Method for measuring bending loss of optical fiber

Abstract

This method for measuring the bending loss of a resin-coated optical fiber comprises: a first step for determining a reference winding number in which the wavelength dependency of the bending loss value of the optical fiber in a region equal to or less than a predetermined transmission wavelength when the optical fiber is wound around a mandrel having a predetermined diameter is in an exponential function shape with respect to the transmission wavelength; a second step for measuring the power of light emitted from the second end of the optical fiber by causing light having a predetermined power to enter the first end of the optical fiber at a predetermined transmission wavelength in a state of being wound around the mandrel by the reference number of windings; a third step for measuring the power of the light emitted from the second end of the optical fiber by causing the light having the predetermined power to enter the first end of the optical fiber at the predetermined transmission wavelength while being wound around the mandrel by a number of windings greater than the reference number of windings; and a fourth step in which, on the basis of the power of the light measured in the second step and the power of the light measured in the third step, a bending loss value at a predetermined transmission wavelength of the optical fiber when the optical fiber is bent at a predetermined diameter is obtained.

Description

Technical Field

[0001] The present invention relates to a method for measuring bending loss of optical fiber. Background Art

[0002] This application claims the benefit of priority based on Japanese Application No. 2022-128675 filed on August 12, 2022, and all the contents described in the above Japanese Application are incorporated herein by reference.

[0003] Patent Document 1 discloses a method for measuring bending loss of an optical fiber. In this method, the bending loss of an optical fiber is measured by obtaining a difference between the transmitted light power when the optical fiber is not bent and the transmitted light power when the optical fiber is bent.

[0004] Patent Document 1: Japanese Patent Application Publication No. 2012-194004 Summary of the invention

[0005] The method for measuring the bending loss of an optical fiber of the present invention measures the bending loss of an optical fiber coated with a resin.

[0006] The optical fiber bending loss measurement method has the following features:

[0007] The first step is to determine a reference winding number at which the wavelength dependence of the bending loss value of the optical fiber in a region below a predetermined transmission wavelength when the optical fiber is wound around a mandrel having a predetermined diameter becomes an exponential function with respect to the transmission wavelength;

[0008] A second step of measuring the power of light emitted from the second end of the optical fiber by injecting light of a predetermined power into the first end of the optical fiber at the predetermined transmission wavelength in a state where the optical fiber is wound around the mandrel at the reference winding number;

[0009] A third step is to measure the power of light emitted from the second end of the optical fiber by causing light of the specified power to be incident on the first end of the optical fiber at the specified transmission wavelength in a state where the optical fiber is wound around the mandrel with a number of turns greater than the reference number of turns; and

[0010] The fourth step is to obtain a bending loss value of the optical fiber at the predetermined transmission wavelength when the optical fiber is bent to have the predetermined diameter, based on the optical power measured in the second step and the optical power measured in the third step. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a diagram showing the flow of the optical fiber bending loss measurement method according to the present embodiment.

[0012] Figure 2This is a diagram for explaining a method of measuring the transmission loss value of an optical fiber.

[0013] Figure 3 This is a graph showing the wavelength dependence of the transmission loss value of an optical fiber.

[0014] Figure 4 This is a graph showing the wavelength dependence of the bending loss value of an optical fiber. DETAILED DESCRIPTION

[0015] [Problems to be Solved by the Invention]

[0016] If part of the light (whispering gallery mode light) leaking from the core to the cladding at the bent portion of the optical fiber undergoes Fresnel reflection at the interface between the cladding and air and recouples with the core mode light, interference between the core mode light and the whispering gallery mode light occurs, making it difficult to accurately measure the bending loss of the optical fiber.

[0017] An object of the present invention is to measure the bending loss of an optical fiber more accurately.

[0018] [Effects of the Invention]

[0019] According to the present invention, the bending loss of an optical fiber can be measured more accurately.

[0020] [Description of Embodiments of the Invention]

[0021] First, the contents of the embodiments of the present invention will be listed and described.

[0022] The method for measuring bending loss of an optical fiber of the present invention,

[0023] (1) The bending loss of the resin-coated optical fiber is measured.

[0024] The optical fiber bending loss measurement method has the following features:

[0025] The first step is to determine a reference winding number at which the wavelength dependence of the bending loss value of the optical fiber in a region below a predetermined transmission wavelength when the optical fiber is wound around a mandrel having a predetermined diameter becomes an exponential function with respect to the transmission wavelength;

[0026] A second step of measuring the power of light emitted from the second end of the optical fiber by injecting light of a predetermined power into the first end of the optical fiber at the predetermined transmission wavelength in a state where the optical fiber is wound around the mandrel at the reference winding number;

[0027] A third step is to measure the power of light emitted from the second end of the optical fiber by causing light of the specified power to be incident on the first end of the optical fiber at the specified transmission wavelength in a state where the optical fiber is wound around the mandrel with a number of turns greater than the reference number of turns; and

[0028] The fourth step is to obtain a bending loss value of the optical fiber at the predetermined transmission wavelength when the optical fiber is bent to have the predetermined diameter, based on the optical power measured in the second step and the optical power measured in the third step.

[0029] According to the above method, the bending loss of the optical fiber is calculated based on the power of two transmitted lights that are not affected by the whispering gallery mode light. This makes it possible to more accurately obtain the bending loss value of the optical fiber.

[0030] (2) In the above (1), the optical fiber can be wound on a core shaft having the specified diameter, and the winding number at which the wavelength dependence of the bending loss value of the optical fiber in the region below the specified transmission wavelength becomes an exponential function with respect to the transmission wavelength can be calculated, and the calculated winding number of the optical fiber can be used as the reference winding number of the first step.

[0031] According to the above method, since the reference winding number is set using the optical fiber to be measured, it is possible to obtain a bending loss value that is not affected by whispering gallery mode light.

[0032] (3) In the above (1), a standard optical fiber can be prepared and wound around a core shaft having the specified diameter, and the number of windings at which the wavelength dependence of the bending loss value of the standard optical fiber in the region below the specified transmission wavelength becomes an exponential function with respect to the transmission wavelength is calculated, and the calculated number of windings of the standard optical fiber is used as the reference number of windings of the first step.

[0033] According to the above method, the reference winding number is set in advance using a standard optical fiber, thereby saving the labor of setting the reference winding number every time the bending loss of the optical fiber is measured.

[0034] (4) In the above (3), the optical fiber can be set as the standard optical fiber when the bending loss value per winding number of the optical fiber obtained based on the bending loss value obtained through the fourth step is smaller than the bending loss value per winding number of the standard optical fiber obtained based on the bending loss value of the standard optical fiber at the specified transmission wavelength.

[0035] According to the above method, the reference winding number is recalculated based on an optical fiber that is more resistant to bending, and therefore a bending loss value that is not affected by whispering gallery mode light can be obtained.

[0036] [Details of the embodiments of the present invention]

[0037] Hereinafter, a specific example of the method for measuring bending loss of an optical fiber according to an embodiment of the present invention will be described with reference to the drawings. The present invention is not limited to these examples but is indicated by the claims, and includes all modifications within the scope of the equivalents to the claims.

[0038] Figure 1 It is a diagram showing the flow of a method for measuring the bending loss of the optical fiber 1 according to the present embodiment. Figure 2 This is a diagram for explaining a method of measuring the power of light that is incident from one end (first end) and emitted from the other end (second end) of an optical fiber 1. The optical fiber 1 to be measured has a core and a cladding made of glass, and the cladding is covered by a coating layer made of resin.

[0039] As a method for measuring the bending loss value of a predetermined transmission wavelength of the optical fiber 1, Figure 1 As shown in the example, first, the reference winding number T1 of the optical fiber 1 wound on the core shaft 2 is determined (STEP 1). The reference winding number T1 is a reference winding number used to determine the bending loss value of the optical fiber 1. The reference winding number T1 is set to a winding number at which the wavelength dependence of the bending loss value of the optical fiber 1 in a region below a specified transmission wavelength is not affected by the whispering gallery mode light. The specified transmission wavelength is, for example, 1550 nm or 1625 nm. Specifically, the reference winding number T1 is set to a winding number at which the wavelength dependence of the bending loss value of the optical fiber 1 in a region below a specified transmission wavelength when the optical fiber 1 is wound on the core shaft 2 becomes an exponential function shape with respect to the transmission wavelength. In addition, the expression "not affected by the whispering gallery mode light" is not limited to a state where there is no influence of the whispering gallery mode light at all, but includes a state where the influence of the whispering gallery mode light is small enough to be negligible.

[0040] The standard winding number T1 can be calculated each time the bending loss value of the optical fiber 1 is obtained. Figure 2 First, when the optical fiber 1 is not wound around the mandrel 2, the power of the light emitted from the other end of the optical fiber 1 by injecting light of a specified power from the light source 3 into one end of the optical fiber 1 (hereinafter referred to as the transmitted light power P0) is measured by the power meter 4. The measurement is performed using light of each wavelength (λ) below the specified transmission wavelength, and the transmitted light power (P0(λ)) for each wavelength (λ) is obtained.

[0041] Next, in a state where the optical fiber 1 is wound around the mandrel 2, the power of light (hereinafter referred to as the transmitted light power P1) emitted from the other end of the optical fiber 1 by injecting light of a specified power from the light source 3 into one end of the optical fiber 1 is measured by the power meter 4. This measurement is performed using light of each wavelength (λ) below the specified transmission wavelength, and the transmitted light power (P1(λ)) for each wavelength (λ) is obtained.

[0042] Next, the bending loss value L(λ) of each wavelength (λ) is calculated. Specifically, the bending loss value L(λ) is calculated by L(λ)[dB]=-10log(P1(λ)[W] / P0(λ)[W]). Thus, the wavelength dependence of the bending loss value of the optical fiber 1 is obtained.

[0043] Then, the wavelength dependence of the bending loss value of the optical fiber 1 in a region below a predetermined transmission wavelength is obtained while changing the number of windings of the optical fiber 1 wound around the mandrel 2 until the wavelength dependence of the bending loss value obtained becomes an exponential function shape with respect to the transmission wavelength. When the wavelength dependence of the bending loss value obtained becomes an exponential function shape with respect to the transmission wavelength, the number of windings of the optical fiber 1 around the mandrel 2 at that time is set as the reference winding number T1.

[0044] Next, the power of light emitted from one end of the optical fiber 1 by injecting light of a specified power at a specified transmission wavelength from the light source 3 while the optical fiber is wound around the core shaft 2 with a reference winding number T1 (hereinafter referred to as the transmitted light power P10) is measured by the power meter 4 (STEP 2).

[0045] Next, the power of light emitted from one end of the optical fiber 1 by injecting light of a specified power at a specified transmission wavelength from the light source 3 into the other end of the optical fiber 1 is measured by the power meter 4 in a state of being wound around the mandrel 2 with the winding number T2 (STEP 3). The winding number T2 is set to be larger than the reference winding number T1.

[0046] Next, based on the transmitted light power P10 measured in STEP 2 and the transmitted light power P11 measured in STEP 3, the bending loss value of the optical fiber 1 at a predetermined transmission wavelength when the optical fiber 1 is bent at a predetermined diameter D is obtained (STEP 4). Specifically, based on the transmitted light power P10 and the transmitted light power P11, the bending loss value L [dB] at the predetermined transmission wavelength of the optical fiber 1 is obtained by L = -10log (P11 [W] / P10 [W]). In addition, when the bending loss value per winding number is obtained, the bending loss value L is divided by the difference between the winding number T2 and the winding number T1.

[0047] Here, the bending loss of the optical fiber 1 is caused by a part of the core mode light leaking into the cladding at the bent portion of the optical fiber 1. A part of the light (whispering gallery mode light) leaking into the cladding is Fresnel reflected at the interface between the cladding and the air and recoupled with the core mode light. During the recoupling, interference between the core mode light and the whispering gallery mode light occurs. Due to this interference, vibration components with a certain optical frequency interval are generated in the transmission spectrum of the bent optical fiber, and as a result, accurate measurement of the bending loss becomes difficult. The smaller the bending diameter of the optical fiber, the more significant the generation of whispering gallery mode light becomes. Therefore, when the bending diameter of the optical fiber 1 is small, it is difficult to accurately measure the bending loss.

[0048] On the other hand, whispering gallery mode light has a property that it is difficult to generate by increasing the number of times the optical fiber 1 is wound around the mandrel 2. Therefore, in the optical fiber bending loss measurement method of the present embodiment, the transmitted light power P10 of the specified transmission wavelength of the optical fiber 1 wound with the reference winding number T1, which generates less whispering gallery mode light, is used as a reference. Then, based on the transmitted light power P10 and the transmitted light power P11 of the specified transmission wavelength of the optical fiber 1 wound with the winding number T2 which is greater than the reference winding number T1, the bending loss value of the specified transmission wavelength of the optical fiber 1 is calculated. That is, based on the two transmitted light powers P10 and P11 which are not affected by the whispering gallery mode light, the bending loss value of the specified transmission wavelength of the optical fiber 1 is calculated. Thus, the bending loss of the optical fiber 1 can be measured more accurately.

[0049] In the present embodiment, the standard winding number T1 is calculated each time the bending loss value of the optical fiber 1 is obtained. However, the standard winding number T1 may be set in advance instead of being calculated each time the bending loss value of the optical fiber 1 is obtained.

[0050] For example, the reference winding number T1 can be obtained by using a standard optical fiber different from the optical fiber to be measured. Specifically, the standard optical fiber is wound on a mandrel 2 having a specified diameter D, and the bending loss value of the standard optical fiber in a region below a specified transmission wavelength is obtained. In addition, the mandrel 2 can be a mandrel used to obtain the bending loss value of the optical fiber 1, or it can be a different mandrel having a specified diameter D. Moreover, the winding number at which the wavelength dependence of the bending loss value of the obtained standard optical fiber becomes an exponential function with respect to the transmission wavelength is set as the reference winding number T1. In addition, the term "standard optical fiber" refers to an optical fiber that serves as a reference for setting the reference winding number T1. For example, as the standard optical fiber, an optical fiber of the same type as the optical fiber to be measured is used.

[0051] Alternatively, the standard winding number used in the past bending loss measurement may be used as the standard winding number T1. In this case, the optical fiber used to calculate the standard winding number in the past bending loss measurement becomes the standard optical fiber used to set the reference winding number T1.

[0052] As described above, by setting the reference winding number T1 in advance using a standard optical fiber, it is possible to save the labor of setting the reference winding number T1 each time the bending loss of the optical fiber 1 is measured.

[0053] Furthermore, when the standard winding number T1 is set in advance, if the bending loss value per winding number of the optical fiber obtained based on the bending loss value obtained in STEP 4 is smaller than the bending loss value per winding number of the standard optical fiber obtained based on the bending loss value of the standard optical fiber at a predetermined transmission wavelength, the reference winding number T1 may be reset based on the optical fiber 1. That is, the optical fiber 1 is set as the standard optical fiber, the winding number at which the bending loss value of the transmission loss value of the optical fiber 1 in a region below the predetermined transmission wavelength becomes an exponential function with respect to the transmission wavelength may be obtained, and the obtained winding number may be used as the reference winding number T1. Thus, the reference winding number T1 is recalculated based on the optical fiber 1 that is more resistant to bending, so that the bending loss value that is not affected by the whispering gallery mode light can be obtained.

[0054] In addition, when obtaining the bending loss value of the standard optical fiber per winding number based on the bending loss value of the standard optical fiber at a specified transmission wavelength, based on the transmitted light power P10 when the standard optical fiber is wound on the core shaft 2 with the reference winding number T1 and the transmitted light power P11 when the standard optical fiber is wound on the core shaft 2 with the winding number T2, the bending loss value L of the standard optical fiber at the specified transmission wavelength is first obtained by L[dB]=-10log(P11[W] / P10[W]), and then it can be obtained by dividing the bending loss value L by the difference between the winding number T2 and the winding number T1.

[0055] (Example)

[0056] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following examples.

[0057] First, the wavelength dependence of the bending loss value of the optical fiber 1 on the number of windings was evaluated. Specifically, using a mandrel 2 with a diameter of 30 mm, the number of windings T of the optical fiber 1 wound on the mandrel 2 was changed, the wavelength of the light incident on the light source 3 was changed, and the bending loss value of the optical fiber 1 at each number of windings was measured. The number of windings T was changed from 2 windings (2T) to 20 windings (20T) in units of 2 windings. The results are shown in Figure 3 Shown.

[0058] like Figure 3 As shown in the figure, the following tendency can be confirmed: the greater the number of windings T of the optical fiber 1, the more the wavelength dependence of the bending loss value of the optical fiber 1 becomes an exponential function shape with respect to the transmission wavelength. This is considered to be because the number of windings of the optical fiber 1 around the mandrel 2 increases, thereby suppressing the generation of whispering gallery mode light, and the bending loss value is not affected by the whispering gallery mode light. In this example, it can be seen that if the number of windings T is 8 or more, the wavelength dependence of the bending loss value of the optical fiber 1 becomes an exponential function shape with respect to the transmission wavelength.

[0059] In addition, the wavelength dependency of the bending loss value of the optical fiber 1 was evaluated. Figure 4 Shown based on Figure 3 The calculated bending loss value of the optical fiber 1 is the wavelength dependency of the calculated bending loss value of the optical fiber 1. Specifically, the bending loss value of the optical fiber 1 is calculated based on the difference between two bending loss values ​​at two different winding numbers.

[0060] like Figure 4 As shown in the figure, it can be seen that the bending loss value calculated by the difference between the bending loss value of 4 turns (4T) and the bending loss value of 2 turns (2T) (the value represented by the index 4T-2T in the figure) cannot ensure the wavelength dependence of the exponential function shape. The reason for this is that the bending loss value with a small number of turns T is affected by the whispering gallery mode light.

[0061] On the other hand, it can be seen that the bending loss value calculated by the difference between the bending loss value of 20 windings (20T) and the bending loss value of 18 windings (18T) (the value represented by the index 20T-18T in the figure) maintains the wavelength dependence of the exponential function shape. The reason for this is that the bending loss value with a large number of windings T is not affected by the whispering gallery mode light.

[0062] The present invention has been described in detail and with reference to specific embodiments, but it is obvious to those skilled in the art that various changes or modifications can be made without departing from the spirit and scope of the present invention. In addition, the number, position, shape, etc. of the structural components described above are not limited to the above embodiments, and can be changed to the number, position, shape, etc. suitable for implementing the present invention.

[0063] In the above-mentioned embodiment, the bending loss value of the optical fiber 1 is calculated based on two bending loss values ​​obtained for the optical fiber 1 having two different winding numbers. However, the bending loss value of the optical fiber 1 may be calculated based on two bending loss values ​​obtained for the optical fiber 1 having one different winding number or three or more different winding numbers.

[0064] Description of the label

[0065] 1: Fiber Optic

[0066] 2: Mandrel

[0067] 3: Light source

[0068] 4: Power meter

[0069] P0: The power of the incident light

[0070] P1: The power of the emitted light

Claims

1. A method for measuring bending loss of an optical fiber, comprising measuring the bending loss of an optical fiber coated with a resin, The optical fiber bending loss measurement method has the following features: The first step is to determine a reference winding number at which the wavelength dependence of the bending loss value of the optical fiber in a region below a predetermined transmission wavelength when the optical fiber is wound around a mandrel having a predetermined diameter becomes an exponential function with respect to the transmission wavelength; A second step of measuring the power of light emitted from the second end of the optical fiber by injecting light of a predetermined power into the first end of the optical fiber at the predetermined transmission wavelength in a state where the optical fiber is wound around the mandrel at the reference winding number; A third step of measuring the power of light emitted from the second end of the optical fiber by causing light of the specified power to be incident on the first end of the optical fiber at the specified transmission wavelength in a state where the optical fiber is wound around the mandrel with a number of turns greater than the reference number of turns; as well as The fourth step is to obtain a bending loss value of the optical fiber at the predetermined transmission wavelength when the optical fiber is bent to have the predetermined diameter, based on the optical power measured in the second step and the optical power measured in the third step.

2. The method for measuring bending loss of an optical fiber according to claim 1, wherein: The optical fiber is wound on a core shaft having the specified diameter, and the winding number at which the wavelength dependence of the bending loss value of the optical fiber in the region below the specified transmission wavelength becomes an exponential function with respect to the transmission wavelength is calculated, and the calculated winding number of the optical fiber is used as the reference winding number of the first step.

3. The method for measuring bending loss of an optical fiber according to claim 1, wherein: Prepare a standard optical fiber and wind the standard optical fiber around a core shaft having the specified diameter, calculate the winding number at which the wavelength dependence of the bending loss value of the standard optical fiber in a region below the specified transmission wavelength becomes an exponential function with respect to the transmission wavelength, and use the calculated winding number of the standard optical fiber as the reference winding number of the first step.

4. The method for measuring bending loss of an optical fiber according to claim 3, wherein: When the bending loss value per winding number of the optical fiber obtained based on the bending loss value obtained through the fourth step is smaller than the bending loss value per winding number of the standard optical fiber obtained based on the bending loss value of the standard optical fiber at the specified transmission wavelength, the optical fiber is set as the standard optical fiber.

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