CN116499575A - phi-OTDR system for cascade suppression of interference fading of electro-optical modulator and working method thereof - Google Patents
phi-OTDR system for cascade suppression of interference fading of electro-optical modulator and working method thereof Download PDFInfo
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Abstract
The invention provides a phi-OTDR system for cascade suppression of interference fading of an electro-optical modulator, which comprises: the device comprises a laser, a first coupler, an electro-optical modulator, a first bias control device, a Mach-Zehnder modulator, a second bias control device, a pulse generation device, an erbium-doped fiber amplifier, a band-pass filter, an optical circulator, a sensing fiber, a second coupler, a balanced photoelectric detector, a data acquisition card and a computer. According to the invention, the electro-optical modulator and the Mach-Zehnder modulator are cascaded, so that the extinction ratio of the optical pulse of the phi-OTDR system is improved, the influence of the extinction ratio on the detection performance of the phi-OTDR system is reduced, the detection dead zone is reduced, the signal to noise ratio of the system is reduced while the phase demodulation precision is improved, and the sensitivity is higher; the offset voltage of the Mach-Zehnder modulator is arranged at the zero point to realize alternate double pulses, so that the Mach-Zehnder modulator has the characteristics of simple structure and convenience in implementation; meanwhile, in a mode of amplitude evaluation, interference fading is reduced by comprehensive analysis and processing.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to a phi-OTDR system for suppressing interference fading by cascading an electro-optical modulator and a working method thereof.
Background
In recent years, phase-sensitive optical time domain reflectometer (Phase-sensitive Optical Time Domain Reflectometer, Φ -OTDR) technology has become a representative technology in distributed optical fiber vibration sensing technology, and has been applied to various fields such as perimeter security, railway transportation, pipeline security detection, natural disaster detection, geophysical prospecting, and the like. The narrow linewidth light source adopted by the sensing system using the phi-OTDR technology has stronger coherence, so that the intensity of the detected Rayleigh backscattering signal has stronger interference fading, and a detection blind area is easy to cause; and the Extinction Ratio (ER) of the optical pulse generated by the phi-OTDR system can also have a certain influence on the detection performance, and a lower ER can generate higher Rayleigh backward scattering noise, so that interference fading is more obvious, a detection blind area is increased, the phase demodulation precision is seriously influenced, and the signal-to-noise Ratio and the sensing precision of the system are reduced.
Common methods for eliminating coherent fading noise in a phi-OTDR system include pi phase shift pulse anti-interference fading technology and frequency domain regulation anti-interference fading technology, which can effectively eliminate fading noise, but have higher requirements on bandwidth and calculation performance of the system, so that the technologies cannot be widely applied to engineering applications requiring large-bandwidth real-time demodulation signals. The use of a non-linear optical environment (Nonlinear Optical Loop Mirror, NOLM) can enhance the extinction ratio of the pulsed light, which is more suitable for engineering applications, but the pulse extinction ratio and spatial resolution obtained in the same sensing range are insufficient, and higher costs are required if better signal-to-noise ratio and sensing accuracy are required. In order to enable the phi-OTDR system to have higher accuracy and sensing precision, further improvement on the existing scheme is always a research hot spot in the field.
Disclosure of Invention
In order to solve the problems, the invention provides a phi-OTDR system for suppressing interference fading by cascading an electro-optical modulator, a working method, a device, a terminal and a storage medium thereof.
The technical scheme of the invention is realized as follows:
an Φ -OTDR system for suppressing interference fading of an electro-optic modulator cascade, comprising: the system comprises a laser, a first coupler, an electro-optical modulator, a first bias control device, a Mach-Zehnder modulator, a second bias control device, a pulse generation device, an erbium-doped optical fiber amplifier, a band-pass filter, an optical circulator, a sensing optical fiber, a second coupler, a balanced photoelectric detector, a data acquisition card and a computer;
the laser, the first coupler, the electro-optical modulator and the Mach-Zehnder modulator are sequentially connected; the pulse generating device is respectively connected with the electro-optical modulator and the Mach-Zehnder modulator, the output of the first bias control device is connected with the electro-optical modulator, and the output of the second bias control device is connected with the Mach-Zehnder modulator;
the double-pulse light sequentially passes through the erbium-doped fiber amplifier, the band-pass filter and a No. 1 port of the optical circulator;
the port No. 2 of the optical circulator is connected with the sensing optical fiber, and the port No. 3 of the optical circulator is sequentially connected with the second coupler, the balance photoelectric detector, the data acquisition card and the computer;
the first coupler divides the narrow linewidth laser output by the laser into two paths: the first path of detection light and the second path of local oscillation light;
the pulse generating means providing a succession of electrical pulses to drive the electro-optic modulator; the first bias control device locks the bias voltage of the electro-optic modulator at the linear value of the transfer function of the electro-optic modulator through feedback control; the first path of detection light output by the first coupler is sent to the electro-optical modulator to be chopped into light pulses;
the pulse generating means providing continuous electrical pulses to drive and mach-zehnder modulators; the second bias control device controls the bias voltage of the Mach-Zehnder modulator through feedback; the Mach-Zehnder modulator receives the optical pulse and then outputs double-pulse light with 0-pi phase shift;
the double-pulse light passes through a band-pass filter after being amplified by the erbium-doped optical fiber amplifier, enters a No. 2 port from a No. 1 port of the optical circulator and is injected into the sensing optical fiber; after vibration is applied to the sensing optical fiber, rayleigh back scattering light carrying vibration information is output at a port 3;
the Rayleigh backward scattered light carrying vibration information is output to the second coupler through a No. 3 port, mixed with the second path of local oscillation light output by the first coupler, detected by the balance photoelectric detector and collected to a computer through the data collection card, and the computer demodulates a vibration phase signal with obviously reduced interference fading through calculating amplitude.
Further, the laser adopts a narrow linewidth optical fiber laser and outputs high-coherence laser with a narrow linewidth and a wavelength of 1550 nm.
Further, the electro-optical modulator and the Mach-Zehnder modulator are both LiNbO3 type modulators.
An operating method of an electro-optical modulator cascade-connected phi-OTDR system for suppressing interference fading is characterized by comprising the following steps:
s1, the laser emits continuous high-coherence laser light, and the continuous high-coherence laser light is divided into two paths by a first coupler: the first path of detection light and the second path of local oscillation light; the first path of detection light is used as initial detection light to be input to the electro-optical modulator, and the second path of local oscillation light is input to the second coupler;
s2, the first path of detection light output by the first coupler is sent to an electro-optical modulator to be chopped into optical pulses;
s3, the Mach-Zehnder modulator receives the optical pulse and then outputs double-pulse light with 0-pi phase shift;
s4, after the optical power of the double-pulse light with 0-pi phase shift is amplified by an erbium-doped optical fiber amplifier, the double-pulse light enters a No. 2 port from a No. 1 port of the optical circulator through a band-pass filter, and the double-pulse light is injected into the sensing optical fiber to output the Rayleigh back-scattered light carrying vibration information;
s5, outputting the Rayleigh backward scattered light carrying vibration information to the input end of the second coupler through a No. 3 port of the optical circulator to be mixed with a second path of local oscillation light output by the first coupler, so as to obtain a beat frequency signal; the beat frequency signal is detected by the balance photoelectric detector and is collected to a computer through the data collection card; the computer calculates the amplitude value and comprehensively optimizes and demodulates the vibration phase signal with obviously reduced interference fading.
Further, the method further comprises: the pulse generating means simultaneously provides successive electrical pulses to the electro-optic modulator in step S2 and the mach-zehnder modulator in step S3, respectively, to drive the electro-optic modulator and the mach-zehnder modulator.
Further, the first bias control device in step S2 locks the first bias voltage of the electro-optical modulator at the linear value of its transfer function by feedback control, so that it operates under a stable state.
Further, the second bias control device locks the second bias voltage of the mach-zehnder modulator at the linear value of the transfer function thereof through feedback control in step S3, so that the second bias control device operates under a stable state.
Further, in step S4, a vibration event is simulated on the sensing optical fiber, so as to cause a change in the intensity and phase of the double-pulse light output by the optical circulator after No. 2, thereby obtaining the rayleigh backscattered light carrying vibration information.
The invention discloses a phi-OTDR system for cascade suppression of interference fading of an electro-optical modulator and a working method thereof, wherein the system comprises the following steps: the system comprises a laser, a first coupler, an electro-optical modulator, a first bias control device, a Mach-Zehnder modulator, a second bias control device, a pulse generation device, an erbium-doped optical fiber amplifier, a band-pass filter, an optical circulator, a sensing optical fiber, a second coupler, a balanced photoelectric detector, a data acquisition card and a computer; the laser, the first coupler, the electro-optical modulator and the Mach-Zehnder modulator are sequentially connected; the output of the first bias control device is connected with the electro-optical modulator, and the output of the second bias control device is connected with the Mach-Zehnder modulator; the double-pulse light sequentially passes through the erbium-doped optical fiber amplifier, the band-pass filter, the port 1 and the port 2 of the optical circulator and is connected with the sensing optical fiber, and the port 3 of the optical circulator is sequentially connected with the second coupler, the balanced photoelectric detector, the data acquisition card and the computer; the first coupler divides the narrow linewidth laser output by the laser into two paths: the first path of detection light and the second path of local oscillation light; the first bias control device locks the bias voltage of the electro-optic modulator at the linear value of the transfer function of the electro-optic modulator through feedback control; the second bias control device controls the bias voltage of the Mach-Zehnder modulator through feedback; the Mach-Zehnder modulator receives the optical pulse and then outputs double-pulse light with 0-pi phase shift; the double-pulse light passes through a band-pass filter after being amplified by the erbium-doped optical fiber amplifier, enters a No. 2 port from a No. 1 port of the optical circulator and is injected into the sensing optical fiber; after vibration is applied to the sensing optical fiber, rayleigh back scattering light carrying vibration information is output at a port 3; the Rayleigh backward scattered light carrying vibration information is output to the second coupler through a No. 3 port, mixed with the second path of local oscillation light output by the first coupler, detected by the balance photoelectric detector and collected to a computer through the data collection card, and the computer demodulates a vibration phase signal with obviously reduced interference fading through calculating amplitude. Therefore, the method can improve the extinction ratio of the optical pulse of the system by cascading the electro-optical modulator and the Mach-Zehnder modulator, further reduce the influence of the extinction ratio on the detection performance of the phi-OTDR system, and has higher sensitivity; and the bias point of the Mach-Zehnder modulator is set at a linear value, namely a zero point, so that the pulse double-phase modulation is realized while the extinction ratio of the optical pulse is increased, and the Mach-Zehnder modulator has a simple structure and is convenient to realize.
Drawings
The invention will be further described with reference to the accompanying drawings and examples, in which:
FIG. 1 is a schematic diagram of a cascade of electro-optic modulators that suppresses interference fading in a phi-OTDR system provided by an embodiment of the present invention;
FIG. 2 is a schematic diagram of operation of a Mach-Zehnder modulator in a cascade of electro-optic modulators suppressing interference fading in a phi-OTDR system according to an embodiment of the present invention;
FIG. 3 is a schematic view of a contrast device according to an embodiment of the present invention;
FIG. 4 is a diagram showing the phase detection effect of a comparison device according to an embodiment of the present invention;
FIG. 5 is a diagram of the phase detection effect of a phi-OTDR system with cascaded electro-optic modulator suppressing interference fading in an embodiment of the present invention;
fig. 6 is a graph showing the comparison of the leakage light suppression effect of an Φ -OTDR system for suppressing interference fading in cascade with a comparison device of an electro-optical modulator according to an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to provide a clearer understanding of the technical features, objects and effects of the present invention. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
Referring to fig. 1, an embodiment of the present invention provides a Φ -OTDR system for suppressing interference fading in cascade connection of electro-optical modulators, including: the system comprises a laser, a first coupler, an electro-optical modulator, a first bias control device, a Mach-Zehnder modulator, a second bias control device, a pulse generation device, an erbium-doped optical fiber amplifier, a band-pass filter, an optical circulator, a sensing optical fiber, a second coupler, a balanced photoelectric detector, a data acquisition card and a computer;
the laser, the first coupler, the electro-optical modulator and the Mach-Zehnder modulator are sequentially connected; the pulse generating device is respectively connected with the electro-optical modulator and the Mach-Zehnder modulator, the output of the first bias control device is connected with the electro-optical modulator, and the output of the second bias control device is connected with the Mach-Zehnder modulator;
the double-pulse light sequentially passes through the erbium-doped fiber amplifier, the band-pass filter and a No. 1 port of the optical circulator; the port No. 2 of the optical circulator is connected with the sensing optical fiber, and the port No. 3 of the optical circulator is sequentially connected with the second coupler, the balance photoelectric detector, the data acquisition card and the computer;
the first coupler divides the narrow linewidth laser output by the laser into two paths: the first path of detection light and the second path of local oscillation light;
the pulse generating means providing a succession of electrical pulses to drive the electro-optic modulator; the first bias control device locks the bias voltage of the electro-optic modulator at the linear value of the transfer function of the electro-optic modulator through feedback control; the first path of detection light output by the first coupler is sent to the electro-optical modulator to be chopped into light pulses; the pulse generating means providing continuous electrical pulses to drive and mach-zehnder modulators; the second bias control device controls the bias voltage of the Mach-Zehnder modulator through feedback; the Mach-Zehnder modulator receives the optical pulse and then outputs double-pulse light with 0-pi phase shift;
the double-pulse light passes through a band-pass filter after being amplified by the erbium-doped optical fiber amplifier, enters a No. 2 port from a No. 1 port of the optical circulator and is injected into the sensing optical fiber; after vibration is applied to the sensing optical fiber, rayleigh back scattering light carrying vibration information is output at a port 3;
the Rayleigh backward scattered light carrying vibration information is output to the second coupler through a No. 3 port, mixed with the second path of local oscillation light output by the first coupler, detected by the balance photoelectric detector and collected to a computer through the data collection card, and the computer demodulates a vibration phase signal with obviously reduced interference fading through calculating amplitude.
Preferably, the laser adopts a single-frequency narrow linewidth laser, and outputs high-coherence laser with the wavelength of 1550nm and narrow linewidth.
Here, the narrow linewidth high coherence laser frequency is 3kHz.
Preferably, the laser emits a continuous laser beam that is injected into the electro-optic modulator.
Preferably, the electro-optical modulator and the mach-zehnder modulator are both LiNbO3 modulators.
Preferably, the pulse generating means generates electrical pulses having a duration of 40ns and a repetition frequency of 20kHz to drive the electro-optic modulator and the mach-zehnder modulator.
The working method of the phi-OTDR system for suppressing interference fading by cascading the electro-optical modulator provided by the embodiment of the invention comprises the following steps:
s1, the laser emits continuous high-coherence laser light, and the continuous high-coherence laser light is divided into two paths by a first coupler: the first path of detection light and the second path of local oscillation light; the first path of detection light is used as initial detection light to be input to the electro-optical modulator, and the second path of local oscillation light is input to the second coupler;
s2, the first path of detection light output by the first coupler is sent to an electro-optical modulator to be chopped into optical pulses;
s3, the Mach-Zehnder modulator receives the optical pulse and then outputs double-pulse light with 0-pi phase shift;
s4, after the optical power of the double-pulse light with 0-pi phase shift is amplified by an erbium-doped optical fiber amplifier, the double-pulse light enters a No. 2 port from a No. 1 port of the optical circulator through a band-pass filter, and the double-pulse light is injected into the sensing optical fiber to output the Rayleigh back-scattered light carrying vibration information;
s5, outputting the Rayleigh backward scattered light carrying vibration information to the input end of the second coupler through a No. 3 port of the optical circulator to be mixed with a second path of local oscillation light output by the first coupler, so as to obtain a beat frequency signal; the beat frequency signal is detected by the balance photoelectric detector and is collected to a computer through the data collection card; the computer calculates the amplitude value and comprehensively optimizes and demodulates the vibration phase signal with obviously reduced interference fading.
Here, the continuous laser emitted by the laser is divided into two parts of continuous laser by the first coupler, namely the first path of detection light and the second path of local oscillation light.
Further, the method further comprises: the pulse generating means simultaneously provides successive electrical pulses to the electro-optic modulator in step S2 and the mach-zehnder modulator in step S3, respectively, to drive the electro-optic modulator and the mach-zehnder modulator.
Preferably, the synchronous modulation frequency of the pulse generating device is 10kHz, and the synchronous modulation frequency is used for synchronizing the working frequency of the two-stage modulator.
Further, the first path of detection light split by the first coupler in S1 sequentially passes through the cascade connection of the electro-optical modulator and the mach-zehnder modulator in S3, and then outputs a double-pulse light with 0-pi phase shift, and the specific working principle is as follows:
first, the received continuous laser light is converted into optical pulses using the Electro-optical Modulators (EOMs). Although the first bias voltage of the electro-optic modulator is adjusted to the minimum value of the EOM transfer function, continuous laser leakage cannot be completely suppressed. Let P be p And P cw Peak value of the light pulse and continuous laser leakage power respectively, extinction ratio E R ＝P p /P cw In the ideal case, P cw =0, wherein E R Is infinite. The balanced photodetector detects the power P of the Rayleigh back scattered light along the optical fiber at the z-position bp (z) can be defined as:
wherein T is p For pulse duration, α is the attenuation coefficient of the single mode fiber, α R For the Rayleigh backscattering coefficient, c is the speed of light in vacuum, n is the refractive index of the fiber, a is an arbitrary constant, k is an integer, and sinh () is a hyperbolic sine function.
Second, since the laser uses a high coherence laser source, the backscattered light from the continuous laser section may interfere with the continuous electrical pulses received by the electro-optic modulator. Adding the interference component L as an input signal to the received signal of the electro-optical modulator, the power P of the interference component L int (z) the calculation formula is as follows:
wherein T is p For pulse duration, α is the attenuation coefficient of the single mode fiber, α R For the Rayleigh backscattering coefficient, c is the speed of light in vacuum, n is the refractive index of the fiber, a is an arbitrary constant, k is an integer, and sinh () is a hyperbolic sine function.
While the power P of the interference component int Which may be considered as noise caused by the finite ER of the electro-optic modulator, the signal-to-noise ratio of the optical pulses output by the electro-optic modulator may be approximated as:
namely, the signal to noise ratio of the optical pulse output by the electro-optical modulator is calculated as follows:
the signal to noise ratio of the optical pulse output by the final electro-optic modulator is as follows:
wherein T is p For pulse duration, α is the attenuation coefficient of the single mode fiber, α R For the Rayleigh backscattering coefficient, c is the speed of light in vacuum, n is the refractive index of the fiber, a is an arbitrary constant, k is an integer, and sinh () is a hyperbolic sine function.
The final signal-to-noise ratio expression of the optical pulse output by the electro-optical modulator almost represents the lowest signal-to-noise ratio in the phi-OTDR system, and when the ER value is higher, the generated optical pulse has a good signal-to-noise ratio, so that the phi-OTDR system has a high spatial resolution.
Thus, in two cascaded modulator arrangements, the first electro-optic modulator is used to modulate the continuous laser light into optical pulses having a finite ER, and the ER value of the optical pulses entering the second stage modulator is further increased as the two stage modulator has achieved synchronization as the optical pulses pass through the second Mach-Zehnder modulator. In this case, the second modulator may be considered an additional gate, further blocking unwanted background light between the light pulses. The ER values obtained for two cascaded electro-optic modulators are determined by the ER of the individual electro-optic modulator.
Thus, the embodiment of the invention can realize the generation of high ER pulse through cascade connection of the electro-optical modulator and the Mach-Zehnder modulator, improves the extinction ratio of the optical pulse, reduces the influence of the extinction ratio on the detection performance of the phi-OTDR system, and has higher detection sensitivity.
Further, the first bias control device in step S2 locks the first bias voltage of the electro-optical modulator at the linear value of its transfer function by feedback control, so that it operates under a stable state.
The feedback control circuit of the first bias control device is illustratively connected to a first bias voltage of the electro-optical modulator, which is maintained at a linear value of the transfer function of the electro-optical modulator by continuously adjusting the first bias voltage by measuring the light intensity.
Further, the second bias control device in S3 locks the second bias voltage of the mach-zehnder modulator at the linear value of its transfer function through feedback control, so that the second bias control device operates under a stable state.
Illustratively, a feedback control circuit of a second bias control device is coupled to a second bias voltage of the mach-zehnder modulator, the second bias voltage being continuously adjusted by measuring light intensity such that the second bias voltage is maintained at a linear value of a transfer function of the mach-zehnder modulator.
Here, the electro-optical modulator and the mach-zehnder modulator are different in type, so that the normal working voltages of the two modulators are different, and the two modulators are respectively and independently controlled by two bias control devices to select the corresponding correct bias voltages, so that the modulation performances of the electro-optical modulator and the mach-zehnder modulator are respectively improved.
Further, as shown in fig. 2, the working principle of the Mach-zehnder modulator (Mach-Zehnder Modulator, MZM) in S3 is that the driving signal of the Mach-zehnder modulator is set to V pi volts, the bias voltages of the upper and lower arms are adjusted to be near the zero point, that is, the bias voltages of the two arms are both set to 0 volts, and bipolar pulses are generated by using the phase modulation characteristic of the Mach-zehnder modulator, so that the double pulse light with 0 pi phase shift can be output.
Preferably, the delay between the upper and lower arms is set to a delay greater than 80 ns.
Therefore, the embodiment of the invention can increase the extinction ratio of the injected light pulse by setting the bias point of the Mach-Zehnder modulator at the zero point, thereby realizing the bi-phase modulation of the high ER value pulse, and having simple structure and convenient realization.
Further, in S4, a piezoelectric ceramic (PZT) capable of generating sinusoidal vibration is placed somewhere on the optical fiber to be measured, that is, the optical fiber is wound on the PZT, so that a vibration event can be simulated on the optical fiber, thereby causing changes in intensity and phase of rayleigh back-scattered light in the optical fiber, and thus the rayleigh back-scattered light carrying vibration information is obtained.
Further, the Rayleigh scattering light carrying vibration information in S5 is output to the input end of the second coupler through a No. 3 port of the optical circulator to be mixed with a second path of local oscillation light output by the first coupler, so that beat frequency signals are obtained; the beat frequency signals are detected by the balance photoelectric detector and are collected to a computer through the data collection card, and the corresponding vibration phase signals are demodulated by the computer.
Specifically, since the rayleigh backward scattered light carrying vibration information has a phase difference of 0-pi, the dead zone positions generated by interference fading are different; and then can gather two different beat frequency signals in turn in data acquisition card department includes: an odd beat signal and an even beat signal, and the envelope of each of the beat signals is quite different. The computer demodulates the amplitude and the phase difference corresponding to each beat frequency signal by executing Fourier integration and calculating a differential phase; some serious fading noise can be observed in the phase difference result demodulated by each beat signal, and the lower the amplitude of the beat signal is, the larger the phase error is; the magnitude of the amplitude of the beat signal is used as a reference to estimate the accuracy of the phase signal calculated at that location, the accuracy of the phase signal being calculated as:
wherein, amp even And Amp (Amp) odd Respectively, the odd precision and the even precision correspond to the corresponding amplitude demodulated by the two sequences; Δφ odd And delta phi even The respective phase differences calculated from the odd and even beat signals, respectively; a is that th Is a threshold that determines whether the beat signal falls within a fading region.
According to the above formula, erroneous phase data caused by signal fading is discarded, and correctly demodulated phase data is retained; and further, a result of correcting the rayleigh backscattered light phase difference track is obtained, wherein most of fading noise is weakened. Therefore, the invention can use the amplitude of the pulse as a threshold value to optimize the phase information of the double pulse, and finally the phase curve obtained by the combination can reduce the possibility of coherent fading.
As an example, the present invention compares the proposed system with other systems or devices. Specific:
referring to fig. 3, there is a comparative device diagram in the case of only one electro-optic modulator. It can be seen that the comparison means provided by the embodiment of fig. 3 differs from the cascade of electro-optical modulators of fig. 1 in that there is a reduced number of electro-optical modulators of the second stage, i.e. mach-zehnder modulators, and the second bias control means corresponding to the mach-zehnder modulators.
For the comparison device provided in fig. 3, the subsequent digital signal processing steps are the same as that of the conventional sensing pulse signal modulation, after the comparison device demodulates the amplitude and the phase of the received rayleigh scattering signal, a phase difference curve obtained according to the conventional method as shown in fig. 4 can be obtained, and fig. 4 shows a phase detection effect diagram of the comparison device, it can be seen that a large interference fading phenomenon is observed according to the phase difference obtained according to the conventional method, and a plurality of vibrations can be observed in the diagram due to the interference fading effect, namely, a plurality of vibration events occur on the optical fiber to be measured, but other observed vibration events are false positives except for the vibration event actually occurring at a certain position.
In the cascade interference fading suppression phi-OTDR system provided by the invention, the amplitude and phase of a beat signal formed by two pulse signals with pi phase difference are respectively demodulated, the amplitude and phase of the beat signal are used as references to estimate the phase information of the position, the amplitude and phase of two opposite interference fields are comprehensively analyzed, a threshold value for determining whether the signals fall into a fading area is set, error phase data caused by signal fading are discarded, correctly demodulated phase data are reserved, and the correction result of a phase difference curve is comprehensively obtained according to the rule, wherein most of fading noise is eliminated. As shown in fig. 5, a phase detection effect diagram of an Φ -OTDR system for suppressing interference fading by cascading an electro-optical modulator provided by an embodiment of the present invention is shown, where only one distance is found to find obvious vibration, and most of interference fading is eliminated.
As an embodiment, comparing the Φ -OTDR system for suppressing interference fading provided by the embodiment of the present invention with the suppression situation of the leakage light of the comparison device provided by fig. 3, a structure shown in fig. 6 may be obtained, where the upper part of the curve in fig. 6 corresponds to the device provided by fig. 3, and the lower part of the curve in fig. 6 corresponds to the Φ -OTDR system for suppressing interference fading provided by the embodiment of fig. 1. As can be seen from fig. 6, the second electro-optic modulator, the mach-zehnder modulator, is added to the embodiment of the present invention, which results in further reduction of the power of the leakage light, thereby verifying the capability of improving the extinction ratio.
It is noted that relational terms such as "first" and "second", and the like, are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements is inherent to. Without further limitation, an element defined by the phrase "comprising one … …" does not exclude the presence of other like elements in a process, method, article, or apparatus that comprises the element.
Although the invention has been described above with reference to specific embodiments, various modifications may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In particular, the features of the disclosed embodiments may be combined with each other in any manner as long as there is no structural conflict, and the exhaustive description of these combinations is not given in this specification merely for the sake of brevity and resource saving. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed herein, but that the invention will include all embodiments falling within the scope of the appended claims.
Claims (8)
1. An Φ -OTDR system for suppressing interference fading of an electro-optic modulator cascade, comprising: the system comprises a laser, a first coupler, an electro-optical modulator, a first bias control device, a Mach-Zehnder modulator, a second bias control device, a pulse generation device, an erbium-doped optical fiber amplifier, a band-pass filter, an optical circulator, a sensing optical fiber, a second coupler, a balanced photoelectric detector, a data acquisition card and a computer;
the laser, the first coupler, the electro-optical modulator and the Mach-Zehnder modulator are sequentially connected; the pulse generating device is respectively connected with the electro-optical modulator and the Mach-Zehnder modulator, the output of the first bias control device is connected with the electro-optical modulator, and the output of the second bias control device is connected with the Mach-Zehnder modulator;
the double-pulse light sequentially passes through the erbium-doped fiber amplifier, the band-pass filter and a No. 1 port of the optical circulator; the port No. 2 of the optical circulator is connected with the sensing optical fiber, and the port No. 3 of the optical circulator is sequentially connected with the second coupler, the balance photoelectric detector, the data acquisition card and the computer;
the first coupler divides the narrow linewidth laser output by the laser into two paths: the first path of detection light and the second path of local oscillation light;
the pulse generating means providing a succession of electrical pulses to drive the electro-optic modulator; the first bias control device locks the bias voltage of the electro-optic modulator at the linear value of the transfer function of the electro-optic modulator through feedback control; the first path of detection light output by the first coupler is sent to the electro-optical modulator to be chopped into light pulses; the pulse generating means providing continuous electrical pulses to drive and mach-zehnder modulators; the second bias control device controls the bias voltage of the Mach-Zehnder modulator through feedback; the Mach-Zehnder modulator receives the optical pulse and then outputs double-pulse light with 0-pi phase shift;
the double-pulse light passes through a band-pass filter after being amplified by the erbium-doped optical fiber amplifier, enters a No. 2 port from a No. 1 port of the optical circulator and is injected into the sensing optical fiber; after vibration is applied to the sensing optical fiber, rayleigh back scattering light carrying vibration information is output at a port 3;
the Rayleigh backscattering light carrying vibration information and the second local oscillation light output to the second coupler through the port No. 3 and output from the first coupler are mixed and then detected by the balanced photoelectric detector and collected to a computer through the data collection card, and the computer demodulates a vibration phase signal with obviously reduced interference fading through calculating amplitude.
2. The system of claim 1, wherein the laser is a narrow linewidth fiber laser, and outputs a narrow linewidth high coherence laser with a wavelength of 1550 nm.
3. An electro-optic modulator cascade connection suppressing interference fading Φ -OTDR system according to claim 1, characterized in that the electro-optic modulator and the mach-zehnder modulator are both LiNbO3 type modulators.
4. A method of operating a Φ -OTDR system for cascade suppression of interference fading of an electro-optic modulator according to claim 1, the method comprising the steps of:
s1, the laser emits continuous high-coherence laser light, and the continuous high-coherence laser light is divided into two paths by a first coupler: the first path of detection light and the second path of local oscillation light; the first path of detection light is used as initial detection light to be input to the electro-optical modulator, and the second path of local oscillation light is input to the second coupler;
s2, the first path of detection light output by the first coupler is sent to an electro-optical modulator to be chopped into optical pulses;
s3, the Mach-Zehnder modulator receives the optical pulse and then outputs double-pulse light with 0-pi phase shift;
s4, after the optical power of the double-pulse light with 0-pi phase shift is amplified by an erbium-doped optical fiber amplifier, the double-pulse light enters a No. 2 port from a No. 1 port of the optical circulator through a band-pass filter, and the double-pulse light is injected into the sensing optical fiber to output the Rayleigh back-scattered light carrying vibration information;
s5, outputting the Rayleigh backward scattered light carrying vibration information to the input end of the second coupler through a No. 3 port of the optical circulator to be mixed with a second path of local oscillation light output by the first coupler, so as to obtain a beat frequency signal; the beat frequency signal is detected by the balance photoelectric detector and is collected to a computer through the data collection card; the computer calculates the amplitude value and comprehensively optimizes and demodulates the vibration phase signal with obviously reduced interference fading.
5. A method of operating a Φ -OTDR system for suppressing interference fading in a cascade of electro-optic modulators according to claim 4, further comprising: the pulse generating means simultaneously provides successive electrical pulses to the electro-optic modulator in step S2 and the mach-zehnder modulator in step S3, respectively, to drive the electro-optic modulator and the mach-zehnder modulator.
6. A method of operating a Φ -OTDR system for cascaded suppression of interference fading of an electro-optical modulator according to claim 4, wherein the first bias control means locks the first bias voltage of the electro-optical modulator at the linear value of its transfer function by feedback control to operate it under a stable state in step S2.
7. An electro-optic modulator cascade connection interference fading suppressing Φ -OTDR system and its operation method according to claim 4, wherein the second bias control means locks the second bias voltage of the mach-zehnder modulator at the linear value of its transfer function by feedback control in step S3 to operate under a stable state.
8. The system and method for operating the same as defined in claim 4 wherein in step S4 a vibration event is simulated on said sensing fiber to cause a change in the intensity and phase of the double pulse light output by said optical circulator No. 2 to obtain said rayleigh backscattered light carrying vibration information.
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| CN117030000A (en) * | 2023-10-10 | 2023-11-10 | 之江实验室 | Distributed acoustic wave sensing polarization control system and polarization fading inhibition method |
| CN118646485A (en) * | 2024-08-14 | 2024-09-13 | 山东派蒙机电技术有限公司 | Laser communication method, device and equipment |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN117030000A (en) * | 2023-10-10 | 2023-11-10 | 之江实验室 | Distributed acoustic wave sensing polarization control system and polarization fading inhibition method |
| CN117030000B (en) * | 2023-10-10 | 2024-01-12 | 之江实验室 | Distributed acoustic wave sensing polarization control system and polarization fading inhibition method |
| CN118646485A (en) * | 2024-08-14 | 2024-09-13 | 山东派蒙机电技术有限公司 | Laser communication method, device and equipment |
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