Core Diameter Testing Platform and Method

Abstract

This invention relates to the field of optical fiber technology, providing a fiber core diameter testing platform and method. The platform includes a light source, a stepped optical fiber under test, a spot detector, and a high-refractive-index coated passive optical fiber. The output end of the high-refractive-index coated passive optical fiber is connected to the input end of the optical fiber under test. The refractive index of the coating of the high-refractive-index coated passive optical fiber is higher than that of the cladding, and the refractive index of the fiber core is higher than that of the cladding. Light generated by the light source enters from the input end of the high-refractive-index coated passive optical fiber and exits from the output end of the optical fiber under test. The spot detector is used to detect the output spot and determine the fiber core diameter or step diameter of the optical fiber under test based on the intensity distribution of the output spot. The fiber core diameter testing platform and method provided by this invention ensure a clear contrast in light intensity between the fiber core and the stepped portion, thereby enabling the identification of the boundary between the fiber core and the step, and solving the problem that traditional testing platforms cannot test the fiber core diameter.

Description

Technical Field

[0001] This invention relates to the field of optical fiber technology, and in particular to a fiber core diameter testing platform and method. Background Technology

[0002] With the development of the optical fiber industry, there are more and more types of optical fibers, and the structure of optical fibers is no longer the traditional core-cladding structure. Stepped optical fibers introduce a ring of steps with a higher refractive index than the cladding around the core, which reduces the refractive index of the core relative to the steps, thereby reducing the numerical aperture of the core. Therefore, stepped optical fibers can ensure good beam quality.

[0003] However, for optical fibers with stepped structures, the core diameter cannot be measured when using traditional testing platforms. Summary of the Invention

[0004] This invention provides a fiber core diameter testing platform and method to address the shortcomings of existing technologies that cannot measure fiber core diameter.

[0005] This invention provides a fiber core diameter testing platform, comprising a light source, a stepped optical fiber under test, and a spot detector. The platform further includes:

[0006] A high-refractive-index coated passive optical fiber, wherein the light-emitting end of the high-refractive-index coated passive optical fiber is connected to the light-incoming end of the optical fiber under test, and the refractive index of the coating of the high-refractive-index coated passive optical fiber is higher than that of the cladding, and the refractive index of the core is higher than that of the cladding.

[0007] The light generated by the light source enters from the light-inlet end of the high-refractive-index coated passive optical fiber and exits from the light-outlet end of the optical fiber under test.

[0008] The light spot detector is used to detect the output light spot and determine the core diameter or step diameter of the optical fiber under test based on the light intensity distribution of the output light spot.

[0009] According to the fiber core diameter testing platform provided by the present invention, the difference between the core diameter of the high-refractive-index coated passive optical fiber and the core diameter of the optical fiber under test is within a first threshold range; or,

[0010] The optical fiber under test has a multi-stage stepped structure, and the difference between the core diameter of the high-refractive-index coated passive optical fiber and the step diameter of any stage in the optical fiber under test is within the first threshold range.

[0011] According to the fiber core diameter testing platform provided by the present invention, the first threshold is ±2 micrometers.

[0012] According to the fiber core diameter testing platform provided by the present invention, the difference between the cladding diameter of the high-refractive-index coated passive fiber and the fiber under test is within a second threshold range; the difference between the fiber core numerical aperture of the high-refractive-index coated passive fiber and the fiber under test is within a third threshold range.

[0013] According to the fiber core diameter testing platform provided by the present invention, the second threshold is ±5 micrometers and the third threshold is ±0.01.

[0014] According to the fiber core diameter testing platform provided by the present invention, the light-emitting end of the high-refractive-index coated passive optical fiber is connected to the light-incoming end of the optical fiber under test by fusion splicing.

[0015] According to the fiber core diameter testing platform provided by the present invention, the step refractive index of the fiber under test is lower than the core refractive index, and the step refractive index is higher than the cladding refractive index.

[0016] According to the fiber core diameter testing platform provided by the present invention, the spot detector includes an image acquisition device and an image processing device. The image acquisition device is used to acquire a spot image of the output spot, and the image processing device is used to determine the light intensity distribution based on the spot image and determine the fiber core diameter of the optical fiber under test based on the light intensity distribution.

[0017] The present invention also provides a method for testing fiber core diameter, the method comprising:

[0018] Construct the aforementioned fiber core diameter testing platform;

[0019] Turn on the light source so that light enters from the light-inlet end of the high-refractive-index coated passive optical fiber and exits from the light-outlet end of the optical fiber under test.

[0020] The output light spot is detected using a light spot detector, and the core diameter or step diameter of the optical fiber under test is determined based on the light intensity distribution of the output light spot.

[0021] According to the fiber core diameter testing method provided by the present invention, if the difference between the fiber core diameter of the high-refractive-index coated passive optical fiber and the fiber core diameter of the optical fiber under test is within a first threshold range, the spot detector determines the fiber core diameter of the optical fiber under test based on the light intensity distribution of the output spot.

[0022] If the optical fiber under test has a multi-stage stepped structure, and the difference between the core diameter of the high-refractive-index coated passive optical fiber and the step diameter of any stage in the optical fiber under test is within the first threshold range, then the spot detector determines the step diameter of any stage in the optical fiber under test based on the light intensity distribution of the output spot.

[0023] The fiber core diameter testing platform and method provided by this invention connects the light-emitting end of a high-refractive-index coated passive optical fiber to the light-inlet end of the optical fiber under test. The light generated by the light source enters from the light-inlet end of the high-refractive-index coated passive optical fiber and exits from the light-emitting end of the optical fiber under test, ensuring that there is a significant contrast in light intensity between the fiber core and the stepped portion, so that the processing software can identify the boundary between the fiber core and the step.

[0024] The fiber core diameter testing platform and method provided by this invention solve the problem that traditional testing platforms cannot test the core diameter of fibers with stepped refractive index distributions. It can also be used to test the diameter of each step in a multi-step structure, thus overcoming the limitation of traditional testing platforms. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the 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.

[0026] Figure 1 This is a schematic diagram of an optical fiber structure in related technologies.

[0027] Figure 2 This is the optical path diagram of the test platform in the related technology.

[0028] Figure 3 This is a light intensity distribution diagram obtained from a conventional optical fiber structure in a conventional testing platform.

[0029] Figure 4 This is a light propagation path diagram of a stepped optical fiber in a traditional testing platform.

[0030] Figure 5 This is the end face light spot pattern of the optical fiber under test.

[0031] Figure 6 This is one of the schematic diagrams of the fiber core diameter testing platform provided by the present invention.

[0032] Figure 7 This is a schematic diagram of the refractive index distribution of the optical fiber under test and the high-refractive-index coated passive optical fiber provided by the present invention.

[0033] Figure 8 This is a light propagation path diagram of the fiber core diameter testing platform provided by the present invention.

[0034] Figure 9 This is a schematic diagram of the light spot obtained by the fiber core diameter testing platform provided by the present invention.

[0035] Figure 10This is the second schematic diagram of the fiber core diameter testing platform provided by the present invention.

[0036] Figure 11 This is a traditional method for testing light spot patterns.

[0037] Figure 12 The image shown is a spot pattern obtained from the testing platform provided by this invention.

[0038] Figure 13 This is a diagram showing the refractive index distribution of the fiber under test, which has a multi-step structure.

[0039] Figure 14 This is a diagram showing the refractive index distribution of the fiber under test with a multi-step structure and a high-refractive-index coated passive fiber. Detailed Implementation

[0040] 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. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0041] In the description of the embodiments of the present invention, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the stated features. In the description of the embodiments of the present invention, "multiple" means two or more, unless otherwise explicitly specified.

[0042] Figure 1 This is a schematic diagram of an optical fiber structure in related technologies, such as... Figure 1 As shown, the left side represents a conventional fiber structure, while the right side represents a stepped fiber structure. The reason for the stepped structure is that this type of fiber needs to meet two requirements: first, it needs a high ion doping concentration to ensure high absorption characteristics; second, it needs a low core numerical aperture to ensure good beam quality.

[0043] Traditional optical fibers cannot simultaneously meet both requirements because while high ion doping concentration ensures high absorption characteristics, higher doping concentrations also lead to higher core refractive index, resulting in an increased numerical aperture in the core and compromising beam quality. Stepped optical fibers, on the other hand, introduce a ring of steps with a higher refractive index than the cladding around the core. This reduces the core's refractive index relative to the steps, thereby decreasing the core's numerical aperture. Therefore, stepped fibers can simultaneously meet both requirements.

[0044] However, the stepped structure presents a problem: the diameter of the fiber core cannot be measured. The reason for this is as follows:

[0045] Figure 2 This is the optical path diagram of a test platform in related technologies. The optical path of a traditional test platform is as follows: Figure 2 As shown, from left to right, the components are the light source, the fiber under test, the light spot detector, and the processing software. During testing, light is passed through one end face of the fiber under test, the coating is removed, and a high-refractive-index adhesive is applied. The light-emitting end face is obtained from the other end of the fiber under test, and the light intensity distribution at the end face is obtained through the light spot detector. The processing software uses the light intensity distribution detected by the light spot detector to determine the core and cladding boundaries based on the obvious light intensity difference boundaries, thereby calculating the core and cladding diameters.

[0046] For traditional optical fibers, traditional testing platforms are applicable. Because the core refractive index of traditional optical fibers is higher than that of the cladding, light entering the core from the end face satisfies the principle of total internal reflection and can propagate stably within the core with minimal light loss. However, light entering the cladding from the end face is subject to total internal reflection due to the high-refractive-index gel coating on the cladding, which disrupts the total internal reflection condition. This causes the light to enter the coating and lose most of its light, resulting in significant light loss. Consequently, the intensity of light in the core and cladding captured by the spot detector will show a significant difference, as indicated by the spot end-face image. Figure 3 As shown, Figure 3 This is a light intensity distribution map obtained from a conventional optical fiber structure on a conventional testing platform. Image processing software can identify the boundaries between the core and cladding by analyzing the differences in light intensity. Then, using a specific algorithm, the diameters of the core and cladding can be calculated.

[0047] However, for optical fibers with a stepped structure, the core diameter cannot be measured using traditional testing platforms. Figure 4 This is a light propagation path diagram of a stepped optical fiber in a traditional testing platform, such as... Figure 4 As shown:

[0048] 1. Light entering the fiber core from the end face of the fiber under test can be transmitted stably with almost no loss because the refractive index of the fiber core is greater than the step refractive index.

[0049] 2. Light entering the step of the fiber under test from the end face can be transmitted stably with almost no loss because the refractive index of the step is greater than that of the cladding and the total internal reflection condition is met.

[0050] 3. Light entering the cladding from the end face cannot be transmitted stably due to the high refractive index coating on the outside of the cladding, which makes the refractive index of the cladding lower than that of the high refractive index, thus disrupting the conditions for total internal reflection and resulting in significant light loss.

[0051] Light can propagate stably in both the core and the stepped sections, but not in the cladding. This results in no intensity difference between the core and the stepped sections, while the cladding section has a weaker intensity. In this situation, the end face of the fiber under test appears as follows: Figure 5 As shown. Because there is almost no difference in light intensity between the cladding and the stepped portion, the processing software cannot find the boundary between the core and the stepped portion, but can only find the boundary between the step and the cladding. Therefore, it can only calculate the diameter of the step and the diameter of the cladding, but cannot calculate the diameter of the core portion.

[0052] To address the above problems, embodiments of the present invention propose a fiber core diameter testing platform. Figure 6 This is one of the schematic diagrams of the fiber core diameter testing platform provided by the present invention, as shown below. Figure 6 As shown, the platform includes a light source, a stepped optical fiber under test, and a spot detector. The platform also includes:

[0053] High-refractive-index coated passive optical fiber, the output end of the high-refractive-index coated passive optical fiber is connected to the input end of the optical fiber under test. The refractive index of the coating of the high-refractive-index coated passive optical fiber is higher than that of the cladding, and the refractive index of the core is higher than that of the cladding.

[0054] The light generated by the light source enters from the light-inlet end of the high-refractive-index passive optical fiber and exits from the light-outlet end of the optical fiber under test.

[0055] A light spot detector is used to detect the output light spot and determine the core diameter or step diameter of the optical fiber under test based on the light intensity distribution of the output light spot.

[0056] Specifically, Figure 7 This is a schematic diagram of the refractive index distribution of the optical fiber under test and the high-refractive-index coated passive optical fiber provided by the present invention, as shown below. Figure 7 As shown, the top diagram illustrates the refractive index distribution of the fiber under test with a stepped structure, while the bottom diagram illustrates the refractive index distribution of the high-refractive-index coated passive fiber. The stepped refractive index of the fiber under test is lower than that of the core, and higher than that of the cladding. The high-refractive-index coated passive fiber itself is a non-stepped structure, meaning it has only two layers: the core and the cladding. The refractive index of the coating in the high-refractive-index coated passive fiber is higher than that of the cladding, and the refractive index of the core is higher than that of the cladding. The fiber under test and the high-refractive-index coated passive fiber can be connected by fusion splicing or coupling; this embodiment of the invention does not specifically limit this connection.

[0057] Figure 8 This is a light propagation path diagram of the fiber core diameter testing platform provided by this invention. (Example) Figure 8As shown in this embodiment, part of the light emitted by the light source enters the cladding of the high-refractive-index coated passive fiber, and part enters the core of the high-refractive-index coated passive fiber. The light entering the cladding cannot be stably transmitted because the refractive index of the cladding is lower than that of the coating, and the cladding does not meet the total internal reflection condition. Therefore, most of the light is lost and cannot enter the step of the fiber under test, resulting in a weak light intensity at the step. However, the light entering the core of the step stripper can be stably transmitted without loss because the refractive index of the core is higher than that of the cladding, thus satisfying the total internal reflection condition. This ensures that the light transmitted in the core of the fiber under test has a strong light intensity, thus ensuring a clear contrast between the light intensity of the core and the step portion, which can be identified by the processing software. Figure 9 This is a schematic diagram of the light spot obtained by the fiber core diameter testing platform provided by the present invention.

[0058] Preferably, the difference between the core diameter of the high-refractive-index coated passive optical fiber and the core diameter of the optical fiber under test is within a first threshold range; or,

[0059] The fiber under test has a multi-step structure, and the difference between the core diameter of the high-refractive-index coated passive fiber and the step diameter of any step in the fiber under test is within the first threshold range.

[0060] Specifically, the fiber core diameter testing platform provided in this embodiment of the invention can not only test the fiber core diameter of the fiber under test with a stepped structure, but also be used to test the diameter of each step in a multi-step structure, thereby solving the problem that traditional testing platforms cannot test the diameter of each step in a multi-step structure.

[0061] If the core diameter of the high-refractive-index coated passive fiber is close to that of the fiber under test, that is, when the difference between the core diameter of the high-refractive-index coated passive fiber and the core diameter of the fiber under test is within the first threshold range, the core diameter of the fiber under test can be tested by light intensity distribution.

[0062] If the core diameter of the high-refractive-index coated passive fiber is close to the step diameter of any step in the fiber under test, that is, when the difference between the core diameter of the high-refractive-index coated passive fiber and the step diameter of any step in the fiber under test is within the first threshold range, the step diameter of any step in the fiber under test can be tested by light intensity distribution.

[0063] For optical fibers with a multi-step structure, the test platform provided in this embodiment of the invention can be used to measure the diameter of any step. It is only necessary to select a high-refractive-index passive optical fiber with a core diameter close to that of the step to be measured to determine the diameter of the step to be measured.

[0064] Here, the first threshold can be preset. Preferably, the first threshold is ±2 micrometers.

[0065] In some embodiments, the difference between the cladding diameter of the high-refractive-index coated passive fiber and the fiber under test is within a second threshold range; the difference between the core numerical aperture of the high-refractive-index coated passive fiber and the fiber under test is within a third threshold range.

[0066] Specifically, when the cladding diameter and core numerical aperture of the high-refractive-index coated passive fiber are relatively close to those of the fiber under test, it can further ensure that most of the light in the fiber under test is stably transmitted in the fiber core, with a very small portion transmitted in the step and cladding. The light intensity in the fiber core is relatively strong, thus ensuring a significant contrast between the light intensity in the fiber core and the step portion, which can be identified by the processing software.

[0067] Here, the second threshold can be represented by the difference in cladding diameter between the high-refractive-index coated passive fiber and the fiber under test, and the third threshold can be represented by the ratio of the core numerical aperture of the high-refractive-index coated passive fiber and the fiber under test. The second and third thresholds can be preset.

[0068] Preferably, the second threshold is ±5 micrometers and the third threshold is ±0.01.

[0069] Preferably, the output end of the high-refractive-index coated passive optical fiber is connected to the input end of the optical fiber under test via fusion splicing. Fusion splicing is quick, convenient, and cost-effective.

[0070] In some embodiments, the spot detector includes an image acquisition device and an image processing device. The image acquisition device is used to acquire a spot image of the output spot, and the image processing device is used to determine the light intensity distribution based on the spot image and to determine the core diameter of the optical fiber under test based on the light intensity distribution.

[0071] Based on any of the above embodiments, a method for testing fiber core diameter is provided, including:

[0072] S1. Construct a core diameter testing platform. Select a high-refractive-index coated passive fiber that matches the fiber under test. This high-refractive-index coated passive fiber is characterized by: first, it is essentially a high-refractive-index coated passive fiber, meaning the refractive index of the fiber coating is higher than that of the cladding; second, the cladding optical stripper itself is a non-stepped structure, meaning it has only two layers: the core and the cladding, and the numerical aperture (NA) of the core is close to that of a stepped-structure fiber. The cladding diameter of the high-refractive-index coated passive fiber is close to that of the fiber under test.

[0073] Take a 0.5-1m length of high-refractive-index passive fiber and fusion splice it with a 0.2-0.5m length of the fiber to be tested for the stepped structure. Position the un-fused end of the high-refractive-index passive fiber towards the light source and fix it in place. Position the un-fused end of the fiber to be tested towards the camera and fix it in place.

[0074] Figure 10 This is a second schematic diagram of the fiber core diameter testing platform provided by the present invention, as shown below. Figure 10 As shown, from left to right, the components are: light source, high-refractive-index coated passive optical fiber, optical fiber under test, and spot detector (including camera and processing software).

[0075] S2, turn on the light source so that light enters from the light inlet of the high-refractive-index passive optical fiber and exits from the light outlet of the optical fiber under test.

[0076] The light source used is a common incandescent lamp. The difference between the cladding diameter of the passive high-refractive-index coated fiber and the cladding diameter of the stepped fiber under test is ±5 micrometers, and the NA difference is ±0.01. In addition, the refractive index of the coating material must be higher than that of the cladding to break the total internal reflection condition of the cladding.

[0077] S3. A spot detector is used to detect the output spot, and the core diameter or step diameter of the fiber under test is determined based on the light intensity distribution of the output spot. A camera is used to photograph the output end face of the fiber under test, and image processing software is used to calculate the core diameter or step diameter of the stepped fiber under test based on the light intensity distribution of the end face.

[0078] If the difference between the core diameter of the high-refractive-index coated passive fiber and the core diameter of the fiber under test is within a first threshold range, the spot detector determines the core diameter of the fiber under test based on the light intensity distribution of the output spot.

[0079] The present invention has already been applied in practice. Different methods were used to test optical fibers with a core design diameter of 25 micrometers, a step design diameter of 42 micrometers, and a cladding design diameter of 300 micrometers. The test results are shown in Table 1.

[0080] Table 1

[0081]

[0082] Figure 11 It is a traditional method for testing light spot patterns. Figure 12 The image shows the light spot pattern obtained using the testing platform provided by this invention. From the light spot patterns and test results of the two methods, it can be seen that using the traditional testing platform, the core diameter measured is 42.2, which is actually the diameter of the step, not the core diameter. However, using the testing platform of this invention, the core diameter is 25.2, which is consistent with the core design value. This is also evident in the light spot pattern; the light spot measured using the testing platform of this invention shows a clear contrast in light intensity between the step portion and the core portion, making it easy to determine their diameter.

[0083] In some embodiments, if the optical fiber under test has a multi-stage stepped structure, and the difference between the core diameter of the high-refractive-index coated passive optical fiber and the step diameter of any stage in the optical fiber under test is within a first threshold range, then the spot detector determines the step diameter of any stage in the optical fiber under test based on the light intensity distribution of the output spot.

[0084] Figure 13 This is a refractive index distribution diagram of the fiber under test with a multi-step structure. For fibers with more complex cores and multi-step structures, if you want to test the fiber core or a certain step, you only need to select a high-refractive-index coated passive fiber with a core diameter close to the diameter of that step to measure its accurate diameter. Figure 14 This is a refractive index distribution diagram of the fiber under test with a multi-step structure and a high-refractive-index coated passive fiber. To test the diameter of step 1, the refractive index distribution of the selected high-refractive-index coated passive fiber is as follows. Figure 14 As shown in the lower middle figure.

[0085] Similarly, using the testing platform of this invention, the diameter of the step to be tested can be determined by selecting the corresponding high-refractive-index coated passive optical fiber.

[0086] The testing platform provided in this invention solves the problem that traditional testing platforms cannot test the core diameter of optical fibers with stepped refractive index distributions. It can also be used to test the diameter of each step in multi-step structures, thus overcoming the limitation of traditional testing platforms.

[0087] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A fiber core diameter testing platform, comprising a light source, a stepped optical fiber under test, and a spot detector, characterized in that, The platform also includes: A high-refractive-index coated passive optical fiber is provided, wherein the output end of the high-refractive-index coated passive optical fiber is connected to the input end of the optical fiber under test, and the refractive index of the coating of the high-refractive-index coated passive optical fiber is higher than that of the cladding, and the refractive index of the core is higher than that of the cladding; the high-refractive-index coated passive optical fiber has a non-step structure. The light generated by the light source enters from the light-inlet end of the high-refractive-index coated passive optical fiber and exits from the light-outlet end of the optical fiber under test. The light spot detector is used to detect the output light spot and determine the core diameter or step diameter of the optical fiber under test based on the light intensity distribution of the output light spot. The difference between the core diameter of the high-refractive-index coated passive optical fiber and the core diameter of the optical fiber under test is within a first threshold range; or, The optical fiber under test has a multi-stage stepped structure, and the difference between the core diameter of the high-refractive-index coated passive optical fiber and the step diameter of any stage in the optical fiber under test is within the first threshold range.

2. The fiber core diameter testing platform according to claim 1, characterized in that, The first threshold is ±2 micrometers.

3. The fiber core diameter testing platform according to claim 1, characterized in that, The difference between the cladding diameter of the high-refractive-index coated passive optical fiber and the optical fiber under test is within a second threshold range; the difference between the core numerical aperture of the high-refractive-index coated passive optical fiber and the optical fiber under test is within a third threshold range.

4. The fiber core diameter testing platform according to claim 3, characterized in that, The second threshold is ±5 micrometers, and the third threshold is ±0.

01.

5. The fiber core diameter testing platform according to claim 1, characterized in that, The output end of the high-refractive-index coated passive optical fiber is connected to the input end of the optical fiber under test by fusion splicing.

6. The fiber core diameter testing platform according to claim 1, characterized in that, The step refractive index of the optical fiber under test is lower than that of the core, and the step refractive index is higher than that of the cladding.

7. The fiber core diameter testing platform according to claim 1, characterized in that, The light spot detector includes an image acquisition device and an image processing device. The image acquisition device is used to acquire and output a light spot image, and the image processing device is used to determine the light intensity distribution based on the light spot image and to determine the core diameter of the optical fiber under test based on the light intensity distribution.

8. A method for testing fiber core diameter, characterized in that, The method includes: Construct a core diameter testing platform as described in any one of claims 1 to 7; Turn on the light source so that light enters from the light-inlet end of the high-refractive-index coated passive optical fiber and exits from the light-outlet end of the optical fiber under test; The output light spot is detected using a light spot detector, and the core diameter or step diameter of the optical fiber under test is determined based on the light intensity distribution of the output light spot.

9. The fiber core diameter testing method according to claim 8, characterized in that, If the difference between the core diameter of the high-refractive-index coated passive optical fiber and the core diameter of the optical fiber under test is within a first threshold range, then the spot detector determines the core diameter of the optical fiber under test based on the light intensity distribution of the output spot. If the optical fiber under test has a multi-stage stepped structure, and the difference between the core diameter of the high-refractive-index coated passive optical fiber and the step diameter of any stage in the optical fiber under test is within the first threshold range, then the spot detector determines the step diameter of any stage in the optical fiber under test based on the light intensity distribution of the output spot.

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