator through the ports c, d, and e. It then reaches the 2x2 fiber coupler through f and g, this repeats to form a series of optical signals with equal optical path difference. The size of the optical path difference between the multiple optical signals described above can be changed by varying the distance between the fiber collimator and the movable optical mirror.

[0013] The optical path autocorrelation detection unit has a single-transmission characteristic, a series of beams from the circular multi-beam generator with equal optical path differences are transmitted only forward and not back to the optical path autocorrelation detection unit. They pass through the input h of the three-port optical circulator and are output from the output port i only, reaching the fiber sensor array via the transmission fiber. After being reflected by the optical fiber sensor, the optical signal carrying the strain information is again input through the output port i and output only from the reflection port j to the photodetector, and is demodulated by the interference signal detecting and processing unit after the photoelectric conversion to obtain the white light interference signal and the strain information of its sensor.

[0014] The fiber sensing array described has the characteristic of the composition of a number of fiber sensors connected in series at the beginning and end, and the fiber sensors are a series of serial arrays connected from beginning to end, which is constructed by varying lengths of single mode fiber, and the single-mode fibers are of any lengths with fiber ferrules at both ends.

[0015] The optical path autocorrelator used for distributed fiber strain sensing measurement, its characteristic is: the fiber devices are all working in the single-mode state.

[0016] The present invention employs a circular multi-beam generator with adjustable optical

path difference to generate two interrogation beams with adjustable optical path difference, by

the introduced optical path delay between the straight-through optical path and the circular

optical path. By introducing a fiber circulator, the light signal emitted by the light source only

has forward transmission characteristics, which avoids feedback of the light signal goes back to

the light source and inhibits the deterioration of the light source. At the same time, the optical

path only has forward transmission characteristics, in addition to the device's intrinsic loss

caused by the optical power attenuation, the light source's power all arrives at the detecting array

to participate in the optical correlation process, increasing the use of light source efficiency.

[0017] The basic principle of the present invention is the interference principle based on low

coherence and broad spectrum light (white light) and the SDM principle. The structure of quasi

distributed fiber white light strain sensor array based on circular multi-beam generator is shown

in FIG. 4, that is, only one sensor 51 is connected in the sensor array. The emitted beam of the

low coherence broad spectrum light source 1 is divided into two beams by the 2x2 fiber coupler

21 in the circular multi-beam generator 2, and one beam is output directly to the optical path

correlation detection unit 3 via the a and b ports. Another beam is fed into the input port d of the

optical circulator 22 via input a of the coupler 21 from output c, arrives at output e from input d,

enters the optical fiber collimator 24, is reflected by the mirror 23. It is then injected from port e,

output from port f, and enters input g of the coupler 21 again. The light signal entering port g is

the same as in the case of port a, where the light source is directly incident, and repeat the

process, so that:

[0018] (1) The optical path of the transmitted beam directly passing through coupler 21:

[0019] a->b;

[0020] (2) The optical path of the transmitted beam travelling through the coupler 21 and the circulator 22 once, respectively:

[0021] a->c->d->e->k->e->f->g>b;

[0022] (3) The optical path of the transmitted beam travelling through the coupler 21 and the circulator 22 two times, respectively:

[0023] a->c->d->e->k->e->f->g->c->d->e->k->e->f->g- >b;

[0024] And so on......

[0025] From this it can be known that: there is a fixed optical path difference

(c->d->e->k->e->f->g) between each light beam.

[0026] Each of the above light beams pass through the input port h of the optical circulator 31 in the optical path autocorrelation detection unit 3, enters the transmission fiber 4 only through the output port i, is reflected by the left reflecting side 1 and the right reflecting side m of the optical fiber sensor 51, reaches the optical circulator 31 again through the transmission fiber 4, and reaches the photodetector 32 and the interferometric signal detecting and processing unit 33 after emitting by the port j only through the port i.

[0027] A series of beams with equal optical path difference c->d->e->k->e->f->g emitted from the circular multi-beam generator 2 are reflected by the left and right surfaces of the fiber sensor, which also causes static optical path difference (without strain loading) 1 -> m. For the signal light emitted from the circular multi-beam generator 2, the optical path b->h->i->l->i->j between the optical path correlation detection unit 3 and the transmittion fiber 4 is a common optical path, so it is negligible for optical autocorrelation, and the additional optical path difference is limited to the 1->m end of the sensor 51. When c->d->e->k->e -f->g and 1->m are nearly equal, they can be precisely matched by the movement of the scanning mirror 23 so that they are exactly equal (1->m = c->d->e->k->e -f->g), i.e., the difference in optical path between the two beams of signal light reflected by the left and right ends of thefiber sensor 51 is fully compensated by the circular multi-beam generator 2. At this time, the reflected signals from the left and right ends of the fiber sensor produce interference, and coherence superposition occurs on the surface of the detector 32, because the coherence length of the broad spectrum low coherence light source is very short, about a few micrometers to tens of micrometers, only when the optical path distance difference of the interference signal is less than the coherence length of the light source, coherence superposition will occur, and white light interference pattern will be output.

I=I1+I2+2I-I-1 (x) -cos(k-x +#) (1)

[0028] Where: Ii, 12 are the signal intensities of the reference beam and the measurement beam, k is the wave number, x is the optical path difference between the two interference signals, (p is the initial phase, y(x) is the light source autocorrelation function.

[0029] Specifically for the optical fiber measurement system of FIG. 4, that is, the accumulated optical path of the measurement signal reflected from the left and right ends of the sensor 51 is equal to the the optical path difference introduced by the reference signal between the circular multi-beam generator reflector 23 and thefiber collimator.

(nL+ 2X)-nL 2 = 2nl (2)

[0030] Where 1 is the length of the fiber sensor between the left and right reflecting surfaces, n is the refractive index of the fiber core, X represents the distance from the fiber collimator 24 to the reflector 23, nLi+2X is the a->c->d->e->k->f->g->b optical path, and L 2 is the a->b optical path.

[0031] As a result of the above analysis:

[0032] (1) The signal light emitted by the light source passes through the circular multi-beam generator 2 and the optical path autocorrelation detection unit 3, the transmission fiber 4 and the sensor 51, and then returns to the optical path autocorrelation detection unit 3, i.e., the light transmission path is a->b(c->d->e->k->e->f->g .. )h->i->l(k)->i->j. Due to the presence of the optical circulators 22 and 31, the optical path has a single-transmission characteristic and the signal light cannot return to the light source, avoiding optical feedback.

[0033] (2) Multiple light beams emitted from the circular multi-beam generator 2 have a fixed optical path difference between them, and when they match the optical path difference caused by the sensor, all the light beams participate in the interference process, which greatly increases the intensity of the interference signal. According to the theory of interferometric signal detection, the enhancement of optical power can optimize the signal-to-noise ratio of the interferometric signal and increase the detection accuracy of the interferometric system.

[0034] The interference strips of a fiber interferometer based on the white light interference principle only occur within a few micrometers to a few dozen micrometers of the optical path match. This feature allows the sensor to be multiplexed without the use of complex time division or frequency division multiplexing techniques, as shown in FIG. 5. The fiber sensors 51 are connected at the beginning and end to form a serial array 5, as shown in FIG. 6. The end faces of each sensor 503 have a certain reflectance. If the length of each sensor is greater than the coherent length of the light source, then only a single white light interference signal exists in the respective coherent length of the interference strips, i.e., the interference strips do not interfere with each other and are independent of each other. By adjusting the scanning mirror 23 in the circular multi-beam generator 2, spatial optical path scanning can be achieved, and if the length of each sensor is different, multiple sensors can be distinguished, so that multiple external physical quantities can be searched and interrogated, making it convenient to realize distributed sensing measurement.

[0035] As can be seen from the above, the basic idea of distributed white light interferometric

sensor array multiplexing and demodulation is that when the optical path difference introduced

by the sensor is compensated by the variable optical path difference generated by the circular

multi-beam generator 2, a one-to-one correspondence of optical path matching occurs, so that the

resulting white light interferometric strips are independent of each other in the optical path

scanning space and do not interfere with each other, thus realizing distributed sensing

measurement.

[0036] When the sensor lj is deformed by external factors such as strain, adjust the variable

parameter Xj to match the optical path, i.e.:

AXi =Anlj j =1, 2,3,... (3)

[0037] Assuming that the length of the fiber sensor changes from to li+Ali, the second sensor

changes from 12 to 1 2 +Al 2 , and the Nth sensor changes from IN to IN+AIN, the strain sensed by each

sensor can be obtained by measuring the change in sensor length.

EN C1-= '2 = 2 -..... N '1 12 /N

[0038] The optical path autocorrelator and optical path autocorrelation detection unit in the

present invention both employ optical circulators with single-transmission characteristics, giving

them the following advantages and features.

[0039] (1) In addition to the optical power attenuation caused by device losses, all the power of

the light source reach the detection array, and the signal power reflected by the sensor array all

reach the detector. In the use of the same light source, even taking into account the attenuation of

the optical circulators, the power reaching the detector is at least doubled, significantly

increasing the light source utilization.

[0040] (2) Since the light signal cannot be returned to the light source, beam feedback is avoided,

which increases the stability of the light source system and the accuracy of the signal

measurement.

[0041] In addition, it has the advantages and characteristics of:

[0042] (3) A distributed fiber white light interferometric sensor system using a circular multi

beam generator, without the use of complex time division multiplexing or frequency division

multiplexing technology, just use the continuous spatial optical path scanning to achieve the

interrogation and measurement of multiple sensor signals, which is a simple technology and is

easy to implement.

[0043] (4) The distributed fiber white light interference sensor array constructed by the present

invention can realize the arraying of fiber sensors, the sensors do not affect each other during

measurement, the sensor length can be determined by the user, its length can be arbitrarily

selected from a few centimeters to several hundred meters, with multi-tasking sensing, multi

sensing, local strain sensing and large scale deformation sensing capabilities.

BRIEF DESCRIPTION OF DRAWINGS

[0044] FIG. 1 is a schematic diagram of the structure of a classic white light interference Michelson interferometer.

[0045] FIG. 2 is a schematic diagram of a classic white light interferometric strip signal.

[0046] FIG. 3 is the schematic diagram of multiplexed optical path structure for fiber sensor based on an unbalanced Michelson interferometer structured optical path autocorrelator.

[0047] FIG. 4 is a schematic diagram of the optical path of a fiber interferometer based on an adjustable-optical-path optical path autocorrelator connecting a single fiber sensor.

[0048] FIG. 5 is the schematic diagram of the optical path when a fiber interferometer based on an adjustable-optical-path optical path autocorrelator is being multiplexed.

[0049] FIG. 6 is the schematic diagram of the structure of a fiber sensor array with a serial topology connected from the beginnings to the ends.

DESCRIPTION OF EMBODIMENTS

[0050] The invention is further described below in connection with the embodiments and drawings:

[0051] A scheme for a distributed fiber strain sensing measurement system based on an annular multi-beam generator is shown in FIG. 5. From FIG. 5, it can be seen that the distributed fiber white light interferometric sensor array comprises a broad spectrum light source 1, a circular multi-beam generator 2, an optical path autocorrelation detection unit 3, a transmission fiber 4, and a fiber sensor array 5, connected in turns from the beginning to the end. The circular multi beam generator 2 comprises a 2x2 fiber coupler 21, a three-port optical circulator 22, a fiber collimator 23, and a movable optical mirror 24. The optical path autocorrelation detection unit 3 comprises a three-port optical circulator 31, an optical detector 32, and an interference signal detecting and processing unit 33.

[0052] As shown in FIG. 5, when the strain sensing system is working, the signal light emitted

from the broad spectrum light source 1 is split into two beams by the 2x2 fiber coupler 21 in the

circular multi-beam generator 2. Abeam is directly output to the optical path correlation

detection unit 3 by port b through input a, and another beam passes the input a of the coupler 21

and is input to input d of the optical circulator 22 via the output c, arriving at output e via input d,

entering fiber collimator 24, and being reflected by mirror 23, it is again incident via port e,

output from port f, and again entering input g of coupler 21. The light signal entering port g is

the same as a, repeat the above and so on. Each of the light beams described above enters the

incident port h of the optical circulator 31 in the optical path autocorrelation detection unit 3 and

enters the transmission fiber 4 only via the output port i. It then is reflected by the front and rear

reflective surfaces of each sensor 51 in the fiber sensor array 5, it reaches the optical circulator

31 again through the transmission fiber, and then reaches the photodetector 32 and the signal

detection and processing unit 33 through only port j from port i. When the optical path difference

introduced by the sensor is compensated by the variable optical path difference generated by the

circular multi-beam generator 2, the detector receives the peak of the interfering AC signal and

tracks the peak of the interfering signal through the dynamic scanning of the optical path to

obtain real-time information on the amount of change in sensor length. By continuously

adjusting the distance between the fiber collimator 24 and the mirror 23, spatial optical path

scanning and tracking can be achieved, and because each fiber sensor has a different length,

multiple sensors can be distinguished, allowing multiple external quantities to be interrogated

and interrogated.

[0053] Fiber sensor 51 is composed of a segment of any length offiber perpendicular to the end

face of the optical fiber with a certain reflectivity at both ends, a typical structure as shown in

FIG. 6, each fiber sensor is composed of a single-mode fiber 504 of approximately the same

length (e.g., sensor length to take 1000 mm long), and both ends are fitted with ceramic ferrule

501, the end faces are polished to obtain fiber end faces perpendicular to the direction of

transmission of light with a reflectance equal to or greater than 1%. The fiber sensor 51 can be

connected to the sensor or to the fiber via a ceramic sleeve 502, which also protects the sensor

end faces. An optically reflective surface 503 of an optical reflectance of 1% to 3% is formed

between two fiber ferrules connected using a fiber sleeve. A number of fiber sensors 51 are

connected at the beginnings and ends to form a serial fiber sensor array 5.

Claims

1. An optical path autocorrelator used for distributed fiber strain sensing measurement. Its characteristic is that: it comprises a broad spectrum light source (1), a circular multi-beam generator (2), an optical path autocorrelation detection unit (3), a transmission fiber (4), and a fiber sensor array (5), connected in turns from the beginning to the end. The circular multi-beam generator (2) comprises a 2x2 fiber coupler (21), a first three-port optical circulator (22), a fiber collimator (23), and a movable optical mirror (24). The optical path autocorrelation detection unit (3) comprises a second three-port optical circulator (31), an optical detector (32), and an interference signal detecting and processing unit (33). The connection relationship of the circular multi-beam generator (2) is as follows: the 2x2 fiber coupler (21) has two optical input signals a and g, and two optical output signals b and c. The spectral ratio between the two optical output signals b and c can be adjusted between 1% and 99%. The first three-port optical circulator (22) has an optical input d, an optical output e and an optical reflection port f. The optical signal input from the optical input d is only output from the optical output e, and the optical signal input from the optical output e is only output from the optical reflection port f. The optical output signal end c and the optical input signal end g of the 2x2 fiber coupler (21) are connected to the optical input end d and the optical reflection port f of the first three-port optical circulator (22), respectively, and the optical output signal end e of the first three-port optical circulator (22) is connected to the fiber collimator (23). A portion of the optical signal entering the optical input signal end a of the 2x2 fiber coupler (21) is output directly from the optical output signal end b, and another portion is reflected by a movable optical mirror (24) after being ejected from the fiber collimator (23) through the optical output signal end c, optical input signal end d, and optical output signal end e ports. It then reaches the 2x2 fiber coupler (21) through the optical reflection port f and optical input signal end g, this repeats to form a series of optical signals with equal optical path difference. The connection relationship of the optical path autocorrelation detection unit (3) is that: a series of beams with equal optical path differences from the circular multi-beam generator (2) pass through the input h of the second three-port optical circulator (31) and are output from the output port i only, reaching the fiber sensor array (5) via the transmission fiber (4). After being reflected by the optical fiber sensor, the optical signal carrying the strain information is again input through the transmission optical fiber (4) from the output port i of the second three-port optical circulator (31) and output only from the reflection port j to the photodetector (32), and is demodulated by the interference signal detecting and processing unit (33) after the photoelectric conversion to obtain the white light interference signal and the strain information of its sensor.

2. As claimed in claim 1, an optical path autocorrelator used for distributed fiber strain

sensing measurement. Its characteristic is that: the fiber sensing array (5) is composed of a

number of fiber sensors (51) connected in series at the beginning and end, and the fiber sensors

(51) are a series of serial arrays connected from beginning to end, which is constructed by

varying lengths of single-mode fiber, and the single-mode fibers are of any lengths with fiber

ferrules at both ends.