JP3257197B2 - Method and apparatus for evaluating single mode optical fiber characteristics - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for evaluating the characteristics of a single-mode optical fiber used for optical communication and the like, and more particularly, to the length of a single-mode optical fiber including a backward Rayleigh scattered light waveform. By measuring the wavelength of the light source and the backward Rayleigh scattered light from both directions of the fiber to be measured, information on the cutoff wavelength and dispersion, which are transmission parameters that can be evaluated from the structural parameters and the structural parameters typified by the refractive index distribution in the direction, is obtained. The purpose is to evaluate structural parameters, transmission parameters, and the like in the longitudinal direction of the one-mode optical fiber.

[0002]

2. Description of the Related Art When introducing a single-mode optical fiber into a subscriber system, it is necessary to cut and branch an optical cable once laid at a portion other than the optical cable connection point. At that time, the structural parameters of the optical fiber housed in the cable are only the values measured at both ends of the optical cable, and the standard parameter values of the structural parameters represented by the refractive index distribution at an arbitrary position in the longitudinal direction of the optical fiber Is assumed to be uniform in the longitudinal direction of the optical fiber, and has not been examined as an actually measured value. Therefore, at present, the standard parameter values can be grasped only after cutting the optical cable. Even in the product stage before the delivery of the optical cable, only the outer diameter of the optical fiber is monitored, and the structural parameters and transmission parameters are not At present, it is not quantitatively evaluated.
As described above, it is necessary to measure the structural parameters and transmission parameters in the longitudinal direction of the optical fiber by cutting the optical fiber by the destruction method and observing the end face. Also, a method of observing fluctuations in structural parameters from the optical loss dependence of an optical fiber by the backward Rayleigh scattering method is considered as a nondestructive method. However, in a single-mode optical fiber, it is difficult to observe structural parameter fluctuations because the backscattered waveform obtained depends on the coherence and incident polarization of the light source used due to the mode birefringence of the optical fiber. Was.

In general, chromatic dispersion factors of a single mode optical fiber include material dispersion and waveguide dispersion (structural dispersion).
There are two types. Here, the material dispersion refers to dispersion caused by a change in the refractive index (and the group velocity) of the fiber material due to a change in the wavelength of light. Also,
Waveguide dispersion is independent of the type of optical fiber material.
This refers to dispersion that occurs because the group velocity of the propagation mode is not constant with respect to the wavelength of light due to the waveguide structure.

[0004] Single-mode fiber is designed to propagate only HE ₁₁ mode is the basic mode, since it is produced, is a major factor in the above-described materials dispersed, waveguide dispersion (waveguide dispersion) determines the chromatic dispersion . That is, when light having different wavelengths propagates through the optical fiber, the propagation speed for each wavelength is different due to the above-described factors, so that when the pulse light propagates through the single-mode optical fiber, waveform distortion occurs.

[0005] The chromatic dispersion (wavelength dispersion) σ is expressed as λ
(Nm), the group delay time per unit length of the optical fiber is τ
(Ps / km), it is defined by the following equation (101).

[0006]

(37)

There are a time domain method and a frequency domain method as a method of measuring the chromatic dispersion σ. The time domain method is a method in which two or more light pulses having different wavelengths are incident on an optical fiber, a group delay time difference at an output end is measured, and chromatic dispersion is obtained according to the definition of Expression (101). In the frequency domain method, light having wavelengths of λ ₁ and λ ₂ is sinusoidally modulated at a modulation frequency f, the modulated signal light is pumped into an optical fiber, and the output light is converted into an electric signal by a light receiver. When propagating through an optical fiber of length L, there is a group delay difference Δτ between the two wavelengths.
A phase difference Δθ occurs between the modulated signals of the emitted light. This phase difference has a relationship with the modulation frequency according to the following equation (102).

[0008]

Δθ = Δτ · 2πf · L (102) Therefore, in the frequency domain method, by obtaining Δθ,
The group delay time difference Δτ of the optical fiber is measured, and the equation (101) is obtained.
According to the definition of, the chromatic dispersion can be measured.

[0009]

However, since the difference in group delay time between the wavelengths λ ₁ and λ ₂ of the single mode optical fiber is small, the measurement of the chromatic dispersion by the conventional measuring method requires a relatively long fiber length. For example, a fiber length of about 1 km was required. Therefore, the conventional measuring method has a problem that it is difficult to measure a fiber length of, for example, 1 km or less.

In order to solve such a problem, an interferometer for measuring chromatic dispersion using a fiber of several meters has been developed. However, in this interferometry, in order to measure the chromatic dispersion in the longitudinal direction of the optical fiber to be measured, it was necessary to cut the optical fiber every several meters and to measure it. Therefore, this method has a problem that the measured optical fiber cannot be used.

The method of measuring the variation in the distribution of the mode field radius in the longitudinal direction and the distribution of the cutoff wavelength, which serve as indices for the specific evaluation of the optical fiber, are described in, for example, MSO'Sullivan and
RSLowe, “Interpretation of SM fiber OTDR signatu
res ”, Proceeding SPIE '86 Optical Testing and Metr
ology, vol. 661, pp. 171-176, 1986, there is a problem that the refractive index distribution is limited to the step type optical fiber and cannot be applied to an arbitrary refractive index distribution.

In view of such circumstances, the present invention provides a single mode optical fiber characteristic evaluation method for non-destructively evaluating a longitudinal characteristic distribution of a single mode optical fiber having an arbitrary refractive index distribution. With the goal.

[0013]

In order to achieve the above object, according to the present invention, the backscattered light intensities S ₁ and S ₁ measured by inputting light pulses of two or more different wavelengths from both ends of an optical fiber are measured. the S ₂ is used. These data and the measured values of refractive index distribution and mode field radius at one end of the optical fiber, or mode field radius W, core radius a, maximum refractive index _nm and maximum relative refractive index difference at one end of the optical fiber. Using the measured value of Δm, the mode field radius W (z) at any position and the cutoff wavelength λ _c
(Z), maximum relative refractive index difference Δm (z), core radius a
(Z), waveguide dispersion σ _w (z), material dispersion σ _m (z), and chromatic dispersion σ _T (z) are obtained. In the present invention, these parameters are evaluated using various relational expressions, but in the manufacturing process of the optical fiber, the refractive index distribution shape standardized by the core radius and the maximum relative refractive index difference is constant in the longitudinal direction. And a practical approximation formula is used.

Further, focusing on controlling the polarization state of the pulse light source to suppress the noise component due to the influence of the polarization of the incident pulse of the backscattered light from the optical fiber, ie, the polarization fluctuation, the single mode fiber The measurement accuracy of the characteristic evaluation method has been improved.

The method for evaluating the characteristics of a single mode optical fiber according to the present invention is specifically as follows.

(First Measuring Method) In the first measuring method, when a pulse light having a wavelength λ is incident from one side of a single-mode optical fiber having an arbitrary length L, a rearward position of the single-mode optical fiber at a position z is obtained. The scattered light intensity S ₁ (z) (unit: dBm) and the scattered light intensity S ₂ (L−z) (unit: dBm) when the pulse light having the wavelength λ similarly enters from the other side of the optical fiber. Is calculated, and the arithmetic mean intensity I (λ, z) of the scattered light intensity measured from both ends is obtained from the following relational expression (1).

[0017]

[Equation 39]

Arithmetic average intensity I at an arbitrary end position z ₀
The normalized arithmetic average intensity I ′ (λ, z) shown in the following equation (2) in which I (λ, z) is normalized by (λ, z ₀ ) is obtained.

[0019]

I ′ (λ, z) = I (λ, z) −I (λ, z ₀ ) (2) Mode field radius W (λ, z ₀ ) at the position z ₀
Is measured, the longitudinal mode field radius of the single mode optical fiber is evaluated by the following relational expression (3).

[0020]

[Equation 41]

(Second Measurement Method) In the second measurement method, the mode field radius W (λ ₁₎ of the single mode optical fiber at any two wavelengths λ ₁ and λ ₂ is obtained using the first measurement method. , Z) and W (λ ₂ , z) are measured, and the position z of any one end of the single mode optical fiber is measured.
₀ , the refractive index distribution in the cross section of the optical fiber is measured, and using the refractive index distribution, an electromagnetic wave equation (where _Tc is a cutoff frequency) with respect to a normalized frequency T represented by the following equation (4): And λ _c is the cutoff wavelength)

[0022]

(Equation 42)

The coefficients c ₀ , c ₁ , c ₂ of the following relational expression representing the cutoff frequency T _c and the ratio (= W / a) of the mode field radius W to the core radius a are obtained,

[0024]

[Equation 43]

The ratio R (λ ₁ , λ ₂ , R ₂₎ of the mode field radius of the _two wavelengths λ ₁ and λ ₂ can be expressed by the following equation (6).
z) (= W (λ ₁ , z) / W (λ ₂ , z))

[0026]

[Equation 44]

Here, ΔI ( λ ₁ , λ ₂ , z) is expressed by the following equation (7).

[0028]

ΔI (λ ₁ , λ ₂ , z) = I ′ (λ ₂ , z) −I ′ (λ ₁ , z) (7) Further, from the following relational expression (8), evaluate.

[0029]

[Equation 46]

(Third Measurement Method) In the third measurement method, the first measurement method
And the second measurement method, the mode field radii W (λ ₁ , z), W (λ ₂ , z) and the cutoff wavelength λ of the single mode optical fiber at any two wavelengths λ ₁ and λ ₂ . _c (z) is measured, and the core radius a (z) in the longitudinal direction of the single mode optical fiber is measured by using one of the following two relational expressions (9) and (10). .

[0031]

[Equation 47]

[0032]

[Equation 48]

(Fourth Measurement Method) In the fourth measurement method, the mode field radii W (λ ₁ , z) and W (λ) are calculated using the first, second and third measurement methods.
₂ , z), cutoff wavelength λ _c (z) and core radius a
(Z) is measured, the refractive index distribution in the cross section of the single mode optical fiber at an arbitrary end position z ₀ is measured, and the shape factor S representing the refractive index distribution shape is obtained from the following relational expression (11). (Where ρ is the normalized radius (= r / a; r is the distance from the center of the optical fiber cross section), and Δm is the maximum relative refractive index difference),

[0034]

[Equation 49]

By substituting the maximum refractive index n _m , S, _Tc in the fiber cross section obtained from the refractive index distribution measurement at an arbitrary end position of the optical fiber into the following relational expression (12), The maximum relative refractive index difference Δm (z) at is measured.

[0036]

[Equation 50]

(Fifth measuring method) In the fifth measuring method, the first
Measurement methods mode field radius W in 3 or more wavelengths with a _{(λ i, z) (i} = 1,2,3 ... n) measure the mode field radius W at respective wavelengths (λ _i, z) And the coefficients g ₀ (z), g satisfying the equation shown in equation (13).
₁ (z) and g ₂ (z)

W (λ, z) = g ₀ (z) + g ₁ (z) λ ^1.5 + g ₂ (z) λ ⁶ (13) From the measurement of the refractive index distribution at any one end of the fiber The maximum refractive index _nm is obtained, and the waveguide dispersion σ _w (λ, z) at the wavelength λ is obtained from the following relational expression (14) (where,
c is the speed of light).

[0039]

(Equation 52)

(Sixth Measurement Method) In the sixth measurement method, the first
Mode field radius W at two wavelengths using the measurement method of
(Λ _i , z) (i = 1, 2) is measured, and the following equation (1) is obtained using the mode field radius W (λ _i , z) at each wavelength.
Coefficients g ₀ (z) and g ₁ satisfying the equation shown in 5)
(Z)

[0041]

W (λ, z) = g ₀ (z) + g ₁ (z) λ ^1.5 (15) The maximum refractive index _nm is obtained from the measurement of the refractive index distribution at an arbitrary end of the fiber. The waveguide dispersion σ _w (λ, z) at the wavelength λ is obtained from the relational expression (16) (where c is the speed of light).

[0042]

(Equation 54)

(Seventh measuring method) In the seventh measuring method, the fourth measuring method
The relative refractive index difference Δm in the longitudinal direction of the optical fiber
(Z) the calculated coefficients A _i and B _i (although the following formula corresponding to Delta] m (z) (17), A _i is ultraviolet and the infrared absorption wavelength, B _i must support the oscillator strength , Which is a constant representing the wavelength dependence of the refractive index) from the material values of the optical fiber, and using equation (17) showing the wavelength dependence of the refractive index,

[0044]

[Equation 55]

The material dispersion σ _m (λ, z) in the longitudinal direction of the optical fiber is evaluated from the following relational expression (18).

[0046]

[Equation 56]

(Eighth Measurement Method) In the eighth measurement method, the fourth measurement method
The relative refractive index difference Δm in the longitudinal direction of the optical fiber
(Z) is obtained, and when the core of the optical fiber is GeO ₂ -doped quartz, the material dispersion σ _m (z) in the longitudinal direction of the optical fiber at the wavelength λ is evaluated using the following relational expression (19). .

[0048]

Equation 57] _{σ m (λ, z) =} -583.36 + 1009.91λ-587.86λ 2 + 121.31λ 3 -Δ m (z) (89.48-152.20λ + 92.13λ 2 - 18.79λ 3) ... (19) ( the Ninth measuring method) In the ninth measuring method, the waveguide dispersion σ _m (λ, z) in the longitudinal direction of the optical fiber is obtained by the fifth method.
The material dispersion σ _{m in} the longitudinal direction of the optical fiber is obtained by the seventh method.
(Λ, z) is obtained, and the chromatic dispersion σ _T (λ, z) in the longitudinal direction of the optical fiber is measured from the following relational expression (20).

[0049]

Σ _T (λ, z) = σ _w (λ, z) + σ _m (λ, z) (20) (Tenth Measurement Method) In the tenth measurement method, the light is obtained by the sixth method. The waveguide dispersion σ _w (λ, z) in the longitudinal direction of the fiber is obtained, and the material dispersion σ _{m in} the longitudinal direction of the optical fiber is obtained by the seventh method.
(Λ, z) is obtained, and the chromatic dispersion σ _T (λ, z) in the longitudinal direction of the optical fiber is measured from the following relational expression (21).

[0050]

Σ _T (λ, z) = σ _w (λ, z) + σ _m (λ, z) (21) (Eleventh Measurement Method) In the eleventh measurement method, the light The waveguide dispersion σ _w (λ, z) in the longitudinal direction of the fiber is obtained, and when the core of the optical fiber is GeO ₂ -doped quartz,
In the eighth method, the material dispersion in the longitudinal direction of the optical fiber σ _m
(Λ, z) is obtained, and the chromatic dispersion σ _T (λ, z) in the longitudinal direction of the optical fiber is measured from the following relational expression (22).

[0051]

Σ _T (λ, z) = σ _w (λ, z) + σ _m (λ, z) (12) (Twelfth Measurement Method) In the twelfth measurement method, the light is The waveguide dispersion σ _w (λ, z) in the longitudinal direction of the fiber is obtained, and when the core of the optical fiber is GeO ₂ -doped quartz,
In the eighth method, the material dispersion in the longitudinal direction of the optical fiber σ _m
(Λ, z) is obtained, and the chromatic dispersion σ _T (λ, z) in the longitudinal direction of the optical fiber is measured from the following relational expression (23).

Σ _T (λ, z) = σ _w (λ, z) + σ _m (λ, z) (13) (Thirteenth Measurement Method) In the thirteenth measurement method, a single length L A mode field radius W (λ _i , z ₀ ) (i = ₃ ) at any three or more wavelengths λ _i at a position z ₀ at one end of the one-mode optical fiber.
1, 2, 3... N; n ≧ 3), the core radius a (z ₀ ) at one end of the fiber is measured, and the cutoff wavelength λ
_c (z ₀ ) is measured, and the cutoff wavelength λ _{c of the} optical fiber is measured.
Using the measured values of (z ₀ ) and W (λ, z ₀ ), the coefficients c ₀ , c ₁ , c ₂ satisfying the following relational expression (24).
,

[0053]

(Equation 24) … (24)

Using the first measurement method, any two wavelengths λ
₁ and lambda ₂ in _{W (λ 1, z),} W (λ 2, z) is measured and expressed by the following equation (25), two wavelengths lambda ₁ and lambda ₂
Of the mode field radii R (λ ₁ , λ ₂ , z) (=
W (λ ₁ , z) / W (λ ₂ , z)) is evaluated,

[0055]

[Equation 63]

However, ΔI ( λ ₁ , λ ₂ , z) is represented by the following equation (26).

[0057]

ΔI (λ ₁ , λ ₂ , z) = I ′ (λ ₂ , z) −I ′ (λ ₁ , z) (26) The cutoff wavelength is evaluated from the following relational expression (27). .

[0058]

[Equation 65]

(Fourteenth Measurement Method) In the fourteenth measurement method,
Using the first and thirteenth measurement methods, the mode field radii W (λ ₁ , z), W (λ ₂ , z) and the cutoff wavelength λ
_c (z) is measured, and two relational expressions (28) and
By using one of (29), the core radius a (z) in the longitudinal direction of the single mode optical fiber is measured.

[0060]

[Equation 66]

[0061]

[Equation 67]

(Fifteenth measuring method) In the fifteenth measuring method,
The cut-off wavelength λ _c (z ₀ ), the core radius a (z ₀ ), and the maximum relative refractive index difference Δm (z ₀ ) at a position z ₀ at any one end of the single mode optical fiber were measured, and Using the thirteenth and fourteenth measurement methods, the mode field radius W (λ
₁ , z), W (λ ₂ , z), cut-off wavelength λ _c (z) and core radius a (z) are measured, and the maximum specific refraction of the core in the longitudinal direction is obtained by using the following relational expression (30). The rate difference Δm (z) is measured.

[0063]

[Equation 68]

(Sixteenth Measurement Method) In the sixteenth measurement method,
The cutoff wavelength λ _c using the thirteenth and fourteenth measurement methods
(Z) and the core radius a (z) are measured to determine the maximum refractive index n _m (z ₀ ) of the core at an arbitrary end of the fiber, and the following relational expression (31) (where c is the speed of light) ) To determine the waveguide dispersion σ _w (λ, z) at the wavelength λ.

[0065]

[Equation 69]

(Seventeenth Measurement Method) In the seventeenth measurement method,
The fifteenth method is used to determine the relative refractive index difference Δm (z) in the longitudinal direction of the optical fiber, and the following equation (3) corresponding to Δm (z) is obtained.
2) Coefficients A _i and B _i (where A _{i corresponds} to the ultraviolet and infrared absorption wavelengths, B _i corresponds to the oscillator strength, and are constants representing the wavelength dependence of the refractive index) Is obtained from the material values constituting the following formula, and using the equation (32) showing the wavelength dependence of the refractive index,

[0067]

[Equation 70]

The material dispersion σ _m (λ, z) in the longitudinal direction of the optical fiber is evaluated from the following relational expression (33).

[0069]

[Equation 71]

(Eighteenth Measurement Method) In the eighteenth measurement method,
Using the fifteenth method, the relative refractive index difference Δm (z) in the longitudinal direction of the optical fiber is obtained. When the core of the optical fiber is GeO ₂ -doped quartz, the wavelength λ is calculated using the following relational expression (34). The material dispersion σ _m (λ, z) in the longitudinal direction of the optical fiber is evaluated.

[0071] _{σ m (λ, z) =} -583.36 + 1009.91λ-587.86λ 2 + 121.31λ 3 -Δ m (z) (89.48-152.20λ + 92.13λ 2 - 18.79λ 3) ... (34) ( 19 In the nineteenth measurement method, the waveguide dispersion σ _w (λ, z) in the longitudinal direction of the optical fiber is obtained by the sixteenth method, and the material dispersion in the longitudinal direction of the optical fiber is obtained by the seventh or eighth method. σ _m (λ, z)
And the chromatic dispersion σ _T (λ, z) in the longitudinal direction of the optical fiber is measured from the following relational expression (35).

[0072]

Σ _T (λ, z) = σ _w (λ, z) + σ _m (λ, z) (20) (20th measurement method) In the twentieth measurement method, light is The waveguide dispersion σ _w (λ, z) in the longitudinal direction of the fiber is obtained, and when the core of the optical fiber is GeO ₂ -doped quartz, the material dispersion σ _{m in} the longitudinal direction of the optical fiber is obtained by the seventh method.
(Λ, z) is obtained, and the chromatic dispersion σ _T (λ, z) in the longitudinal direction of the optical fiber is measured from the following relational expression (36).

[0073]

Σ _T (λ, z) = σ _w (λ, z) + σ _m (λ, z) (36) On the other hand, the first characteristic evaluation device according to the present invention employs a rear Rayleigh A pulsed light source having two or more wavelengths for generating scattered light, a wavelength switching means for switching a wavelength of the light source incident on the optical fiber to be measured, and a polarized light for controlling a polarization state of the pulse light source. Wave control means;
The light emitted from the polarization control means is guided to the fiber to be measured, and the multiplexing / demultiplexing means for extracting the backward Rayleigh scattered light generated in the optical fiber to be measured is emitted from the multiplexing / demultiplexing means. Optical switching means for causing light to be incident on one of both ends of the fiber to be measured, signal processing means for analyzing a temporal waveform of the backward Rayleigh scattered light detected by the multiplexing means, An arithmetic processing means for analyzing the rear Rayleigh scattered waveform measured from both ends of two or more wavelengths of the optical fiber to be measured by using any one of the first to twentieth methods; An output processing device for outputting a result, switching means for the incident position of the measured fiber, and control means for controlling the wavelength switching means of the light source. .

Further, the second characteristic evaluation apparatus according to the present invention comprises a pulse light source having two or more wavelengths for generating backward Rayleigh scattered light on the fiber to be measured, and a light source incident on the optical fiber to be measured. Wavelength switching means for switching a wavelength, polarization control means for controlling the polarization state of the pulse light source, light guided from the polarization control means to the fiber under measurement, and Multiplexing / demultiplexing means for extracting backward Rayleigh scattered light generated in the optical fiber, and optical switching means for causing light emitted from the multiplexing / demultiplexing means to be incident on one of both ends of the fiber to be measured. A reference fiber whose fiber characteristics are known for guiding the light of the light source to both ends of the wavelength switching means and the fiber to be measured, and a backward Rayleigh scattered light detected by the multiplexing means. Signal processing means for analyzing a dynamic waveform, and analyzing and processing the backward Rayleigh scattering waveform measured from both ends of two or more wavelengths of the measured optical fiber by using any one of the first to twentieth methods. Arithmetic processing means, an output processing device for outputting a result processed by the arithmetic processing means, switching means for the incident position of the measured fiber, and control means for controlling the wavelength switching means of the light source And characterized in that:

[0075]

In the present invention, chromatic dispersion, waveguide dispersion, material dispersion,
The distribution characteristics of the relative refractive index difference, the core diameter, and the mode field radius in the longitudinal direction can be evaluated. Such a distribution characteristic in the longitudinal direction becomes important when designing a transmission system. Further, the present invention has an advantage that it can be applied to an arbitrary refractive index distribution manufactured under realistic conditions.

[0076]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.

Embodiment 1 FIG. 1 is a process chart showing an embodiment of a method for measuring the dispersion and fiber parameters in the longitudinal direction of an optical fiber according to the present invention. As shown in the figure,
The present invention uses a backscattered light measurement device (OTDR) using two or more light sources having different wavelengths in order to non-destructively measure the longitudinal dispersion characteristic distribution of a single mode optical fiber. Is measured, and the dispersion in the longitudinal direction and the distribution of fiber parameters are evaluated by analyzing the measured OTDR waveform. The details of the present invention will be described below.

A method for evaluating the characteristics of the mode field radius in the longitudinal direction is described in, for example, the document [MSO'Sull.
ivan and RSLowe, “Interpretation of SM fiber OTD
R signatures ”, Proceeding SPIE '86 Optical Testin
g and Metrology, vol.661, pp.171-176, 1986. ].

The backscattered light intensity P ( λ, z) of wavelength λ at the point z from the incident end is expressed by the following equation (37). However, P ₀
Is the light intensity at the wavelength λ excited by the optical fiber, α _s
(Λ, z) is the scattering coefficient of wavelength λ at z, B (λ, z) is the capture rate of backscattered light of wavelength λ, and γ (λ, z) is z
Represents the loss coefficient of the wavelength λ.

[0080]

[Equation 75]

The length z measured from both ends of the fiber length L
The backscattered light intensity at S ₁ ( λ, z) and S ₂ ( λ, L
−z) (unit: dBm), these are represented by the following equations (38) and (39), respectively.

[0082]

[Equation 76]

[0083]

[Equation 77]

The scattered light intensity S ₁ measured from both ends
Arithmetic mean intensity I of (λ, z) and S ₂ (λ, Lz)
(Λ, z) can be described by the following equation (40).

[0085]

[Equation 78]

Where a ₂ is given by the following equation (41):
This is a constant independent of z.

[0087]

[Expression 79]

Therefore, I (λ, z) is proportional to the scattering coefficient α _s (λ, z) at z and the capture rate B (λ, z).
The capture rate B (λ, z) can be expressed by the following equation (42). Here, λ is the wavelength, n is the refractive index of the core, and W (λ, z) represents the mode field radius at the wavelength λ at z.

[0089]

[Equation 80]

Since the influence of the fluctuation of the scattering coefficient α _s (z) and the refractive index n in the longitudinal direction on I (λ, z) is smaller than the contribution of the fluctuation of the mode field radius, they are constant in the longitudinal direction. It can be assumed that Therefore,
I (λ, z) can be expressed by the following equation (43) as a constant in which a ₂ ′ does not depend on z.

[0091]

[Equation 81]

Next, when equation (43) is normalized by the value of I (λ, z ₀ ) at an arbitrary point z = z ₀ , the following equation (44) can be rewritten.

[0093]

(Equation 82)

Here, the position z = z _{0 at} one end of the fiber
The mode field radius W (λ, z
₀ ), the mode field radius W (λ, z)
Can be evaluated by the following equation (45).

[0095]

[Equation 83]

Next, a method for obtaining the cutoff wavelength λ _c (z) will be described. Using the method described above at any two wavelengths λ ₁ and λ ₂ , W (λ ₁ , z), W (λ ₂ ,
Measure z). Next, the refractive index distribution in the cross section of the single-mode optical fiber at a position z ₀ at any one end is measured,
The shape factor S of the refractive index distribution defined by the following equation (46) is obtained. Here, a represents a core radius, ρ (= r / a) represents a normalized radius, and Δm represents a maximum relative refractive index difference.

[0097]

[Equation 84]

Next, a normalized frequency T that can be arbitrarily applied to the refractive index distribution is defined by the following equation (47). Here, T _c is a cutoff frequency, k (= 2π / λ) represents a wave number, and λ
_c is the cutoff wavelength.

[0099]

[Equation 85]

The relationship between the shape factor and the T value in the equation (46) can be expressed by the following equation (48). However, n _m denotes a maximum refractive index.

[0101]

[Equation 86]

Accordingly, since the T value is S = 1 in the step type fiber, the V value (a kn ₁ (2Δ) ^1/2 : n ₁ )
Is the same as the refractive index of the core), and can be regarded as an effective V value.

The cut-off frequency T _c can be obtained by solving the electromagnetic wave equation once the refractive index distribution is known.

Here, the mode field radius W is Ma
An empirical approximation formula as shown below is derived from rcuse for the case of a step type fiber (refer to the document [D. Marcuse, “Loss analysis of single mode f
iber splices ”, Bell System Technical Journal, vol.
56, pp.703-718, 1977]). It is also known that this relational expression can be applied to any refractive index distribution (M. Ohashi, K. Kitayama, Y. Ishida, and Y. Negishi,
“Simple approximations for chromatic diapersion i
n single-mode fibers with various index profile
s ”, IEEE / OSA J. Lightwave Technol., vol.LT-3, pp.1
10-115, 1985]).

Coefficients c ₀ and c of the following relational expression (49) representing the ratio (W / a) of mode field radius W to core radius a:
₁ and c ₂ are constants depending only on the refractive index distribution, and can be obtained by changing the T value and solving the electromagnetic wave equation.

[0106]

[Equation 87]

Normally, when the optical fiber preform is manufactured, the maximum relative refractive index difference Δm sometimes fluctuates in the longitudinal direction.
/ Δm can be seen to be almost constant. In the process of drawing an optical fiber by drawing such a base material, the core diameter and the relative refractive index difference may fluctuate in the longitudinal direction, but the change in the refractive index distribution shape can be approximated by similar deformation. Therefore, the shape coefficient S normalized by the maximum relative refractive index difference Δm and the core radius a can be regarded as constant in the longitudinal direction.

Then, the normalized arithmetic average scattering intensities I ′ (λ ₁ , z) and I ′ (λ ₂ , z) at the respective wavelengths of the measured backscattering waveforms at the _two wavelengths λ ₁ and λ ₂ are obtained. )
Is obtained, and the difference is represented by ΔI (λ ₁ , λ ₂ , z).

[0109]

[Equation 88]

Next, the mode field radius ratio R (λ
₁ , (λ ₂ , z) (= W (λ ₁ , z) / W (λ ₂ , z)) is obtained by the following equation (51).

[0111]

[Equation 89]

Further, the ratio R (λ ₁ , λ ₂ , z) of the mode field radius at two wavelengths can be expressed as follows using equation (49).

[0113]

[Equation 90]

Therefore, by measuring the refractive index distribution at one end of the fiber and determining T _c , c ₀ , c ₁ and c ₂ by the above-described method, the optical fiber having an arbitrary refractive index distribution can be obtained. The cutoff wavelength λ _c (z) can be determined.

Next, a method for evaluating the core radius a (z) will be described. In the manner described above, the mode field radius W
By measuring (λ, z) and λ _c (z) and using the following relational expression (53), the core radius a (z) in the longitudinal direction of the single mode optical fiber can be measured. .

[0116]

[Equation 91]

Further, using the above-described measuring method, the wavelength λ ₁
And W at _{λ 2 (λ 1, z)} , W (λ 2, z),
Measure λ _c (z) and a (z). Then any
The refractive index distribution in the cross section of the single mode optical fiber at the position z ₀ at one end is measured, and the shape factor S defined by the equation (46) is obtained. Knowing the values of these parameters, the relationship in equation (48) and the maximum refractive index measured at one end of the fiber
with n _m, it can be evaluated at the maximum relative refractive index difference Δm a (z) following equation (54).

[0118]

(Equation 25) … (54)

Next, a method for evaluating variance will be described.

In general, the chromatic dispersion σ _T can be expressed as the sum of the material variance σ _m and the structural variance σ _w as in the following equation (55).

[0121]

Σ _T = σ _m + σ _w (55) where the material dispersion and the structural dispersion can be described by the following equations (56) and (57), respectively (where N ₁ is the group refractive index at the core) And c represents the speed of light).

[0122]

[Equation 94]

[0123]

[Equation 95]

First, a method for measuring the dispersion of the waveguide will be described. From equation (57), it can be seen that the waveguide dispersion can be evaluated from the wavelength dependence of the mode field radius of the optical fiber. From the relational expression of Expression (49), the wavelength dependence of the mode field radius can be expressed by the following Expression (58).

[0125]

W = g ₀ + g ₁ λ ^1.5 + g ₂ λ ⁶ (58) Here, the second term of the equation (58) is larger than the third term, and this is expressed by the following equation (59). Can also be approximated.

W = g ₀ + g ₁ λ ^1.5 (59) Accordingly, equation (57) can be modified as follows.

[0127]

(Equation 26) … (60)

On the other hand, using the method for measuring the mode field radius described above, the mode field radius W
_{(Λ i, z) (i} = 1,2,3 ... n) is measured, the coefficient _g 0 satisfying the equation (58) using the mode field radius W at respective wavelengths (lambda _i, z), g ₁ and g ₂
Can be requested. Next, coefficients g ₁ obtained by a maximum refractive index n _m of the refractive index distribution measured at the fiber end as described above _{(z), g 2 (z} ) and (6
The waveguide dispersion σ _w (λ, z) can be obtained from the following equation (61) obtained by substituting equation (58) into (0).

[0129]

[Equation 99]

On the other hand, when determining the waveguide dispersion at two wavelengths, first, the mode field radius W (λ _i , z) (i
= 1, 2), and the coefficients g ₀ (z) and g ₁ (z) satisfying the expression (59) are obtained using the mode field radius W (λ _i , z) at each wavelength. Next, obtained by substituting equation (58) to the coefficient g ₁ obtained by a maximum refractive index n _m of the refractive index distribution measured at the fiber end as described above _(z) and (60) The waveguide dispersion σ _w (λ, z) can be obtained from the following equation (62).

[0131]

[Equation 100]

Next, a method for measuring material dispersion will be described. Generally, the material dispersion of an optical fiber is given by the following equation (63).
It is given by the relational expression as shown by.

[0133]

[Equation 101]

To determine this value, the wavelength dependence of the refractive index is necessary. The wavelength dependence of the refractive index is given by the following equation (6)
It can be obtained by using the Sellmeier relational expression as shown in 4).

[0135]

[Equation 102]

Equation (64) is an equation showing the physical content representing the wavelength dependence of the refractive index, where A _{i corresponds} to the ultraviolet and infrared absorption wavelengths and B _i corresponds to the oscillator strength. It is known that the measured values are in good agreement with the measured values in a wide wavelength range including a wavelength having a large dispersion. It is known that sufficient accuracy can be obtained when the value of k is used up to three terms (references B. Brinxner, J. Op.
t. Soc. Am., vol. 55, pp. 1205, 1967). The Cellmeier coefficients A _i and B _i of glass used for optical fibers are described in the literature (S. Kobayashi, S. Shibata,
N.Shibata, and T.Izawa, International Conference o
n Integrated Optics and Optical Fiber Communicatio
n, Tokyo, 1977, Technical Paper P309).

Therefore, the material dispersion is obtained by converting equation (64) into equation (6).
It can be obtained by substituting into 3). That is, the maximum relative refractive index difference in the longitudinal direction of the optical fiber using the method of measuring the maximum relative refractive index difference Delta] m (z) mentioned above Delta] m
(Z) is obtained, and the coefficients A _i and B _i of the equation (64) corresponding to Δm (z) are obtained from the material values constituting the optical fiber, whereby the material dispersion σ _m in the longitudinal direction of the optical fiber is obtained. ( Λ, z) can be evaluated.

When the core of the optical fiber is GeO ₂ -doped quartz glass, an approximate expression of the following expression (65) can be derived using the expressions (63) and (64).

[0139]

Equation 103] _{σ m (λ, z) =} -583.36 + 1009.91λ-587.86λ 2 + 121.31λ 3 -Δ m (z) (89.48-152.20λ + 92.13λ 2 - 18.79λ 3) ... (65) Therefore, When the core is GeO ₂ -doped quartz glass,
By substituting Δm (z) in equation (65), the material variance σ _m (λ, z) can be obtained.

Next, a method for obtaining chromatic dispersion will be described. In the manner described above, the waveguide dispersion σ _w (λ, z) and the material dispersion σ _m (λ,
z), the chromatic dispersion σ _T (λ, z) in the longitudinal direction of the optical fiber is measured by substituting the waveguide dispersion and the material dispersion into the following relational expression (66). Can be.

[0141]

Σ _T (λ, z) = σ _w (λ, z) + σ _m (λ, z) (66) (Embodiment 2) In the second embodiment of the present invention, two wavelengths are 1.3.
A method for measuring dispersion and fiber parameters from backscattered waveforms at 1 μm and 1.55 μm is shown.

FIGS. 2 and 3 each show 1.31 μm
And about 1.55 μm stepped fiber to be measured
It is a figure of the result measured from both ends of a backscattering waveform of 0km. From these figures, when the longitudinal characteristics of the mode field radius at 1.31 μm and 1.55 μm were evaluated using equations (44) and (45), the results shown in FIGS. 4 and 5 were obtained. . Note that W ( λ _i , z
₀ ) used the mode field radius at one end of the fiber. Next, FIG. 6 shows the result of obtaining the cutoff wavelength λ _c (z) using the equations (49) to (52).

Next, T _c , c calculated using the measurement results of the mode field radius at z, the cutoff wavelength, and the refractive index distribution at one end of the fiber obtained as described above.
_The core radius a (z) was determined by substituting ₀ , c ₁ and c ₂ into equation (53). FIG. 7 shows the result. Further, in the equation (54), S = 1, the refractive index distribution maximum refractive index n _m obtained from the measurement, and the above-described manner using the core radius obtained Δm results of evaluation of the (z) Figure FIG.

Further, the waveguide dispersion was derived as follows. Using the measurements of the mode field radius at two wavelengths, the coefficients g ₀ (z) and g ₁ (z) of equation (59) were determined. The mode field radii of 1.31 μm and 1.55 μm at z are given by W (1.31 , z ) and W
Assuming (1.55 , z ), g ₀ (z) and g ₁
(Z) is uniquely obtained as in the following equations (67) and (68).

[0145]

[Equation 27] … (67)

[0146]

[Equation 28] … (68)

Therefore, the mode field radius, _nm , g
By substituting ₁ (z), the speed of light c, and the wavelength λ into the equation (62), the waveguide dispersion σ _w (λ, z) can be evaluated. FIG. 9 shows the distribution characteristics in the longitudinal direction at this time.

On the other hand, since the present optical fiber is a GeO ₂ -doped quartz core fiber, the material dispersion can be evaluated using the equation (65). FIG. 10 shows the distribution characteristics of the material dispersion at 1.55 μm.

The chromatic dispersion σ _T at the wavelength λ is obtained by the sum of the above-described material dispersion and waveguide dispersion. Therefore, it can be obtained by adding waveguide dispersion and material dispersion, and has a wavelength of 1.55 μm.
FIG. 11 shows the result of chromatic dispersion at m.

[0150] In a third embodiment (Embodiment 3) The present invention, the mode field radius W at one end of the fiber to be measured, the core radius a, the maximum refractive index n _m
And the measured value of the maximum relative refractive index difference Δm, the cutoff wavelength λ _c (z) at an arbitrary position, the maximum relative refractive index difference Δ
m (z), core radius a (z), waveguide dispersion σ _w ( λ,
z), material variance σ _m ( λ, z) and chromatic variance σ _T
A method for obtaining ( λ, z) will be described. FIG.
Is a process diagram showing an example of a method for measuring the dispersion and fiber parameters in the longitudinal direction of the optical fiber of the present invention,
The description will be made according to this.

The measurement of the mode field radius in the longitudinal direction is as described in the first and second embodiments.

Next, a method for obtaining a cutoff wavelength will be described.

The mode field radius W is equal to Marcus
e, an empirical approximation formula as shown below is derived for the case of the step type fiber (reference [D.
Marcuse, “Loss analysis of single mode fiber splic
es ”, Bell System Technical Journal, vol.56, pp.70
3-718, 1977.]). It is also known that this relational expression can be applied to any refractive index distribution (reference [M. Ohas
hi, K. Kitayama, Y. Ishida, and Y. Negishi, “Simple a
pproximations for chromatic diapersion insingle-mo
de fibers with various index profiles ”, IEEE / OSA
J. Lightwave Technol., Vol.LT-3, pp.110-115, 1985
]). Coefficients b ₀ and b of the following relational expression (69) representing the ratio (W / a) of mode field radius W to core radius a:
₁ and b ₂ are constants depending on the refractive index distribution, and can be obtained by changing the V value and solving the electromagnetic field wavelength equation.

[0154]

[Equation 107]

The V value is a parameter called a normalized frequency, and is defined by the following equation (70). Where V _c
Represents a cut-off frequency, the lambda _c is the cutoff wavelength, _{n m} is the maximum refractive index of the core, Delta] m is the maximum relative refractive index difference _{(= (n} ^{m 2} -
n ₂ ² ) / 2 _nm ² ; n ₂ is the refractive index of the cladding.

[0156]

[Equation 108]

The above equation is applicable to a step type fiber. Therefore, a standardized frequency T applicable to an arbitrary refractive index distribution is defined as in the following equation (71). Here, T _c is a cutoff frequency depending on the refractive index distribution, and n
Here, _m and Δm represent the maximum refractive index and the maximum relative refractive index difference of the core. K represents a wave number (= 2π / λ).

[0158]

(Equation 109)

S is a parameter representing the refractive index distribution shape and is defined by the following equation (72).

[0160]

[Equation 110]

Using the above relational expression, the relation of W / a for an arbitrary refractive index distribution can be expressed by the following expression (73).

[0162]

(Equation 111)

Here, the coefficients c ₀ and c in the equation (73)
₁ and c ₂ are constants depending on an arbitrary refractive index distribution shape factor S, cut-off frequency λ _c , maximum refractive index _nm , maximum relative refractive index difference Δm, and the like.

Usually, the maximum relative refractive index difference Δm may fluctuate in the longitudinal direction during the production of the optical fiber preform.
It can be seen that (r) / Δm is almost constant. Also,
In the process of drawing an optical fiber by drawing such a base material, the core diameter and the relative refractive index difference may fluctuate in the longitudinal direction, but the change in the refractive index distribution shape can be closely approximated by similar deformation. The shape factor S normalized by the relative refractive index difference Δm and the core radius a can be considered constant in the longitudinal direction. Therefore, the mode field radius W at an arbitrary wavelength λ can be obtained by evaluating the coefficients c ₀ , c ₁ and c ₂ in the equation (73).

Next, a method of determining the coefficient will be described. First, the core radius a (z ₀ ), the cutoff wavelength λ _c (z ₀ ) and the arbitrary 3
The mode field radius W (λ, z ₀ ) is measured at one or more wavelengths. Here, the core radius a (z ₀ ) is, for example, NFP
(Near Field Pattern) method, RNFP (Refractive Near Fiel
d Pattern) method. And 3
From the measured values of the mode field radius and the core radius at one or more wavelengths, c ₀ , which satisfies the following equation (74):
c ₁ and c ₂ can be determined.

[0166]

[Equation 112]

[0167] Next, using the methods described above for any two wavelengths lambda ₁ and _{λ 2, W (λ 1,} z), W (λ 2,
Measure z). And the two wavelengths λ ₁ and λ
_The normalized arithmetic average scattering intensities I ′ (λ ₁ , z) and I ′ (λ
₂ , z) and the difference is represented by ΔI (λ ₁ , λ ₂ , z), which can be expressed by the following equation (75).

[0168]

[Equation 113]

Next, the mode field radius ratio R (λ
_{1, λ 2, z) (} = W (λ 1, z) / W (λ 2, z))
Is obtained as in the following equation (76).

[0170]

[Equation 114]

The ratio R (λ ₁ , λ ₂ , z) of the mode field radii at these two wavelengths can be expressed by the following equation (77) using the equation (73).

[0172]

[Equation 115]

The cutoff wavelength λ _c (z) at an arbitrary position z can be obtained by solving equation (77). Accordingly, c ₀ , c _1, and c ₂ are determined from the measured values of the core radius and cutoff wavelength at one end of the fiber by the above-described method, so that the cutoff wavelength can be set for an optical fiber having an arbitrary refractive index distribution. You can ask.

Next, a method for evaluating the core radius will be described.
The mode field radii W (λ, z) and λ _c (z) are measured by the method described above, and by using the following relational expression (78), the core radius a in the longitudinal direction of the single mode optical fiber is obtained. (Z) can be measured.

[0175]

[Equation 116]

Further, using the measurement method described above, W (λ ₁ , z), w (λ ₂ , z), and W (λ ₂ , z) at wavelengths λ ₁ and λ ₂ are obtained.
Measure λ _c (z) and a (z).

The normalized frequency T (z ₀ ) at an arbitrary end position z ₀ is calculated by the following equation (7) using the core radius, maximum refractive index, maximum relative refractive index difference, and shape factor at one end of the fiber.
9).

[0178]

[Formula 117]

Also, the normalized frequency T at an arbitrary position z
(Z) can be similarly expressed by the following equation (80).

[0180]

[Equation 118]

[0181] Here, the change in the maximum refractive index n _m of the longitudinal direction assumed to be small relative to the change of Δm, n _m (z) =
Using an approximation of n _m (z _0). This assumption is a sufficient approximation for ordinary fibers. Therefore, equation (7)
From (9) and (80), the maximum relative refractive index difference Δm (z) at any position can be evaluated using the following equation (81). In this method, it is not necessary to accurately measure the refractive index distribution shape.

[0182]

[Equation 119]

Next, a method for evaluating variance will be described.

In general, the chromatic dispersion σ _T can be expressed as the sum of the material variance σ _m and the structural variance σ _w .

[0185]

Σ _T = σ _m + σ _w (82) where the material variance and the structural variance are expressed by the following equations (83),
This can be described in (84). Here, N ₁ represents the group refractive index at the core, and c represents the speed of light.

[0186]

[Equation 121]

[0187]

[Equation 122]

First, a method of measuring the dispersion of the waveguide will be described. From equation (84), it can be seen that the waveguide dispersion can be evaluated from the wavelength dependence of the mode field radius of the optical fiber. From the relational expression of Expression (73), the wavelength dependence of the mode field radius can be expressed by the following Expression (85).

[0189]

[Equation 123]

Accordingly, the mode field radius W (z, λ) can be evaluated by measuring a (z) and λ _c (z). Equation (84) can be transformed into the following equation (86).

[0191]

(Equation 29) … (86)

On the other hand, by substituting equation (85) into equation (86), the following equation (87) for evaluating waveguide dispersion at an arbitrary wavelength λ can be obtained.

[0193]

[Equation 125]

Therefore, the waveguide dispersion σ _w (z, λ) at an arbitrary position z and a wavelength λ is calculated as c ₀ ,
The coefficients can be determined by determining the coefficients of c ₁ and c ₂ , measuring a (z) and λ _c (z) at arbitrary positions, and substituting those values into equation (87).

Next, a method for measuring the material dispersion will be described. Generally, the material dispersion of an optical fiber is given by the following equation (88).
It is given by the relational expression as shown by.

[0196]

[Equation 126]

To obtain this value, the wavelength dependence of the refractive index is required. The wavelength dependence of the refractive index is given by the following equation (8)
It can be obtained by using the Sellmeier relational expression as shown in 9).

[0198]

[Equation 127]

Equation (89) is an equation showing the physical content representing the wavelength dependence of the refractive index, where A _{i corresponds} to the ultraviolet and infrared absorption wavelengths and B _i corresponds to the oscillator strength. It is known that measured values are well measured in a wide wavelength range including a wavelength having a large dispersion. It is known that sufficient accuracy can be obtained when the value of k is used up to three terms (references B. Brinxner, J. Op.
t. Soc. Am., vol. 55, pp. 1205, 1967). The Cellmeier coefficients A _i and B _i of glass used for optical fibers are described in the literature (S. Kobayashi, S. Shibata,
N.Shibata, and T.Izawa, International Conference o
n Integrated Optics and Optical Fiber Communicatio
n, Tokyo, 1977, Technical Paper P309).

Therefore, the material dispersion is obtained by converting equation (89) into equation (8)
It can be obtained by substituting into 8).

Using the method for measuring the relative refractive index difference Δm (z) described above, the relative refractive index difference Δ in the longitudinal direction of the optical fiber is used.
m (z) is determined, and the coefficients A _i and B _i in equation (89) corresponding to Δm (z) are determined from the material values constituting the optical fiber, thereby obtaining the material dispersion σ in the longitudinal direction of the optical fiber. _m ( λ, z) can be evaluated.

When the core of the optical fiber is made of GeO ₂ -doped quartz glass, the following equation (90) can be derived using equations (88) and (89).

[0203]

Equation 128] _{σ m (λ, z) =} -583.36 + 1009.91λ-587.86λ 2 + 121.31λ 3 -Δ m (z) (89.48-152.20λ + 92.13λ 2 - 18.79λ 3) ... (90) Therefore, When the core is GeO ₂ -doped quartz glass,
By substituting Δm (z) and wavelength λ in equation (90), material dispersion σ _m (λ, z) can be obtained.

Next, a method for obtaining chromatic dispersion will be described. Since the waveguide dispersion σ _w (λ, z) and the material dispersion σ _m (λ, z) in the longitudinal direction of the optical fiber can be obtained by the method as described above, the waveguide relation is given by the following relational expression (91). By substituting the dispersion and the material dispersion, the chromatic dispersion σ _T (λ, z) in the longitudinal direction of the optical fiber can be measured.

[0205]

Σ _T (λ, z) = σ _w (λ, z) + σ _m (λ, z) (91) (Embodiment 4) FIG. 13 illustrates a fourth embodiment of the present invention. It is a block diagram. Here, reference numeral 1 denotes a pulse light source having a pulse width suitable for a distance resolution for evaluating the characteristics of the fiber to be measured. When the distance resolution is about 1 m, the pulse light source needs to have a pulse width of 10 ns. 2
Is an n × 1 optical switch for switching the wavelength of the pulse light source 1. Reference numeral 3 denotes a polarization control device for adjusting the polarization of the pulse light incident on the optical fiber 7 to be measured.
Reference numeral 4 denotes a multiplexer / demultiplexer for demultiplexing the light incident on the measured fiber 7 and the backscattered light from the measured fiber 7. 5
Is a 1.times.2 optical switch for making light incident on one of the two ends of the optical fiber to be measured. 9 is a photodetector 8
This is a signal processing device for receiving an electric signal from a computer and analyzing a temporal waveform of the signal. Reference numeral 10 denotes an arithmetic processing unit for accumulating or processing and analyzing the backward Rayleigh scattering waveform measured from both ends of the optical fiber 7 to be measured at two or more wavelengths of the fiber 7 to be measured. Reference numeral 11 denotes a control device for controlling the optical switches 2 and 5, and the polarization controller 3. Reference numeral 12 denotes an output device for outputting a processing result.

The operation of this evaluation device is as follows.

An optical pulse emitted from the pulse light source 1 having the wavelength λ ₁ enters the polarization controller 3. The incident light pulse is controlled to be linearly polarized light in any one of a plurality of polarization directions in which the polarization is set in advance. Next, the light passes through the directional coupler 4 and enters the optical fiber 7 to be measured via the optical switch 5. Backscattered light from the measured fiber 7 is detected by the photodetector 8 via the optical switch 5 and the directional coupler 4. One measurement waveform obtained by the detector 8 is obtained in a form as shown in FIG. The time t until the backward Rayleigh scattered light generated at the distance z from the measured fiber 7 is detected by the photodetector 8 is expressed by the following formula (92), where V is the speed of light in the optical fiber. Therefore, this one measurement waveform can be written as Si (λ ₁ , z).

[0208]

T = 2z / V (92) Here, a plurality of measurement waveforms can be obtained one after another by repeatedly inputting the pulse light from the optical palace light source 1 to the fiber 7 to be measured. In general, when the backward Rayleigh scattered light is received in the measurement fiber, a component that fluctuates due to the polarization as shown in FIG. 15 is generated due to a change in the polarization state of the propagating light. At this time,
The polarization controller 3 changes the linear polarization state from 0 to 360 degrees,
The operation is changed according to a fixed time, or the linear polarization state is switched between 0 degree and 90 degrees.

For example, a function for switching the linear polarization state between 0 ° and 90 ° using a polarization controller will be described below.

Consider the case of the configuration shown in FIG. Here, a case where a polarizer is used as the polarization controller 3 is considered for simplicity. First, a Jones matrix M describing the polarization state is defined as in the following equation (93).

[0211]

M = R (θ) F ₀ R (−θ) (93) where R (θ) is a rotation vector represented by the following equation (94), and θ represents the principal axis angle of the fiber.

[0212]

(Equation 132)

F ₀ is the following equation (9) representing the passage of the fiber.
5) is the Jones matrix, where φ represents the magnitude of the birefringence of the fiber.

[0214]

[Equation 133]

The AO Jones matrix is given by the following equation (9)
6).

[0216]

[Equation 134]

Here, k is generally not 1. The power P _X of the light receiver when X-polarized light is incident from the light source is expressed by the following equation (97).

[0218]

[Equation 135]

Similarly, the received light power P _Y when the Y polarized light E _Y is incident is expressed by the following equation (98).

[0220]

[Equation 136]

Therefore, when the sum of the respective polarization components is P, the following equation (99) is obtained.

[0222]

137

From equation (99), it can be seen that the sum of the polarization components is a constant independent of φ and θ. Accordingly, in this case, the polarization fluctuation in the fiber can be compensated. Therefore, the characteristics of the fiber can be evaluated with high accuracy by the polarization control.

FIGS. 17 and 18 show examples of actually measuring the backscattered light waveform of the optical fiber. FIGS. 17 and 18 show values obtained by compensating the fiber loss at each length. FIG. 17 shows the result when the polarization control is not performed. When the pulse width is 100 ns, the fluctuation has about 0.08 dB, and when the pulse width is 1 μs, the fluctuation component of about 0.05 dB has a fluctuation component of about 0.08 dB. is there. On the other hand, FIG.
In the system as shown in FIG. 16, the result of measuring the backscattered light while switching the angle of the polarizer between the light source and the directional coupler between 0 degree and 90 degrees (orthogonal method) and rotating the polarizer The result (rotation method) of the measurement is shown. By comparing FIG. 17 with FIG. 18, it can be seen that the magnitude of the fluctuation is drastically reduced by clearly controlling the polarization.

Under the above configuration, O
When the TDR waveform can be measured, the results can be used to evaluate the characteristics of the single mode optical fiber according to the steps shown in FIGS.

(Embodiment 5) FIG. 19 is a block diagram for explaining a fifth embodiment of the present invention. Here, reference numeral 21 denotes a pulse light source having a pulse width suitable for the distance resolution for evaluating the characteristics of the fiber to be measured. When the distance resolution is about 1 m, it is necessary to have a pulse width of 10 ns. Reference numeral 22 denotes an n × 1 optical switch for switching the wavelength of the pulse light source 21. Reference numeral 23 denotes a polarization control device for adjusting the polarization of the pulse light incident on the measured optical fiber 27. Reference numeral 24 denotes a multiplexer / demultiplexer for splitting the light incident on the measured fiber 27 and the backscattered light from the measured fiber 27. Reference numeral 25 denotes a 1 × 2 optical switch for making light incident on one of both ends of the optical fiber 27 to be measured. Reference numeral 26 denotes a reference fiber used for analyzing characteristics of the optical fiber under measurement 27. 29 is
This is a signal processing device for receiving an electric signal from the photodetector 28 and analyzing a temporal waveform of the signal. Reference numeral 20 denotes an arithmetic processing unit for accumulating or analyzing the rear Rayleigh scattered waveform measured from both ends of the reference fiber 26 and the measured optical fiber 27 at two or more wavelengths of the measured fiber 27. . 31 is an optical switch 22
And 25, and a control device for controlling the polarization controller 23. Reference numeral 32 denotes an output device for outputting a processing result.

The operation of this evaluation device is the same as that of the fourth embodiment, and a description thereof will be omitted. In this embodiment, since the reference fiber 26 is connected to both ends of the measured fiber 27, it is not necessary to measure the mode field diameter of the measured fiber 27. That is, reference fiber 2
By measuring the wavelength dependence of the mode field diameter in 6 in advance, it is possible to measure the mode field diameter distribution and dispersion distribution of a fiber having an arbitrary refractive index distribution.

[0228]

As described above, the present invention provides a method for measuring the backscattering waveform and the mode field radius and the refractive index distribution at one end of a fiber using a light source having two or more wavelengths. Non-destructive evaluation of the longitudinal properties of a single mode optical fiber using measured values of mode field radius W, core radius a, maximum refractive index _nm and maximum relative refractive index difference Δm at one end of the fiber. This has the effect that it can be performed. Further, the present invention has an advantage that it can be applied to a fiber having any refractive index distribution. Furthermore, in this measurement device, the polarization fluctuation can be greatly improved by controlling the polarization of the incident pulse and measuring the backscattered light of the optical fiber. It has the advantage that measurement accuracy can be improved.

[Brief description of the drawings]

FIG. 1 is a process chart showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a backscattering waveform at 1.31 μm.

FIG. 3 is a diagram showing a backscattering waveform at 1.55 μm.

FIG. 4 is a diagram showing a mode field diameter in the longitudinal direction at 1.31 μm.

FIG. 5 is a diagram showing the mode field diameter in the longitudinal direction at 1.55 μm.

FIG. 6 is a diagram showing a measurement result of a cutoff wavelength in a longitudinal direction.

FIG. 7 is a diagram showing a core diameter 2a in a longitudinal direction.

FIG. 8 is a diagram showing a relative refractive index difference Δ in a longitudinal direction.

FIG. 9 is a diagram illustrating a waveguide dispersion characteristic in a longitudinal direction.

FIG. 10 is a diagram showing material dispersion characteristics in a longitudinal direction.

FIG. 11 is a diagram illustrating chromatic dispersion characteristics in the longitudinal direction.

FIG. 12 is a process chart showing a third embodiment of the present invention.

FIG. 13 is a block diagram showing a fourth embodiment of the present invention.

FIG. 14 is a diagram illustrating an example of a backscattered waveform.

FIG. 15 is a diagram showing a backscattered waveform having polarization fluctuation.

FIG. 16 is a diagram for explaining the effect of polarization control.

FIG. 17 is a diagram showing a conventional backscattering waveform when polarization control is not performed.

FIG. 18 is a diagram illustrating a backscattering waveform when polarization control is performed.

FIG. 19 is a block diagram showing a fifth embodiment of the present invention.

[Explanation of symbols]

 1,21 pulse light source 2,22 n × 1 optical switch 3,23 polarization controller 4,24 directional coupler 5,25 1 × 2 optical switch 7 fiber under test 8,28 photo detector 9,29 Signal processing device 10, 30 Arithmetic processing device 11, 31, Control device 12, 32 Output device 26 Reference fiber

──────────────────────────────────────────────────続 き Continued on the front page (56) Reference FELIX P. KAPRON et al. , Fiber-Optic Reflection Measurements Using OCWR and OTDR Technologies, J. et al. Lightwave Technology. , USA, IEEE, Vol. 7, No. 8,1224-1241 (58) Field surveyed (Int. Cl. ⁷ , DB name) G01M 11/00 JICST file (JOIS)

Claims (22)

(57) [Claims] 1. A backscattered light intensity S ₁ ( λ, z) at a position z of a single-mode optical fiber having an arbitrary length L when a pulse light having a wavelength λ is incident from one side of the optical fiber.
(Unit: dBm) and the scattered light intensity S ₂ when the pulse light having the wavelength λ similarly enters from the other side of the optical fiber.
( Λ, Lz) (unit: dBm), and the arithmetic mean intensity I (λ,
z) is obtained from the following relational expression (1). ... (1) arithmetic mean intensity at position _{z 0} of any one I (lambda, z
₀ ), the normalized arithmetic average intensity I ′ (λ, z) shown in the following equation (2) obtained by standardizing I (λ, z) is obtained. I ′ (λ, z) = I (λ, z) −I (λ, z ₀ ) (2) The mode field radius W (λ, z ₀ ) at the position z _0
0 ), the longitudinal mode field radius of the single mode optical fiber is determined by the following relational expression (3):
A method for evaluating characteristics of a single mode optical fiber, characterized in that: (Equation 2) (3) 2. Using the measurement method of claim 1, wherein, any two wavelengths lambda ₁ and lambda ₂ in a single mode optical fiber mode field radius _{W (λ 1, z),} W (λ 2,
z) is measured, and any one end of the single mode optical fiber is measured.
The refractive index distribution in the cross section of the optical fiber is measured at the position z ₀ , and the electromagnetic wave equation (where T _c is cut off) for the normalized frequency T represented by the following equation (4) using the refractive index distribution: Is the frequency, and λ _c is the cut-off wavelength). (4) The coefficients c ₀ , c ₁ , and c ₂ of the following relational expression (5) representing the cut-off frequency T _c and the ratio of the mode field radius W to the core radius a (= W / a) are obtained. 4] ... (5) The ratio R (λ ₁ , λ ₂ , z) of the mode field radius of the _two wavelengths λ ₁ and λ ₂ expressed by the following equation (6) (= W
(Λ ₁ , z) / W (λ ₂ , z)) is evaluated, and (6) where ΔI ( λ ₁ , λ ₂ , z) is expressed by the following equation (7). ΔI (λ ₁ , λ ₂ , z) = I ′ (λ ₂ , z) −I ′
(Λ ₁ , z) (7) A single-mode optical fiber characteristic evaluation method characterized by evaluating a cutoff wavelength according to the following relational expression (8). (Equation 6) … (8) 3. The mode field radius W (λ ₁ , λ ₁₎ of a single-mode optical fiber at any two wavelengths λ ₁ and λ ₂ using the measurement method according to claim 1 and 2.
z), W (λ ₂ , z) and cutoff wavelength λ _c (z) are measured, and one of the following two relational expressions (9) and (10) is used to obtain the single mode optical fiber. Measuring the core radius a (z) in the longitudinal direction of the single-mode optical fiber. (Equation 7) ... (9) … (10) 4. A mode field radius W (λ ₁ , z), W using the measuring method according to claim 1, 2, or 3.
(Λ ₂ , z), cutoff wavelength λ _c (z) and core radius a
(Z) is measured, the refractive index distribution in the cross section of the single mode optical fiber at an arbitrary end position z ₀ is measured, and the shape factor S representing the refractive index distribution shape is obtained from the following relational expression (11). (Where ρ is the normalized radius (= r / a; r is the distance from the center of the optical fiber cross section), and Δm is the maximum relative refractive index difference). (11) The maximum refractive index _nm , S, and _Tc in the fiber cross section obtained from the refractive index distribution measurement at an arbitrary end position of the optical fiber are substituted into the following relational expression (12). And measuring the maximum relative refractive index difference Δm (z) in the longitudinal direction. (Equation 10) … (12) 5. A mode field radius W (λ _i , z) (i = 3) at three or more wavelengths using the measurement method according to claim 1.
1, 2, 3,... N), and coefficients g ₀ (z), g ₁ (g ₁ ) satisfying the equation (13) using the mode field radius W (λ _i , z) at each wavelength. z) and g
₂ (z) and calculated, W (λ, z) = g 0 (z) + g 1 (z) λ 15 + g 2 (z) λ 6 ... (13) of the refractive index distribution at the position of any one of the fiber The maximum refractive index _nm is obtained from the measurement, and the waveguide dispersion σ _w (λ, z) at the wavelength λ is obtained from the following relational expression (14) (where,
c is the speed of light). [Equation 11] … (14) 6. A mode field radius W (λ _i , z) (i = 1, 2) at two wavelengths using the measurement method according to claim 1.
2) and measure the mode field radius W at each wavelength.
Using (λ _i , z), coefficients g ₀ (z) and g ₁ (z) satisfying the following equation (15) are obtained, and W (λ, z) = g ₀ (z) + g ₁ ( z) λ ^1.5 (15) The maximum refractive index _nm is obtained from the measurement of the refractive index distribution at an arbitrary end of the fiber, and the waveguide dispersion σ _w at the wavelength λ is obtained from the following relational expression (16). (Λ, z) (where,
c is the speed of light). (Equation 12) … (16) 7. The method according to claim 4, wherein a relative refractive index difference Δm (z) in the longitudinal direction of the optical fiber is obtained, and Δm (z)
Constant coefficients A _i and B _{i (however,} A _i is the ultraviolet and infrared absorption wavelengths, representing B _i must support the oscillator strength, the wavelength dependence of the refractive index of the following equation corresponding (17) to Is obtained from the material values of the optical fiber,
Using the equation (17) showing the wavelength dependence of the refractive index, (17) A single mode optical fiber characteristic evaluation method characterized by evaluating the material dispersion σ _m (λ, z) in the longitudinal direction of the optical fiber from the following relational expression (18). [Equation 14] … (18) 8. The method according to claim 4, wherein a relative refractive index difference Δm (z) in the longitudinal direction of the optical fiber is obtained. When the core of the optical fiber is GeO ₂ -doped quartz, the following relational expression (19) is obtained. ) Is used to evaluate the material dispersion σ _m (z) in the longitudinal direction of the optical fiber at the wavelength λ. _{σ m (λ, z) =} -583.36 + 1009.91λ-587.86λ 2 + 121.31λ 3 -Δ m (z) (89.48-152.20λ + 92.13λ 2 - 18.79λ 3) ... (19) 9. The method according to claim 5, wherein a waveguide dispersion σ _w (λ, z) in the longitudinal direction of the optical fiber is obtained, and the material dispersion σ _{m in} the longitudinal direction of the optical fiber is obtained by the method according to claim 7.
A single-mode optical fiber characteristic evaluation method, wherein (λ, z) is obtained, and chromatic dispersion σ _T (λ, z) in the longitudinal direction of the optical fiber is measured from the following relational expression (20). σ _T (λ, z) = σ _w (λ, z) + σ _m (λ, z) (20) 10. The method according to claim 6, wherein a waveguide dispersion σ _w (λ, z) in a longitudinal direction of the optical fiber is obtained.
The material dispersion σ _{m in} the longitudinal direction of the optical fiber is obtained by the method described.
A single-mode optical fiber characteristic evaluation method characterized by determining (λ, z) and measuring chromatic dispersion σ _T (λ, z) in the longitudinal direction of the optical fiber from the following relational expression (21). σ _T (λ, z) = σ _w (λ, z) + σ _m (λ, z) (21) 11. The method according to claim 8, wherein the waveguide dispersion σ _w (λ, z) in the longitudinal direction of the optical fiber is obtained by the method according to claim 5, and the core of the optical fiber is GeO ₂ -doped quartz. And the material dispersion σ _{m in} the longitudinal direction of the optical fiber
A single-mode optical fiber characteristic evaluation method characterized by determining (λ, z) and measuring chromatic dispersion σ _T (λ, z) in the longitudinal direction of the optical fiber from the following relational expression (22). σ _T (λ, z) = σ _w (λ, z) + σ _m (λ, z) (22) 12. The method according to claim 8, wherein the waveguide dispersion σ _w (λ, z) in the longitudinal direction of the optical fiber is obtained by the method according to claim 6, and the core of the optical fiber is GeO ₂ -doped quartz. And the material dispersion σ _{m in} the longitudinal direction of the optical fiber
A single-mode optical fiber characteristic evaluation method characterized by determining (λ, z) and measuring chromatic dispersion σ _T (λ, z) in the longitudinal direction of the optical fiber from the following relational expression (23). σ _T (λ, z) = σ _w (λ, z) + σ _m (λ, z) (23) 13. A mode field radius W (λ _i , z ₀ ) (i = 1, 2) at any one end z ₀ of any length L of a single mode optical fiber and at any three or more wavelengths λ _i. , 3
.. N; n ≧ 3), the core radius a (z ₀ ) at one end of the fiber is measured, and the cutoff wavelength λ _c (z ₀ )
Is measured, and the following relational expression (2) is obtained using the measured values of the cutoff wavelengths λ _c (z ₀ ) and W (λ, z ₀ ) of the optical fiber.
The coefficients c ₀ , c ₁ , and c ₂ that satisfy 4) are obtained, and .. (24) Any two wavelengths λ ₁ using the measurement method according to claim _1.
And lambda ₂ in _{W (λ 1, z),} W (λ 2, z) is measured and expressed by the following equation (25), two wavelengths lambda ₁ and lambda ₂
Of the mode field radii of R (λ ₁ , λ ₂ , z)
(= W (λ ₁ , z) / W (λ ₂ , z)) is evaluated, and (25) where ΔI ( λ ₁ , λ ₂ , z) is expressed by the following equation (26). ΔI (λ ₁ , λ ₂ , z) = I ′ (λ ₂ , z) −I ′ (λ ₁ , z) (26) Characteristically, the cutoff wavelength is evaluated by the following relational expression (27). Method for evaluating single mode optical fiber characteristics. [Equation 17] … (27) 14. The method of claim 1, wherein the mode field radii W (λ ₁ , z), W (λ
₂ , z) and the cutoff wavelength λ _c (z) are measured, and one of the following two relational expressions (28) and (29) is used to determine the core radius in the longitudinal direction of the single mode optical fiber. A method for evaluating characteristics of a single mode optical fiber, comprising measuring a (z). (Equation 18) ... (28) … (29) 15. A cutoff wavelength λ _c (z ₀ ) at a position z ₀ at an arbitrary end of a single mode optical fiber, and a core radius a
(Z ₀ ) and the maximum relative refractive index difference Δm (z ₀ ) are measured, and using the measuring methods according to claims 1, 13 and 14,
Mode field radii W (λ ₁ , z), W (λ ₂ ,
z), the cutoff wavelength λ _c (z) and the core radius a (z) are measured, and the maximum relative refractive index difference Δm (z) of the core in the longitudinal direction is measured using the following relational expression (30). A method for evaluating characteristics of a single mode optical fiber, comprising the steps of: (Equation 20) … (30) 16. A cutoff wavelength λ _c (z) and a core radius a (z) using the measuring method according to claim 13.
Is measured, the maximum refractive index n _m (z ₀ ) of the core at an arbitrary end position of the fiber is obtained, and the following relational expression (31) is obtained.
A method for evaluating single-mode optical fiber characteristics, wherein a waveguide dispersion σ _w (λ, z) at a wavelength λ is obtained from (where c is the speed of light). (Equation 21) … (31) 17. The method according to claim 15, wherein a relative refractive index difference Δm (z) in the longitudinal direction of the optical fiber is obtained.
Coefficients A _i and B _{i of} the following equation (32) corresponding to (z)
(Where A _{i corresponds} to the ultraviolet and infrared absorption wavelengths, and B _i corresponds to the oscillator strength, and is a constant representing the wavelength dependence of the refractive index) from the material values of the optical fiber. Using the equation (32) showing the wavelength dependence of the refractive index, (32) A single mode optical fiber characteristic evaluation method characterized by evaluating the material dispersion σ _m (λ, z) in the longitudinal direction of the optical fiber from the following relational expression (33). (Equation 23) … (33) 18. The method according to claim 15, wherein a relative refractive index difference Δm (z) in the longitudinal direction of the optical fiber is obtained. When the core of the optical fiber is GeO ₂ -doped quartz, the following relational expression (34) is obtained. ) Is used to evaluate the material dispersion σ _m (λ, z) in the longitudinal direction of the optical fiber at the wavelength λ. _{σ m (λ, z) =} -583.36 + 1009.91λ-587.86λ 2 + 121.31λ 3 -Δ m (z) (89.48-152.20λ + 92.13λ 2 - 18.79λ 3) ... (34) 19. A method according to claim 16, wherein a waveguide dispersion σ _w (λ, z) in a longitudinal direction of the optical fiber is obtained.
The material dispersion σ _m (λ, z) in the longitudinal direction of the optical fiber is obtained by the method described in 7 or 8 , and the chromatic dispersion σ _T (λ, z) in the longitudinal direction of the optical fiber is obtained from the following relational expression (35). A single mode optical fiber characteristic evaluation method characterized by measuring. σ _T (λ, z) = σ _w (λ, z) + σ _m (λ, z) (35) 20. The method according to claim 16, wherein a waveguide dispersion σ _w (λ, z) in the longitudinal direction of the optical fiber is obtained, and when the core of the optical fiber is GeO ₂ -doped quartz.
The material dispersion σ _{m in} the longitudinal direction of the optical fiber is obtained by the method described.
A single-mode optical fiber characteristic evaluation method characterized by determining (λ, z) and measuring chromatic dispersion σ _T (λ, z) in the longitudinal direction of the optical fiber from the following relational expression (36). σ _T (λ, z) = σ _w (λ, z) + σ _m (λ, z) (36) 21. A pulsed light source having two or more wavelengths for generating backward Rayleigh scattered light on a measured fiber, a wavelength switching unit for switching a wavelength of the light source incident on the measured optical fiber, a polarization control means for controlling the polarization state of the pulse light source, the light emitted from the polarization control unit guided to the fiber to be measured, and taken out backward Rayleigh scattered light generated in該被measured optical fiber Multiplexing / demultiplexing means, optical switching means for causing light emitted from the multiplexing / demultiplexing means to be incident on one of both ends of the fiber under measurement, and a rear Rayleigh detected by the multiplexing means. 21. A signal processing means for analyzing a temporal waveform of scattered light, and a rear Rayleigh scattered waveform measured from both ends of two or more wavelengths of the measured optical fiber according to any one of claims 1 to 20. Processing means for performing analysis processing using a method, an output processing device for outputting a result processed by the calculation processing means, means for switching the incident position of the measured fiber, and the wavelength of the light source A single mode optical fiber characteristic evaluation device, comprising: control means for controlling a switching means. 22. A pulsed light source having two or more wavelengths for generating backward Rayleigh scattered light on the measured fiber; a wavelength switching unit for switching a wavelength of the light source incident on the measured optical fiber; A polarization control unit for controlling the polarization state of the pulse light source, and a device for guiding light emitted from the polarization control unit to the fiber to be measured and extracting backward Rayleigh scattered light generated in the optical fiber to be measured. Multiplexing / demultiplexing means; optical switching means for causing light emitted from the multiplexing / demultiplexing means to be incident on one of both ends of the measured fiber; and both ends of the wavelength switching means and the measured fiber A reference fiber having a known fiber characteristic for guiding the light of the light source, and a signal processing means for analyzing a temporal waveform of the backward Rayleigh scattered light detected by the multiplexing means. 21. An arithmetic processing means for analyzing the backward Rayleigh scattering waveform measured from both ends of two or more wavelengths of the optical fiber to be measured using any one of the methods according to claim 1; An output processing device for outputting the result processed by the means, a switching means for switching the incident position of the fiber to be measured, and a control means for controlling the wavelength switching means of the light source. Single-mode optical fiber characteristics evaluation device.