Optical frequency domain reflection distributed measurement system based on complex beat frequency signal reconstruction and application method thereof

Abstract

The invention discloses an optical frequency domain reflection distributed measurement system based on complex beat frequency signal reconstruction and an application method thereof. The system comprises a linear sweep frequency laser, a first optical coupler, an erbium-doped optical fiber amplifier, an optical circulator, an optical waveguide to be measured, a polarization controller, a second optical coupler, a photoelectric detector and a data acquisition and processing module. According to the invention, the reconstruction of the complex beat frequency signal can be realized without introducing a new photoelectric device, the coherent fading and cross interference noise in the low-frequency range of the OFDR can be effectively suppressed, the interference of positive and negative spectrum aliasing noise can be avoided, and the measured signal-to-noise ratio and the measurement precision can be obviously improved without increasing the system hardware complexity.

Description

Technical Field

[0001] This invention relates to the field of distributed optical fiber sensing and measurement, specifically to a distributed optical frequency domain reflection measurement system based on reconstructed multiplex frequency signals and its application method. Background Technology

[0002] Currently, optical devices and photonic chips have become the core foundation of the next-generation information industry, and their performance directly determines the development level of key areas such as communication systems, quantum computing, and intelligent sensing. However, various structural damages and defects generated during production, preparation, transportation, assembly, and operation will seriously affect their performance, becoming a significant bottleneck restricting the yield of optical devices and photonic chips. As optical devices and photonic chips continue to develop towards high performance and multifunctionality, the complexity and integration of their internal structures are increasing exponentially, posing significant technical challenges to the accurate detection of their internal characteristics. Current mainstream optical detection technologies mainly rely on Optical Vector Analyzer (OVA) technology to analyze the network transmission parameters of devices. This method can only provide end-to-end network parameters and cannot obtain the continuous distribution of the internal structural characteristics of optical devices and photonic chips. Facing the ever-increasing demand for detection of highly complex optical devices and photonic chips, the limitations of this method are particularly prominent—it cannot reflect the overall performance uniformity of the device, nor can it locate randomly distributed microscopic defects.

[0003] Optical Frequency Domain Reflectometry (OFDR), as a distributed measurement and sensing technology, has been applied in fields such as fiber optic link fault detection, structural health monitoring of components and equipment, transportation safety maintenance, and testing and verification of new materials. In recent years, its unique advantages over traditional optical temporal domain reflectometry distributed sensing technology, including high precision, high spatial resolution, and high sensitivity, have gradually become an important way to overcome the aforementioned bottlenecks, demonstrating irreplaceable advantages in the field of optical device and photonic chip measurement.

[0004] According to the OFDR measurement principle, when microscopic defects and structural damage exist in an optical waveguide, the amplitude and phase of the Rayleigh backscattering (RBS) signal generated during the transmission of the frequency-modulated continuous wave probe signal will change. By using optical coherent demodulation and spectral analysis, and relying on the mapping relationship between beat frequency and spatial location, the spatiotemporal distribution of various optical parameters along the light wave propagation path can be obtained, thereby enabling distributed analysis of the waveguide structural characteristics.

[0005] Due to random interference from Rectangular Baselines (RBSs) during demodulation, problems such as coherent fading and cross-interference encountered in actual measurements will severely affect the measurement accuracy of OFDR. The former mainly stems from the coherent superposition of RBSs within the same spatial resolution during demodulation, causing drastic fluctuations in the beat frequency signal strength. Cross-interference of RBSs within different spatial resolutions leads to signal crosstalk and an increase in the noise floor, primarily concentrated near the low frequencies. Furthermore, the aliasing of positive and negative spectral noise caused by the Fourier transform during demodulation will also degrade low-frequency noise. Therefore, to meet the demands for high-precision and high-spatial-resolution distributed measurements in optical device and photonic chip testing, it is necessary to overcome the noise bottlenecks caused by the above factors.

[0006] The main current technical approaches to address this are orthogonal demodulation and heterodyne frequency shifting. The former uses a 90° optical mixer to construct a dual-channel optical orthogonal coherent demodulation to directly acquire the two orthogonal components of the beat frequency signal, thereby suppressing spectral aliasing caused by Fourier transform by directly constructing an analytic signal. The latter applies a fixed frequency offset by adding a frequency shifting device to the reference path of the measured interference structure, thus constituting heterodyne demodulation to achieve frequency domain isolation of interference crosstalk. Both of these approaches are effective at suppressing only certain types of noise and require specific device support. Therefore, they increase the complexity of the system structure and make it susceptible to introducing other noise, while also increasing the pressure on the demodulation and processing stages.

[0007] Therefore, how to effectively suppress coherent fading and cross-interference in the low-frequency range of OFDR without increasing the complexity of the system hardware structure can significantly improve the performance of OFDR technology in the testing and characterization of optical devices and photonic chips, which will help promote the development of related fields of optical devices and photonic chips and has important application value. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention proposes a distributed optical frequency domain reflectance measurement method based on the reconstruction of beat frequency signals. This method requires no additional hardware components and can directly reconstruct the phase of the beat frequency signal to form its analytical form. In this process, it effectively suppresses the influence of coherent fading, cross-interference, and other noise in the low-frequency range of OFDR measurements. Specifically, by employing a beat frequency phase reconstruction algorithm and establishing a nonlinear mapping relationship between the amplitude and phase of the beat frequency signal, the phase can be recovered using the time-domain beat frequency intensity signal received by the photodetector. This enables accurate reconstruction of the beat frequency signal and effectively suppresses coherent fading and cross-interference noise in the low-frequency range of OFDR measurements. Furthermore, since the OFDR demodulation process based on this method can directly analyze the beat frequency signal, the corresponding single-sideband spectral information avoids aliasing noise caused by positive and negative spectra.

[0009] To achieve the above objectives, the present invention provides a distributed optical frequency domain reflectance measurement system based on reconstructed multiplexed frequency signals, comprising: a linearly swept laser, a first optical coupler, an erbium-doped fiber amplifier, an optical circulator, a waveguide under test, a polarization controller, a second optical coupler, a photodetector, and a data acquisition and processing module;

[0010] Among them, the linear sweep laser is used to generate a linear sweep optical signal. The first optical coupler splits the generated optical signal and injects it into the measurement path and the reference path respectively. The optical signal in the measurement path is amplified by the erbium-doped fiber amplifier and then enters the optical circulator. The optical signal in the measurement path enters the optical waveguide under test through the optical circulator, generates an echo signal in the optical waveguide under test, and then adjusts the polarization state through the polarization controller after passing through the optical circulator.

[0011] The optical signals from the measurement path and the reference path enter the second optical coupler to generate a beat frequency signal; the beat frequency optical signal is converted into an electrical signal by a photodetector and then connected to the data acquisition and processing module for data acquisition and processing.

[0012] Preferably, the generated linearly swept optical signal includes:

[0013]

[0014] in, The starting optical frequency; The sweep slope; The instantaneous phase of the light source; denoted as the amplitude of the optical signal; t represents the transmission time of the optical signal; e represents the base of the natural logarithm; and j represents the imaginary unit.

[0015] Preferably, the optical field of the measured path echo signal is represented as follows:

[0016]

[0017] Where R0 represents reflectivity; E r This represents the linearly swept frequency light produced by the laser; This indicates the time delay difference between the reference path signal and the measured path signal.

[0018] Preferably, the photocurrent generated after the optical signals from the measurement path and the reference path enter the second optical coupler is expressed as:

[0019]

[0020] in, This indicates the signal after interference between the measurement path and the reference path; This indicates cross-interference noise.

[0021] Preferably, the beat frequency signal generated after the photocurrent is interfered with by the second optical coupler is represented as:

[0022]

[0023] in, Indicates the phase after the beat frequency; This indicates a single-sideband signal; i represents the imaginary unit.

[0024] The present invention also provides a measurement method, wherein the measurement method uses the above-mentioned optical frequency domain reflectance distributed measurement system based on the reconstruction of multiple beat frequency signals to perform the measurement, and the steps include:

[0025] S1. Acquire the amplitude of the beat frequency signal in the time domain by collecting the electrical signal output by the photodetector.

[0026] S2. Recover the phase of the beat frequency signal using the relationship between the amplitude and phase of the beat frequency signal. ;

[0027] S3, based on and The complete beat frequency signal was reconstructed;

[0028] S4. Fourier transform the reconstructed beat frequency signal to the frequency domain to obtain the OFDR trace, thus completing the measurement.

[0029] Preferably, its expression includes: for We obtain the following by performing a Hilbert transform:

[0030]

[0031] in, τ represents the Hilbert transform; τ represents the integral variable over time.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] This invention can reconstruct the repeating frequency signal without introducing new optoelectronic devices, effectively suppress coherent fading and cross-interference noise in the low-frequency range of OFDR, avoid interference from positive and negative spectrum aliasing noise, and significantly improve the signal-to-noise ratio and measurement accuracy without increasing the system hardware complexity. Attached Figure Description

[0034] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 This is a schematic diagram of the system structure according to an embodiment of the present invention;

[0036] Figure 2 This is a schematic diagram of the test results of an embodiment of the present invention; wherein, Figure 2 (a) in the diagram represents a schematic diagram of the test results of the traditional OFDR demodulation scheme; (b) represents a schematic diagram of the test results of the measurement method of the present invention. Detailed Implementation

[0037] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0039] like Figure 1 The diagram shown is a schematic of the system structure of this embodiment, including: a linearly swept laser, a first optical coupler, an erbium-doped fiber amplifier, an optical circulator, a waveguide under test, a polarization controller, a second optical coupler, a photodetector, and a data acquisition and processing module.

[0040] Specifically, a linearly swept laser is used to generate a linearly swept optical signal to provide a light source for the system. In this embodiment, the sweep frequency range of the linearly swept laser is 4 GHz. This sweep frequency range is only for example and can be other sweep frequency ranges. A first optical coupler splits the linearly swept optical wave into the measurement path and reference path respectively. The splitting ratio of the first optical coupler is 95 / 5, with 95% of the light entering the measurement path. The light wave in the measurement path is amplified by an erbium-doped fiber amplifier and then enters an optical circulator. The light wave passes through the optical circulator and enters the waveguide under test, generating an echo signal. After passing through the optical circulator, the polarization state is adjusted by a polarization controller. The signals from the measurement path and the reference path interfere with each other to generate a beat frequency signal. The splitting ratio of the second optical coupler is 50 / 50. The beat frequency signal is converted into an electrical signal by a photodetector and then connected to the data acquisition and processing module for data acquisition and processing.

[0041] The modulated linear sweep frequency optical signal of the laser can be expressed as:

[0042]

[0043] in, The starting optical frequency; The sweep slope; The instantaneous phase of the light source; denoted as the amplitude of the optical signal; t represents the transmission time of the optical signal; e represents the base of the natural logarithm; and j represents the imaginary unit.

[0044] Considering the echo signal generated at z0 in the optical waveguide under test in the measurement path, the time delay difference between the reference path signal and the measurement path signal is: In the formula, c is the speed of light in vacuum, and n is the refractive index of the optical waveguide under test. Therefore, the signal frequency during the beat frequency interference of two optical signals can be derived as: If the reflectivity at z0 is R0, then the optical field of the measured echo signal can be expressed as:

[0045]

[0046] Among them, E r This refers to a linearly swept frequency light source generated by a laser.

[0047] The photocurrent generated after the optical signals from the measurement path and the reference path enter the second optical coupler and undergo beat frequency interference is expressed as:

[0048]

[0049] in, This indicates the signal after interference between the measurement path and the reference path; This indicates cross-interference noise.

[0050] The beat frequency signal generated after interference can be expressed as:

[0051]

[0052] in, Indicates the phase after the beat frequency; This represents a single-sideband signal; 'i' represents the imaginary unit. Let:

[0053]

[0054] in, Indicates single-sideband signal The complex signal obtained by taking the natural logarithm.

[0055] but The Fourier transform of can be expressed as Further to We can obtain the following by performing an inverse Fourier transform:

[0056]

[0057] in, Represents the time integral variable; This represents the Cauchy principal value.

[0058] The phase of the beat frequency signal can then be expressed as:

[0059]

[0060] in, This represents the Hilbert transform.

[0061] Example 2

[0062] This embodiment provides a measurement method, which uses the measuring device provided in Embodiment 1, and includes the following steps:

[0063] S1. Acquire the amplitude of the beat frequency signal in the time domain by collecting the output electrical signal of the photodetector. ;

[0064] S2. Recover the phase of the beat frequency signal by utilizing the relationship between the amplitude and phase of the beat frequency signal, i.e., for By performing the Hilbert transform, we can obtain:

[0065]

[0066] in, This represents the Hilbert transform.

[0067] S3, The amplitude of the known beat frequency signal and phase Then the complete beat frequency signal can be reconstructed, that is:

[0068] .

[0069] S4. The reconstructed beat frequency signal is Fourier transformed to the frequency domain to obtain the OFDR trace. It can be observed that coherent fading and cross-interference noise in the low frequency range are effectively suppressed.

[0070] like Figure 2 As shown, the optical frequency domain reflectance distributed measurement method based on reconstructed repeater frequency signal in this embodiment effectively suppresses coherent fading and cross-interference noise in the low frequency range, and significantly improves the measurement performance of OFDR in the low frequency range.

[0071] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. An optical frequency domain reflectometry distributed measurement system based on reconstruction of a complex beat frequency signal, characterized in that, The application relates to a linear sweep frequency laser, a first optical coupler, an erbium-doped fiber amplifier, an optical circulator, a to-be-measured optical waveguide, a polarization controller, a second optical coupler, a photodetector and a data acquisition and processing module. The linear sweep frequency laser is used for generating a linear sweep frequency optical signal, the first optical coupler is used for splitting the generated optical signal and injecting the optical signal into a measurement path and a reference path, the optical signal in the measurement path is amplified by the erbium-doped fiber amplifier and then enters the optical circulator. The optical signal in the measurement path enters the to-be-measured optical waveguide through the optical circulator, and a backwave signal is generated in the to-be-measured optical waveguide; the backwave signal passes through the optical circulator and is adjusted by the polarization controller. The optical signals in the measurement path and the reference path enter the second optical coupler to generate a beat frequency signal; the beat frequency signal is converted into an electric signal by the photodetector and is connected to the data acquisition and processing module to perform data acquisition and processing. The generated linear sweep frequency optical signal comprises:

2. The optical frequency domain reflectometry distributed measurement system based on reconstruction of beat frequency signals according to claim 1, characterized in that, The optical field of the backwave signal in the measurement path is represented as: wherein, is the starting optical frequency; is the sweep slope; is the instantaneous phase of the light source; is the amplitude of the optical signal; t represents the transmission time of the optical signal; e represents the base of the natural logarithm; and j represents the imaginary unit.

3. The optical frequency domain reflectometry distributed measurement system based on reconstruction of beat frequency signals according to claim 2, characterized in that, After the optical signals in the measurement path and the reference path enter the second optical coupler, the generated photocurrent is represented as: where R0 represents reflectivity; E r represents a linearly swept light generated by a laser; represents a delay difference between a reference path signal and a measurement path signal.

4. The optical frequency domain reflectometry system based on reconstruction of beat frequency signals according to claim 3, wherein, The beat frequency signal generated after the photocurrent is interfered by the second optical coupler is represented as: wherein represents the signal measured after interference of the road with the reference road; represents the cross-interference noise.

5. The optical frequency domain reflectometry system based on reconstruction of beat frequency signals according to claim 4, wherein, The application further relates to a method for measuring an optical waveguide. wherein denotes the phase after the beat frequency; denotes the single sideband signal; i denotes the imaginary unit.

6. A method of measurement employing the optical frequency domain reflectometry distributed measurement system based on reconstruction of beat frequency signals according to any of claims 1 to 5, characterized by the steps of S4, Fourier transform the reconstructed beat frequency signal to the frequency domain to obtain an OFDR trace, and the measurement is completed. S1, collecting the electrical signal output by the photodetector to obtain the amplitude of the beat frequency signal in the time domain S2, recovering the phase of the beat signal using the relationship between the amplitude and the phase of the beat signal ; S3, based on and reconstruct a complete beat signal; ​ 7. The method of measuring according to claim 6, wherein, Recovering the phase of the beat signal using a data processing module The expression thereof includes: performing a Hilbert transform on to obtain wherein denotes the Hilbert transform; τ denotes the integration variable of time.

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