CN116499575A - A Φ-OTDR system in which electro-optic modulators are cascaded to suppress interference fading and its working method - Google Patents

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

A Φ-OTDR system with electro-optic modulators cascaded to suppress interference fading and its operation method

technical field

The present invention relates to but not limited to the technical field of optical fiber sensing, in particular to a Φ-OTDR system in which electro-optic modulators are cascaded to suppress interference fading and a working method thereof.

Background technique

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 in perimeter security, railway transportation, pipeline Safety detection, natural disaster detection, geophysical exploration and other fields. The narrow-linewidth light source used in the sensor system using Φ-OTDR technology has strong coherence, resulting in strong interference fading in the detected Rayleigh backscattering signal intensity, which is easy to cause detection blind spots; and Φ-OTDR system The extinction ratio (Extinction Ratio, ER) of the generated optical pulse will also have a certain impact on its detection performance. A lower ER will generate higher Rayleigh backscattering noise, making the interference fading more obvious and increasing the detection performance. The dead zone seriously affects the accuracy of phase demodulation and reduces the system signal-to-noise ratio and sensing accuracy.

Common methods used to eliminate coherent fading noise in Φ-OTDR systems include π-phase-shift pulse anti-interference fading technology and frequency-domain regulation anti-interference fading technology. These technologies can effectively eliminate fading noise, but at the same time affect the system bandwidth and computing performance There are higher requirements, which prevent these technologies from being widely used in engineering applications that require large-bandwidth real-time demodulation signals. The use of Nonlinear Optical Loop Mirror (NOLM) can enhance the extinction ratio of pulsed light, which is more suitable for engineering applications, but the pulse extinction ratio and spatial resolution obtained in the same sensing range are not enough, such as Needing to obtain better signal-to-noise ratio and sensing accuracy requires higher cost. In order to make the Φ-OTDR system have higher accuracy and sensing precision, the further improvement of the existing scheme has always been a research hotspot in this field.

Contents of the invention

In order to solve the above problems, the present invention provides a Φ-OTDR system in which electro-optic modulators are cascaded to suppress interference fading and its working method, device, terminal and storage medium.

Technical scheme of the present invention is realized like this:

A Φ-OTDR system in which electro-optic modulators are cascaded to suppress interference fading, comprising: a laser, a first coupler, an electro-optic modulator, a first bias control device, a Mach-Zehnder modulator, a second bias Control device, pulse generating device, erbium-doped fiber amplifier, band-pass filter, optical circulator, sensing fiber, second coupler, balanced photodetector, data acquisition card, computer;

The laser, the first coupler, the electro-optic modulator, and the Mach-Zehnder modulator are connected in sequence; the pulse generation device is connected to the electro-optic modulator and the Mach-Zehnder modulator respectively, and the first bias control device The output is connected to the electro-optic modulator, and the output of the second bias control device is connected to the Mach-Zehnder modulator;

The double-pulse light sequentially passes through the No. 1 port of the erbium-doped fiber amplifier, bandpass filter, and optical circulator;

The No. 2 port of the optical circulator is connected to the sensing fiber, and the No. 3 port of the optical circulator is connected to the second coupler, a balanced photodetector, a data acquisition card, and a computer in sequence;

The first coupler divides the narrow-linewidth laser output from the laser into two paths: the first path of detection light and the second path of local oscillator light;

The pulse generation device provides continuous electric 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 The first detection light output by the first coupler is sent to the electro-optic modulator to be chopped into light pulses;

The pulse generating means provides continuous electrical pulses to drive the Mach-Zehnder modulator; the second bias control means controls the bias voltage of the Mach-Zehnder modulator through feedback; the Mach-Zehnder modulator The device outputs double-pulse light with a 0-π phase shift after receiving the light pulse;

The double-pulse light is amplified by an erbium-doped fiber amplifier and then passed through a band-pass filter, and injected into the sensing fiber from port 1 of the optical circulator to port 2; After the vibration is applied to the sensing fiber, the Rayleigh backscattered light carrying the vibration information is output at port 3;

The Rayleigh backscattered light carrying vibration information is output to the second coupler through port 3, mixed with the second local oscillator light output by the first coupler, and then sent to the balanced photodetector It is detected and collected to a computer through the data acquisition card, and the computer demodulates a vibration phase signal with significantly reduced interference fading by calculating the amplitude.

Further, the laser adopts a narrow-linewidth fiber laser, which outputs a narrow-linewidth high-coherence laser with a wavelength of 1550 nm.

Further, both the electro-optic modulator and the Mach-Zehnder modulator are LiNbO3 modulators.

A kind of working method of the Φ-OTDR system of electro-optical modulator cascade suppression interference fading, it is characterized in that, comprises the following steps:

S1. The continuous high-coherence laser light emitted by the laser is divided into two paths by the first coupler: the first path of detection light and the second path of local oscillator light; the first path of detection light is input to the electro-optical modulator as the initial detection light , the second local oscillator light input to the second coupler;

S2. The first detection light output by the first coupler is sent to the electro-optic modulator to be chopped into light pulses;

S3. The Mach-Zehnder modulator outputs double-pulse light with a 0-π phase shift after receiving the light pulse;

S4. After the double pulsed light with 0-π phase shift is amplified through the erbium-doped fiber amplifier for optical power, then enters from the No. 1 port of the optical circulator to the No. 2 port through a bandpass filter, and injects it into the said optical circulator. output the Rayleigh backscattered light carrying vibration information after the sensing fiber;

S5. The Rayleigh backscattered light carrying vibration information is output to the input end of the second coupler through port 3 of the optical circulator to mix with the second local oscillator light output by the first coupler frequency to obtain a beat frequency signal; the beat frequency signal is detected by the balanced photodetector and collected to the computer through the data acquisition card; the computer calculates the amplitude and comprehensively optimizes and demodulates the vibration with significantly reduced interference fading phase signal.

Further, the method further includes: the pulse generating device simultaneously provides continuous electric pulses to the electro-optic modulator in step S2 and the Mach-Zehnder modulator in step S3, so as to drive the electro-optic modulator and the Mach-Zehnder modulator. -Zehnder Modulator.

Further, in step S2, the first bias control device locks the first bias voltage of the electro-optic modulator at the linear value of its transfer function through feedback control, so that it works under a stable state.

Further, in step S3, the second bias control device locks the second bias voltage of the Mach-Zehnder modulator at the linear value of its transfer function through feedback control, so that it works in a stable state under.

Further, in step S4, a vibration event is simulated on the sensing fiber, causing changes in the intensity and phase of the double-pulse light output after No. 2 of the optical circulator, and obtaining the Rui Backscattered light.

The invention discloses a Φ-OTDR system in which electro-optic modulators are cascaded to suppress interference fading and a working method thereof, including: a laser, a first coupler, an electro-optic modulator, a first bias control device, and a Mach-Zehnder modulator , a second bias control device, a pulse generating device, an erbium-doped fiber amplifier, a bandpass filter, an optical circulator, a sensing fiber, a second coupler, a balanced photodetector, a data acquisition card, a computer; the laser, The first coupler, the electro-optic modulator, and the Mach-Zehnder modulator are connected in sequence; the output of the first bias control device is connected to the electro-optic modulator, and the output of the second bias control device is connected to the Mach-Zehnder modulator ; The double-pulse light passes through the erbium-doped fiber amplifier, the band-pass filter, the No. 1 port and the No. 2 port of the optical circulator in turn to connect with the sensing fiber, and the No. 3 port of the optical circulator is connected to the optical circulator. The second coupler, balanced photodetector, data acquisition card, and computer are connected in sequence; the first coupler divides the narrow linewidth laser output from the laser into two paths: the first path of detection light and the second path of local oscillator light ; the first bias control means 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 means controls the Mach-Zeng The bias voltage of the Del modulator; the Mach-Zehnder modulator outputs double pulse light with 0-π phase shift after receiving the optical pulse; the double pulse light is amplified by the optical power of the erbium-doped fiber amplifier and then Through a band-pass filter, it is injected into the sensing fiber from port No. 1 of the optical circulator into port No. 2; after vibration is applied to the sensing fiber, the Rayleigh carrying vibration information is reversed The scattered light is output at port No. 3; the Rayleigh backscattered light carrying vibration information is output to the second coupler through port No. 3 for mixing with the second local oscillator light output by the first coupler After being detected by the balanced photodetector and collected to the computer through the data acquisition card, the computer demodulates the vibration phase signal with significantly reduced interference fading by calculating the amplitude. In this way, the present invention can improve the optical pulse extinction ratio of the system by cascading the electro-optic modulator and the Mach-Zehnder modulator, thereby reducing the impact of the extinction ratio on the detection performance of the Φ-OTDR system, It has higher sensitivity; and the bias point of the Mach-Zehnder modulator is set at a linear value, that is, at the zero point, so as to increase the extinction ratio of the optical pulse and at the same time realize the dual-phase modulation of the pulse, the structure is simple, and the implementation is convenient.

Description of drawings

The present invention will be further described below in conjunction with accompanying drawing and embodiment, in the accompanying drawing:

1 is a schematic structural diagram of a Φ-OTDR system in which electro-optical modulators are cascaded to suppress interference fading provided by an embodiment of the present invention;

Fig. 2 is a working principle diagram of a Mach-Zehnder modulator in a Φ-OTDR system in which an electro-optic modulator is cascaded to suppress interference fading provided by an embodiment of the present invention;

Fig. 3 is the schematic diagram of a kind of comparison device in the embodiment of the present invention;

Fig. 4 is a phase detection effect diagram of a comparison device provided by an embodiment of the present invention;

5 is a phase detection effect diagram of a Φ-OTDR system in which electro-optic modulators are cascaded to suppress interference fading in an embodiment of the present invention;

Fig. 6 is a comparison diagram of the leakage light suppression effect of a Φ-OTDR system in which electro-optic modulators are cascaded to suppress interference fading provided by an embodiment of the present invention and a comparison device.

Detailed ways

In order to have a clearer understanding of the technical features, purposes and effects of the present invention, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

Please refer to Fig. 1, an embodiment of the present invention provides a Φ-OTDR system in which electro-optic modulators are cascaded to suppress interference fading, including: a laser, a first coupler, an electro-optic modulator, a first bias control device, a Mach-Zeng Del modulator, second bias control device, pulse generator, erbium-doped fiber amplifier, band-pass filter, optical circulator, sensing fiber, second coupler, balanced photodetector, data acquisition card, computer;

The laser, the first coupler, the electro-optic modulator, and the Mach-Zehnder modulator are connected in sequence; the pulse generation device is connected to the electro-optic modulator and the Mach-Zehnder modulator respectively, and the first bias control device The output is connected to the electro-optic modulator, and the output of the second bias control device is connected to the Mach-Zehnder modulator;

The double-pulse light sequentially passes through the No. 1 port of the erbium-doped fiber amplifier, the bandpass filter, and the optical circulator; the No. 2 port of the optical circulator is connected to the sensing fiber, and the No. 2 port of the optical circulator is connected to the sensing fiber. No. 3 port is sequentially connected with the second coupler, balanced photodetector, data acquisition card, and computer;

The first coupler divides the narrow-linewidth laser output from the laser into two paths: the first path of detection light and the second path of local oscillator light;

The pulse generation device provides continuous electric 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 The first detection light output by the first coupler is sent to the electro-optic modulator to be chopped into optical pulses; the pulse generating device provides continuous electric pulses to drive the Mach-Zehnder modulator; the second bias control The device controls the bias voltage of the Mach-Zehnder modulator through feedback; the Mach-Zehnder modulator outputs double-pulse light with a 0-π phase shift after receiving the light pulse;

The double-pulse light is amplified by an erbium-doped fiber amplifier and then passed through a band-pass filter, and injected into the sensing fiber from port 1 of the optical circulator to port 2; After the vibration is applied to the sensing fiber, the Rayleigh backscattered light carrying the vibration information is output at port 3;

The Rayleigh backscattered light carrying vibration information is output to the second coupler through port 3, mixed with the second local oscillator light output by the first coupler, and then sent to the balanced photodetector It is detected and collected to a computer through the data acquisition card, and the computer demodulates a vibration phase signal with significantly reduced interference fading by calculating the amplitude.

Preferably, the laser is a single-frequency narrow-linewidth laser, which outputs a highly coherent laser with a wavelength of 1550 nm and a narrow linewidth.

Here, the frequency of the narrow-linewidth high-coherence laser is 3 kHz.

Preferably, the laser emits continuous laser light and injects it into the electro-optic modulator.

Preferably, both the electro-optic modulator and the Mach-Zehnder modulator are LiNbO3 modulators.

Preferably, the pulse generating device generates electrical pulses with a duration of 40 ns and a repetition rate of 20 kHz to drive the electro-optic modulator and the Mach-Zehnder modulator.

An embodiment of the present invention provides a working method of a Φ-OTDR system in which electro-optic modulators are cascaded to suppress interference fading, including the following steps:

S1. The continuous high-coherence laser light emitted by the laser is divided into two paths by the first coupler: the first path of detection light and the second path of local oscillator light; the first path of detection light is input to the electro-optical modulator as the initial detection light , the second local oscillator light input to the second coupler;

S2. The first detection light output by the first coupler is sent to the electro-optic modulator to be chopped into light pulses;

S3. The Mach-Zehnder modulator outputs double-pulse light with a 0-π phase shift after receiving the light pulse;

S4. After the double pulsed light with 0-π phase shift is amplified through the erbium-doped fiber amplifier for optical power, then enters from the No. 1 port of the optical circulator to the No. 2 port through a bandpass filter, and injects it into the said optical circulator. output the Rayleigh backscattered light carrying vibration information after the sensing fiber;

S5. The Rayleigh backscattered light carrying vibration information is output to the input end of the second coupler through port 3 of the optical circulator to mix with the second local oscillator light output by the first coupler frequency to obtain a beat frequency signal; the beat frequency signal is detected by the balanced photodetector and collected to the computer through the data acquisition card; the computer calculates the amplitude and comprehensively optimizes and demodulates the vibration with significantly reduced interference fading phase signal.

Here, the continuous laser light emitted by the laser is divided into two parts of continuous laser light by the first coupler, that is, the first path of detection light and the second path of local oscillator light.

Further, the method further includes: the pulse generating device simultaneously provides continuous electrical pulses to the electro-optic modulator in step S2 and the Mach-Zehnder modulator in step S3 respectively, so as to drive the electro-optic modulator and the Mach-Zehnder modulator. -Zehnder Modulator.

Preferably, the synchronous modulation frequency of the pulse generating device is 10 kHz, which is used to synchronize the working frequency of the two-stage modulator.

Further, the first path of probe light split by the first coupler in S1 is sequentially cascaded with the electro-optic modulator and the Mach-Zehnder modulator in S3, and the output has a 0-π phase Shifted double pulse light, the specific working principle is as follows:

First, the received continuous laser light is converted into optical pulses using the Electro-Optic Modulators (EOM). 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. Assuming that P _p and P _cw are the peak value of the optical pulse and the continuous laser leakage power respectively, then the extinction ratio E _R =P _p /P _cw , under ideal conditions, P _cw =0, where E _R is infinite. The power P _bp (z) of the Rayleigh backscattered light detected by the balanced photodetector along the optical fiber at the z position can be defined as:

Among them, T _p is the pulse duration, α is the attenuation coefficient of the single-mode fiber, α _R is 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, and k is an integer , sinh() is a hyperbolic sine function.

Second, since the laser uses a highly coherent laser source, the backscattered light from the continuous laser portion interferes with the continuous electrical pulses received by the electro-optic modulator. The interference component L is added to the receiving signal of the electro-optic modulator as an input signal, and the power P _int (z) of the interference component L is calculated as follows:

Among them, T _p is the pulse duration, α is the attenuation coefficient of the single-mode fiber, α _R is 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, and k is an integer , sinh() is a hyperbolic sine function.

And, the power P _int of the interference component can be considered as the noise caused by the finite ER of the electro-optic modulator, then the signal-to-noise ratio of the optical pulse output by the electro-optic modulator can be approximated as:

That is, the calculation formula of the signal-to-noise ratio of the optical pulse output by the electro-optic modulator is:

After simplifying the above formula, it can be obtained that the final signal-to-noise ratio of the optical pulse output by the electro-optic modulator is:

Among them, T _p is the pulse duration, α is the attenuation coefficient of the single-mode fiber, α _R is 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, and k is an integer , sinh() is a hyperbolic sine function.

The final signal-to-noise ratio expression of the optical pulse output by the electro-optic modulator almost represents the lowest signal-to-noise ratio in the Φ-OTDR system. When the ER value is high, the generated optical pulse will have a good The signal-to-noise ratio makes the Φ-OTDR system have a high spatial resolution.

Thus, in a two-cascaded modulator setup, the first EO modulator is used to modulate the CW laser into light pulses with finite ER, and when the light pulses pass through the second Mach-Zehnder modulator, due to the two The first-order modulator has been synchronized, and the ER value of the light pulse entering the second-order modulator is further increased. In this case, the second modulator can be thought of as an additional gate, further blocking unwanted background light between light pulses. The ER value obtained by two cascaded EO modulators is determined by the ER of a single EO modulator.

In this way, the embodiments of the present invention can realize the generation of high ER pulses by cascading the electro-optic modulator and the Mach-Zehnder modulator, improve the extinction ratio of optical pulses, and reduce the extinction ratio caused by the detection performance of the Φ-OTDR system. The resulting impact has a higher detection sensitivity.

Further, in step S2, the first bias control device locks the first bias voltage of the electro-optic modulator at the linear value of its transfer function through feedback control, so that it works under a stable state.

Exemplarily, the feedback control circuit of the first bias control device is connected to the first bias voltage of the electro-optic modulator, and the first bias voltage is continuously adjusted by measuring the light intensity, so that the first bias The voltage is maintained at the linear value of the transfer function of the electro-optic modulator.

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 it works between a stable state Down.

Exemplarily, the feedback control circuit of the second bias control device is connected to the second bias voltage of the Mach-Zehnder modulator, and the second bias voltage is continuously adjusted by measuring the light intensity, so that the first Two bias voltages are maintained at the linear value of the transfer function of the Mach-Zehnder modulator.

Here, since the types of the electro-optic modulator and the Mach-Zehnder modulator are different, the normal operating voltages of the two will also be different, so the two modulators are independently controlled by two bias control devices to select The corresponding correct bias voltage can improve the modulation performance of the electro-optic modulator and the Mach-Zehnder modulator respectively.

Further, the working principle of the Mach-Zehnder Modulator (MZM) in S3 is shown in Figure 2, through the feedback control of the second bias control device, the Mach-Zehnder The drive signal of the modulator is set to Vπ volts, and the bias voltages of the upper and lower arms are adjusted to be near zero, that is, the bias voltages of the two arms are both set to 0 volts, and the phase modulation characteristics of the Mach-Zehnder modulator are used to generate dual Polar pulses, and then can output the double pulse light with 0-π phase shift.

Preferably, the delay between the upper and lower arms is set to a delay greater than 80 ns.

In this way, the embodiment of the present invention can increase the extinction ratio of the injected light pulse by setting the bias point of the Mach-Zehnder modulator at zero, and realize the dual-phase modulation of the pulse with high ER value, which has a simple structure and is convenient to implement.

Further, in S4, a piezoelectric ceramic (PZT) capable of generating sinusoidal vibrations is placed somewhere in the optical fiber to be tested, that is, the optical fiber is wound on the PZT, and a vibration event can be simulated on the optical fiber, thereby causing Rayleigh backscattering in the optical fiber Changes in light intensity and phase, so as to obtain the Rayleigh backscattered light carrying vibration information.

Further, the Rayleigh scattered light carrying the vibration information in S5 is output to the input end of the second coupler through the No. 3 port of the optical circulator, and the second local oscillator light output by the first coupler is carried out. Frequency mixing to obtain a beat frequency signal; the beat frequency signal is detected by the balanced photodetector and collected to the computer through the data acquisition card, and the corresponding vibration phase signal is demodulated by the computer.

Specifically, since the Rayleigh backscattered light carrying vibration information has a phase difference of 0-π, the position of the blind area produced by its interference fading will be different; and then the data acquisition card can alternately collect two different beat frequency signals, including odd beat frequency signals and even beat frequency signals, and the envelopes of each beat frequency signal are completely different. The computer demodulates the amplitude and phase difference corresponding to each of the beat frequency signals by performing Fourier integration and calculating the differential phase; in the phase difference results demodulated by each of the beat frequency signals Some severe fading noise can be observed, the lower the amplitude of the beat signal, the larger the phase error; the magnitude of the amplitude of the beat signal is used as a reference to estimate the accuracy of the phase signal calculated at this position, so The formula for calculating the accuracy of the phase signal is as follows:

Among them, Amp _even and Amp _odd are odd precision and _even precision respectively, corresponding to the corresponding amplitudes demodulated by _the two sequences; Phase difference; A _th is the threshold to determine whether the beat signal falls into the fading region.

According to the above formula, the erroneous phase data caused by signal fading is discarded, and the correctly demodulated phase data is retained; and then the correction result of the Rayleigh backscattered light phase difference trajectory is obtained, in which most of the fading noise is weakened up. In this way, the present invention can use the amplitude of the pulses as a threshold to optimize the phase information of the double pulses, and finally the integrated phase curve can reduce the possibility of coherent fading.

As an example, the present invention compares the proposed system with other systems or devices. specific:

Please refer to Figure 3, a diagram of a comparative device with only one electro-optic modulator. It can be seen that the difference between the comparison device provided in the embodiment of Figure 3 and the Φ-OTDR system in which the electro-optic modulators are cascaded to suppress interference fading provided in Figure 1 is that the second-stage electro-optic modulator, that is, the Mach - a Zehnder modulator, and said second bias control means corresponding to said Mach-Zehnder modulator.

For the comparison device provided in Figure 3, the subsequent digital signal processing steps are the same as the traditional sensing pulse signal modulation. After the comparison device demodulates the received Rayleigh scattering signal in amplitude and phase, it can be obtained As shown in Figure 4, according to the phase difference curve obtained by the traditional method, Fig. 4 provides the phase detection effect diagram of the comparison device, as can be seen, the phase difference curve obtained according to the traditional method can be observed to have a large Due to the interference fading effect, multiple vibrations can be observed in the figure, that is, multiple vibration events occur on the optical fiber to be tested, but except for the vibration event that actually occurs at a certain position, other observed of vibration events are false positives.

However, the Φ-OTDR system in which the electro-optical modulators are cascaded to suppress interference fading provided by the present invention performs amplitude and phase demodulation on the beat frequency signals formed by the received two pulse signals with a phase difference of π, respectively, The magnitude of the amplitude is used as a reference to estimate the phase information of the position. Finally, the amplitude and phase of the two opposite interference fields are comprehensively analyzed, and the threshold for determining whether the signal falls into the fading area is set. The wrong phase data caused by signal fading will be discarded, and the correctly demodulated phase data is retained, and according to this rule, the correction result of the phase difference curve is synthesized, and most of the fading noise is eliminated. As shown in Figure 5, it is a phase detection effect diagram of a Φ-OTDR system in which electro-optic modulators are cascaded to suppress interference fading provided by an embodiment of the present invention. It can be found that only one distance is found to have obvious vibration, while Most of the interference fading is eliminated.

As an example, compare a Φ-OTDR system in which electro-optic modulators are cascaded to suppress interference fading provided in FIG. 1 of the embodiment of the present invention and the suppression of leaked light of the comparison device provided in FIG. 3 , and it can be obtained The structure shown in Figure 6, the upper curve in Figure 6 corresponds to the device provided in Figure 3, and the lower curve in Figure 6 corresponds to the Φ- OTDR system. It can be seen from FIG. 6 that the embodiment of the present invention further reduces the power of the leaked light due to the addition of the second electro-optic modulator, the Mach-Zehnder modulator, thereby verifying the ability to increase the extinction ratio.

It should be noted that relative terms such as the terms "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any relationship between these entities or operations. There is such an actual relationship or order between them. Furthermore, the terms "comprising", "comprising" or any other variation thereof are intended to cover a non-exclusive inclusion such that elements inherent in a process, method, article, or apparatus including a series of elements are included. Without further limitations, an element defined by the phrase "comprising a" does not exclude the presence of additional identical elements in the process, method, article or apparatus comprising said element.

While the invention has been described above with reference to specific embodiments thereof, various modifications may be made and equivalents may be substituted for parts thereof without departing from the scope of the invention. In particular, as long as there is no structural conflict, all the features in the specific embodiments disclosed in the present invention can be used in combination with each other in any way, and these combinations are not exhaustively described in this specification. In consideration of omitting space and saving resources. Therefore, the present invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims (8)

1. A Φ-OTDR system in which electro-optic modulators are cascaded to suppress interference fading, comprising: laser, the first coupler, electro-optic modulator, the first bias control device, Mach-Zehnder modulator, the first 2. Bias control device, pulse generating device, erbium-doped fiber amplifier, band-pass filter, optical circulator, sensing fiber, second coupler, balanced photodetector, data acquisition card, computer; The laser, the first coupler, the electro-optic modulator, and the Mach-Zehnder modulator are connected in sequence; the pulse generation device is connected to the electro-optic modulator and the Mach-Zehnder modulator respectively, and the first bias control device The output is connected to the electro-optic modulator, and the output of the second bias control device is connected to the Mach-Zehnder modulator; The double-pulse light sequentially passes through the No. 1 port of the erbium-doped fiber amplifier, the bandpass filter, and the optical circulator; the No. 2 port of the optical circulator is connected to the sensing fiber, and the No. 2 port of the optical circulator is connected to the sensing fiber. No. 3 port is sequentially connected with the second coupler, balanced photodetector, data acquisition card, and computer; The first coupler divides the narrow-linewidth laser output from the laser into two paths: the first path of detection light and the second path of local oscillator light; The pulse generation device provides continuous electric 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 The first detection light output by the first coupler is sent to the electro-optic modulator to be chopped into optical pulses; the pulse generating device provides continuous electric pulses to drive the Mach-Zehnder modulator; the second bias control The device controls the bias voltage of the Mach-Zehnder modulator through feedback; the Mach-Zehnder modulator outputs double-pulse light with a 0-π phase shift after receiving the light pulse; The double-pulse light is amplified by an erbium-doped fiber amplifier and then passed through a band-pass filter, and injected into the sensing fiber from port 1 of the optical circulator to port 2; After the vibration is applied to the sensing fiber, the Rayleigh backscattered light carrying the vibration information is output at port 3; The Rayleigh backscattered light carrying vibration information and the second local oscillator light output to the second coupler via port 3 and the second local oscillator light output by the first coupler are mixed and then balanced by the The photoelectric detector detects and collects it to the computer through the data acquisition card, and the computer demodulates the vibration phase signal with significantly reduced interference fading by calculating the amplitude. 2. the Φ-OTDR system of a kind of electro-optical modulator cascade suppression interference fading according to claim 1, it is characterized in that, described laser adopts narrow-linewidth fiber laser, output wavelength is the high coherence of the narrow linewidth of 1550nm laser. 3. A Φ-OTDR system in which electro-optic modulators are cascaded to suppress interference fading according to claim 1, wherein both the electro-optic modulator and the Mach-Zehnder modulator are LiNbO3 modulators. 4. the working method of the Φ-OTDR system of a kind of electro-optical modulator cascade suppression interference fading according to claim 1, it is characterized in that, the method comprises the following steps: S1. The continuous high-coherence laser light emitted by the laser is divided into two paths by the first coupler: the first path of detection light and the second path of local oscillator light; the first path of detection light is input to the electro-optical modulator as the initial detection light , the second local oscillator light input to the second coupler; S2. The first detection light output by the first coupler is sent to the electro-optic modulator to be chopped into light pulses; S3. The Mach-Zehnder modulator outputs double-pulse light with a 0-π phase shift after receiving the light pulse; S4. After the double pulsed light with 0-π phase shift is amplified through the erbium-doped fiber amplifier for optical power, then enters from the No. 1 port of the optical circulator to the No. 2 port through a bandpass filter, and injects it into the said optical circulator. output the Rayleigh backscattered light carrying vibration information after the sensing fiber; S5. The Rayleigh backscattered light carrying vibration information is output to the input end of the second coupler through port 3 of the optical circulator to mix with the second local oscillator light output by the first coupler frequency to obtain a beat frequency signal; the beat frequency signal is detected by the balanced photodetector and collected to the computer through the data acquisition card; the computer calculates the amplitude and comprehensively optimizes and demodulates the vibration with significantly reduced interference fading phase signal. 5. the working method of the Φ-OTDR system of a kind of electro-optic modulator cascade suppression interference fading according to claim 4, it is characterized in that, the method also comprises: described pulse generation device provides continuous electric pulse to step respectively simultaneously The electro-optic modulator in S2 and the Mach-Zehnder modulator in S3, to drive the electro-optic modulator and the Mach-Zehnder modulator. 6. The working method of a Φ-OTDR system in which electro-optical modulators are cascaded to suppress interference fading according to claim 4, wherein the first bias control device in step S2 controls the electro-optical The first bias voltage of the modulator is locked at the linear value of its transfer function, making it work under a stable state. 7. A kind of electro-optic modulator cascaded suppress interference fading Φ-OTDR system and working method thereof according to claim 4, it is characterized in that, described in step S3, described second bias control device controls described by feedback The second bias voltage of the Mach-Zehnder modulator is locked at the linear value of its transfer function, making it work in a stable state. 8. the Φ-OTDR system and working method thereof of a kind of electro-optic modulator cascade suppression interference fading according to claim 4, it is characterized in that, on the sensing fiber described in step S4, simulate a vibration event, cause described The intensity and phase changes of the double-pulse light output after No. 2 of the optical circulator are used to obtain the Rayleigh backscattered light carrying vibration information.