Optical fiber sensing phase demodulation method and system

Abstract

The invention discloses an optical fiber sensing phase demodulation method and system, and belongs to the technical field of optical sensing. According to the invention, line width lt is adopted; a 1MHz laser is used as a light source, a low-frequency or high-frequency current modulation signal introduced at the light source is eliminated by introducing a reference interferometer and combining a cross subtraction algorithm and arc tangent operation, the requirement for the line width of the light source is reduced, and the cost and complexity of the system are remarkably reduced; a large-amplitude phase excitation signal is introduced through a low-frequency or high-frequency current modulation signal, ellipse closing is forced, the limitation that the signal amplitude is larger than or equal to pi / 2 in a traditional ellipse fitting method is avoided, the problem of parameter mismatch is solved, meanwhile, introduction of a modulator or a frequency shifter is avoided, and the size and power consumption of the system are further reduced. A reference interferometer with a symmetrical structure is introduced, a light source modulation signal and frequency jitter noise are eliminated in combination with a cross subtraction algorithm, and the demodulation precision and stability are improved.

Description

Technical Field

[0001] The present invention relates to the field of optical sensing technology, and in particular to an optical fiber sensing phase demodulation method and system. Background Art

[0002] Fiber optic sensing systems are gaining more and more attention in the industrial and engineering fields due to their high sensitivity and strong resistance to electromagnetic interference. Fiber-based sensing mechanisms are developing step by step and are expected to replace existing electrical sensing mechanisms in many fields. Current fiber optic sensing systems are generally based on Mach-Zehnder or Michelson interferometer structures. One or two arms of the interferometer are wrapped around the sensitive element. When the interferometer senses the external signal to be measured, a phase difference will be generated on the two fiber optic arms. The interference signals generated by the two arms are detected and demodulated, and the signal to be measured can be demodulated. The system generally uses narrow-linewidth semiconductor lasers or narrow-linewidth fiber lasers with a laser linewidth of about 1kHz. Various networking technologies are then used to form a large-scale sensor array.

[0003] In order to achieve low system phase noise and high phase resolution, current fiber optic sensing systems require semiconductor lasers or fiber lasers with a line width of the kHz order, and often use phase generation carrier method or heterodyne method for demodulation, or demodulation schemes based on ellipse fitting methods, etc. Fiber optic sensing systems based on phase generation carrier method or heterodyne method require the use of lasers with very narrow line widths, generally in the kHz order, and also require the introduction of modulators or frequency shifters inside the light source or in the sensing optical path, which not only increases the complexity of the system, but also increases 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 balanced interferometer method to reduce the functional requirements for the line width 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 arc of the fitted ellipse is not less than 1 / 4 of a circle. Otherwise, the system cannot meet the small signal pickup. In order to achieve small signal pickup, large signal excitation is also used to complete an ellipse, and then the signal is demodulated. However, the use of the one-time ellipse fitting method will cause mismatch in the parameters of the one-time ellipse fitting due to fluctuations in the light source power and other reasons as the working time of the sensor system increases, resulting in poor demodulation stability. Summary of the invention

[0004] In view of the above problems, a fiber optic sensing phase demodulation method is provided. The present invention reduces the requirements on the line width of the light source through the design of a quasi-balanced interferometer, significantly reducing the cost and complexity of the system; introduces a large phase excitation signal through a low-frequency or high-frequency current modulation signal to force the ellipse to close, avoid the limitation of the traditional ellipse fitting method on the signal amplitude (≥π / 2), and solve the parameter mismatch problem, while avoiding the introduction of a modulator or frequency shifter, further reducing the size and power consumption of the system; introduces a reference interferometer with a symmetrical structure, combined with a cross-subtraction algorithm to eliminate the light source modulation signal and frequency jitter noise, thereby 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 method for optical fiber sensing phase demodulation comprises 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 of <1MHz; Step 2: A sensor interferometer and a reference interferometer with the same structural parameters and an arm length difference of ΔL=0.5~2cm are constructed through a 3×3 coupler to respectively collect a sensor signal containing a signal to be measured and a current modulation signal and a reference signal containing only a current modulation signal, and output two sensor light signals and two reference light signals accordingly; Step 3: Perform ellipse fitting operation on the two sensing light signals and the two reference light signals to obtain two pairs of orthogonal signals with a phase difference of π / 2; Step 4: The low-frequency or high-frequency current modulation signal introduced at the light source is eliminated by performing a cross-subtraction algorithm and an inverse tangent operation on the two pairs of orthogonal signals to obtain a phase signal to be measured.

[0007] Preferably, the two sensing light signals and the two reference light signals in step 2 can be expressed as: in, A S is the DC term of the sensor interferometer output, B S is the AC term of the sensor interferometer output, A r is the DC term of the reference interferometer output, B r is the AC term of the reference interferometer output, 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 caused by the frequency jitter of the light source in the output of the sensing interferometer and the reference interferometer.

[0008] Preferably, the phase difference between the two sensing light signals and the two reference light signals in step 2 is 2π / 3.

[0009] Preferably, the two pairs of orthogonal signals obtained by ellipse fitting calculation in step 3 can be expressed as: in, is the phase difference generated by the target signal, is the phase difference generated by the current modulation signal, and is the phase difference caused by the frequency jitter of the light source after the output of the sensing interferometer and the reference interferometer respectively.

[0010] Preferably, the cross subtraction algorithm and the inverse tangent operation in step 4 can be respectively expressed as: in, is the phase difference of the signal to be measured, S 1 and S 2 is a pair of orthogonal signals corresponding to the output of the sensor interferometer; R 1 and R 2 is a pair of orthogonal signals corresponding to the output of the reference interferometer; Z 1 and Z 2 is a pair of orthogonal signals obtained after the cross-subtraction algorithm is used.

[0011] Preferably, the sensor interferometer comprises a sensor arm wound on the signal sensitive element and a reference arm wound on the signal insensitive element or the signal sensitive element.

[0012] Preferably, both arms of the reference interferometer are wound around a signal-insensitive element.

[0013] An optical fiber sensing phase demodulation system comprises a wide-linewidth semiconductor laser with a linewidth of less than 1 MHz; a power drive and modulation module for outputting a low-frequency or high-frequency current modulation signal and a drive signal other than a signal to be measured to the semiconductor laser; an isolator for isolating the return light in the optical path; a 1×2 coupler for splitting light; a sensing interferometer and a reference interferometer with the same structural parameters and an arm length difference ΔL=0.5~2cm constructed based on a 3×3 coupler; a photoelectric converter for converting an optical signal into an electrical signal and a signal acquisition and processing module for calculating and outputting a phase signal to be measured using the above-mentioned phase demodulation method.

[0014] Preferably, the frequency band of the signal to be tested is 10 Hz to 3 kHz, and the low-frequency current modulation signal output by the corresponding power drive and modulation module is less than 10 Hz or the high-frequency current modulation signal is greater than 3 kHz.

[0015] Preferably, the low-frequency current modulation signal is 3 Hz or the high-frequency current modulation signal is 3.5 kHz.

[0016] Due to the adoption of the above technical solution, the present invention has the following beneficial effects.

[0017] (1) The present invention is based on the principle of optical interference. The signal to be measured is converted into a phase change of light by a quasi-balanced fiber interferometer (including but not limited to a Michelson interferometer and a Mach-Zehnder interferometer) based on a 3×3 coupler, thereby generating two interference signals with a phase difference of 2π / 3. The quasi-balanced fiber interferometer can relax the requirements on the line width of the laser and realize low-frequency or high-frequency current modulation of the light source.

[0018] (2) The present invention introduces a high-frequency or low-frequency phase signal outside the bandwidth of the signal to be measured by current modulation of the semiconductor laser, so that the elliptical signal output by the interferometer can reach a closed state.

[0019] (3) The present invention introduces a reference interferometer with equal structural parameters into the sensor interferometer structure, modulates the laser through a low-frequency or high-frequency current modulation signal, introduces a large phase excitation signal, and forces the ellipse to close. The output signals of the sensor interferometer and the reference interferometer are combined to eliminate the interference of the modulation signal and realize real-time ellipse fitting demodulation.

[0020] (4) The present invention uses a laser with a linewidth of <1MHz (the existing technology requires a kHz-level narrow linewidth light source) as the light source, and eliminates the low-frequency or high-frequency current modulation signal introduced at the light source by introducing a reference interferometer and combining it with a cross-subtraction algorithm and an inverse tangent operation, thereby reducing the requirements for the linewidth of the light source and significantly reducing the cost and complexity of the system; a large-scale phase excitation signal is introduced through a 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) and solving the parameter mismatch problem, while avoiding the introduction of a modulator or frequency shifter, further reducing the size and power consumption of the system; a reference interferometer with a symmetrical structure is introduced and combined with a cross-subtraction algorithm to eliminate the light source modulation signal and frequency jitter noise, thereby improving the demodulation accuracy and stability.

[0021] (5) The present invention utilizes a quasi-balanced fiber interferometer composed of 3×3 fiber couplers to achieve signal detection. The difference between the sensing arm and the reference arm of the interferometer is at the centimeter level. This can reduce the requirement that the sensing system use a very narrow linewidth laser (generally at the kHz level) in order to achieve high-precision signal detection, and can also modulate the laser.

[0022] (6) The present invention adopts a quasi-balanced dual interferometer design, which is compatible with lasers with a linewidth of <1 MHz, relaxes the requirement for narrow linewidth of the light source, and can significantly reduce the cost of the light source; the current modulation signal is forced to close by the current modulation signal, avoiding the parameter mismatch problem of traditional single ellipse fitting; the current modulation signal is accurately detected by separating the reference interferometer with the same structural parameters, and the light source modulation signal and frequency jitter noise are eliminated by combining the cross-subtraction algorithm, thereby improving the demodulation accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The following is a detailed discussion of the making and application of the preferred embodiments of the present invention. However, it should be understood that the present invention provides many applicable inventive concepts that can be embodied in various specific environments. The specific embodiments discussed are only for the purpose of illustrating the specific ways of making 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 also be obtained based on these drawings without creative work.

[0024] Figure 1 The figure is a flow chart of the phase demodulation method of the present invention.

[0025] Figure 2 This is a comparison diagram of the closed Lissajous figure caused by the low-frequency current modulation signal of the present invention.

[0026] Figure 3 It is a schematic diagram of the structure of the system of the present invention.

[0027] Figure 4 This is a comparison chart of the data processing effect after the monitoring method of the present invention.

[0028] Figure 5 This is a comparison chart of the amplitude stability differences output by the primary ellipse fitting algorithm and the real-time ellipse fitting algorithm of the present invention. DETAILED DESCRIPTION

[0029] The following is a detailed discussion of the making and application of the preferred embodiments of the present invention. However, it should be understood that the present invention provides many applicable inventive concepts that can be embodied in various specific environments. The specific embodiments discussed are only intended to illustrate specific ways to make and use the present invention and do not limit the scope of the present invention.

[0030] The present invention adopts a laser with a line width of <1MHz as a light source, and eliminates the low-frequency or high-frequency current modulation signal introduced at the light source by introducing a reference interferometer and combining a cross-subtraction algorithm and an inverse tangent operation, thereby reducing the requirements for the line width of the light source and significantly reducing the cost and complexity of the system; a large-scale phase excitation signal is introduced through a low-frequency or high-frequency current modulation signal to force the ellipse to close, thereby avoiding the limitation of the traditional ellipse fitting method on the signal amplitude (≥π / 2) and solving the parameter mismatch problem, while avoiding the introduction of a modulator or a frequency shifter, further reducing the size and power consumption of the system; a reference interferometer with a symmetrical structure is introduced and combined with a cross-subtraction algorithm to eliminate the light source modulation signal and frequency jitter noise, thereby improving the demodulation accuracy and stability.

[0031] The following is combined with Figure 1-5 Further elaboration.

[0032] Example 1

[0033] like Figure 1 The optical fiber sensing phase demodulation method shown includes the following steps: Step 1: applying 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 of <1 MHz.

[0034] Step 2: A sensing interferometer and a reference interferometer with the same structural parameters and an arm length difference of ΔL=0.5~2cm are constructed through a 3×3 coupler to respectively collect sensing signals containing the measured signal and the current modulation signal and a reference signal containing only the current modulation signal, and correspondingly output two sensing light signals and two reference light signals, wherein the phase difference between the two sensing light signals and the two reference light signals is 2π / 3.

[0035] The sensor interferometer comprises a sensor arm wound on a signal sensitive element and a reference arm wound on a signal insensitive element or a signal sensitive element. Both arms of the reference interferometer are wound on the signal insensitive element. The two sensing light signals and the two reference light signals can be expressed as: in, A S is the DC term of the sensor interferometer output, B S is the AC term of the sensor interferometer output, A r is the DC term of the reference interferometer output, B r is the AC term of the reference interferometer output, 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 caused by the frequency jitter of the light source in the output of the sensing interferometer and the reference interferometer.

[0036] Step 3: Perform ellipse fitting operation on the two sensing light signals and the two 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: in, is the phase difference generated by the target signal, is the phase difference generated by the current modulation signal, and is the phase difference caused by the frequency jitter of the light source after the output of the sensing interferometer and the reference interferometer respectively.

[0037] 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 inverse tangent operation on the two pairs of orthogonal signals to obtain a phase signal to be measured. The cross-subtraction algorithm and the inverse tangent operation can be expressed as: in, is the phase difference generated by the signal to be measured, S 1 and S 2 is a pair of orthogonal signals corresponding to the output of the sensor interferometer; R 1 and R 2 is a pair of orthogonal signals corresponding to the output of the reference interferometer; Z 1 and Z 2 is a pair of orthogonal signals obtained after the cross-subtraction algorithm is used.

[0038] Example 2

[0039] like Figure 3 The optical fiber sensing phase demodulation system shown includes a wide-linewidth semiconductor laser with a linewidth of <1MHz, which provides a light source with a linewidth of <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 optical signals into electrical signals and a signal acquisition and processing module that uses the above phase demodulation method to calculate and output the phase signal to be measured.

[0040] A power drive and modulation module for outputting a low-frequency or high-frequency current modulation signal and a driving signal other than a signal to be tested to a semiconductor laser, wherein the monitoring frequency range of the signal to be tested, i.e., the frequency band, is 10 Hz to 3 kHz; the low-frequency or high-frequency current modulation signal refers to a low-frequency current modulation signal < 10 Hz or a high-frequency current modulation signal > 3 kHz. In this embodiment, the low-frequency current modulation signal emitted by the power drive and modulation module is 3 Hz.

[0041] It also includes a sensing interferometer and a reference interferometer with the same structural parameters and an arm length difference ΔL=0.5cm 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 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 interferometer. Both arms of the reference interferometer are wound around the signal insensitive element. The sensing interferometer and the reference interferometer use a Michelson interferometer or a Mach-Zehnder interferometer, and the sensing interferometer and the reference interferometer also include two corresponding Faraday rotator mirrors FRM for reflecting incident light.

[0042] One path on the left side of the 3×3 coupler of the sensing interferometer is used as the input end of the signal after splitting, and the other two paths are used to pick up the external vibration signal and the low-frequency or high-frequency current modulation signal after the sensing interferometer. In this embodiment, two paths are used to pick up the external vibration signal and the 3Hz low-frequency current modulation signal after 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 is used as the input end of the signal after splitting, and the other two paths are used to pick up the external vibration signal and the low-frequency or high-frequency current modulation signal after the reference interferometer. In this embodiment, two paths are used to pick up the 3Hz low-frequency current modulation signal after the reference interferometer, and form two output ports of the interference signal in the 3×3 coupler.

[0043] A large phase signal is introduced in this system, so that the elliptical signals output by the quasi-balanced sensing interferometer composed of 3×3 couplers and the quasi-balanced reference interferometer composed of 3×3 couplers are closed. The elliptical signal output by the quasi-balanced sensing interferometer not only contains a 3Hz current modulation signal but also a vibration acceleration signal of the signal to be measured with a frequency range of 10Hz~3000Hz. The elliptical signal output by the quasi-balanced reference interferometer only contains a 3Hz current modulation signal. After a cross-subtraction algorithm, a vibration acceleration signal with a frequency range of 10Hz~3000Hz can be obtained.

[0044] The principle and process of this optical fiber sensing phase demodulation system are as follows.

[0045] The power drive and modulation module generates a driving signal for the semiconductor laser, driving the semiconductor laser to generate a laser with a line width of less than 1 MHz. At the same time, the power drive and modulation module generates a low-frequency or high-frequency current modulation signal. The low-frequency current modulation signal emitted by the power drive and modulation module in this embodiment is 3 Hz. The current modulation signal is a large-amplitude modulation signal, which can make the two-way ellipse fitting signal output by the sensor interferometer reach a closed state. The Lissajous figure without the modulation signal and the Lissajous figure after adding the 3 Hz low-frequency current modulation signal are as follows: Figure 2 As shown, it can be clearly seen that the two-way ellipse fitting signals output by the sensor interferometer after adding the current modulation signal generate a complete ellipse closed shape, which can make the two-way ellipse fitting signals output by the sensor interferometer and the reference interferometer perform ellipse fitting in real time. The power drive and modulation module outputs the drive signal and the low-frequency or high-frequency current modulation signal, which can cause the semiconductor laser to generate a laser with a line width of less than 1MHz containing a low-frequency or high-frequency current modulation signal.

[0046] The laser is split into sensing light and reference light after passing through an isolator and a 1×2 coupler in sequence. The sensing light enters the sensing arm and reference arm of the sensing interferometer through a 3×3 coupler, and picks up the sensing light signal containing the signal to be measured and the low-frequency or high-frequency current modulation signal with a phase difference of 2π / 3 after being reflected by the Faraday rotating mirror FRM. Two sensing light signals are formed in the 3×3 coupler and enter the corresponding photoelectric converter PD respectively. The light signal is converted into an electrical signal and then collected by the signal acquisition and processing module. The reference light enters the sensing arm and reference arm of the reference interferometer through a 3×3 coupler, and picks up the reference light signal containing the low-frequency or high-frequency current modulation signal with a phase difference of 2π / 3 after being reflected by the Faraday rotating mirror FRM. Two reference light signals are formed in the 3×3 coupler and enter the corresponding photoelectric converter PD respectively. The light signal is converted into an electrical signal and then collected by the signal acquisition and processing module. The comparison diagram of the sensing interferometer and the reference interferometer and their corresponding subtraction is shown as follows. Figure 4 shown.

[0047] After the signal acquisition and processing module acquires two paths of sensing light signals and two paths of reference light signals outputted by the sensing interferometer and the reference interferometer respectively, the two paths of sensing light signals and the two paths of reference light signals can be expressed as: in, A S is the DC term of the sensor interferometer output, B S is the AC term of the sensor interferometer output, A r is the DC term of the reference interferometer output, B r is the AC term of the reference interferometer output, is the phase difference of the signal to be measured, is the phase difference generated by the current modulation signal, and is the phase difference caused by the frequency jitter of the light source in the output of the sensing interferometer and the reference interferometer.

[0048] The signal acquisition and processing module performs ellipse fitting operation on two sensing light signals and two 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: in, is the phase difference generated by the target signal, is the phase difference generated by the current modulation signal, and is the phase difference caused by the frequency jitter of the light source after the output of the sensing interferometer and the reference interferometer respectively.

[0049] Then the signal acquisition and processing module performs a cross-subtraction algorithm on the two pairs of orthogonal signals to obtain a new pair of orthogonal signals: in, is the phase difference of the signal to be measured, S 1 and S 2 is a pair of orthogonal signals corresponding to the output of the sensor interferometer; R 1 and R 2 is a pair of orthogonal signals corresponding to the output of the reference interferometer; Z 1 and Z 2 is a pair of orthogonal signals obtained after the cross-subtraction algorithm is used.

[0050] Finally, the signal acquisition and processing module performs an inverse tangent operation on the new pair of orthogonal signals to obtain the phase signal to be measured. : .

[0051] The present invention utilizes a quasi-balanced fiber interferometer composed of 3×3 fiber couplers to realize signal detection. The difference between the sensing arm and the reference arm of the interferometer is at the centimeter level. This can reduce the requirement that the sensing system needs to use a laser with a very narrow line width (generally at the kHz level) in order to achieve high-precision signal detection, and can also modulate the laser.

[0052] Example 3

[0053] like Figure 3The optical fiber sensing phase demodulation system shown includes a wide-linewidth semiconductor laser with a linewidth of <1MHz, which provides a light source with a linewidth of <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 optical signals into electrical signals and a signal acquisition and processing module that uses the above phase demodulation method to calculate and output the phase signal to be measured.

[0054] A power drive and modulation module for outputting a low-frequency or high-frequency current modulation signal and a driving signal other than a signal to be tested to a semiconductor laser, wherein the monitoring frequency range of the signal to be tested, i.e., the frequency band, is 10 Hz to 3 kHz; the low-frequency or high-frequency current modulation signal refers to a low-frequency current modulation signal < 10 Hz or a high-frequency current modulation signal > 3 kHz, respectively. In this embodiment, the high-frequency current modulation signal emitted by the power drive and modulation module is 3.5 kHz.

[0055] It also includes a sensing interferometer and a reference interferometer with the same structural parameters and an arm length difference ΔL=2cm constructed based on a 3×3 coupler. This embodiment also uses a sensing interferometer and a reference interferometer with an arm length difference ΔL=1cm and 1.5cm for testing; 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 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 interferometer. Both arms of the reference interferometer are wound around the signal insensitive element. The sensing interferometer and the reference interferometer use a Michelson interferometer, and may also use a Mach-Zehnder interferometer. The sensing interferometer and the reference interferometer also include two corresponding Faraday rotator mirrors FRM for reflecting incident light.

[0056] One path on the left side of the 3×3 coupler of the sensing interferometer is used as the input end of the signal after splitting, and the other two paths are used to pick up the external vibration signal and the low-frequency or high-frequency current modulation signal after the sensing interferometer. In this embodiment, two paths are used to pick up the external vibration signal and the 3.5kHz high-frequency current modulation signal after 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 is used as the input end of the signal after splitting, and the other two paths are used to pick up the external vibration signal and the low-frequency or high-frequency current modulation signal after the reference interferometer. In this embodiment, two paths are used to pick up the 3.5kHz high-frequency current modulation signal after the reference interferometer, and form two output ports of the interference signal in the 3×3 coupler.

[0057] A large phase signal is introduced in this system, so that the elliptical signals output by the quasi-balanced sensing interferometer composed of 3×3 couplers and the quasi-balanced reference interferometer composed of 3×3 couplers are closed. The elliptical signal output by the quasi-balanced sensing interferometer not only contains a 3.5kHz current modulation signal but also a vibration acceleration signal of the signal to be measured with a frequency range of 10Hz~3000Hz. The elliptical signal output by the quasi-balanced reference interferometer only contains a 3.5kHz current modulation signal. After a cross-subtraction algorithm, a vibration acceleration signal with a frequency range of 10Hz~3000Hz can be obtained.

[0058] The principle and process of this optical fiber sensing phase demodulation system are as follows.

[0059] The power drive and modulation module generates a driving signal for the semiconductor laser, driving the semiconductor laser to generate a laser with a line width of less than 1 MHz. At the same time, the power drive and modulation module generates a low-frequency or high-frequency current modulation signal. The high-frequency current modulation signal emitted by the power drive and modulation module in this embodiment is 3.5 kHz. The current modulation signal is a large-amplitude modulation signal, which can make the two-way ellipse fitting signal output by the sensor interferometer reach a closed state. The Lissajous figure without the modulation signal and the Lissajous figure after adding the 3.5 kHz high-frequency current modulation signal are as follows: Figure 2 As shown, it can be clearly seen that the two-way ellipse fitting signals output by the sensor interferometer after adding the current modulation signal generate a complete ellipse closed shape, which can make the two-way ellipse fitting signals output by the sensor interferometer and the reference interferometer perform ellipse fitting in real time. The power drive and modulation module outputs the drive signal and the low-frequency or high-frequency current modulation signal, which can cause the semiconductor laser to generate a laser with a line width of less than 1MHz containing a low-frequency or high-frequency current modulation signal.

[0060] The laser is split into sensing light and reference light after passing through an isolator and a 1×2 coupler in sequence. The sensing light enters the sensing arm and reference arm of the sensing interferometer through a 3×3 coupler, and picks up the sensing light signal containing the signal to be measured and the low-frequency or high-frequency current modulation signal with a phase difference of 2π / 3 after being reflected by the Faraday rotating mirror FRM. Two sensing light signals are formed in the 3×3 coupler and enter the corresponding photoelectric converter PD respectively. The light signal is converted into an electrical signal and then collected by the signal acquisition and processing module. The reference light enters the sensing arm and reference arm of the reference interferometer through a 3×3 coupler, and picks up the reference light signal containing the low-frequency or high-frequency current modulation signal with a phase difference of 2π / 3 after being reflected by the Faraday rotating mirror FRM. Two reference light signals are formed in the 3×3 coupler and enter the corresponding photoelectric converter PD respectively. The light signal is converted into an electrical signal and then collected by the signal acquisition and processing module. The comparison diagram of the sensing interferometer and the reference interferometer and their corresponding subtraction is shown as follows. Figure 4 shown.

[0061] After the signal acquisition and processing module acquires two paths of sensing light signals and two paths of reference light signals outputted by the sensing interferometer and the reference interferometer respectively, the two paths of sensing light signals and the two paths of reference light signals can be expressed as: in, A S is the DC term of the sensor interferometer output, B S is the AC term of the sensor interferometer output, A r is the DC term of the reference interferometer output, B r is the AC term of the reference interferometer output, 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 caused by the frequency jitter of the light source in the output of the sensing interferometer and the reference interferometer.

[0062] The signal acquisition and processing module performs ellipse fitting operation on two sensing light signals and two 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: in, is the phase difference generated by the target signal, is the phase difference generated by the current modulation signal, and is the phase difference caused by the frequency jitter of the light source after the output of the sensing interferometer and the reference interferometer respectively.

[0063] Then the signal acquisition and processing module performs a cross-subtraction algorithm on the two pairs of orthogonal signals to obtain a new pair of orthogonal signals: in, is the phase difference generated by the signal to be measured, S 1 and S 2 is a pair of orthogonal signals corresponding to the output of the sensor interferometer; R 1 and R 2 is a pair of orthogonal signals corresponding to the output of the reference interferometer; Z 1 and Z 2 is a pair of orthogonal signals obtained after the cross-subtraction algorithm is used.

[0064] Finally, the signal acquisition and processing module performs an inverse tangent operation on the new pair of orthogonal signals to obtain the phase signal to be measured. :

[0065] The demodulation amplitude stability is used to represent the demodulation advantages and disadvantages of this scheme and the traditional ellipse fitting algorithm. The demodulation amplitude stability is obtained by (maximum amplitude - minimum amplitude) / average amplitude. The phase signal to be measured after real-time ellipse fitting by the technical solution of the present invention is The comparison of the fitting results and the traditional ellipse fitting algorithm is shown in the figure below. Figure 5 It can be clearly seen that the demodulation amplitude stability of the phase signal to be measured by the technical solution 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 solution of the present invention can greatly improve the stability and accuracy of the demodulated signal.

[0066] The present invention adopts a quasi-balanced dual interferometer design, is compatible with lasers with a linewidth of less than 1 MHz, relaxes the requirement for narrow linewidth of the light source, and can significantly reduce the cost of the light source; forces the ellipse to close through the current modulation signal, avoiding the parameter mismatch problem of traditional single ellipse fitting; accurately detects the current modulation signal through separation of 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.

[0067] Although the specification has been described in detail, it should be understood that various changes, substitutions and modifications may be made without departing from the spirit and scope of the invention as defined by the appended claims. In addition, the specific embodiments described are not intended to limit the scope of the invention, and those of ordinary skill in the art can easily understand based on the present invention that currently existing or later to be developed processes, machines, manufactures, material compositions, means, methods, or steps can perform substantially the same functions as the embodiments of the present invention or obtain substantially the same results. Therefore, the appended claims are intended to include such processes, machines, manufactures, material compositions, means, methods or steps within their scope.

Claims

1. A fiber optic sensing phase demodulation method, characterized in that: The following steps are involved: 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 of <1MHz; Step 2: A sensor interferometer and a reference interferometer with the same structural parameters and an arm length difference of ΔL=0.5~2cm are constructed through a 3×3 coupler to respectively collect a sensor signal containing a signal to be measured and a current modulation signal and a reference signal containing only a current modulation signal, and output two sensor light signals and two reference light signals accordingly; Step 3: Perform ellipse fitting operation on the two sensing light signals and the two reference light signals to obtain two pairs of orthogonal signals with a phase difference of π / 2; Step 4: The low-frequency or high-frequency current modulation signal introduced at the light source is eliminated by performing a cross-subtraction algorithm and an inverse tangent operation on the two pairs of orthogonal signals to obtain a phase signal to be measured.

2. The optical fiber sensing phase demodulation method according to claim 1, characterized in that: The two sensing light signals and two reference light signals in step 2 can be expressed as: in, A S is the DC term of the sensor interferometer output, B S is the AC term of the sensor interferometer output, A r is the DC term of the reference interferometer output, B r is the AC term of the reference interferometer output, 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 caused by the frequency jitter of the light source in the output of the sensing interferometer and the reference interferometer.

3. A fiber optic sensing phase demodulation method as claimed in claim 2, characterized in that: The phase difference between the two sensing light signals and the two reference light signals in step 2 is 2π / 3.

4. The optical fiber sensing phase demodulation method according to claim 1, characterized in that: The two pairs of orthogonal signals obtained by the ellipse fitting operation in step 3 can be expressed as: in, is the phase difference generated by the target signal, is the phase difference generated by the current modulation signal, and is the phase difference caused by the frequency jitter of the light source after the output of the sensing interferometer and the reference interferometer respectively.

5. The optical fiber sensing phase demodulation method according to claim 1, characterized in that: The cross subtraction algorithm and the inverse tangent operation in step 4 can be expressed as: in, is the phase difference generated by the signal to be measured, S 1 and S 2 is a pair of orthogonal signals corresponding to the output of the sensor interferometer; R 1 and R 2 is a pair of orthogonal signals corresponding to the output of the reference interferometer; Z 1 and Z 2 is a pair of orthogonal signals obtained after the cross-subtraction algorithm is used.

6. A fiber optic sensing phase demodulation method as claimed in claim 5, characterized in that: The sensor interferometer comprises a sensor arm wound on a signal sensitive element and a reference arm wound on a signal insensitive element or a signal sensitive element.

7. The optical fiber sensing phase demodulation method according to claim 5, characterized in that: Both arms of the reference interferometer are wrapped around a signal-insensitive element.

8. An optical fiber sensing phase demodulation system, characterized in that: Including a wide-linewidth semiconductor laser with a linewidth of <1MHz; a power drive and modulation module for outputting a low-frequency or high-frequency current modulation signal and a drive signal other than a signal to be measured to the semiconductor laser; An isolator for isolating the returning light in the optical path; A 1×2 coupler for splitting light; a sensing interferometer and a reference interferometer with the same structural parameters and an arm length difference ΔL=0.5~2cm constructed based on a 3×3 coupler; a photoelectric converter that converts optical signals into electrical signals and a signal acquisition and processing module that uses the phase demodulation method as described in any one of claims 1-7 to calculate and output the phase signal to be measured.

9. The optical fiber sensing phase demodulation system according to claim 8, characterized in that: The frequency band of the signal to be tested is 10 Hz to 3 kHz, and the low-frequency current modulation signal output by the corresponding power drive and modulation module is less than 10 Hz or the high-frequency current modulation signal is greater than 3 kHz.

10. The optical fiber sensing phase demodulation system according to claim 9, characterized in that: The low frequency current modulation signal is 3 Hz or the high frequency current modulation signal is 3.5 kHz.

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