CN108562237B - A device and method for spectral calibration in an optical frequency domain reflectometry sensing system using an HCN gas cell - Google Patents

Abstract

The invention discloses a device and a method for performing spectrum calibration in an optical frequency domain reflection sensing system by adopting an HCN (hydrogen cyanide) gas chamber. The device and the method adopted by the invention reduce the performance requirement on the existing tunable laser, reduce the error of the spectral movement amount and effectively improve the distributed measurement precision. The invention provides an optical fiber strain measurement method based on OFDR technology, which utilizes an OFDR method to realize the real-time measurement of strain or temperature of each position, utilizes the characteristic of accurate calibration of a sweep frequency signal of an adjustable laser by an HCN gas chamber, overcomes the problem of insufficient accuracy of the output wavelength of the adjustable laser, and realizes the real-time and accurate measurement of the strain of an optical fiber.

Description

A method for spectral calibration in optical frequency domain reflectometry system using HCN gas cell device and method

technical field

The invention belongs to the technical field of distributed optical fiber sensing, and in particular relates to a device and method for spectrum calibration in an optical frequency domain reflection sensing system by using an HCN gas chamber.

Background technique

High-precision distributed sensing is widely used in many fields such as structural health detection, surface measurement, and optical fiber three-dimensional shape sensing, and the real-time and accurate measurement of optical fiber strain or temperature is particularly important in distributed sensing. In 1998, Mark Froggatt proposed that high-spatial-resolution distributed strain measurement in optical fiber with Rayleigh scatter can be realized by using the spectral shift of single-mode fiber Rayleigh scattering in optical frequency domain reflectometry (OFDM). This technology requires that the laser frequency sweep range and the starting wavelength are strictly consistent during the two measurement processes (one is the reference state and the other is the state where the physical quantity has changed), that is, the spectra of the two frequency sweeps are strictly aligned. The inconsistency of the swept spectrum of the light source will cause the calculated Rayleigh scattering spectrum to change, reduce the sensing accuracy, and cause the measurement error of this technology. However, the output wavelength of the current tunable laser may be different from the theoretically set wavelength due to the interference of itself and the environment, and the frequency sweep repeatability of the tunable light source is difficult to achieve. Take a tunable laser from Yenista in France as an example, its tuning repeatability is ±5pm, while a laser with high repeatability, such as Agilent 81607A, has a repeatability within 1pm, but its price is very expensive. It is of great significance and value to find an effective and economical application for spectral alignment in optical frequency domain reflectometry to improve the measurement accuracy of physical quantities.

SUMMARY OF THE INVENTION

According to the problems existing in the prior art, the present invention discloses a device for spectral calibration in an optical frequency domain reflectometry system using an HCN gas chamber, including: a scanning light source for providing a light source for the optical frequency domain reflectometry system, so The light source outlet end of the scanning light source is connected with an optical coupler, and the optical coupler is connected with the main interferometer through the c port, and the main interferometer is connected with the input end of the data acquisition card; the optical coupling The first is connected with the coupler through the b port, the coupler is connected with the HCN wavelength calibration module through the b port, the HCN wavelength calibration module is connected with the data acquisition card, and the HCN wavelength calibration module is provided with HCN gas chamber and photodetector, the output end of the HCN gas chamber is connected to the photodetector; the third optical coupler is connected to the auxiliary interferometer through the c port, and the output end of the auxiliary interferometer is connected to the data acquisition card connected, the data acquisition card is connected with the computer.

The main interferometer includes an optical coupler, a circulator, a second optical coupler for sensing fiber, and a balanced detector. The input end of the optical coupler is connected to the c port of the optical coupler one, and the optical coupler The output end of the circulator is connected with the circulator and the second optocoupler, the second optocoupler is connected with the balanced detector, and the output end of the balanced detector is connected with the input end of the data acquisition card; the annular The circulator is connected with the sensing fiber, and the output end of the circulator is connected with the second optical coupler.

The auxiliary interferometer includes a fourth optical coupler, a first Faraday rotating mirror, a second Faraday rotating mirror, a first photodetector and a clock shaping module, and the output end of the fourth optical coupler is connected to the first Faraday rotating mirror. connected with the second Faraday rotating mirror, the input end of the fourth optical coupler is connected with the first photodetector, the first photodetector is connected with the clock shaping module, and the output end of the clock shaping module Connect with the data acquisition card.

A method for spectral calibration in an optical frequency domain reflectometry sensing system using an HCN gas cell, comprising the following steps:

S1: Turn on the acquisition system, and set the zero-crossing point of the auxiliary interferometer signal as the external trigger signal _Sa ;

S2: Set the sweep start wavelength of the tunable laser as λ _st , the end wavelength as λ _en , the sweep frequency is turned on, and the acquisition card synchronously records the main interferometer sensing signal _{A 1M} and the HCN gas chamber under the trigger of the trigger signal Sa data A _1H ;

S3: Fit the HCN gas chamber signal A _1H to obtain each attenuation peak of the gas absorption spectrum, compare the fitted attenuation peak with the gas absorption spectrum, find the number corresponding to the corresponding attenuation peak, determine the corresponding wavelength, and search The sampling point position corresponding to the center of the attenuation peak, select one of the absorption peak wavelengths as λ ₀ , record its corresponding sampling point position P ₁ , and take the P _1th data in the sensing data A _1M of the main interferometer as the current time the starting position of the measurement;

S4: Select the wavelengths λ _a and λ _b corresponding to the two attenuation peaks, calculate the number of zero-crossing points of the additional interferometer signal as N ₀ , and calibrate to obtain the wavelength interval Δλ _min of the additional interferometer zero-crossing points, Δλ _min =(λ _b -λ _a )/N ₀ ;

S5: In the main interferometer, select the point N behind the point P ₁ as the sampled data recorded this time, and the obtained one-dimensional sequence of length N is recorded as the reference signal S ₁ , and the recording frequency sweep range Δλ=N× _Δλmin is obtained. ;

S6: After the strain or temperature change occurs, repeat S2, S3 and S5 to obtain another dimensional sequence of length N, which is recorded as the reference signal measurement signal S ₂ ;

S7: Perform fast Fourier transform on S ₁ and S ₂ respectively, convert the optical frequency domain information to the distance domain information of each position in the sensing fiber, use the moving window to scan the signal of each position in the sensing fiber, use the Fourier transform The inverse Liye transform is converted to the optical frequency domain, corresponding to the Rayleigh scattering spectral signal of the reference signal and the measurement signal at each position;

S8: Cross-correlation operation is performed on the Rayleigh scattering spectral signal of the reference signal and the measurement signal to obtain the deviation of the cross-correlation peak value of the measurement signal relative to each position of the reference signal, thereby obtaining the measured strain value.

Due to the adoption of the above technical solution, the present invention provides a device and method for spectral alignment in the optical frequency domain reflection sensing technology, which adds an HCN gas chamber to the traditional optical frequency domain reflection sensing device for synchronous acquisition, The scan start and stop wavelengths are established by the peak position of the HCN absorption line to ensure the consistency of the frequency range of each scan. The device and method adopted in the present invention reduce the performance requirements for the existing tunable laser, reduce the error of the spectral shift amount, and effectively improve the distributed measurement accuracy. The invention provides an optical fiber strain measurement method based on OFDR technology which utilizes the HCN gas chamber for wavelength calibration. The present invention utilizes the OFDR method to realize real-time measurement of the strain or temperature at each position, and utilizes the HCN gas chamber to measure the tunable laser. The characteristic of accurate calibration of the swept frequency signal overcomes the problem of insufficient accuracy of the output wavelength of the tunable laser, and realizes the real-time and accurate measurement of the strain of the optical fiber.

Description of drawings

In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the accompanying drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only These are some embodiments described in this application. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings without any creative effort.

1 is a flow chart of a method for spectral alignment using an HCN gas cell in optical frequency domain reflectometry;

2 is a schematic diagram of a device for spectral alignment using an HCN gas cell in the optical frequency domain reflectometry technology;

Fig. 3 is the standard spectral line diagram of HCN gas absorption line after normalization;

Reference signs in the figure: 1, computer, 2, scanning light source, 3, optical coupler, 4, optical coupler, 5, circulator, 6, sensing fiber, 7, second optical coupler, 8, Balanced Detector, 9 Third Optical Coupler, 10 Fourth Optical Coupler, 11, First Faraday Rotating Mirror, 12, Second Faraday Rotating Mirror, 13 First Photodetector, 14, Clock Shaping Module, 15, HCN Air chamber, 16, photoelectric detector, 17, data acquisition card, 18, HCN wavelength calibration module, 19, auxiliary interferometer, 20, main interferometer.

Detailed ways

In order to make the technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention:

As shown in FIG. 1, a device for spectral calibration in an optical frequency domain reflectometry system using an HCN gas cell includes: a scanning light source 2 for providing a light source for the optical frequency domain reflectometry system, wherein the light of the scanning light source 2 The frequency can be linearly scanned, the light source outlet end of the scanning light source 2 is connected to the optical coupler 1 3, the optical coupler 1 3 is connected to the main interferometer 20 through the c port, and the main interferometer 20 is connected to the input end of the data acquisition card 17 The optical coupler-3 is connected with the coupler 9 through the b port, and the coupler 9 is connected with the HCN wavelength calibration module 18 through the b port, and the HCN wavelength calibration module 18 is connected with the data acquisition card 17, and the HCN wavelength The calibration module 18 is provided with an HCN gas chamber 15 and a photodetector 16, and the output end of the HCN gas chamber 15 is connected to the photodetector 16; the third optical coupler 9 is connected to the auxiliary interferometer 19 through the c port, and the auxiliary interference The output end of the instrument 19 is connected with the data acquisition card 17 , and the data acquisition card 17 is connected with the computer 1 .

Further, the main interferometer 20 includes an optical coupler 4, a circulator 5, a sensing fiber 6, a second optical coupler 7 and a balanced detector 8, and the input end of the optical coupler 4 is connected to the c of the optical coupler-3. port is connected, the output end of the optical coupler 4 is connected with the circulator 5 and the second optical coupler 7, the second optical coupler 7 is connected with the balanced detector 8, and the output end of the balanced detector 8 is connected It is connected with the input end of the data acquisition card 17 ; the circulator 5 is connected with the sensing fiber 6 , and the output end of the circulator 5 is connected with the second optical coupler 7 .

Further, the auxiliary interferometer 19 includes a fourth optical coupler 10 , a first Faraday rotating mirror 11 , a second Faraday rotating mirror 12 , a first photodetector 13 and a clock shaping module 14 . The output end is connected to the first Faraday rotating mirror 11 and the second Faraday rotating mirror 12 , the input end of the fourth optical coupler 10 is connected to the first photodetector 13 , and the first photodetector 13 is connected to the clock shaping module 14 . connected, the output end of the clock shaping module 14 is connected with the data acquisition card 17 .

A method for spectral calibration in an optical frequency-domain reflectometry sensing system using an HCN gas cell, comprising the following steps:

S1: Turn on the acquisition system, and set the zero-crossing point of the auxiliary interferometer signal as the external trigger signal _Sa ;

S2: Set the sweep start wavelength of the tunable laser as λ _st , the end wavelength as λ _en , the sweep frequency is turned on, and the acquisition card synchronously records the main interferometer sensing signal _{A 1M} and the HCN gas chamber under the trigger of the trigger signal Sa data A _1H ;

S3: Fit the HCN gas chamber signal A _1H to obtain each attenuation peak of the gas absorption spectrum, compare the fitted attenuation peak with the gas absorption spectrum, find the number corresponding to the corresponding attenuation peak, determine the corresponding wavelength, and search The sampling point position corresponding to the center of the attenuation peak, select one of the absorption peak wavelengths as λ ₀ , record its corresponding sampling point position P ₁ , and take the P _1th data in the sensing data A _1M of the main interferometer as the current time the starting position of the measurement;

S4: Select the wavelengths λ _a and λ _b corresponding to the two attenuation peaks, calculate the number of zero-crossing points of the additional interferometer signal as N ₀ , and calibrate to obtain the wavelength interval Δλ _min of the additional interferometer zero-crossing points, Δλ _min =(λ _b -λ _a )/N ₀ ;

S5: In the main interferometer, select the point N behind the point P ₁ as the sampled data recorded this time, and the obtained one-dimensional sequence of length N is recorded as the reference signal S ₁ , and the recording frequency sweep range Δλ=N× _Δλmin is obtained. ;

S6: After the strain or temperature change occurs, repeat S2, S3 and S5 to obtain another dimensional sequence of length N, which is recorded as the reference signal measurement signal S ₂ ;

S7: Perform fast Fourier transform on S ₁ and S ₂ respectively, convert the optical frequency domain information to the distance domain information of each position in the sensing fiber, use the moving window to scan the signal of each position in the sensing fiber, use the Fourier transform The inverse Liye transform is converted to the optical frequency domain, corresponding to the Rayleigh scattering spectral signal of the reference signal and the measurement signal at each position;

S8: Cross-correlation operation is performed on the Rayleigh scattering spectral signal of the reference signal and the measurement signal to obtain the deviation of the cross-correlation peak value of the measurement signal relative to each position of the reference signal, thereby obtaining the measured strain value.

Example:

Step 1. Determine the starting wavelength of the scanning range of the scanning light source. Since the HCN gas cell 15 is within a certain wavelength range (1525nm-1565nm), it has the ability to absorb light of certain specific wavelengths and has high precision. This selects the wavelength λ ₁ corresponding to a certain absorption line peak of the HCN gas cell, where λ ₁ is taken as 1540.43120 nm. The starting wavelength λ ₂ of the tunable laser is set to be λ 3 and the end wavelength is λ ₃ . In order to make the accurate calibration wavelength, the starting wavelength λ ₂ should be slightly smaller than λ ₁ , and λ ₁ can be selected to be 1540.00 nm according to the accuracy conditions of the scanning light source. ;

Step 2. Turn on the acquisition system, set the zero-crossing point of the signal of the auxiliary interferometer 19 as the external trigger signal, the interference signal generated by the auxiliary interferometer is subjected to photoelectric conversion and clock shaping, and the interference beat frequency signal is shaped into a square wave. The data measurement processing of the sensing fiber 6 needs to use fast Fourier transform, so that the sampling frequency interval should be consistent. The signal can be used as the external clock signal of the acquisition device;

Step 3: The laser frequency sweep is turned on, and the sensing data A _1M of the main interferometer and the data A _1H of the HCN gas chamber 15 are synchronously recorded. The sensing data of the main interferometer 20 are formed by the sensing segment in the sensing fiber 6 through Rayleigh scattering The HCN gas chamber 15 has the ability to absorb light of certain specific wavelengths within a certain wavelength range (1525nm-1565nm), and has high precision, and the data of the HCN gas chamber 15 meets the accurate wavelength calibration conditions;

Step 4: Fit the HCN data A _1H to obtain each attenuation peak of the gas absorption spectrum, compare the fitted attenuation peak with the gas absorption spectrum in the figure, find the number corresponding to the corresponding attenuation peak, and determine the corresponding attenuation peak. wavelength, find the sampling point position corresponding to the center of the attenuation peak, record the sampling point position P ₁ corresponding to the wavelength of λ ₁ , and the P _1th data in the interferometer sensing data A _1M as the starting position of this measurement; To make the sampling zero-crossing wavelength interval as accurate as possible, it is necessary to take as many points as possible, that is, the wavelength between the selected two attenuation peaks is as long as possible. Here, the wavelengths λ ₁ corresponding to the two attenuation peaks are selected as 1540.43120nm and λ ₂ is 1544.51503 nm, calculate the number of zero-crossing points of the additional interferometer signal, count as N ₀ , and calibrate to obtain the additional interferometer zero-crossing point wavelength interval Δλ _min ;

Step 5: To measure the strain value and temperature change value of the sensing fiber 6, it is necessary to use the unchanging sensing fiber as the reference value, so point N after P ₁ is selected as the sampling data recorded this time, that is, the reference signal S ₁ , obtain the recording sweep frequency range Δλ=N× _Δλmin ;

Step 6, after the strain or temperature change occurs, repeat steps 3 to 5 to obtain the measurement signal S ₂ ;

Step 7: Perform fast Fourier transform on S ₁ and S ₂ respectively, convert the optical frequency domain information to the distance domain information of each position in the sensing fiber, and use the moving window to scan the signal of each position in the sensing fiber, Use inverse Fourier transform to convert to the optical frequency domain, corresponding to the reference signal and the Rayleigh scattering spectral signal of the measurement signal at each position;

Step 8: Perform cross-correlation operation on the Rayleigh scattering spectral signals of the reference signal and the measurement signal to obtain the deviation of the cross-correlation peak value of the measurement signal relative to the reference signal at each position, and obtain the measured physical quantity, that is, the corresponding strain and temperature amount of change.

To sum up, the present invention utilizes the OFDR method to achieve real-time measurement of the strain at each position, and utilizes the HCN gas chamber 15 to accurately calibrate the frequency sweep signal of the tunable laser, which overcomes the accuracy of the output wavelength of the tunable laser. Insufficient problem, the real-time accurate measurement of the strain and temperature change of the optical fiber is realized.

The above scheme will be described in detail below in conjunction with a specific strain sensing device, see the following description for details:

The scanning light source 2 outputs continuous wavelength light at a certain scanning rate. The light emitted by the scanning light source 2 is input from the a channel port of the 90/10 optical coupler 3, and is divided into two beams of light signals by the 90/10 optical coupler 3. A beam of optical signals is output from the b channel port, enters the optical coupler 11 through the a channel port of the 50/50 optical coupler 11, and is divided into two optical signals with equal light intensity by the 50/50 optical coupler 11, one of which is The optical signal is output from the b channel port of the 50/50 optical coupler 11 and enters the HCN gas chamber 17. The optical signal in the fixed frequency range is absorbed by the gas molecules in the gas chamber, and the absorbed optical signal is converted into electricity through the photodetector 18. The signal is used to calibrate the wavelength range of the light source, and the analog signal is collected by adjusting the sampling rate of the digital acquisition card 19. Another beam of light is output from the c-channel port of the 50/50 optical coupler 11, the light enters from the a-channel port of the 50/50 optical coupler 12, and exits from the c and d ports of the optical coupler, respectively. A Faraday rotating mirror 13 and a second Faraday rotating mirror 14 reflect and return to the c and d ports of the coupler 12. The two beams of light interfere in the optical coupler, and are output from the b port of the optical coupler 12. The outgoing light enters The detector 15 converts the detected optical signal into an interference beat frequency signal and transmits it to the clock shaping module. The clock shaping module shapes the interference beat frequency signal into a square wave, and the shaped signal is transmitted to the digital acquisition card 19 as the acquisition card 19. The device's external clock signal. A strong beam of light emitted from the 90/10 optical coupler 3 is output from the c-channel port of 3, enters the optical coupler 4 through the a-channel port of the 50/50 optical coupler 4, and is transmitted by the 50/50 optical coupler. 4 is divided into two optical signals with equal light intensity, one of which is output from the b channel port of the 50/50 optical coupler 4 and enters the a channel port of the 50/50 optical coupler 7, and the other optical signal is from the 50 /50 The output of the c-channel port of the optical coupler 4 enters the a-channel port of the circulator 5, and enters the sensing fiber from the c port of the circulator 5, and the backscattered light of the sensing fiber enters from the c port of the circulator 5. , output from the b port of the circulator 4, enter the b channel port of the 50/50 optical coupler 7 and the optical signal entering the a channel port to combine to form beat frequency interference and transmit from the c port of the 50/50 optical coupler 7 The and d ports are output to the first polarization beam splitter 8 and the second polarization beam splitter 9, and then enter the balance detector 10, and the balance detector 10 transmits the output analog electrical signal to the digital acquisition card 19, and the digital acquisition card 19 is in the Under the action of the external clock signal formed by the clock shaping module 21, the collected analog electrical signal is transmitted to the computer 1, and the computer 1 performs data processing on the interference signal collected by the digital acquisition card 19, and realizes the wavelength calibration based on the HCN gas chamber based on OFDR technology Fiber Strain Measurements.

The data processing process of the real-time wavelength calibration method used in the wavelength scanning system includes processing the data collected after passing through the gas chamber. According to the function of the gas chamber, it can be known that the gas molecules only absorb the energy of the optical signal of a specific wavelength, and different wavelengths can be absorbed. The degree of optical power attenuation is different at the wavelength position. Figure 3 shows the wavelength position of the optical power decline after passing through the gas chamber and the degree of decline after normalization. In the figure, the attenuation peaks after gas absorption are numbered and divided into There are two parts R and P, the wavelength corresponding to the center of each attenuation peak can be obtained by looking up Table 1

Table 1

According to the function of the gas chamber, the collected data needs to be fitted first, and each attenuation peak of the HCN gas absorption line is fitted, and the Voigt curve is used to fit each attenuation peak of the gas absorption line, The subsequent attenuation peaks are compared with the gas absorption lines in Fig. 3, and the number corresponding to the corresponding attenuation peak can be found. By looking up Table 1, the wavelength corresponding to the center of the attenuation peak can be known. By looking up the sample corresponding to the center of the attenuation peak According to the sampling rate, the sampling time corresponding to the sampling point, the wavelength and sampling time corresponding to the center of the attenuation peak can be known, and the real-time calibration of the scanning light source can be realized.

The above description is only a preferred embodiment of the present invention, but the protection scope of the present invention is not limited to this. The equivalent replacement or change of the inventive concept thereof shall be included within the protection scope of the present invention.

Claims (1)

1. A method for performing spectrum calibration in an optical frequency domain reflection sensing system by using an HCN gas chamber is characterized by comprising the following steps: the method comprises the following steps:

s1: opening the acquisition system and setting the zero crossing point of the auxiliary interferometer signal as an external trigger signal S_a；

S2: setting the sweep frequency starting wavelength of the tunable laser as lambda_stTerminating wavelength of λ_enWhen the sweep frequency is started, the acquisition card triggers a signal S_aSynchronously recording the sensing signal A of the main interferometer under the trigger_1MAnd HCN gas cell data A_1H；

S3: for HCN air chamber signal A_1HFitting to obtain each attenuation peak of the gas absorption spectral line, comparing the fitted attenuation peak with the gas absorption spectral line, finding out the number corresponding to the corresponding attenuation peak, determining the corresponding wavelength, searching the position of the sampling point corresponding to the center of the attenuation peak, and selecting one of the absorption peak wavelengths as lambda₀Record the corresponding sampling point position P₁Simultaneously sensing data A by the main interferometer_1MP in (1)₁Taking the data as the initial position of the measurement;

s4: selecting the wavelength lambda corresponding to two attenuation peaks_aAnd λ_bCalculating the number of zero crossings of the additional interferometer signal therein as N₀And obtaining the zero crossing point wavelength interval Delta lambda of the additional interferometer by calibration_min，Δλ_min＝(λ_b-λ_a)/N₀；

S5: choosing P in the main interferometer₁Taking N points behind the point as sampling data of the record, and recording the obtained one-dimensional number sequence with the length of N as a reference signal S₁Obtaining the recorded sweep frequency range delta lambda as NxDelta lambda_min；

S6: after the strain or temperature change has occurred, S2, S3 and S5 are repeated to obtain another one-dimensional series with length N as the measurement signal S₂；

S7: are respectively paired with S₁And S₂Performing fast Fourier transform to convert optical frequency domain information to distance domain information at each position in the sensing fiber, scanning signals at each position in the sensing fiber with a moving window, and converting to fast Fourier transformAn optical frequency domain corresponding to the rayleigh scattering spectrum signals of the reference signal and the measurement signal at each position;

s8: and performing cross-correlation operation on the Rayleigh scattering spectrum signals of the reference signal and the measurement signal to obtain the deviation of the cross-correlation peak value of the measurement signal relative to each position of the reference signal from the central quantity, thereby obtaining the measured strain value.

Images