Fiber mzi free spectral range measurement method and system based on broadband light source

Abstract

The application provides a kind of free spectral range measurement method and system of fiber MZI based on broadband light source, it is related to optical fiber communication and optical measurement technical field, including: the broadband light source is input to the Mach-Zehnder interferometer of to-be-measured optical fiber, interference optical signal output by the Mach-Zehnder interferometer of to-be-measured optical fiber is collected, and it is converted into electrical signal;The AC coupling processing is carried out to electrical signal, and the direct current component and low frequency noise component in electrical signal are filtered out, and time domain signal is obtained;The time domain signal is carried out spectrum analysis, and power spectral density diagram is obtained, and feature frequency peak value is extracted from power spectral density diagram, and feature frequency peak value is the free spectral range of the Mach-Zehnder interferometer of to-be-measured optical fiber.The application effectively suppresses low frequency noise and baseline drift interference in photoelectric detection link under the premise of not depending on expensive narrow linewidth light source, and realizes the stable measurement of high precision, low cost to the MHz order free spectral range of fiber MZI.

Description

Technical Field

[0001] This invention relates to the fields of optical fiber communication and optical measurement technology, and in particular to a method and system for measuring the free spectral range of optical fiber MZI based on a broadband light source, which is especially suitable for high-precision and low-cost testing of optical fiber interferometers with free spectral ranges in the MHz range. Background Technology

[0002] The fiber optic Mach-Zehnder interferometer (MZI) is an important optical device based on the principle of two-beam interference. Due to its simple structure, high sensitivity, and strong resistance to electromagnetic interference, it is widely used in fiber optic sensing, optical filtering, wavelength division multiplexing systems, and precision measurement. Among the various performance parameters of the MZI, the free spectral range (FSR) is one of the most critical technical indicators. It is defined as the frequency interval between adjacent interference orders and directly determines the frequency resolution and measurement accuracy of the device.

[0003] Currently, the following methods are mainly used to measure the free spectral range of optical fiber MZI: (I) Narrow-linewidth laser frequency sweep method. This method uses a narrow-linewidth tunable laser to scan the wavelength of an interferometer. By recording the interference fringes where the output light intensity changes with wavelength, the frequency interval between adjacent interference peaks is calculated to obtain the free spectral range. However, this method has significant limitations: it places extremely stringent requirements on the laser's linewidth and the linearity of the frequency sweep, and when the free spectral range of the interferometer under test is large (e.g., reaching the MHz level), it requires extremely high sampling rates and long scan times, significantly increasing system complexity. Therefore, this method is difficult to meet the requirements of low-cost, high-efficiency engineering measurements.

[0004] (II) Point-by-point scanning method with tunable lasers. This method involves changing the operating wavelength of the laser point by point and recording the corresponding output light intensity to plot a complete interference spectrum. While this method reduces the requirement for frequency sweep linearity to some extent, the measurement speed is extremely slow, and it has stringent requirements for the wavelength and power stability of the light source. In addition, environmental disturbances during point-by-point scanning can easily introduce additional measurement errors, affecting the repeatability and accuracy of the measurement results.

[0005] (III) Broadband Light Source Combined with Spectrometer Method. This method uses a broadband light source (such as a superluminescent diode or an erbium-doped fiber amplifier spontaneous emission source) as input. A spectrometer is used to directly observe the transmission or reflection spectrum of the interferometer, and the free spectral range is calculated from the intervals between the interference fringes in the spectral domain. This method does not require expensive, high-precision tunable lasers, resulting in relatively low system cost. However, the resolution of existing spectrometers is typically on the order of GHz. For fiber MZI with a free spectral range on the order of MHz, the spectrometer cannot resolve its fine interference fringe structure, leading to invalid measurement results.

[0006] In summary, existing technologies for measuring the free spectral range of fiber MZI have the following problems: On the one hand, methods relying on narrow-linewidth swept-frequency light sources are complex, costly, and slow in scanning speed; on the other hand, the resolution of broadband light sources combined with spectrometers is limited, resulting in low measurement accuracy and failing to meet the high-precision measurement requirements of the MHz-level free spectral range. Furthermore, existing methods generally do not consider the interference of low-frequency noise in the photoelectric detection link (such as inherent intensity noise of the light source and baseline drift introduced by DC coupling of the oscilloscope) on measurement accuracy. When the measured signal is weak or the free spectral range is small, these noise factors may lead to significant measurement deviations or even erroneous results, resulting in unstable measurement results and poor repeatability. Summary of the Invention

[0007] To address the aforementioned problems, this invention provides a method and system for measuring the free spectral range of fiber MZI based on a broadband light source. This solves the technical problem of effectively suppressing low-frequency noise and baseline drift interference in the photoelectric detection link without relying on expensive narrow-linewidth light sources, and achieves high-precision, low-cost, and stable measurement of the MHz-level free spectral range of fiber MZI.

[0008] This invention provides a method for measuring the free spectral range of fiber MZI based on a broadband light source, the method comprising: A broadband light source is input into the fiber optic Mach-Zehnder interferometer under test, the interference light signal output by the fiber optic Mach-Zehnder interferometer under test is collected, and it is converted into an electrical signal. The electrical signal is subjected to AC coupling processing to filter out the DC component and low-frequency noise component, thereby obtaining a time-domain signal. The time-domain signal is subjected to spectral analysis to obtain a power spectral density map, and the characteristic frequency peak is extracted from the power spectral density map. The characteristic frequency peak is the free spectral range of the fiber Mach-Zehnder interferometer under test.

[0009] Furthermore, the spectral width of the broadband light source is greater than the estimated frequency interval corresponding to the free spectral range estimated by the fiber-to-optical Mach-Zehnder interferometer under test.

[0010] Furthermore, using an AC coupling circuit to perform AC coupling processing on the electrical signal filters out the DC component and low-frequency noise components with frequencies below the cutoff frequency, retaining only the interferometric AC components with frequencies above the cutoff frequency; and the cutoff frequency is lower than the theoretical minimum value of the free spectral range of the fiber optic Mach-Zehnder interferometer under test.

[0011] Furthermore, AC coupling circuits are used to perform AC coupling processing on electrical signals, specifically including: The output electrical signal is directly connected to an oscilloscope or data acquisition card with a built-in AC coupling circuit, and its input coupling mode is set to AC coupling mode. The built-in AC coupling circuit filters out DC components and low-frequency noise components, and the signal is acquired at a set sampling rate to obtain a time-domain signal.

[0012] Furthermore, the set sampling rate is no less than twice the theoretical maximum value of the free spectral range of the fiber optic Mach-Zehnder interferometer under test.

[0013] Furthermore, spectral analysis is performed on the time-domain signal to obtain the power spectral density map, specifically including: The time-domain signal is divided into multiple overlapping or non-overlapping data segments; Perform a Fast Fourier Transform on each data segment to transform the data segment from the time domain to the frequency domain, thereby obtaining the sub-spectrum of each data segment; The power spectrum of each data segment is obtained by taking the square of the modulus of the sub-spectrum of each data segment. The power spectral density estimate is obtained by averaging the power spectra of all data segments. Plot a power spectral density diagram with frequency on the x-axis and power spectral density value on the y-axis.

[0014] Furthermore, extracting the characteristic frequency peak from the power spectral density map specifically includes: traversing all frequency points in the power spectral density map, searching for the frequency point corresponding to the maximum amplitude, and determining the frequency value corresponding to that frequency point as the characteristic frequency peak.

[0015] Furthermore, after the step of traversing all frequency points in the power spectral density map, searching for the frequency point corresponding to the maximum amplitude, and determining the frequency value corresponding to the frequency point as the characteristic frequency peak, the method further includes: interpolating and fitting multiple frequency points near the currently obtained characteristic frequency peak to obtain the corrected characteristic frequency peak.

[0016] Furthermore, interpolation fitting is performed on multiple frequency points near the currently obtained characteristic frequency peak. Specifically, this includes: taking the currently obtained characteristic frequency peak and several frequency points adjacent to it on the left and right, using its frequency value and amplitude data as samples, and performing interpolation fitting using any one of parabolic interpolation, Gaussian interpolation or centroid interpolation, and taking the frequency value corresponding to the vertex of the fitted curve as the corrected characteristic frequency peak.

[0017] On the other hand, the present invention also provides a fiber optic MZI free spectral range measurement system based on a broadband light source, including a memory, a processor, and a computer program stored in the memory. The processor executes the computer program to implement the steps of any of the methods described above.

[0018] In summary, this invention provides a method and system for measuring the free spectral range of fiber MZI based on a broadband light source, which achieves the following advantages compared with the prior art: (1) This invention uses a broadband light source instead of a traditional tunable laser as the test light source, eliminating the need for complex and expensive narrow-linewidth tunable lasers or high-resolution spectrometers, thus significantly reducing measurement complexity and hardware costs. At the same time, by using AC coupling processing to directly filter out the DC component and low-frequency noise in the electrical signal, the interference of inherent low-frequency noise and baseline drift of the light source on the measurement results is effectively eliminated. This avoids the need for additional lock-in amplifiers or high-precision DC bias compensation circuits in traditional schemes, fundamentally preventing the problem of false peak misjudgment caused by low-frequency noise, and ensuring the accuracy and reliability of the measurement results. Finally, by directly extracting the characteristic frequency peak as the free spectral range through spectrum analysis, low-cost and high-precision measurement of the free spectral range of fiber MZI is achieved.

[0019] (2) The spectral width of the broadband light source of the present invention is greater than the estimated frequency interval corresponding to the estimated free spectral range of the fiber optic Mach-Zehnder interferometer under test, which ensures that the interferometer can generate a sufficiently strong interference signal under different arm length differences. At the same time, the cutoff frequency of the AC coupling processing is set to be lower than the theoretical minimum value of the free spectral range of the interferometer under test, which effectively filters out low-frequency noise without affecting the integrity of the target characteristic frequency signal, thus taking into account both noise suppression and signal fidelity.

[0020] (3) This invention achieves rapid and high-precision extraction of the free spectrum range by traversing all frequency points in the power spectral density map, searching for the frequency point corresponding to the maximum amplitude, and determining the frequency value corresponding to the frequency point as the characteristic frequency peak. Furthermore, by interpolating and fitting multiple frequency points near the characteristic frequency peak, the quantization error caused by the limited frequency resolution of the discrete Fourier transform is overcome, and the measurement accuracy is improved to the MHz level. Thus, the measurement accuracy of the free spectrum range is significantly improved without increasing the sampling time and data volume. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in this invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the method flow of a fiber MZI free spectral range measurement method and system based on a broadband light source provided by the present invention; Figure 2 This is a schematic diagram of the system principle of a fiber MZI free spectral range measurement method and system based on a broadband light source provided by the present invention; Figure 3 The present invention relates to an optical fiber MZI with a 20m arm length difference in segment length 2. 12 The power spectral density plot and its measurement results are shown below; Figure 4 The present invention relates to an optical fiber MZI with a 20m arm length difference in segment length 2. 13 The power spectral density plot and its measurement results are shown below; Figure 5 The present invention relates to an optical fiber MZI with a 20m arm length difference in segment length 2. 14 The power spectral density plot and its measurement results are shown below; Figure 6 The present invention relates to an optical fiber MZI with a 20m arm length difference in segment length 2. 15 The power spectral density plot and its measurement results are shown below; Figure 7 The present invention relates to an optical fiber MZI with a 20m arm length difference in segment length 2. 16 The power spectral density plot and its measurement results are shown below; Figure 8 The present invention relates to an optical fiber MZI with a 20m arm length difference in segment length 2. 17 The power spectral density plot and its measurement results are shown below; Figure 9 The present invention relates to an optical fiber MZI with a 20m arm length difference in segment length 2. 18 The power spectral density plot and its measurement results are shown below; Figure 10 The present invention relates to an optical fiber MZI with a 20m arm length difference in segment length 2. 19 The power spectral density plot and its measurement results are shown below; Figure 11 The present invention relates to an optical fiber MZI with a 40m arm length difference in segment length 2. 10 The power spectral density plot and its measurement results are shown below; Figure 12 The present invention relates to an optical fiber MZI with a 40m arm length difference in segment length 2. 11 The power spectral density plot and its measurement results are shown below; Figure 13 The present invention relates to an optical fiber MZI with a 40m arm length difference in segment length 2. 12 The power spectral density plot and its measurement results are shown below; Figure 14 The present invention relates to an optical fiber MZI with a 40m arm length difference in segment length 2. 13 The power spectral density plot and its measurement results are shown below; Figure 15 The present invention relates to an optical fiber MZI with a 40m arm length difference in segment length 2. 14 The power spectral density plot and its measurement results are shown below; Figure 16 The present invention relates to an optical fiber MZI with a 40m arm length difference in segment length 2. 15 The power spectral density plot and its measurement results are shown below; Figure 17 The present invention relates to an optical fiber MZI with a 40m arm length difference in segment length 2. 16 The power spectral density plot and its measurement results are shown below; Figure 18 The present invention relates to an optical fiber MZI with a 40m arm length difference in segment length 2. 17 The power spectral density plot and its measurement results are shown below; 100 - Broadband light source module; 200 - Fiber optic Mach-Zehnder interferometer under test; 300 - Photodetector; 400 - AC coupling module; 500 - Data processing module. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0024] It should be noted that, in the description of the embodiments of the present invention, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a method, step, or apparatus that includes a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to the method, step, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of additional identical elements in the method, step, or apparatus that includes the element.

[0025] To address the technical challenge of effectively suppressing low-frequency noise and baseline drift interference in photoelectric detection links without relying on expensive narrow-linewidth light sources, this invention provides a method and system for measuring the free spectral range of fiber MZI based on a broadband light source. This method utilizes a broadband light source instead of a traditional narrow-linewidth swept-frequency laser, and combines AC coupling processing and spectral analysis to directly extract the free spectral range of fiber MZI from the time-domain signal. This achieves high-precision, low-cost, and stable measurement of the MHz-level free spectral range of fiber MZI.

[0026] like Figure 1 As shown, the method specifically includes: S100: Input a broadband light source into the fiber optic Mach-Zehnder interferometer under test, collect the interference light signal output by the fiber optic Mach-Zehnder interferometer under test, and convert it into an electrical signal.

[0027] Specifically, a broadband light source is turned on to output a stable optical signal, which is then input into the MZI optical fiber under test to obtain an interference optical signal.

[0028] It should be noted that the MZI under test will have a fixed free spectral range (FSR, unit: Hz) due to the length difference ΔL between its two interferometer arms, and this free spectral range corresponds to a frequency interval value on the optical frequency axis. In other words, the MZI of the fiber under test corresponds to a frequency interval value because the length difference ΔL between the two interference arms is ΔL. Although there is a definite mathematical relationship between the free spectral range (FSR) and the arm length difference (ΔL). ,in, The speed of light in a vacuum. The effective refractive index of the optical fiber. However, in practical applications, the arm length difference ΔL of the MZI of the optical fiber under test is often an unknown parameter, or although the nominal value is known, factors such as the wavelength dependence of the fiber refractive index, manufacturing tolerances, packaging stress, and changes in ambient temperature cause the nominal ΔL to fail to accurately reflect the true optical path difference, thus making it impossible to directly calculate the accurate free spectral range using formulas. Therefore, this invention does not rely on the arm length difference ΔL, but directly extracts the free spectral range from the power spectral density map of the interference signal, achieving high-precision, real-time measurement of the free spectral range.

[0029] As an example, the spectral width of the broadband light source is greater than the estimated frequency interval corresponding to the free spectral range estimated by the fiber-to-the-beam Mach-Zehnder interferometer.

[0030] In other words, the spectral width of the broadband light source of the present invention Should meet: Among them, the spectral width of a broadband light source usually refers to the output spectrum of the light source. The 3dB full width at half maximum (FWHM) is converted to the width of the frequency domain. It should be noted that in practical applications, the arm length difference ΔL of fiber MZI is usually within a roughly known range, such as 1mm to 50mm. The corresponding free spectral range can be estimated, but not accurately. Furthermore, the corresponding frequency interval can be estimated.

[0031] When the spectral width of the broadband light source is greater than the frequency interval corresponding to the free spectral range of the MZI fiber under test, the interferometer can form effective interference modulation throughout the entire emission band of the broadband light source. This results in a complete periodic change in the interference output light intensity with frequency. Consequently, after AC coupling and power spectral density analysis, a clear and stable characteristic frequency peak appears in the power spectral density diagram. This characteristic frequency peak can then be directly used as the measured value of the free spectral range. If the spectral width of the light source is less than or equal to the frequency interval, the interference fringes are insufficient to complete a complete cycle within one light source bandwidth, leading to unclear spectral characteristics or even the inability to extract the correct free spectral range. Therefore, the spectral width of the broadband light source must be greater than the estimated frequency interval corresponding to the estimated free spectral range of the MZI fiber under test.

[0032] Preferably, the broadband light source employs a superluminescent diode (SLD) or an erbium-doped fiber amplifier spontaneous emission source (ASE / EDFA). Specifically, the broadband light source has a full width at half maximum (FWHM) of not less than 30 nm, corresponding to a spectral width typically greater than 3 THz, which is far larger than the free spectral range estimated by the interferometer, thus meeting the measurement requirements of common fiber MZI. For example, the broadband light source uses an ASE light source with a center wavelength of 1550 nm and a FWHM of 35 nm.

[0033] It should be noted that, regardless of whether a superluminescent diode or an erbium-doped fiber amplifier spontaneous emission source is used, in order to generate stable interference fringes between the two arms of the interferometer and ensure that the interference light signal has sufficient contrast, the spectral width of the broadband light source is greater than the estimated frequency interval corresponding to the estimated free spectral range of the fiber Mach-Zehnder interferometer under test.

[0034] Traditional methods rely on expensive narrow-linewidth lasers or tunable lasers, resulting in complex and costly systems. In contrast, this invention uses a broadband light source instead of a narrow-linewidth swept-frequency laser. By using common broadband light sources such as ASE or EDFA, no precision wavelength scanning device is required, which can significantly reduce costs.

[0035] The fiber under test, MZI, is composed of a first fiber coupler and a second fiber coupler connected end to end. The two interferometer arms have a length difference ΔL, which in turn produces an optical path difference. This is a well-known interferometer structure in the field of fiber optics, and will not be elaborated here.

[0036] For example, a 2×2 50:50 fiber coupler is used to construct the MZI of the fiber under test, with different delay fiber lengths set for the two arms, such as 20m and 40m, to form an arm length difference ΔL; a polarization controller (PC) is added to the input end or the interferometer arm to optimize the interference contrast; the output end of the MZI of the fiber under test is connected to the input end of the photodetector through fiber jumpers or fusion splices, so that the interference light signal generated by the interferometer is directly incident on the photodetector, and the interference light signal is converted into an electrical signal.

[0037] It should be noted that the conversion of the interference optical signal into an electrical signal is achieved through a photodetector; the photodetector can be an InGaAs PIN photodiode, which converts the incident light power into a proportional current or voltage signal based on the photogenerated carrier effect.

[0038] Specifically, the converted electrical signal includes: a DC component generated by the average power of the broadband optical wave, a periodic interference AC component caused by the difference in the length of the MZI arms of the optical fiber, and a low-frequency noise component; wherein the frequency of the interference AC component is equal to the free spectral range of the interferometer. Subsequently, the electrical signal is AC-coupled to filter out the DC component and the low-frequency noise component, retaining the interference AC component for spectral analysis. The conversion of optical signals into electrical signals using photodetectors is a well-known technique in fiber optics and photoelectric detection, and will not be elaborated upon here.

[0039] S200: Performs AC coupling processing on the electrical signal, filtering out the DC component and low-frequency noise component in the electrical signal to obtain the time-domain signal.

[0040] It should be noted that time-domain electrical signals acquired under DC coupling may exhibit spurious frequency peaks due to low-frequency noise and baseline drift. However, this invention acquires time-domain signals under AC coupling, effectively eliminating these spurious frequency peaks and ensuring that the extracted characteristic frequencies closely match the theoretically calculated values ​​within the free spectral range. They match perfectly.

[0041] AC coupling processing essentially utilizes the principle of high-pass filtering to remove DC components and low-frequency noise components while retaining the interfering AC components.

[0042] Specifically, an AC coupling circuit is used to process the electrical signal, filtering out the DC component and low-frequency noise components below the cutoff frequency, retaining only the interfering AC components above the cutoff frequency. This results in a time-domain signal with the same frequency as the free spectral range, allowing subsequent spectral analysis to clearly identify the characteristic frequency peaks corresponding to the free spectral range.

[0043] To filter out low-frequency noise and DC drift, and to prevent them from generating spurious peaks in the spectrum, as an example, the cutoff frequency is lower than the theoretical minimum of the free spectral range of the fiber optic Mach-Zehnder interferometer under test. For example, the cutoff frequency of the AC coupling circuit can be from 0.03 Hz to 30 Hz, far below the free spectral range in the MHz range, thus ensuring that the AC interference components caused by the arm length difference are not attenuated.

[0044] As one embodiment, the AC coupling circuit is used to perform AC coupling processing on the electrical signal, specifically including: The output electrical signal is directly connected to an oscilloscope or data acquisition card with a built-in AC coupling circuit, and its input coupling mode is set to AC coupling mode. The built-in AC coupling circuit filters out DC components and low-frequency noise components, and the signal is acquired at a set sampling rate to obtain a time-domain signal.

[0045] It should be noted that the AC coupling circuit can be an RC-coupled high-pass filter, which isolates the DC component in the electrical signal through a series capacitor, and filters out low-frequency noise components and baseline drift by setting an appropriate cutoff frequency, retaining only the AC component containing interference information.

[0046] Acquiring a signal at a set sampling rate means sampling and quantizing the analog electrical signal after AC coupling processing at equal intervals according to a preset time interval, converting it into a discrete voltage value sequence that varies with time, which is digitized waveform data.

[0047] To prevent spectral aliasing and ensure accurate measurement of the free spectral range, as an embodiment of the present invention, the sampling rate is set to be no less than twice the theoretical maximum value of the free spectral range of the fiber optic Mach-Zehnder interferometer under test.

[0048] For example, the sampling rate is set based on the estimated range of arm length difference of the interferometer under test. For instance, if the planned measurement is of an MZI with an arm length difference range of 1m to 40m, the theoretical minimum value of the free spectral range is 2.58 MHz, corresponding to a 40m arm length difference; the theoretical maximum value is 103 MHz, corresponding to a 1m arm length difference. In this case, the AC coupling cutoff frequency should be less than 2.58 MHz to ensure that the free spectral range of all measured MZIs is not attenuated; the sampling rate should be greater than or equal to 2 × 103 MHz = 206 MHz to ensure that the free spectral range of the measured MZI does not experience spectral aliasing.

[0049] S300: Performs spectral analysis on the time-domain signal to obtain the power spectral density map, and extracts the characteristic frequency peak from the power spectral density map. The characteristic frequency peak is the free spectral range of the fiber Mach-Zehnder interferometer under test.

[0050] As an example, spectral analysis is performed on the time-domain signal to obtain the power spectral density map, specifically including: Divide the time-domain signal into multiple overlapping or non-overlapping data segments; Perform a Fast Fourier Transform on each data segment to transform the data segment from the time domain to the frequency domain, thereby obtaining the sub-spectrum of each data segment; The power spectrum of each data segment is obtained by taking the square of the modulus of the sub-spectrum of each data segment. The power spectral density estimate is obtained by averaging the power spectra of all data segments. Plot a power spectral density diagram with frequency on the x-axis and power spectral density value on the y-axis.

[0051] In this embodiment, a time-domain signal of length N (essentially a digital signal sequence) is divided into K overlapping or non-overlapping data segments, each of length M. The purpose of segmentation is to allow for multiple estimations and averaging, thereby reducing the variance of the spectral estimation. The segment length M can be selected according to actual needs, with the selection principle being: while ensuring sufficient frequency resolution to distinguish characteristic peaks within the free spectral range, to maximize the stability and signal-to-noise ratio of the power spectral density estimation. For example, the optimal parameters can be determined by observing the stability of characteristic peaks when changing the segment length. 20 If the value is too large, it will cause peak blurring, requiring a rollback. Therefore, the data segment length M is determined to be within 2. 10 Up to 2 19 Choose from the options.

[0052] To suppress spectral leakage caused by signal truncation, as an example, before performing Fast Fourier Transform (FFT) on each data segment, the method includes: first applying a window function to each data segment; then performing FFT on each windowed data segment. This reduces the interference of sidelobe energy on the localization of the main lobe peak frequency, improves the stability and anti-interference capability of spectral analysis, and is particularly suitable for applications with low signal-to-noise ratios or limited measurement time.

[0053] Specifically, the window function can be any one of the Hanning window, Heming window, Blackman window, or Kaiser window.

[0054] As an example, extracting the characteristic frequency peak from the power spectral density map specifically includes: traversing all frequency points in the power spectral density map, searching for the frequency point corresponding to the maximum amplitude, and determining the frequency value corresponding to the frequency point as the characteristic frequency peak, that is, the free spectral range of the fiber Mach-Zehnder interferometer under test. This method is simple to implement and has a fast calculation speed.

[0055] Based on this, in order to further improve the measurement accuracy, after traversing all frequency points in the power spectral density map, searching for the frequency point corresponding to the maximum amplitude, and determining the frequency value corresponding to the frequency point as the characteristic frequency peak, the method further includes: interpolating and fitting multiple frequency points near the currently obtained characteristic frequency peak to obtain the corrected characteristic frequency peak.

[0056] Furthermore, interpolation fitting is performed on multiple frequency points near the currently obtained characteristic frequency peak. Specifically, this includes: taking the currently obtained characteristic frequency peak and several frequency points adjacent to it, using their frequency and amplitude data as samples, and performing interpolation fitting using any one of parabolic interpolation, Gaussian interpolation, or centroid interpolation. The frequency value corresponding to the vertex of the fitted curve is taken as the corrected characteristic frequency peak. This overcomes the limitation of the frequency resolution of the Fast Fourier Transform and improves the measurement accuracy to the sub-frequency resolution unit level.

[0057] For example, parabolic interpolation fitting can be used to obtain a more accurate characteristic frequency peak position by using the amplitude and frequency coordinates of the peak point and its two adjacent points on the left and right through quadratic polynomial fitting.

[0058] It should be noted that the characteristic frequency peak is the free spectral range of the MZI fiber under test. This is because the MZI fiber under test has a length difference ΔL between its two interference arms, causing interference of the input broadband light. The output light intensity varies periodically with the optical frequency, and its period in the frequency domain is the free spectral range. After the photodetector performs square-law detection on the interference light signal, the interference modulation is converted into an interference AC component in the electrical signal. The oscillation frequency of this interference AC component is equal to the free spectral range. After AC coupling processing to remove the DC component and low-frequency noise, this interference AC component is retained as the effective signal, i.e., the time-domain signal. When performing power spectral density analysis on the time-domain signal, since the spectral width of the broadband light source is much larger than the free spectral range, the coherent beat frequency introduced by the interference appears as the only significant spectral peak in the power spectral density plot. The frequency corresponding to this peak is the frequency of the interference AC component, which is also the free spectral range of the MZI in the fiber under test. Therefore, the characteristic frequency peak extracted from the power spectral density plot can be directly used as the measurement value of the free spectral range, and the measurement accuracy can reach the MHz level.

[0059] On the other hand, the present invention also provides a fiber optic MZI free spectral range measurement system based on a broadband light source, including a memory, a processor, and a computer program stored in the memory. The processor executes the computer program to implement the steps of any of the methods described above.

[0060] Specifically, such as Figure 2 As shown, the system may further include, in sequence: a broadband light source module 100, a fiber optic Mach-Zehnder interferometer under test 200, a photodetector 300, an AC coupling module 400, and a data processing module 500.

[0061] Broadband light source module 100 is used to output optical signals.

[0062] The input end of the fiber optic Mach-Zehnder interferometer 200 under test is connected to the output end of the broadband light source module 100 via a single-mode fiber fusion splice or an FC / APC connector, and is used to receive the optical signal output by the broadband light source and output the interference optical signal.

[0063] The input end of the photodetector 300 is connected to the output end of the fiber optic Mach-Zehnder interferometer 200 under test, and is used to convert the interference light signal into an electrical signal.

[0064] The response bandwidth of the photodetector 300 should cover the free spectral range of the fiber optic Mach-Zehnder interferometer 200 under test to ensure complete acquisition of the interference signal. For example, an InGaAs PIN photodetector with a response bandwidth of DC-1GHz can be used.

[0065] The input terminal of the AC coupling module 400 is connected to the output terminal of the photodetector 300. It is used to perform AC coupling processing on the electrical signal, filter out the DC component and low-frequency noise component in the electrical signal, and output a time-domain signal.

[0066] The AC coupling module 400 can be implemented using an active high-pass filter composed of an RC coupling circuit or an operational amplifier. For example, the cutoff frequency of the AC coupling module 400 can be set to 0.1Hz, which is far below the theoretical minimum of the free spectral range of a typical fiber MZI. Therefore, it will not cause attenuation to the target signal, while effectively filtering out low-frequency drift of the light source and baseline fluctuations caused by slow changes in the environment.

[0067] The input of the data processing module 500 is connected to the output of the AC coupling module 400. It is used to perform spectral analysis on the time-domain signal to obtain the power spectral density map, and extract the characteristic frequency peak from the power spectral density map. The characteristic frequency peak is the precise free spectral range of the fiber optic Mach-Zehnder interferometer 200 under test.

[0068] The data processing module 500 can be an FPGA, DSP, ARM processor or general-purpose computer, with FPGA preferred for implementing real-time spectrum analysis.

[0069] The technical features of the system are consistent with those of the method, and will not be elaborated here.

[0070] To verify the technical effect of the present invention, fiber MZIs with arm length differences of 20m and 40m were selected to measure their free spectral range, and the measured values ​​were compared with the theoretical values.

[0071] In a specific embodiment, the broadband light source is an ASE light source; the fiber MZI consists of two 2×2 50:50 fiber couplers, one arm being the standard optical path and the other arm connected to a 20m delay fiber, with a theoretical free spectral range of approximately 7.9MHz; a polarization controller (PC) is set at the input end of the MZI, and a photodetector (PD) is connected to the output end, followed by an oscilloscope. The 2×2 50:50 fiber coupler refers to a passive fiber optic device with two input ports and two output ports, with a splitting ratio of 50:50, meaning the input optical power is evenly distributed to the two output ports, each accounting for 50%. The first 2×2 50:50 fiber coupler is used to split the broadband light source input light into two equal paths, which are then fed into the two interference arms of the MZI; the second 2×2 50:50 fiber coupler is used to combine and split the beams returning from the two interference arms, with one path output to the photodetector for subsequent signal processing.

[0072] Connect the ASE light source to the MZI input terminal, adjust the polarization controller to maximize the interference contrast, and simultaneously set the oscilloscope to AC coupling mode with a sampling rate of 3.125×10⁻⁶. 8 Hz, sampling time 500ms; acquire time-domain signal and save as .mat format data file; perform spectrum analysis on the signal, identify the characteristic peak located at about 7.9MHz in the power spectral density plot, which is the free spectrum range of the MZI of the fiber under test.

[0073] It should be noted that the choice of coupling mode has a decisive impact on the accuracy of the measurement results. In DC coupling mode, the inherent low-frequency noise of the ASE light source and baseline drift are introduced into the signal, causing significant deviations in the free spectral range measurement results. For example, for an interferometer with a 20 m arm length difference, the measured value is only 4 MHz, far lower than the theoretical value of approximately 7.9 MHz. However, switching to AC coupling mode effectively filters out low-frequency noise interference. Retest results show that the free spectral range measurement value corresponding to the 20 m arm length difference is approximately 7.9 MHz, which is in high agreement with the theoretical calculation value, verifying the effectiveness of this system in suppressing noise and improving measurement accuracy.

[0074] Furthermore, in order to verify the stability and reliability of the present invention, such as Figures 3-10 As shown, the power spectral density plots and their measurement results are displayed for different segment lengths. Since when the segment length increases to 2... 20 At this point, if the frequency resolution is too high, the signal-to-noise ratio decreases, and the characteristic peaks become blurred. Therefore, the maximum segment length is only 2. 19 In other words, the segment length can be 2. 12 2 13 2 14 2 15 2 16 2 17 2 18 2 19 .

[0075] Experiments show that, under different segment lengths, i.e., at different FFT frequency resolutions, the characteristic peak at approximately 7.9 MHz can be clearly identified in the power spectral density plot, and the measurement results are in high agreement with the theoretical values. Even when the signal-to-noise ratio decreases due to larger segment lengths and extremely high frequency resolution, this invention can still effectively avoid peak blurring, accurately extract the peak, and thus obtain a high-precision free spectral range, ensuring accuracy and stability in practical applications.

[0076] As another specific embodiment, the broadband light source is an ASE light source; the fiber MZI consists of two 2×2 50:50 fiber couplers, one arm is the standard optical path, and the other arm is connected to a 40m delay fiber, with a theoretical free spectral range of approximately 4.4MHz; similarly, a polarization controller (PC) is set at the input end of the MZI, and a photodetector (PD) is connected to the output end, followed by an oscilloscope.

[0077] Connect the ASE light source to the MZI input terminal, adjust the polarization controller to maximize the interference contrast, and simultaneously set the oscilloscope to AC coupling mode with a sampling rate of 1.25 × 10⁻⁶. 9Hz, sampling time 200ms; acquire time-domain signal and save as .mat format data file; perform spectrum analysis on the signal, identify the characteristic peak located at about 4.4MHz in the power spectral density plot, which is the free spectrum range of the MZI of the fiber under test.

[0078] Similarly, such as Figures 11-18 As shown, the power spectral density plots and their measurement results are displayed for different segment lengths. Since when the segment length increases to 2... 18 At this point, excessively high frequency resolution leads to a decrease in signal-to-noise ratio, blurring of characteristic peaks, and a reduction in measurement accuracy; therefore, the maximum segment length is only 2. 17 In other words, the segment length can be 2. 10 2 11 2 12 2 13 2 14 2 15 2 16 2 17 .

[0079] The experiment also showed that, for different segment lengths, the characteristic peak at approximately 4.4 MHz could be clearly identified in the power spectral density plot, and the measurement results were in high agreement with the theoretical values.

[0080] As can be seen, for a 20m arm length difference, this invention can accurately measure the free spectral range to be approximately 7.9MHz; for a 40m arm length difference, it can accurately measure the free spectral range to be approximately 4.4MHz. These results not only perfectly match the theoretical calculations but also exhibit good linear response characteristics, proving the measurement consistency of this invention under different physical lengths. Furthermore, regardless of whether it's in the higher frequency band (7.9MHz) corresponding to 20m or the lower frequency band (4.4MHz) corresponding to 40m, this invention can accurately extract characteristic peaks through its processing mechanism even when facing a decrease in signal-to-noise ratio due to the increased segment length. This fully demonstrates that this invention does not rely on specific arm length parameters or specific frequency ranges, possesses extremely strong noise resistance and cross-frequency band adaptability, and can be applied as a general high-precision free spectral range measurement scheme in practical engineering.

[0081] Furthermore, the equipment used in this invention is low-cost and fast, requiring only 200–500 ms for a single measurement, making it suitable for real-time measurements in scenarios such as rapid calibration and batch testing. It can be extended not only to synchronous calibration of Q-values ​​and dispersion coefficients of microcavity resonators, but also widely used in factory testing of passive fiber optic devices, integrated optical chip testing, fiber optic sensing system calibration, and optical resonator parameter characterization.

[0082] In summary, the present invention provides a method and system for measuring the free spectral range of fiber MZI based on a broadband light source. By organically combining a broadband light source, AC coupling processing, and spectrum analysis, it achieves low-cost, high-precision, and highly interference-resistant free spectral range measurement, and has good prospects for industrial application.

[0083] It should be noted that, for the sake of simplicity, the foregoing embodiments are all described as a series of actions. However, those skilled in the art should understand that this application is not limited to the described order of actions, as some steps may be performed in other orders or simultaneously according to this application. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and modules involved are not necessarily essential to this application.

[0084] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0085] In the several embodiments provided in this application, it should be understood that the disclosed methods or systems can be implemented in other ways. For example, the embodiments described above are merely illustrative. For instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed.

[0086] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0087] Furthermore, the functional units in the various embodiments of this application can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0088] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage device. Based on this understanding, the technical solution of this application, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a memory and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of this application.

[0089] Those skilled in the art will understand that all or part of the circuits in the above embodiments can be implemented by a program instructing related hardware. The program can be stored in a computer-readable storage medium, which may include: a flash drive, a read-only memory (ROM), a random access memory (RAM), a magnetic disk, or an optical disk, etc.

[0090] The foregoing description is merely an exemplary embodiment of this disclosure and should not be construed as limiting the scope of this disclosure. Any equivalent changes and modifications made in accordance with the teachings of this disclosure shall still fall within the scope of this disclosure. Those skilled in the art will readily conceive of embodiments of this disclosure upon considering the specification and practicing the disclosure herein. This application is intended to cover any variations, uses, or adaptations of this disclosure that follow the general principles of this disclosure and include common knowledge or customary techniques in the art not described herein. The specification and embodiments are to be considered exemplary only, and the scope and spirit of this disclosure are defined by the claims.

[0091] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0092] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for measuring the free spectral range of fiber optic MZI based on a broadband light source, characterized in that, The method includes: A broadband light source is input into the fiber optic Mach-Zehnder interferometer under test, the interference light signal output by the fiber optic Mach-Zehnder interferometer under test is collected, and it is converted into an electrical signal. The electrical signal is subjected to AC coupling processing to filter out the DC component and low-frequency noise components with frequencies below the cutoff frequency, retaining only the interferometric AC component with a frequency above the cutoff frequency to obtain a time-domain signal; wherein the cutoff frequency is lower than the theoretical minimum value of the free spectral range of the fiber optic Mach-Zehnder interferometer under test. The time-domain signal is subjected to spectral analysis to obtain a power spectral density map, and the characteristic frequency peak is extracted from the power spectral density map. The characteristic frequency peak is the free spectral range of the fiber Mach-Zehnder interferometer under test. The spectral width of the broadband light source is greater than the estimated frequency interval corresponding to the free spectral range estimated by the fiber-to-optical Mach-Zehnder interferometer under test. The step of performing spectral analysis on the time-domain signal to obtain the power spectral density map specifically includes: The time-domain signal is divided into multiple overlapping or non-overlapping data segments; Apply a window function to each data segment, and then perform a fast Fourier transform on each windowed data segment to transform the data segment from the time domain to the frequency domain, thereby obtaining the sub-spectrum of each data segment; The power spectrum of each data segment is obtained by taking the square of the modulus of the sub-spectrum of each data segment. The power spectral density estimate is obtained by averaging the power spectra of all data segments. Plot a power spectral density diagram with frequency on the x-axis and power spectral density value on the y-axis.

2. The method for measuring the free spectral range of fiber MZI based on a broadband light source according to claim 1, characterized in that, An AC coupling circuit is used to perform AC coupling processing on electrical signals.

3. The method for measuring the free spectral range of fiber MZI based on a broadband light source according to claim 2, characterized in that, The AC coupling process for electrical signals using AC coupling circuits specifically includes: The output electrical signal is directly connected to an oscilloscope or data acquisition card with a built-in AC coupling circuit, and its input coupling mode is set to AC coupling mode. The built-in AC coupling circuit filters out DC components and low-frequency noise components, and the signal is acquired at a set sampling rate to obtain a time-domain signal.

4. The method for measuring the free spectral range of fiber MZI based on a broadband light source according to claim 3, characterized in that, The set sampling rate is no less than twice the theoretical maximum value of the free spectral range of the fiber optic Mach-Zehnder interferometer under test.

5. The method for measuring the free spectral range of fiber MZI based on a broadband light source according to claim 1, characterized in that, Extracting the characteristic frequency peak from the power spectral density map specifically includes: traversing all frequency points in the power spectral density map, searching for the frequency point corresponding to the maximum amplitude, and determining the frequency value corresponding to the frequency point as the characteristic frequency peak.

6. The method for measuring the free spectral range of fiber MZI based on a broadband light source according to claim 5, characterized in that, After the step of traversing all frequency points in the power spectral density map, searching for the frequency point corresponding to the maximum amplitude, and determining the frequency value corresponding to the frequency point as the characteristic frequency peak, the method further includes: interpolating and fitting multiple frequency points near the currently obtained characteristic frequency peak to obtain the corrected characteristic frequency peak.

7. The method for measuring the free spectral range of fiber MZI based on a broadband light source according to claim 6, characterized in that, The interpolation fitting is performed on multiple frequency points near the currently obtained characteristic frequency peak. Specifically, it includes: taking the currently obtained characteristic frequency peak and several frequency points to its left and right, using their frequency value and amplitude data as samples, and performing interpolation fitting using any one of parabolic interpolation, Gaussian interpolation or centroid interpolation, and taking the frequency value corresponding to the vertex of the fitted curve as the corrected characteristic frequency peak.

8. A fiber optic MZI free spectral range measurement system based on a broadband light source, comprising a memory, a processor, and a computer program stored in the memory, characterized in that, The processor executes the computer program to implement the steps of the method according to any one of claims 1 to 7.

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