JP4795896B2 - Evaluation method and apparatus for group delay time difference of optical fiber - Google Patents

Description

  The present invention relates to an evaluation method and an evaluation apparatus for an optical fiber group delay time difference, and more particularly to an evaluation method and an evaluation apparatus for an optical fiber group delay time difference for evaluating propagation modes in a two-mode propagation wavelength region.

  When a signal having a wavelength shorter than the cutoff wavelength is incident on the optical fiber, a light wave of a higher mode is propagated in addition to the fundamental mode. In general, the fundamental mode and the higher-order mode have different propagation speeds in the optical fiber. For this reason, in optical communication using a wavelength equal to or shorter than the cutoff wavelength, transmission characteristics deteriorate due to a group delay time difference between modes propagating in the optical fiber, and an applicable transmission speed or transmission distance is limited. Therefore, in optical communication using a wavelength equal to or shorter than the cut-off wavelength, it is necessary to grasp the group delay time difference in the optical fiber with high accuracy.

  The group delay time difference in the optical fiber is obtained by selectively or simultaneously exciting the fundamental mode and the higher order mode of the optical fiber to be measured, and sampling the propagation time difference between the fundamental mode and the higher order mode at the output end of the optical fiber to be measured. It can be measured (hereinafter also referred to as evaluation) by directly observing with an oscilloscope or the like. In addition, it is known that the excitation conditions of the fundamental mode and the higher-order mode of the optical fiber can be varied by applying a measurement light to the optical fiber to be measured with a slight deviation from the central axis of the cross section of the optical fiber to be measured. For example, FIGS. 6 and 7 of Non-Patent Document 1 and the description thereof disclose that changes in the propagation state of a plurality of modes can be observed by varying the excitation position in the axial direction of the cross section of the optical fiber. ing.

P.F.Kolesar, D.J.Mazzarese, `` Understanding multimodebandwidth and differential mode delay measurements and their applications '' Proc. Of 51st IWCS, November 18-21, 2002, pp.453-460

  However, when evaluating the group delay time difference of the measured optical fiber, if the group delay time difference Δτ (λ) at the measurement wavelength λ in the measured optical fiber is very small, the time difference between the propagation modes is observed. Therefore, there is a problem that a sufficient length of the fiber is required or a measurement light source having a pulse width that is extremely short, that is, a pulse width smaller than the difference in the group delay time of the optical fiber.

  In general, higher-order modes in an optical fiber have a larger propagation loss than the fundamental mode. Therefore, if a fiber length sufficient to observe the time difference between propagation modes is given to the measured optical fiber, the measured optical fiber As the fiber length increases, it may become difficult to observe higher-order modes after propagation.

  The present invention solves such a problem, and it is possible to easily and easily evaluate a group delay time difference of an optical fiber regardless of the magnitude of the group delay time difference depending on a measurement wavelength. An object is to provide an evaluation method and an evaluation apparatus.

The method for evaluating the group delay time difference of the optical fiber according to the first invention of the present invention for solving the above-mentioned problems is as follows.
In the evaluation method for evaluating the group delay time difference, which is the difference between the group delay time of the fundamental mode light propagating in the optical fiber and the group delay time of the higher order mode light,
An optical signal oscillated from a pulse light source that oscillates in a single mode at a wavelength λ is shifted from the central axis of a reference optical fiber having an arbitrary length L _R having a group delay time difference Δτ _R (λ) at the wavelength λ. Is incident and
The light emitted from the reference optical fiber is incident on an optical fiber to be measured having an arbitrary length L,
Measure the group delay time difference Δτ _M (λ) at the output end of the optical fiber to be measured,
The group delay time difference Δτ (λ) at the wavelength λ of the optical fiber to be measured is calculated from the following relational expression.
Δτ (λ) = {Δτ _M (λ) × (L _R + L) −Δτ _R (λ) × L _R } / L
The method for evaluating the group delay time difference of the optical fiber according to the second invention of the present invention for solving the above-described problems is as follows.
In the evaluation method of the group delay time difference of the optical fiber according to the first invention,
The pulse width P _W of the optical signal oscillated by the pulse light source, the length L _{R of} the reference optical fiber, the group delay time difference Δτ _R (λ) at the wavelength λ of the reference optical fiber, the length of the optical fiber to be measured L and a group delay time difference Δτ (λ) at the wavelength λ of the optical fiber to be measured satisfy the following relational expression.
P _W ≦ │Δτ _R (λ) × L _R │
P _W ≦ | Δτ _R (λ) × L _R + Δτ (λ) × L |

An apparatus for evaluating a group delay time difference of an optical fiber according to a third aspect of the present invention,
In an evaluation apparatus for evaluating a group delay time difference that is a difference between a group delay time of light in a fundamental mode propagating in an optical fiber and a group delay time of light in a higher-order mode,
A pulsed light source that oscillates in a single mode at a wavelength λ;
Has a differential group delay Δτ _R (λ) at the wavelength lambda, the optical signal emitted from the pulsed light source is incident, any length you incident light emitted to the measured optical fiber of any length L L _R reference optical fiber,
A coupling function for injecting the signal light from the pulsed light source with an axial shift with respect to a central axis of the reference optical fiber;
And an optical signal detection function of detecting an optical signal on the time axis of the exit end of the optical fiber to be measured,
Characterized by comprising an arithmetic processing function of calculating the group delay time difference .DELTA..tau (lambda) at a wavelength lambda of the optical fiber to be measured from the optical signal detecting function equation below based on the result of detection by the.
Δτ (λ) = {Δτ _M (λ) × (L _R + L) −Δτ _R (λ) × L _R } / L
An apparatus for evaluating a group delay time difference of an optical fiber according to a fourth aspect of the present invention for solving the above-described problems is as follows.
In the evaluation apparatus for the group delay time difference of the optical fiber according to the third invention,
The pulse width P _W of the optical signal oscillated by the pulse light source, the length L _{R of} the reference optical fiber, the group delay time difference Δτ _R (λ) at the wavelength λ of the reference optical fiber, the length of the optical fiber to be measured L and a group delay time difference Δτ (λ) at the wavelength λ of the optical fiber to be measured satisfy the following relational expression.
P _W ≦ │Δτ _R (λ) × L _R │
P _W ≦ | Δτ _R (λ) × L _R + Δτ (λ) × L |

According to the present invention, when measuring the group delay time difference Δτ (λ) of the optical fiber to be measured having an arbitrary length L, the arbitrary length L _R having the group delay time difference Δτ _R (λ) at the wavelength λ. By using the reference optical fiber, the group delay time difference Δτ (λ) of the optical fiber to be measured can be evaluated with high accuracy regardless of the magnitude of the group delay time difference Δτ (λ). The effect of becoming is obtained.

Furthermore, according to the group delay time difference evaluation method of the present invention, the group delay time difference Δτ _R (λ) at the wavelength λ of the reference optical fiber can be adjusted according to the type or length of the reference optical fiber, and an arbitrary pulse width P _W can be set. It is possible to obtain an effect that the present invention can be easily applied to a measurement light source and an arbitrary type of optical fiber to be measured having an arbitrary length L.

  Furthermore, the group delay time difference evaluation apparatus according to the present invention is a simple configuration using a general-purpose pulse light source, a detection function of an optical signal on a time axis, and a calculation processing function of a group delay time difference. The effect that the evaluation of the group delay time difference can be realized.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to FIG. FIG. 1 is a conceptual diagram showing a configuration example of an apparatus for evaluating a group delay time difference of optical fibers according to the present embodiment.

As shown in FIG. 1, in this embodiment, the group delay time difference evaluation apparatus has a pulse light source 11 that oscillates in a single mode at a wavelength λ and a group delay time difference Δτ _R (λ) at a wavelength λ. any and reference optical fiber 12 of a length L _R, and the measured optical fiber 13, the reference optical fiber 12, or any length optical signal on the time axis at the output end of the optical fiber under test 13 L connected to the And an arithmetic processing unit 15 for calculating the group delay time difference Δτ (λ) at the wavelength λ of the optical fiber 13 to be measured using the result detected by the optical signal detection unit 14. Yes.

The pulse light source 11 has a function of generating an optical pulse train on the time axis with a pulse width P _W. The measurement light having the wavelength λ oscillated from the pulse light source 11 is incident on a reference optical fiber 12 having an arbitrary length L _R having a group delay time difference Δτ _R (λ) at the wavelength λ. Here, at the coupling portion between the pulse light source 11 and the reference optical fiber 12, for example, a function of allowing the measurement light from the pulse light source 11 to enter the reference optical fiber 12 with an arbitrary amount of axial deviation, etc. It is desirable that a function for changing the state is provided.

Any type of optical fiber having an arbitrary length L _R can be applied to the reference optical fiber 12. For example, a 1.3 μm band zero-dispersion optical fiber, a dispersion-shifted fiber, a non-optical fiber used in normal optical communication can be used. Zero-dispersion fibers, cut-off shift fibers, multimode fibers, and the like are applicable. The group delay time difference Δτ _R (λ) at the wavelength λ of the reference optical fiber 12 can be adjusted by selecting the type or length of the reference optical fiber 12.

  The optical signal detector 14 uses the signal light at the output end of the optical fiber to be measured 13 having an arbitrary length L to which the output light of the reference optical fiber 12 or the reference optical fiber 12 is incident as an optical signal string on the time axis. To detect.

The arithmetic processing unit 15 is connected to the group delay time difference Δτ _R (λ) at the wavelength λ of the reference optical fiber 12 and the reference optical fiber 12 from the optical signal sequence on the time axis detected by the optical signal detection unit 14. The group delay time difference Δτ _M (λ) at the measurement wavelength λ at the output end of the optical fiber 13 to be measured is measured. Further, based on the evaluation results of these group delay time differences Δτ _R (λ) and Δτ _M (λ), the group delay time difference Δτ (λ) at the wavelength λ of the optical fiber 13 to be measured is calculated using the relationship of the following equation (1). calculate.
Δτ (λ) = {Δτ _M (λ) × (L _R + L) −Δτ _R (λ) × L _R } / L (1)

The pulse width P _W of the pulse light source 11, the group delay time difference Δτ _R (λ) at the wavelength λ of the reference optical fiber 12 having an arbitrary length L _R , and the wavelength of the measured optical fiber 13 having an arbitrary length L It is desirable that the group delay time difference Δτ (λ) at λ satisfies the relationship of the following expressions (2) and (3).
P _W ≦ | Δτ _R (λ) × L _R | (2)
P _W ≦ | Δτ _R (λ) × L _R + Δτ (λ) × L | (3)

  Next, a procedure for evaluating the group delay time difference Δτ (λ) at the wavelength λ of the optical fiber 13 to be measured using the group delay time difference evaluation apparatus shown in FIG. 1 will be described in detail with reference to FIG. FIG. 2 is a flowchart showing a group delay time difference evaluation procedure according to this embodiment.

As shown in FIG. 2, first, calculation from the result by the optical signal detection unit 14 detects the signal light at the output end of the reference optical fiber 12 of any length L _R as an optical signal sequence on the time axis, is detected The processing unit 15 measures the group delay time difference Δτ _R (λ) at the wavelength λ of the reference optical fiber 12, and stores the measurement result in a data storage unit (not shown) (step S1).

Next, the reference optical fiber 12 is connected to the optical fiber 13 to be measured having a length L, the light emitted from the reference optical fiber 12 is incident on the optical fiber 13 to be measured, and the optical signal detector 14 Signal light from the emission end is detected as an optical signal train on the time axis. Then, the arithmetic processing unit 15 measures the group delay time difference Δτ _M (λ) at the wavelength λ of the emission end of the optical fiber 13 to be measured from the optical signal sequence detected by the optical signal detection unit 14, and the measurement result is stored in the data storage unit. (Step S2).

Finally, the group delay time difference Δτ _R (λ) at the wavelength λ of the reference optical fiber 12 obtained at step S1 and the group delay time difference Δτ _M at the wavelength λ of the output end of the measured optical fiber 13 obtained at step S2. (Λ) is read from the data storage unit, and the calculation processing unit 15 performs a calculation based on the above equation (1) to calculate the group delay time difference Δτ (λ) of the measured optical fiber 13 at the wavelength λ (step) S3).

According to the present embodiment described above, when measuring the group delay time difference Δτ (λ) at the wavelength λ of the optical fiber 13 to be measured, the reference optical fiber 12 having the group delay time difference Δτ _R (λ) at the wavelength λ is used. With this configuration, the group delay time difference Δτ (λ) at the measurement wavelength λ of any optical fiber 13 to be measured can be evaluated with high accuracy regardless of the magnitude of the group delay time difference Δτ (λ).

Examples of the present invention are shown below. This example has the same configuration as that of the above-described embodiment shown in FIGS. 1 and 2, uses a non-zero dispersion fiber having a length L _R = 28 m as the reference optical fiber 12, and has a length as the measured optical fiber 13. A 1.3 μm-band zero-dispersion fiber with L = 100 m is used, and the group delay time difference Δτ (λ) of the measured fiber 13 is evaluated when the wavelength λ of the pulse light source 11 is 853 nm and 1064 nm, respectively. . In the following, the same members as those shown in FIG.

FIG. 3 and FIG. 4 show the evaluation results obtained by measuring the group delay time difference Δτ _R (λ) of the reference optical fiber 12 at the wavelengths λ = 853 nm and 1064 nm by the process shown in step S1 of FIG.

As shown in FIGS. 3 and 4, in this example, the evaluation results of the group delay time difference Δτ _R (λ) at wavelengths λ = 853 nm and 1064 nm of the reference optical fiber 12 are −20.8 (ns / km), respectively. It was -19.2 (ns / km).

5 and 6 show the evaluation results obtained by measuring the group delay time difference Δτ _R (λ) at the wavelengths λ = 853 nm and 1064 nm of the reference optical fiber 12 by the processing shown in step S2 of FIG. .

As shown in FIGS. 5 and 6, the group delay time differences Δτ _M (λ) at wavelengths λ = 853 nm and 1064 nm at the output end of the measured optical fiber 13 are −2.7 (ns / km) and −4.2, respectively. (Ns / km).

From the above evaluation results, the group delay time difference Δτ (λ) at the wavelengths λ = 853 nm and 1064 nm of the measured optical fiber 13 in this example is calculated by the calculation processing unit 15 in step S3 of FIG. The group delay time difference Δτ _R (λ) of the reference optical fiber 12 shown in FIGS. 3 and 4 and the group delay time difference Δτ _M (λ) at the output end of the optical fiber 13 to be measured shown in FIGS. ) Is calculated as 2.4 (ns / km) and −0.03 (ns / km), respectively.
Δτ (λ) = {Δτ _M (λ) × (L _R + L) −Δτ _R (λ) × L _R } / L (4)

  In this embodiment, the measurement light from the pulse light source 11 is incident on the coupling portion between the pulse light source 11 and the reference optical fiber 12 with an arbitrary axial deviation from the central axis of the reference optical fiber 12. It was assumed that the function was applied, and specifically, an axial deviation of 2.5 μm was given in the evaluation shown in FIGS.

In addition, the pulse light source 11 used in this example has a pulse width P _W of 100 to 300 (ps), and realizes a more preferable measurement condition represented by the following equations (5) and (6). ing.
P _W ≦ | Δτ _R (λ) × L _R | (5)
P _W ≦ | Δτ _R (λ) × L _R + Δτ (λ) × L | (6)

  Below, the effect of a present Example is demonstrated. Table 1 shows the result of calculating the group delay time difference at the wavelengths of 850 nm and 1060 nm of the reference optical fiber and the optical fiber to be measured used in this example by numerical calculation.

  Note that the group delay time difference of each optical fiber was calculated by a general multilayer division approximation method, and the measurement result of the refractive index distribution of the reference optical fiber and the optical fiber to be measured was taken into consideration.

From Table 1, the group delay time difference Δτ _R (λ) of the reference optical fibers at the wavelengths λ = 853 nm and 1064 nm calculated by the group delay time difference evaluation method and evaluation apparatus according to the above-described embodiment, and the group of optical fibers to be measured, respectively. It can be seen that the evaluation result of the delay time difference Δτ (λ) is in good agreement with the numerical calculation result.

7 and 8 show a reference optical fiber in a conventional method for directly observing the propagation time difference between the fundamental mode and the higher-order mode of the measured optical fiber 13, such as the processing shown in step S1 of FIG. This is an evaluation result of the group delay time difference Δτ (λ) at wavelengths λ = 853 nm and 1064 nm of the measured optical fiber 13 calculated by replacing 12 with the measured optical fiber 13. The measurement light from the pulse light source 11 is incident with a 2.5 μm axial shift with respect to the central axis of the reference optical fiber 12, and the pulse width P _W of the pulse light source 11 is set to 100 to 300 (ps). .

As shown in FIG. 7, the group delay time difference Δτ (λ) at the wavelength λ = 853 nm of the optical fiber 13 to be measured is 2.4 ns / km, which is in good agreement with the results in Table 1. However, when the group delay time difference Δτ (λ) at the wavelength λ = 1064 nm of the measured optical fiber 13 is measured using a pulse light source having a pulse width P _W = 100 to 300 (ps) in the conventional configuration, As shown in Table 1, the group delay time difference Δτ (λ) at the wavelength λ = 1060 nm of the measured optical fiber 13 is −0.4 (ns / km) with respect to the pulse width in each mode of the fundamental mode and the higher-order mode. As shown in FIG. 8, the respective modes overlap, and it can be seen that the group delay time difference Δτ (λ) cannot be evaluated.

  That is, in the conventional measurement of the group delay time difference Δτ (λ) at the wavelength λ of the optical fiber 13 to be measured, evaluation is possible if the value of the group delay time difference Δτ (λ) is large, but the group delay time difference Δτ When (λ) is very small, the evaluation cannot be performed.

On the other hand, as shown in FIGS. 5 and 6, the group delay time difference Δτ _M (λ) at the output end of the optical fiber 13 to be measured to which the reference optical fiber 12 is connected can be sufficiently observed. According to the present embodiment using the reference optical fiber, it is possible to measure the group delay time difference Δτ (λ) of the measured optical fiber 13 that could not be determined with the conventional configuration with high accuracy. I understand.

As described above, according to the group delay time difference evaluation method and evaluation apparatus according to the present embodiment, a reference optical fiber having an arbitrary length L _R having a group delay time difference of Δτ _R (λ) at a wavelength λ is used. By using it, the group delay time difference of an arbitrary optical fiber to be measured can be evaluated easily and accurately regardless of the delay time difference depending on the measurement wavelength and the length of the optical fiber.

  The present invention relates to an optical fiber evaluation method, and more particularly to an evaluation method and apparatus for a group delay time difference between propagation modes in a two-mode propagation wavelength region.

It is a conceptual diagram which shows the structural example of the evaluation apparatus of the group delay time difference of the optical fiber which concerns on embodiment of this invention. It is a flowchart which shows the evaluation procedure of the group delay time difference of the optical fiber which concerns on embodiment of this invention. It is a graph which shows the evaluation result of the group delay time difference (DELTA) (tau) _R ((lambda)) of the reference optical fiber in wavelength (lambda) = 853nm measured with the evaluation method in the Example of this invention. It is a graph which shows the evaluation result of group delay time difference (DELTA) (tau) _R ((lambda)) of the reference optical fiber of wavelength (lambda) = 1064nm measured with the evaluation method in the Example of this invention. It is a graph which shows the evaluation result of group delay time difference (DELTA) (tau) _M ((lambda)) in the output end of the to-be-measured optical fiber of wavelength (lambda) = 853nm measured with the evaluation method in the Example of this invention. It is a graph which shows the evaluation result of group delay time difference (DELTA) (tau) _M ((lambda)) in the output end of the to-be-measured optical fiber of wavelength (lambda) = 1064nm measured with the evaluation method in the Example of this invention. It is a graph which shows the evaluation result of group delay time difference (DELTA) (tau) (lambda) of the to-be-measured optical fiber in wavelength (lambda) = 853nm measured with the conventional evaluation method. It is a graph which shows the evaluation result of the group delay time difference (DELTA) (tau) (lambda) of the to-be-measured optical fiber in wavelength (lambda) = 1064nm measured with the conventional evaluation method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Pulse light source 12 Reference optical fiber 13 Optical fiber to be measured 14 Optical signal detection part 15 Arithmetic processing part

Claims (4)

In the evaluation method for evaluating the group delay time difference, which is the difference between the group delay time of the fundamental mode light propagating in the optical fiber and the group delay time of the higher order mode light,
An optical signal oscillated from a pulse light source that oscillates in a single mode at a wavelength λ is shifted from the central axis of a reference optical fiber having an arbitrary length L _R having a group delay time difference Δτ _R (λ) at the wavelength λ. Is incident and
The light emitted from the reference optical fiber is incident on an optical fiber to be measured having an arbitrary length L,
Measure the group delay time difference Δτ _M (λ) at the output end of the optical fiber to be measured,
A method of evaluating a group delay time difference of an optical fiber, wherein a group delay time difference Δτ (λ) at a wavelength λ of the optical fiber to be measured is calculated from the following relational expression.
Δτ (λ) = {Δτ _M (λ) × (L _R + L) −Δτ _R (λ) × L _R } / L In the evaluation method of the group delay time difference of the optical fiber according to claim 1,
Pulse width P of an optical signal oscillated by the pulse light source _W , Length L of the reference optical fiber _R , Group delay time difference Δτ at the wavelength λ of the reference optical fiber _R (Λ), a length L of the optical fiber to be measured, and a group delay time difference Δτ (λ) at the wavelength λ of the optical fiber to be measured satisfy the following relational expression: Evaluation method of group delay time difference of fiber.
P _W ≦ │Δτ _R (Λ) × L _R │
P _W ≦ │Δτ _R (Λ) × L _R + Δτ (λ) × L│ In an evaluation apparatus for evaluating a group delay time difference that is a difference between a group delay time of light in a fundamental mode propagating in an optical fiber and a group delay time of light in a higher-order mode,
A pulsed light source that oscillates in a single mode at a wavelength λ;
Has a differential group delay Δτ _R (λ) at the wavelength lambda, the optical signal emitted from the pulsed light source is incident, any length you incident light emitted to the measured optical fiber of any length L L _R reference optical fiber,
A coupling function for injecting the signal light from the pulsed light source with an axial shift with respect to a central axis of the reference optical fiber;
And an optical signal detection function of detecting an optical signal on the time axis of the exit end of the optical fiber to be measured,
And an arithmetic processing function for calculating a group delay time difference Δτ (λ) at a wavelength λ of the optical fiber to be measured from the following relational expression based on a result detected by the optical signal detection function. Group delay time evaluation device.
Δτ (λ) = {Δτ _M (λ) × (L _R + L) −Δτ _R (λ) × L _R } / L In the evaluation apparatus of the group delay time difference of the optical fiber according to claim 3,
Pulse width P of an optical signal oscillated by the pulse light source _W , Length L of the reference optical fiber _R , Group delay time difference Δτ at the wavelength λ of the reference optical fiber _R (Λ), a length L of the optical fiber to be measured, and a group delay time difference Δτ (λ) at the wavelength λ of the optical fiber to be measured satisfy the following relational expression: Evaluation device for difference in group delay time of fiber.
P _W ≦ │Δτ _R (Λ) × L _R │
P _W ≦ │Δτ _R (Λ) × L _R + Δτ (λ) × L│

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