Optical fiber sensor high-speed demodulation device based on cascaded Mach-Zehnder interferometer

Abstract

The invention discloses an optical fiber sensor high-speed demodulation device based on a cascaded Mach-Zehnder interferometer, which adopts a two-stage cascaded Mach-Zehnder interferometer structure, and obtains four-channel output with equal central wavelength intervals by accurately aligning two-stage central wavelengths. And the four-channel signal is utilized to carry out orthogonal demultiplexing on a spectral signal of the optical fiber sensor. In a digital domain, an orthogonal demodulation algorithm for phase demodulation of an optical fiber sensor is provided, and the demodulation speed is only limited by a digital sampling rate. Compared with a traditional spectrum demodulation algorithm, the scheme has the advantages of being high in demodulation speed, wide in working frequency band, easy to implement and the like.

Description

[0001] Project Funding Statement The research work of this invention was supported by the Zhejiang Provincial Department of Education Research Project (Y202457133). Technical Field

[0002] This invention belongs to the field of optical sensing technology, specifically relating to a high-speed demodulation device for fiber optic sensors based on a cascaded Mach-Zehnder interferometer. Background Technology

[0003] Fiber optic sensing, with its characteristics of large dynamic range, low loss, resistance to electromagnetic interference, and high sensitivity, is widely used in weak signal sensing, solving the problems of complex structure, small dynamic range, susceptibility to electromagnetic interference, and insufficient sensitivity of traditional electrical sensors.

[0004] In fiber optic sensor systems, interferometric sensors are widely considered to be the most promising type. For sensors, demodulation of weak signals is crucial, directly affecting the overall performance of the interferometer system. Common phase demodulation techniques for interferometric fiber optic sensors include PGC demodulation, 3×3 coupler demodulation, dual-wavelength demodulation, three-wavelength demodulation, and spectral demodulation. PGC demodulation, 3×3 coupler demodulation, dual-wavelength demodulation, and three-wavelength demodulation all construct two orthogonal signals and employ differential cross-multiplication or arctangent algorithms for phase demodulation.

[0005] Because PGC demodulation uses a narrow-linewidth light source, it is easily affected by the environment when demodulating low-frequency signals. Furthermore, the introduction of a carrier wave places high demands on the data sampling rate. 3×3 coupler demodulation heavily relies on the properties of the devices. Dual-wavelength demodulation uses an ellipse fitting algorithm for demodulation; however, at low signal strengths, the ellipse degenerates into a straight line, making it difficult to accurately fit the DC component and introducing significant errors. Three-wavelength demodulation requires that the DC components of the three interference signals have equal AC amplitudes, necessitating strict control over the interferometer's reflectivity and contrast, the intensity of the incident light, insertion loss during transmission, and the sensitivity of the photodetector. Spectral demodulation is mainly used with Fabry-Poirot sensors; limited by the resolution of current spectral acquisition equipment, the demodulated signal frequency is typically no more than a few kHz, making it suitable for demodulating low-frequency signals. Summary of the Invention

[0006] In view of the above, the present invention provides a high-speed demodulation device for fiber optic sensors based on a cascaded Mach-Zehnder interferometer. This method has the advantages of fast demodulation speed, wide operating bandwidth, and ease of implementation.

[0007] A high-speed demodulation device for fiber optic sensors based on cascaded Mach-Zehnder interferometers is characterized by comprising a fiber optic sensing system and a high-speed demodulation system. The fiber optic sensing system responds to an external signal to be detected and converts it into a phase signal, which is then modulated onto an optical signal. The high-speed demodulation system demodulates the interference optical signal from the fiber optic sensing system to obtain the external signal to be detected. This scheme modulates the signal to be detected onto an optical signal using a fiber optic sensor and utilizes a demodulation system composed of two cascaded Mach-Zehnder interferometers to detect the interference optical signal from the sensing system, thereby achieving high-speed demodulation of the target signal.

[0008] Furthermore, the fiber optic sensing system includes: Broadband light source, used to generate low-coherence broadband lasers; A bandpass filter is used to bandpass filter broadband optical signals; A circulator is used to input broadband laser light into an optical fiber sensor and input the sensor's reflection spectrum into a high-speed demodulation system for demodulation. Interferometric fiber optic sensors are used to modulate the original signal to be detected onto an optical signal and convert it into phase information through interference effects, such as Fabry-Perot fiber optic sensors and Michelson fiber optic sensors. Furthermore, the high-speed demodulation system includes: The first Mach-Zehnder interferometer is used to split the filtered optical signal into two paths and generate two interference signals through the fixed optical path difference between its two arms. The two signals have the same free spectral range, and their peaks and troughs interweave, and are output to the second and third Mach-Zehnder interferometers, respectively. The second and third Mach-Zehnder interferometers are used to receive two signals from the first Mach-Zehnder interferometer and align them with their peaks, and further interfere with the signals through their internal optical path difference, and finally each outputs two interference light signals. The first and second balanced photodetectors are used to receive the interference light signals output by the second and third Mach-Zehnder interferometers, respectively, and convert them into differential electrical signals. Data acquisition card: Used to convert electrical signals into digital signals and perform high-speed demodulation using an internally integrated digital signal processing unit to obtain the external signal to be detected; Furthermore, the second and third Mach-Zehnder interferometers have the same free spectral range and are twice that of the first Mach-Zehnder interferometer.

[0009] Furthermore, the center wavelengths of the two-stage interference signals of the cascaded Mach-Zehnder interferometer are precisely aligned to obtain four-way interference outputs with equal optical frequency intervals.

[0010] Furthermore, the bandwidth of the bandpass filter is consistent with the free spectral range of the second and third Mach-Zehnder interferometers.

[0011] Furthermore, the two outputs of adjacent center wavelengths of the cascaded Mach-Zehnder interferometer have the same phase difference.

[0012] Furthermore, when the center wavelength of the cascaded Mach-Zehnder interferometer shifts, it causes a phase shift. The shift in center wavelength can be obtained from the output spectrum, and the phase shift can then be calculated. This effect can then be eliminated using trigonometric sum and difference formulas.

[0013] Furthermore, the digital signal processing unit includes a phase difference cancellation module, a division module, an arctangent module, and a phase decoupling module. The balanced photodetector converts the two input optical signals into differential electrical signals and performs digital sampling using a data sampling card. The outputs of the two balanced photodetectors have a constant phase difference. The phase difference cancellation module performs trigonometric function operations on one of the signals to eliminate the influence of this phase difference. This result, along with the output signal from the other balanced photodetector, is then divided in the division module and input to the arctangent module. The result is then input to the phase decoupling module for phase decoupling, thereby demodulating the external signal to be detected.

[0014] The device of this invention employs a two-stage cascaded Mach-Zehnder interferometer structure. By precisely aligning the center wavelengths of the two stages, a four-channel output with equal center wavelength spacing is obtained. This four-channel signal is then used to orthogonally demultiplex the spectral signal of the fiber optic acoustic sensor. In the digital domain, an orthogonal demodulation algorithm for phase demodulation of the fiber optic sensor is proposed, whose demodulation speed is limited only by the digital sampling rate. Compared with traditional spectral demodulation algorithms, this scheme has advantages such as fast demodulation speed, wide operating bandwidth, and ease of implementation. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the specific structure of the device of the present invention.

[0016] In the diagram: 101—Broadband light source, 102—Bandpass filter, 103—Circulator, 104—Fiber optic acoustic sensor, 105—First Mach-Zehnder interferometer, 106—Second Mach-Zehnder interferometer, 107—Third Mach-Zehnder interferometer, 108—First balanced photodetector, 109—Second balanced photodetector, 110—Data acquisition card.

[0017] Figure 2 This is a schematic diagram illustrating the implementation principle of the high-speed demodulation algorithm of the device of the present invention.

[0018] Figure 3This is a flowchart of the algorithm for correcting center wavelength offset in the device of the present invention.

[0019] Figure 4 This is a schematic diagram of the fiber optic acoustic sensor used in the device of the present invention.

[0020] In the diagram: 401—single-mode fiber, 402—fiber collimator, 403—ceramic ferrule, 404—epoxy resin adhesive, 405—brass encapsulation component, 406—titanium-aluminum-titanium sensing film.

[0021] Figure 5 This refers to the output characteristics of the two-stage cascaded Mach-Zehnder interferometer in the device of this invention.

[0022] Figure 6 The simulation demodulation results of the high-speed demodulation algorithm provided by the device of the present invention are shown. (a) shows the phase change information in the time domain; (b) shows the corresponding spectrum. Detailed Implementation

[0023] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0024] like Figure 1 As shown, the present invention relates to a high-speed demodulation device for fiber optic vibration sensors based on cascaded Mach-Zehnder interferometers. The entire device includes a broadband light source 101, a bandpass filter 102, a circulator 103, an interferometric fiber optic sensor 104, a first Mach-Zehnder interferometer 105, a second Mach-Zehnder interferometer 106, a third Mach-Zehnder interferometer 107, a first balanced photodetector 108, a second balanced photodetector 109, and a data acquisition card 110.

[0025] In this embodiment, the optical signal emitted by the broadband light source 101 is filtered by the bandpass filter 102, then passes through the circulator 103 and enters the interferometric fiber optic acoustic sensor 10. The sensor responds to the external acoustic signal, generating a vibration signal that causes a change in cavity length. The interfered signal is then processed by a first Mach-Zehnder interferometer 105, and cascaded second and third Mach-Zehnder interferometers 106 and 107 to obtain four outputs. A first balanced photodetector 108 and a second balanced photodetector 109 receive the optical signals from the second and third Mach-Zehnder interferometers, respectively, and differentially amplify and convert them into electrical signals, which are then input to the data acquisition card 110 for further sampling and demodulation. Specifically, the interference signal obtained by the fiber optic acoustic sensor can be expressed as...

[0026] In the formula, A It is the DC component. B It's about stripe contrast;n= 1.0003 is the refractive index of air. The wavelength of the broadband light source, This is the initial cavity length of the fiber optic acoustic sensor. The cavity length change is caused by the vibration signal generated by the sensor in response to external sound wave signals. After passing through a cascaded Mach-Zehnder interferometer, the obtained four-channel signal can be represented as...

[0027] In the formula, No. i The center wavelength of each channel i = 1, 2, 3, and 4. Among them, and The output of the second Mach-Zehnder interferometer and This is the output of the third Mach-Zehnder interferometer. Let...

[0028]

[0029] In the formula, j = 2, 3 and 4; It is the center wavelength of the broadband light source. Since the center light frequencies of each channel in a cascaded Mach-Zehnder interferometer are uniformly spaced, the center wavelength can be considered approximately uniformly spaced, and thus we can obtain...

[0030]

[0031] The four-channel signal can be rewritten as follows:

[0032] In the formula, The value depends on the center wavelength of the light source. Output wavelength spacing of cascaded Mach-Zehnder interferometers Difference between the initial arm length and the hydrophone Furthermore, both of these are known quantities. For a specific fiber optic hydrophone, the initial arm length difference of the fiber optic hydrophone... It is certain, at this time. It is a constant.

[0033] To obtain the external signal to be detected, according to one aspect of the present invention, an orthogonal demodulation algorithm for phase demodulation of an optical fiber sensor is provided, comprising: S1: Four optical signals with equal phase difference are output by using a cascaded Mach-Zehnder interferometer.

[0034] S2: By performing balanced detection on the output optical signals of the second and third Mach-Zehnder interferometers respectively, two differential signals can be obtained, defined as intermediate variables. and , can be represented as

[0035]

[0036] In the formula, It is a constant.

[0037] S3: Yes and By performing algebraic operations, we can obtain two orthogonal signals, defined as intermediate variables ES and OS, which can be expressed as follows:

[0038]

[0039] S4: By performing arctangent operation on the two obtained orthogonal signals, we can obtain...

[0040] S5: Finally, the external signal to be detected is obtained through phase convolution operation.

[0041] The implementation principle of the high-speed demodulation algorithm based on this four-channel is as follows: Figure 2 As shown.

[0042] Considering that in actual fabrication, due to factors such as light source flatness or incomplete alignment of the center wavelengths of the two-stage Mach-Zehnder interferometer, the transmittance of the four channels may be inconsistent or the center wavelength may have a certain offset, correction is necessary. For the issue of inconsistent transmittance, normalization can be achieved by dividing by the transmittance of each channel. Furthermore, when there is an offset between channels 2 and 4, the output of the second balanced detector will have a phase offset, which can be expressed as...

[0043] In the formula This refers to the phase shift, which needs to be corrected. Specifically, S2 includes: S201: Obtain the center wavelength of the four-channel output through spectral output. ~ .

[0044] S202: Calculate the offset of the center wavelength based on the principle of equal wavelength spacing, and you can obtain...

[0045] S203: The resulting phase shift can be calculated using the wavelength shift, and the following can be obtained:

[0046] S204: Then eliminate using the trigonometric sum and difference formulas. The effect, and thus the correction. ES , can be represented as

[0047] S205: Utilizing the revised ES Execute the orthogonal demodulation algorithms for S3 and S4.

[0048] S206: Receive the external signal to be detected.

[0049] The algorithm flow for correcting center wavelength offset is as follows: Figure 3 As shown.

[0050] Figure 4 A schematic diagram of the fiber optic acoustic sensor used in the device of this invention is shown, including a single-mode fiber 401, a fiber collimator 402, a ceramic ferrule 403, an epoxy resin adhesive 404, a brass encapsulation 405, and a titanium-aluminum-titanium sensing film 406. This acoustic sensor uses the sensing film as a reflective surface, forming an eigenfibrillary-Perot interferometer with the fiber optic cable. The optical signal input to the single-mode fiber 401 is collimated by the fiber collimator 402 before entering the fiber optic acoustic sensor. The ceramic ferrule 403 is used to fix the fiber optic cable, making it perpendicular to the sensing film 406. The brass encapsulation 405 is used to fix the boundary of the titanium-aluminum-titanium composite sensing film; the effective diameter of the sensing film depends on the inner diameter of the brass encapsulation. The titanium-aluminum-titanium sensing film 406 undergoes forced vibration under the influence of the acoustic signal, changing its distance from the fiber end face, thereby converting the acoustic signal into phase information of the interference optical signal.

[0051] The output characteristics of a cascaded Mach-Zehnder interferometer with four channels are as follows: Figure 5 As shown, the four-channel outputs have the same optical frequency spacing. When the interference signal from the fiber optic sensor is input into this cascaded Mach-Zehnder interferometer, adjacent channels have the same phase difference.

[0052] In one embodiment, the demodulation results of a phase signal with a frequency of 1 kHz and an amplitude of 3 rad were simulated. The time-domain phase change signal and its spectrum obtained from the simulation demodulation are shown below. Figure 6 (a) and Figure 6 As shown in (b).

[0053] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. It will be apparent to those skilled in the art that various modifications can be made to the above embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A high-speed demodulation device for fiber optic sensors based on a cascaded Mach-Zehnder interferometer, characterized in that: The system comprises a fiber optic sensing system and a high-speed demodulation system. The fiber optic sensing system responds to the external signal to be detected and converts it into a phase signal, which is then modulated onto an optical signal. The high-speed demodulation system demodulates the interference optical signal from the fiber optic sensing system to obtain the external signal to be detected. This scheme uses a fiber optic sensor to modulate the signal to be detected onto an optical signal and utilizes a demodulation system consisting of two cascaded Mach-Zehnder interferometers to detect the interference optical signal from the sensing system, thereby achieving high-speed demodulation of the target signal.

2. The demodulation device according to claim 1, characterized in that: The fiber optic sensing system includes: Broadband light source, used to generate low-coherence broadband lasers; A bandpass filter is used to bandpass filter broadband optical signals; A circulator is used to input broadband laser light into an optical fiber sensor and input the sensor's reflection spectrum into a high-speed demodulation system for demodulation. Interferometric fiber optic sensors are used to modulate the original signal to be detected onto an optical signal and convert it into phase information through interference effects, such as Fabry-Perot fiber optic sensors and Michelson fiber optic sensors.

3. The demodulation device according to claim 1, characterized in that: The high-speed demodulation system includes: The first Mach-Zehnder interferometer is used to split the input optical signal into two paths and generate two interference signals through the fixed optical path difference between its two arms. The two signals have the same free spectral range, and their peaks and troughs interweave, and are output to the second and third Mach-Zehnder interferometers, respectively. The second and third Mach-Zehnder interferometers are used to receive two signals from the first Mach-Zehnder interferometer and align them with their peaks. They further interfere with the signals through their internal optical path difference and finally output two interference light signals respectively. The first and second balanced photodetectors are used to receive the interference light signals output by the second and third Mach-Zehnder interferometers, respectively, and convert them into differential electrical signals. Data acquisition card: Used to convert electrical signals into digital signals and perform high-speed demodulation using an internally integrated digital signal processing unit to obtain the external signal to be detected.

4. The demodulation device according to claim 3, characterized in that: The second and third Mach-Zehnder interferometers have the same free spectral range and are twice that of the first Mach-Zehnder interferometer.

5. The demodulation device according to claim 3, characterized in that: The center wavelengths of the interference signals from the two stages of the cascaded Mach-Zehnder interferometer are precisely aligned, resulting in four interference outputs with equal optical frequency intervals.

6. The demodulation device according to claim 3, characterized in that: The bandwidth of the bandpass filter is consistent with the free spectral range of the second and third Mach-Zehnder interferometers.

7. The demodulation device according to claim 3, characterized in that: The two outputs of the cascaded Mach-Zehnder interferometer at adjacent center wavelengths have the same phase difference.

8. The demodulation device according to claim 3, characterized in that: The digital signal processing unit includes a phase difference cancellation module, a division module, an arctangent module, and a phase decoupling module. The balanced photodetector converts the two input optical signals into differential electrical signals and performs digital sampling using a data sampling card. The outputs of the two balanced photodetectors have a constant phase difference. The phase difference cancellation module performs trigonometric function operations on one of the signals to eliminate the influence of this phase difference. This result, along with the output signal from the other balanced photodetector, is then divided in the division module and input to the arctangent module. The result is then input to the phase decoupling module for phase decoupling, thereby demodulating the external signal to be detected.

9. The demodulation device according to claim 3, characterized in that: When the center wavelength of the cascaded Mach-Zehnder interferometer shifts, it causes a phase shift. The shift in center wavelength can be obtained from the output spectrum, and the phase shift can then be calculated. The effect can then be eliminated using trigonometric sum and difference formulas.

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