JP3207323B2 - Optical fiber test method - 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 for testing an optical fiber using an optical time domain reflectometer (hereinafter referred to as OTDR), and more particularly, to a transmission loss, a Rayleigh scattering coefficient, and a backscatter of an optical fiber used as a communication means. The present invention relates to a method for testing characteristics such as a change in the collection rate.

[0002]

2. Description of the Related Art One of means for measuring transmission loss at a specific wavelength of an optical fiber is a method of displaying an optical intensity waveform of an optical pulse incident on the optical fiber by OTDR. FIG.
As shown in (1), an optical pulse 21a is incident from one end of the optical fiber 20, and the optical pulse 21a undergoes Rayleigh scattering during transmission through the fiber 20. A part of the scattered light returns and the backward scattered light 2 from the incident end of the light pulse 21a.
Emitted as 1b. The OTDR measures the time from when the light pulse 21a enters the optical fiber 20 to when the backscattered light 21b is emitted, and the intensity of the backscattered light 21b. Dividing the measured time by the speed of light gives the light pulse 21
The scattering position 30 of a can be calculated. Characteristics, in particular the transmission loss of the optical fiber 20 from the intensity of the backscattered light 21b can be evaluated.

[0003] P.Di Vita and U.Rossi: "Backscattering
Measurements in Optical Fibers: Separation of Power
Decay from Imperfection Contribution ", Electron.Le
According to tt., 1979, 15, pp. 467-469, the intensity of the backscattered light is represented by the following equation (1).

[0004]

(Equation 1)

[0005] where z is the longitudinal position of the fiber, P
₀ is the intensity of the incident light, α _s (z) is the Rayleigh scattering coefficient, B
(z) indicates the backscatter capture fraction (backscatter collection rate), and γ (x) indicates the loss at the fiber position x.

The light intensity waveform indicated by OTDR is
The light intensity is represented on a dB scale. Assuming that this waveform is S (z), S (z) is represented by the following equation (2).

[0007]

(Equation 2)

When the backscattering collection rate B (z) and the loss term γ (x) are stable in the longitudinal direction, S (z) is linearly decreased in the longitudinal direction as shown in FIG. The transmission loss per unit length can be obtained from the slope of this straight line.

When B (z) and γ (x) vary in the longitudinal direction, a straight line (solid line) monotonically decreasing in the longitudinal direction of S (z) becomes a wavy curve (dotted line). . However, in equation (2), since both α _s (z) and B (z) and γ (x) exist, the undulation of the waveform is caused by the fluctuation of α _s (z) and B (z). It was unclear whether it was swelling or swelling caused by a change in γ (x).

[0010]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and the wave of a waveform displayed by an OTDR (optical time domain reflectometer) has a Rayleigh scattering coefficient α _s (z). Or the variation in the backscattering collection rate B (z), the transmission loss γ
It is an object of the present invention to provide an optical fiber test method that can easily determine whether the change is caused by the variation of (x).

[0011]

FIG. 1 shows a method for testing an optical fiber according to the present invention which has been made to achieve the above object. An optical pulse 21a is incident from one end of the optical fiber 20, is scattered in the optical fiber 20, and the waveform of the backscattered light 21b propagating in a direction opposite to the incident direction is represented by an optical pulse 2
Observed from one end where 1a is incident , the time of the backscattered light
The intensity at the scattering position 30 calculated by dividing the difference by the speed of light is calculated. Light pulse 2 from one end opposite to the one end
2a is incident, and another backscattered light 2 scattered back
Observing the waveform of 2b from the opposite end ,
Position 3 calculated by dividing the time difference between the scattered lights by the speed of light
To calculate the intensity at 0. 2 calculated by the above
The optical characteristics of the optical fiber 20 are obtained by calculating the intensity of the waveform at which the two scattering positions 30 coincide.

Furthermore, obtaining the local transmission loss in Nozomu Tokoro position of the optical fiber 20 by the calculation processing.

[0013]

When the optical pulse 21a is incident from one end of the optical fiber 20, the optical pulse 21a is scattered in the optical fiber 20. A part of the scattered light propagates in the direction opposite to the incident direction, and the back scattered light 21
It emits as b. The scattering position 30 and intensity are calculated by observing the waveform of the backscattered light 21b.

An optical pulse 2 is applied from one end opposite to the one end.
2a is incident. Similarly, backscattered light 22b is emitted from one end where the light pulse 22a is scattered and incident. The scattering position 30 and the intensity are calculated by observing the waveform of the backscattered light 22b. By comparing the calculated two scattering positions 30,
By adjusting the intensity of the waveform at which the scattering position 30 coincides, an equation is obtained in which the swell component caused by the fluctuation of the Rayleigh scattering coefficient or the backscatter collection rate and the swell component caused by the fluctuation of the transmission loss are separated. The transmission loss of the fiber 20 is determined.

A pulse is applied from one end of the optical fiber,
The intensity of the waveform displayed by the OTDR is defined as S ₁ (z). S ₁ (z) is given by the following equation (3)

[0016]

(Equation 3)

[0017] Where z is the longitudinal position of the fiber, P ₀ is the intensity of the incident light, α _s (z) is the Rayleigh scattering coefficient, B (z) is the backscatter capture fraction (backscatter collection rate), γ (x) indicates the loss at the fiber position x.

Further, a pulse is applied from one end opposite to the one end, and the intensity of the waveform measured by OTDR is defined as S ₂ (LZ). S ₂ (LZ) is given by the following equation (4)

[0019]

(Equation 4)

[0020] L indicates the length of the optical fiber.

The sum of S ₁ (z) and S ₂ (LZ) is 2I (z), S
Assuming that the difference between ₁ (z) and S ₂ (LZ) is 2D (z), I (z) is expressed by Expression (5) from Expressions (3) and (4), and D (z) is expressed by Expression (5). Expression (6) is obtained from expression (3) and expression (4).

[0022]

(Equation 5)

[0023]

(Equation 6)

Equation (5) is an equation in which γ (x) is separated, and Equation (6) is an equation in which α _s (z) and B (z) are separated. t ₁ and t ₂ are constants. By formulating α _s (z) or B (z) and γ (x) separately, if the waveform becomes undulating, it is clear whether the undulation is due to scattering or transmission loss. Can be

[0025]

Embodiments of the present invention will be described below in detail. FIG. 1 is a schematic diagram showing the principle of an optical fiber testing method to which the present invention is applied. As shown in FIG.
The light pulse 21a is incident from one end of the zero. Light pulse 2
1a is scattered in the optical fiber 20. Part of the scattered light propagates in the direction opposite to the incident direction, and is emitted as backscattered light 21b from one end where the light pulse 21a is incident. The scattering position 30 and intensity are calculated by observing the waveform of the backscattered light 21b.

Light pulse 2 is applied from one end opposite to the one end.
2a is incident. The optical pulse 22a is
Scattered within. Part of the scattered light is emitted as back scattered light 22b from one end where the light pulse 22a is incident. The scattering position 30 and the intensity are calculated by observing the waveform of the backscattered light 22b. By comparing the calculated two scattering positions 30,
The transmission loss of the optical fiber 20 is obtained by adjusting the intensity of the waveform whose scattering position 30 is relatively coincident.

FIG. 2 is a schematic view of an optical fiber test apparatus. As shown in FIG. 1, a panel 2, a display 8, a random access memory 7 (hereinafter, RAM), and a waveform measuring device 10 are connected to the controller 3. The parameters necessary for the measurement are input to the controller 3, and the controller 3 stores these information in the RAM 7. Waveform measuring device 10
Is composed of a splicer 1, a light source 4, a detector 5, and a calculator 6. One end of an optical fiber 20 is connected to the splicer 1.

First, one end of an optical fiber 20 is connected to the splicer 1 of the waveform measuring device 10, and a command to start waveform measurement is sent from the panel 2 to the controller 3. In response to this command, the controller 3 operates and sends the same command to the waveform measuring device 10. The light source 4 in the waveform measuring device 10 causes an optical pulse signal to be incident on one end of the optical fiber 20 connected to the joining device 1 according to a command from the controller 3. The optical pulse signal is scattered in the optical fiber 20 and becomes backscattered light, which is sent to the detector 5 via the splicer 1. The time difference and the intensity of the backscattered light are calculated and processed by the calculator 6,
It is sent to the controller 3. The scattering position is calculated by dividing the time difference of the scattered light by the speed of light. The controller 3 stores the scattering position and intensity data in the RAM 7 and displays the waveform on the display 8.

In the waveforms measured by applying light pulses from different ends, the positional relationship between the two ends of the fiber 20 is opposite to each other. The controller 3 determines the positions of the ends of the two waveforms by the Fresnel reflection occurring in the fiber 20. By adjusting the intensity of the waveform where the scattering positions 30 (see FIG. 1) in the fiber 20 coincide, the equation (5) described above is used.
Formula (6) is created. From the equations (5) and (6), the Rayleigh scattering coefficient α _s (z), the backscatter collection rate B (z), and the transmission loss γ (x) are calculated. The controller 3 displays, on the display 8, the waveform I (z) obtained by separating the γ (x) component and the waveform D (z) obtained by separating the _αs (z) and B (z) components. .

A dispersion-shifted single-mode optical fiber 20 having a length of 20.4 km is connected to the splicer 1 of the waveform measuring instrument 10 and an optical pulse having a wavelength of 1550 nm and a pulse width of 1 μs is incident on the fiber 20. The waveform was measured. The sampling interval of the backscattered light was 80 ns.

FIG. 3 shows the measured light intensity waveform. S
₁ (z) and S ₂ (LZ) are waveform intensities when an optical pulse is applied from another end of the optical fiber. As shown in the figure, OTD is used for S ₁ (z) and S ₂ (LZ).
The directions of z indicating the positions of the fibers of the waveform indicated by R are opposite to each other. Therefore, S ₁ (z) and S
_After setting one end with ₂ (LZ), calculate the intensity in alternate directions so that each end matches. The fiber end was detected for each waveform based on the position of Fresnel reflection. S ₁ where the scattering positions of the fibers are the same
(z) that calculates the difference of the sum and S ₁ and (z) S ₂ and (LZ) and S ₂ (LZ).

FIG. 4 shows a waveform having an intensity of (S (z) + S (LZ)) / 2 and a waveform having an intensity of (S (z) -S (LZ)) / 2. I (z) shown in the figure is (S (z) + S (L-
Z)) / 2, and D (z) is (S (z) −S (LZ)) /
2. For I (z), the term indicating the transmission loss term is separated, and for D (z), the Rayleigh scattering coefficient and the backscatter collection rate are separated.

Furthermore, to calculate the fine frequency coefficients of transmission loss term in the waveform, showing changes in the longitudinal direction of the local transmission loss in FIG.

[0034]

By adjusting the intensity of the waveform having the same scattering position, the undulation of the light intensity waveform displayed by the OTDR (Optical Time Domain Reflectometer) is reduced by the Rayleigh scattering coefficient α _s (z) and the backward. It is possible to determine whether it is based on the scattering collection rate B (z) or the transmission loss γ (x). For this reason, the transmission loss of the optical fiber can be easily evaluated.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing the principle of an optical fiber test method to which the present invention is applied.

FIG. 2 is a schematic diagram of an optical fiber test apparatus.

FIG. 3 is a diagram showing a light intensity waveform measured by a test method to which the present invention is applied.

FIG. 4 is measured by a test method applying the present invention;
It is the figure which adjusted the light intensity waveform which the scattering position matched relatively.

FIG. 5 is a diagram showing a change in local transmission loss in a longitudinal direction of an optical fiber.

FIG. 6 is a schematic view showing the principle of a conventional optical fiber testing method.

FIG. 7 is a diagram showing a light intensity waveform measured by a conventional method.

[Explanation of symbols]

1 is a junction device, 2 is a panel, 3 is a controller, 4 is a light source, 5 is a detector, 6 is a calculator, 7 is a random access memory, 8
Is a display, 10 is a waveform measuring instrument, 20 is an optical fiber, 21
a · 22a is a light pulse, 21b · 22b is backscattered light,
30 is a scattering position.

Claims (2)

(57) [Claims] An optical pulse is made incident from one end of an optical fiber, scattered in the optical fiber, and the waveform of backscattered light propagating in a direction opposite to the incident direction is observed from one end where the optical pulse is made incident. Calculated by dividing the time difference of the backscattered light by the speed of light
The intensity at scattering position calculated to be, and the end applying light pulses from the opposite end, scattering the different waveform of the backscattered light that has returned to be observed from one end of the opposite side, the Calculated by dividing the time difference of another backscattered light by the speed of light
That the intensity in the scattered position is calculated, the method of testing an optical fiber, characterized in that to determine the optical characteristics of the optical fiber by the two scattered positions calculated to computing the intensity of the matched waveform by the. 2. An optical pulse is incident from one end of an optical fiber, scattered in the optical fiber, and the waveform of backscattered light propagating in a direction opposite to the incident direction is observed from one end where the optical pulse is incident. Calculate the scattering position and intensity, apply a light pulse from one end opposite to the one end, observe the waveform of another backscattered light that has been scattered and returned from one end on the opposite side, and observe the scattering position and intensity. calculates, to obtain the local transmission loss in Nozomu Tokoro position of the optical fiber by the two scattering position calculated by said to computing the differential coefficient <br/> transmission loss term in the identical waveforms A method for testing an optical fiber, comprising: