An Optical Fiber Sensing Phase Demodulation Method and System

Abstract

The present invention discloses an optical fiber sensing phase demodulation method and system, belonging to the field of optical sensing technology. The present invention uses a laser with a linewidth <1 MHz as the light source, eliminates the low-frequency or high-frequency current modulation signals introduced at the light source by introducing a reference interferometer and combining the cross-subtraction algorithm and the arctangent operation, reduces the requirement for the linewidth of the light source, and significantly reduces the system cost and complexity; a large-phase excitation signal is introduced through the low-frequency or high-frequency current modulation signal to force the ellipse to close, avoiding the limitation of the traditional ellipse fitting method on the signal amplitude ≥π / 2, solving the parameter mismatch problem, and at the same time avoiding the introduction of a modulator or a frequency shifter, further reducing the volume and power consumption of the system; by introducing a reference interferometer with a symmetric structure and combining the cross-subtraction algorithm to eliminate the light source modulation signal and the frequency jitter noise, the demodulation accuracy and stability are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of optical sensing, and particularly to a fiber optic sensing phase demodulation method and system. Background Art

[0002] Fiber optic sensing systems have attracted increasing attention and importance in the industrial and engineering fields due to their high sensitivity and strong anti-electromagnetic interference ability. The sensing mechanism based on optical fibers is gradually developing and is expected to replace the existing electrical sensing mechanisms in multiple fields. Currently, fiber optic sensing systems generally consist of Mach-Zehnder or Michelson interferometer structures. One or both arms of the interferometer are wound around the sensitive element. When the interferometer senses the external signal to be measured, a phase difference will be generated between the two fiber arms. By detecting and demodulating the interference signals generated by the two arms, the signal to be measured can be demodulated. The system generally uses a narrow linewidth semiconductor laser or a narrow linewidth fiber laser, with a laser linewidth of about 1 kHz magnitude, and then uses various networking technologies to form a large-scale sensing array.

[0003] In current fiber optic sensing systems, in order to achieve low system phase noise and high phase resolution, semiconductor lasers or fiber lasers with a linewidth of kHz magnitude are required. The phase generated carrier method or the heterodyne method is mostly used for demodulation, or a demodulation scheme based on the ellipse fitting method, etc. Fiber optic sensing systems based on the phase generated carrier method or the heterodyne method both require the use of lasers with a very narrow linewidth, generally with a linewidth of kHz magnitude, and a modulator or a frequency shifter needs to be introduced inside the light source or in the sensing optical path. This will not only increase the complexity of the system, but also increase the cost and power consumption of the system, which is not conducive to the miniaturization and integration of the system. The demodulation scheme based on the ellipse fitting method can use the method of a balanced interferometer to reduce the requirement for the linewidth of the light source. However, in order to achieve high-precision fitting, it is generally required that the signal phase change is not less than π / 2, and the fitted ellipse arc is not less than 1 / 4 of a circular arc. Otherwise, the system cannot meet the pickup of small signals. In order to achieve the pickup of small signals, a large signal excitation is also used to complete an ellipse first, and then the signal is demodulated. However, when using the method of one-time ellipse fitting, with the increase of the working time of the sensing system, due to reasons such as the fluctuation of the light source power, the parameters of the one-time ellipse fitting will be mismatched, resulting in poor demodulation stability. Summary of the Invention

[0004] To address the above problems, a fiber optic sensing phase demodulation method is provided. The present invention reduces the requirement for the line width of the light source through the design of a quasi-balanced interferometer, significantly reducing the system cost and complexity; a large phase excitation signal is introduced through a low-frequency or high-frequency current modulation signal, forcing the ellipse to close, avoiding the limitation of the signal amplitude (≥π / 2) in the traditional ellipse fitting method, and solving the problem of parameter mismatch. At the same time, the introduction of a modulator or a frequency shifter is avoided, further reducing the volume and power consumption of the system; by introducing a reference interferometer with a symmetric structure and combining the cross-subtraction algorithm, the light source modulation signal and the frequency jitter noise are eliminated, improving the demodulation accuracy and stability.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows.

[0006] A fiber optic sensing phase demodulation method includes the following steps:

[0007] Step 1: Apply a low-frequency or high-frequency current modulation signal outside the frequency band of the signal to be measured to a wide-linewidth light source with a line width <1 MHz.

[0008] Step 2: Construct a sensing interferometer and a reference interferometer with the same structural parameters and an arm length difference ΔL = 0.5 - 2 cm through a 3×3 coupler to collect a sensing signal containing the signal to be measured and the current modulation signal and a reference signal containing only the current modulation signal respectively, and output two sensing optical signals and two reference optical signals correspondingly.

[0009] Step 3: Perform ellipse fitting operations on the two sensing optical signals and the two reference optical signals to obtain two pairs of orthogonal signals with a phase difference of π / 2.

[0010] Step 4: Eliminate the low-frequency or high-frequency current modulation signal introduced at the light source through cross-subtraction algorithm and arctangent operation on the two pairs of orthogonal signals to obtain the signal phase to be measured.

[0011] Preferably, the two sensing optical signals and the two reference optical signals in Step 2 can be expressed as:

[0012]

[0013] Where, A S Is the DC term output by the sensing interferometer, B S Is the AC term output by the sensing interferometer, A r Is the DC term output by the reference interferometer, B r Is the AC term output by the reference interferometer, Is the phase difference generated by the signal to be measured, Is the phase difference generated by the current modulation signal, And is the phase difference generated by the light source frequency jitter at the outputs of the sensing interferometer and the reference interferometer respectively.

[0014] Preferably, the phase differences of the two sensing optical signals and the two reference optical signals in step 2 are both 2π / 3.

[0015] Preferably, the two pairs of orthogonal signals obtained by the ellipse fitting operation in step 3 can be expressed as:

[0016]

[0017] where is the phase difference generated by the target signal, is the phase difference generated by the current modulation signal, and are the phase differences generated after the light source frequency jitter at the outputs of the sensing interferometer and the reference interferometer respectively.

[0018] Preferably, the cross-subtraction algorithm and the arctangent operation in step 4 can be expressed as:

[0019]

[0020]

[0021] where is the phase difference generated by the signal to be measured, S 1 and S 2 are a pair of orthogonal signals corresponding to the output of the sensing interferometer; R 1 and R 2 are a pair of orthogonal signals corresponding to the output of the reference interferometer; Z 1 and Z 2 are a pair of orthogonal signals obtained after the cross-subtraction algorithm operation.

[0022] Preferably, the sensing interferometer includes a sensing arm wound around a signal-sensitive element and a reference arm wound around a signal-insensitive element or a signal-sensitive element.

[0023] Preferably, both arms of the reference interferometer are wound around a signal-insensitive element.

[0024] An optical fiber sensing phase demodulation system includes a wide-linewidth semiconductor laser with a linewidth < 1 MHz; a power supply driving and modulation module for outputting low-frequency or high-frequency current modulation signals and driving signals other than the signal to be measured to the semiconductor laser; an isolator for isolating the return light in the optical path; a 1×2 coupler for beam splitting; a sensing interferometer and a reference interferometer based on a 3×3 coupler with the same structural parameters and an arm length difference ΔL = 0.5 - 2 cm; a photoelectric converter for converting the optical signal into an electrical signal, and a signal acquisition and processing module for calculating and outputting the signal to be measured by using the above phase demodulation method.

[0025] Preferably, the frequency band of the signal to be measured is 10 Hz - 3 kHz, and the corresponding low-frequency current modulation signal output by the power supply driving and modulation module is < 10 Hz or the high-frequency current modulation signal is > 3 kHz.

[0026] Preferably, the low-frequency current modulation signal is 3 Hz or the high-frequency current modulation signal is 3.5 kHz.

[0027] Due to the above technical solutions, the present invention has the following beneficial effects.

[0028] (1) Based on the optical interference principle, the present invention converts the signal to be measured into a phase change of light through a quasi-balanced fiber optic interferometer (including but not limited to Michelson interferometer, Mach-Zehnder interferometer) composed of a 3×3 coupler, generating two interference signals with a phase difference of 2π / 3. By using the quasi-balanced fiber optic interferometer, the requirement for the linewidth of the laser can be relaxed, and at the same time, low-frequency or high-frequency current modulation of the light source can be realized.

[0029] (2) By modulating the current of the semiconductor laser, the present invention introduces high-frequency or low-frequency phase signals outside the bandwidth of the signal to be measured, so that the elliptical signal output by the interferometer can reach a closed state.

[0030] (3) A reference interferometer with the same structural parameters is introduced into the structure of the sensing interferometer. By modulating the laser with a low-frequency or high-frequency current modulation signal, a large-phase excitation signal is introduced to force the ellipse to close. By combining the output signals of the sensing interferometer and the reference interferometer, the interference of the modulation signal is eliminated, and real-time ellipse fitting demodulation is realized.

[0031] (4) The present invention uses a laser with a linewidth < 1 MHz (a narrow linewidth light source at the kHz level is required in the prior art) as the light source. By introducing a reference interferometer and combining the cross-subtraction algorithm and the arctangent operation, the low-frequency or high-frequency current modulation signals introduced at the light source are eliminated, the requirement for the linewidth of the light source is reduced, and the system cost and complexity are significantly reduced; by introducing a large-phase excitation signal through the low-frequency or high-frequency current modulation signal, the ellipse is forced to close, avoiding the limitation of the signal amplitude (≥π / 2) in the traditional ellipse fitting method, solving the parameter mismatch problem, and at the same time avoiding introducing a modulator or a frequency shifter, further reducing the volume and power consumption of the system; by introducing a reference interferometer with a symmetric structure and combining the cross-subtraction algorithm to eliminate the light source modulation signal and the frequency jitter noise, the demodulation accuracy and stability are improved.

[0032] (5) The present invention uses a fiber quasi-balanced fiber interferometer composed of a 3×3 fiber coupler to realize signal detection. There is a centimeter-level difference between the sensing arm and the reference arm of the interferometer, which can not only reduce the requirement for using a laser with a very narrow linewidth (generally at the kHz level) for the sensing system to achieve high-precision signal detection, but also modulate the laser.

[0033] (6) Through the quasi-balanced dual-interferometer design, the present invention is compatible with a laser with a linewidth < 1 MHz, relaxes the requirement for a narrow linewidth of the light source, and can significantly reduce the light source cost; through the current modulation signal, the ellipse is forced to close, avoiding the parameter mismatch problem in the traditional single ellipse fitting; through the reference interferometer with the same structural parameters, the current modulation signal is separated for accurate detection, and the light source modulation signal and the frequency jitter noise are eliminated by combining the cross-subtraction algorithm, improving the demodulation accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The following details the fabrication and application of the preferred embodiments of the present invention. It should be understood, however, that the present invention provides many applicable inventive concepts that can be embodied in various specific environments. The specific embodiments discussed are only for illustrating the specific ways of manufacturing and using the present invention and do not limit the scope of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0035] Figure 1 It is a flowchart of the phase demodulation method of the present invention.

[0036] Figure 2 It is a comparison diagram of the Lissajous figure closed by the low-frequency current modulation signal of the present invention.

[0037] Figure 3 It is a schematic structural diagram of the system of the present invention.

[0038] Figure 4 It is a comparison diagram of the data processing effect after the monitoring method of the present invention.

[0039] Figure 5 This is a comparison chart of the amplitude stability differences between the first-order ellipse fitting algorithm and the real-time ellipse fitting algorithm of the present invention. Detailed implementation manners

[0040] The following will discuss in detail the fabrication and application of the preferred embodiments of the present invention. However, it should be understood that the present invention provides many applicable inventive concepts, which can be embodied in various specific environments. The specific embodiments discussed are only for illustrating the specific ways of manufacturing and using the present invention, and do not limit the scope of the present invention.

[0041] The present invention uses a laser with a linewidth < 1 MHz as the light source, eliminates the low-frequency or high-frequency current modulation signals introduced at the light source by introducing a reference interferometer and combining the cross-subtraction algorithm and the arctangent operation, reduces the requirements for the light source linewidth, and significantly reduces the system cost and complexity; introduces a large-phase excitation signal through the low-frequency or high-frequency current modulation signal, forces the ellipse to close, avoids the limitation of the traditional ellipse fitting method on the signal amplitude (≥π / 2), solves the parameter mismatch problem, and at the same time avoids introducing a modulator or a frequency shifter, further reducing the volume and power consumption of the system; introduces a reference interferometer with a symmetric structure, combines the cross-subtraction algorithm to eliminate the light source modulation signal and the frequency jitter noise, and improves the demodulation accuracy and stability.

[0042] The following will be further elaborated in conjunction with the attached Figures 1-5 figures.

[0043] Embodiment 1

[0044] As Figure 1 shown, a fiber optic sensing phase demodulation method includes the following steps: Step 1: Apply a low-frequency or high-frequency current modulation signal outside the frequency band of the signal to be measured to a broadband light source with a linewidth < 1 MHz.

[0045] Step 2: Construct a sensing interferometer and a reference interferometer with the same structural parameters and an arm length difference ΔL = 0.5 - 2 cm through a 3×3 coupler to respectively collect a sensing signal containing the signal to be measured and the current modulation signal and a reference signal containing only the current modulation signal, and correspondingly output two sensing optical signals and two reference optical signals, and the phase difference between the two sensing optical signals and the two reference optical signals is 2π / 3.

[0046] The sensing interferometer includes a sensing arm wound around a signal-sensitive element and a reference arm wound around a signal-insensitive element or a signal-sensitive element. The two arms of the reference interferometer are both wound around the signal-insensitive element. The two sensing optical signals and the two reference optical signals can be expressed as:

[0047]

[0048] Among them, A S is the DC term output by the sensing interferometer, B S is the AC term output by the sensing interferometer, A r is the DC term output by the reference interferometer, B r is the AC term output by the reference interferometer, is the phase difference generated by the signal to be measured, is the phase difference generated by the current modulation signal, and are the phase differences generated by the light source frequency jitter at the outputs of the sensing interferometer and the reference interferometer respectively.

[0049] Step 3: Perform elliptical fitting operations on the two paths of sensing optical signals and the two paths of reference optical signals to obtain two pairs of orthogonal signals with a phase difference of π / 2. The two pairs of orthogonal signals can be expressed as:

[0050]

[0051] Among them, is the phase difference generated by the target signal, is the phase difference generated by the current modulation signal, and are the phase differences generated by the light source frequency jitter at the outputs of the sensing interferometer and the reference interferometer respectively after the output.

[0052] Step 4: Eliminate the low-frequency or high-frequency current modulation signals introduced at the light source by performing cross-subtraction algorithms and arctangent operations on the two pairs of orthogonal signals to obtain the phase signal to be measured. The cross-subtraction algorithms and arctangent operations can be expressed as:

[0053]

[0054]

[0055] Among them, is the phase difference generated by the signal to be measured, S 1 and S 2 are a pair of orthogonal signals corresponding to the output of the sensing interferometer; R 1 and R 2 are a pair of orthogonal signals corresponding to the output of the reference interferometer; Z 1 and Z 2 are a pair of orthogonal signals obtained after the cross-subtraction algorithm operation.

[0056] Embodiment 2

[0057] Such as Figure 3An optical fiber sensing phase demodulation system as shown includes a wide-linewidth semiconductor laser with a linewidth < 1MHz, which provides a light source with a linewidth < 1MHz for the sensing system. An isolator for isolating the return light in the optical path. A 1×2 coupler for splitting light. A photoelectric converter PD for converting an optical signal into an electrical signal, and a signal acquisition and processing module that uses the above phase demodulation method to calculate and output the phase signal to be measured.

[0058] A power supply drive and modulation module for outputting a low-frequency or high-frequency current modulation signal and a drive signal other than the signal to be measured to the semiconductor laser. The monitoring frequency range, i.e., frequency band, of the signal to be measured is 10Hz to 3kHz; the low-frequency or high-frequency current modulation signals respectively refer to a low-frequency current modulation signal < 10Hz or a high-frequency current modulation signal > 3kHz. In this embodiment, the low-frequency current modulation signal emitted by the power supply drive and modulation module is 3Hz.

[0059] It further includes a sensing interferometer and a reference interferometer with the same structural parameters and an arm length difference ΔL = 0.5cm constructed based on a 3×3 coupler; the two reference arms and the sensing arm of the sensing interferometer are respectively wound around two signal-sensitive elements to form a push-pull fiber optic interferometer; the sensing arm of the sensing interferometer is wound around the signal-sensitive element, and the other reference arm is wound around the signal-insensitive element to form a non-push-pull fiber optic interferometer. Both arms of the reference interferometer are wound around the signal-insensitive element. The sensing interferometer and the reference interferometer can adopt a Michelson interferometer or a Mach-Zehnder interferometer. The sensing interferometer and the reference interferometer also include corresponding 2 Faraday rotation mirrors FRM for reflecting the incident light.

[0060] One path on the left side of the 3×3 coupler of the sensing interferometer serves as the input end of the signal after splitting light, and the other two paths are for picking up the external vibration signal and the low-frequency or high-frequency current modulation signal after passing through the sensing interferometer. In this embodiment, the two paths are for picking up the external vibration signal and the 3Hz low-frequency current modulation signal after passing through the sensing interferometer, and form two output ports of the interference signal in the 3×3 coupler; one path on the left side of the 3×3 coupler of the reference interferometer serves as the input end of the signal after splitting light, and the other two paths are for picking up the external vibration signal and the low-frequency or high-frequency current modulation signal after passing through the reference interferometer. In this embodiment, the two paths are for picking up the 3Hz low-frequency current modulation signal after passing through the reference interferometer, and form two output ports of the interference signal in the 3×3 coupler.

[0061] In this system, a large-phase signal is introduced, so that the elliptical signals output by the quasi-balanced sensing interferometer composed of a 3×3 coupler and the quasi-balanced reference interferometer composed of a 3×3 coupler are closed. The elliptical signal output by the quasi-balanced sensing interferometer contains not only a 3Hz current modulation signal but also a vibration acceleration signal with a frequency range of 10Hz to 3000Hz of the signal to be measured. The elliptical signal output by the quasi-balanced reference interferometer only contains a 3Hz current modulation signal. Through the cross-subtraction algorithm, a vibration acceleration signal with a frequency range of 10Hz to 3000Hz can be obtained.

[0062] The principle and process of this fiber optic sensing phase demodulation system are as follows.

[0063] The power supply drive and modulation module generates a drive signal for the semiconductor laser, driving the semiconductor laser to generate a laser with a linewidth <1MHz. At the same time, the power supply drive and modulation module generates a low-frequency or high-frequency current modulation signal. The low-frequency current modulation signal emitted by the power supply drive and modulation module in this embodiment is 3Hz. This current modulation signal is a large-amplitude modulation signal, which can make the two elliptical fitting signals output by the sensing interferometer reach a closed state. The Lissajous figure without the modulation signal and the Lissajous figure after adding the 3Hz low-frequency current modulation signal are as Figure 2 shown. It can be clearly seen that the two elliptical fitting signals output by the sensing interferometer after adding the current modulation signal generate a complete elliptical closed form, which can make the two elliptical fitting signals output by the sensing interferometer and the reference interferometer perform elliptical fitting in real time. By outputting a drive signal and a low-frequency or high-frequency current modulation signal through the power supply drive and modulation module, the semiconductor laser can be prompted to generate a laser with a linewidth <1MHz and containing a low-frequency or high-frequency current modulation signal.

[0064] The laser is sequentially split into sensing light and reference light by an isolator and a 1×2 coupler. The sensing light enters the sensing arm and the reference arm of the sensing interferometer through a 3×3 coupler, and after being reflected by the Faraday rotation mirror FRM respectively, it picks up the sensing light signals containing the signal to be measured and the low-frequency or high-frequency current modulation signal with a phase difference of 2π / 3, and forms two sensing light signals in the 3×3 coupler and enters the corresponding photoelectric converters PD respectively. After converting the optical signal into an electrical signal, it is collected by the signal acquisition and processing module. The reference light enters the sensing arm and the reference arm of the reference interferometer through a 3×3 coupler, and after being reflected by the Faraday rotation mirror FRM respectively, it picks up the reference light signals containing the low-frequency or high-frequency current modulation signal with a phase difference of 2π / 3, and forms two reference light signals in the 3×3 coupler and enters the corresponding photoelectric converters PD respectively. After converting the optical signal into an electrical signal, it is collected by the signal acquisition and processing module. The sensing interferometer, the reference interferometer and their comparison diagrams after corresponding subtraction are as Figure 4 shown.

[0065] After the signal acquisition and processing module acquires the two sensing optical signals and two reference optical signals corresponding to the output of the sensing interferometer and the reference interferometer, the two sensing optical signals and two reference optical signals can be expressed as:

[0066]

[0067] Wherein, A S is the DC term output by the sensing interferometer, B S is the AC term output by the sensing interferometer, A r is the DC term output by the reference interferometer, B r is the AC term output by the reference interferometer, is the phase difference generated by the signal to be measured, is the phase difference generated by the current modulation signal, and are the phase differences generated by the light source frequency jitter at the outputs of the sensing interferometer and the reference interferometer respectively.

[0068] The signal acquisition and processing module performs an ellipse fitting operation on the two sensing optical signals and two reference optical signals to obtain two pairs of orthogonal signals with a phase difference of π / 2. The two pairs of orthogonal signals can be expressed as:

[0069]

[0070] Wherein, is the phase difference generated by the target signal, is the phase difference generated by the current modulation signal, and are the phase differences generated by the light source frequency jitter at the outputs of the sensing interferometer and the reference interferometer respectively after output.

[0071] Then the signal acquisition and processing module performs an operation of the cross subtraction algorithm on the above two pairs of orthogonal signals to obtain a new pair of orthogonal signals:

[0072]

[0073] Wherein, is the phase difference generated by the signal to be measured, S 1 and S 2 are a pair of orthogonal signals corresponding to the output of the sensing interferometer; R 1 and R 2 are a pair of orthogonal signals corresponding to the output of the reference interferometer; Z 1 and Z 2 are a pair of orthogonal signals obtained after the operation of the cross subtraction algorithm.

[0074] Finally, the signal acquisition and processing module performs an arctangent operation on the above new pair of orthogonal signals to obtain the phase signal to be measured. :

[0075] .

[0076] The present invention uses a fiber-optic quasi-balanced fiber-optic interferometer composed of a 3×3 fiber coupler to achieve signal detection. There is a centimeter-level difference between the sensing arm and the reference arm of the interferometer. In this way, the requirement for using a laser with a very narrow linewidth (generally in the kHz level) for the sensing system to achieve high-precision signal detection can be reduced, and at the same time, the laser can be modulated.

[0077] Embodiment 3

[0078] Such as Figure 3 A fiber-optic sensing phase demodulation system as shown includes a wide-linewidth semiconductor laser with a linewidth <1 MHz, which provides a light source with a linewidth <1 MHz for the sensing system. An isolator for isolating the return light in the optical path. A 1×2 coupler for splitting light. A photoelectric converter PD for converting the optical signal into an electrical signal, and a signal acquisition and processing module that uses the above phase demodulation method to calculate and output the phase signal to be measured.

[0079] A power supply drive and modulation module for outputting a low-frequency or high-frequency current modulation signal and a drive signal other than the signal to be measured to the semiconductor laser. The monitoring frequency range, i.e., the frequency band, of the signal to be measured is 10 Hz to 3 kHz; the low-frequency or high-frequency current modulation signals respectively refer to a low-frequency current modulation signal <10 Hz or a high-frequency current modulation signal >3 kHz. In this embodiment, the high-frequency current modulation signal emitted by the power supply drive and modulation module is 3.5 kHz.

[0080] It further includes a sensing interferometer and a reference interferometer with the same structural parameters and an arm length difference ΔL = 2 cm constructed based on a 3×3 coupler. In this embodiment, sensing interferometers and reference interferometers with arm length differences ΔL = 1 cm and 1.5 cm are also used for experiments; the two reference arms and the sensing arm of the sensing interferometer are respectively wound around two signal-sensitive elements to form a push-pull fiber-optic interferometer; the sensing arm of the sensing interferometer is wound around the signal-sensitive element, and the other reference arm is wound around the signal-insensitive element to form a non-push-pull fiber-optic interferometer. The two arms of the reference interferometer are both wound around the signal-insensitive element. The sensing interferometer and the reference interferometer use a Michelson interferometer or can also use a Mach-Zehnder interferometer. The sensing interferometer and the reference interferometer further include 2 corresponding Faraday rotation mirrors FRM for reflecting the incident light.

[0081] One path on the left side of the 3×3 coupler of the sensing interferometer serves as the input end of the signal after beam splitting. The other two paths are for picking up external vibration signals and low-frequency or high-frequency current modulation signals after passing through the sensing interferometer. In this embodiment, the two paths are for picking up external vibration signals and a 3.5 kHz high-frequency current modulation signal after passing through the sensing interferometer, and form two output ports of the interference signal in the 3×3 coupler; One path on the left side of the 3×3 coupler of the reference interferometer serves as the input end of the signal after beam splitting. The other two paths are for picking up external vibration signals and low-frequency or high-frequency current modulation signals after passing through the reference interferometer. In this embodiment, the two paths are for picking up a 3.5 kHz high-frequency current modulation signal after passing through the reference interferometer, and form two output ports of the interference signal in the 3×3 coupler.

[0082] A large phase signal is introduced into this system, making the elliptical signals output by the quasi-balanced sensing interferometer composed of a 3×3 coupler and the quasi-balanced reference interferometer composed of a 3×3 coupler closed. Among them, the elliptical signal output by the quasi-balanced sensing interferometer not only contains a 3.5 kHz current modulation signal but also contains a vibration acceleration signal with a frequency range of 10 Hz to 3000 Hz of the signal to be measured. The elliptical signal output by the quasi-balanced reference interferometer only contains a 3.5 kHz current modulation signal. Through the cross-subtraction algorithm, a vibration acceleration signal with a frequency range of 10 Hz to 3000 Hz can be obtained.

[0083] The principle and process of this fiber optic sensing phase demodulation system are as follows.

[0084] The power supply drive and modulation module generates a drive signal for the semiconductor laser, driving the semiconductor laser to generate a laser with a linewidth < 1 MHz. At the same time, the power supply drive and modulation module generates a low-frequency or high-frequency current modulation signal. In this embodiment, the high-frequency current modulation signal emitted by the power supply drive and modulation module is 3.5 kHz. This current modulation signal is a large modulation signal, which can make the two elliptical fitting signals output by the sensing interferometer reach a closed state. The Lissajous figure without the modulation signal and the Lissajous figure after adding a 3.5 kHz high-frequency current modulation signal are as Figure 2 shown. It can be clearly seen that the two elliptical fitting signals output by the sensing interferometer after adding the current modulation signal generate a complete elliptical closed form, which can make the two elliptical fitting signals output by the sensing interferometer and the reference interferometer perform elliptical fitting in real time. By outputting the drive signal and the low-frequency or high-frequency current modulation signal through the power supply drive and modulation module, the semiconductor laser can be prompted to generate a laser with a linewidth < 1 MHz and containing a low-frequency or high-frequency current modulation signal.

[0085] The laser light is split into sensing light and reference light successively through an isolator and a 1×2 coupler. The sensing light enters the sensing arm and the reference arm of the sensing interferometer through a 3×3 coupler, and after being reflected by the Faraday rotation mirror FRM respectively, picks up sensing light signals containing the signal to be measured and low-frequency or high-frequency current modulation signals with a phase difference of 2π / 3. Then, two paths of sensing light signals are formed in the 3×3 coupler and enter the corresponding photoelectric converters PD respectively. After converting the optical signals into electrical signals, they are collected by the signal acquisition and processing module. The reference light enters the sensing arm and the reference arm of the reference interferometer through a 3×3 coupler, and after being reflected by the Faraday rotation mirror FRM respectively, picks up reference light signals containing low-frequency or high-frequency current modulation signals with a phase difference of 2π / 3. Then, two paths of reference light signals are formed in the 3×3 coupler and enter the corresponding photoelectric converters PD respectively. After converting the optical signals into electrical signals, they are collected by the signal acquisition and processing module. The comparison diagrams of the sensing interferometer, the reference interferometer and their corresponding subtraction are as shown in Figure 4 shown.

[0086] After the signal acquisition and processing module collects the two paths of sensing light signals and two paths of reference light signals respectively output by the sensing interferometer and the reference interferometer, the two paths of sensing light signals and two paths of reference light signals can be expressed as:

[0087]

[0088] where, A S is the DC term output by the sensing interferometer, B S is the AC term output by the sensing interferometer, A r is the DC term output by the reference interferometer, B r is the AC term output by the reference interferometer, is the phase difference generated by the signal to be measured, is the phase difference generated by the current modulation signal, and are the phase differences generated by the light source frequency jitter at the outputs of the sensing interferometer and the reference interferometer respectively.

[0089] The signal acquisition and processing module performs elliptical fitting operations on the two paths of sensing light signals and two paths of reference light signals to obtain two pairs of orthogonal signals with a phase difference of π / 2. The two pairs of orthogonal signals can be expressed as:

[0090]

[0091] where, is the phase difference generated by the target signal, is the phase difference generated by the current modulation signal, and is the phase difference generated by the light source frequency jitter after the outputs of the sensing interferometer and the reference interferometer respectively.

[0092] Then the signal acquisition and processing module performs an operation of cross-subtraction algorithm on the above two pairs of orthogonal signals to obtain a new pair of orthogonal signals:

[0093]

[0094] Wherein, is the phase difference generated by the signal to be measured, S 1 and S 2 are a pair of orthogonal signals corresponding to the output of the sensing interferometer; R 1 and R 2 are a pair of orthogonal signals corresponding to the output of the reference interferometer; Z 1 and Z 2 are a pair of orthogonal signals obtained after the operation of the cross-subtraction algorithm.

[0095] Finally, the signal acquisition and processing module performs an arctangent operation on the above new pair of orthogonal signals to obtain the phase signal to be measured :

[0096]

[0097] The demodulation amplitude stability is used to represent the comparison of the demodulation advantages and disadvantages between this scheme and the traditional ellipse fitting algorithm. The demodulation amplitude stability is obtained by (maximum amplitude - minimum amplitude) / average value of the amplitude. The phase signal to be measured obtained by real-time ellipse fitting through the technical scheme of the present invention The comparison diagram of the fitting results and the traditional ellipse fitting algorithm for sequential ellipse fitting is as Figure 5 shown. It can be clearly seen that the demodulation amplitude stability of the phase signal to be measured through the technical scheme of the present invention is maintained at 0.31%, while the demodulation amplitude stability of the phase signal to be measured using the traditional ellipse fitting algorithm is maintained at 1.02%, proving that the demodulation method of the technical scheme of the present invention can greatly improve the stability and accuracy of the demodulation signal.

[0098] The present invention designs a quasi-balanced dual interferometer, which is compatible with lasers with a linewidth < 1 MHz, relaxes the requirement for a narrow linewidth of the light source, and can greatly reduce the light source cost; closes the ellipse forcibly through the current modulation signal to avoid the parameter mismatch problem of the traditional single ellipse fitting; separates and accurately detects the current modulation signal through a reference interferometer with the same structural parameters, and combines the cross-subtraction algorithm to eliminate the light source modulation signal and frequency jitter noise, thereby improving the demodulation accuracy.

[0099] Although the specification has been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the appended claims. In addition, the specific embodiments described do not limit the scope of the invention, and those of ordinary skill in the art can readily understand based on the present invention that currently existing or later to be developed processes, machines, manufactures, compositions of matter, means, methods, or steps can perform functions substantially the same as those of the embodiments of the present invention or achieve substantially the same results. Therefore, the appended claims are intended to include such processes, machines, manufactures, compositions of matter, means, methods, or steps within their scope.

Claims

1. An optical fiber sensing phase demodulation method, characterized in that: It includes the following steps: Step 1: Apply a low-frequency or high-frequency current modulation signal outside the frequency band of the signal to be measured to a wide-linewidth light source with a linewidth < 1 MHz. The low-frequency or high-frequency current modulation signal forces the optical signal output by the interferometer to have an elliptical closure; Step 2: Construct a sensing interferometer and a reference interferometer with the same structural parameters and an arm length difference ΔL = 0.5 - 2 cm using a 3×3 coupler to respectively collect a sensing signal containing the signal to be measured and the current modulation signal and a reference signal containing only the current modulation signal, and correspondingly output two paths of sensing optical signals and two paths of reference optical signals. Among them, both arms of the reference interferometer are wound around signal-insensitive elements; Step 3: Perform elliptical fitting operations on the two paths of sensing optical signals and the two paths of reference optical signals to obtain two pairs of orthogonal signals with a phase difference of π / 2; Step 4: Eliminate the low-frequency or high-frequency current modulation signal introduced at the light source by performing a cross-subtraction algorithm and an arctangent operation on the two pairs of orthogonal signals to obtain the signal phase to be measured. The cross-subtraction algorithm is expressed as: Among them, is the phase difference generated by the signal to be measured; S1 and S2 are a pair of orthogonal signals corresponding to the output of the sensing interferometer; R1 and R2 are a pair of orthogonal signals corresponding to the output of the reference interferometer; Z1 and Z2 are a pair of orthogonal signals obtained after the cross-subtraction algorithm operation.

2. The fiber optic sensing phase demodulation method according to claim 1, characterized in that: The two paths of sensing optical signals and the two paths of reference optical signals in Step 2 can be expressed as: Among them, A S is the DC term output by the sensing interferometer, B S is the AC term output by the sensing interferometer, A r is the DC term output by the reference interferometer, B r is the AC term output by the reference interferometer, is the phase difference generated by the signal to be measured, is the phase difference generated by the current modulation signal, and are the phase differences generated by the light source frequency jitter at the outputs of the sensing interferometer and the reference interferometer respectively.

3. The fiber optic sensing phase demodulation method according to claim 2, characterized in that: The phase differences of the two paths of sensing optical signals and the two paths of reference optical signals in Step 2 are both 2π / 3.

4. The fiber optic sensing phase demodulation method according to claim 1, wherein: The two pairs of orthogonal signals obtained through elliptical fitting operations in Step 3 can be expressed as: Among them, is the phase difference generated by the target signal, is the phase difference generated by the current modulation signal, and are the phase differences generated after the light source frequency jitter outputs from the sensing interferometer and the reference interferometer, respectively.

5. The fiber optic sensing phase demodulation method according to claim 1, characterized in that: The arctangent operation in Step 4 is expressed as:

6. The fiber optic sensing phase demodulation method according to claim 4, characterized in that: The sensing interferometer includes a sensing arm wound around a signal-sensitive element and a reference arm wound around a signal-insensitive element or a signal-sensitive element.

7. An optical fiber sensing phase demodulation system, characterized in that: It includes a wide-linewidth semiconductor laser with a linewidth < 1 MHz; a power supply driving and modulation module for outputting a low-frequency or high-frequency current modulation signal and a driving signal other than the signal to be measured to the semiconductor laser; An isolator for isolating the return light in the optical path; a 1×2 coupler for beam splitting; a sensing interferometer and a reference interferometer with the same structural parameters and an arm length difference ΔL = 0.5 - 2 cm constructed based on a 3×3 coupler; a photoelectric converter for converting the optical signal into an electrical signal and a signal acquisition and processing module for calculating and outputting the signal phase to be measured by using the phase demodulation method described in any one of claims 1 - 6; 8. The fiber optic sensing phase demodulation system according to claim 7, characterized in that: If the frequency band of the signal to be measured is 10 Hz - 3 kHz, then the low-frequency current modulation signal output by the corresponding power supply driving and modulation module < 10 Hz or the high-frequency current modulation signal > 3 kHz.

9. The fiber optic sensing phase demodulation system according to claim 8, wherein: The low-frequency current modulation signal is 3 Hz or the high-frequency current modulation signal is 3.5 kHz.

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