JP2003329544A - Effective nonlinear constant measuring method and apparatus for single-mode optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and an apparatus for accurately evaluating the effective nonlinear constant of an optical fiber to be measured at a desired wavelength by using the measurement result of the distribution of refraction of a single-mode optical fiber section and the distribution of an electric field. <P>SOLUTION: The distribution of a refractive index on an arbitrary section of the single-mode optical fiber, and that of the electric field at a desired wavelength are measured. The distribution of a nonlinear diffraction index is obtained by using the distribution of the refractive index and an additive dependency of the nonlinear refractive index determined by a glass material in the single- mode optical fiber. Further, arithmetic processing with the distribution of the electric field is performed, thus accurately evaluating the effective nonlinear refractive index and the effective nonlinear constant of the single-mode optical fiber to be measured. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for measuring an effective nonlinear refractive index and an effective nonlinear constant of a single mode optical fiber.

[0002]

2. Description of the Related Art In recent years, in an optical communication system using an optical amplifying device, as the intensity of light propagating in a single-mode optical fiber increases, deterioration of a propagation waveform due to optical nonlinearity in the single-mode optical fiber, etc. Is a problem. The optical non-linearity in the single mode optical fiber is due to the fact that the refractive index n (P) of the single mode optical fiber changes according to the intensity P of the light according to the relationship represented by the following formula (1). n (P) = n ₀ + n ₂ P (1) where n ₀ represents a linear refractive index and n ₂ represents a non-linear refractive index.

Further, the influence of various optical non-linearities changes depending on the non-linear constant representing the optical non-linearity of the single mode optical fiber. The non-linear constant is given by n ₂ / A _eff using the non-linear refractive index n ₂ of the single mode optical fiber and the effective area A _eff which is a parameter representing the energy density of light. Therefore, in long-distance, large-capacity optical communication using the optical amplifier, it is necessary to accurately know the nonlinear refractive index and the nonlinear constant of the single-mode optical fiber used.

The nonlinear constant of a single-mode optical fiber can be measured by observing various optical nonlinear phenomena occurring in the single-mode optical fiber with respect to a change in input light intensity. Various proposals have been made as shown in the documents 1 to 5. Reference 1: Method using self-phase modulation effect (RHStolen
and C. Lin, "Self-phase-modulation in silica optica
l fibers ", Phy. Rev., vol. 17, No. 4, pp. 1448-1453, 1978.
, Reference 2: Method using self-phase modulation effect (A. Boskovic
et al., "Direct continuous-wave measurement of n2
in various types of telecommunication fiberat 1.55
μm ", Opt.Lett., vol.21, No.24, pp.1966-1968, 1996)
, Reference 3: Method using cross phase modulation effect (A. Wada et.
al., "Measurement of nonlinear-index coefficients o
f optical fibers through the cross-phase modulatio
n using delayed-self-heterodyne technique ", ECOC" 9
3, MoB1.2, pp.45-48, 1993.), Reference 4: Method using four-wave mixing (L.Prigent and J.
P.Hamaide, "Measurement of fiber nonlinear Kerr coef
ficient by four-wave mixing ", Photon.Technol.Lett.,
vol.5, No.9, pp.1092-1095,1993.) Reference 5: Method using modulation instability (M. Artiglia et.
al., "Using modulation instability to determine Kerr
coefficient in optical fibers ", Electron.Lett., vo
l.31, No.12, pp.1012-1013, 1995. )

The effective area A _eff of the single-mode optical fiber at the desired wavelength λ can be evaluated by measuring the electric field distribution on the end face of the single-mode optical fiber using the measuring light of the desired wavelength λ. It is proposed in Reference 6 below. Reference 6: (GPAgrawal, "Nonlinear fiber optics", A
cademic Press.)

Therefore, the nonlinear refractive index difference of the single mode optical fiber can be obtained from the measurement results of the effective area A _eff and the nonlinear constant.

[0007]

However, the measuring method using pulsed light as the measuring light has a problem that the measurement accuracy of the nonlinear constant strongly depends on the pulse shape of the pulsed light.

Even when the pulse shape is properly adjusted, or when continuous light is used as the measuring light, the measurement accuracy of the nonlinear constant depends on the fiber length, wavelength dispersion, and effective of the single mode optical fiber to be measured. There is a problem that it changes depending on the cross-sectional area and the like.

The present invention uses the measurement result of the refractive index distribution in an arbitrary cross section of a single mode optical fiber and the electric field distribution at a desired wavelength λ to determine the nonlinear refraction determined by the glass material used in the optical fiber. It is an object of the present invention to provide an effective non-linear constant measuring method for a single mode optical fiber that is highly accurate and independent of measurement conditions by performing a numerical calculation in consideration of the additive dependence of the index. To do.

[0010]

The first invention of the present invention for solving the above problems is to measure the refractive index distribution in an arbitrary cross section of a single mode optical fiber and the electric field distribution at a desired wavelength λ, Using the additive dependence of the nonlinear refractive index determined by the refractive index distribution and the glass material in the single mode optical fiber, the nonlinear refractive index distribution of the measured single mode optical fiber is obtained, and at the desired wavelength λ A single mode characterized by highly accurately evaluating an effective nonlinear refractive index and an effective nonlinear constant at a desired wavelength λ of a single mode optical fiber to be measured by performing calculation processing with an electric field distribution. It is a method for measuring the effective nonlinear constant of an optical fiber.

According to the present invention, the measurement accuracy in the conventional measuring method using pulsed light strongly depends on the shape of the optical pulse, and also depends on the length, wavelength dispersion and effective cross-sectional area of the optical fiber to be measured. It becomes possible to solve such a problem and obtain a highly accurate nonlinear constant.

A second aspect of the present invention is a means for injecting a measurement light having a desired measurement wavelength into a single mode optical fiber, a means for measuring a refractive index distribution in an arbitrary section of the single mode optical fiber, and a single mode. A means for measuring the electric field distribution at a desired measurement wavelength of the optical fiber, the conversion of the refractive index distribution into a nonlinear refractive index distribution, and the numerical integration with the electric field distribution to obtain the effective nonlinear refractive index of the single mode optical fiber to be measured. And an arithmetic means for calculating the effective nonlinear constant, and an effective nonlinear constant measuring device for a single-mode optical fiber.

That is, the present invention has a function of measuring the refractive index distribution in an arbitrary cross section of a single mode optical fiber and the electric field distribution at a desired wavelength λ. The nonlinear refractive index distribution is obtained by using the additive dependence of the nonlinear refractive index peculiar to the glass material of
Furthermore, the effective non-linear refractive index of the single mode optical fiber to be measured is calculated by numerical calculation processing with the electric field distribution.

Further, the effective non-linear constant of the single mode optical fiber to be measured is calculated by using the effective area A _eff obtained by the numerical calculation of the electric field distribution.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments according to the present invention will be described below, but the present invention is not limited to these embodiments.

The method of evaluating the effective nonlinear constant of the single mode optical fiber according to the embodiment of the present invention is as follows.

In the evaluation method of this embodiment, the radius method r and the refractive index distribution n ₀ (r, λ) at the measurement wavelength λ in an arbitrary cross section of the single mode optical fiber to be measured are determined by the NFP method or the RNFP method. And the addition amount distribution M (r) of the glass material in the single-mode optical fiber to be measured is calculated using the relationship represented by the equation (2) shown in the following "Equation 1". Here, the measurement using the NFP method or the RNFP method is disclosed in the following Documents 7 and 8. Reference 7: D. Marcuse and HMPresby, "Automatic geomet
ric measurement of single-mode and multimode optic
al fibers ", Appl.Opt., vol.18, No.3, pp.402-409,1979. Reference 8: KIWhite," Practical application of the re
fracted nearfield technique for the measurement of
optical fiber refractive index profiles ", Opt.Quan
tum Electron., vol.11, p.185,1979.

[0018]

[Equation 1] Here, n _os (λ) in the equation represents the refractive index of pure quartz at the measurement wavelength λ. Further, K (λ) represents the addition amount dependency of the refractive index with respect to the specific additive at the measurement wavelength λ, and at the wavelength λ of the glass in which the specific additive is added to pure quartz by M ₀ (unit: mol%). By knowing the refractive index n ₀ (M ₀ , λ), the refractive index n ₀ (M ₀ , λ) can be determined using the following equation (3) shown in “Equation 2” below.

[0019]

[Equation 2] The refractive index n ₀ (M ₀ , λ) of the glass in which a specific additive M _{0 is} added to pure quartz at an arbitrary wavelength λ can be calculated by the following formula (4). , A _i and B _i (i = 1, 2, 3) in the equation represent coefficients peculiar to the glass material.

The measurement of the refractive index n ₀ (M ₀ , λ) of a glass in which a specific additive M _{0 is} added to pure quartz at an arbitrary wavelength λ can be performed, for example, in “Reference 9” (JWFleming, “Material di”).
spersion in lightguide glasses ", Electron.Lett., vo
It can be known from measurement examples such as l.14, No.11, pp.326-328, 1978.).

[0021]

[Equation 3]

Next, M in the equation (2) shown in the above "Equation 1"
(r), three reference wavelengths, refractive index distributions at 0.48613, 0.58756, and 0.65627 μm, respectively, n _F (r)
, N _D (r), and n _C (r) are obtained using the relationship of the following equation (5) shown in the following “Equation 4”.

[0023]

[Equation 4]

Further, using the empirical formula expressed by the formula (6) shown in the following "Equation 5", the nonlinear refractive index distribution n ₂ (r) of the single mode optical fiber to be measured [unit: × 10 ^-20] m ² / W] is calculated.

As a reference for calculating the nonlinear refractive index n ₂ from the linear refractive index n ₀ of quartz glass by using the empirical formula expressed by the equation (6) shown in the following “Equation 5”, for example, “Reference 1” is given.
0 "(NLBoling et al," Emprical relationships for
predicting nonlinear refractive index changes in
optical solids ", J.Quantum Electron., QE-14, pp.601-6
08,1978.) Can be mentioned.

[0026]

[Equation 5] Here, v _D (r) in the equation is the equation (7) shown in the following “Equation 6”.
Represents the Abbe number given by.

[0027]

[Equation 6]

Next, the near-field pattern at the desired wavelength λ of the single mode optical fiber to be measured or
The far field pattern is calculated by the NFP method, the FFP method, etc. (Reference 11: Y. Murakami et al., "Cut-off w").
avelength measurements for single-mode optical fiber
rs ", Appl.Opt., vol.18, p.1101,1979., Reference 12: ART
ynes et al., "Low v-number optical fiber: secondary
maxims in the far-field radiation pattern ", J.Opt, S
oc.Am., vol.69, No.11, pp.1587-1596,1979.), and the electric field distribution E at any cross section of the measured fiber.
Calculate (r, λ).

The far field by the FFP method
When the pattern is measured, the electric field distribution E of the single mode optical fiber to be measured is converted by Hankel conversion of the measurement result.
(R, λ) can be calculated.

Next, the above-mentioned nonlinear refractive index distribution n ₂ (r),
And the measurement result of the electric field distribution E (r, λ) are shown in the following "Formula 7".
The effective nonlinear refractive index n _{2 (eff)} (λ) of the fiber to be measured is calculated by performing the arithmetic processing using the relational expression represented by the equation (8).

[0031]

[Equation 7]

Further, it is obtained by the equation (8) shown in the above "Equation 7".
N_{2 (eff)}(Λ) is the effective area A _effDivide by (λ)
And the effective nonlinear constant n of the optical fiber under test is
_{2 (eff)}(Λ) / A_effCalculate (λ). Incidentally, A
_eff(Λ) uses the above E (r, λ) and
8 ”and is calculated by the following equation (9).

[0033]

[Equation 8]

As described above, in the measuring method of the present invention, the effective nonlinear refractive index and the effective refractive index of the single mode optical fiber to be measured are used by using the measurement results of the linear refractive index distribution and the electric field distribution at the desired measurement wavelength. The non-linear constant can be evaluated.

Since the linear refractive index distribution and the electric field distribution can be measured using a continuous wave light source having a desired measurement wavelength and a short optical fiber, the characteristics of the measuring light source and the single mode light to be measured can be measured. It is possible to avoid deterioration of measurement accuracy due to the characteristics of the fiber.

[0036]

EXAMPLES Preferred examples for confirming the effects of the present invention will be described below, but the present invention is not limited thereto.

In the embodiment of the present invention, the RNFP method and the FFP are used.
Method is used to measure the refractive index distribution of a 1.3 μm band zero dispersion fiber (SMF) and the electric field distribution at a desired wavelength of 1.55 μm, and evaluate the effective nonlinear refractive index and effective nonlinear constant of the measured SMF. did.

FIG. 1 is a flow chart showing a processing procedure of an effective nonlinear constant measuring method for a single mode optical fiber according to an embodiment of the present invention.

Step 1: First, as shown in FIG. 1, in the effective nonlinear constant measurement of the single mode optical fiber in the present embodiment, the measurement wavelength λ at an arbitrary cross section of the single mode optical fiber to be measured is measured. Refractive index distribution n ₀ (r,
λ) is measured using the RNFP method or the NFP method (S101).

Step 2: Next, the field pattern in an arbitrary cross section of the single-mode optical fiber to be measured is measured using a measurement light source having a desired wavelength λ and the FFP method or the NFP method (S102).

Step 3: Next, the above Step 1
Refractive index distribution n obtained in (S101) ₀(r, λ) and formula
Using the relationships of (2) to (4), the measured single mode light
A fiber addition amount distribution M (r) is calculated (S103).
The SMF used in the measurement of this example was made of pure quartz and germanium.
It consists of a core part with um added and a pure quartz clad part.
Therefore, in this embodiment, the coefficient A of the equation (4) is _i, B_i
(I = 1, 2, 3)
A calculation process was performed using the values.

At this time, the refractive index n _0s (0.67 μm) of pure quartz at the measurement wavelength of 0.67 μm is 1.4563, and the germanium addition amount dependency K (0.67 μm) of the refractive index is 1.585 × 1.
It becomes 0 ^-3 (unit: 1 / mol%).

[0043]

[Table 1]

Step 4: Next, the above Step 3
Using the addition amount distribution M (r) obtained in (S103) and the relational expression (5), the refractive index distribution n _{F at} three reference wavelengths is calculated.
(r), n _D (r), and n _C (r) are calculated, and the nonlinear refractive index distribution n ₂ (r) is calculated using the relationships of the expressions (6) and (7) (S104).

Step 5: Further, the above Step 2
The far field pattern obtained in (S102) is subjected to Hankel conversion to calculate the electric field distribution E (r, λ) of the measured single mode optical fiber at the desired wavelength λ (S).
105). In step 2 (S102), N
When the near field pattern is measured by the FP method, the calculation process of step 5 (S105) is unnecessary.

Step 6: Next, Step 4
Nonlinear refractive index distribution n in (S104)₂(r) and step
5 (S105), the electric field distribution E (r, λ) is replaced by the equation (8).
The single-mode optical fiber to be measured at the desired wavelength λ.
Effective nonlinear refractive index n_{2 (ef f)}Evaluate (λ)
(S106).

Step 7: Further, the above Step 5
The effective cross-sectional area A _eff (λ) of the measured single-mode optical fiber at the desired wavelength λ is calculated using the relationship between the electric field distribution E (r, λ) in (S105) and the equation (9) (S10).
7).

Step 8: Step 6 (S10)
Using the effective nonlinear refractive index n _{2 (eff)} (λ) of 6) and the effective cross-sectional area A _eff (λ) of step 7, the effective nonlinearity of the measured single-mode optical fiber at the desired wavelength λ. The constant n _{2 (eff)} (λ) / A _eff (λ) is evaluated (S10).
8).

FIG. 2 is a schematic view showing the schematic arrangement of an apparatus for carrying out the method for measuring the nonlinear constant of a single-mode optical fiber according to the embodiment of the present invention. In FIG. 2, reference numeral 11 is a refractive index distribution measuring device, 12 is a field pattern measuring device,
Reference numerals 13 respectively denote arithmetic processing units.

FIG. 3 shows the measurement wavelength λ of the SMF to be measured (= 0.67).
is a graph showing measurement results of the refractive index distribution n ₀ (r, λ) in [mu] m). Here, in FIG. 3, the horizontal axis represents the radius r in an arbitrary measurement cross section of the SMF to be measured, and the vertical axis represents the linear refractive index n _0.
Is represented.

FIG. 4 shows the wavelength λ (= 1.55 μ) of the SMF to be measured.
It is a figure which shows the far field pattern measurement result in m). Here, in FIG. 4, the horizontal axis represents the measured SM.
The rotation angle of the FFP light receiving element in an arbitrary measurement section of F, and the vertical axis represents the normalized light receiving intensity.

FIG. 5 is a diagram showing the calculation processing result of the addition amount distribution M (r) of the glass material of the SMF to be measured, which is calculated by the processing procedure described above using the result of FIG. Here, in FIG. 5, the horizontal axis represents the radius r in an arbitrary measurement cross section of the SMF to be measured, and the vertical axis represents the addition amount M of germanium.

FIG. 6 shows the nonlinear refractive index distribution n ₂ (r) of the SMF to be measured and the electric field intensity distribution E at a wavelength of 1.55 μm, which are calculated by the above-described processing procedure using the results of FIGS. 4 and 5. It is a figure which shows the calculation processing result of ² (r, (lambda)).
Here, in FIG. 6, the horizontal axis represents the radius r in an arbitrary measurement cross section of the SMF to be measured, and the vertical axis represents the nonlinear refractive index n _{2 and the} normalized electric field strength E ² .

By calculating the result of FIG. 6 using the relations of equations (8) and (9), the wavelength 1.
The effective nonlinear refractive index n _{2 (eff)} (λ) and the nonlinear constant n _{2 (eff)} (λ) / A _eff (λ) at 55 μm can be evaluated as follows.

Effective Nonlinear Refractive Index n _{2 (eff)} (λ) 2.66 × 10 ^-20 m ² / W Effective Nonlinear Constant n _{2 (eff)} (λ) / A _eff (λ) 3.36 × 10 ^-10 W ^-1

[0056]

As described above, according to the present invention,
The effective non-linear refractive index of the measured single mode optical fiber is calculated by processing the refractive index distribution measured in an arbitrary cross section of the measured single mode optical fiber and the field pattern at the desired measurement wavelength. The effective nonlinear constant can be evaluated independent of the characteristics of the single mode optical fiber under test.

According to the present invention, the measurement accuracy in the conventional measuring method using pulsed light strongly depends on the shape of the optical pulse, and also depends on the length, wavelength dispersion, and effective sectional area of the optical fiber to be measured. It becomes possible to solve such a problem and obtain a highly accurate nonlinear constant.

Further, since the refractive index distribution and the field pattern can be measured using a continuous wave light source having a desired wavelength, there is no problem that the measurement accuracy varies depending on the characteristics of the measurement light source.

[Brief description of drawings]

FIG. 1 is a flowchart showing a processing procedure of an effective nonlinear constant measuring method for a single mode optical fiber according to an embodiment of the present invention.

FIG. 2 is a diagram showing a schematic configuration of a measuring apparatus that implements an effective nonlinear constant measuring method for a single-mode optical fiber according to an embodiment of the present invention.

FIG. 3 is a diagram showing a measurement result of a refractive index distribution n ₀ (r, λ) at a measurement wavelength of 0.67 μm of an SMF to be measured in an example of the present invention.

FIG. 4 is a diagram showing a measurement result of a far field pattern at a desired wavelength of 1.55 μm of an SMF to be measured in an example of the present invention.

FIG. 5 is a diagram showing a calculation processing result of a glass material addition amount distribution M (r) of a measured SMF in an example of the present invention.

FIG. 6 is a diagram showing a calculation processing result of a nonlinear refractive index distribution n ₂ (r) of an SMF to be measured and an electric field intensity distribution E ² (r, λ) at a desired wavelength of 1.55 μm in an example of the present invention.

[Explanation of symbols]

11 Refractive index distribution measuring device 12 field pattern measuring device 13 Arithmetic processing unit

Claims (2)

[Claims] 1. A non-linearity determined by measuring a refractive index distribution in an arbitrary cross section of a single mode optical fiber and an electric field distribution at a desired wavelength λ and determining a refractive index distribution and a glass material in the single mode optical fiber. Using the additive dependence of the refractive index, the nonlinear refractive index distribution of the single-mode optical fiber to be measured is calculated, and the calculation is performed with the electric field distribution at the desired wavelength λ to obtain the single-mode optical fiber to be measured. A method for measuring an effective nonlinear constant of a single-mode optical fiber, which comprises highly accurately evaluating an effective nonlinear refractive index and an effective nonlinear constant at a desired wavelength λ. 2. Means for injecting measurement light at a desired measurement wavelength into a single-mode optical fiber, means for measuring a refractive index distribution in an arbitrary cross section of the single-mode optical fiber, and desired for the single-mode optical fiber. Means for measuring the electric field distribution at the measurement wavelength of, the effective non-linear refractive index of the single-mode optical fiber to be measured, and the effective Effective non-linear constant measuring device for a single-mode optical fiber, comprising: