CN114598394A - Real-time compensation method for frequency modulation signal intensity error of OFDR system - Google Patents
Real-time compensation method for frequency modulation signal intensity error of OFDR system Download PDFInfo
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- 238000000034 method Methods 0.000 title claims abstract description 24
- 238000002168 optical frequency-domain reflectometry Methods 0.000 title claims 5
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- GQYHUHYESMUTHG-UHFFFAOYSA-N lithium niobate Chemical group [Li+].[O-][Nb](=O)=O GQYHUHYESMUTHG-UHFFFAOYSA-N 0.000 description 1
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- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
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Abstract
The invention discloses a real-time compensation method for frequency modulation signal intensity error of an Optical Frequency Domain Reflectometer (OFDR) system, belonging to the field of optical fiber sensing. The method comprises the following steps: A. collecting frequency modulation signals of an OFDR system and extracting signal data according to a signal period; B. extracting the amplitude variation of each period data; C. superimposing the periodic data; D. moving average downsampling; E. linear interpolation and downsampling again to make the final sampling point number consistent with the frequency modulation signal point number to obtain compensation amount; F. the compensation quantity feedback control modulation signal generation unit is adopted to realize the real-time compensation of the intensity error; G. the compensation effect is evaluated. The invention aims to compensate the intensity error of the frequency modulation signal of the OFDR system by collecting the intensity information of the frequency modulation signal in real time and combining the method of the invention, effectively reduce the attenuation of the intensity of the frequency modulation signal along with the increase of the frequency, improve the signal-to-noise ratio of the system and further improve the measurement range, the measurement precision and the spatial resolution of the OFDR system.
Description
Technical Field
The invention relates to the field of optical fiber sensing, in particular to a real-time compensation method for frequency modulation signal intensity error of an OFDR system.
Background
In recent years, with the development of internet and mobile communication technologies, internet of things technology for "everything interconnection" has received more and more attention. The high-performance sensor is used as a core sensing layer of the Internet of things, and has more and more market demands. The optical reflectometer can be used for realizing distributed sensing as an important component in the field of optical fiber sensing, and is widely applied to an internet of things system, and the Optical Frequency Domain Reflectometer (OFDR) technology becomes an indispensable part of the optical frequency domain reflectometer with unique advantages.
The OFDR technique is based on Frequency Modulated Continuous Wave (FMCW) technique, and adopts a frequency domain positioning mode, which is mainly divided into incoherent detection and coherent detection. In a mainstream coherent detection method, a beam of linear sweep light with high coherence is generally divided into two paths, one path is used as measurement light to be injected into an optical fiber to be detected, the other path is used as reference light to be used for coherent amplification, and the frequency of a radio frequency signal formed by interference between the measurement light and the reference light is different through different arrival times of rayleigh scattering return light signals of the measurement light at different positions of the optical fiber, so that position discrimination is realized on a frequency domain.
One of the most important components in OFDR systems is a swept-frequency light source, and the performance of the swept-frequency light source directly determines the performance of the whole OFDR system. Swept optical sources typically have a linear sweep characteristic, i.e., the instantaneous frequency of the output optical signal varies linearly with time. The implementation method of the frequency sweeping light comprises internal modulation and external modulation, wherein the internal modulation adopts a tunable laser, and the frequency modulation is realized by changing the driving current or the temperature of the laser; the external modulation adopts a single-frequency laser, and the modulation signal loaded on the external optical modulation device is changed to realize frequency modulation.
The external modulation scheme avoids the use of expensive tunable lasers, has the advantage of low cost, and has wider application scenes. In an external modulation scheme, the performance of a modulation signal greatly affects the overall performance of an OFDR system, the attenuation of the intensity of the modulation signal to a certain degree along with the increase of frequency is one of the inevitable problems, the signal-to-noise ratio of the system is reduced due to the imbalance of the intensity, and it is important to compensate for the intensity error of the modulation signal in order to realize the measurement of long distance and high spatial resolution.
Disclosure of Invention
The invention aims to provide a real-time compensation method for frequency modulation signal intensity errors of an OFDR system aiming at the problem that the modulation signal intensity changes along with frequency generally existing in the conventional external modulation OFDR system.
The invention provides the following technical scheme:
a real-time compensation method for intensity error of frequency modulation signals of an OFDR system comprises the following steps:
A. obtaining frequency modulation signal data of an OFDR system by adopting an analog-to-digital converter, and extracting the data into { signal according to a signal periodi}nWherein, signaliFrequency modulation signal data representing the ith period, wherein n represents the total number of periods and is more than 2;
B. respectively performing Hilbert transform on the frequency modulation signal data of each period to obtain amplitude information of each period, and recording the amplitude information as { envelopei}nWherein, envelopeiAmplitude information representing frequency modulated signal data of an ith cycle;
C. since the noise in the amplitude information conforms to the Gaussian distribution and the independent equal distribution, the average value is 0, and each envelope is usediThe addition can reduce the influence of noise
The width of the existing evenlope _ mean is less than any evenlopeiAnd is in envelopeiThe center of (a);
D. and (3) carrying out moving average downsampling on the envelope _ mean: taking a sliding window with the window size of w, taking the average from the first point of an envelope _ mean to the w point of the first down-sampling point, taking the average from the w-k-w/2 point of the envelope _ mean to the w-k + w/2 point of the kth down-sampling point, and taking the average from the last point to the last w point of the envelope _ mean; recording the sampled point as envelope _ downlink sample;
E. performing linear interpolation on the endpoint of the envelope _ down sample, interpolating the endpoint to m endpoints according to the requirement, down-sampling to the actual needed point number of the frequency modulation signal, and marking as the envelope _ complex, namely the compensation reference quantity;
F. measuring the original amount { envelope } of the frequency-modulated signali}nAnd taking the endpoint of the envelope _ complex as a reference, and performing feedback control on the frequency modulation signal generation unit to obtain a new compensated frequency modulation signal.
Preferably, the method further comprises a step G, collecting the compensated frequency modulation signal, and repeating the steps A-G if the compensation requirement is not met.
Preferably, the OFDR system adopts an external modulation method, the frequency modulation signal is used for modulating a single frequency signal into a linear frequency sweep signal, and the modulation signal is loaded on an external optical modulation device as a driving signal to realize scanning of optical frequency and realize frequency modulation continuous wave ranging.
Preferably, the frequency modulation signal generation unit is composed of an FPGA and a DDS, and the control of the frequency modulation signal intensity is realized by adjusting the amplitude of the DDS output signal in real time according to the compensation reference quantity.
Compared with the prior art, the method and the device have the advantages that the intensity error of the frequency modulation signal of the OFDR system is compensated in real time, the attenuation trend of the compensation reference quantity and the intensity of the modulation signal is good in consistency, the attenuation of the intensity of the frequency modulation signal along with the increase of the frequency is effectively reduced, the signal-to-noise ratio of the OFDR system is improved, and the measurement range, the measurement precision and the spatial resolution of the OFDR system are finally improved.
Drawings
Fig. 1 is a schematic diagram of a linear swept optical signal generation principle in an OFDR system.
Fig. 2 is a time domain diagram of a frequency modulated signal before compensation.
Fig. 3 is a time domain plot of a frequency modulated signal for a single period prior to compensation.
FIG. 4 is a comparison of envelope and envelope _ mean.
FIG. 5 is an effect diagram of envelope _ complex and envelope _ mean.
Fig. 6 is a time domain diagram of the compensated fm signal.
Detailed Description
The invention obtains the compensation reference quantity by modulating the signal intensity information in real time and combining the corresponding compensation algorithm, and controls the modulation signal generating unit according to the feedback of the compensation reference quantity to realize the real-time compensation of the modulation signal intensity error.
The method for compensating the strength error of the frequency modulation signal of the OFDR system in real time is specifically described below with reference to the accompanying drawings.
As shown in fig. 1, the generation principle of the linear swept-frequency optical signal in the externally modulated OFDR system is as follows: the single-frequency laser emits a single-frequency optical signal and enters the optical modulation unit, the modulation signal generation unit generates a modulation signal and inputs the modulation signal into the optical modulation unit, the single-frequency optical signal is modulated into a linear frequency-swept optical signal, meanwhile, the modulation signal is processed by the compensation unit in real time, and the compensation reference quantity obtained after processing feeds back and controls the modulation signal generation unit to realize real-time compensation of the intensity error of the modulation signal. The single-frequency laser adopted in the embodiment has the wavelength of 1550nm and the line width of 1kHz, the optical modulation unit is a lithium niobate IQ modulator, and the modulation signal generation unit mainly comprises an FPGA, a DDS and a processing circuit.
The real-time compensation method for the intensity error of the frequency modulation signal of the OFDR system adopted by the compensation unit comprises the following steps:
A. obtaining frequency modulation signal data of an OFDR system by adopting an analog-to-digital converter, and extracting the data into { signal according to signal periodi}nWherein, the lower subscript i represents the ith period, n represents the total number of periods, and n is 8 in this example.
B. Data signal for each periodiPerforming Hilbert transform to obtain amplitude information of data in each period, and recording the amplitude information as { envelope } correspondinglyi}n。
C. Since the noise in the amplitude information conforms to the Gaussian distribution and the independent equal distribution, the average value is 0, and each envelope is usediThe addition can reduce the influence of noise
The width of the existing envelope _ mean is less than any envelopeiAnd is substantially in envelopeiThe envelope _ mean represents the common part of the intensity errors in the n periods after the noise interference is eliminated, namely the part needing compensation.
D. Carrying out moving average down-sampling on the envelope _ mean, namely taking a sliding window with the window size of w, taking the average from the first point of the envelope _ mean to the w-th point of the first down-sampling point, taking the average from w-k-w/2 to w-k + w/2 of the envelope _ mean from the k-th down-sampling point, and taking the average from the last point to the last w point of the envelope _ mean; and recording the point after down sampling as eventope _ downlink sample.
E. And performing linear interpolation on the endpoint of the envelope _ down sample, interpolating the endpoint to m endpoints according to the requirement, down-sampling to the actual needed point number of the frequency modulation signal, and marking as the envelope _ complex, namely the compensation reference quantity.
F. And (3) taking the original amount of the frequency modulation signal as a reference point of the envelope _ complex to feedback control the frequency modulation signal generating unit to obtain a new compensated frequency modulation signal.
G. And C, collecting the compensated frequency modulation signal, and repeating the steps A-G if the frequency modulation signal does not meet the requirement. The requirements to be met can be set by those skilled in the art according to actual conditions.
FIG. 2 is a time domain diagram of a periodic modulation signal before compensation; as shown in fig. 3, which is a time domain diagram of the modulation signal of a single period before compensation, wherein the abscissa is time and also represents frequency, it can be seen that the intensity of the modulation signal fluctuates and attenuates obviously as the frequency increases.
As shown in fig. 4, which is a comparison graph of envelope and envelope _ mean, it can be seen that the influence of noise can be effectively reduced by superposition.
As shown in fig. 5, which is an effect diagram of the envelope _ compensation and the envelope _ mean, it can be seen that the compensation reference has good consistency with the attenuation trend of the modulation signal intensity, and therefore, the modulation signal intensity error can be effectively compensated by feeding back the compensation reference to the modulation signal generating unit.
As shown in fig. 6, which is a time domain diagram of the compensated modulation signal, it can be seen that the modulation signal intensity error is effectively compensated by the compensation method of the present invention, and the signal intensity is uniformly distributed on different frequency components.
In summary, the modulation signal strength error in the OFDR system can be compensated in real time by the method of the present invention. The compensation method provided by the invention is established on the basis of simulation and experiment, and has a good compensation effect.
The above description is only a preferred embodiment of the present invention, and the scope of the claimed invention is not limited thereto. The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and it is therefore intended that all such changes and modifications as fall within the true spirit and scope of the invention as defined by the appended claims be interpreted in accordance with the breadth to which the invention pertains.
Claims (4)
1. A real-time compensation method for frequency modulation signal intensity error of an OFDR system is characterized by comprising the following steps:
A. obtaining frequency modulation signal data of an OFDR system by adopting an analog-to-digital converter, extracting signal data according to a signal period, and marking the signal data as { signali}nWherein, signaliRepresenting the frequency modulation signal data of the ith period, wherein n represents the total number of periods and is more than 2;
B. respectively carrying out Hilbert conversion on the frequency modulation signal data of each period to obtain amplitude information of each period, and recording the amplitude information as { envelopei}nWherein, envelopeiAmplitude information representing the i-th cycle of the frequency modulated signal data;
C. since the noise in the amplitude information conforms to the Gaussian distribution and the independent equal distribution, the average value is 0, and each envelope is usediThe addition can reduce the influence of noise
The width of the existing evenlope _ mean is less than any evenlopeiAnd is in envelopeiThe center of (a);
D. and (3) carrying out moving average downsampling on the envelope _ mean: taking a sliding window with the window size of w, taking the average from the first point of an envelope _ mean to the w point of the first down-sampling point, taking the average from the w-k-w/2 point of the envelope _ mean to the w-k + w/2 point of the kth down-sampling point, and taking the average from the last point to the last w point of the envelope _ mean; recording the sampled point as envelope _ downlink sample;
E. performing linear interpolation on the endpoint of the envelope _ down sample, interpolating the endpoint to m endpoints according to the requirement, then down-sampling to the actual needed endpoint of the frequency modulation signal, and marking as the envelope _ compensate, namely the compensation reference quantity;
F. measuring the original amount { envelope } of the frequency-modulated signali}nAnd taking the point of the compensation reference quantity as a reference, and performing feedback control on the frequency modulation signal generation unit to obtain a new compensated frequency modulation signal.
2. The method for compensating the intensity error of the frequency-modulated signal in the OFDR system of claim 1, further comprising a step G of collecting the compensated frequency-modulated signal, and repeating the steps a to G if the compensation requirement is not met.
3. The method of claim 1, wherein the OFDR system employs an external modulation method, the fm signal is used to modulate a single frequency signal into a linear swept frequency signal, and the modulation signal is applied as a driving signal to an external optical modulation device to achieve optical frequency sweeping and fm continuous wave ranging.
4. The method of claim 1, wherein the fm signal generating unit comprises an FPGA and a DDS, and the control of the fm signal intensity is achieved by adjusting the amplitude of the DDS output signal in real time according to the compensation reference.
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Citations (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5168213A (en) * | 1990-03-13 | 1992-12-01 | Hewlett-Packard Company | Swept signal analysis instrument and method |
| JP2001308815A (en) * | 2000-04-18 | 2001-11-02 | Victor Co Of Japan Ltd | Method and device for generating transmission signal |
| US20130022351A1 (en) * | 2010-04-06 | 2013-01-24 | Nec Corporation | Optical transmitting/receiving system and timing extracting method in optical transmitting/receiving system |
| CN110487313A (en) * | 2019-08-02 | 2019-11-22 | 哈尔滨工业大学 | Light source frequency sweep Nonlinear Self-tuning method in optical frequency domain reflection technology |
| CN111397644A (en) * | 2020-03-26 | 2020-07-10 | 南京大学 | Laser nonlinear tuning effect compensation system and compensation method for optical frequency domain reflectometer |
| CN113804404A (en) * | 2021-08-16 | 2021-12-17 | 广东工业大学 | Light source frequency sweep nonlinear correction method for optical frequency domain polarization crosstalk measurement |
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Patent Citations (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5168213A (en) * | 1990-03-13 | 1992-12-01 | Hewlett-Packard Company | Swept signal analysis instrument and method |
| JP2001308815A (en) * | 2000-04-18 | 2001-11-02 | Victor Co Of Japan Ltd | Method and device for generating transmission signal |
| US20130022351A1 (en) * | 2010-04-06 | 2013-01-24 | Nec Corporation | Optical transmitting/receiving system and timing extracting method in optical transmitting/receiving system |
| CN110487313A (en) * | 2019-08-02 | 2019-11-22 | 哈尔滨工业大学 | Light source frequency sweep Nonlinear Self-tuning method in optical frequency domain reflection technology |
| CN111397644A (en) * | 2020-03-26 | 2020-07-10 | 南京大学 | Laser nonlinear tuning effect compensation system and compensation method for optical frequency domain reflectometer |
| CN113804404A (en) * | 2021-08-16 | 2021-12-17 | 广东工业大学 | Light source frequency sweep nonlinear correction method for optical frequency domain polarization crosstalk measurement |
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