Mode field diameter measuring device and mode field diameter measuring method

Abstract

To provide a mode field diameter measuring device and a mode field diameter measuring method with which it is possible to accurately measure the mode field diameter of a discretionary waveguide mode.SOLUTION: A mode field diameter measuring device 100 pertaining to the present invention measures the mode field diameter of each waveguide mode of a few-mode fiber 10 (optical fiber to be tested) by a VA method using a variable aperture (VA) 14. The mode field diameter measuring device is characterized by having an aligner that adjusts the alignment with the few-mode fiber using a fundamental mode or a waveguide mode where the light intensity at mode field center is maximum, before measuring the mode field diameter of each waveguide mode of the few-mode fiber.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a mode field diameter measurement device and a measurement method for measuring the mode field diameter of an optical fiber. [Background technology]

[0002] Few-mode fiber is one of the promising optical fibers for realizing future high-capacity optical communications. Because few-mode fiber uses multiple guided modes as transmission channels, it is important to understand the transmission characteristics of each guided mode. The transmission characteristics of optical fiber are closely related to the electric field distribution of the guided modes. The mode field diameter (MFD) is a parameter that represents the electric field extent of the fundamental mode (LP01 mode). This parameter allows us to estimate the fundamental mode splice loss, chromatic dispersion, backscattered light capture efficiency, etc., making it an important parameter for understanding the transmission characteristics of conventional single-mode fiber. Furthermore, the splice loss, chromatic dispersion, backscattered light capture efficiency, etc. of higher-order modes can also be estimated from the electric field extent (MFD), making the MFD an important parameter in few-mode fiber as well.

[0003] The variable aperture method (VA method) is known as a method for measuring the mode field diameter of a single-mode fiber. The VA method measures the output light intensity from an optical fiber under test through a variable aperture. Non-Patent Document 1 discloses a method for measuring the mode field diameter of each mode in a few-mode fiber by the VA method.

[0004] On the other hand, the VA method has the problem that if there is a misalignment between the center of the mode field of the object to be measured and the center of the variable aperture, the measurement accuracy deteriorates and it becomes impossible to obtain the correct mode field diameter. To address this problem, Non-Patent Document 2 discloses a procedure in which, in order to align the center of the mode field of the object to be measured with the center of the variable aperture, "the positions of the optical fiber to be measured and the variable aperture are aligned before measurement so that the transmitted light intensity is maximized." [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Nakamura et al., "Applicability of the VA method to MFD measurements of multi-core fibers," IEICE Technical Report, vol. 120, no. 369, pp. 72-75, 2021. [Non-patent document 2] IEC 60793-1-45, “Optical fibers - Part 1-45: Measurement methods and test procedures - Mode field diameter,” 2017. [Non-patent document 3] A. Nakamura et al., “Mode field diameter definitions for few-mode fibers based on spot size of higher-order Gaussian mode,” IEEE Photonics Journal, vol. 12, no. 2, 7200609, 2020. Summary of the Invention [Problem to be solved by the invention]

[0006] FIG. 1 is a diagram illustrating the problem to be solved by the present invention. The light intensity Lid of the fundamental mode is greatest at the center of the mode field (≒ center of the core) (see FIG. 1(A)). Therefore, if the light intensity transmitted through the small aperture is aligned to be at its maximum, the center of the mode field will coincide with the center of the variable aperture 14. In other words, the method disclosed in Non-Patent Document 2 was effective for measuring the mode field diameter of the fundamental mode.

[0007] However, the light intensity Lid of a higher-order mode is not necessarily greatest at the mode field center (see the example of the LP11 mode in Figure 1(B)). For this reason, even if the light intensity transmitted through a small opening is aligned to be at its maximum, the mode field center and the center of the variable opening 14 do not coincide. In other words, the method disclosed in Non-Patent Document 2 was not effective when measuring the mode field diameter of a higher-order mode. As such, the conventional technology had the problem that it was difficult to accurately measure the mode field diameter depending on the waveguide mode.

[0008] Therefore, in order to solve the above-mentioned problems, an object of the present invention is to provide a mode field diameter measurement apparatus and a mode field diameter measurement method that can accurately measure the mode field diameter of any guided mode. [Means for solving the problem]

[0009] In order to achieve the above object, the mode field diameter measurement device according to the present invention aligns the positions of the optical fiber to be measured and the variable aperture so that the transmitted light intensity is maximized when the fundamental mode or the guided mode in which the light intensity at the center of the mode field is maximized is incident, and then measures the mode field diameter of an arbitrary guided mode.

[0010] Specifically, the mode field diameter measurement device according to the present invention is a mode field diameter measurement device that measures the mode field diameter of each guided mode of a few-mode fiber by a variable aperture (VA) method, and is characterized by including an aligner that aligns the few-mode fiber with the fundamental mode or the guided mode having the maximum light intensity at the mode field center before measuring the mode field diameter of each guided mode of the few-mode fiber.

[0011] Furthermore, a mode field diameter measurement method according to the present invention is a method for measuring the mode field diameter of each guided mode of a few-mode fiber by a variable aperture (VA) method, characterized in that, before measuring the mode field diameter of each guided mode of the few-mode fiber, the few-mode fiber is aligned with a measurement device using the fundamental mode or the guided mode having the maximum light intensity at the mode field center.

[0012] Here, the aligner changes its relative position with the few-mode fiber, and the relative position at which the light intensity of the light received by the optical receiver from the few-mode fiber is maximized is set as the measurement position for the mode field diameter.

[0013] For accurate alignment, it is preferable that the aligner minimizes the numerical aperture of the variable aperture when performing the alignment.

[0014] The above inventions can be combined as much as possible. [Effects of the Invention]

[0015] The present invention can provide a mode field diameter measurement device and a mode field diameter measurement method that can accurately measure the mode field diameter of any guided mode. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram illustrating a problem to be solved by the present invention. [Figure 2] 1 is a diagram illustrating a mode field diameter measurement device according to the present invention. FIG. [Figure 3] 1A to 1C are process diagrams illustrating a mode field diameter measurement method according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] The following description of the preferred embodiments of the present invention will be given with reference to the accompanying drawings. The preferred embodiments described below are examples of the present invention, and the present invention is not limited to the preferred embodiments. In this specification and the drawings, components having the same reference numerals are intended to represent the same components.

[0018] FIG. 2 is a diagram illustrating a mode field diameter measurement apparatus 100 according to this embodiment. The mode field diameter measurement apparatus 100 is an apparatus that measures the mode field diameter of each guided mode of a few-mode fiber (optical fiber under test) 10 by a VA method using a variable aperture (VA) 14. The mode field diameter measurement apparatus 100 is a test light input unit 100A that selectively inputs test light from a test light input end 10a into a core to be measured in an optical fiber 10 under test in a mode to be measured; a light intensity measuring unit 100B for measuring the light intensity of the output light from the test light output terminal 10b passing through the opening 14a of the variable aperture 14; a mode field diameter calculation unit 100C that calculates a mode field diameter of the mode incident as test light by using the mode orders in the azimuthal direction and the radial direction of the mode incident as test light by the test light incident unit 100A and the dependence of the light intensity measured by the light intensity measurement unit 100B on the aperture angle θ; Equipped with.

[0019] The test light input unit 100A has a light source 11 and a mode exciter 12. Continuous light output from the light source 11 is converted into a mode to be measured by the mode exciter 12. The mode exciter 12 is, for example, a mode multiplexer / demultiplexer. The continuous light converted into the mode to be measured by the mode exciter 12 is input from the test light input end 10a of the optical fiber 10 under test to the core to be measured.

[0020] The optical intensity measuring unit 100B has a variable aperture 14 having an aperture 14a, an optical lens 15, a photodetector 16, and a control unit 17. The test light output from the test light output end 10b of the optical fiber 10 under test passes through the aperture 14a of the variable aperture 14, is collected by the optical lens 15, and is photoelectrically converted by the photodetector 16. At this time, the aperture angle θ of the aperture 14a is changed in accordance with a signal from the control unit 17. Also, an aligner 13 is installed at the test light output end 10b of the optical fiber 10 under test to align the central axis of the optical fiber 10 under test with the center of the aperture 14a of the variable aperture 14.

[0021] The mode field diameter calculation unit 100C has an A / D (analog / digital) converter 18 and a signal processing unit 19. The signal relating to the light intensity output from the photodetector 16 is converted into digital data by the A / D (analog / digital) converter 18. The signal processing unit 19 acquires the light intensity for the aperture angle θ of the opening 14a of the variable aperture 14 based on the signal relating to the aperture angle θ from the control unit 17 and the digital data converted by the converter 18. Furthermore, the signal processing unit 19 performs arithmetic processing to calculate the mode field diameter using the light intensity for the aperture angle θ and the mode orders in the azimuthal and radial directions of the mode incident as the test light.

[0022] In addition, when the optical fiber 10 under test is a multi-core fiber (MCF) having multiple cores, the mode field diameter obtained by this measurement method and measurement device depends on the distance z between the test light output end 10b of the optical fiber 10 under test and the variable aperture 14, so the distance z needs to be set appropriately depending on the desired measurement accuracy (see Non-Patent Document 1).

[0023] 3 is a process diagram illustrating the mode field diameter measurement method performed by the mode field diameter measurement apparatus 100. This mode field diameter measurement method includes a test light generation step S1, an alignment step S2, a test light incidence step S3, a light intensity measurement step S4, and a mode field diameter calculation step S5.

[0024] In the test light generation step S1, the light source 11 generates test light having a desired wavelength.

[0025] In alignment step S2, the aligner 13 performs alignment to align the center of the core under test with the center of the variable aperture 14. First, the mode exciter 12 converts the test light into a fundamental mode or a guided mode in which the light intensity at the mode field center is maximized, and then inputs the converted test light into the core under test from the test light input end 10a of the optical fiber 10 under test. The aligner 13 aligns the position of the optical fiber 10 under test (the position of the test light output end 10b) so that the light intensity of the test light output from the test light output end 10b of the optical fiber 10 under test passing through the opening 14a of the variable aperture 14 is maximized. Note that "maximizing the light intensity" has the following meaning: When the positional relationship between the optical fiber 10 under test and the opening 14 is changed, how the light intensity changes is obtained, and the maximum value of this change is taken as the maximum light intensity.

[0026] In the alignment step S2, in order to improve alignment accuracy, the minimum numerical aperture that can be set in the variable aperture 14 is set. For example, it is desirable to set the numerical aperture to 0.02 or less.

[0027] In the test light injection step S3, the mode exciter 12 converts the test light into a guided mode under measurement, and the test light is injected into the core under measurement from the test light input end 10a of the optical fiber 10 under test.

[0028] In the light intensity measurement procedure S4, the photodetector 16 measures the light intensity of the test light output from the optical fiber 10 under test that passes through the opening 14a of the variable aperture 14. Here, based on instructions from the control unit 17, the variable aperture 14 changes the numerical aperture of the opening 14a, and the photodetector 16 measures the light intensity at that time, thereby obtaining the numerical aperture dependency of the light intensity.

[0029] In the mode field diameter calculation step S5, the signal processing unit 19 calculates the mode field diameter from the numerical aperture dependency of the light intensity. Specifically, the signal processing unit 19 calculates the mode field diameter in the waveguide mode of the test light using the azimuthal and radial mode orders of the mode incident as the test light in the test light incident step S3, the numerical aperture dependency of the light intensity measured in the light intensity measurement step S4, and the following formula (1).

number

[0030] Before measuring the mode field diameter of each guided mode of the non-measurement optical fiber (few-mode fiber) 10, the mode field diameter measurement device 100 aligns the variable aperture 14 and the non-measurement optical fiber (few-mode fiber) 10 using the fundamental mode or a guided mode in which the light intensity at the mode field center is maximum. Therefore, the mode field diameter measurement device 100 can align the optical axes of the non-measurement optical fiber 10 and the variable aperture 14 even in a higher-order mode such as the LP11 mode in which the light intensity at the mode field center is not maximum, and can accurately measure the mode field diameter by the VA method even in a higher-order mode.

[0031] [appendix] In this chapter, we explain the derivation of equation (1).

[0032] Linearly polarized modes (LP) with azimuthal and radial orders ν and μ, respectively νμ The electric field distribution in the far field of the radiated beam (mode) is expressed by the following equation:

number

[0033] In this case, the light intensity passing through the variable aperture 14 with an aperture angle α can be expressed by the following equation:

number

number

[0034] From equation (2), F νμ is a function that takes the divergence angle as an argument, so F on the right side of equation (4) νμ The α in (α) represents the divergence angle. On the other hand, P on the left side of equation (4) νμ The α in (α) represents the aperture angle. Therefore, equation (4) expresses the radial electric field distribution F at the divergence angle α. νμ (α) is the light intensity P νμ This means that it can be expressed as (α).

[0035] On the other hand, according to Non-Patent Document 3, LP νμ The mode field diameter of the mode can be expressed by the following equation (5) using the radial electric field distribution in the far field, where θ is the divergence angle.

number

[0036] By applying equation (4) with α replaced by θ to equation (5), the equation for calculating the mode field diameter can be rewritten as follows:

number

[0037] 10: Optical fiber under test (several mode fiber) 10a: Test light input terminal 10b: Test light output terminal 11:Light source 12: Mode exciter 13: Aligner 14: Variable aperture 14a: Opening 15: Optical lens 16:Receiver 17: Control unit 18: A / D converter 19: Signal processing section 100: Mode field diameter measurement device 100A: Test light input section 100B: Light intensity measurement unit 100C: Mode field diameter calculation section

Claims

1. A mode field diameter measuring device for measuring the mode field diameter of each guided mode of a few-mode fiber by a VA method using a variable aperture (VA), comprising: a light source that outputs continuous light; a mode exciter that converts the continuous light into an arbitrary guided mode and inputs the converted light into one end of the few-mode fiber; an aligner for adjusting the relative position between the variable aperture and the other end of the few-mode fiber; a control unit that, before measuring the mode field diameter of each guided mode of the few-mode fiber, converts the continuous light into a fundamental mode or a guided mode having a maximum light intensity at a center of a mode field by the mode exciter, and controls the aligner to align the continuous light with the few-mode fiber using the guided mode; A mode field diameter measurement device comprising:

2. 2. The mode field diameter measurement device according to claim 1, wherein the aligner changes a relative position with respect to the few-mode fiber, and the relative position at which the intensity of light received by the optical receiver from the few-mode fiber is maximized is set as the measurement position for the mode field diameter.

3. 3. The mode field diameter measurement apparatus according to claim 1, wherein the aligner minimizes the numerical aperture of the variable aperture when performing the alignment.

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