CN107328429B - Device and method for improving proximity sensing stability in optical frequency domain reflection technology - Google Patents
Device and method for improving proximity sensing stability in optical frequency domain reflection technology Download PDFInfo
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Abstract
The invention relates to a device and a method for improving the stability of close-range sensing in the optical frequency domain reflection technology, wherein the device comprises a linear sweep frequency laser, an optical fiber beam splitter, a main interferometer, an auxiliary interferometer, a data acquisition card and a computer, wherein the main interferometer is used for leading sweep frequency laser entering the main interferometer to generate beat frequency interference and generating a first beat frequency signal; the auxiliary interferometer causes the sweep frequency laser entering the auxiliary interferometer to generate beat frequency interference, and generates a second beat frequency signal which is converted to be used as an external clock of the high-speed data acquisition card; the data acquisition card samples the first beat frequency signal at equal frequency domain intervals under the triggering of an external clock. On the premise of fixed measurement distance, the physical distance of the detected beat frequency signal of the main interferometer from high frequency to low frequency is from near to far, and the detected beat frequency signal at the near position is a high frequency signal. The invention can effectively reduce the interference of the beat frequency signal in the proximity sensing by the external low frequency signal, and enhance the system stability.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to a device and a method for improving the close-range sensing stability of an optical frequency domain reflection technology.
Background
The optical frequency domain reflection technology has the advantages of high precision, long distance, wide dynamic range and the like, and is widely applied to the fields of aerospace, national defense and military, civil engineering, energy and power and the like. In engineering application, especially in the case of severe environment, the optical frequency domain reflection technology is easy to be interfered by external environment in endpoint detection and distributed temperature and strain sensing. When using optical frequency domain reflective devices, users typically utilize only a small fraction of the device's measurable range, and are therefore most commonly used in close range measurements. The external environment noise is mainly low-frequency noise, and is easy to generate crosstalk with low-frequency signals, so that the spatial resolution, precision and stability of the system are affected. Therefore, in the optical frequency domain reflection technology, it is necessary to reduce the interference of external noise and improve the stability of the system.
Disclosure of Invention
Aiming at the defects or improvement demands of the prior art, the invention provides a device and a method which are based on an optical frequency domain reflection technology, can reduce external noise interference and improve the stability of the proximity sensing.
The invention relates to a device capable of improving the stability of close-range sensing in the optical frequency domain reflection technology, which comprises a linear sweep frequency laser, an optical fiber beam splitter, a main interferometer, an auxiliary interferometer, a data acquisition card and a computer, wherein:
the linear sweep frequency laser is used for emitting sweep frequency laser with the laser wavelength periodically changing linearly;
the optical fiber beam splitter is used for dividing the sweep frequency laser into two paths, and respectively entering the auxiliary interferometer and the main interferometer;
the main interferometer is used for enabling sweep frequency laser entering the main interferometer to generate beat frequency interference and generating a first beat frequency signal;
the auxiliary interferometer is used for enabling sweep frequency laser entering the auxiliary interferometer to generate beat frequency interference and generating a second beat frequency signal, and the second beat frequency signal is converted and then used as an external clock of the high-speed data acquisition card;
the data acquisition card is used for sampling the first beat frequency signal at equal frequency domain intervals under the triggering of an external clock;
and the computer is used for processing and analyzing the acquired first beat frequency signals.
By adopting the technical scheme, the main interferometer comprises a sensing arm and a reference arm, wherein the reference arm is longer than the sensing arm, and the tail end of the reference arm is connected with the Faraday rotary mirror.
By adopting the technical scheme, the main interferometer further comprises a first optical isolator and a first optical fiber coupler, one end of the first optical isolator is connected with the optical fiber beam splitter, the other end of the first optical fiber isolator is connected with one end of the first optical fiber coupler, the other end of the first optical fiber coupler is connected with the sensing arm and the reference arm, and the reflected signals of the sensing arm and the reference arm are subjected to beat frequency interference at the first optical fiber coupler.
By adopting the technical scheme, the main interferometer further comprises a first photoelectric detector, one end of the first photoelectric detector is connected with the first optical fiber coupler, and the other end of the first photoelectric detector is connected with the data acquisition card.
By adopting the technical scheme, the auxiliary interferometer comprises a second optical isolator, a second optical fiber coupler, two paths of single-mode optical fibers and a second photoelectric detector, wherein the tail ends of the two paths of single-mode optical fibers are connected with Faraday rotary mirrors, the two paths of light are reflected by the two Faraday rotary mirrors to return along the path, beat frequency interference occurs at the second optical fiber coupler, and a generated second beat frequency signal enters the second photoelectric detector.
With the technical scheme, the optical fiber beam splitter specifically divides the sweep frequency laser into 10:90 two paths.
By adopting the technical scheme, the linear sweep frequency laser is a narrow linewidth laser, the scanning range is 1520nm-1630nm, and the sweep frequency speed is 2nm/s-2000nm/s.
The invention also provides a method for improving the stability of the close-range sensing in the optical frequency domain reflection technology based on the device, which comprises the following steps:
the linear sweep laser emitted by the linear sweep laser is divided into two paths through the optical fiber coupler, one path enters the main interferometer, and the other path enters the auxiliary interferometer;
in the main interferometer, light is divided into two paths through a first optical fiber coupler, and one path is in an optical fiber to be detected and is used as a signal arm; the other path enters a reference arm with a fixed length and a Faraday rotating mirror arranged at the tail end, a Rayleigh backscattering signal in a signal arm and an end surface reflection signal in the reference arm interfere in an optical fiber coupler, and as the optical paths of the two paths of return signals are different, time delay is introduced, and the interference signal contains a beat frequency signal;
in the auxiliary interferometer, light is divided into two paths through a second optical fiber coupler, and the two paths of light reflected by the Faraday rotating mirror generate beat frequency interference at the second optical fiber coupler, and beat frequency signals are used as an external clock of a data acquisition card and used for triggering and acquiring beat frequency signals of the main interferometer;
the data acquisition card performs equal frequency domain interval sampling on the beat frequency signals of the main interferometer, the computer performs processing analysis, the physical distance of the detected beat frequency signals of the main interferometer from high frequency to low frequency is from short to long, and the corresponding beat frequency signals at the short distance are high frequency signals.
Compared with the prior art, the invention has the beneficial effects that: the invention provides a device and a method for improving the short-distance sensing stability of an optical frequency domain reflection technology, which adopt beat frequency signals generated by an auxiliary interferometer as an external clock of a data acquisition card, realize equal-frequency interval sampling on the beat frequency signals of a main interferometer, and the detected physical distance of the beat frequency signals of the main interferometer, which are mapped from high frequency to low frequency, is from short to long, and the corresponding beat frequency signals at a short distance are high-frequency signals. The invention has the advantages of long measurement distance, high spatial resolution, high system repeatability and good stability. Especially in short-distance measurement, especially under the condition of severe external environment, the interference of the external environment can be effectively reduced. The distributed temperature and strain sensing function can be applied to endpoint detection of optical network devices, and can also be applied to the fields of aerospace, national defense and military, civil engineering, energy power and the like.
Drawings
For the purpose of promoting an understanding of the nature of the invention, reference will now be made in detail to the embodiments of the invention, examples of which are illustrated in the accompanying drawings.
FIG. 1 is a diagram of a distributed optical frequency domain reflectometry sensor device;
FIG. 2 is a schematic diagram showing a specific structure of a distributed optical frequency domain reflection sensor.
In the figure: 1 is a linear scanning laser, 2 is an optical fiber beam splitter (10:90), 3 is a main interferometer, 4 is an auxiliary interferometer, 5 is a high-speed data acquisition card, 6 is a computer, 7 is an optical isolator, 8 is an optical fiber coupler (2 x 2), 9 is a sensing arm, 10 is a reference arm, 11 is a single-mode optical fiber, 12 is a reflection point R1, 13 is a reflection point R2, 14 is a single-mode optical fiber, 15 is a Faraday rotary mirror, and 16 is a photoelectric detector. 17 is an optical isolator, 18 is an optical fiber coupler (2 x 2), 19 is a faraday rotation mirror, 20 is a single mode optical fiber, 21 is a faraday rotation mirror, and 22 is a photodetector.
Detailed Description
The invention is further described in the following examples with reference to the accompanying drawings.
As shown in fig. 1, the device for improving the stability of the proximity sensing in the optical frequency domain reflection technology of the present invention comprises a linear sweep laser 1, an optical fiber beam splitter 2, a main interferometer 3, an auxiliary interferometer 4, a data acquisition card 5 and a computer 6, wherein:
the linear sweep frequency laser is used for emitting sweep frequency laser with the laser wavelength periodically changing linearly;
the optical fiber beam splitter is used for dividing the sweep frequency laser into two paths, and respectively entering the auxiliary interferometer and the main interferometer;
the main interferometer is used for enabling sweep frequency laser entering the main interferometer to generate beat frequency interference and generating a first beat frequency signal;
the auxiliary interferometer is used for enabling sweep frequency laser entering the auxiliary interferometer to generate beat frequency interference and generating a second beat frequency signal, and the second beat frequency signal is converted and then used as an external clock of the high-speed data acquisition card;
the data acquisition card is used for sampling the first beat frequency signal at equal frequency domain intervals under the triggering of an external clock;
and the computer is used for processing and analyzing the acquired first beat frequency signals.
Further, the main interferometer comprises a sensing arm 9 and a reference arm 10, wherein the reference arm is longer than the sensing arm, and the tail end of the reference arm is connected with a Faraday rotary mirror.
The main interferometer further comprises an optical isolator 7 and an optical fiber coupler 8, one end of the optical isolator 7 is connected with the optical fiber beam splitter, the other end of the optical fiber coupler is connected with one end of the optical fiber coupler 8, the other end of the optical fiber coupler 8 is connected with a sensing arm and a reference arm, and beat frequency interference occurs on reflected signals of the sensing arm and the reference arm at the position of the optical fiber coupler 8.
The main interferometer also comprises a photoelectric detector 16, one end of which is connected with the optical fiber coupler 8, and the other end of which is connected with the data acquisition card.
The auxiliary interferometer comprises an optical isolator 17, an optical fiber coupler 18, two paths of single-mode optical fibers and a photoelectric detector 22, wherein the tail ends of the two paths of single-mode optical fibers are respectively connected with Faraday rotary mirrors 19 and 21, the two paths of light are reflected by the two Faraday rotary mirrors to return along the paths, beat frequency interference occurs at the optical fiber coupler 18, and a generated second beat frequency signal enters the photoelectric detector 22.
In the embodiment of the invention, the optical fiber beam splitter specifically divides the sweep laser into 10:90 two paths.
The linear sweep frequency laser is a narrow linewidth laser, the scanning range is 1520nm-1630nm, and the sweep frequency speed is 2nm/s-2000nm/s.
The invention relates to a method for improving the stability of close-range sensing in the optical frequency domain reflection technology based on the device, which comprises the following steps:
the linear sweep laser emitted by the linear sweep laser is divided into two paths through the optical fiber coupler, one path enters the main interferometer, and the other path enters the auxiliary interferometer;
in the main interferometer, light is divided into two paths through a first optical fiber coupler, and one path is in an optical fiber to be detected and is used as a signal arm; the other path enters a reference arm with a fixed length and a Faraday rotating mirror arranged at the tail end, a Rayleigh backscattering signal in a signal arm and an end surface reflection signal in the reference arm interfere in an optical fiber coupler, and as the optical paths of the two paths of return signals are different, time delay is introduced, and the interference signal contains a beat frequency signal;
in the auxiliary interferometer, light is divided into two paths through a second optical fiber coupler, and the two paths of light reflected by the Faraday rotating mirror generate beat frequency interference at the second optical fiber coupler, and beat frequency signals are used as an external clock of a data acquisition card and used for triggering and acquiring beat frequency signals of the main interferometer;
the data acquisition card performs equal frequency domain interval sampling on the beat frequency signals of the main interferometer, the computer performs processing analysis, the physical distance of the detected beat frequency signals of the main interferometer from high frequency to low frequency is from short to long, and the corresponding beat frequency signals at the short distance are high frequency signals.
The invention adopts the beat frequency signal generated by the auxiliary interferometer as the external clock of the data acquisition card to realize equal-frequency interval sampling on the beat frequency signal of the main interferometer.
The basic principle is based on optical heterodyne interference technology. Specifically, the linear sweep frequency laser emitted by the narrow linewidth laser is divided into two paths through the optical fiber coupler, one path enters the main interferometer system, and the other path enters the auxiliary interferometer system.
In a main interferometer system, light is divided into two paths through an optical fiber coupler, and one path is used as a signal arm in a device to be tested (namely an optical fiber to be tested); the other path enters an optical fiber link with a fixed length and a Faraday rotating mirror arranged at the tail end and is used as a reference arm. The rayleigh backscattering signal in the signal arm interferes with the end-face reflection signal in the reference arm in the coupler.
Because the optical paths of the two return signals are different, time delay is introduced, and the interference signal contains beat frequency signals.
In the auxiliary interferometer, light is divided into two paths through an optical fiber coupler, the optical fiber coupler is designed into an M-Z interferometer, and a Faraday rotating mirror is arranged at the tail end of the optical fiber coupler. The light enters the auxiliary interferometer, and the two paths of light reflected by the Faraday rotating mirror generate beat frequency interference at the optical fiber coupler. The beat frequency signal is used as an external clock of the high-speed data acquisition card and used for triggering and acquiring the beat frequency signal of the main interferometer. According to the sampling theorem, the beat frequency of the auxiliary interferometer determines the maximum measurable distance of the optical frequency domain reflecting device.
The beat frequency signals of the two interferometer systems are converted into electric signals after passing through the photoelectric detector. The main interferometer beat frequency signal is connected with an input channel of the high-speed data acquisition card, and the auxiliary interferometer beat frequency signal is connected with an external clock channel of the high-speed data acquisition card.
And the main interferometer electrical signals acquired by the high-speed data acquisition card are processed and analyzed by a PC. The measured beat signal frequency can be mapped into a physical distance, and the reflection point position information can be expressed by the following formula according to the optical frequency domain reflection technology positioning principle:
(wherein z is the difference in distance between the position of the reflection point and the position of the Faraday rotator mirror of the reference arm, f b For the beat frequency of the beat frequency signal, gamma is the sweep rate of the linear light source, c is the light speed, and n is the refractive index of the optical fiber. )
The concrete steps are as follows: the measured beat frequency signal frequency and the distance difference between the position of the reflecting point in the sensing arm and the position of the Faraday rotary mirror of the reference arm are in a linear relation. And the beat signal power reflects the reflectivity of its corresponding reflection point.
In distributed temperature and strain sensing, beat signals mapped by all reflection points are selected and analyzed in a nearby interval (the size of the interval before and after the position to be measured is determined according to the measurement spatial resolution and the measurement accuracy) at the position to be measured. When the temperature and the strain at the position to be measured change, the frequency spectrum corresponding to the beat frequency signal mapped by the selected interval is translated integrally, and the translation amount is linearly related to the change amount of the temperature and the strain. The overall translation quantity of the frequency spectrum corresponding to the beat frequency signal mapped by the selected interval can be calculated by performing cross-correlation operation on the two groups of data before and after the temperature and the strain change, so that the temperature and the strain at the position are measured. When the operation mode is adopted along the whole optical fiber to be detected one by one, distributed temperature and strain sensing can be realized.
The key method for improving the short-distance sensing stability in the optical frequency domain reflection technology is as follows: compared with the traditional optical frequency domain reflecting device, the invention makes the reference arm of the main interferometer be reflective, and lengthens the reference arm so that the reference arm is larger than the sensing arm. On the premise of fixed measurement distance, the optical frequency domain reflection system can map the detected main interferometer beat frequency signal from high frequency to low frequency to have a physical distance from near to far.
When using optical frequency domain reflective devices, users typically utilize only a small fraction of the device's measurable range, and are therefore most commonly used in close range measurements. Therefore, the short-distance beat frequency signal in the main interferometer detected by the invention is a high-frequency signal, the influence of the low-frequency signal of the external environment is reduced, and the system stability is improved.
The key technology of the invention is as follows: the reference arm 10 in the optical frequency domain reflectometer main interferometer 3 is designed such that the reference arm 10 is reflective and a faraday rotator mirror 15 is added at the end.
Preferably, the reference arm 10 is made longer than the sensor arm 9. Therefore, the detected short-distance beat frequency signal in the main interferometer is a high-frequency signal, the interference of external low-frequency noise is reduced, and the system stability is improved.
The working principle of the optical frequency domain reflecting device is as follows: the linear scanning laser 1 emits laser light with the laser wavelength periodically changing linearly, and the laser light enters the optical fiber beam splitter (10:90) 2 to be split into two paths of light (the invention adopts the optical fiber beam splitter of 10:90 to enable more optical signals to enter the main interferometer).
One light enters the main interferometer 3 (90% output) and one light enters the auxiliary interferometer 4. In the main interferometer 3, an optical isolator 7 is placed in the 1-port optical path in the fiber coupler (2 x 2) 8 to prevent the 1-port reflected light from entering the linear scanning laser 1. Light enters the 1 port of the optical fiber coupler (2 x 2) 8, exits from the 3 port and the 4 port, and enters the sensing arm 9 and the reference arm 10 respectively. The backward rayleigh scattering in the sensor arm 9 returns to the fiber coupler (2 x 2) 8 along the single-mode fiber 11, and the light in the reference arm 10 is reflected by the end faraday rotator mirror 15 back to the fiber coupler (2 x 2) 8 along the single-mode fiber 14. Because the lengths of the single-mode optical fiber 11 and the single-mode optical fiber 14 are different, the optical paths of the two optical return signals are different, so that the two optical signals returned from the sensing arm 9 and the reference arm 10 generate beat interference on the optical fiber coupler (2 x 2) 8 and exit from the 2 port in the optical fiber coupler (2 x 2) 8 to enter the photoelectric detector 16.
In the auxiliary interferometer 4, an optical isolator 17 is placed in the 1-port optical path of the fiber coupler (2 x 2) 18 to prevent the 1-port reflected light from entering the linear scanning laser 1. Light enters the 3-port and 4-port through the 1-port, and a single-mode fiber 20 is connected to one arm of the auxiliary interferometer 4 as a delay fiber. The two paths of light are reflected back along the paths by faraday rotation mirrors 19 and 21, interfere with beat frequencies at fiber coupler (2 x 2) 18 and exit at the 2 port into photodetector 22.
The beat frequency signal of the auxiliary interferometer 4 is converted into an electric signal through the photoelectric detector 22 and then used as an external clock of the high-speed data acquisition card 5 to trigger acquisition of the beat frequency signal in the main interferometer 3, so that equal frequency domain interval sampling is realized. The acquired beat frequency signals are led into a computer 6 for analysis and processing.
The measured beat signal frequency can be mapped to a physical distance, while the beat signal power reflects the reflectivity of its corresponding reflection point. And selecting set data points before and after the temperature and strain changes at the position to be measured, performing cross-correlation operation analysis, and sensing the temperature and the strain at the position to be measured.
In the main interferometer 3, the reference arm 9 is made longer than the sensor arm 10. It is assumed that there are two reflection points R in the sensor arm 9 1 12 and reflection point R 2 13, the distance between the two reflection points is d, and the reflection point R 1 The arm difference at 12 and the position of faraday rotator mirror 15 in reference arm 10 is z. Reflection point R 1 The beat frequency interference frequency at the Faraday rotator mirror 15 in the reference arm 10 and 12 can be given by formula (1)
The method comprises the following steps:
(where z is the difference in distance between the position of the reflection point and the position of the reference arm Faraday rotator mirror, γ is the linear light source sweep rate, c is the speed of light, and n is the refractive index of the fiber.)
And the reflection point R 2 The beat interference frequency at faraday rotator mirror 15 in reference arm 10 is of the magnitude:
(where z is the difference in distance between the position of the reflection point and the position of the Faraday rotator mirror of the reference arm, d is the distance between the two reflection points, γ is the linear light source sweep rate, c is the speed of light, n is the refractive index of the fiber.)
Analysis of equations (2) and (3) can yield the reflection point R 1 The beat frequency interference frequency of the reference arm 10 and 12 is larger than the reflection point R 2 13 and the magnitude of the beat interference frequency of the reference arm 10. Therefore, the physical distance of the high-frequency-to-low-frequency mapping of the detected beat frequency signal of the main interferometer is from short to long on the premise of fixed measurement distance. Because the external environment noise is mainly the low frequency signal, the short-distance beat frequency signal in the main interferometer detected is the high frequency signal, and the influence of the external environment low frequency signal is reduced, and the system stability is improved.
When using an optical frequency domain reflective device, the user typically uses only a small fraction of the device's measurable range, and is therefore most commonly used in close range measurements. On the premise that hardware equipment does not need to be changed, the invention can effectively reduce the interference of external environment and improve the system stability in short-distance measurement, especially under the condition of severe external environment.
It will be readily understood by those skilled in the art that the drawings and examples described herein are illustrative only and not limiting of the present invention, and that any modifications, equivalents, and improvements made within the spirit and principles of the present invention are intended to be encompassed within the scope of the claimed invention.
Claims (7)
1. The device capable of improving the stability of the close-range sensing in the optical frequency domain reflection technology is characterized by comprising a linear sweep frequency laser, an optical fiber beam splitter, a main interferometer, an auxiliary interferometer, a data acquisition card and a computer, wherein:
the linear sweep frequency laser is used for emitting sweep frequency laser with the laser wavelength periodically changing linearly;
the optical fiber beam splitter is used for dividing the sweep frequency laser into two paths, and respectively entering the auxiliary interferometer and the main interferometer;
the main interferometer is used for enabling sweep frequency laser entering the main interferometer to generate beat frequency interference and generating a first beat frequency signal;
the auxiliary interferometer is used for enabling sweep frequency laser entering the auxiliary interferometer to generate beat frequency interference and generating a second beat frequency signal, and the second beat frequency signal is converted and then used as an external clock of the high-speed data acquisition card; the main interferometer comprises a sensing arm and a reference arm, wherein the reference arm is longer than the sensing arm, and the tail end of the reference arm is connected with a Faraday rotary mirror;
the data acquisition card is used for sampling the first beat frequency signal at equal frequency domain intervals under the triggering of an external clock;
and the computer is used for processing and analyzing the acquired first beat frequency signals.
2. The apparatus of claim 1, wherein the main interferometer further comprises a first optical isolator and a first optical fiber coupler, one end of the first optical isolator is connected to the optical fiber splitter, the other end is connected to one end of the first optical fiber coupler, the other end of the first optical fiber coupler is connected to the sensing arm and the reference arm, and the reflected signals of the sensing arm and the reference arm interfere with each other at the first optical fiber coupler.
3. The apparatus of claim 2, wherein the main interferometer further comprises a first photodetector having one end connected to the first fiber optic coupler and another end connected to the data acquisition card.
4. A device as claimed in any one of claims 1 to 3 wherein the auxiliary interferometer comprises a second optical isolator, a second optical fibre coupler, two single-mode optical fibres and a second photodetector, the ends of the two single-mode optical fibres being connected to a faraday rotator mirror, the two paths of light being reflected by the two faraday rotator mirrors back along the path, beat interference occurring at the second optical fibre coupler, the resulting second beat signal entering the second photodetector.
5. The apparatus of claim 4, wherein the fiber optic beam splitter specifically splits the swept laser light into 10:90 two paths.
6. The apparatus of claim 4, wherein the linear swept laser is a narrow linewidth laser with a scan range of 1520nm-1630nm and a sweep speed of 2nm/s-2000nm/s.
7. A method for improving the stability of proximity sensing based on the optical frequency domain reflection technique of claim 1, comprising the steps of:
the linear sweep laser emitted by the linear sweep laser is divided into two paths through the optical fiber coupler, one path enters the main interferometer, and the other path enters the auxiliary interferometer;
in the main interferometer, light is divided into two paths through a first optical fiber coupler, and one path is in an optical fiber to be detected and is used as a signal arm; the other path enters a reference arm with a fixed length and a Faraday rotating mirror arranged at the tail end, a Rayleigh backscattering signal in a signal arm and an end surface reflection signal in the reference arm interfere in an optical fiber coupler, and as the optical paths of the two paths of return signals are different, time delay is introduced, and the interference signal contains a beat frequency signal;
in the auxiliary interferometer, light is divided into two paths through a second optical fiber coupler, and the two paths of light reflected by the Faraday rotating mirror generate beat frequency interference at the second optical fiber coupler, and beat frequency signals are used as an external clock of a data acquisition card and used for triggering and acquiring beat frequency signals of the main interferometer;
the data acquisition card performs equal frequency domain interval sampling on the beat frequency signals of the main interferometer, the computer performs processing analysis, the physical distance of the detected beat frequency signals of the main interferometer from high frequency to low frequency is from short to long, and the corresponding beat frequency signals at the short distance are high frequency signals.
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