OFDR detection distance extension system and method based on real-time adaptive SOGI spread spectrum technology

Abstract

The invention discloses an OFDR detection distance prolonging system and method based on a real-time adaptive SOGI spread spectrum technology, and the method comprises the steps: obtaining main components and orthogonal components of an original signal through an SOGI algorithm, obtaining a signal phase through triangular calculation, achieving the frequency multiplication, obtaining an instantaneous frequency through a differential algorithm and short-time frequency measurement, and forming adaptive frequency feedback. The phase signal after frequency multiplication is unwound and then is used for equal optical frequency sampling, so that the distance utilization rate of the auxiliary interferometer can be improved. The algorithm is realized on a field programmable gate array (FPGA), and a real-time processing mechanism is designed according to the characteristics of FPGA pipeline calculation. And the detection distance of the OFDR is effectively prolonged under the condition of meeting real-time processing. According to the invention, the detection distance of the OFDR is increased mainly aiming at the data utilization rate of the OFDR under the condition that the optical path is not changed.

Description

Technical Field

[0001] This invention relates to optical frequency domain reflectometers and real-time data processing, specifically to an OFDR detection range extension system and method based on real-time adaptive SOGI (Second Order Generalized Integrator) spread spectrum technology. Background Technology

[0002] Optical Frequency Domain Reflectometry (OFDR), as a fiber optic sensing technology, has attracted increasing attention since its inception due to its advantages such as high spatial resolution, high sensitivity, and wide precision range. OFDR is widely used in high-precision fiber optic breakpoint detection, short-to-medium distance high-precision fiber optic sensing, and grating array detection. The detection range of OFDR is a focus of much research. Improving the detection range of OFDR without altering the existing optical path structure, increasing processing time, and meeting real-time requirements would be a crucial performance enhancement for both breakpoint detection and fiber optic sensing.

[0003] Since OFDR often employs a dual-interferometer detection strategy, the length of the auxiliary interferometer significantly limits the detection distance. According to the sampling theorem, OFDRs using this method often cannot exceed half the difference in arm length between the auxiliary interferometers. Furthermore, directly using the signal from the auxiliary interferometer as the sampling clock, while simple, results in extremely low data utilization.

[0004] Existing technology 1 [Wang Huiwen et al., Device and Method for Long-Distance Measurement via OFDR Segmented Acquisition, CN111578971B] proposes a device and method for long-distance measurement via OFDR segmented acquisition. It employs an auxiliary interferometer, using the generated beat frequency signal as the external clock of the data acquisition card to sample the beat frequency signal of the main interferometer at equal frequency intervals. Segmented measurement is used to acquire the beat frequency signal, and the beat frequency signal is simultaneously spliced ​​in the distance domain to obtain the long-distance OFDR beat frequency signal. This invention achieves long-distance OFDR measurement, but it requires modification of the existing optical path, involves multiple interferometers, is relatively complex, and significantly increases the data volume, making real-time processing difficult.

[0005] Existing Technology 2 [Shen Zhen et al. Research on improving the spatial resolution of externally modulated OFDR by frequency domain interpolation and window width optimization [J]. Acta Optica Sinica, 2023, 36(10).] This paper proposes an externally modulated OFDR system, which can improve spatial resolution by reducing the sliding window in the data processing process. Frequency domain interpolation and window width optimization methods are proposed. In the data processing flow, frequency domain interpolation is used to improve the positioning accuracy of the system. By comprehensively evaluating the positioning error of the system and balancing the window width and the number of interpolation bits, the spatial resolution of the system can be further improved while taking into account the positioning accuracy. This method can optimize data processing, but its focus is on spatial resolution, and the improvement in detection distance is relatively small.

[0006] In summary, improving OFDR data utilization and extending detection range without changing the existing optical path or increasing processing time is a crucial performance enhancement. Summary of the Invention

[0007] To overcome the shortcomings of existing technologies, this invention proposes an OFDR detection range extension algorithm based on real-time adaptive SOGI spread spectrum technology. The SOGI algorithm obtains the main and orthogonal components of the original signal, and the signal phase is obtained through trigonometric calculations to achieve frequency doubling. The instantaneous frequency is then obtained using a differential algorithm and short-time frequency measurement, forming an adaptive frequency feedback. The frequency-doubled phase signal is decoupled and then used for equal-frequency sampling, which improves the range utilization of the auxiliary interferometer. Furthermore, a real-time processing mechanism is designed based on the characteristics of FPGA pipeline computing, enabling effective extension of the OFDR detection range while meeting real-time processing requirements.

[0008] The technical solution of the present invention is as follows:

[0009] On the one hand, the present invention provides an OFDR detection range extension system based on real-time adaptive SOGI spread spectrum technology, characterized in that it includes:

[0010] The instantaneous frequency feedforward module is used to measure the short-time frequency of a signal using the zero-crossing counting method and to provide frequency feedforward for the SOGI calculation module.

[0011] The SOGI calculation module is used to convert the filter transfer function in the analog domain into the difference equation in the digital domain according to the difference equation, calculate the main components and orthogonal components of the signal in real time, and adjust the SOGI coefficients according to the instantaneous input frequency to achieve dynamic frequency tracking.

[0012] The phase angle calculation module, including the Atan calculation module, the unwinding module, and the frequency multiplication module, is used to calculate the phase of the signal and perform unwinding and frequency multiplication processing.

[0013] The instantaneous frequency feedback module, including a differentiating module and a low-pass filter module, is used to convert the input phase angle value into an instantaneous frequency value, perform filtering processing, and then input the weighted synthesis with the feedforward instantaneous frequency into the SOGI calculation module.

[0014] The OFDR data processing module is used to resample the OFDR signal and store the processed data in an off-chip memory chip.

[0015] Furthermore, the instantaneous frequency feedforward module counts within the signal period using an incrementing counter and latches the count value at the edge of the signal period, obtaining the feedforward instantaneous frequency of the signal through reciprocal calculation.

[0016] Furthermore, the SOGI calculation module includes a D flip-flop, a multiplier, an adder, and a bit-width adjuster, used to realize real-time calculation of the SOGI algorithm.

[0017] Furthermore, the Atan calculation module in the phase angle calculation module calculates the phase angle using the CORDIC method, the unwinding module converts the Atan output into a continuous phase angle, and the frequency multiplication module amplifies the phase angle value by an equal multiple and limits it to the range of 0 to 2π through remainder processing.

[0018] Furthermore, the instantaneous frequency feedback module converts the phase angle value into an instantaneous frequency value through a differentiating module, and a low-pass filter filters the instantaneous frequency value to prevent high-frequency noise interference.

[0019] Furthermore, the OFDR data processing module performs equal-frequency sampling on the signal after SOGI processing to improve the distance utilization of the auxiliary interferometer.

[0020] On the other hand, the present invention also provides a method for extending the OFDR detection range based on real-time adaptive SOGI spread spectrum technology, characterized by comprising the following steps:

[0021] a) Using a continuously tuned laser as a light source, the output continuously tuned light passes through a coupler and enters the auxiliary interferometer and the main interferometer respectively. Through interference and photoelectric conversion, the optical signal is converted into an electrical signal, and then converted into a digital signal by a sampling module, and enters the FPGA data processing system.

[0022] b) Measure the short-time frequency of the digital signal and obtain the feedforward instantaneous frequency of the signal;

[0023] c) Input the digital signal into the SOGI calculation module, calculate the SOGI expression, and obtain two output signals;

[0024] d) Connect the output signal of the SOGI calculation module to the phase angle calculation module to calculate the continuous phase angle and multiply the phase angle signal to obtain the signal after unwinding and phase frequency multiplication;

[0025] e) The unwound signal is fed into the differential module for numerical differential calculation, then passed through a low-pass filter and added to the output of the instantaneous frequency acquisition unit, and written back to the SOGI calculation module to achieve real-time frequency adjustment.

[0026] f) Delay adjustment is performed on another digital signal to achieve synchronization with the processed signal, then OFDR data processing is performed, and the processed data is written to the off-chip memory unit.

[0027] Furthermore, step b) specifically includes: using a numerical comparator, a D flip-flop, a counter, a digital register, and logic gates to capture the rising edge corresponding to the zero crossing point, and using the output of the logic gates to achieve synchronous counter clearing and numerical register latching.

[0028] Furthermore, in step c), the SOGI calculation module consists of two second-order difference equation calculation units, each corresponding to two outputs. Each second-order difference equation calculation unit consists of a delay unit, a bit width adjuster, and an adder.

[0029] Furthermore, in step d), the phase angle calculation module calculates Atan through the CORDIC calculation unit and outputs the winding phase angle from 0 to 2π. Then, it performs unwinding and phase frequency multiplication. The unwinding part uses an output register, an adder / subtractor, a numerical comparator, and a data selector, while the frequency multiplication part uses a multiplier and a 2N remainder unit.

[0030] Furthermore, in step e), the differential module consists of a numerical register, a subtractor, and a multiplier. It adopts the second-order differential calculation principle, subtracts N rounds of data from N-2 rounds of data, and then multiplies the result by 0.5 to obtain the numerical differential calculation result.

[0031] Furthermore, the delay adjustment unit in step f) is for adjusting the delay of the digital signal to achieve signal synchronization and ensure that no misalignment occurs during the processing. The OFDR data processing and writing module adopts the classic OFDR digital signal processing method.

[0032] The beneficial effects of this invention are: by calculating the phase angle using SOGI and phase angle, the phase is directly extracted and frequency multiplied, which can effectively improve data utilization and extend the detection range of OFDR without changing the existing optical path. Furthermore, because this processing is synchronized with signal sampling and is real-time, the input and output signals only have a fixed clock beat difference, which has no impact on the overall processing time of OFDR. Attached Figure Description

[0033] Figure 1 The amplitude and phase response of SOGI

[0034] Figure 2Schematic diagram of real-time adaptive SOGI

[0035] Figure 3 OFDR optical path schematic diagram for real-time adaptive SOGI

[0036] Figure 4 Logic diagram of the zero-crossing counter method

[0037] Figure 5 Schematic diagram of the SOGI computing module

[0038] Figure 6 Logic diagram of the phase angle calculation module

[0039] Figure 7 Logic diagram of the differential calculation module Detailed Implementation

[0040] To facilitate understanding of the embodiments of the present invention, the most representative embodiments will be further explained and described below with reference to the accompanying drawings. However, these embodiments do not constitute a limitation on the embodiments of the present invention.

[0041] The amplitude-frequency response and phase-frequency response of the real-time adaptive SOGI proposed in this embodiment are as follows: Figure 1 As shown in the figure, SOGI is a set of orthogonal filters, and the output of the filters has bandpass filter characteristics. The integrator has two outputs, A and B. The output signals are filtered by the bandpass filter in terms of amplitude and are 90° out of phase. For orthogonal signals processed by the filter, the phase angle can be obtained using Atan. The schematic diagram of the detection distance extension method proposed in this embodiment, implemented using FPGA, is shown below. Figure 2 It mainly includes an OFDR optical path and signal acquisition unit 1, an FPGA data processing system 2, and an off-chip storage unit 3. The FPGA data processing system 2 can be further subdivided into a delay adjustment unit 21, an instantaneous frequency acquisition unit 22, a reciprocal calculation unit 23, an SOGI calculation module 24, a phase angle calculation module 27, a differential calculation module 25, a low-pass filter 26, and an OFDR data processing and writing module 28.

[0042] The proposed method for extending the detection range based on real-time adaptive SOGI mainly includes five steps:

[0043] 1. For example Figure 3A continuously tuned laser 10 serves as the light source. The continuously tuned light output by the laser passes through coupler 111 and then enters the auxiliary interferometer and the main interferometer, respectively. In the auxiliary interferometer, the laser beam is split into two beams by coupler 114. One beam, after passing through delay fiber 13, interferes in coupler 115. The length of delay fiber 13 limits the detection distance of the OFDR. The interference result is converted into an electrical signal 152 by photodetector 142. In the main interferometer, the situation is similar. The laser beam is split into two beams by coupler 112. One beam passes through circulator 12 and enters the detection fiber 16. The returning light passes through circulator 12 and interferes in coupler 113. The interference result is converted into an electrical signal 151 by photodetector 141. Electrical signals 151 and 152 are converted into digital signals 171 and 172 by sampling module 17 and then enter the FPGA data processing system 2.

[0044] 2. For digital signal 172, the short-time frequency of the signal is measured using the instantaneous frequency acquisition unit 22. This method uses an incremental counting method within one period of the signal, with the counter's clock being the signal's sampling clock. The counter is latched at the edge of the signal period and then cleared. The specific implementation steps of this step are as follows: Figure 5 This design utilizes a numerical comparator, two D flip-flops, a counter, a digital register, and several logic gates to capture the rising edge corresponding to the zero-crossing point. Based on the outputs of the logic gates, synchronous counter clearing and digital register latching are achieved. The output of the digital register is connected to the reciprocal calculation unit 23 to obtain the instantaneous feedforward frequency of the signal.

[0045] 3. Digital signal 172 is input to SOGI calculation module 24, which is responsible for calculating the SOGI expression. Its schematic diagram is shown below. Figure 5 This module consists of two second-order difference equation calculation units, corresponding to outputs A and B respectively. Each second-order difference equation calculation unit comprises a delay unit, a bit-width adjuster, and an adder. The outputs of the two second-order difference equation calculation units are digital signal 241 and digital signal 242, respectively.

[0046] 4. Digital signals 241 and 242 are connected to phase angle calculation module 27. This module is responsible for calculating continuous phase angles and multiplying the phase angle signals. Its schematic diagram is shown below. Figure 6Digital signals 241 and 242 are processed by a CORDIC calculation unit to output a winding phase angle of 0 to 2π. The CORDIC calculation unit is a classic FPGA method for calculating Atan, typically implemented using an IP core. The output winding phase angle is then unwound and frequency-multiplied. In the unwound section, the output register latches the previous data, and an adder / subtractor calculates the difference between the two data sets. A comparator checks if the difference is greater than 0. If the difference is greater than 0, the data selector outputs the difference directly; if the difference is less than 0, it is added to 2π and then output by the data selector. The difference output by the data selector is then accumulated and output to signal 271. In the frequency multiplication section, a multiplier first doubles the phase, and then... N The remainder circuit maps the data that exceeds the 2π range after doubling to the range of 0 to 2π, achieving phase frequency multiplication, and the output signal is signal 272.

[0047] 5. Signal 271 is connected to the differential calculation module 25, and its logic diagram is as follows. Figure 7 It consists of three numerical registers, a subtractor, and a multiplier. Employing the principle of second-order differential calculation, it subtracts N rounds of data from N-2 rounds of data, then multiplies the result by 0.5 to obtain the numerical differential calculation result. The output result is passed through a low-pass filter 26, added to the output of the instantaneous frequency acquisition unit 22, and written back to the SOGI calculation module 24. The SOGI calculation module 24 uses this value to modify the center frequency of the integrator, achieving real-time frequency adjustment.

[0048] 6. Digital signal 171 is input to the OFDR data processing and writing module 28 via delay adjustment unit 21. Delay adjustment unit 21 adjusts the delay between digital signal 272 and digital signal 172. Since the processed signal 272 has a longer clock cycle delay compared to 171, delaying 171 ensures signal synchronization and prevents misalignment during processing. OFDR data processing and writing module 28 is primarily responsible for subsequent data processing of the signal, employing classic OFDR digital signal processing methods. The processed data is then written to external storage unit 3 for later use.

[0049] The foregoing detailed description of some embodiments of the present invention with reference to the accompanying drawings is provided, but the present invention is not limited to the implementation methods described above. Various modifications or variations made without departing from the spirit of the present invention are all within the scope of this patent. The scope of protection of the present invention should not be limited thereto.

Claims

1. An OFDR detection range extension system based on real-time adaptive SOGI spread spectrum technology, characterized in that, include: The instantaneous frequency feedforward module is used to measure the short-time frequency of a signal using the zero-crossing counting method and to provide frequency feedforward for the SOGI calculation module. The SOGI calculation module is used to convert the filter transfer function in the analog domain into the difference equation in the digital domain according to the difference equation, calculate the main components and orthogonal components of the signal in real time, and adjust the SOGI coefficients according to the instantaneous input frequency to achieve dynamic frequency tracking. The phase angle calculation module, including the Atan calculation module, the unwinding module, and the frequency multiplication module, is used to calculate the phase of the signal and perform unwinding and frequency multiplication processing. The instantaneous frequency feedback module, including a differentiating module and a low-pass filter module, is used to convert the input phase angle value into an instantaneous frequency value, perform filtering processing, and then input the weighted synthesis with the feedforward instantaneous frequency into the SOGI calculation module. The OFDR data processing module is used to resample the OFDR signal and store the processed data in an off-chip memory chip.

2. The OFDR detection distance extension system according to claim 1, characterized in that, The instantaneous frequency feedforward module counts within the signal period using an incrementing counter and latches the count value at the edge of the signal period, obtaining the feedforward instantaneous frequency of the signal through reciprocal calculation.

3. The OFDR detection distance extension system according to claim 1, characterized in that, The SOGI calculation module includes a D flip-flop, a multiplier, an adder, and a bit-width adjuster, used to realize the real-time calculation of the SOGI algorithm.

4. The OFDR detection distance extension system according to claim 1, characterized in that, The Atan calculation module in the phase angle calculation module uses the CORDIC method to calculate the phase angle, the unwinding module converts the Atan output into a continuous phase angle, and the frequency multiplication module amplifies the phase angle value by an equal multiple and limits it to the range of 0 to 2π through remainder processing.

5. The OFDR detection distance extension system according to claim 1, characterized in that, The instantaneous frequency feedback module converts the phase angle value into an instantaneous frequency value through a differentiating module, and the low-pass filter filters the instantaneous frequency value to prevent high-frequency noise interference.

6. The OFDR detection distance extension system according to claim 1, characterized in that, The OFDR data processing module performs equal-frequency sampling on the signal after SOGI processing to improve the distance utilization of the auxiliary interferometer.

7. A method for extending the detection range of OFDR based on real-time adaptive SOGI spread spectrum technology, characterized in that, Includes the following steps: a) Using a continuously tuned laser as a light source, the output continuously tuned light passes through a coupler and enters the auxiliary interferometer and the main interferometer respectively. Through interference and photoelectric conversion, the optical signal is converted into an electrical signal, and then converted into a digital signal by a sampling module, and enters the FPGA data processing system. b) Measure the short-time frequency of the digital signal and obtain the feedforward instantaneous frequency of the signal; c) Input the digital signal into the SOGI calculation module, calculate the SOGI expression, and obtain two output signals; d) Connect the output signal of the SOGI calculation module to the phase angle calculation module to calculate the continuous phase angle and multiply the phase angle signal to obtain the signal after unwinding and phase frequency multiplication; e) The unwound signal is fed into the differential module for numerical differential calculation, then passed through a low-pass filter and added to the output of the instantaneous frequency acquisition unit, and written back to the SOGI calculation module to achieve real-time frequency adjustment. f) Delay adjustment is performed on another digital signal to achieve synchronization with the processed signal, then OFDR data processing is performed, and the processed data is written to the off-chip memory unit.

8. The OFDR detection distance extension method according to claim 1, characterized in that, Step b) specifically includes: using a numerical comparator, D flip-flop, counter, digital register and logic gate to capture the rising edge corresponding to the zero crossing point, and realizing synchronous counter clearing and numerical register latching based on the output of the logic gate.

9. The OFDR detection distance extension method according to claim 1, characterized in that, In step c), the SOGI calculation module consists of two second-order difference equation calculation units, each corresponding to two outputs. Each second-order difference equation calculation unit consists of a delay unit, a bit width adjuster, and an adder.

10. The method according to claim 1, characterized in that, In step d), the phase angle calculation module calculates Atan through the CORDIC calculation unit and outputs the winding phase angle from 0 to 2π. Then, it performs unwinding and phase frequency multiplication. The unwinding part uses an output register, adder / subtractor, numerical comparator and data selector, and the frequency multiplication part uses a multiplier and a 2N remainder.

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