A method for on-line detection and feedback of growth defects of sapphire optical fiber

Abstract

The application discloses a method for on-line detection and feedback of growth defects of sapphire optical fibers, which is characterized by the following steps: matching the optical path of the target length to be detected, and ingeniously using the end-round-table-shaped sapphire optical fiber seed crystal as a connecting device to realize real-time detection of the backscattering signals in the sapphire optical fiber during the growth process, and to obtain information about internal defects and unevenness, so as to provide accurate quantitative data for the preparation process and subsequent application of the sapphire optical fiber. The application realizes a new application of the white-light interference distributed testing of the optical fiber, and also lays a solid foundation for the technical development of the sapphire optical fiber.

Description

Technical fields:

[0001] This invention relates to a method for online detection and feedback of growth defects in sapphire optical fibers, belonging to the field of optical fiber testing. Background Technology

[0002] Sapphire fiber is an excellent high-temperature waveguide with a melting point as high as 2043℃, exhibiting excellent high-temperature resistance. Furthermore, sapphire fiber possesses superior light transmission properties in the ultraviolet to mid-infrared band, giving it a unique advantage in high-temperature sensing applications. Simultaneously, due to its high laser damage threshold, sapphire fiber is also widely used in high-power laser devices. However, factors such as internal defects, inhomogeneities, bubbles, and fiber diameter fluctuations generated during single-crystal growth can lead to increased transmission loss and degraded optical performance in sapphire fiber, which is detrimental to signal reception and demodulation in high-temperature sensors.

[0003] The growth conditions of sapphire optical fibers determine their quality. Currently, most commercially available sapphire optical fibers are prepared using the laser heated substrate method (LHPG). In sapphire crystal growth, the main types of defects include inhomogeneous growth regions and bubbles, and the presence of defects in the crystal significantly affects the optical and mechanical properties of sapphire optical fibers. (Wang D, Hou W, NaL I, et al. Defects and Optical Property of Single-crystal Sapphire Fibers Grown by Edge-defined Film-fed Growth Method[J]. Journal of Inorganic Materials, 2020, 35(9):573.). When preparing sapphire optical fibers using the LHPG method, factors such as the feed rate of the source rod, the movement rate of the seed crystal, the power stability of the laser heating source, and the focal position directly affect the crystal growth quality of the sapphire optical fiber (Rey-García F, R, Angurel LA, et al. Laser floating zone growth: Overview, singular materials, broad applications, and future perspectives[J]. Crystals, 2020, 11(1):38.). Defect detection in sapphire optical fibers is usually performed by direct observation using optical microscopy or scanning electron microscopy (Liu B, Yu Y, Bera S, et al. Study of molten zone profile and defect formation during laser heated pedestal growth[C] / / Micro-and Nanotechnology Sensors, Systems, and Applications XI.SPIE, 2019, 10982:418-426.), or by detecting bubbles using machine vision and image recognition methods (Yang X, Qiao T, Zhang H, et al. Research on image recognition and detection method of sapphire bubbles[J]. Journal of Instrumentation, 2019, 14(12):P12013.). However, these detection methods are generally only applicable to finished sapphire crystal products. In sapphire fiber growth control systems, high-speed cameras are often used to acquire images of the sapphire fiber diameter and feed them back to the control system to reduce dimensional fluctuations during growth. However, this method cannot detect internal defects in the sapphire fiber in real time. By using a distributed fiber waveguide detection method, abnormal reflection signals caused by internal defects such as bubbles and uneven growth regions within the grown sapphire fiber can be detected and fed back in real time. This allows for timely adjustment and optimization of single-crystal growth conditions, ultimately improving the growth quality and fabrication efficiency of sapphire fiber, avoiding material waste, and saving costs. Summary of the Invention

[0004] The purpose of this invention is to improve the growth quality and efficiency of sapphire optical fibers by proposing an online detection and feedback method for defects in sapphire optical fiber growth. This invention uses a fiber optic white light interferometry distributed testing device to achieve real-time detection and feedback of defects in sapphire optical fiber growth. The defect signals and location information measured by the fiber optic white light interferometry distributed testing device are fed back to the sapphire optical fiber growth and preparation system to assist in optimizing growth process parameters.

[0005] The objective of this invention is achieved as follows:

[0006] A method for online detection and feedback of defects in sapphire optical fiber growth involves feeding back defect signals and locations measured by a fiber white light interferometry distributed testing device to the sapphire optical fiber growth and fabrication system to assist in optimizing growth process parameters. The method specifically includes the following steps:

[0007] Step 1: Based on the expected length of the sapphire fiber to be grown, design the length of the matching fiber in the fiber white light interferometric distributed testing device to form the fiber white light interferometric distributed testing device 9 to be measured; and install a circulator, as well as a recovery optical attenuator 7 and an optical filter 8 on the measuring arm of the fiber white light interferometric distributed testing device 9 to facilitate the intensity adjustment of backscattered or reflected light and the filtering of interference light.

[0008] Step 2: Based on the dimensions of the single-mode fiber 6 at the test end interface of the fiber optic white light interferometry distributed testing device 9, the upper surface of the sapphire fiber seed crystal 1 is ground into a frustum 2 and polished. Depending on the target diameter of the sapphire fiber to be prepared, the diameter of the sapphire fiber seed crystal 1 will vary in the actual growth process. The diameter of the upper circular cross-section of the frustum 2 is less than or equal to the cladding diameter of the single-mode fiber 6 and greater than or equal to the core diameter of the single-mode fiber 6, in order to increase optical coupling efficiency, constrain the optical transmission mode, and enhance the reception of reflected light signals.

[0009] Step 3: Fusion the frustum 2 and the single-mode fiber 6 together, and fix the sapphire fiber seed crystal 1 on the fixture of the heating base.

[0010] Step 4: Start the fiber white light interferometry distributed testing device 9 to perform a pre-test on the sapphire fiber seed crystal 1 in its initial state and calibrate the optical path position of the sapphire fiber seed crystal 1 in its initial state.

[0011] Step 5: Start the CO2 laser source 5 used for heating during the sapphire fiber growth process, gradually increase the power of the CO2 laser, and adjust the recovery optical attenuator 7 and filter 8 to ensure that the recovery power of the detection signal light is moderate and to ensure a high signal-to-noise ratio of the detection signal of the fiber white light interferometry distributed testing device.

[0012] Step Six: After the molten zone 4 at the top of the sapphire crystal source rod 3 stabilizes, the lower end of the sapphire fiber seed crystal 1 sinks into the molten growth zone at the top of the sapphire crystal source rod 3 and is pulled upward at a certain pulling speed. At the same time, the sapphire crystal source rod is fed upward at a certain speed; the sapphire fiber begins to grow, and the fiber white light interferometry distributed testing device 9 is used for formal testing. Combined with the real-time data processing results, the online detection of defect distribution during the growth process of the sapphire fiber is realized.

[0013] Step 7: In the sapphire fiber growth stage of Step 6, based on the feedback from the online detection results of the defect distribution of the fiber white light interferometry distributed testing device 9, the growth process parameters such as the feed rate of the source rod, the moving rate of the seed crystal, and the power of the CO2 laser are finely adjusted in real time. After obtaining the variable parameter with the fewest sapphire defects, the variable parameter is kept unchanged in the subsequent growth process until a sapphire fiber is grown.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0015] (1) A method for online detection of fiber distribution during the growth and preparation of sapphire optical fiber is proposed for the first time. This method can detect the quality of sapphire optical fiber and also quantitatively improve the sapphire optical fiber preparation process based on real-time detection signal feedback.

[0016] (2) Innovatively, a frustum-shaped sapphire fiber seed crystal is used as a connection device between the single-mode fiber and the sapphire fiber under test, which solves the problem of core diameter mismatch and does not affect the traditional sapphire growth and preparation process.

[0017] (3) By utilizing the performance advantages of the fiber white light interferometry distributed testing device, high spatial resolution testing of internal defects in sapphire optical fibers can be effectively achieved, providing an important reference for parameter setting during the growth process of sapphire optical fibers.

[0018] (4) By analyzing the real-time detection signal of the fiber white light interference distributed testing device, the defect type of sapphire fiber, such as bubbles, uneven growth region of crystal, etc., can be determined. Finally, the feedback is fed back to the sapphire fiber growth and preparation system to optimize the growth process parameters in real time, such as the pulling speed of sapphire fiber seed crystal, the feeding speed of sapphire crystal source rod, the focus and power stability of CO2 laser, etc.

[0019] (5) This invention not only plays an important role in optimizing the growth process parameters of sapphire optical fiber and improving the growth quality of sapphire optical fiber, but also achieves the purpose of timely adjustment and optimization of growth parameters through real-time detection and feedback of defects, which can effectively reduce the defect rate and save raw materials and costs. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the grinding shape of the top end face of the sapphire fiber seed crystal according to the present invention;

[0021] Figure 2 This is a schematic diagram of the real-time detection device for growth defects in sapphire optical fibers according to the present invention.

[0022] Figure 3 A schematic diagram of typical defect types and distributed test signals in sapphire optical fiber;

[0023] Figure 4 This is a schematic diagram of the process for real-time detection and feedback control of growth parameters for sapphire optical fiber growth defects as described in this invention.

[0024] Among them, 1 is the sapphire fiber seed crystal, 2 is the upper part of the sapphire fiber seed crystal polished into a frustum shape, 3 is the sapphire crystal source rod, 4 is the fusion zone on the upper part of the sapphire crystal source rod, 5 is the CO2 laser source, 6 is the single-mode fiber, which is the measurement end interface of the fiber white light interferometry distributed testing device 9; 7 is the optical attenuator, 8 is the filter, 9 is the fiber white light interferometry distributed testing device, and 10 is the sapphire fiber. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings:

[0026] A method for online detection and feedback of growth defects in sapphire optical fibers includes the following steps:

[0027] Step 1: Based on the expected length of the sapphire fiber to be grown, design the matching length of the fiber in the fiber-optic distributed interferometer to construct a fiber-optic white light interferometer distributed testing device to match the target to be measured. A circulator, as well as a recovery optical attenuator and an optical filter, are added to the measuring arm of the fiber-optic white light interferometer distributed testing device to facilitate the intensity adjustment of backscattered or reflected light and the filtering of interference light.

[0028] Step 2: Based on the fiber dimensions of the test end interface of the fiber-optic white light interferometry distributed testing device, a frustum-shaped sapphire fiber seed crystal is prepared by grinding and polishing. The diameter of the sapphire fiber seed crystal varies depending on the target diameter of the prepared sapphire fiber. The diameter of the upper circular cross-section of the frustum at the top of the seed crystal is less than or equal to the cladding diameter of the quartz fiber at the test end of the fiber-optic white light interferometry distributed testing device, and greater than or equal to the core diameter of the quartz fiber at the test end of the fiber-optic distributed measurement instrument. This is to increase optical coupling efficiency, constrain the optical transmission mode, and enhance the reception of reflected light signals.

[0029] Step 3: The top surface of the frustum-shaped sapphire fiber seed obtained in Step 2 is fused together with the test end interface fiber of the fiber white light interference distributed testing device, and the sapphire fiber seed is fixed on the clamp of the heating base.

[0030] Step 4: Start the fiber optic white light interferometry distributed testing device to perform pre-tests on the sapphire fiber seed crystal in its initial state and calibrate the optical path position of the sapphire fiber seed crystal in its initial state.

[0031] Step 5: Start the CO2 laser used for heating during the sapphire fiber growth process, gradually increase the power of the CO2 laser, and adjust the recovery light attenuator of the fiber white light interferometry distributed testing device to ensure that the recovery power of the detection signal light is moderate and to ensure a high signal-to-noise ratio of the detection signal of the fiber white light interferometry distributed testing device.

[0032] Step 6: Continue increasing the CO2 laser power to create a stable molten zone at the top of the sapphire crystal source rod. The lower end of the sapphire fiber seed crystal sinks into the molten growth zone at the top of the sapphire crystal source rod and is pulled upwards at a certain speed. Simultaneously, the sapphire crystal source rod is fed upwards at an appropriate speed. After meeting the conditions required for sapphire fiber growth, testing is officially conducted using a fiber optic white light interferometry distributed testing device. Combined with real-time data processing results, online detection of defect distribution during the sapphire fiber growth process is completed.

[0033] Step 7: In the initial stage of the sapphire fiber growth process in Step 6, the feed rate of the source rod, the moving rate of the seed crystal, and the power of the CO2 laser are finely adjusted respectively. Based on the online detection results of the defect distribution in Step 6, the variable parameter with the fewest sapphire defects is obtained, and this variable parameter is kept unchanged in the subsequent growth process until a sapphire fiber is grown.

[0034] Given the low coherence characteristics of broadband white light sources, precise matching of the optical path lengths of the measuring arm and the reference arm is necessary. Therefore, the fiber length and scanning range of the scanning mirror must be matched to the length range of the fiber under test. Furthermore, since the growth of sapphire fiber involves the release of significant heat and light radiation from the melting of the crystal material by a high-power laser, it is essential to filter and attenuate the recovered light. This protects the fiber optic white light interferometry distributed testing device while simultaneously improving its signal detection capabilities.

[0035] The bottom surface of the frustum is the same size as the sapphire fiber seed crystal, and the diameter of the bottom surface of the frustum is less than or equal to the size of the fiber at the test end interface of the fiber-optic white light interferometry distributed testing device. Because the fibers in the fiber-optic white light interferometry distributed testing device are all single-mode fibers, designing the seed crystal as a frustum ensures that the fiber-optic white light interferometry distributed testing device is suitable for online testing of sapphire fibers of any size, reducing the impact of core diameter mismatch.

[0036] The purpose of calibrating the initial optical path position of the sapphire fiber seed crystal is to use the seed crystal as a reference to accurately position the subsequently grown sapphire fiber.

[0037] (1) First, based on the expected length of the sapphire fiber to be grown, the length of the matching fiber in the fiber distributed tester is designed to form the fiber white light interferometric distributed test device 9 to match the target to be measured. A circulator, as well as a recovery optical attenuator 7 and an optical filter 8 are added to the measuring arm of the fiber white light interferometric distributed test device 9 to facilitate the intensity adjustment of backscattered or reflected light and the filtering of interference light.

[0038] (2) Based on the dimensions of the fiber 6 at the test end interface of the fiber-optic white light interferometry distributed testing device 9, the upper surface of the sapphire fiber seed crystal 1 is ground into a frustum shape 2 and then polished. Depending on the target diameter of the sapphire fiber being prepared, the diameter of the sapphire fiber seed crystal 1 varies during actual growth. The diameter of the upper circular cross-section of the frustum 2 at the top of the seed crystal is less than or equal to the cladding diameter of the quartz fiber 6 at the test end of the fiber-optic white light interferometry distributed testing device, and greater than or equal to the core diameter of the quartz fiber 6 at the test end of the fiber-optic distributed measuring instrument, in order to increase optical coupling efficiency, constrain the optical transmission mode, and enhance the reception of reflected light signals.

[0039] (3) The top surface 2 of the obtained frustum-shaped sapphire fiber seed crystal is fused together with the test end interface fiber 6 of the fiber white light interference distributed test device, and the sapphire fiber seed crystal 1 is fixed on the fixture of the heating base.

[0040] (4) Start the fiber white light interference distributed test device 9 to perform a pre-test on the sapphire fiber seed crystal 1 in the initial state and calibrate the optical path position of the sapphire fiber seed crystal 1 in the initial state.

[0041] (5) Start the CO2 laser source 5 used for heating during the growth of sapphire fiber, gradually increase the power of the CO2 laser, and adjust the recovery light attenuator 7 and filter 8 to ensure that the recovery power of the detection signal light is moderate and to ensure the high signal-to-noise ratio of the detection signal of the fiber white light interference distributed test device.

[0042] (6) After a stable molten zone 4 is formed at the top of the sapphire crystal source rod, the lower end of the sapphire fiber seed crystal sinks into the molten growth zone at the top of the sapphire crystal source rod and is pulled upwards at a certain pulling speed. At the same time, the sapphire crystal source rod is fed upwards at a certain speed. The sapphire fiber begins to grow, and the fiber white light interferometry distributed testing device is used for formal testing. Combined with the real-time data processing results, online detection of defect distribution during the sapphire fiber growth process is realized.

[0043] (7) In the sapphire fiber growth stage in step six, the growth process parameters such as the feed rate of the source rod, the moving rate of the seed crystal, and the power of the CO2 laser are finely adjusted in real time based on the feedback of the online detection results of the defect distribution of the fiber white light interference distributed testing device. After obtaining the variable parameter with the fewest sapphire defects, the variable parameter is kept unchanged in the subsequent growth process until a sapphire fiber is grown.

[0044] Application Example 1: When the seed crystal pulling speed of sapphire fiber is too fast, bubbles will form inside the growing sapphire fiber. These bubbles generate strong light reflection, allowing the fiber optic distributed measurement instrument to detect sharp abrupt peaks in the reflected signal. Based on the scanning optical path position, the location of the bubble within the sapphire fiber can be pinpointed. Figure 3 As shown, after observing sharp reflection signal peaks, real-time fine-tuning to slow down the seed crystal pulling speed can reduce the generation of bubbles in the sapphire fiber. This optimizes the growth parameters and avoids further waste of raw materials during the growth process.

[0045] Application Example 2: When the source rod feed speed is too fast or the seed crystal pulling speed is too slow, densely packed regions of crystal growth will occur during sapphire fiber growth, causing fluctuations in density and refractive index. This uneven growth region manifests in the measurement signal of the fiber optic distributed measurement instrument as an overall increase in the reflected signal within an optical path length equivalent to that region, such as... Figure 3 As shown. At this point, the feed rate of the source rod should be slightly reduced or the pulling speed of the seed crystal should be appropriately increased to ensure uniform growth of the sapphire crystal.

[0046] Furthermore, this invention is also applicable to defect detection in finished sapphire optical fibers. The sapphire fiber seed crystal can be replaced with a pre-fabricated sapphire optical fiber, and the sapphire end can be similarly... Figure 1 The process involves grinding and polishing the sapphire fiber in a specific manner, and then connecting it to a fiber optic distributed measurement instrument. This allows for the detection of internal defects in finished sapphire fiber optic products. Based on the detection information, the part with the fewest defects can be selected for use, such as in the manufacture of sapphire fiber optic grating high-temperature sensors or Fabry-Perot interferometer high-temperature strain / temperature sensors.

[0047] To address the issue of defects that easily occur during the growth of sapphire optical fibers, this invention proposes a distributed online detection method for the sapphire optical fiber growth process based on a distributed white-light interferometry testing device. By designing the optical path to match the length of the target fiber and cleverly utilizing the frustum-shaped sapphire fiber seed crystal at the end as a connecting device, the backscattering signal inside the sapphire fiber during growth is detected in real time, acquiring information such as internal defects and non-uniformity. This provides accurate quantitative data for the fabrication process and subsequent applications of sapphire optical fibers. This invention realizes a new application of distributed white-light interferometry testing and lays a solid foundation for the technological development of sapphire optical fibers.

Claims

1. A method for online detection and feedback of growth defects in sapphire optical fibers, characterized in that, The defect signal and location information measured by the fiber white light interferometry distributed testing device are fed back to the sapphire fiber growth and fabrication system to assist in optimizing the growth process parameters. The specific steps include the following: Step 1: Based on the expected length of the sapphire fiber to be grown, design the length of the matching fiber in the fiber white light interferometric distributed test device to form the fiber white light interferometric distributed test device (9) to be measured; and install a circulator, as well as a recovery optical attenuator (7) and an optical filter (8) on the measuring arm of the fiber white light interferometric distributed test device (9) to facilitate the intensity adjustment of backscattered or reflected light and the filtering of interference light. Step 2: Based on the dimensions of the single-mode fiber (6) at the test end interface of the fiber white light interference distributed testing device (9), the upper surface of the sapphire fiber seed crystal (1) is ground into a frustum (2) and polished. Depending on the target diameter of the sapphire fiber to be prepared, the diameter of the sapphire fiber seed crystal (1) is also different in the actual growth process. The diameter of the upper circular cross section of the frustum (2) is less than or equal to the cladding diameter of the single-mode fiber (6) and greater than or equal to the core diameter of the single-mode fiber (6) to increase the optical coupling efficiency, constrain the optical transmission mode and enhance the reception of reflected light signals. Step 3: Fusion the frustum (2) and the single-mode fiber (6) together, and fix the sapphire fiber seed crystal (1) on the fixture of the heating base; Step 4: Start the fiber white light interference distributed test device (9) to perform a pre-test on the sapphire fiber seed crystal (1) in the initial state and calibrate the optical path position of the sapphire fiber seed crystal (1) in the initial state. Step 5: Start the CO2 laser source (5) used for heating during the sapphire fiber growth process, gradually increase the power of the CO2 laser, and adjust the recovery light attenuator (7) and filter (8) to ensure that the recovery power of the detection signal light is moderate and to ensure the high signal-to-noise ratio of the detection signal of the fiber white light interference distributed test device. Step 6: After the molten zone (4) at the top of the sapphire crystal source rod (3) stabilizes, the lower end of the sapphire fiber seed crystal (1) sinks into the molten growth zone at the top of the sapphire crystal source rod (3) and is pulled upward at a certain pulling speed. At the same time, the sapphire crystal source rod is fed upward at a certain speed. The sapphire fiber begins to grow, and the fiber white light interference distributed testing device (9) is used for testing. Combined with the real-time data processing results, the online detection of defect distribution during the growth process of the sapphire fiber is realized. Step 7: In the sapphire fiber growth stage in Step 6, based on the feedback from the online detection results of the defect distribution of the fiber white light interference distributed testing device (9), the feed rate of the source rod, the moving rate of the seed crystal, and the CO2 laser power growth process parameters are finely adjusted in real time. After obtaining the variable parameter with the fewest sapphire defects, the variable parameter is kept unchanged in the subsequent growth process until a sapphire fiber is grown.

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