A hollow optical fiber drawing on-line OFDR detection system and method for communication

Abstract

This invention discloses an online OFDR inspection system and method for hollow optical fiber drawing, relating to the field of optical fiber manufacturing quality inspection technology. The online OFDR inspection system for hollow optical fiber drawing includes: a front optical coupling unit located near the preform, which injects a linearly swept laser non-contactly into the preform end face through a 1-2 mm air gap, moving the detection point forward to the drawing source; an OFDR main unit performing dual polarization diversity coherent reception to eliminate polarization fading; and a defect identification and location unit for analyzing the electrical signal to be processed. This invention, by moving the detection point from the traditional finished fiber end to the near end of the preform, achieves online closed-loop control of immediate defect detection and adjustment, solving problems such as detection lag, contact coupling damage to the fiber, near-end blind zone, polarization fading, and signal crosstalk in existing technologies, significantly reducing material scrap rate.

Description

Technical Field

[0001] This invention relates to the field of optical fiber manufacturing quality inspection technology, specifically to an online OFDR inspection system and method for drawing hollow optical fibers for communication. Background Technology

[0002] Hollow optical fiber for communication uses air as its core and guides light through its cladding microstructure. It is a new generation of ultra-low loss and ultra-low latency transmission medium, and its transmission performance depends entirely on the nanometer-level precision of the cladding microstructure. Defects such as microstructural distortion, microbending, microcracks, hollow core contamination, and uneven wall thickness during the fiber drawing process will directly lead to product scrap. Therefore, implementing online defect detection in the fiber drawing stage is crucial.

[0003] The limitations of existing technologies include at least the following problems: Existing detection schemes place the detection point at the end of the fiber drawing process, i.e., at the finished fiber, allowing only post-processing detection of solidified defects. This makes it difficult to detect and prevent defects from occurring in the first instance, resulting in significant material waste. This scheme relies on contact coupling to introduce detection light into the fiber. Physical contact easily causes the microstructure of hollow fibers to collapse or become contaminated. Furthermore, the fiber segment from the hot zone exit to the coupling point is a detection blind zone, meaning defects cannot be monitored during the most critical stage of fiber formation. Rayleigh scattering in hollow fibers is much weaker than in solid fibers, and strong birefringence induces polarization fading, resulting in a low signal-to-noise ratio for backscattered signals. Simultaneously, the detection light propagates along the entire fiber segment, and the backscattered signals accumulated on the long-distance finished fiber on the winding reel can alias into the effective detection frequency band, masking the scattering characteristics of weak defects near the end. These combined factors make it difficult for existing technologies to meet the actual requirements of online detection of anti-resonant hollow fibers and photonic bandgap hollow fibers in terms of detection sensitivity, signal-to-noise ratio, and defect location accuracy. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides an online OFDR inspection system and method for hollow optical fiber drawing for communication applications. It solves the problems in existing technologies, such as the difficulty in real-time defect detection due to the rear placement of inspection points, easy damage to optical fibers due to contact coupling, near-end inspection blind zone, polarization fading, far-end signal crosstalk masking near-end defect characteristics, and difficulty in closed-loop linkage between inspection and process.

[0005] To achieve the above objectives, the present invention provides the following technical solution: an online OFDR inspection system for hollow optical fiber drawing, comprising: a front optical coupling unit, located at the non-fused cold end of a hollow optical fiber preform, for injecting a linearly swept laser non-contactly into the end face of the preform through an air gap; an OFDR main unit, connected to the front optical coupling unit, including a dual polarization diversity coherent receiving module, for receiving backscattered signals and forming interference beat frequency signals, and performing dual polarization diversity coherent reception and photoelectric conversion on the interference beat frequency signals to form an electrical signal to be processed; a defect identification and location unit, for analyzing the electrical signal to be processed to obtain the distance domain loss distribution trace of the near end of the drawn optical fiber to be inspected, identifying defects and determining their axial positions based on the distance domain loss distribution trace; and a production linkage closed-loop unit, connected to the defect identification and location unit and the drawing tower main control system, for sending process parameter adjustment commands to the drawing tower main control system based on the detected defect information.

[0006] Furthermore, the front optical coupling unit includes a three-dimensional closed-loop adjustment frame and an aspherical lens mounted on the adjustment frame. The adjustment frame is used to perform real-time closed-loop adjustment of the relative position between the aspherical lens and the end face of the preform in the X, Y, and Z axes to maintain the air gap at 1-2 mm.

[0007] Furthermore, the front optical coupling unit also includes a redundant dual-optical-path backup module. The redundant dual-optical-path backup module includes a main detection optical path, a backup detection optical path, and an optical switch connected to both. When the output optical power, coupling efficiency, interference beat frequency signal amplitude, or signal-to-noise ratio of the main detection optical path is detected to be lower than a preset threshold, the optical switch switches to the backup detection optical path.

[0008] Furthermore, it also includes: a range-matched filtering unit, used to determine the filter cutoff frequency based on the sweep rate of the linear sweep laser, the effective refractive index of the hollow fiber, and the maximum detection distance of the near-end of the drawn fiber to be tested, so as to retain the beat frequency component corresponding to the near-end of the drawn fiber to be tested, and suppress the non-interested distance beat frequency component or over-range beat frequency component corresponding to the long-distance finished fiber accumulated on the rear take-up reel.

[0009] Furthermore, it also includes: a wire drawing synchronization trigger unit, which is electrically connected to the encoder of the wire drawing tower traction wheel, and is used to generate OFDR scanning trigger pulses according to the real-time pulse signal output by the encoder, so that the scanning trigger time of the linear sweep laser is hard synchronized with the axial spatial position of the optical fiber to be tested.

[0010] An online OFDR inspection method for hollow optical fiber drawing for communication includes the following steps: injecting a linearly swept laser non-contactly through an air gap from the non-molten cold end of a hollow optical fiber preform, allowing the linearly swept laser to propagate along the continuous optical fiber during the drawing process; receiving backscattered signals and performing dual polarization diversity coherent reception and photoelectric conversion to form an electrical signal to be processed; performing data processing on the electrical signal to be processed to obtain the distance domain loss distribution trace of the near-end of the drawn optical fiber to be inspected; identifying defects and determining their axial positions based on the distance domain loss distribution trace; generating process parameter adjustment instructions based on the axial position and type of the defects and sending them to the drawing tower main control system.

[0011] Furthermore, the data processing includes determining a filter cutoff frequency based on the sweep rate of the linear sweep laser, the effective refractive index of the hollow fiber, and the maximum detection distance of the near-end of the drawn fiber to be tested, and filtering the electrical signal to be processed based on the filter cutoff frequency to retain the beat frequency component corresponding to the near-end of the drawn fiber to be tested, and suppressing the non-interested distance beat frequency component or over-range beat frequency component corresponding to the long-distance finished fiber accumulated on the subsequent take-up reel.

[0012] Furthermore, it also includes: acquiring real-time pulse signals from the encoder of the drawing tower traction wheel, and generating OFDR scanning trigger pulses based on the real-time pulse signals, so as to achieve hard synchronization between the scanning trigger time of the linear sweep laser and the axial spatial position of the continuous optical fiber.

[0013] Furthermore, the process parameter adjustment command is used to adjust at least one of the following in real time: drawing speed, heating furnace temperature, tension, and winding parameters.

[0014] The present invention has the following beneficial effects:

[0015] (1) The online OFDR detection system for hollow optical fiber drawing for communication moves the detection point from the traditional finished optical fiber end to the drawing source by setting the front optical coupling unit at the non-molten cold end of the preform and injecting detection light non-contactly through the air gap. At the same time, the system is coordinated with the production linkage closed-loop unit to convert the identified defect information into process parameter adjustment instructions in real time. This realizes online closed-loop control of detecting and adjusting defects as soon as they are generated, blocking the continuous production of defective optical fibers from the source and significantly reducing the material scrap rate.

[0016] (2) The online OFDR detection system for hollow fiber drawing used in communication separates the interference beat frequency signal into two orthogonal polarization states through a dual polarization diversity coherent receiver module and performs balanced photoelectric conversion on each state. This avoids the interference of random evolution of polarization state introduced by drawing stress on signal reception stability. At the same time, the range matching filter unit selectively strips the non-interest distance beat frequency component or the over-range beat frequency component corresponding to the far-end accumulated finished fiber. This significantly weakens the masking of the far-end echo on the scattering characteristics of the near-end weak defects at the front end of the signal link, and improves the detection sensitivity and signal-to-noise ratio.

[0017] Of course, any product implementing this invention does not necessarily need to achieve all of the advantages described above at the same time. Attached Figure Description

[0018] Figure 1 This is a block diagram of an online OFDR detection system for hollow optical fiber drawing for communication according to the present invention.

[0019] Figure 2 This is a flowchart of an online OFDR detection method for hollow optical fiber drawing for communication according to the present invention. Detailed Implementation

[0020] Please see Figure 1 This invention provides a technical solution: an online OFDR inspection system for hollow optical fiber drawing, comprising: a front optical coupling unit located at the non-fused cold end of a hollow optical fiber preform, used to inject linearly swept laser light non-contactly into the end face of the preform through an air gap; an OFDR main unit connected to the front optical coupling unit, including a dual polarization diversity coherent receiving module, used to receive backscattered signals and form interference beat frequency signals, and to perform dual polarization diversity coherent reception and photoelectric conversion on the interference beat frequency signals to form an electrical signal to be processed; a defect identification and location unit, used to analyze the electrical signal to be processed to obtain the distance domain loss distribution trace of the near end of the drawn optical fiber to be inspected, and to identify defects and determine their axial positions based on the distance domain loss distribution trace; and a production linkage closed-loop unit connected to the defect identification and location unit and the drawing tower main control system, used to send process parameter adjustment commands to the drawing tower main control system based on the detected defect information.

[0021] Specifically, the front optical coupling unit includes a three-dimensional closed-loop adjustment frame and an aspherical lens mounted on the adjustment frame. The adjustment frame is used to perform real-time closed-loop adjustment of the relative position between the aspherical lens and the end face of the preform in the X, Y, and Z axes to maintain an air gap of 1 to 2 mm.

[0022] In some embodiments, the adjustment frame includes precision displacement actuators respectively arranged along the X-axis, Y-axis and Z-axis, with the X / Y direction used for lateral coupling deviation correction and the Z direction used for defocus correction;

[0023] The controller uses a PID closed-loop control algorithm to correct the three-axis displacement in real time based on the detected light injection power or the amplitude of the interference beat frequency signal, and adjusts the frequency to no less than 5Hz.

[0024] In some embodiments, the pre-optical coupling unit is further equipped with an injection state monitoring module. This module collects at least one parameter in real time from the preform end face reflected light intensity, near-end backscattered signal intensity, coupling efficiency fluctuation, and interference beat frequency signal amplitude, and compares these parameters with a preset stability threshold. When an abnormal increase in reflected light intensity, a decrease in backscattered signal intensity, a coupling efficiency fluctuation exceeding a preset range, or an interference beat frequency signal amplitude falling below a preset threshold is detected, it is determined that the detection light has not stably entered the hollow fiber guide path. At this time, the controller drives the three-dimensional closed-loop adjustment frame to fine-tune the X-axis, Y-axis, and Z-axis positions or incident angle of the aspherical lens until the detection light is re-coupled stably into the preform end face and propagates along the continuous hollow fiber during the fiber drawing process.

[0025] In this implementation scheme, the aspherical lens is adjusted in real time by a three-dimensional closed-loop adjustment frame, which can compensate for the injection deviation caused by mechanical vibration, thermal drift or changes in the posture of the preform, maintain the stability of the detection light injection state, and improve the coupling efficiency.

[0026] In an optional embodiment, the front-end optical coupling unit further includes a redundant dual-optical-path backup module. The redundant dual-optical-path backup module includes a main detection optical path, a backup detection optical path, and an optical switch. The system monitors the output optical power, coupling efficiency, interference beat frequency signal amplitude, or signal-to-noise ratio of the main detection optical path in real time. When any of these parameters falls below a preset threshold or remains abnormal for more than a preset time, the system controls the optical switch to switch from the main detection optical path to the backup detection optical path. Preferably, the switching time does not exceed 1 ms. This structure maintains continuous online detection even when the main detection optical path experiences abnormal optical power, device failure, or coupling instability, reducing the risk of downtime due to a single-path failure.

[0027] In this implementation plan, by setting up redundant dual-optical-path backup, when the main detection optical path experiences abnormal optical power, signal interruption, device failure, or coupling instability, it can quickly switch to the backup optical path to continue performing online detection, reducing the risk of downtime caused by a single path failure.

[0028] Specifically, it also includes: a range-matched filtering unit, used to determine the filter cutoff frequency based on the sweep rate of the linear sweep laser, the effective refractive index of the hollow fiber, and the maximum detection distance of the near-end of the drawing section to be tested, so as to retain the beat frequency component corresponding to the near-end of the drawing section to be tested, and suppress the non-interested distance beat frequency component or over-range beat frequency component corresponding to the long-distance finished fiber accumulated on the rear take-up reel.

[0029] The OFDR host has a nominal range of 200m, which is used to cover the maximum distance range that the system can detect; the actual length of the section of interest of the fiber to be tested at the near end of the drawn fiber can be 0.5m to 5m.

[0030] The range-matched filtering unit prioritizes determining the filter cutoff frequency based on the maximum detection distance of the actual segment of interest. Under the condition that it does not exceed the frequency range corresponding to the nominal range of the OFDR host, it retains the beat frequency component of the near-end segment of interest and suppresses the non-interest distance component or over-range component corresponding to the long-distance finished optical fiber accumulated on the reel.

[0031] For back-accumulated finished optical fibers that exceed the OFDR nominal range or are not within the range of interest for near-end fiber drawing detection, the corresponding backscattered echoes appear as over-range or non-interest distance components in the interferometric beat frequency signal. The range-matched filter unit is used to suppress these components to reduce their masking of the scattering characteristics of weak defects near the fiber drawing end.

[0032] In this implementation scheme, the range-matched filtering unit determines the filter cutoff frequency based on the maximum detection distance of the fiber segment to be tested near the drawing end, and reserves margins based on the sampling bandwidth, nominal range, and spatial resolution of the OFDR host. For the long-distance finished fiber that has accumulated on the reel, the backscattered echo generated by it corresponds to the beat frequency component of the non-interest distance or the beat frequency component of the over-range. The range-matched filtering unit suppresses such components through low-pass filtering, digital filtering, or out-of-band suppression, while retaining the beat frequency component corresponding to the interest segment near the drawing end, thereby reducing the masking of the scattering characteristics of weak defects at the near end by the accumulated echo at the far end.

[0033] Specifically, it also includes: a wire drawing synchronization trigger unit, which is electrically connected to the encoder of the wire drawing tower traction wheel, and is used to generate OFDR scanning trigger pulses according to the real-time pulse signal output by the encoder, so that the scanning trigger time of the linear sweep laser is hard synchronized with the axial spatial position of the fiber to be tested.

[0034] In this implementation scheme, the detection signal acquisition time is synchronized with the actual axial position of the optical fiber under test by the wire drawing synchronous triggering unit, which reduces the position calibration error caused by wire drawing speed fluctuations or asynchronous acquisition and improves the axial positioning accuracy of defects.

[0035] Specifically, the dual polarization diversity coherent receiver module includes a polarization beamsplitter and two balanced photodetectors. The polarization beamsplitter separates the interference beat frequency signal into two mutually orthogonal polarization state signals, and the two polarization state signals are respectively input into the corresponding balanced photodetectors for photoelectric conversion.

[0036] Among them, the polarization beam splitter adopts a polarization-maintaining polarization beam splitter, with the working wavelength matching the sweep frequency range of the linear sweep laser, an extinction ratio of not less than 30dB, and an insertion loss of not more than 0.5dB;

[0037] The balanced photodetector is a dual-channel balanced photodetector with a response wavelength of 1500nm-1600nm, an electrical bandwidth of 250MHz-1GHz, and a conversion gain of 10V / W-100V / W.

[0038] In this implementation scheme, the interference beat frequency signal is decomposed into two orthogonal polarization components by a polarization beam splitter and sent to a balanced photodetector for photoelectric conversion. This avoids the interference of random evolution of polarization state introduced by the drawing stress of hollow optical fiber on the stability of signal reception.

[0039] Please see Figure 2 This invention provides a technical solution: an online OFDR detection method for hollow optical fiber drawing for communication, comprising the following steps: injecting a linearly swept laser non-contactly through an air gap from the non-molten cold end of a hollow optical fiber preform, allowing the linearly swept laser to propagate along the continuous optical fiber during the drawing process; receiving backscattered signals and performing dual polarization diversity coherent reception and photoelectric conversion to form an electrical signal to be processed; processing the electrical signal to be processed to obtain the distance domain loss distribution trace of the near-end of the fiber to be tested; identifying defects and determining their axial positions based on the distance domain loss distribution trace; generating process parameter adjustment instructions based on the axial position and type of defects and sending them to the drawing tower main control system.

[0040] Specifically, the steps for dual-polarization diversity coherent reception and photoelectric conversion of the interferometric beat frequency signal are as follows:

[0041] The interference beat frequency signal is input into the polarization beam splitter, which separates the interference beat frequency signal into two mutually orthogonal polarization state signals. The two polarization state signals are then input into the balanced photodetector and output the corresponding balanced electrical signal.

[0042] The two mutually orthogonal polarization state signals are respectively horizontal polarization state signals. and vertical polarization state signal ;

[0043] in, ;

[0044] ;

[0045] , These represent the amplitudes of the two polarization-state signals, The center frequency of the interference beat frequency signal. , These are the instantaneous phases of the two polarization signals, respectively;

[0046] A balanced photodetector converts a horizontally polarized signal into a horizontally polarized balanced electrical signal. Converting vertically polarized signals into vertically polarized balanced electrical signals The transformation relation satisfies:

[0047] ;

[0048] ;

[0049] To balance the conversion gain of the photodetector, , These are the inherent noises of the two balanced electrical signals, respectively.

[0050] The amplitude of the two balanced electrical signals is normalized using the maximum amplitude normalization method to calculate the maximum amplitude of the two balanced electrical signals. , The maximum value of the two is taken as the normalization benchmark. Then, normalization operations are performed on the two balanced electrical signals respectively to obtain the normalized horizontally polarized electrical signals. Normalized vertically polarized electrical signal .

[0051] The two balanced electrical signals, after amplitude normalization, are combined into a single signal to be processed. This merging is performed using a non-coherent amplitude superposition method to avoid phase cancellation between the two signals. The merging formula is as follows:

[0052] ;

[0053] in, This is the merged electrical signal that needs to be processed.

[0054] In this implementation scheme, the two balanced electrical signals are normalized in amplitude and then combined into one electrical signal to be processed by the sum of squares. This avoids the phase cancellation attenuation that may occur when directly adding them together, and ensures that the amplitude of the combined signal remains stable.

[0055] Specifically, data processing includes filtering out the over-range high-frequency beat frequency components corresponding to the long-distance finished optical fiber accumulated on the rear reel, retaining the beat frequency components within the nominal range corresponding to the near-end of the fiber to be tested, and obtaining the filtered electrical signal. The specific steps are as follows:

[0056] The electrical signal to be processed is subjected to a low-pass filter. The cutoff frequency of the low-pass filter is set according to the sweep rate of the linear sweep laser and the nominal range of the OFDR detection. The calculation formula is:

[0057] ;

[0058] in, At the speed of light, For the nominal range, A coefficient related to the sweep frequency period. The sweep rate of the linear sweep laser is determined by the range, spatial resolution, and sampling bandwidth of the OFDR host.

[0059] The filter cutoff frequency is calculated based on the maximum detection distance of the section to be detected near the wire drawing end, with a certain margin reserved.

[0060] The signal after low-pass filtering is sequentially subjected to aliasing residual component removal and baseline drift correction.

[0061] The aliasing residual component removal process employs an adaptive notch filter algorithm with a notch frequency range of 0.9. ~1.1 The notch depth is not less than 30dB, used to remove aliased signals with frequencies close to the cutoff frequency that remain after filtering out over-range components.

[0062] The baseline drift correction process uses the sliding window mean method to calculate the signal mean within the window of each sampling point, and subtracts the mean of the corresponding window from the original signal to eliminate slow baseline drift in the signal.

[0063] Output the filtered electrical signal after baseline drift correction.

[0064] In this implementation, the adaptive notch filtering and sliding window baseline drift correction processes performed sequentially after filtering eliminate aliasing residues near the cutoff frequency and remove the influence of slow drift on the signal baseline.

[0065] Specifically, data processing of the electrical signal to be processed also includes: data acquisition and frequency sweep nonlinear correction of the filtered electrical signal, and analytically obtaining the distance domain loss distribution trace of the near-end of the drawn optical fiber under test. The specific steps are as follows:

[0066] The filtered electrical signal is sampled to obtain the corresponding time-domain sampling sequence. Simultaneously, the fixed optical path difference interference signal generated by the auxiliary interferometer is acquired. The auxiliary interferometer is a Michelson interferometer, which consists of a beam splitter, a fixed mirror, and an adjustable mirror. The optical path difference is fixed at 1m-5m. Taking the zero-crossing point of the fixed optical path difference interference signal generated by the auxiliary interferometer as the reference, the time-domain sampling sequence is resampled and subjected to equal optical frequency interval normalization to obtain the electrical signal sequence that has completed the frequency sweep nonlinear correction.

[0067] Perform a Fast Fourier Transform (FFT) on the electrical signal sequence after frequency sweep nonlinear correction, and map the frequency domain result after the FFT to the loss value corresponding to the axial position of the fiber. The mapping formula is as follows: ;

[0068] in, The effective refractive index of the hollow fiber is 1.001 to 1.01, depending on the type of cladding structure of the hollow fiber. For example, the effective refractive index of the anti-resonant hollow fiber is about 1.005.

[0069] The formula for calculating losses is:

[0070] ;

[0071] Abnormal numerical points that exceed the preset loss range in the mapping results are removed. The remaining numerical sequence is processed using a smooth fitting algorithm to output the distance domain loss distribution trace of the fiber to be tested at the near end of the fiber.

[0072] In this implementation scheme, the aforementioned auxiliary interferometer zero-crossing resampling, equal-frequency spacing regularization, fast Fourier transform, and axial position mapping are specific implementation methods for the defect identification and positioning unit to analyze the electrical signal to be processed. Through this processing flow, the sweep rate fluctuation and wavelength nonlinearity generated by the linear sweep laser during actual scanning can be corrected, so that the sampling sequence maintains a relatively uniform interval in the optical frequency domain, thereby facilitating the subsequent Fourier transform to obtain a stable distance domain loss distribution trace. This processing method can reduce the distance domain broadening and spurious peak interference caused by sweep frequency nonlinearity, making local loss anomalies such as microcracks and microbending clearer in the distance domain trace.

[0073] Specifically, defects are identified and their axial positions are determined based on the distance domain loss distribution traces. An adaptive residual statistical processing algorithm is then used to process the distance domain loss distribution traces. The specific steps are as follows:

[0074] The axial offset is obtained by cross-correlation calculation between the distance domain loss distribution trace and the defect-free reference trace. The two traces are then aligned point by point according to the axial offset. The defect-free reference trace is a pre-acquired distance domain reference trace of a defect-free hollow fiber with the same specifications as the hollow fiber to be tested. It is acquired under the same frequency sweep parameters and receiving parameters as the online test, and is used as a comparison benchmark after temperature drift, tension fluctuation and baseline difference compensation.

[0075] Calculate the loss residual value between the two aligned traces point by point and form a loss residual value sequence. Perform background fluctuation removal processing on the loss residual value sequence.

[0076] For each location point in the loss residual value sequence, extract all loss residual values ​​in the axial neighborhood centered on that location point and calculate the mean value. Use this mean value as the background fluctuation estimate for that location point.

[0077] The difference between the original loss residual value and the background fluctuation estimate at the corresponding location point is calculated.

[0078] The result of the difference operation is subjected to random noise suppression processing, which includes at least one of median filtering, moving average filtering or wavelet denoising, and the output is the loss residual value after removing background fluctuations.

[0079] For each location point, extract the loss residual value after removing background fluctuations in its axial neighborhood, calculate the mean and standard deviation of the loss residual value in the neighborhood, and use the sum of the mean and the preset multiple standard deviation as the defect judgment threshold for the location point. The preset multiple k ranges from 2 to 3, and k=2 in this embodiment.

[0080] The loss residual value at each location point is compared with the corresponding defect judgment threshold one by one, and the location points whose loss residual value exceeds the defect judgment threshold are marked as candidate defect points. Spatial clustering and merging processing is performed on the candidate defect points to merge candidate defect points with an axial spacing of less than a preset merging distance into the same defect region;

[0081] Remove defect regions whose length is less than a preset length threshold, and output the axial position of the remaining defect regions as the axial position of microcracks and microbending loss defects.

[0082] In this implementation scheme, the adaptive residual statistical processing algorithm is a specific method for the defect identification and localization unit to identify defects and determine their axial positions based on the distance domain loss distribution trace. By cross-correlating and aligning the distance domain loss distribution trace obtained from online detection with the defect-free reference trace, the influence of axial offset caused by wire drawing speed fluctuations, temperature drift, or tension changes can be reduced. By calculating the loss residual value point by point and combining it with the neighborhood mean to remove background fluctuations and the median filter to suppress random noise, the loss mutation characteristics caused by microcracks, microbending, or local structural anomalies can be highlighted. The defect judgment threshold is dynamically determined based on the local mean and standard deviation, so that the judgment threshold can adaptively change with local background fluctuations. This improves the detection capability of weak defects in stable background areas and reduces the false alarm probability in areas with strong background fluctuations. Through spatial clustering and merging and region length filtering, multiple candidate points caused by the same defect can be merged into a complete defect region, and finally the axial position range of the defect is output.

[0083] Specifically, it also includes: acquiring real-time pulse signals from the encoder of the drawing tower traction wheel, and generating OFDR scanning trigger pulses based on the real-time pulse signals, so as to achieve hard synchronization between the scanning trigger time of the linear sweep laser and the axial spatial position of the continuous optical fiber.

[0084] Specifically, the process parameter adjustment command is used to adjust at least one of the following in real time: drawing speed, heating furnace temperature, tension, and winding parameters. The production linkage closed-loop unit is connected to the defect identification and positioning unit and the drawing tower main control system, respectively. It converts the detected defect type, defect axial position, and other defect information into corresponding process deviation information and outputs the process adjustment command to the drawing tower main control system. The defect identification delay does not exceed 100ms, and the process parameter adjustment response time does not exceed 200ms.

[0085] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0086] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A hollow optical fiber drawing on-line OFDR detection system for communication, characterized in that, include: A front optical coupling unit is located at the non-molten cold end of the hollow fiber preform and is used to inject linear sweep laser into the end face of the preform non-contactly through an air gap. The OFDR main unit, connected to the front optical coupling unit, includes a dual polarization diversity coherent receiving module for receiving backscattered signals and forming interferometric beat frequency signals, and performing dual polarization diversity coherent receiving and photoelectric conversion on the interferometric beat frequency signals to form an electrical signal to be processed. The defect identification and location unit is used to analyze the electrical signal to be processed, obtain the distance domain loss distribution trace of the fiber to be tested at the near end of the drawing, identify the defect and determine its axial position based on the distance domain loss distribution trace. The production linkage closed-loop unit is connected to the defect identification and location unit and the drawing tower main control system, respectively, and is used to send process parameter adjustment instructions to the drawing tower main control system based on the detected defect information.

2. The hollow core fiber drawing on-line OFDR sensing system for communication according to claim 1, characterized in that: The front optical coupling unit includes a three-dimensional closed-loop adjustment frame and an aspherical lens mounted on the adjustment frame. The adjustment frame is used to perform real-time closed-loop adjustment of the relative position between the aspherical lens and the end face of the preform in the X, Y, and Z axes to maintain the air gap at 1-2 mm.

3. The hollow core fiber drawing on-line OFDR sensing system for communication according to claim 1, characterized in that, The front optical coupling unit also includes a redundant dual-optical-path backup module. The redundant dual-optical-path backup module includes a main detection optical path, a backup detection optical path, and an optical switch connected to both. When the output optical power, coupling efficiency, interference beat frequency signal amplitude, or signal-to-noise ratio of the main detection optical path is detected to be lower than a preset threshold, the optical switch is used to switch to the backup detection optical path.

4. The online OFDR inspection system for hollow optical fiber drawing for communication as described in claim 1, characterized in that, Also includes: The range-matched filtering unit is used to determine the filter cutoff frequency based on the sweep rate of the linear sweep laser, the effective refractive index of the hollow fiber, and the maximum detection distance of the near-end of the fiber drawing section to be tested, so as to retain the beat frequency component corresponding to the near-end of the fiber drawing section and suppress the non-interested distance beat frequency component or over-range beat frequency component corresponding to the long-distance finished fiber accumulated on the subsequent take-up reel.

5. The online OFDR inspection system for hollow optical fiber drawing for communication as described in claim 1, characterized in that, Also includes: The wire drawing synchronization trigger unit is electrically connected to the encoder of the wire drawing tower traction wheel. It is used to generate OFDR scanning trigger pulses based on the real-time pulse signal output by the encoder, so that the scanning trigger time of the linear sweep laser is hard synchronized with the axial spatial position of the optical fiber to be tested.

6. The online OFDR inspection system for hollow optical fiber drawing for communication as described in claim 1, characterized in that, The dual polarization diversity coherent receiving module includes a polarization beamsplitter and two balanced photodetectors. The polarization beamsplitter separates the interferometric beat frequency signal into two mutually orthogonal polarization state signals, and the two polarization state signals are respectively input into the corresponding balanced photodetectors for photoelectric conversion.

7. A method for online OFDR testing of hollow optical fiber drawing for communication, using the online OFDR testing system for hollow optical fiber drawing according to any one of claims 1-6, characterized in that, Includes the following steps: A linear sweeping laser is injected non-contactly from the non-molten cold end of a hollow fiber preform through an air gap, allowing the linear sweeping laser to propagate along the continuous optical fiber during the fiber drawing process. The backscattered signal is received and subjected to dual polarization diversity coherent reception and photoelectric conversion to form an electrical signal to be processed. The electrical signal to be processed is processed to obtain the distance domain loss distribution trace of the near end of the drawn fiber to be tested. Defects are identified and their axial positions are determined based on the distance domain loss distribution traces. Based on the axial position and type of the defect, process parameter adjustment instructions are generated and sent to the drawing tower main control system.

8. The hollow optical fiber drawing on-line OFDR detection method for communication according to claim 7, characterized in that: The data processing includes determining the filter cutoff frequency based on the sweep rate of the linear sweep laser, the effective refractive index of the hollow fiber, and the maximum detection distance of the near-end of the drawn fiber to be tested, and filtering the electrical signal to be processed based on the filter cutoff frequency to retain the beat frequency component corresponding to the near-end of the drawn fiber to be tested, and suppressing the non-interested distance beat frequency component or over-range beat frequency component corresponding to the long-distance finished fiber accumulated on the reel.

9. The hollow optical fiber drawing on-line OFDR sensing method for communication according to claim 7, characterized in that, Also includes: The encoder of the drawing tower traction wheel acquires real-time pulse signals, and OFDR scanning trigger pulses are generated based on the real-time pulse signals to achieve hard synchronization between the scanning trigger time of the linear sweep laser and the axial spatial position of the continuous optical fiber.

10. The hollow optical fiber drawing on-line OFDR sensing method for communication according to claim 7, characterized in that, The process parameter adjustment command is used to adjust at least one of the following in real time: drawing speed, heating furnace temperature, tension, and winding parameters.

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