JP2021135179A - Optical fiber mode group delay characteristic evaluation method and evaluation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mode group delay characteristic evaluation method and an evaluation apparatus for evaluating a mode group delay characteristic of a number mode fiber using a fiber parameter obtained by a method simpler than a group delay characteristic. The purpose.
The present disclosure is a mode group delay characteristic evaluation method for evaluating a mode group delay characteristic of an optical fiber under test, and when the difference in refractive index between a core and a clad is small, the mode group of the optical fiber under test is described. Based on the fact that an approximate solution of the delay characteristic can be derived using the group refractive index of the core, the group refractive index of the cladding, the normalized frequency, and the normalized propagation constant, the mode group delay characteristic of the propagation mode in the optical fiber under test is obtained. This is a mode group delay characteristic evaluation method having a mode group delay calculation procedure for calculation.
[Selection diagram] Fig. 1

Description

The present disclosure relates to methods and devices for evaluating mode group delay characteristics of optical fibers.

Due to the widespread use of a wide variety of Internet services, the amount of traffic flowing through a single optical fiber is rapidly increasing year by year. On the other hand, the transmission capacity that can be propagated by an optical fiber is finite, and it is predicted that the single-mode fiber (SMF) that is widely used at present cannot cope with the future increase in traffic. In order to overcome this situation, a spatial multiplex transmission system is attracting attention, and a mode multiplex transmission system using an optical fiber capable of propagating a plurality of modes (hereinafter referred to as several mode fibers) is being studied.

In a mode multiplex transmission system using several mode fibers, the mode group delay characteristic is one of the important characteristics. The reason is that the larger the group delay difference (DMD: Differential Mode Delay) between modes, the larger the amount of digital signal processing required for restoring the transmission signal. Another reason is that the smaller the DMD, the higher the efficiency of four-wave mixing (FWM) generation between modes. Therefore, in order to design a mode multiplex transmission system, it is necessary to understand the group delay characteristics and the local group delay characteristics over the entire length of the several-mode fiber.

N. Shibata, M.D. Ohashi, R.M. Maruyama, and N. et al. Kuwaki, “Measurement of differential group delay and chromatic dispersion for LP01 and LP11 mods of foot-mode fibers with depressed” Review, vol. 22, no. 1, pp. 65-70, 2015. S. Ohno, D. Iida, K.K. Toge, and T.M. Manabe, "High-resolution measurement of differential mode delay of fiber-mode fiber using phase reference technology technology for factor. Fiber Technology. , Vol. 40, pp. 56-61, 2018. M. Ohashi et al. , "Longitudinal fiber parameter measurement of two-mode fiber links by using OTDR," ECOC2014, Th. 1.4.5, 2014. A. Nakamura et al. , "Effective mode field diameter for LP11 mode and it's measurement technique," Photon. Technol. Lett. , Vol. 28, no. 22, pp. 2553-2556, 2016. B. Brixner, "Refractive-Index Interpolation for Forced Silica," J. Mol. Opt. Soc. Am. , Vol. 57, no. 5, pp. 674-676, 1967. I. H. Malitzon, “Interspecimen Company of the Refractive Index of Forced Silica,” J. Mol. Opt. Soc. Am. , Vol. 55, no. 10, pp. 1205-1209, 1965. S. Kobayashi, N.K. Shibata, S.M. Shibata, and T. et al. Izawa, "Characteristics of Optical Fibers in Infrared Wavelength Region," REVIEW OF THE ELECTRICAL COMMUNICATION LABORATORIES, vol. 26, pp. 453-467, 1978. D. Krumbholz, E.I. Brinkmeyer, and E.I. -G. Neumann, "Core / grading power distribution, promotion constant, and group delay: Simple distribution for power-low graded-index fiber," J. et al. Opt. Soc. Am. , Vol. 70, no. 2, pp. 179-183, 1980. D. Logge, "Weekly Guiding Fibers," Appl. Opt. vol. 10, no. 10, pp. 2252-2258, 1971. J. D. Love and C.I. D. Hussey, "Variational approximations for higher-order modes of weekend-guiding fibers," Optical and Quantum Electronics, vol. 16, pp. 41-48, 1984.

For example, Non-Patent Document 1 and Non-Patent Document 2 disclose a method for directly evaluating the average value and distribution characteristics of group delay characteristics in a number mode fiber. On the other hand, when designing a transmission system using SMF, fiber parameters are usually used. Therefore, in order to design a mode multiplex transmission system using several mode fibers by the same method as the conventional transmission system using SMF, a method of evaluating the mode group delay characteristics from the fiber parameters is required.

The present disclosure has focused on the above circumstances, and is a mode group delay characteristic evaluation method for evaluating the mode group delay characteristics of several mode fibers using fiber parameters obtained by a method simpler than the group delay characteristics. It is an object of the present invention to provide an evaluation device.

In order to achieve the above object, the mode group delay characteristic evaluation method and the evaluation device according to the present invention have a specific refractive index difference and MFD (Mode) that can be measured with a device configuration that is cheaper than a device that directly measures the mode group delay characteristic. It was decided to evaluate the mode group delay characteristics from fiber parameters such as Field Diameter).

Specifically, the mode group delay characteristic evaluation method according to the present disclosure is described.
This is a mode group delay characteristic evaluation method for evaluating the mode group delay characteristics of an optical fiber.
When the difference in refractive index between the core and the clad of the optical fiber is small, the approximate solution of the mode group delay characteristic of the optical fiber is derived using the group refractive index of the core, the group refractive index of the clad, the normalized frequency and the normalized propagation constant. It is characterized by having a mode group delay calculation procedure for calculating the mode group delay characteristic of the propagation mode in the optical fiber under test based on what can be done.

Specifically, the mode group delay characteristic evaluation device according to the present disclosure is
A mode group delay characteristic evaluation device that evaluates the mode group delay characteristics of optical fibers.
When the difference in refractive index between the core and the clad of the optical fiber is small, the approximate solution of the mode group delay characteristic of the optical fiber is derived using the group refractive index of the core, the group refractive index of the clad, the normalized frequency and the normalized propagation constant. It is characterized by comprising a mode group delay calculating means for calculating the mode group delay characteristic of the propagation mode in the optical fiber under test based on what can be done.

In the present disclosure, it is possible to provide a mode group delay characteristic evaluation method and an evaluation device for evaluating the mode group delay characteristics of a number mode fiber with an apparatus configuration inexpensive as compared with a direct measurement method.

It is a figure explaining the mode group delay characteristic evaluation apparatus of 1st Embodiment. It is a figure explaining the mode group delay characteristic evaluation apparatus of 2nd Embodiment.

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The present disclosure is not limited to the embodiments shown below. Examples of these implementations are merely examples, and the present disclosure can be implemented in various modifications and improvements based on the knowledge of those skilled in the art. In addition, the components having the same reference numerals in the present specification and the drawings shall indicate the same components.

(First Embodiment)
FIG. 1 is a diagram illustrating a mode group delay characteristic evaluation device of the present embodiment. This mode group delay characteristic evaluation device is a mode group delay characteristic evaluation device that evaluates the mode group delay characteristics of an optical fiber.
Fiber parameter acquisition means 11 for acquiring the mode field radius, specific refractive index difference, core refractive index, clad refractive index, core group refractive index, clad group refractive index, and core radius of the desired propagation mode in the optical fiber under test.
An electric field distribution approximating means 12 for obtaining an approximate electric field distribution of the propagation mode using the mode field radius and a higher-order Gaussian function, and
The mode field radius, the specific refractive index difference, the core refractive index, the clad refractive index, the core group refractive index, the clad group refractive index, the core radius, and the electric field distribution approximation acquired by the fiber parameter acquisition means. Using the electric field distribution acquired by the means, the mode group delay calculating means 13 for calculating the mode group delay characteristic of the propagation mode in the optical fiber under test, and the mode group delay calculating means 13.
It is characterized by having.

Here, the fiber parameter acquisition means 11 includes the acquired mode order and mode field radius information a1 of the desired propagation mode, and the acquired specific refractive index difference, core refractive index, clad refractive index, core group refractive index, and clad group. The information a2 of the refractive index and the core radius is output. The electric field distribution approximating means 12 receives the information a1 of the mode order and the mode field radius, calculates the approximate electric field distribution of the propagation mode using them and the higher-order Gaussian function, and outputs the electric field distribution information a3. .. The mode group delay calculation means 13 receives the information a2 and the information a3, calculates the mode group delay using the information and the mode group delay evaluation formula, and outputs the mode group delay information a4. The electric field distribution approximating means 12 may output the information a1, and the mode group delay calculating means 13 may further receive the information a1.

This mode group delay characteristic evaluation method is a mode group delay characteristic evaluation method for evaluating the mode group delay characteristics of an optical fiber.
A fiber parameter acquisition procedure for acquiring the mode field radius, specific refractive index difference, core refractive index, clad refractive index, core group refractive index, clad group refractive index, and core radius of the desired propagation mode in the optical fiber under test.
An electric field distribution approximation procedure for obtaining an approximate electric field distribution for the propagation mode using the mode field radius and a higher-order Gaussian function, and
The mode field radius, the specific refractive index difference, the core refractive index, the clad refractive index, the core group refractive index, the clad group refractive index, the core radius, and the electric field distribution approximation acquired in the fiber parameter acquisition procedure. Using the electric field distribution acquired in the procedure, the mode group delay calculation procedure for calculating the mode group delay characteristic of the propagation mode in the optical fiber under test, and the mode group delay calculation procedure.
It is characterized by performing.

First, the fiber parameter acquisition means 11 acquires the mode field radius, specific refractive index difference, core refractive index, clad refractive index, core group refractive index, clad group refractive index, and core radius of the desired propagation mode in the optical fiber under test. do.

As a means for realizing the acquisition of the mode field radius, for example, the NFP (Refractive Near Field) method, the FFP (Far Field Field) method, the VA (Vertical Aligned) method, or the like, the light emitted from the optical fiber to be tested. A method of obtaining the mode field radius from the light intensity distribution can be used.

Further, as a means for acquiring the specific refractive index difference, the core refractive index, and the clad refractive index, a method for acquiring the refractive index distribution of the optical fiber such as the RNFP method can be used. Thus, the relative refractive index of the radial coordinate r difference delta (r), it is possible to obtain the relative refractive index difference delta ₀ of the optical fiber center.

これらの値を用いることにより、被試験光ファイバ長手方向のモード群遅延特性を取得することも可能となる。 Further, as a means for realizing the acquisition of the mode field radius and the specific refractive index difference, for example, the optical time domain reflection measurement methods described in Non-Patent Document 3 and Non-Patent Document 4 can be used. By using such a method, the distribution w (z) of the mode field radius and the distribution Δ (z) of the specific refractive index difference with respect to the longitudinal direction (z direction) of the optical fiber under test can be obtained. Further, from the specific refractive index difference Δ (z) and the cutoff wavelength, when the optical fiber is a step type, the core radius a (z) can be obtained by the following equation.

By using these values, it is also possible to acquire the mode group delay characteristics in the longitudinal direction of the optical fiber under test.

ここで、ｎ_１（λ）は波長λでのコアの屈折率、ｎ_２（λ）は波長λでのクラッドの屈折率である。 [Details of refractive index and group refractive index calculation procedure]
Next, a method of calculating the group refractive index will be described.
_{The group refractive indexes N 1} (λ) and N ₂ (λ) of the core and the clad in the optical fiber are expressed by the following equations.

Here, n ₁ (λ) is the refractive index of the core at the wavelength λ, and n ₂ (λ) is the refractive index of the cladding at the wavelength λ.

In order to obtain the value of the group refractive index, the wavelength dependence of the refractive index is necessary. The wavelength dependence of the refractive index can be obtained by using the relational expression of selmire as shown in the following equation (3).

Equation (3) is an equation in which Ai corresponds to the ultraviolet and infrared absorption wavelengths and Bi corresponds to the oscillator strength, and shows the physical content indicating the wavelength dependence of the refractive index. It is known that the measured values match well in a wide wavelength range including. Further, as shown in Non-Patent Document 5 and Non-Patent Document 6, it is known that the value of k can be sufficiently obtained by using up to 3 terms. Further, the coefficients Ai and Bi of the cermier of the glass used for the optical fiber are shown in Non-Patent Document 7.

Therefore, the group refractive index of the core and the clad can be obtained by substituting the equation (3) into the equation (2). That is, the specific refractive index difference Δ of the optical fiber is obtained by using the method for measuring the specific refractive index difference Δ described above, and the coefficients Ai and Bi of the equation (3) corresponding to Δ are set as the materials constituting the optical fiber. The group refractive index of the core and clad of the optical fiber can be derived by obtaining from the value.

Ｅ_ｌｍは周方向のモード次数がｌでありかつ径方向のモード次数がｍのときの電界分布、ｒ及びθは光ファイバの中心を原点とする極座標、ｎ（ｒ）は光ファイバの中心から距離ｒでの屈折率、ｋは真空中の波数、β_ｌｍはＬＰ_ｌｍモードの伝搬定数である。 [Details of mode group delay characteristic calculation procedure]
Next, the arithmetic processing for calculating the mode group delay characteristic will be described.
The wave equation in an optical fiber can be described by the following equation.

E _lm is the electric field distribution when the mode order in the circumferential direction is l and the mode order in the radial direction is m, r and θ are the polar coordinates with the center of the optical fiber as the origin, and n (r) is from the center of the optical fiber. The refractive index at the distance r, k is the number of waves in vacuum, and β _lm is the propagation constant of _{LP lm mode.}

Here, assuming that the solution of the wave equation of equation (4) is as follows:

Substituting equation (5) into equation (4) gives the following equation.

ここで、ａはコア半径を表し、ｎ_１及びｎ_２はそれぞれコア及びクラッドの屈折率を表す。 The refractive index distribution of the optical fiber is as follows.

Here, a represents the core radius, and n ₁ and n ₂ represent the refractive indexes of the core and the clad, respectively.

Normalized propagation constant _{b lm} of _{LP lm} modes in an optical fiber is given by the following equation.

ｋは波数を、Δは比屈折率差を表し次式で定義されている。

Here, v represents the normalized frequency and is defined by the following equation.

k represents the wave number and Δ represents the difference in the specific refractive index, which are defined by the following equations.

_{Multiply the} equation (2) by Elm and integrate with the cross-sectional area A of the optical fiber.

Equation (12) can be arranged as the following equation.

Therefore, the normalized propagation constant _blm can be expressed as the following equation.

The waveguide dispersion can be expressed by the following equation using the normalized frequency v and the normalized propagation constant _blm (see, for example, Non-Patent Document 8).

Therefore, d (vb _lm ) / dv can be obtained by the following equation.

ここで、cは光速を表し、β_ｌｍはＬＰ_ｌｍモードの伝搬定数を表す。 On the other hand, the group delay τ _{lm in the LP lm} mode can be described by the following equation.

Here, c represents the speed of light, and β _lm represents the propagation constant of the LP _{lm mode.}

Although the normalized propagation constant _blm is defined by the equation (8), the difference in refractive index between the core and the clad of the optical fiber, which is of practical interest, is often 1% or less, so that the core and the clad have a refractive index difference of 1% or less. Using the weakly guiding approximation (see, for example, Non-Patent Document 9) that the difference in refractive index is small, the propagation constant β _lm can be described by the following equation.

Differentiating equation (18) with wave number k

Moreover, the approximation of the following equation is used here.

Therefore, equation (19) can be approximated as the following equation.

Therefore, the group delay τ _lm can be approximated by the following equation.

ただし、β_ｌｍはＬＰ_ｌｍモードの伝搬定数、ｋは真空中の波数、ｃは光速、Δ（ｒ）は半径方向の座標ｒの比屈折率差、Δ_０は光ファイバ中心の比屈折率差、ｖは正規化周波数、Ａは光ファイバ断面積を表す。 Next, by substituting d (vb _lm ) / dv calculated above into the equation (22), the group delay τ _{lm in the LP lm} mode can be described by the following equation.

However, β _lm is _{the propagation constant in LP lm} mode, k is the number of waves in vacuum, c is the speed of light, Δ (r) is the difference in the specific refractive index of the radial coordinates r, and Δ ₀ is the difference in the specific refractive index in the center of the optical fiber. , V represent the normalized frequency, and A represents the optical fiber cross-sectional area.

Therefore, substituting the specific refractive index differences Δ (r) and Δ ₀ , the core group refractive index N ₁ , the clad group refractive index N ₂ , and the core radius a into _{the above equation (23), and further substituting the electric field distribution Elm.} , LP _lm mode group delay τ _lm can be obtained.

ｒ及びθは光ファイバの中心を原点とする極座標、ｗ_ｌｍはＬＰ_ｌｍモードのモードフィールド半径、Ｌ^ｌ _ｍ−１（ｘ）はラゲールの倍多項式である。 Here, in the electric field distribution approximation procedure, the electric field distribution approximation means 12 _{can derive the electric field distribution Elm} by using a higher-order Gaussian function (see, for example, Non-Patent Document 10).

r and θ are polar coordinates with the center of the optical fiber as the origin, w _lm is the mode field radius in _{LP lm} ^{mode, and L l} _m-1 (x) is a Laguerre polynomary.

Therefore, in the mode group delay calculation procedure, the mode group delay calculation means has the mode field radius w _lm , the specific refractive index difference Δ (r) and Δ ₀ , the core refractive index n ₁ , and the clad refractive index acquired in the fiber parameter acquisition procedure. Mode group delay characteristics _{of propagation mode LP lm} in the optical fiber under test using n ₂ , core group refractive index N ₁ , clad group refractive index N ₂ , core radius a, and electric field distribution E _{lm obtained by the electric field distribution approximation procedure.} Can be calculated. Further, the DMD can be calculated from the difference in the group delay characteristics of each mode.

ここで、Ｓｚは時間平均ポインティングベクトルの軸成分である。

として、式（２５）を以下のように書き換える。

The derivation process of equation (15) is as follows.
The following equations can be obtained from the equations (1) and (4) in Non-Patent Document 8.

Here, Sz is an axis component of the time average pointing vector.

As a result, the equation (25) is rewritten as follows.

Consider the left side of equation (27).

By substituting the equation (28) into the equation (27) and rearranging it, the desired equation (15) can be obtained.

(Second Embodiment)
FIG. 2 is a diagram illustrating a mode group delay characteristic evaluation device of the present embodiment. The mode group delay characteristic evaluation device of the present embodiment includes a fiber parameter acquisition means 11 and a mode group delay calculation means 13. By combining equations (23) and (24), an analytical solution for mode group delay can be obtained.

For example, when the refractive index distribution of the optical fiber under test is a step type, Δ (r) = 0. Therefore, the term of integration in the optical fiber cross-sectional area in the equation (23) can be expressed by the following equation.

In the LP ₀₁ mode, the electric field distribution E ₀₁ is expressed by the following equation from the equation (24).

In the LP ₁₁ mode, the electric field distribution E ₁₁ is expressed by the following equation from the equation (24).

Therefore, the group delay τ _{01 of the LP 01} mode shown in Eq. (23) can be expressed by the following equation. The same applies to the group delay τ _{11 in the LP 11 mode.}

By substituting the specific refractive index difference, group refractive index, core radius, and mode field radius into the above equations (32) and (33), the LP ₀₁ mode and LP _{11 of the optical fiber under test having a stepped refractive index distribution.} The group delay characteristic of the mode can be obtained. Further, the DMD can be calculated from the difference in the group delay characteristics of each mode.

As described above, in the present embodiment, the fiber parameter acquisition means 11 acquires the specific refractive index difference, the group refractive index, the core radius, and the mode field radius. Then, the mode group delay calculation means 13 obtains the group delay τ _lm of the _{LP lm} mode by using the analytical solution of the mode group delay obtained by combining equations (23) and (24). In this embodiment _{, examples of LP 01} mode and LP ₁₁ mode are shown, but the present disclosure can be applied to arbitrary values of l and m.

The derivation process of equation (32) is as follows.
In the LP ₀₁ mode, equation (29) can be written as follows.

Substituting equation (30) into the first term of equation (34),

Substituting equation (30) into the second term of equation (34),

When the equation (35) and the equation (36) are substituted into the equation (34) and applied to the equation (23), the equation (32) can be derived.

The derivation process of equation (33) is as follows.
In the LP ₁₁ mode, equation (29) can be written as follows.

となるように正規化する。

Since the final result is the same regardless of whether the function in the circumferential direction is cos θ or sin θ, the calculation proceeds using the case of cos θ as an example.

Normalize so that

Substituting equation (40) into the first term of equation (39),

Substituting equation (40) into the second term of equation (39),

Substituting equation (40) into the third term of equation (37),

By substituting Eqs. (41), (42) and (43) into Eq. (37) and applying it to Eq. (23), Eq. (33) can be derived.

The present invention is not limited to the above embodiment, and can be variously modified and implemented without departing from the gist of the present invention.

In short, the present invention is not limited to the above-described embodiment as it is, and at the implementation stage, the components can be modified and embodied within a range that does not deviate from the gist thereof. In addition, various inventions can be formed by an appropriate combination of the plurality of components disclosed in the above-described embodiment. For example, some components may be removed from all the components shown in the embodiments. In addition, components from different embodiments may be combined as appropriate.

This disclosure can be applied to the information and communication industry.

11: Fiber parameter acquisition means 12: Electric field distribution approximation means 13: Mode group delay calculation means

Claims (8)

This is a mode group delay characteristic evaluation method for evaluating the mode group delay characteristics of an optical fiber.
When the difference in refractive index between the core and the clad of the optical fiber is small, the approximate solution of the mode group delay characteristic of the optical fiber is derived using the group refractive index of the core, the group refractive index of the clad, the normalized frequency and the normalized propagation constant. It is characterized by having a mode group delay calculation procedure for calculating the mode group delay characteristic of the propagation mode in the optical fiber under test based on what can be done.
Mode group delay characteristic evaluation method. A fiber parameter acquisition procedure for acquiring the mode field radius, specific refractive index difference, core refractive index, clad refractive index, core group refractive index, clad group refractive index, and core radius of the desired propagation mode in the optical fiber under test.
An electric field distribution approximation procedure for obtaining an approximate electric field distribution for the propagation mode using the mode field radius and a higher-order Gaussian function, and
With more
In the mode group delay calculation procedure, the mode field radius acquired in the fiber parameter acquisition procedure, the specific refractive index difference, the core refractive index, the clad refractive index, the core group refractive index, the clad group refractive index, the above. Using the core radius and the electric field distribution obtained in the electric field distribution approximation procedure, the mode group delay characteristic of the propagation mode in the optical fiber under test is calculated.
The mode group delay characteristic evaluation method according to claim 1. The electric field distribution approximation procedure is characterized in that an approximate electric field distribution of the propagation mode is obtained by using the following equation.
The mode group delay characteristic evaluation method according to claim 2.

However, l and m are the mode orders in the circumferential direction and the radial direction of the _{optical fiber under test, w lm} is the mode field radius _{of the LP lm} mode in the optical fiber under test ^{, and L l} _m-1 (x) is Lager. The multiple polynomials, r and θ of are polar coordinates with the center of the optical fiber under test as the origin. The mode group delay calculation procedure is characterized in that the mode group delay is calculated using the following equation.
The mode group delay characteristic evaluation method according to any one of claims 1 to 3.

However, β _lm is _{the propagation constant of LP lm} mode, k is the number of waves in vacuum, c is the speed of light, Δ (r) is the difference in the specific refractive index of the radial coordinate r, and Δ ₀ is the center of the optical fiber under test. The difference in the refractive index of the above, v is the normalized frequency, and A is the cross-sectional area of the optical fiber under test. In the mode group delay calculation procedure, an analytical solution of the mode group delay is obtained by combining the equation of the electric field distribution represented by the number C41 and the equation of the mode group delay represented by the number C42, and the analytical solution is used. It is characterized by calculating the mode group delay,
The mode group delay characteristic evaluation method according to claim 1.

However, l and m are the mode orders in the circumferential direction and the radial direction of the optical fiber to be tested, w _lm is the mode field radius of the LP _lm ^{mode, and L l} _m-1 (x) is the multiple polypoly of Lager, r and. θ is a polar coordinate with the center of the optical fiber under test as the origin.

However, β _lm is _{the propagation constant of LP lm} mode, k is the number of waves in vacuum, c is the speed of light, Δ (r) is the difference in the specific refractive index of the radial coordinate r, and Δ ₀ is the center of the optical fiber under test. The difference in the specific refractive index of, v is the normalized frequency, and A is the cross-sectional area of the optical fiber under test. Wherein when the refractive index distribution of the test optical fiber is stepped, the analytical solution of the group delay tau ₁₁ group delay tau ₀₁ and the first higher-order mode of the fundamental mode (LP ₀₁ mode) at a wavelength lambda (LP ₁₁ mode) The feature is that it is calculated using the following equation.
The mode group delay characteristic evaluation method according to claim 5.

In the fiber parameter acquisition procedure, the mode field radius distribution and the specific refractive index difference distribution in the longitudinal direction of the optical fiber under test are acquired from the backward scattered light generated when the test light is incident on the optical fiber under test. The core radius is derived from the specific refractive index difference and the cutoff wavelength acquired in advance.
The mode group delay characteristic evaluation method according to claim 2 or 3. A mode group delay characteristic evaluation device that evaluates the mode group delay characteristics of optical fibers.
When the difference in refractive index between the core and the clad of the optical fiber is small, the approximate solution of the mode group delay characteristic of the optical fiber is derived using the group refractive index of the core, the group refractive index of the clad, the normalized frequency and the normalized propagation constant. It is characterized by comprising a mode group delay calculation means for calculating the mode group delay characteristic of the propagation mode in the optical fiber to be tested based on what can be done.
Mode group delay characteristic evaluation device.

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