JP6393563B2 - Optical fiber evaluation method and evaluation apparatus - Google Patents

Description

  The present invention is a method and apparatus for evaluating the properties of an optical fiber. The present invention is preferably used in the information communication field.

  In general, characteristics such as a mode field diameter (MFD), a cutoff wavelength, and chromatic dispersion are required for an optical fiber. On the other hand, these characteristics vary in the longitudinal direction of the optical fiber due to a change in the refractive index distribution in the longitudinal direction that occurs in the optical fiber manufacturing process. Therefore, even if various characteristics are evaluated at both ends of the optical cable, the values are not always guaranteed over the entire longitudinal direction. Therefore, for example, the backscattered light intensity waveform measured from both directions of the single mode fiber (SMF) as in Patent Documents 1 and 2 is analyzed, and the distribution characteristics in the longitudinal direction of the MFD and chromatic dispersion in the optical fiber are determined. A technique for evaluating by destruction is disclosed.

JP-A-6-213770 JP 2007-298335 A

  In recent years, several mode fibers (FMF, Few mode fiber) have been proposed as one of large-capacity transmission technologies that exceed the existing transmission technologies using SMF. In SMF, one mode propagates. On the other hand, FMF is an optical fiber through which a plurality of modes propagate. In a transmission system using FMF (mode division multiplex transmission), by loading a signal in each propagation mode, multiplicity (specifically, spatial multiplicity) can be improved and transmission capacity can be increased.

  Similar to the SMF, the FMF is required to evaluate the distribution characteristics in the longitudinal direction. However, the characteristics of FMF cannot be evaluated by the same method as SMF. This is because, in the FMF, a phenomenon called mode coupling occurs in which optical signals placed in each mode interfere with each other, and the mode coupling becomes a noise component.

As another problem, there is a method for evaluating an effective area (A _eff ) of a higher-order mode propagating through the FMF. In constructing mode division multiplex transmission using FMF, it is important to establish a method for evaluating A _eff of higher-order modes. In SMF (only the basic mode is propagated), the far field pattern (FFP) method or the like is used for the evaluation of A _eff . However, in FMF, since the electric field distributions of the fundamental mode and the higher order mode are different, A _{eff of} the higher order mode cannot be evaluated. As another evaluation method of A _eff , there is a near field pattern (NFP) method. The NFP method can evaluate A _eff even in a higher-order mode, but the NFP method has a problem that accuracy is low.

  This invention is made | formed in view of the said situation, and makes it a subject to provide the evaluation method and evaluation apparatus of an optical fiber which can evaluate the characteristic of several mode fiber.

As a solution to the above problem, the present inventor measured i) the backscattered light intensity waveform when incident from both directions, and ii) the mode coupling characteristics, and developed these results and the conventional bidirectional theory. It was invented that the characteristics in the longitudinal direction of FMF can be evaluated by using theory. Furthermore, it has been found that using this evaluation method, it is possible to evaluate higher-order mode A _eff , which has been difficult to measure conventionally.

In order to solve the above-mentioned problems, the present invention is directed to an optical fiber transmission line having two different types of reference fibers and several mode fibers, and a pulse light having a wavelength λ is incident on the optical fiber transmission line in the longitudinal direction z. As a function, backscattered light intensities S ₁ (λ, z) and S ₂ (λ, z) (unit: dB) from both ends of the optical fiber transmission line are measured, and the backscattered light intensity S ₁ (λ , Z) and S ₂ (λ, z) and the mode coupling coefficient of the number mode fiber measured by an arbitrary method, the loss component I (λ, z) depending on the structural component of the number mode fiber is obtained. An evaluation method of an optical fiber, characterized by being evaluated.

In addition, the present invention is a method in which pulsed light having a wavelength λ is incident on an optical fiber transmission line having two different types of reference fibers and a several mode fiber, and as a function of the longitudinal position z of the optical fiber transmission line, The backscattered light intensities S ₁ (λ, z) and S ₂ (λ, z) (unit: dB) from both ends of the optical fiber transmission line are measured, and the backscattered light intensities S ₁ (λ, z) and S ₂ (λ, z) and a mode coupling coefficient of the number mode fiber measured by an arbitrary method, a loss component I (λ, z) depending on the structure component of the number mode fiber is derived, and the structure A mode field at an arbitrary position z in the optical fiber transmission line using a loss component I (λ, z) depending on the component and a loss component depending on the mode field diameter and the structural component of the two different types of reference fibers. Diameter, ratio There is provided an optical fiber evaluation method characterized by evaluating a distribution characteristic selected from a refractive index difference, waveguide dispersion, and wavelength dispersion.

Further, the present invention is directed to an optical fiber transmission line having a wavelength λ incident on an optical fiber transmission line having a first reference fiber and a second reference fiber, which are two different types of reference fibers, and a number mode fiber. As a function of the longitudinal position z, backscattered light intensities S ₁ (λ, z) and S ₂ (λ, z) (unit: dB) from both ends of the optical fiber transmission line are measured and the backscatter is measured. From the light intensities S ₁ (λ, z) and S ₂ (λ, z) and the mode coupling coefficient of the number mode fiber measured by an arbitrary method, the loss component I depending on the structural component of the number mode fiber is obtained. (Λ, z) and the loss C (z) due to mode coupling of the several mode fiber are derived, and the loss component I (λ, z) depending on the structural component and the loss C (z) due to mode coupling are different from the above. 2 types of references Using the mode field diameter of the fiber, the loss component depending on the structural component, and the loss C (z) due to the mode coupling, the mode field diameter of the first reference fiber is 2w (λ, z ₀ ), and the structural component of the first reference fiber I (λ, z ₀ ), the loss C (z ₀ ) due to mode coupling of the first reference fiber, the mode field diameter of the second reference fiber is 2w (λ, z ₁ ), and the second reference fiber. The distribution of the mode field diameter at an arbitrary position z of the optical fiber transmission line is defined as I (λ, z ₁ ) and the loss C (z ₁ ) due to mode coupling of the second reference fiber. An evaluation method of an optical fiber is provided, which is evaluated according to an expression (11) described later.

In order to solve the above-mentioned problems, the present invention is directed to an optical fiber transmission line having two different types of reference fibers and several mode fibers, and a pulse light having a wavelength λ is incident on the optical fiber transmission line in the longitudinal direction z. Means for measuring the backscattered light intensities S ₁ (λ, z) and S ₂ (λ, z) (unit: dB) from both ends of the optical fiber transmission line, and the backscattered light intensity S ₁ From (λ, z) and S ₂ (λ, z) and the mode coupling coefficient of the number mode fiber measured by an arbitrary method, the loss component I (λ, z) depending on the structural component of the number mode fiber. And an optical fiber evaluation device.

In addition, the present invention is a method in which pulsed light having a wavelength λ is incident on an optical fiber transmission line having two different types of reference fibers and a several mode fiber, and as a function of the longitudinal position z of the optical fiber transmission line, Means for measuring backscattered light intensities S ₁ (λ, z) and S ₂ (λ, z) (unit: dB) from both ends of the optical fiber transmission line; and the backscattered light intensity S ₁ (λ, z) And S ₂ (λ, z) and a mode coupling coefficient of the number mode fiber measured by an arbitrary method to derive a loss component I (λ, z) depending on the structural component of the number mode fiber And a loss component I (λ, z) depending on the structural component, and a loss component depending on the mode field diameter and the structural component of the two different types of reference fibers, and an arbitrary position of the optical fiber transmission line Mode at z And a means for evaluating a distribution characteristic selected from a field diameter, a relative refractive index difference, waveguide dispersion, and wavelength dispersion.

Further, the present invention is directed to an optical fiber transmission line having a wavelength λ incident on an optical fiber transmission line having a first reference fiber and a second reference fiber, which are two different types of reference fibers, and a number mode fiber. Means for measuring backscattered light intensities S ₁ (λ, z) and S ₂ (λ, z) (unit: dB) from both ends of the optical fiber transmission line as a function of the longitudinal position z of The loss depending on the structural component of the number mode fiber from the backscattered light intensities S ₁ (λ, z) and S ₂ (λ, z) and the mode coupling coefficient of the number mode fiber measured by an arbitrary method. Means for deriving a component I (λ, z) and a loss C (z) due to mode coupling of the number mode fiber; a loss component I (λ, z) depending on the structural component; and a loss C (z) due to mode coupling And the different The mode field diameter of the first reference fiber is set to 2w (λ, z ₀ ), the first reference fiber mode field diameter, the loss component depending on the structural component, and the loss C (z) due to mode coupling. The loss component depending on the structural component of the reference fiber is I (λ, z ₀ ), the loss C (z ₀ ) due to the mode coupling of the first reference fiber, and the mode field diameter of the second reference fiber is 2w (λ, z ₁ ). , The loss component depending on the structural component of the second reference fiber is I (λ, z ₁ ), and the loss C (z ₁ ) is due to mode coupling of the second reference fiber. An apparatus for evaluating an optical fiber, comprising: means for evaluating a distribution of field diameters using an expression (11) described later.

The optical fiber evaluation apparatus may include a mode converter and a mode multiplexer / demultiplexer as means for connecting between the OTDR capable of switching a light receiving port by a switch and the OTDR and the optical fiber transmission line. Good. This evaluation apparatus measures the backscattered light intensity S ₁ (λ, z) and S ₂ (λ, z) (unit: dB) by bidirectional OTDR, and obtains each of the light-receiving ports obtained for each mode. From the OTDR waveform, the mode coupling coefficient of the several mode fiber can be obtained.

  ADVANTAGE OF THE INVENTION According to this invention, the novel optical fiber evaluation method and evaluation apparatus which can evaluate the characteristic of several mode fiber can be provided.

It is a schematic diagram which shows an example of the measurement system of bidirectional | two-way OTDR. It is a schematic diagram which shows an example of the measurement system which calculates | requires C (z). In Example 1, it is a graph of the OTDR waveform measured from both directions. In Example 1, it is a graph of the measurement result of the loss component depending on the structural component. In Example 1, it is a graph of the OTDR waveform of a 1st mode and a 2nd mode. In Example 1, it is a graph of the measurement result of MFD distribution. In Example 1, it is a graph of the measurement result of relative refractive index difference (DELTA) distribution.

As an embodiment, a method including the following first to third steps will be described.
As a first step, the backscattered light intensity is measured by OTDR from both ends of the optical fiber transmission line including the FMF. As a second step, the mode coupling coefficient of FMF is measured. As a third step, the characteristics of the FMF are evaluated from the results of the first and second steps. The order of the first step and the second step is not limited, and either may be performed first.

  In this specification, OTDR refers to optical time domain reflectometry (OTDR method) or optical time domain reflectometer (OTDR apparatus).

When the end of an optical fiber (transmission path) is specified and referred to as the backscattered light intensity from that end, it means the backscattered light intensity measured by connecting an OTDR to the end.
The backscattered light intensity from both ends means the backscattered light intensity from one end and the backscattered light intensity from the other end (generic name for both). The backscattered light intensity from both ends of the optical fiber transmission line is two backscattered light intensities obtained by bidirectional OTDR measurement of the optical fiber transmission line.

  When specifying the position on the coordinates along the longitudinal direction of the optical fiber (transmission path) (including the case where the position does not coincide with the end and the case where the position coincides with the end), and the backscattered light intensity at that position Means the backscattered light intensity measured at the position after being scattered back to the OTDR.

  FIG. 1 shows an example of a bidirectional OTDR measurement system. In this measurement system, the optical fiber transmission line 10 is obtained by connecting the two reference fibers 11 and 12 to the test fiber 13 to be measured. The reference fibers 11 and 12 are the same FMF as the test fiber 13. Both ends of the optical fiber transmission line 10 are defined as a first end 14 and a second end 15, respectively. The optical fiber transmission line 10 may be, for example, an optical fiber link in an optical fiber transmission system. The optical fiber transmission line is not necessarily composed of only an optical fiber, and can include various optical components.

  An OTDR 21 for measuring the intensity of backscattered light from the first end 14 is connected to the first end 14 of the optical fiber transmission line 10. The second end 15 is connected to an OTDR 22 for measuring the backscattered light intensity from the second end 15. Since bidirectional OTDR measurement can be performed one end at a time, it is not necessary to connect both OTDRs 21 and 22 to the optical fiber transmission line 10 at the same time.

To simplify the explanation, consider the case where the FMF propagates in two modes. For convenience of explanation, one of the two modes (basic mode and higher-order mode) is called a first mode, and the other is called a second mode. The coordinate position along the longitudinal direction of the optical fiber transmission line 10 is z, the total length of the optical fiber transmission line 10 including the reference fibers 11 and 12 is L, and the wavelength of the pulsed light incident on the optical fiber transmission line 10 is Expressed as λ. At this time, the backscattered light intensity from both ends of the optical fiber transmission line 10 having the length L can be described by the following expressions (1) and (2). S ₁ = 10logP ₁ (λ, z) in equation (1) represents the backscattered light intensity (unit: dB) from the end of z = 0, and S ₂ = 10logP ₂ (λ, z) in equation (2). ) Represents the backscattered light intensity (unit: dB) from the end of z = L.

Here, _{P 1} of _{P 0} of the formula (1) (2) is the incident power (constant) from OTDR. α _s is the fiber scattering coefficient, B (λ, z) is the capture rate for the backscattered light, α is the fiber loss, and h is the mode coupling coefficient. log is a common logarithm (base 10 logarithm). e is the exponential function exp or the base of the natural logarithm ln (Napier's number).

  With bidirectional OTDR, the loss component I (λ, z) depending on the structural component of the fiber can be described by the following equation (3).

  In Formula (3), the same symbols as in Formula (1) and Formula (2) have the same meaning. C (z) is defined by the following formula (4).

  The capture rate B (λ, z) can be described by the following equation (5).

  Here, n represents the refractive index of the core of the optical fiber, and 2w (λ, z) represents the mode field diameter (MFD) at the wavelength λ and the distance z. π is the circumference ratio. Substituting equation (5) into equation (3) and rearranging results in the following equation (6).

Here, a ₀ represents a constant term in which components independent of z are collected.
Next, Equation (6) is normalized as Equation (7) with a loss component I (λ, z ₀ ) depending on the structural component at the reference point z = z ₀ on the first reference fiber.

  Here, the equation (7) is rewritten as the following equation (8).

Equation (7) also holds at the reference point z = z ₁ on the second reference fiber. For this reason, A (λ) and B (λ) in Equation (8) are obtained as Equation (9) and Equation (10), respectively.

  Therefore, MFD of FMF is calculated | required by following Formula (11) from Formula (8)-(10).

  According to equation (11), C (z) needs to be obtained in order to obtain MFD. C (z) can be obtained by measuring the backscattered light intensity of the first mode and the second mode when the first mode is excited using, for example, an OTDR and a mode exciter.

  FIG. 2 shows an example of a measurement system for obtaining C (z). The OTDRs 31 and 34 receive pulsed light incident on the optical fiber transmission line 10 and receive backscattered light returning from the optical fiber transmission line 10. The mode exciters 32 and 33 include ports 32 a and 33 a for exciting the first mode, ports 32 b and 33 b for exciting the second mode, and ports 32 c and 33 c connected to the optical fiber transmission line 10. Have.

  The OTDRs 31 and 34 include switches (not shown) that switch the light receiving ports of the OTDRs 31 and 34. With this switch (optical switch), the OTDR 31 or 34 can receive the backscattered light at the desired port (32a or 32b in the case of OTDR31, 33a or 33b in the case of OTDR34). Mode exciters 32 and 33 are provided as means for connecting between the OTDRs 31 and 34 and the optical fiber transmission line 10 (the first end 14 or the second end 15 thereof). The mode exciters 32 and 33 may include a mode converter and a mode multiplexer / demultiplexer (not shown). The mode multiplexer / demultiplexer is used to excite, multiplex, or separate optical signals having different modes in, for example, mode multiplexing transmission. In the present embodiment, a mode multiplexer / demultiplexer can be used to separate the first mode from the optical fiber transmission line 10 into the ports 32a and 33a and the second mode into the ports 32b and 33b. In addition, when the optical transmission path on the OTDR 31 and 34 side is an SMF, the mode can be converted by a mode converter so that light can efficiently propagate between the SMF and the FMF on the optical fiber transmission path 10 side. preferable. Mode conversion may be performed for all modes among a plurality of modes in which FMF can propagate, and mode conversion may be performed for only some modes (for example, higher-order modes).

  When the mode is excited from the port 32 a of the mode exciter 32 connected to the first end 14 of the optical fiber transmission line 10, the first mode light obtained from the OTDR 31 through the mode exciter 32 is the reference fibers 11, 12. Through the test fiber 13. When backscattered light is generated in the test fiber 13, the backscattered light is separated into the first mode and the second mode in the mode multiplexer / demultiplexer of the mode exciter 32. The backscattered light in the first mode is received by the OTDR 31 via the port 32a, and the backscattered light in the second mode is received via the port 32b.

  It is also possible to make pulsed light incident on the second end 15 of the optical fiber transmission line 10. In this case, the first mode light obtained from the mode exciter 33 is incident on the test fiber 13 and the backscattered light is received by the OTDR 34. The configurations and functions of the OTDR 34 and the mode exciter 33 are the same as those of the OTDR 31 and the mode exciter 32, respectively. According to this configuration, in addition to C (z), bi-directional OTDR (see FIG. 1) can be measured using the apparatus of FIG. In the bidirectional OTDR measurement, the backscattered light intensity of the first mode when the first mode (for example, LP01) is excited may be used. Incidentally, the above formula (1) and the formula (12) described later are equivalent to each other.

The backscattered light intensity (unit: dB) of the first mode when the first mode is excited is expressed as S ₁ (z) = 10 logP ₁ (z), and the backscatter of the second mode when the first mode is excited. When the light intensity (unit: dB) is represented by S ₃ (z) = 10 log P ₃ (z), S ₁ (z) and S ₃ (z) are described as in Expression (12) and Expression (13), respectively. it can.

Here, P ₀ is the incident power, α _s is the scattering coefficient, B (z) is the capture rate, α is the fiber loss, and h is the mode coupling coefficient. When the capture rate in the first mode is represented by B ₁ and the capture rate in the second mode is represented by B ₂ , the capture rate B is equal to B ₁ = B ₂ . Therefore, from the equations (12) and (13), the apparent inter-mode crosstalk XT _otdr is obtained by the following equation (14).

Therefore, the average mode coupling coefficient h ^* is obtained from the equation (14). By substituting h ^* into h in formula (4), C (z) can be obtained.

From the above, it can be seen that the longitudinal distribution of MFD in FMF can be evaluated. Note that the method of measuring the average mode coupling coefficient h ^* is not limited to the method described above. Other measurement methods include an impulse response method (see Reference 1) and an intensity modulation method (Reference 2).

Reference 1: Masahiro Ikeda, “Transmission characteristics of multimode optical fiber”, Nippon Telegraph and Telephone Public Corporation Research Report on Practical Use, vol. 26, no. 9, pp. 2621-2645 (1977).
Reference 2: T Mizuno, et al. "Modal Cross Measurement Based on Intensity Tone for Few-Mode Fiber Transmission Systems", OFC 2014, W3D. 5, 2014.

  Further, as with the SMF, the distribution characteristics of the relative refractive index difference, the waveguide dispersion, and the chromatic dispersion can be evaluated by further expanding the equation.

First, a method for evaluating the relative refractive index difference will be described. The loss component I _n (λ, z) depending on the normalized structural component can be rewritten as the following equation (15).

Here, with respect to the core refractive index n (z) of the test fiber and the core refractive index n (z ₀ ) of the reference fiber, it is assumed that the squares of both are equal to each other as in the following equation (16).

Further, the scattering coefficient α _s is proportional to the Rayleigh scattering coefficient R expressed by the following equation (17).

Here, R ₀ represents the Rayleigh scattering coefficient of pure quartz, Δ represents the relative refractive index difference, and k is a proportional constant.

  Therefore, when Expression (17) is substituted into Expression (15), the following Expression (18) is obtained.

Further, the relative refractive index difference values Δ (z ₀ ) and Δ (z (z) at the two reference points z ₀ and z ₁ are derived in the same manner as the MFD is derived from the equations (8) to (11). _{When 1} ) is used, the longitudinal distribution Δ (z) of the relative refractive index difference of the test fiber can be evaluated by the following equation (19).

  From the above, the relative refractive index difference (z) at the position z is a loss depending on the relative refractive index difference and MFD values at the two reference points, the MFD longitudinal distribution characteristics of the test fiber, and the standardized structural components. It can be determined from the components. This method uses the fact that the scattering coefficient depends on the relative refractive index difference.

Next, a method for evaluating the chromatic dispersion distribution using these evaluation results will be described.
In general, the wavelength dispersion D is as in Equation (20), given by the sum of the material dispersion D _m and the waveguide dispersion D _w.

Each material dispersion _{D m} and waveguide dispersion _{D w,} can be evaluated by the following equation (21) and (22).

Here, λ is the wavelength in vacuum, c is the speed of light in vacuum, n is the refractive index of the core, w is the mode field radius (MFR), and π is the circumference. The material dispersion D _m can be evaluated from the dopant concentration of the optical fiber by using a Selmeier relational expression. The Selmeier relational expression is an approximate expression representing the wavelength dependence of the refractive index, and the coefficient value can be determined for each material. The dopant concentration can be obtained when the relative refractive index difference Δ of the optical fiber is known. On the other hand, waveguide dispersion _{D w,} from equation (22), the wavelength dependence of MFR, i.e., can be evaluated by knowing the w (lambda, z).

An empirical formula (see Reference 3) such as the following formula (23) is given to the wavelength dependence of MFR.
Reference 3: D. Marcus, “Loss analysis of single-mode fiber splices,” Bell Syst. Tech. J. et al. , Vol. 56, pp. 703-718, 1977.

However, a represents a core radius, λ _c represents a cutoff wavelength, and v represents a normalized frequency. p ₀ , p ₁ , p ₂ , q ₀ , q ₁ , q ₂ are coefficients. Here, by replacing q ₀ = g ₀ / a, q ₁ = g ₁ λ _c ^1.5 / a, and q ₂ = g ₂ λ _c ⁶ / a, the MFR is a function w (λ) of the wavelength λ. Is approximated by the equation (24).

The coefficients g ₀ , g ₁ , and g ₂ can be calculated by evaluating MFD at three wavelengths or more. Then, when Expression (24) is substituted into Expression (22), Expression (25) is obtained. At this time, w in equation (22) and w (λ) in equation (24) are replaced with w (λ, z).

Therefore, by obtaining the coefficients g ₀ , g ₁ , and g ₂ , the waveguide dispersion D _w at the position z can be evaluated using w (λ, z).
As described above, the distribution characteristics of MFD, relative refractive index difference, waveguide dispersion, and chromatic dispersion can be evaluated by measuring the longitudinal characteristics (OTDR waveform) of backscattered light intensity and the mode coupling coefficient in FMF.

As mentioned above, although this invention has been demonstrated based on suitable embodiment, this invention is not limited to the above-mentioned embodiment, A various change is possible in the range which does not deviate from the summary of this invention.
As an optical fiber evaluation apparatus (system), in addition to an OTDR apparatus and a mode multiplexer / demultiplexer, an apparatus provided with a program, a controller, a computer, etc. as means for performing calculations necessary for carrying out the above-described evaluation method can be cited. It is done. The evaluation apparatus preferably includes means for measuring the mode coupling coefficient of the several mode fiber.

  Hereinafter, the present invention will be specifically described with reference to examples.

Example 1
In order to confirm the effectiveness of the measurement method according to the present invention, the distribution of MFD and relative refractive index difference Δ of an optical fiber transmission line (see FIG. 1) composed of one test fiber and two reference fibers was measured. Both the test fiber and the reference fiber are two-mode fibers that propagate only the LP01 mode and the LP11 mode in the 1.55 μm band.

  Table 1 shows the parameters of each fiber used for the measurement. MFD is the result of measurement by the FFP method using a number m at the end of the optical fiber as a sample. Similarly, the relative refractive index difference is a result of measurement by a refractive near field (RNF) method using a few meters at the end of the optical fiber as a sample.

  The OTDR waveform measured from both directions of the optical fiber transmission line is shown in FIG. The wavelength of OTDR is 1.55 μm, the pulse width is 100 ns, and the averaging time is 3 minutes. All optical fiber connections are fusion splices.

  Next, the loss component depending on the structural component was obtained from the measurement result of FIG. The result is shown in FIG. This loss component includes a loss component due to mode coupling.

Next, the backscattered light intensity S ₁ (z) in the first mode and the backscattered light intensity S ₃ (z) in the second mode are measured using a measurement system (see FIG. 2) including an OTDR and a mode exciter. did. The measurement results of these OTDR waveforms are shown in FIG. S ₁ (z) represents the LP01 mode backscattered light, and S ₃ (z) represents the backscattered light that has passed through the LP11 mode mode coupler. The measurement wavelength is 1.55 μm. As in the above equation (14), the mode coupling coefficient can be derived from the difference between S ₁ (z) and S ₃ (z), and the loss component due to mode coupling can be evaluated.

  FIG. 6 shows the measurement result of the MFD distribution obtained by the above equation (11). This result is in good agreement with the result of the FFP method (see Table 1). FIG. 7 shows the measurement result of the relative refractive index difference Δ distribution obtained by the above equation (19). It can be seen that this result substantially matches the result of the RNF method (see Table 1).

DESCRIPTION OF SYMBOLS 10 ... Optical fiber transmission line 11, 12 ... Reference fiber, 13 ... Test fiber (several mode fiber), 14 ... 1st end, 15 ... 2nd end, 21, 22, 31, 34 ... OTDR, 32, 33 ... Mode exciter, 32a, 32b, 32c, 33a, 33b, 33c ... port.

Claims (7)

A pulse light having a wavelength λ is incident on an optical fiber transmission line having two different types of reference fibers and a number mode fiber, and the both ends of the optical fiber transmission line are a function of the longitudinal position z of the optical fiber transmission line. Backscattered light intensity S ₁ (λ, z) and S ₂ (λ, z) (unit: dB) from
The backscattered light intensity S ₁ (λ, z) and S ₂ (λ, z) and the mode coupling coefficient of the number mode fiber measured by an arbitrary method depended on the structural components of the number mode fiber. An evaluation method of an optical fiber, characterized by evaluating a loss component I (λ, z). A pulse light having a wavelength λ is incident on an optical fiber transmission line having two different types of reference fibers and a number mode fiber, and the both ends of the optical fiber transmission line are a function of the longitudinal position z of the optical fiber transmission line. Backscattered light intensity S ₁ (λ, z) and S ₂ (λ, z) (unit: dB) from
The backscattered light intensity S ₁ (λ, z) and S ₂ (λ, z) and the mode coupling coefficient of the number mode fiber measured by an arbitrary method depended on the structural components of the number mode fiber. Deriving the loss component I (λ, z)
Using the loss component I (λ, z) depending on the structural component and the loss component depending on the mode field diameter and the structural component of the two different types of reference fibers, at an arbitrary position z of the optical fiber transmission line An evaluation method of an optical fiber, characterized by evaluating a distribution characteristic selected from a mode field diameter, a relative refractive index difference, waveguide dispersion, and wavelength dispersion. An optical fiber evaluation method according to claim 2,
Pulse light of wavelength λ is incident on an optical fiber transmission line having two different types of reference fibers, a first reference fiber and a second reference fiber, and a number mode fiber, and the longitudinal position z of the optical fiber transmission line Measure backscattered light intensities S ₁ (λ, z) and S ₂ (λ, z) (unit: dB) from both ends of the optical fiber transmission line as a function of
The backscattered light intensity S ₁ (λ, z) and S ₂ (λ, z) and the mode coupling coefficient of the number mode fiber measured by an arbitrary method depended on the structural components of the number mode fiber. Deriving a loss component I (λ, z) and a loss C (z) due to mode coupling of the number mode fiber;
The loss component I (λ, z) depending on the structural component and the loss C (z) due to mode coupling, the mode field diameter of the two different types of reference fibers, the loss component depending on the structural component, and the loss C due to mode coupling (Z), the mode field diameter of the first reference fiber is 2w (λ, z ₀ ), the loss component depending on the structural component of the first reference fiber is I (λ, z ₀ ), and the first reference fiber The loss C (z ₀ ) due to mode coupling, the mode field diameter of the second reference fiber is 2w (λ, z ₁ ), the loss component depending on the structural component of the second reference fiber is I (λ, z ₁ ), the second As the loss C (z ₁ ) due to the mode coupling of the reference fiber, the distribution of the mode field diameter at an arbitrary position z in the optical fiber transmission line is expressed by the following equation (1)

An evaluation method for an optical fiber, characterized by: A pulse light having a wavelength λ is incident on an optical fiber transmission line having two different types of reference fibers and a number mode fiber, and the both ends of the optical fiber transmission line are a function of the longitudinal position z of the optical fiber transmission line. Means for measuring the backscattered light intensity S ₁ (λ, z) and S ₂ (λ, z) (unit: dB) from
The backscattered light intensity S ₁ (λ, z) and S ₂ (λ, z) and the mode coupling coefficient of the number mode fiber measured by an arbitrary method depended on the structural components of the number mode fiber. Means for evaluating the loss component I (λ, z);
An optical fiber evaluation apparatus comprising: A pulse light having a wavelength λ is incident on an optical fiber transmission line having two different types of reference fibers and a number mode fiber, and the both ends of the optical fiber transmission line are a function of the longitudinal position z of the optical fiber transmission line. Means for measuring the backscattered light intensity S ₁ (λ, z) and S ₂ (λ, z) (unit: dB) from
The backscattered light intensity S ₁ (λ, z) and S ₂ (λ, z) and the mode coupling coefficient of the number mode fiber measured by an arbitrary method depended on the structural components of the number mode fiber. Means for deriving the loss component I (λ, z);
Using the loss component I (λ, z) depending on the structural component and the loss component depending on the mode field diameter and the structural component of the two different types of reference fibers, at an arbitrary position z of the optical fiber transmission line Means for evaluating distribution characteristics selected from mode field diameter, relative refractive index difference, waveguide dispersion, and chromatic dispersion;
An optical fiber evaluation apparatus comprising: An optical fiber evaluation apparatus according to claim 5,
Pulse light of wavelength λ is incident on an optical fiber transmission line having two different types of reference fibers, a first reference fiber and a second reference fiber, and a number mode fiber, and the longitudinal position z of the optical fiber transmission line Means for measuring backscattered light intensities S ₁ (λ, z) and S ₂ (λ, z) (unit: dB) from both ends of the optical fiber transmission line as a function of
The backscattered light intensity S ₁ (λ, z) and S ₂ (λ, z) and the mode coupling coefficient of the number mode fiber measured by an arbitrary method depended on the structural components of the number mode fiber. Means for deriving a loss component I (λ, z) and a loss C (z) due to mode coupling of the number mode fiber;
The loss component I (λ, z) depending on the structural component and the loss C (z) due to mode coupling, the mode field diameter of the two different types of reference fibers, the loss component depending on the structural component, and the loss C due to mode coupling (Z), the mode field diameter of the first reference fiber is 2w (λ, z ₀ ), the loss component depending on the structural component of the first reference fiber is I (λ, z ₀ ), and the first reference fiber The loss C (z ₀ ) due to mode coupling, the mode field diameter of the second reference fiber is 2w (λ, z ₁ ), the loss component depending on the structural component of the second reference fiber is I (λ, z ₁ ), the second As the loss C (z ₁ ) due to the mode coupling of the reference fiber, the distribution of the mode field diameter at an arbitrary position z in the optical fiber transmission line is expressed by Equation (2).

Means to evaluate according to
An optical fiber evaluation apparatus comprising: An optical fiber evaluation apparatus according to any one of claims 4 to 6,
As a means for connecting between the OTDR whose light-receiving port can be switched by a switch and the OTDR and the optical fiber transmission line, a mode converter and a mode multiplexer / demultiplexer are provided, and the backscattered light is obtained by bidirectional OTDR. The intensity S ₁ (λ, z) and S ₂ (λ, z) (unit: dB) are measured, and the mode coupling coefficient of the number mode fiber is obtained from each OTDR waveform obtained from the light receiving port for each mode. An apparatus for evaluating an optical fiber, characterized in that

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