A bend-insensitive high-resolution image-guiding optical fiber and a method for manufacturing the same

Abstract

This invention belongs to the field of image transmission fiber technology and discloses a bend-insensitive high-resolution image transmission fiber and its fabrication method. The image transmission fiber provided by this invention comprises several image transmission cores and a co-cladding layer between the image transmission cores; each image transmission core includes, from the inside out, a core layer, a recessed layer, an inner cladding layer, and a light-absorbing layer; the light-absorbing layer is quartz glass containing absorber ions and fluorine. The fabrication method of the image transmission fiber provided by this invention includes preparing a light-absorbing core rod, drawing the light-absorbing core rod to obtain a light-absorbing capillary rod, stacking several light-absorbing capillary rods in a pure quartz glass sleeve to obtain an image transmission fiber preform, and drawing the image transmission fiber preform to obtain a bend-insensitive high-resolution image transmission fiber. This invention can improve the imaging resolution of the image transmission fiber, and the fabrication method has the advantages of simple and convenient process and strong production stability.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber technology, and more specifically, relates to a bend-insensitive high-resolution optical fiber and its fabrication method. Background Technology

[0002] Imaging fiber, also known as a transmission fiber, is a passive fiber optic device used in endoscopes. It plays a sampling role within the endoscope. An imaging fiber contains thousands or even tens of thousands of cores, each of which can be considered an optical lens, transmitting the optical information of the sample from one end to the other, achieving real-time, high-resolution image transmission. Due to its high resolution, high flexibility, extremely small size, real-time imaging, interference resistance, and relatively low cost, imaging fiber has found increasing applications in many fields.

[0003] Image-transmitting optical fibers typically have a diameter of several hundred micrometers, containing thousands to tens of thousands of cores. The distance between these cores is usually only a few micrometers. With such close spacing, crosstalk between the signal light in each core is inevitable. The imaging principle of image-transmitting optical fibers dictates that improving image transmission resolution requires minimizing the distance between the cores. However, excessively close spacing exacerbates crosstalk, leading to a decrease in resolution. Therefore, to further improve the imaging resolution of image-transmitting optical fibers, it is necessary to suppress crosstalk while simultaneously reducing the spacing between cores; otherwise, it will be difficult to overcome the resolution bottleneck and may even result in a decline in image quality.

[0004] In existing technologies, there are several ways to suppress inter-core crosstalk and improve resolution: (1) By designing a specific structure to confine the waveguide of light; however, the fiber fabrication process used in this type of method is usually quite complex; (2) By using heterogeneous fiber cores with different geometric dimensions and refractive index profiles, so that every two adjacent fiber cores are heterogeneous structures, inter-core crosstalk is suppressed; however, the fiber fabrication process used in this type of method is usually extremely complex, and may also lead to a decrease in fiber imaging quality or mechanical properties. In addition, considering the conventional application scenarios of image transmission fibers, it is necessary to ensure that the image transmission fiber still has good imaging quality when it is bent. Summary of the Invention

[0005] This invention provides a bend-insensitive high-resolution image transmission fiber and its fabrication method, thereby solving the problems of insufficient imaging resolution and complex fabrication processes in existing image transmission fibers.

[0006] This invention provides a bend-insensitive high-resolution imaging fiber. The imaging surface of the imaging fiber includes a plurality of imaging cores and a co-cladding layer between the imaging cores. Each of the imaging cores includes, from the inside out, a core layer, a recessed layer, an inner cladding layer, and a light-absorbing layer. The light-absorbing layer is quartz glass containing light-absorbing ions and fluorine.

[0007] Preferably, the light-absorbing ions include one or more transition metal ions, and the concentration of the light-absorbing ions is from 100 ppm to 2000 ppm.

[0008] Preferably, the core layer is germanium-doped quartz glass, and the relative refractive index difference Δ1 of the core layer ranges from 2% to 3.5%. The recessed layer is fluorine-doped quartz glass, and the relative refractive index difference Δ2 of the recessed layer ranges from -1% to -0.4%. The inner cladding is made of pure quartz glass, and the relative refractive index difference Δ3 of the inner cladding is 0. The relative refractive index difference Δ4 of the light-absorbing layer is 0, with an error range of less than ±0.07%. The co-cladding is pure quartz glass, and the relative refractive index difference Δ5 of the co-cladding is 0.

[0009] Preferably, the bend-insensitive high-resolution image transmission fiber further includes: an outer cladding layer and a coating layer; the outer cladding layer is located outside the image transmission surface, the outer cladding layer is pure quartz glass, and the image transmission surface and the outer cladding layer constitute the glass portion of the image transmission fiber; the coating layer is located outside the glass portion of the image transmission fiber.

[0010] Preferably, the radius R1 of the core layer ranges from 0.5 μm to 1 μm, the radius R2 of the recessed layer ranges from 0.55 μm to 1.1 μm, the radius R3 of the inner cladding layer ranges from 0.7 μm to 1.4 μm, and the radius R4 of the light-absorbing layer ranges from 0.75 μm to 1.5 μm; the inter-core spacing Λ between image transmission cores ranges from 2 μm to 4 μm, and the number of image transmission cores is from 1,000 to 100,000; the radius R5 of the image transmission surface ranges from 50 μm to 500 μm; the radius R6 of the outer cladding layer ranges from 75 μm to 525 μm; and the radius R7 of the coating layer ranges from 100 μm to 700 μm.

[0011] Preferably, the bending-insensitive high-resolution imaging fiber has a numerical aperture of 0.3 to 0.45 and a resolution ≥143 lp / mm; after 10 turns with a radius of 20 mm, the resolution is ≥128 lp / mm; and the visible light transmittance of the 3m fiber is >90%.

[0012] On the other hand, the present invention provides a method for fabricating a bend-insensitive high-resolution imaging optical fiber as described above, comprising the following steps: S1. Prepare light-absorbing core rod; A fluorine-doped silica porous layer is deposited on the inner wall of a pure quartz liner tube. A light-absorbing agent solution is added for immersion. After immersion and thorough drying, the liner tube is sintered to vitrify the porous layer, forming a light-absorbing layer doped with light-absorbing agent ions. An inner cladding layer, a recessed layer, and a core layer are deposited sequentially, and the light-absorbing core rod is obtained after melting and shrinking. S2. The light-absorbing core rod is drawn to obtain a light-absorbing capillary rod; S3. Stack several of the light-absorbing capillary rods in a pure quartz glass sleeve to obtain an image transmission fiber preform. S4. The image transmission fiber preform is drawn to obtain a high-resolution image transmission fiber that is insensitive to bending.

[0013] Preferably, in S1, the soaking time for adding the light absorber solution is 3h to 12h; the concentration of the light absorber in the light absorber solution is 0.5wt% to 3wt%, and the light absorber is one or more of the soluble salts corresponding to transition metals.

[0014] Preferably, in step S1, nitrogen is used for purging and drying for 6 to 24 hours to achieve thorough drying.

[0015] Preferably, in S2, the radius of the light-absorbing capillary rod is 0.2 mm to 1 mm.

[0016] One or more technical solutions provided in this invention have at least the following technical effects or advantages: (1) In terms of fiber design, the bending-insensitive high-resolution imaging fiber provided by this invention comprises several imaging cores and a co-cladding layer between the imaging cores; each imaging core comprises, from the inside out, a core layer, a recessed layer, an inner cladding layer, and an absorbing layer; the absorbing layer is quartz glass containing absorber ions and fluorine. By introducing an absorbing layer containing absorber ions, this invention ensures that each imaging core in the imaging fiber is uniformly wrapped by the absorbing layer. The absorber ions in the absorbing layer can fully absorb the signal light leaking from the fiber core, especially in a bent state, ensuring that the signal light in the imaging core does not propagate to adjacent imaging cores, effectively reducing inter-core crosstalk and improving imaging resolution. That is, due to the presence of the absorbing layer, the imaging fiber has a low crosstalk level even with a very small inter-core spacing, increasing the core density and further improving the resolution.

[0017] (2) In terms of optical fiber design, the present invention also designs an inner cladding layer inside the light-absorbing layer. On the one hand, it is used to form the conditions for total internal reflection of light, and on the other hand, it is used to isolate the light-absorbing layer from the recessed layer and the core layer. This can prevent the light-absorbing agent ions in the light-absorbing layer from diffusing to the recessed layer or the core layer, thereby preventing the decrease in optical fiber transmittance.

[0018] (3) Regarding the fabrication of optical fibers, the fabrication method provided by this invention includes fabricating an absorbing core rod, drawing the absorbing core rod to obtain an absorbing capillary rod, stacking several absorbing capillary rods in a pure quartz glass liner to obtain an image transmission fiber preform, and drawing the image transmission fiber preform to obtain a bend-insensitive high-resolution image transmission fiber. Specifically, in fabricating the absorbing core rod, a fluorine-doped silica porous layer is deposited on the inner wall of the pure quartz liner, and an absorber solution is added for immersion. The liner, after immersion and thorough drying, is sintered to vitrify the porous layer, forming an absorbing layer doped with absorber ions. An inner cladding, a recessed layer, and a core layer are deposited sequentially, and the absorbing core rod is obtained after melting and shrinking. This invention introduces an absorbing layer containing absorber ions around the fiber core using an improved chemical vapor deposition (MCVD) method combined with a solution immersion process. This absorbs the signal light overflowing from the fiber core, thereby significantly reducing crosstalk between image transmission cores and improving the resolution of the image transmission fiber. Furthermore, the preparation method provided by this invention has the advantages of simple and convenient process and strong production stability. Attached Figure Description

[0019] Figure 1 A process flow diagram corresponding to a method for fabricating a bend-insensitive high-resolution imaging optical fiber provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the structure of the light-absorbing core rod or light-absorbing capillary rod prepared in the method for preparing a bend-insensitive high-resolution imaging optical fiber according to an embodiment of the present invention. Figure 3 A schematic diagram of the structure of a single-core image transmission fiber prepared in a method for preparing a bend-insensitive high-resolution image transmission fiber according to an embodiment of the present invention. Figure 4 A schematic diagram of the structure of the image-transmitting fiber prepared in the method for preparing a bend-insensitive high-resolution image-transmitting fiber provided in an embodiment of the present invention. Figure 5 This is a schematic diagram of the refractive index profile of the image transmission core and its surrounding area. Detailed Implementation

[0020] To better understand the above technical solutions, the following will provide a detailed explanation of the technical solutions in conjunction with the accompanying drawings and specific implementation methods.

[0021] In a first aspect, the present invention provides a method for fabricating a bend-insensitive high-resolution imaging optical fiber, see [link to previous section]. Figure 1 This includes the following steps: S1. Prepare light-absorbing core rod.

[0022] Specifically, S1 includes the following sub-steps: (1) Deposited loose layer.

[0023] Using the MCVD process, a loose layer of fluorine-doped silica is deposited on the inner wall of a pure quartz liner.

[0024] (2) Soaking in solution.

[0025] After deposition, remove the liner with the loose layer, then add an absorbent solution (e.g., an ethanol solution containing the absorbent) to soak it, allowing the loose layer to fully adsorb the absorbent ions. After soaking, pour out the remaining solution and dry it thoroughly.

[0026] The soaking time for adding the light-absorbing agent solution is 3 to 12 hours. The concentration of the light-absorbing agent in the solution is 0.5 wt% to 3 wt%, and the light-absorbing agent is one or more soluble salts of transition metals. Nitrogen purging and drying can be performed for 6 to 24 hours to achieve thorough drying.

[0027] For example, prepare a 0.5 wt% FeCl3 ethanol solution and add it to a liner with a loose layer to soak for 3 hours. After soaking, pour out the remaining solution and dry it by purging with nitrogen for 6 hours.

[0028] (3) Sintering.

[0029] After soaking and thorough drying, the liner is sintered to vitrify the porous layer, forming a light-absorbing layer doped with light-absorbing ions.

[0030] This invention controls the thickness of the light-absorbing layer by changing the deposition amount of the porous layer, and controls the concentration of light-absorbing ions in the light-absorbing layer by changing the concentration of the light-absorbing agent solution and the soaking time. Introducing light-absorbing ions into pure silica increases the refractive index of the glass; therefore, during the deposition of the porous layer, a certain amount of fluorine needs to be added to balance the change in refractive index, ensuring that the total relative refractive index difference of the light-absorbing layer is zero. That is, during the deposition of the porous layer, the amount of fluorine deposited is determined by its contribution to the relative refractive index difference provided by the light-absorbing agent, which satisfies: Δ 吸光剂 +Δ 氟 ≈0.

[0031] (4) MCVD / melting.

[0032] The light-absorbing core rod is obtained by sequentially depositing an inner cladding layer, a recessed layer, and a core layer using the MCVD process and then melting and shrinking the core.

[0033] The structure of the light-absorbing core rod prepared is as follows: Figure 2 As shown, from the inside out, the layers are core layer 1-1, sunken layer 1-2, inner cladding layer 1-3, light-absorbing layer 1-4, and cladding layer 1-5.

[0034] S2. The light-absorbing core rod is drawn to obtain a light-absorbing capillary rod.

[0035] The radius of the light-absorbing capillary rod is 0.2 mm to 1 mm.

[0036] For example, the light-absorbing core rod is stretched at high temperature into light-absorbing capillary rods with the same radius (0.5 mm).

[0037] The structure of the prepared light-absorbing capillary rod can also be found in [reference needed]. Figure 2 .

[0038] S3. Stack several of the light-absorbing capillary rods in a pure quartz glass sleeve to obtain an image transmission fiber preform.

[0039] In this process, several light-absorbing capillary rods are stacked and filled in a pure quartz glass sleeve with a specific inner diameter and wall thickness until it is completely filled and no further filling is possible. Then, the light-absorbing capillary rods are fixed in the sleeve to obtain an image transmission fiber preform. During stacking, the capillary rods should be kept as tightly packed as possible in a hexagonal shape to avoid missing cores. The number of capillary rods stacked depends on the core count requirement.

[0040] For example, approximately 2,000 of the light-absorbing capillary rods are tightly stacked and fixed in a pure quartz sleeve to obtain an image transmission fiber preform.

[0041] S4. The image transmission fiber preform is drawn (i.e. drawn at high temperature) to obtain a high-resolution image transmission fiber that is insensitive to bending.

[0042] In addition, see Figure 4 Combined with conventional fabrication processes, the final image-transmitting optical fiber may also include an outer cladding layer 3 and a coating layer 4; the outer cladding layer 3 is located outside the image-transmitting surface 2, and the outer cladding layer 3 is pure quartz glass. The image-transmitting surface 2 and the outer cladding layer 3 constitute the glass portion of the image-transmitting optical fiber; the coating layer 4 is located outside the glass portion of the image-transmitting optical fiber.

[0043] See Figure 2 , Figure 3 and Figure 4The glass portion of the bend-insensitive high-resolution image transmission fiber obtained by the above preparation method includes an image transmission surface 2 and an outer cladding 3. The image transmission surface 2 is composed of an image transmission single core and a co-cladding 2-5 formed by stretching several light-absorbing capillary rods. The core layer 2-1, the recessed layer 2-2, the inner cladding 2-3, and the light-absorbing layer 2-4 of the image transmission single core correspond to the core layer 1-1, the recessed layer 1-2, the inner cladding 1-3, and the light-absorbing layer 1-4 of the light-absorbing core rod and the light-absorbing capillary rod, respectively. The cladding 1-5 of the light-absorbing core rod or the light-absorbing capillary rod is fused to form the co-cladding 2-5 between the image transmission single cores.

[0044] The bending-insensitive high-resolution imaging fiber obtained by the above preparation method includes several imaging cores and a co-cladding between the imaging cores; each imaging core includes a core layer, a recessed layer, an inner cladding layer and an absorbing layer from the inside out; the absorbing layer is quartz glass containing absorber ions and fluorine.

[0045] See Figures 3 to 5 The bending-insensitive high-resolution imaging fiber obtained by the above preparation method has the following characteristics: the core layer is germanium-doped quartz glass, and the relative refractive index difference Δ1 of the core layer ranges from 2% to 3.5%; the recessed layer is fluorine-doped quartz glass, and the relative refractive index difference Δ2 of the recessed layer ranges from -1% to -0.4%; the inner cladding is pure quartz glass, and the relative refractive index difference Δ3 of the inner cladding is 0; the relative refractive index difference Δ4 of the light-absorbing layer is 0, with an error range of less than ±0.07%; and the co-cladding is pure quartz glass, and the relative refractive index difference Δ5 of the co-cladding is 0.

[0046] In this invention, the core layer is doped with germanium to increase its refractive index, and the recessed layer is doped with fluorine to decrease its refractive index. These doping methods create a refractive index difference between the inside and outside of the optical fiber, thus establishing conditions for total internal reflection. This invention forms a recessed refractive index layer, which can suppress crosstalk between the optical signal in the core layer and adjacent fiber cores over a certain length.

[0047] The target relative refractive index difference of the light-absorbing layer in this invention is 0. The light-absorbing agent will cause the refractive index of the glass material to increase slightly. Therefore, the refractive index is controlled by adding a small amount of fluorine.

[0048] See Figures 3 to 5The radius R1 of the core layer ranges from 0.5 μm to 1 μm, the radius R2 of the recessed layer ranges from 0.55 μm to 1.1 μm, the radius R3 of the inner cladding layer ranges from 0.7 μm to 1.4 μm, and the radius R4 of the light-absorbing layer ranges from 0.75 μm to 1.5 μm. The radius R4 of the light-absorbing layer is also the radius of the image transmission core, and the radial thickness of the light-absorbing layer is from 0.05 μm to 0.1 μm. The inter-core spacing Λ (the distance between the geometric center of a certain image transmission core and the geometric center of the nearest adjacent image transmission core) ranges from 2 μm to 4 μm, and the number of image transmission cores is from 1,000 to 100,000. The radius R5 of the image transmission surface ranges from 50 μm to 500 μm; the radius R6 of the outer cladding layer ranges from 75 μm to 525 μm; and the radius R7 of the coating layer ranges from 100 μm to 700 μm.

[0049] The bending-insensitive high-resolution imaging fiber has a numerical aperture (NA) of 0.3 to 0.45 and a resolution ≥143 lp / mm; after 10 turns with a 20 mm radius, the resolution is ≥128 lp / mm; and the visible light transmittance of the 3 m fiber is >90%.

[0050] Secondly, the present invention provides a bend-insensitive high-resolution imaging optical fiber, which can be obtained by the preparation method provided in the first aspect of the present invention, or by other processes.

[0051] The bending-insensitive high-resolution imaging fiber provided by the present invention comprises a plurality of imaging cores and a co-cladding layer between the imaging cores; each of the imaging cores comprises, from the inside out, a core layer, a recessed layer, an inner cladding layer and a light-absorbing layer; the light-absorbing layer is quartz glass containing light-absorbing agent ions and fluorine.

[0052] The light-absorbing ions include one or more transition metal ions, for example, the light-absorbing agent is one or more transition metal elements such as iron, cobalt, and nickel; the concentration of the light-absorbing ions is from 100 ppm to 2000 ppm.

[0053] Furthermore, the bending-insensitive high-resolution imaging fiber may also include an outer cladding and a coating.

[0054] The relative refractive index difference, dimensions, and other parameters of each layer of the bend-insensitive high-resolution imaging fiber provided by this invention can be found in the description of the first aspect of this invention, and will not be repeated here.

[0055] The following 10 embodiments further illustrate the present invention. The process parameters, geometric parameters, and optical parameters (main performance parameters) of the image transmission optical fiber corresponding to the 10 embodiments (corresponding to serial numbers 1 to 10 in Tables 1, 2, and 3) are shown in Tables 1, 2, and 3, respectively.

[0056] Table 1 Process Parameters

[0057] Table 2 Geometric Parameters

[0058] Table 3 Optical Parameters

[0059] Based on the above data, it can be seen that this invention solves the technical problem of low resolution in current imaging fibers, especially severe intercore crosstalk and poor imaging quality under bending conditions, and provides a new solution for bending-insensitive high-resolution imaging fibers. The imaging fiber prepared by the method provided by this invention has high contrast, high resolution, few defects, and good bending resistance. The preparation method provided by this invention also has the advantages of simple and convenient process and strong production stability.

[0060] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A bend-insensitive high-resolution imaging optical fiber, characterized in that, The image transmission surface of the image transmission fiber includes several image transmission single cores and a co-cladding layer between the image transmission single cores; each image transmission single core includes a core layer, a recessed layer, an inner cladding layer and a light-absorbing layer from the inside out; the light-absorbing layer is quartz glass containing light-absorbing agent ions and fluorine.

2. The bend-insensitive high-resolution imaging fiber according to claim 1, characterized in that, The light-absorbing ions include one or more transition metal ions, and the concentration of the light-absorbing ions is from 100 ppm to 2000 ppm.

3. The bend-insensitive high-resolution imaging fiber according to claim 1, characterized in that, The core layer is germanium-doped quartz glass, and the relative refractive index difference Δ1 of the core layer ranges from 2% to 3.5%. The recessed layer is fluorine-doped quartz glass, and the relative refractive index difference Δ2 of the recessed layer ranges from -1% to -0.4%. The inner cladding is made of pure quartz glass, and the relative refractive index difference Δ3 of the inner cladding is 0. The relative refractive index difference Δ4 of the light-absorbing layer is 0, with an error range of less than ±0.07%. The co-cladding is pure quartz glass, and the relative refractive index difference Δ5 of the co-cladding is 0.

4. The bend-insensitive high-resolution imaging fiber according to claim 1, characterized in that, Also includes: Outer cladding and coating; The outer cladding layer is located outside the image transmission surface, and the outer cladding layer is pure quartz glass. The image transmission surface and the outer cladding layer constitute the glass portion of the image transmission optical fiber. The coating layer is located outside the glass portion of the image transmission optical fiber.

5. The bend-insensitive high-resolution imaging fiber according to claim 4, characterized in that, The radius R1 of the core layer ranges from 0.5 μm to 1 μm, the radius R2 of the recessed layer ranges from 0.55 μm to 1.1 μm, the radius R3 of the inner cladding layer ranges from 0.7 μm to 1.4 μm, and the radius R4 of the light-absorbing layer ranges from 0.75 μm to 1.5 μm; the inter-core spacing Λ between image transmission cores ranges from 2 μm to 4 μm, and the number of image transmission cores ranges from 1,000 to 100,000; the radius R5 of the image transmission surface ranges from 50 μm to 500 μm; the radius R6 of the outer cladding layer ranges from 75 μm to 525 μm; and the radius R7 of the coating layer ranges from 100 μm to 700 μm.

6. The bend-insensitive high-resolution imaging optical fiber according to claim 1, characterized in that, The bending-insensitive high-resolution imaging fiber has a numerical aperture of 0.3 to 0.45 and a resolution ≥143 lp / mm; after 10 turns with a radius of 20 mm, the resolution is ≥128 lp / mm; and the visible light transmittance of the 3m fiber is >90%.

7. A method for fabricating a bend-insensitive high-resolution imaging optical fiber as described in any one of claims 1 to 6, characterized in that, Includes the following steps: S1. Prepare light-absorbing core rod; A loose layer of fluorine-doped silica is deposited on the inner wall of a pure quartz liner. A light-absorbing agent solution is added and the liner is soaked. After soaking and drying, the liner is sintered to vitrify the loose layer and form a light-absorbing layer doped with light-absorbing agent ions. The light-absorbing core rod is obtained by sequentially depositing an inner cladding layer, a recessed layer, and a core layer, followed by melting and shrinking. S2. The light-absorbing core rod is drawn to obtain a light-absorbing capillary rod; S3. Stack several of the light-absorbing capillary rods in a pure quartz glass sleeve to obtain an image transmission fiber preform. S4. The image transmission fiber preform is drawn to obtain a high-resolution image transmission fiber that is insensitive to bending.

8. The method for fabricating a bend-insensitive high-resolution imaging optical fiber according to claim 7, characterized in that, In S1, the soaking time for adding the light absorber solution is 3h to 12h; the concentration of the light absorber in the light absorber solution is 0.5wt% to 3wt%, and the light absorber is one or more of the soluble salts corresponding to transition metals.

9. The method for fabricating a bend-insensitive high-resolution imaging optical fiber according to claim 7, characterized in that, In S1, nitrogen is used for purging and drying for 6 to 24 hours to achieve thorough drying.

10. The method for fabricating a bend-insensitive high-resolution imaging optical fiber according to claim 7, characterized in that, In S2, the radius of the light-absorbing capillary rod is 0.2 mm to 1 mm.

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