CN109323750B - Distributed optical fiber vibration sensing system and phase demodulation method - Google Patents
Distributed optical fiber vibration sensing system and phase demodulation method Download PDFInfo
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Abstract
The invention discloses a distributed typeAn optical fiber vibration sensing system and a phase demodulation method belong to the technical field of optical fiber sensing. The invention is based on
The optical fiber vibration sensing principle designs a phase demodulation optical path system and a demodulation algorithm. The distributed optical fiber vibration sensing system of the present invention comprises: the optical fiber laser device comprises a narrow-line-width laser, a first coupler, an acousto-optic modulator, a second coupler, an electro-optic modulator, a first erbium-doped optical fiber amplifier, a circulator, a third coupler, a first tunable optical attenuator, a fourth coupler, a fifth coupler, a second tunable optical attenuator, a sixth coupler, a second erbium-doped optical fiber amplifier, a photoelectric detector, a filter and a digital acquisition processor. The invention introduces two difference frequencies from the same source in the traditional phi-OTDR system, modulates the vibration signal at a certain position of the optical fiber to the first heterodyne frequency, and eliminates the first heterodyne frequency through the other heterodyne frequency. The method simultaneously avoids the influence of low-frequency noise and also avoids the use of complex down-conversion circuit for demodulating information such as phase and the like.
Description
Technical Field
The invention relates to the technical field of distributed optical fiber Rayleigh scattering vibration sensing systems, in particular to a phase demodulation system and a phase demodulation method.
Background
The distributed optical fiber vibration sensor is an optical fiber sensing system which is developed in recent decades and is used for measuring the spatial vibration information distribution in real time. phi-OTDR distributed fiber optic vibration sensing systems utilize Rayleigh scattered signals, a technique that typically employs optical heterodyne to improve sensitivity. The heterodyne method is an interference measurement method, which changes the frequency of a reference signal to generate a frequency difference with a measurement signal, and after the reference signal interferes with the measurement signal, the phase of the interference signal includes a phase modulation term (carrier) and a measured term, and the interference signal is demodulated to obtain the measured phase. This method of introducing a carrier wave in the phase of the interference signal is called heterodyne.
The phase and amplitude are demodulated by the heterodyne method, and the external frequency difference Δ f needs to be determined, for example, in the following patent: CN 207036249U, CN 107976248A, CN 102628698A and the like. Since the external frequency difference generated by the device generating the external frequency difference is absolutely not a fixed value, for example, the british specification of acousto-optic modulators will indicate 80MHZ ± 0.1%. Even if the error of the external frequency difference is only one thousandth, the influence on the result is fatal, so that a plurality of techniques are provided for determining the error of the external frequency difference Δ f (namely Δ w (t) which is different by a constant), and detailed methods are shown in related papers, such as: the collection and demodulation method of heterodyne laser interference signals is summarized in J. laser journal, 2018(1): 20-24. It can be seen that the process of determining the external frequency difference Δ f is relatively complicated, and certain errors exist.
Disclosure of Invention
Aiming at the defects or improvement requirements of the existing distributed vibration sensing system and the phase demodulation method, the invention provides a novel coherent reception distributed vibration sensing system and a phase demodulation method, aiming at more simply demodulating the relevant information of the vibration signal phase along the optical fiber to be detected according to the accuracy.
The technical scheme adopted by the invention for solving the technical problems is as follows:
the distributed optical fiber vibration sensing system comprises a narrow linewidth laser, a first coupler, an acousto-optic modulator, a second coupler, an electro-optic modulator, a first erbium-doped optical fiber amplifier and a circulator which are connected in sequence, wherein one port of the circulator is connected with a sensing optical cable;
the narrow-linewidth laser is connected with the input end of a first coupler, the first output end of the first coupler is connected with the input end of an acousto-optic modulator, the output end of the acousto-optic modulator is connected with the input end of a second coupler, the first output end of the second coupler is connected with the input end of an electro-optic modulator, the output end of the electro-optic modulator is connected with the input end of a first erbium-doped fiber amplifier, the output end of the first erbium-doped fiber amplifier is connected with the first port of a circulator, and the second port of the circulator is connected with a sensing optical cable;
the sensing system also comprises a third coupler, a first tunable optical attenuator, a fourth coupler, a fifth coupler, a second tunable optical attenuator and a sixth coupler;
the second output end of the first coupler is connected with the input end of a third coupler, the first output end of the third coupler is connected with the input end of the first tunable optical attenuator, and the second output end of the third coupler and the second output end of the second coupler are respectively connected with two input ends of a fourth coupler; the output end of the first tunable optical attenuator and the third port of the circulator are respectively connected with two input ends of a fifth coupler, the output end of a fourth coupler is connected with the input end of a second tunable optical attenuator, and the output end of the fifth coupler and the output end of the second tunable optical attenuator are respectively connected with two input ends of a sixth coupler;
the sensing system also comprises a second erbium-doped fiber amplifier, a photoelectric detector, a filter and a signal acquisition processor which are connected in sequence; the input end of the second erbium-doped fiber amplifier is connected with the output end of the sixth coupler.
According to the technical scheme, the third coupler, the fourth coupler, the fifth coupler and the sixth coupler are all equal-division couplers.
According to the technical scheme, laser output by the narrow linewidth laser is divided into two paths through the first coupler, the two paths of laser comprise a first main path light and a first reference light, the main path light enters the acousto-optic modulator to generate a frequency shift quantity, and output light is continuous light; the output light of the acousto-optic modulator enters the second coupler and is divided into two paths, the two paths of output light comprise a second main path light and a second path of reference light, the second main path light enters the electro-optic demodulator, and the second path of reference light enters the fourth coupler.
The invention also provides a phase demodulation method based on the distributed optical fiber vibration sensing optical fiber of claim 1, which comprises the following steps:
laser output by the narrow linewidth laser is divided into two paths through a first coupler, the two paths of laser comprise a first main path light and a first reference light, the main path light enters an acousto-optic modulator to generate a frequency shift quantity, and output light is continuous light; the output light of the acousto-optic modulator enters a second coupler and is divided into two paths, wherein the two paths of output light comprise a second main path light and a second path of reference light, the second main path light enters an electro-optic demodulator, and the second path of reference light enters a fourth coupler;
the second main path light enters the electro-optical modulator to generate pulse light, and the pulse light is amplified by the first erbium-doped fiber amplifier and enters the sensing fiber through the circulator;
a first reference light signal E10 × exp [ -iwt ] of the second output end light of the first coupler is split by the third coupler, the first split light of the first reference light enters the first tunable optical attenuator to be subjected to power adjustment, the amplitude of the optical signal is changed, and the phase of the optical signal is unchanged;
the second output end of the second coupler outputs the second path of reference light signal
E20*exp{-i[w+Δw(t)]t}；
The second path of light splitting of the first path of reference light and the second path of reference light pass through a fourth coupler, and the fourth coupler outputs optical signals as follows: e14 exp [ -iwt ] + E21 exp { -i [ w + Δ w (t) ] t };
the optical signal output by the fourth coupler enters a second tunable optical attenuator to carry out power regulation, the amplitude of the optical signal is changed, and the phase is unchanged;
the signal light reflected from the third port of the circulator carries the vibration information of the external optical cable, and the optical signal is
The attenuated first path of light splitting of the first path of reference light is coupled with the signal light reflected from the third port of the circulator at a fifth coupler, and the output optical signal of the fifth coupler is as follows:
the attenuated first path of reference light, the attenuated second path of reference light and the reflected signal light are mixed at a sixth coupler, and an optical signal output by the sixth coupler is as follows:
the optical signal output by the sixth coupler is amplified by the second erbium-doped fiber amplifier, and the phase is unchanged;
the amplified optical signal is subjected to photoelectric conversion by a photoelectric detector, and the output is an electrical signal:
after passing through a band-pass filter, the electric signal is filtered to remove direct current and high frequency, and the signal is
The filtered signals enter a data acquisition processor for processing and outputting phase related information;
in the above formula, E10, E13, E14, E16, E20, E21, E23, E30, E31 and E32 are all amplitudes, w is optical frequency, Δ w (t) is heterodyne frequency shift amount of the modulator, the heterodyne frequency shift amount varies randomly with time,
for the phase change caused by the intrusion, D is the amplitude,
is the phase of the signal.
The invention has the following beneficial effects: the invention adopts a method of two paths of reference light, eliminates the influence of the precision of solving the external frequency difference on the phase demodulation result, avoids the complex process of determining the external frequency difference and can obtain the information of phase, amplitude and the like.
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 distributed optical fiber vibration sensing system according to an embodiment of the present invention;
fig. 2 is a schematic diagram of coherent reception principle according to an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The distributed optical fiber vibration sensing system comprises a narrow-linewidth laser 1, a first coupler 2, an acousto-optic modulator 3, a second coupler 4, an electro-optic modulator 5, a first erbium-doped optical fiber amplifier 6 and a circulator 7, and further comprises a third coupler 8, a first tunable optical attenuator 9, a fourth coupler 11, a fifth coupler 10, a second tunable optical attenuator 12, a sixth coupler 13, a second erbium-doped optical fiber amplifier 14, a photoelectric detector 15, a filter 16 and a signal acquisition processor 17.
The narrow linewidth laser 1 is connected with the input end of a first coupler 2, the first output end of the first coupler is connected with the input end of an acousto-optic modulator (AOM)3, the output end of the acousto-optic modulator (AOM)3 is connected with the input end of a second coupler 4, the first output end of the second coupler 4 is connected with the input end of an electro-optic modulator (EOM)5, the output end of the electro-optic modulator (EOM)5 is connected with the input end of a first erbium-doped fiber amplifier (EDFA)6, the first erbium-doped fiber amplifier (EDFA)6 is connected with the first port of a circulator 7, and the second port of the circulator 7 is connected with a sensing optical cable. A second output end of the first coupler 2 is connected with an input end of a third coupler 8, a first output end of the third coupler 8 is connected with an input end of a first tunable optical attenuator (VOA)9, and a second output end of the third coupler 8 and a second output end of the second coupler 4 are respectively connected with two input ends of a fourth coupler 11. An output end of the first tunable optical attenuator (VOA)9 and a third port of the circulator 7 are respectively connected with two input ends of a fifth coupler 10, an output end of a fourth coupler 11 is connected with a second tunable optical attenuator (VOA)12, and an output end of the fifth coupler 10 and an output end of the second tunable optical attenuator (VOA)12 are respectively connected with two output ends of a sixth coupler 13. The output end of the sixth coupler 13 is connected with a second erbium-doped fiber amplifier (EDFA)14, a Photoelectric Detector (PD)15, a filter 16 and a digital acquisition processor 17 respectively. And the third coupler to the sixth coupler are all equal-division couplers.
When the system is in operation, the narrow linewidth laser 1 outputs high-quality laser, the high-quality laser is divided into two paths (main path light and first path reference light) through the first coupler 2, the main path light enters the acousto-optic modulator (AOM)3 to generate a frequency shift quantity, and the output light is continuous light. Light output from the acousto-optic modulator (AOM)3 enters the second coupler 4 to be divided into two paths (main path light and second path reference light), and the electro-optic modulator (EOM)5 is used for generating pulses. Then enters a first erbium-doped fiber amplifier (EDFA)6 for amplification, and enters the sensing fiber through a circulator 7. The signal light reflected from the third port of the circulator 7 is mixed with the first path of reference light and the second path of reference light and demodulated.
The optical signal at the second output end of the first coupler 2 is E10 × exp [ -iwt ] and is split by the third coupler 8, the power of the first tunable optical attenuator (VOA)9 is adjusted, the amplitude of the optical signal is changed, and the phase of the optical signal is not changed.
The optical signal at the second output of the second coupler 4 is (note that the external frequency difference Δ w (t) is not a quantitative one, and strictly speaking, varies randomly with time): e20 exp { -i [ w + Δ w (t) ] t }.
The light is split by the fourth coupler 11, the power of the second tunable optical attenuator (VOA)12 is adjusted, the amplitude of the optical signal is changed, and the phase of the optical signal is unchanged.
The third port of the circulator 7 carries vibration information of an external optical cable, and the optical signal is
The output optical signal of the third port of the circulator 7 is coupled with the output signal light of the first tunable attenuator 9 at the fifth coupler 10, and the output optical signal is:
one path of split light of the third coupler 8 is coupled with the second path of reference light at the fourth coupler 11, and the output optical signal is: e14 exp [ -iwt ] + E21 exp { -i [ w + Δ w (t) ] t }
The attenuated first path of reference light, the attenuated second path of reference light, and the reflected signal light are mixed at the sixth coupler 13, and the output optical signal of the sixth coupler 13 is:
the optical signal is amplified by a second Erbium Doped Fiber Amplifier (EDFA)14, with a constant phase.
The Photodetector (PD)15 outputs as electrical signals:
where A is the DC current, Δ w (t) is typically in the MHz range,
the frequency is typically at KHz or lower.
After passing through the band-pass filter 16, the dc and high frequency components can be removed, and the signal is:
wherein D is the amplitude of the vibration,
is the phase of the signal.
The filtered signal is simply processed in the data acquisition processor 17 to output phase-related information.
Compared with the traditional coherent demodulation method, the method of the invention additionally increases the frequency mixing of the frequency-shifted light and the signal light emitted by the laser, and extracts the heterodyne frequency shift quantity which randomly changes along with the time. The traditional coherence comprises heterodyne frequency shift quantity which randomly changes along with time, and the extracted randomly changing along with time is subtracted by a quadratic frequency mixing method, so that the influence of the precision of solving the external frequency difference on a phase demodulation result is eliminated, and the complicated process of determining the external frequency difference is avoided
From the above process, it can be seen that: 1) the heterodyne method avoids the influence of low-frequency noise; 2) the two paths of heterodyne signals are subjected to heterodyne again, so that the complex process of solving the real-time heterodyne frequency is avoided, and the precision is higher; 3) phase, amplitude, etc. information can be demodulated.
It will be understood that modifications and variations can be made by persons skilled in the art in light of the above teachings and all such modifications and variations are intended to be included within the scope of the invention as defined in the appended claims.
Claims (4)
1. A distributed optical fiber vibration sensing system is characterized by comprising a narrow linewidth laser, a first coupler, an acousto-optic modulator, a second coupler, an electro-optic modulator, a first erbium-doped optical fiber amplifier and a circulator which are connected in sequence;
the narrow-linewidth laser is connected with the input end of a first coupler, the first output end of the first coupler is connected with the input end of an acousto-optic modulator, the output end of the acousto-optic modulator is connected with the input end of a second coupler, the first output end of the second coupler is connected with the input end of an electro-optic modulator, the output end of the electro-optic modulator is connected with the input end of a first erbium-doped optical fiber amplifier, the output end of the first erbium-doped optical fiber amplifier is connected with the first port of a circulator, and the second port of the circulator is connected with a sensing optical cable;
the sensing system also comprises a third coupler, a first tunable optical attenuator, a fourth coupler, a fifth coupler, a second tunable optical attenuator and a sixth coupler;
the second output end of the first coupler is connected with the input end of a third coupler, the first output end of the third coupler is connected with the input end of the first tunable optical attenuator, and the second output end of the third coupler and the second output end of the second coupler are respectively connected with two input ends of a fourth coupler; the output end of the first tunable optical attenuator and the third port of the circulator are respectively connected with two input ends of a fifth coupler, the output end of a fourth coupler is connected with the input end of a second tunable optical attenuator, and the output end of the fifth coupler and the output end of the second tunable optical attenuator are respectively connected with two input ends of a sixth coupler;
the sensing system also comprises a second erbium-doped fiber amplifier, a photoelectric detector, a filter and a signal acquisition processor which are connected in sequence; and the input end of the second erbium-doped fiber amplifier is connected with the output end of the sixth coupler.
2. The distributed optical fiber vibration sensing system of claim 1, wherein the third coupler, the fourth coupler, the fifth coupler, and the sixth coupler are all equal-division couplers.
3. The distributed optical fiber vibration sensing system according to claim 1, wherein the laser light output from the narrow linewidth laser is divided into two paths by the first coupler, the two paths include a first main path light and a first reference light, the main path light enters the acousto-optic modulator to generate a frequency shift amount, and the output light is continuous light; the output light of the acousto-optic modulator enters the second coupler and is divided into two paths, the two paths of output light comprise a second main path light and a second path of reference light, the second main path light enters the electro-optic demodulator, and the second path of reference light enters the fourth coupler.
4. A phase demodulation method for a distributed optical fiber vibration sensing system according to claim 1, comprising the steps of:
laser output by the narrow linewidth laser is divided into two paths through a first coupler, the two paths of laser comprise a first main path light and a first reference light, the main path light enters an acousto-optic modulator to generate a frequency shift quantity, and output light is continuous light; the output light of the acousto-optic modulator enters a second coupler and is divided into two paths, wherein the two paths of output light comprise a second main path light and a second path of reference light, the second main path light enters an electro-optic demodulator, and the second path of reference light enters a fourth coupler;
the second main path light enters the electro-optical modulator to generate pulse light, and the pulse light is amplified by the first erbium-doped fiber amplifier and enters the sensing fiber through the circulator;
the second output end of the first coupler outputs a first path of reference light signal E10 × exp [ -iwt ], the first path of reference light signal passes through the third coupler for light splitting, the first path of reference light signal enters the first tunable optical attenuator for power adjustment, the amplitude of the optical signal is changed, and the phase is unchanged;
the second output end of the second coupler outputs the second path of reference light signal
E20*exp{-i[w+Δw(t)]t}；
The second path of light splitting of the first path of reference light and the second path of reference light pass through a fourth coupler, and the fourth coupler outputs optical signals as follows: e14 exp [ -iwt ] + E21 exp { -i [ w + Δ w (t) ] t };
the optical signal output by the fourth coupler enters a second tunable optical attenuator to carry out power regulation, the amplitude of the optical signal is changed, and the phase is unchanged;
the signal light reflected from the third port of the circulator carries the vibration information of the external optical cable, and the optical signal is
The attenuated first path of light splitting of the first path of reference light is coupled with the signal light reflected from the third port of the circulator at a fifth coupler, and the output optical signal of the fifth coupler is as follows:
the attenuated first path of reference light, the attenuated second path of reference light and the reflected signal light are mixed at a sixth coupler, and an optical signal output by the sixth coupler is as follows:
the optical signal output by the sixth coupler is amplified by the second erbium-doped fiber amplifier, and the phase is unchanged;
the amplified optical signal is subjected to photoelectric conversion by a photoelectric detector, and the output is an electrical signal:
wherein A is the DC current, Δ w (t) is at a frequency in the order of MHz,
the frequency is in the order of KHz or lower;
after passing through a band-pass filter, the electric signal is filtered to remove direct current and high frequency, and the signal is
The filtered signals enter a data acquisition processor for processing and outputting phase related information;
in the above formula, E10, E13, E14, E16, E20, E21, E23, E30, E31 and E32 are amplitudes, w is optical frequency, Δ w (t) is heterodyne frequency shift amount of the acousto-optic modulator, the heterodyne frequency shift amount varies randomly with time,
for the phase change caused by the intrusion, D is the amplitude.
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