CN114964329A - Double Sideband Optical Frequency Domain Reflectometer - Google Patents

Abstract

A double-sideband optical frequency domain reflectometer, comprising: a modulation signal generating unit, a sensing signal receiving unit and a signal processing unit, the modulation signal generating unit generates double-sideband detection light waves by means of external modulation, and divides the detection light waves into two paths , one way is input to the optical fiber under test as the probe light, and the other way is used as the local optical input sensing signal receiving unit. The fiber under test couples the change of the external physical quantity to the probe light wave, and uses the back Rayleigh scattering signal generated by it as the signal light It is sent back to the sensing signal receiving unit. The sensing signal receiving unit uses the frequency shift method or IQ receiving method to divide the double sideband signal light from two different beat frequency signals generated by the two sideband frequency sweeps. The two different beat signals are aligned in the time domain according to the sweep frequency range and then Fourier transform is performed to obtain the intensity and phase information of Rayleigh scattering on the fiber. The present invention uses IQ demodulation or local optical frequency shifting to demodulate the backscattered signals generated by the two sideband detection optical signals respectively, so as to realize the purpose of expanding the frequency sweeping range and improving the spatial resolution and measurement range of the reflector. .

Description

Double Sideband Optical Frequency Domain Reflectometer

technical field

The invention relates to a technology in the field of optical sensing, in particular to a double-sideband optical frequency-domain reflectometer which adopts two frequency sweep sidebands to obtain the distribution information of backscattered signals on the optical fiber on the optical fiber.

Background technique

Distributed optical fiber sensing technology uses light reflectometer technology to locate the backscattered signal. Among them, the Optical Frequency Domain Reflectometer (OFDR) system uses a linearly swept optical signal as the probe light and the local light. The back Rayleigh scattering signal generated by the fiber to be tested is proportional to the local light beat frequency, the frequency of which is proportional to The time delay of backscattered light determines the location of backscattered Rayleigh scattering by the size of the beat frequency. Its spatial resolution depends on the frequency tuning range of the light source, and high spatial resolution can be achieved.

The OFDR system requires a linear swept frequency light source as the probe light and local light. The generation method of the swept frequency light source can be divided into two types: internal modulation and external modulation. The internal modulation scheme directly uses a tunable laser to generate a swept-frequency optical signal, and the frequency tuning range of the light source can reach tens of THz. However, the internal modulation laser generally has a wide linewidth and is difficult to suppress the swept frequency nonlinearity, resulting in the use of the internal modulation scheme. The phase noise of the OFDR system is relatively large, and its measurement distance is relatively small. The OFDR system of the external modulation scheme uses a frequency-stabilized laser as the seed light, and the frequency of the seed laser is changed by a modulator to achieve linear frequency sweep. The external modulation method can obtain a swept frequency light source with low phase noise and high linearity, and realize long-distance sensing. Electro-optic modulators are commonly used in OFDR systems, and their frequency modulation range is limited by the frequency bandwidth of the modulator and the driving electrical signal; moreover, the existing electro-optic modulator schemes require the use of expensive single-sideband modulators, additional optical filters or Only solutions such as injection locking can generate stable linear frequency sweep light, which makes the OFDR system complicated and difficult to work stably.

SUMMARY OF THE INVENTION

Aiming at the above-mentioned shortcomings of the prior art, the present invention proposes a double-sideband optical frequency domain reflectometer. Through seed light and an electro-optical modulator, two positive and negative sidebands with different frequency sweeping directions generated by the electro-optical modulator are simultaneously processed. As a useful signal, instead of a single linear frequency sweep light in the classical OFDR system, IQ demodulation or local optical frequency shifting is used to demodulate the backscattered signals generated by the two sideband detection optical signals respectively, so as to realize the enlarged sweep. frequency range, and improve the spatial resolution and measurement range of the reflectometer.

The present invention is achieved through the following technical solutions:

The invention relates to a double-sideband optical frequency domain reflectometer, comprising: a modulated signal generating unit, a sensing signal receiving unit and a signal processing unit, wherein: the modulating signal generating unit generates double-sideband detection light waves by means of external modulation, and detects The light wave is divided into two paths, one is input to the optical fiber under test as the detection light, and the other is used as the local optical input sensing signal receiving unit. The scattered signal is sent back to the sensing signal receiving unit as signal light, and the sensing signal receiving unit uses the frequency shift method or IQ receiving method to distinguish the beat frequency signal generated from the two sideband frequency sweeps from the double sideband signal light, and the signal processing unit The beat frequency signals generated by the two sidebands are aligned in the time domain according to the sweep frequency range and Fourier transform is performed to obtain the intensity and phase information of Rayleigh scattering on the fiber.

The modulated signal generating unit includes: an electro-optical intensity modulator, a single-frequency laser, a signal generator and a fiber coupler connected to it, and an erbium-doped fiber amplifier and a fiber circulator connected in turn, wherein: the single-frequency laser A beam of seed light with a fixed frequency is generated, and the signal generator generates a radio frequency sweep signal whose frequency varies linearly to drive the electro-optical intensity modulator. After the seed light is modulated by the electro-optical intensity modulator, a double-sideband swept lightwave signal with a frequency symmetrical on both sides of the seed light frequency, and the difference between the seed light frequency and the frequency of the seed light is equal to the same RF frequency sweep signal, and the double-sideband swept frequency lightwave passes through the fiber coupler. Each of them is divided into two beams, one of which is input to the erbium-doped fiber amplifier, which is guided through the fiber circulator and input to the fiber to be tested as the probe light, and the other is input to the sensing signal receiving unit as the local reference light. The measured physical quantity affects the transmission of light waves in it by affecting the physical parameters of the fiber under test, so the phase and intensity of the back Rayleigh scattering signal generated on the fiber under test will change. The backward Rayleigh scattering signal is guided by the optical fiber circulator and is input to the sensing signal receiving unit as signal light.

The single frequency laser is preferably a narrow linewidth laser.

The electro-optical intensity modulator is preferably a Mach-Zehnder modulator (MZM).

The sensor signal receiving unit is any one of the following structures:

① The sensing signal receiving unit includes: an acousto-optic frequency shifter and a polarization diversifier connected to the modulation signal generating unit, a balanced photodetector connected to the polarization diversifier, and a multi-channel analog-to-digital conversion device; when the sensing When the signal receiving unit uses the frequency shift method to separate the positive and negative sidebands from the double-sideband signal light, a modulated signal with a fixed frequency is loaded on the acousto-optic frequency shifter, and the reference light input to the sensing signal receiving unit passes through the frequency of the acousto-optic frequency shifter. Increase or decrease, the change value is the frequency of the modulated signal on the acousto-optic frequency shifter. The signal light is input into the polarization diversifier and is divided into a first polarization state and a second polarization state, so that the signal light of different polarization states can interfere with the reference light of the same polarization state respectively. After the frequency-shifted reference light is adjusted by the polarization state, it interferes with the signal light of the first polarization state and the second polarization state in the polarization diversifier, respectively, to obtain two sets of beat frequency signals respectively representing two different optical signals. Linearly polarized and output to two balanced photodetectors respectively, after square detection and filtering out common mode components, beat frequency signals of different polarization states are obtained. The electrical signal output by the photodetector is input into the multi-channel analog-to-digital conversion equipment, converted into a digital signal, and sent to the signal processing unit through the data transmission medium; the frequency-shifted reference light and the signal light returned by the Rayleigh scattering spectrum are in polarization diversity. The frequency is mixed according to different polarization states in the device. The balanced photodetector completes square detection, and the result of the mutual beat frequency of the two sidebands in the reference light and the signal light is obtained. The beat frequency signal of the negative first-order signal light is filtered out by the pre-filter in the multi-channel analog-to-digital conversion device, that is, the digital signal collected by the analog-to-digital conversion device only contains the beat frequency of the positive first-order signal light and the positive first-order reference light. The beat frequency signal of the signal and the negative first-order reference light and the negative first-order signal light.

The frequency mixing refers to: the signal light field in the input optical mixer is E _S , the reference light field is E _L , and the two output light fields obtained according to the input-output relationship of the mixer are:

The square detection means that the balanced photodetector uses the built-in photodiode to perform square detection, and the photocurrent signals corresponding to the two input light fields are obtained as:

The beat frequency signal obtained by beating each other in pairs refers to the differential mode component between the photocurrent signals after square detection in the balanced photodetector:

②The sensing signal receiving unit includes: a polarization diversity IQ receiver, a balanced photodetector and a multi-channel analog-to-digital conversion device connected with the modulation signal generating unit; When the positive and negative sidebands are separated in the signal light, the signal light is input into the polarization diversity IQ receiver and then separated by the polarization beam splitter according to the first polarization state and the second polarization state, and two sets of beat frequency signals are obtained respectively representing two different optical signals. Linearly polarized, output to different 90° optical mixers, respectively, to mix with local light. The output light of each 90° optical mixer is input into two different balanced photodetectors, and after square detection and filtering out the common mode component, beat frequency complex signals of different polarization states are obtained. The electrical signal output by the photodetector is input into the multi-channel analog-to-digital conversion device, converted into a digital signal, and sent to the signal processing unit through the data transmission medium; the reference light and the signal light returned by the Rayleigh scattering spectrum are in the polarization diversity IQ receiver. Separate IQ mixing according to different polarization states. The balanced photodetector completes the square detection of the IQ signal, and obtains the result that the two sidebands in the reference light and the signal light beat each other in pairs; the beat signal of the positive first-order signal light and the negative first-order reference light and the positive first-order reference light The beat frequency signal of the light and the negative first-order signal light is filtered out by the pre-filter in the multi-channel analog-to-digital conversion device. The digital signal collected by the analog-to-digital conversion device only contains the beat frequency of the positive first-order signal light and the positive first-order reference light. frequency signal and the beat frequency signal of the negative first-order reference light and the negative first-order signal light.

The IQ mixing refers to: the signal light field in the input 90° optical mixer is E _S , the reference light field is E _L , and the output four light fields are obtained according to the input-output relationship of the IQ mixer. They are:

The square detection of the IQ signal means that the balanced photodetector uses the built-in photodiode to perform square detection, and the photocurrent signals corresponding to the four input light fields are obtained as follows:

即分别为同一个复信号的实部和虚部。The beat frequency signal obtained by beating each other in pairs refers to the differential mode component between the photocurrent signals after square detection in the balanced photodetector:

That is, the real and imaginary parts of the same complex signal, respectively.

The signal processing unit includes: a signal processing device. The signal processing device may be a computing device with high performance, such as a server, a personal computer, an industrial computer, a DSP board or an FPGA board.

The spatial resolution of the double-sideband optical frequency domain reflectometer not only depends on the frequency coverage of the two sidebands themselves, but also depends on the maximum frequency difference between the two sidebands. In order to avoid the influence of the mutual beat frequency between the two sidebands, the frequency coverage of the two sidebands will be discontinuous, which is equivalent to the sweep formed by the maximum frequency difference between the two sidebands in the frequency sweep process. A window function in the optical frequency domain (time domain) is superimposed on the frequency signal. This window function will further improve the spatial resolution under the premise of broadening the frequency spectrum, so that the final spatial resolution improvement is greater than the original single sideband. Twice the frequency sweep effect, that is, the width of the spectrum in the distance domain when the maximum value of the beat frequency spectrum generated by a scattering point drops by 3dB.

technical effect

The invention adopts the simultaneous frequency sweep of positive and negative sidebands at the transmitting end, adopts frequency shift reception or IQ receiving scheme technology at the receiving end to realize the identification of the frequency sweeping signals of the positive and negative sidebands, and at the signal processing end, the positive and negative frequency sweeping signals are The Fourier transform is performed after the frequency arrangement.

Compared with the prior art, the present invention uses a double-sideband frequency sweep light source generated by external modulation, and proposes a corresponding double-sideband demodulation method, thereby realizing a distributed OFDR sensing system. The invention can expand the equivalent sweep frequency range of the optical frequency domain to more than twice the sweep frequency range of the radio frequency signal, and realizes that the spatial resolution is improved to more than twice that of the prior art.

Description of drawings

Figure 1 is a schematic diagram of the scheme ① of the double-sideband optical frequency domain reflectometer;

Fig. 2 is the demodulation schematic diagram of the double-sideband optical frequency domain reflectometer in Fig. 1;

Figure 3 is a schematic diagram of the double-sideband optical frequency domain reflectometer scheme ②;

4 is a schematic diagram of an IQ receiver formed by a 90° optical mixer and a balanced photodetector (BPD) of the double-sideband optical frequency domain reflectometer in FIG. 3;

Fig. 5 is the demodulation schematic diagram of the double-sideband optical frequency domain reflectometer in Fig. 3;

In the figure: 1 modulated signal generating unit, 2 sensing signal receiving unit, 3 signal processing unit, 4 fiber to be measured, 10 single frequency laser, 11 electro-optical intensity modulator, 12 signal generator, 13 fiber coupler, 14 erbium-doped Fiber amplifiers, 15 fiber circulators, 20 balanced photodetectors, 21 multi-channel analog-to-digital conversion equipment, 30 signal processing equipment, 40 acousto-optic frequency shifters, 41 polarization dividers, 42 polarization diversity IQ receivers;

Fig. 6 is embodiment 1 using scheme 1. realizes the power spectral density of the beat frequency signal that double-sideband signal receives;

Fig. 7 is embodiment 2 using scheme 2. realizes the power spectral density of the beat frequency signal that double-sideband signal receives;

8 is a schematic diagram of the Rayleigh scattering spectrum of the fiber to be tested after being processed by the signal processing unit;

FIG. 9 is a schematic diagram showing the comparison of the spatial resolution of the Rayleigh scattering spectrum using only a single sideband data and the Rayleigh scattering spectrum using all two sideband data.

Detailed ways

Example 1

As shown in FIG. 1, an OFDR system using an external modulation method for demodulating double-sideband signals using a frequency shift method includes a modulation signal generating unit 1, a sensing signal receiving unit 2 and a signal processing unit 3 that are connected in sequence, wherein: The modulated signal generating unit 1 outputs modulated signals to the sensing signal receiving unit 2 and the signal processing unit 3 respectively. The sensing signal receiving unit 2 accepts the sensing information returned by the fiber to be tested 4 and completes the separation and mode of the positive and negative sideband signals. Digital conversion, the digital signal is transmitted to the signal processing unit 3, and the signal processing unit 3 demodulates the distribution information of the backscattered signal on the optical fiber according to the modulation signal and the sensing signal.

The modulated signal generating unit 1 includes: a single-frequency fiber laser 10, a signal generator 12, a fiber coupler 13, an erbium-doped fiber amplifier 14, and a fiber circulator 15, which are respectively connected with the electro-optical intensity modulator 11, wherein: the fiber ring The detector 15 outputs the detection signal to the fiber under test 4, and the output light of the single-frequency fiber laser 10 is output to the electro-optical intensity modulator 11 and is divided into two sub-branches: the phase of one channel is directly controlled by the input RF signal, and the phase of the other channel is controlled by The reversed RF signal and a DC signal are controlled in series; adjusting the DC signal to keep the phase difference between the two sub-branches by π enables the intensity modulator to suppress the carrier and even harmonics.

The power of the RF signal input to the electro-optical intensity modulator 11 is adjusted to 20 dBm, so that the output light of the electro-optical intensity modulator 11 is mainly composed of two first-order sidebands, and the power of the high-order sidebands and the carrier can be ignored.

Preferably, when the radio frequency signal loaded on the electro-optical intensity modulator 11 is a frequency sweep signal, the two first-order sidebands of the output light are the frequency sweep light with the same frequency sweep rate and frequency sweep range, except that the negative The sweep direction of the first-order sideband is opposite to that of the positive first-order sideband.

The sensing signal receiving unit 2 includes: an acousto-optic frequency shifter 40 and a polarization diversifier 41 connected to the modulation signal generating unit 1, a balanced photodetector 20 connected to the polarization diversifier 41, and a multi-channel analog-to-digital conversion device twenty one.

As shown in Figure 2, the abscissa represents time, the ordinate represents the frequency of the optical signal in different states, s1 is the local reference light emitted by the second branch of the fiber coupler 13, and s2 is the local reference light that has undergone acousto-optic frequency shifting The process in which the frequency of the device 40 is shifted to change its frequency, s3 is the process in which the probe light emitted by the first branch of the fiber coupler 13 passes through the erbium-doped fiber amplifier 14 and the fiber circulator 15 to reach the Rayleigh scattering point P, and s4 is the Rayleigh scattering point P. The process of the backward Rayleigh scattered light generated on the scattering point P reaching the polarization _diversifier 41 as the signal light _; The starting frequency, Ω is the frequency shift amount of the acousto-optic frequency shifter 40 . τ _p is the time when the probe light emitted from the fiber circulator 15 reaches a certain Rayleigh scattering point P on the fiber to be tested, and the backward Rayleigh scattering light generated at this point passes through the fiber circulator 15 and reaches the polarization diversifier 41. The time is also τ _p . Since the time for the light wave emitted from the second branch of the fiber coupler 13 to pass through the acousto-optic frequency shifter 40 to reach the polarization diversifier 41 is negligible, the backward Rayleigh scattered light generated by the Rayleigh scattering point P and the reference light reach the polarization The time delay difference of the diversifier 41 is 2τ _p . Δf ₁ is the frequency obtained by the beat frequency between the backward Rayleigh astigmatism generated by the positive first-order tape probe light reaching the Rayleigh scattering point P and the positive first-order sideband of the local light, and Δf ₂ is the sum of the negative first-order sideband. The frequency obtained by the mutual beat frequency;

In this embodiment, the acousto-optic frequency shifter 40 is configured to shift the frequency positively by 60 MHz.

In this embodiment, the multi-channel analog-to-digital conversion device 21 is configured in a dual-channel mode with a sampling rate of 256 MHz.

In this embodiment, the signal processing device 30 is a personal computer.

In this embodiment, the output wavelength of the single-frequency laser 10 is 1550.12 nm, that is, the frequency is 193.40 THz. The start frequency of the sweep signal generated by the signal generator 12 is 4GHz, the end frequency of the sweep is 20GHz, and the sweep time is set to 1.4ms, so the sweep rate is 11.43THz/s.

In this embodiment, the fiber 4 to be tested uses an ordinary single-mode fiber with a total length of about 470 m. The corresponding maximum beat frequency is about 52.60MHz.

Figure 6 shows the power spectral density of the beat signal obtained by the double-sideband OFDR system that uses the frequency shift method to receive the double-sideband signal in Embodiment 1. In the figure, the horizontal axis is the frequency of the beat signal, and the vertical axis is the frequency of the beat signal. Power spectral density, the vertical axis uses the logarithmic axis. Use scheme ① The received beat frequency signal is a real signal, so the frequency range is 0 Hz to (Fs/2) Hz, and Fs is the sampling rate used by the multi-channel analog-to-digital conversion in Example 1.

The contour of the power spectral density of the beat signal in the figure is symmetrical with the frequency loaded by the acousto-optic frequency shifter at 60MHz. The left side is the beat spectrum generated by the negative first-order sideband sweep signal, and the right side is the positive first-order sideband sweep signal. the beat spectrum. Since the information contained in the different beat spectra on the left and right sides comes from the probe light in different frequency ranges, they contain non-overlapping information.

The signal processing unit 3 extracts the different beat signals from the beat frequency domain, aligns them according to the sweep frequency range in the optical frequency domain (time domain), and then calculates their Fourier transform. Obtain a resolution-enhanced Rayleigh scattering spectrum.

Figure 8 shows the Rayleigh scattering spectrum processed by the signal processing unit 3. In order to better compare the spatial resolution, a Fresnel reflection peak with a signal-to-noise ratio of about 101 m in the scattering spectrum is selected.

As shown in Figure 9, the discriminative ability of spatial resolution is improved by means of Fourier interpolation. According to the experimental results, the common OFDR system can achieve a spatial resolution of 5.74mm according to the frequency sweep range set in this embodiment, and the double-sideband OFDR system using the frequency shift method can achieve a 2.12mm frequency sweep range according to the same frequency sweep range. spatial resolution. The improvement of the spatial resolution of the double-sideband OFDR system is more than twice that of the ordinary OFDR system.

Example 2

As shown in FIG. 3 , compared with the first embodiment, the present embodiment relates to an OFDR system that uses the IQ receiving method to realize double-sideband signal demodulation. The sensing signal receiving unit 2 includes: Polarization diversity IQ receiver 42 , balanced photodetector 20 and multi-channel analog-to-digital conversion device 21 .

As shown in Figure 5, the abscissa represents time, the ordinate represents the frequency of the optical signal in different states, s1 is the local reference light emitted by the second branch of the fiber coupler 13, and s3 is the first branch of the fiber coupler 13. The process of the signal detection light from the channel reaching the Rayleigh scattering point P, s4 is the process of the backward Rayleigh scattering light generated on the Rayleigh scattering point P as the signal light reaching the polarization diversity IQ receiver 42; ν ₀ is the single frequency The frequency of the light wave output by the laser 10, f ₀ is the starting frequency of the radio frequency sweep signal generated by the signal generator 12, τ _p is the detection light from the fiber circulator 15 to reach a certain Rayleigh scattering point P on the fiber under test. time, and the backward Rayleigh scattered light generated at this point passes through the fiber circulator 15 and reaches the polarization diversity IQ receiver 42 with a time delay difference of 2τ _p . Δf ₃ is the frequency obtained by the beat frequency between the backward Rayleigh astigmatism generated by the positive first-order tape probe light reaching the Rayleigh scattering point P and the positive first-order sideband of the local light, and Δf ₄ is the sum of the negative first-order sideband. The frequency obtained by the mutual beat frequency;

In this embodiment, the parameters of the modulated signal generating unit 1 are exactly the same as those in the first embodiment.

In this embodiment, the signal processing unit 3 is exactly the same as that of the first embodiment.

In this embodiment, the multi-channel analog-to-digital conversion device 21 is configured in a four-channel mode with a sampling rate of 125 MHz. Since the sampling rate is halved when the number of channels is increased, for most freely configurable four-channel analog-to-digital conversion devices, the performance requirements have not changed compared to Example 1.

FIG. 7 shows the power spectral density of the beat signal obtained by the double-sideband OFDR system using the IQ method to realize double-sideband signal reception in Embodiment 2. In the figure, the horizontal axis is the frequency of the beat signal, the vertical axis is the power spectral density of the beat signal, and the vertical axis uses the logarithmic coordinate axis. The beat frequency signal received by using ② is a complex signal, so the frequency range is (-Fs/2) Hz～(Fs/2) Hz, and Fs is the sampling rate used by the multi-channel analog-to-digital conversion in Embodiment 2.

Since the received signal is a complex signal in this embodiment, the positive and negative frequencies do not affect each other. In the figure, the profile of the power spectral density of the beat signal is symmetrical with the frequency of 0MHz. The left side is the beat spectrum generated by the negative first-order sideband sweep signal, and the right side is the beat spectrum generated by the positive first-order sideband sweep signal. Since the information contained in the different beat spectra on the left and right sides comes from the probe light in different frequency ranges, they contain non-overlapping information.

Compared with Embodiment 1, this embodiment needs to use more photodetectors, but also saves the frequency shifting operation.

Whether it is the frequency shifting method or the IQ receiving method, the Rayleigh scattering spectrum of the positive and negative sidebands can be easily filtered out from the Rayleigh scattering spectrum containing the information of the two sidebands by using the band-pass filtering. After splicing the Rayleigh signals of the positive and negative sidebands, the equivalent frequency sweep range can be increased to improve the spatial resolution and the dynamic range of the measurement.

The parameters of the optical fiber 4 to be tested in this embodiment are exactly the same as those in Embodiment 1, so the obtained Rayleigh scattering spectrum data are also identical to those in Embodiment 1, as shown in FIG. 8 and FIG. 9 .

In Fig. 8, the horizontal axis is the distance, corresponding to the distance from a certain position on the fiber to its input end, and the vertical axis is the power spectral density of the Rayleigh scattering spectrum of the fiber to be tested, corresponding to the intensity of backward Rayleigh scattering at a certain position of the fiber to be tested , and the vertical axis uses a logarithmic axis.

In order to compare with the prior art, two curves are drawn in Fig. 8: the first thin solid line uses only a single sideband data, corresponding to the demodulation method of OFDR in the prior art; the second thin dashed line is the use of all The two sideband data correspond to the demodulation method of the double sideband OFDR in the present invention. The outlines of the thin solid line and the thin dashed line are consistent, indicating that the double-sideband OFDR is consistent with the Rayleigh scattering spectrum of the optical fiber obtained by OFDR in the prior art.

In order to better observe the difference in spatial resolution in Figure 8, Figure 9 selects the Fresnel reflection peaks at about 101m in Figure 8 generated by the APC plug in the fiber under test, and magnifies them and places them in two sub-images.

In Fig. 9, the horizontal axis is the distance, and the vertical axis is the power spectral density of the Rayleigh scattering spectrum of the fiber under test, corresponding to the intensity of the backward Rayleigh scattering at a certain position of the fiber under test. In order to better observe the change of spatial resolution, Normalize using its maximum value, and use the linear axis for the vertical axis.

The solid line with triangular marks in Figure 9 is the Rayleigh scattering spectrum after scaling, the triangular mark is the frequency sampling point of the Rayleigh scattering spectrum by discrete Fourier transform, and the dotted line is the frequency sampling value using the existing Rayleigh scattering spectrum The calculated Fourier interpolation curve is used to better calculate the change in spatial resolution. Fourier interpolation can be consistent with the discrete-time Fourier transform of actual data, and its 3dB spectral width can more accurately characterize the spatial resolution. The spatial resolution calculated from the 3dB spectral width is marked in Figure 9.

According to the experimental results, according to the parameters set in this embodiment, the spatial resolution of the Rayleigh scattering spectrum obtained by the double-sideband OFDR using the IQ method is 2.12 mm, which is 2.7 times that of the 5.74 mm obtained by ordinary OFDR, indicating that the bilateral The improvement of the spatial resolution of the system with OFDR is more than twice that of the ordinary OFDR system.

Compared with the prior art, the measuring range of the device is expanded to more than twice the original, and the spatial resolution is improved to more than twice the original.

The above-mentioned specific implementation can be partially adjusted by those skilled in the art in different ways without departing from the principle and purpose of the present invention. The protection scope of the present invention is subject to the claims and is not limited by the above-mentioned specific implementation. Each implementation within the scope is bound by the present invention.

Claims (7)

1. a double-sideband optical frequency domain reflectometer, is characterized in that, comprises: modulation signal generation unit, sensing signal receiving unit and signal processing unit, wherein: modulation signal generation unit produces double-sideband detection light wave by the mode of external modulation, The detection light wave is divided into two paths, one is input to the optical fiber under test as the detection light, and the other is used as the local optical input sensing signal receiving unit. The Rayleigh scattering signal is sent back to the sensing signal receiving unit as signal light, and the sensing signal receiving unit uses the frequency shift method or the IQ receiving method to divide the double sideband signal light from the two different sideband frequency sweeps. For the beat frequency signal, the signal processing unit performs Fourier transform on the two different beat frequency signals according to the sweep frequency range after alignment in the time domain, to obtain the intensity and phase information of Rayleigh scattering on the optical fiber; The sensor signal receiving unit is any one of the following structures: ① The sensing signal receiving unit includes: an acousto-optic frequency shifter and a polarization diversifier connected to the modulation signal generating unit, a balanced photodetector connected to the polarization diversifier, and a multi-channel analog-to-digital conversion device, or ②The sensing signal receiving unit includes: a polarization diversity IQ receiver, a balanced photodetector and a multi-channel analog-to-digital conversion device connected with the modulation signal generating unit. 2. double-sideband optical frequency domain reflectometer according to claim 1, is characterized in that, described modulation signal generation unit, comprises: electro-optical intensity modulator and the single frequency laser that is connected with it respectively, signal generator and optical fiber A coupler, and an erbium-doped fiber amplifier and a fiber circulator connected in sequence, wherein: a single-frequency laser generates a beam of seed light with a fixed frequency, and a signal generator generates a radio frequency sweep signal whose frequency changes linearly to drive an electro-optical intensity modulator; the seed; After the light is modulated by the electro-optical intensity modulator, a double-sideband swept lightwave signal with a frequency symmetrical on both sides of the seed light frequency, and the difference between the seed light frequency and the frequency of the seed light is equal to the same as the RF swept signal, is generated. Divided into two beams of light, one of which is input to an erbium-doped fiber amplifier, which is guided through a fiber circulator and input to the fiber to be measured as the probe light, and the other is input to the sensing signal receiving unit as the local reference light; The physical parameters of the fiber affect the transmission of light waves in it, so the phase and intensity of the back Rayleigh scattering signal generated on the fiber to be tested will change; the back Rayleigh scattering signal is guided by the fiber circulator and used as signal light, input Sensing signal receiving unit. 3. double-sideband optical frequency domain reflectometer according to claim 1, is characterized in that, when described sensing signal receiving unit uses frequency shift method to separate positive and negative sidebands from double-sideband signal light, acousto-optic shift A modulated signal with a fixed frequency is loaded on the frequency converter, and the reference light input to the sensor signal receiving unit increases or decreases in frequency after passing through the acousto-optic frequency shifter, and the changed value is the frequency of the modulated signal on the acousto-optic frequency shifter; the signal light input polarization diversity In the device, it is divided into a first polarization state and a second polarization state, so that the signal light of different polarization states can interfere with the reference light of the same polarization state respectively; The diversifier interferes the beat frequency with the signal light of the first polarization state and the second polarization state, respectively, and obtains two sets of beat frequency signals representing two different linear polarizations of the optical signal, respectively, and output to two balanced photodetectors respectively. Square detection and filter out the common mode component to obtain beat frequency signals of different polarization states; the electrical signal output by the photodetector is input into the multi-channel analog-to-digital conversion equipment, converted into a digital signal, and sent to the signal processing unit through the data transmission medium; The frequency-shifted reference light and the signal light returned by the Rayleigh scattering spectrum are respectively mixed in the polarization diversifier according to different polarization states; the balanced photodetector completes the square detection, and the two sidebands in the reference light and the signal light are obtained, respectively. The result of the beat frequency, in which the beat frequency signals of the positive first-order signal light and the negative first-order reference light and the beat frequency signals of the positive first-order reference light and the negative first-order signal light are filtered by the pre-filter in the multi-channel analog-to-digital conversion device. In addition, the digital signal collected by the analog-to-digital conversion device only includes the beat frequency signal of the positive first-order signal light and the positive first-order reference light and the beat frequency signal of the negative first-order reference light and the negative first-order signal light. 4. The double-sideband optical frequency domain reflectometer according to claim 3, wherein the frequency mixing refers to: the signal light field in the input optical mixer is E _S , and the reference light field is E _L , according to the input-output relationship of the mixer, the two output light fields are:

The square detection means that the balanced photodetector uses the built-in photodiode to perform square detection, and the photocurrent signals corresponding to the two input light fields are obtained as:

The beat frequency signal obtained by beating each other in pairs refers to the differential mode component between the photocurrent signals after square detection in the balanced photodetector:

. 5. double-sideband optical frequency domain reflectometer according to claim 1, is characterized in that, when described sensing signal receiving unit uses IQ receiving method to separate positive and negative sidebands from double-sideband signal light, signal light input After the polarization diversity IQ receiver is separated by the polarization beam splitter according to the first polarization state and the second polarization state, two sets of beat frequency signals representing two different linear polarizations of the optical signal are obtained, respectively, and output to different 90° optical mixing frequencies. The output light of each 90° optical mixer is input into two different balanced photodetectors, after square detection and filtering out the common mode component, beat frequency complex signals of different polarization states are obtained ;The electrical signal output by the photodetector is input into the multi-channel analog-to-digital conversion device, converted into a digital signal, and sent to the signal processing unit through the data transmission medium; the reference light and the signal light returned by the Rayleigh scattering spectrum are sent to the polarization diversity IQ receiver According to different polarization states, the IQ frequency is mixed separately; the balanced photodetector completes the square detection of the IQ signal, and the result that the two sidebands in the reference light and the signal light beat each other in pairs; the positive first-order signal light and the negative first-order signal light The beat frequency signal of the reference light and the beat frequency signals of the positive first-order reference light and the negative first-order signal light are filtered out by the pre-filter in the multi-channel analog-to-digital conversion device, and the digital signal collected by the analog-to-digital conversion device only contains the positive first-order signal. The beat frequency signal of the signal light and the positive first-order reference light and the beat frequency signal of the negative first-order reference light and the negative first-order signal light. 6. The double-sideband optical frequency domain reflectometer according to claim 5, wherein the IQ mixing refers to: the signal light field in the input 90° optical mixer is _ES , reference light The field is E _L . According to the input-output relationship of the IQ mixer, the four output light fields are:

The square detection of the IQ signal means that the balanced photodetector uses the built-in photodiode to perform square detection, and the photocurrent signals corresponding to the four input light fields are obtained as follows:

The beat frequency signal obtained by beating each other in pairs refers to the differential mode component between the photocurrent signals after square detection in the balanced photodetector:

That is, the real and imaginary parts of the same complex signal, respectively. 7. The double-sideband optical frequency-domain reflectometer according to claim 1, wherein the spatial resolution of the double-sideband optical frequency-domain reflectometer not only depends on the frequency coverage of the two sidebands themselves, but also depends on The maximum frequency difference between the two sidebands; in order to avoid the influence of the mutual beat frequency between the two sidebands, the frequency coverage of the two sidebands will be discontinuous, which is equivalent to sweeping the two sidebands. A window function in the optical frequency domain (time domain) is superimposed on the sweep signal formed by the maximum frequency difference in the process. The increase in resolution is more than twice that of the original single sideband sweep.

Images