Method for Measuring Carrier Modulation Depth and Phase Delay and Carrier Demodulation System

Abstract

The present invention provides a method for measuring the modulation depth and phase delay of a carrier wave, comprising: obtaining the interference fringes of the carrier wave; calculating the frequency point energy of the interference fringes corresponding to each multiple frequency at at least 3 preset multiple frequencies to obtain at least 3 frequency point energies; calculating the modulation depth M and phase delay of the carrier wave according to the at least 3 frequency point energies. The measurement method provided by the present invention can effectively reduce the operation complexity of measuring the modulation depth of the carrier wave and the carrier phase delay. At the same time, the present invention also helps to improve the real-time performance of measuring the modulation depth of the carrier wave and the carrier phase delay.

Description

Technical Field

[0001] The present invention relates to the technical field of fiber optic sensing, and particularly to a method for measuring carrier modulation depth and phase delay and a carrier demodulation system. Background Art

[0002] In recent years, the phase-generated carrier (PGC) demodulation scheme has been a research hotspot in the field of fiber optic sensing and has been widely used in signal extraction of fiber optic sensors such as fiber optic hydrophones, fiber optic seismometers, and fiber optic microphones. The phase-generated carrier demodulation scheme has the advantages of large dynamic range, good stability, simple networking structure, and easy implementation of an all-optical sensing network. However, the phase-generated carrier scheme requires the modulation depth to be maintained at a fixed value (2.63 or 2.37) and the carrier phase delay to be 0; otherwise, the consistency and stability of the demodulation result will be affected. Therefore, it is necessary to measure the carrier modulation depth and the carrier phase delay.

[0003] Many of the currently proposed measurement methods for modulation depth and carrier phase delay have the disadvantages of limited measurement accuracy, high complexity and large computational amount, and poor real-time performance. Summary of the Invention

[0004] (1) Technical Problems to be Solved

[0005] In view of the problems of poor real-time performance, complex operations, and measurement accuracy existing in the current carrier modulation depth and carrier phase delay measurement schemes, the present invention realizes the synchronous measurement of the carrier modulation depth and the carrier phase delay by calculating the energy of specific frequency points of interference fringes, with a simple process, small computational amount, and real-time measurement.

[0006] (2) Technical Solutions

[0007] The present invention provides a method for measuring carrier modulation depth and phase delay, including:

[0008] Obtaining the interference fringes of the carrier; calculating the frequency point energy of the interference fringes corresponding to each of at least 3 preset multiple frequencies to obtain at least 3 frequency point energies; calculating the modulation depth M and the phase delay of the carrier according to the at least 3 frequency point energies

[0009] Optionally, the calculating the frequency point energy of the interference fringes corresponding to each of at least 3 preset multiple frequencies includes: expanding the interference fringes based on the Bessel function; determining the at least 3 preset multiple frequencies according to the order of the expanded Bessel function.

[0010] Optionally, calculating the frequency point energy of the interference fringes corresponding to each multiple frequency includes: calculating the frequency point energy of the interference fringes corresponding to each multiple frequency by using at least 3 groups of parallel Goertzel algorithms.

[0011] Optionally, calculating the modulation depth M of the carrier according to the at least 3 frequency point energies includes:

[0012] M = 2iV i / [V i-1 +V i+1

[0013] where i is the order of the Bessel function, and V i is the frequency point energy of the i-th multiple frequency of the interference fringes.

[0014] Optionally, calculating the phase delay of the carrier according to the at least 3 frequency point energies includes:

[0015]

[0016] where i is the order of the Bessel function, and V i is the frequency point energy of the i-th multiple frequency of the interference fringes, Im(V i ) is the imaginary part of the complex number V i , and Re(V i ) is the real part of the complex number V i .

[0017] Optionally, calculating the modulation depth M of the carrier according to the at least 3 frequency point energies further includes: selecting different i values to obtain multiple groups of M = 2iV i / [V i-1 +V i+1 ; calculating the modulation depth M through the multiple groups of M = 2iV i / [V i-1 +V i+1 .

[0018] Optionally, the calculating the modulation depth M through the multiple groups of M = 2iV i / [V i-1 +V i+1 includes:

[0019]

[0020] where i takes 2, 3, 4, 5.

[0021] Optionally, the calculating the modulation depth M through the multiple groups of M = 2iV i / [V i-1 +V i+1 includes:​

[0022]

[0023] Among them, i takes 2, 3, and 4.

[0024] On the other hand, the present invention provides a carrier demodulation system, including:

[0025] A tunable narrow linewidth laser 10, suitable for providing narrow linewidth laser;

[0026] An optical isolator 20, suitable for preventing the narrow linewidth laser from being interfered by external factors;

[0027] An optical attenuator 30, suitable for adjusting the optical power of the narrow linewidth laser within the response range of a photodetector 60;

[0028] An optical fiber circulator 40, suitable for transmitting the narrow linewidth laser to an interferometric fiber optic sensor 50 and transmitting the interference fringes of the interferometric fiber optic sensor 50 to the photodetector (60);

[0029] An interferometric fiber optic sensor 50, suitable for converting the narrow linewidth laser into the interference fringes;

[0030] A photodetector 60, suitable for converting the interference fringes into an electrical signal;

[0031] A data acquisition card 70, suitable for converting the electrical signal into a digital signal;

[0032] An arithmetic unit 80, suitable for receiving the digital signal and measuring the carrier modulation depth and carrier phase delay by using the measurement method described above;

[0033] A carrier signal generator 90, suitable for generating a carrier signal of the modulation depth and the carrier phase, wherein the carrier signal is used to modulate the tunable narrow linewidth laser 10.

[0034] (III) Beneficial effects

[0035] The measurement method provided by the present invention can effectively reduce the arithmetic complexity of measuring the carrier modulation depth and carrier phase delay. At the same time, the present invention also calculates the energies of multiple specific frequency points in real time through multiple groups of parallel Goertzel algorithms, which helps to improve the real-time performance of measuring the carrier modulation depth and carrier phase delay.

[0036] In the phase-generated carrier demodulation system provided by the present invention, according to the carrier modulation depth and carrier phase delay calculated in real time, the amplitude and phase of the output sine wave of the carrier signal generator 90 are adjusted in real time, so that the carrier modulation depth and carrier phase delay are stabilized at the ideal values, thereby improving the consistency and stability of the fiber optic sensor demodulation system. Brief Description of the Drawings

[0037] Figure 1 The flowchart of the measurement method provided by the present invention is shown.

[0038] Figure 2 The schematic diagram of the measurement system provided by the present invention is schematically shown. Detailed Description of the Preferred Embodiments

[0039] To make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to specific embodiments and the accompanying drawings.

[0040] The present invention provides a method for measuring the modulation depth and phase delay of a carrier wave. Referring to Figure 1 , it includes:

[0041] S101. Obtain the interference fringes of the carrier wave.

[0042] S102. Calculate the frequency point energy of the interference fringes corresponding to each multiple frequency at at least 3 preset multiple frequencies to obtain at least 3 frequency point energies.

[0043] S103. Calculate the modulation depth M and phase delay of the carrier wave according to the at least 3 frequency point energies

[0044] In step S101 of the present invention, the interference fringes output by the fiber optic interferometer are obtained, which can be expressed by the following formula:

[0045]

[0046] where A is the DC component of the interference fringes, B is the amplitude of the AC component of the interference fringes, M is the modulation depth, ω M is the modulation angular frequency, is the carrier phase delay, is the sensing signal.

[0047] In step 102 of the present invention, at at least 3 preset multiple frequencies, the frequency point energy of the interference fringes corresponding to each multiple frequency is calculated to obtain at least 3 frequency point energies, which includes the following sub-steps:

[0048] Expand the interference fringes based on the Bessel function;

[0049] Determine the at least 3 preset multiple frequencies according to the order of the expanded Bessel function.

[0050] First, expand using the Bessel function J i (M) to obtain:

[0051]

[0052] Then, according to the recurrence relation of Bessel functions, the modulation depth M can be expressed as:

[0053] M = 2iJ i (M) / [J i-1 (M) + J i+1 (M)]

[0054] where i is the order of the Bessel function.

[0055] Next, let the frequency point energy of the i - multiple frequency of the carrier modulation frequency be V i , and since V i ∝ 2J i (M), the expression of M can also be represented as:

[0056] M = 2iV i / [V i-1 + V i+1

[0057] Therefore, the present invention can determine at least three preset multiple frequencies according to the order of the Bessel function to calculate the adjustment depth M according to the above formula. For example, if the order i of the Bessel function is taken as 2, the corresponding preset multiple frequencies are 1, 2, 3, and the corresponding frequency point energies are V1, V2, V3, then M = 4V2 / (V1 + V3).

[0058] By separately analyzing the frequency point energy of the i - multiple frequency of the interference fringes of the carrier, the phase delay of the carrier can be obtained according to the phase angle characteristics and its formula is expressed as follows:

[0059]

[0060] Im(V i ) is the imaginary part of the complex number V i , Re(V i ) is the real part of the complex number V i , and atan() is the arctangent function. Therefore, by substituting the real part and the imaginary part of V i into the above formula, the phase delay

[0061] ​Further, in step S103 of the present invention, calculating the frequency point energy of the interference fringes corresponding to each multiple frequency under multiple preset multiple frequencies includes: calculating the frequency point energy of the interference fringes corresponding to each multiple frequency by using at least 3 groups of parallel Goertzel algorithms. The present invention proposes a method of implementing multiple groups of parallel Goertzel algorithms to calculate the frequency point energy of the preset multiple frequencies of the carrier in real time and synchronously. The Goertzel algorithm can selectively obtain the energy of a specific frequency signal from the sampled data. When only calculating the energy of a few frequency points of interest, the Goertzel algorithm based on recursive calculation is faster and more flexible than the FFT algorithm. Its transfer function is as follows:

[0062]

[0063] In an embodiment of the present invention, based on the parallel operation characteristics of the FPGA, a parallel implementation method of the Goertzel algorithm based on the FPGA is proposed. After the FPGA module obtains the interference fringes of the carrier, multiple groups of Goertzel algorithms are synchronously implemented and run inside.

[0064] From M = 2iV i / [V i-1 +V i+1 , it can be seen that in the present invention, through the order i of the Bessel function, the required frequency point energy V i 、V i+1 、V i-1 can be determined, and thus the adjustment depth M of the carrier is calculated. This method can effectively reduce the computational complexity of the carrier modulation depth and carrier phase delay measurement. At the same time, the present invention also calculates the energy of multiple specific frequency points in real time through multiple groups of parallel Goertzel algorithms, which helps to improve the real-time performance of the carrier modulation depth and carrier phase delay measurement.

[0065] Further, in the measurement method proposed by the present invention, calculating the modulation depth M of the carrier according to at least 3 frequency point energies further includes: selecting different i values to obtain multiple groups of M = 2iV i / [V i-1 +V i+1 ; calculating the modulation depth M through the multiple groups of M = 2iV i / [V i-1 +V i+1 combinations. Based on this, the measurement method provided by the present invention can avoid large measurement errors caused by the situation that (V i+1 +V i-1 ) is too small or zero, effectively eliminate errors, and make the measurement result more accurate.

[0066] The energies of six specific frequency points, including the energies V1 to V6 of the 1st to 6th harmonic frequency points, can be calculated in real time through multiple groups of parallel Goertzel algorithms.

[0067] In an embodiment of the present invention, when i takes 2, 3, 4, or 5, the calculation formula for the adjustment depth M can be obtained as follows:

[0068]

[0069] Furthermore, it can be obtained that:

[0070]

[0071] Thus, it can be obtained that:

[0072]

[0073] That is:

[0074]

[0075] Substituting the energies V1 to V6 of the above 1st to 6th harmonic frequency points into the above formula, the more accurate value of the modulation depth M can be obtained.

[0076] In another embodiment of the present invention, by selecting the calculation formulas of M with any three adjacent Bessel orders, it can be obtained that:

[0077]

[0078] Therefore, there is:

[0079]

[0080] Eliminating V i and V i+2 in the above formula, it is obtained that:

[0081]

[0082] Taking i as 2, it is obtained that

[0083]

[0084] That is, the above formula is calculated by combining the calculation formulas of M for 2, 3, and 4.

[0085] The present invention also provides a phase-generated carrier demodulation system, including:

[0086] A tunable narrow linewidth laser 10, suitable for providing narrow linewidth laser;

[0087] An optical isolator 20, suitable for preventing the narrow linewidth laser from being interfered by external factors;

[0088] An optical attenuator 30, which is suitable for adjusting the optical power of the narrow linewidth laser within the response range of the photodetector 60;

[0089] An optical fiber circulator 40, which is suitable for transmitting the narrow linewidth laser to the interferometric fiber optic sensor 50 and transmitting the interference fringes of the interferometric fiber optic sensor 50 to the photodetector (60);

[0090] An interferometric fiber optic sensor 50, which is suitable for converting the narrow linewidth laser into the interference fringes;

[0091] A photodetector 60, which is suitable for converting the interference fringes into an electrical signal;

[0092] A data acquisition card 70, which is suitable for converting the electrical signal into a digital signal;

[0093] An arithmetic unit 80, which is suitable for receiving the digital signal and measuring the carrier modulation depth and carrier phase delay by using the measurement method described above;

[0094] A carrier signal generator 90, which is suitable for generating a carrier signal of the modulation depth and the carrier phase, wherein the carrier signal is used to modulate the tunable narrow linewidth laser 10.

[0095] As Figure 2 shown, the tunable narrow linewidth laser 10 emits a single-frequency laser, i.e., a carrier, which is transmitted to the incident port of the optical fiber circulator 40 through the optical attenuator 20 and the optical isolator 30, and then enters the interferometric fiber optic sensor 50 from the first output port of the optical fiber circulator 40. The basic principle of the interferometric fiber optic sensor 50 is a Michelson interferometer, which consists of an optical fiber coupler, two optical fibers, and two Faraday rotators. The input laser is split into two by the optical fiber coupler, and then transmitted in the optical fiber and reflected by the two Faraday rotators, and then exits from the optical fiber coupler. The function of the Faraday rotator is to resist polarization fading; the return light of the interferometric fiber optic sensor 50 is the interference fringes of the carrier, which enters through the first output port of the optical fiber circulator 40 and then exits from the second output port of the optical fiber circulator 40. The carrier signal generator 90 is used to generate a sine modulation signal of a specific frequency to modulate the tunable narrow linewidth laser 10.

[0096] The output light of the optical fiber circulator 40 enters the photodetector 60 to realize the conversion of the optical signal into an electrical signal; then, the analog electrical signal output by the photodetector 60 is converted into a digital signal by the signal acquisition card 70, and at this time, the acquisition of the interference fringes is completed; the digital signal output by the signal acquisition card 70 enters the arithmetic unit 80 to realize the low-complexity and high-real-time calculation of the carrier modulation depth and carrier phase delay. The arithmetic unit 80 can be a field programmable gate array (FPGA).

[0097] In the system provided by the present invention, according to the real-time calculated M and the amplitude and phase of the output sine wave of the carrier signal generator 90 are adjusted in real time, so that M and are stabilized at the ideal values, thereby improving the consistency and stability of the fiber optic sensor demodulation system.

[0098] The specific embodiments described above further elaborate on the purpose, technical solutions, and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for measuring the modulation depth and phase delay of a carrier wave, characterized in that, Including: Obtaining the interference fringes of the carrier wave; Expanding the interference fringes based on the Bessel function, and determining at least three preset multiple frequencies according to the order of the expanded Bessel function; At at least three preset multiple frequencies, using at least three groups of parallel Goertzel algorithms to calculate the frequency point energy of the interference fringes corresponding to each preset multiple frequency, and obtaining at least three frequency point energies; According to calculate the modulation depth M of the carrier wave; according to calculate the phase delay of the carrier wave , where i is the order of the Bessel function, V i is the frequency point energy of the i-th harmonic of the interference fringe, Im(V i ) is the imaginary part of the complex number V i , and Re(V i ) is the real part of the complex number V i .

2. The measurement method according to claim 1, characterized in that, Calculating the modulation depth M of the carrier wave according to the at least three frequency point energies further includes: Select different values of i to obtain multiple groups of ; Through the multiple groups calculate the modulation depth M through combined calculation.

3. The measuring method according to claim 2, characterized in that, The above-mentioned through the above-mentioned multiple groups The combined calculation of the modulation depth M includes: where i takes 2, 3, 4, 5.

4. The measuring method according to claim 2, wherein The above-mentioned multi-group Combined calculation of the modulation depth M includes: where i takes 2, 3, 4.

5. A carrier wave demodulation system, including: A tunable narrow linewidth laser (10), suitable for providing narrow linewidth laser light; An optical isolator (20), suitable for preventing the narrow linewidth laser light from being interfered by external factors; An optical attenuator (30), suitable for adjusting the optical power of the narrow linewidth laser light within the response range of a photodetector (60); An optical fiber circulator (40), suitable for transmitting the narrow linewidth laser light to an interferometric fiber optic sensor (50), and transmitting the interference fringes of the interferometric fiber optic sensor (50) to the photodetector (60); An interferometric fiber optic sensor (50), suitable for converting the narrow linewidth laser light into the interference fringes; A photodetector (60), suitable for converting the interference fringes into an electrical signal; A data acquisition card (70), suitable for converting the electrical signal into a digital signal; An operation unit (80), suitable for receiving the digital signal and measuring the carrier wave modulation depth and the carrier wave phase delay by using the measurement method according to any one of claims 1 to 4; A carrier wave signal generator (90), suitable for emitting a carrier wave signal of the modulation depth and the carrier wave phase, wherein the carrier wave signal is used for modulating the tunable narrow linewidth laser (10).

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