JP2001074597A - Testing method for branched optical lines, and the branched optical lines with testing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inexpensively estimate loss distribution of each branched optical fiber after branched, in a branched optical line. SOLUTION: Pulsated testing lights with plural wavelengths are made incident from a wavelength-variable OTDR testing device 11 to an optical fiber 19 before branched as a branched optical line, to detect the reflected light from optical fibers 21 after being branch connected via a branching element 20. Simultaneous linear equations showing the relation between the sum of detected lights from the respective optical fibers 21 after being branched and the respective lights from the respective optical fibers 21 after branched based on respective branching ratios are solved, using a measured result hereinbefore and the branching ratios to respective branched lines in the wavelengths of the testing lights in the branching element 20, so as to estimate loss distribution in the longitudinal direction of the each optical fiber 21 after being branched.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a 1-input N-output branch element, a pre-branch optical fiber connected to the input side end of the 1-input N-output branch element, and the 1-input N-output branch element. In a branch optical line composed of N optical fibers after branching connected to each end on the output side of the optical fiber, a pulse-like test light is sent to the optical fiber before branching from each point in the longitudinal direction of the optical fiber after branching. The present invention relates to a test method for estimating the loss distribution in the longitudinal direction of the branched optical fiber by detecting reflected light of the optical fiber, and a branch optical line including a test device for realizing the test method.

[0002]

2. Description of the Related Art Various network configuration technologies are being studied for FTTH, which provides high-speed video and data transmission services to homes and offices. As a technology to provide an inexpensive FTTH network,
Practical use of a branch optical line in which a core optical fiber is branched into N-core optical fibers on the subscriber side by a branch element is being studied. By the way, for the purpose of promptly detecting a fault occurring in an optical line and guaranteeing the reliability of communication, a maintenance technique for the optical line, especially a technique for estimating a loss distribution in the longitudinal direction of the optical line is important. OTDR is one of the optical line maintenance technologies.
Techniques have been established for estimating the loss distribution in the longitudinal direction of an optical line using a test apparatus.

[0003] An optical line test method using an OTDR test apparatus is usually such that an OTDR test apparatus is installed in a station, and optical fibers to be tested are sequentially selected by an optical switch.
The test light is transmitted from the OTDR test device, is incident on the optical fiber, and the reflected light, which is the backscattered light from each point of the optical fiber, is detected by a photodetector incorporated in the OTDR test device, and the waveform is measured by a CRT. By observing the above, the loss distribution in the longitudinal direction of the optical line is estimated.

[0004] When a test system using a conventional OTDR test apparatus is applied to a branch optical line, there are the following problems. In the branch optical line, 1
Since the optical fiber before branching is connected to a plurality of optical fibers after branching at the subscriber side via the branching element, the test light sent to the optical fiber before branching at the station side is transmitted by the branching element. Demultiplexed and incident on the optical fiber after a plurality of branches,
The reflected light from each point in the longitudinal direction of the optical fiber after each branch is multiplexed again by the branching element, and detected by the light receiver of the OTDR test apparatus placed in the station. That is, since reflected light from a plurality of branched optical fibers is superimposed and detected by a light receiver, for example, a failure occurs in a specific single-fiber branched optical fiber and changes to reflected light detected by the light receiver. However, it is not possible to specify which post-branch optical fiber causes the change.

The loss distribution of each post-branch optical fiber in the branch optical line is measured by arranging a pulse-like test light on one of the pre-branch optical fibers at the station side, detecting the reflected light, and detecting the reflected light.
As a conventional technique of estimating by observing at 199,
There is a technique described in the IEICE General Conference Proceedings B-672 "Remote individual loss distribution measurement of branch optical line". FIG. 6 shows the test method described in the above document. In FIG. 6, reference numeral 61 denotes a variable wavelength OTDR, 62 denotes an optical fiber before branching, 63 denotes an optical fiber after branching, and 64 denotes a wavelength-selective optical branching circuit. The branch optical line in FIG. 6 is composed of 14 post-branch optical fibers.

[0006] Wavelength tunable OTDR61 sequentially oscillates pulsed test light having a wavelength of lambda ₁₄ from lambda _1. Wavelength selective optical branching circuit 64 serves spectrometer directing the lambda ₁ from lambda ₁₄ wavelength of the pulsed test light from the first, respectively the 14 th branch halo fiber. Therefore, since the test light of wavelength λ _i is guided only to the i-th post-branch optical fiber, if the reflected light is detected, only the reflected light from the i-th post-branch optical fiber is detected. And the loss distribution in the longitudinal direction of the i-th branched optical fiber can be estimated.

[0007]

However, the prior art for estimating the loss distribution of a branch optical line using such a wavelength-selective optical branch circuit has the following problems.
As the wavelength-selective optical branching circuit 64, it is necessary to use an element having excellent coupling with an optical fiber having a spectral function.
Although a waveguide-type spectroscopy element is used, AWG has a disadvantage that the price for forming a branch optical line is high because of its high price. Since the AWG needs to be used for all of the branch portions of the branch optical line, for example, in the branch optical line of FIG. One AWG
Therefore, the advantage of using the branch optical line to configure the subscriber system at a low cost is offset by the high price of the AWG, which is not practical. Also,
Since the AWG has a steep spectral characteristic of guiding light of a specific wavelength only to a specific branched optical fiber, the spectral characteristic changes according to a change in the surrounding environmental temperature. Therefore, AW
In actual use of G, it is necessary to devise a method for maintaining its environmental temperature constant.

The present invention provides a 1-input N-output branch element, a pre-branch optical fiber connected to an input end of the 1-input N-output branch element, and an output side of the 1-input N-output branch element. Of a branch optical line comprising N optical fibers after branch connected to respective ends of the optical fiber, and a branch optical line which realizes estimation of the loss distribution in the longitudinal direction of each of the optical fibers after branch using a wavelength tunable OTDR at low cost. It is an object of the present invention to provide a test method and a branch optical line including a test apparatus for realizing the test method.

[0009]

SUMMARY OF THE INVENTION The present invention provides a 1-input N-output branch element, a pre-branch optical fiber connected to an input end of the 1-input N-output branch element, and a 1-input N-output. A pulse-like test light is transmitted to the pre-branch optical fiber of the branch optical line composed of the N-fibers of the post-branch optical fiber connected to the respective ends on the output side of the branch element, and is transmitted to the pre-branch optical fiber via the branch element. And the reflected light from each point in the longitudinal direction of each of the branched optical fibers is detected, and the waveform of the reflected light is observed on a CRT, whereby the longitudinal loss distribution of the branched optical fiber is measured. The pulsed test light has a variable wavelength, and the branching ratio of the optical power from the pre-branch optical fiber to the post-branch optical fiber of the branching element is respectively Exam A plurality of wavelengths of the pulse-like test light to be transmitted to the pre-branch optical fiber so as to be different from each other at a wavelength of the reflected light from the respective post-branch optical fibers with respect to each of the plurality of test lights. Detecting a summation, calculating reflection light from each of the post-branch optical fibers based on the branching ratio, and estimating a loss distribution of each of the post-branch optical fibers. And a branch optical line provided with a test apparatus for realizing the test method.

That is, the branching element does not need a function as a spectroscope for guiding light of a specific wavelength to an optical fiber after a specific branch as shown in the prior art of FIG. It is only necessary that the branching ratio to the optical fiber after each branching changes gradually. As a result, the requirements for the performance required for the branch element are greatly relaxed, and the configuration of the branch optical line is realized at low cost. Also, since the branching ratio to the optical fiber after each branching changes gradually with the change in wavelength,
The change of the branching ratio with respect to the change of the environmental temperature is small, and it is not necessary to keep the environmental temperature of the branch element constant.

[0011]

FIG. 1 shows a configuration of a method for testing a branch optical line according to the present invention. In FIG. 1, 11 is OTDR
A light source 1 which is a test device and emits pulsed test light.
3. Photodetector 1 for detecting reflected light from optical fiber after branch
2. It comprises a first optical fiber coupler 14. O
The TDR test apparatus 11 is connected to a first fiber selector 17 via an optical fiber 22. The first optical fiber coupler 14 transmits the test light from the light source 13 to the optical fiber 22.
And a function of guiding the reflected light returning from the first fiber selector 17 via the optical fiber 22 to the light receiver 12.

The second fiber selector 15 is equipped with N fiber Bragg gratings 16 having different reflection wavelengths, and can select a fiber Bragg grating of a specific wavelength. That is, when a specific pair of terminals of the second fiber selector 15 is selected, the fiber Bragg grating 16 having a specific reflection wavelength whose reflection wavelength is λ _i is set to the OTDR test apparatus 1.
1 terminal 11a.

The light source 13 is a Fabry-Perot type laser diode for generating a pulse-like test light, and the transmittance of the light emitting side end face 13a is approximately 100%. In this case, the fiber Bragg grating 16 and the end face 13b on the opposite side of the light emitting side end face 13a constitute a laser resonator, and the fiber Bragg grating 16
Is selected as the output end 15 of the second fiber selector 15
From b, in the pulse light having a wavelength lambda _i which corresponds to the Bragg wavelength of the fiber Bragg grating 16 is emitted.

The first fiber selector 17 selects a specific one-core optical fiber 17 a, so that the pulse light is transmitted to a specific pre-branch optical fiber 19 connected via a second optical fiber coupler 18. Be guided. That is, the first
Has a function of selecting a specific pre-branch optical fiber from a large number of pre-branch optical fibers. The selected specific pre-branch optical fiber 19 is connected to N post-branch optical fibers 21 via a branching element 20. By the way, in optical communication, light having a wavelength of 1.31 μm is usually used as communication light. In the test of the optical branch line, a test light having a wavelength different from the communication light is used. Is used. For the communication light, it is desirable that the ratio of the signal light distributed to the optical fiber after each branch, that is, the branch ratio is equal. On the other hand, in order to estimate the loss distribution of the optical fiber after branching, the branching ratio of the branching element needs to be different for each wavelength of the plurality of test lights.

As a method of realizing such a one-input / N-output branch element having such wavelength dependence, there is a method of forming a one-input / two-output optical fiber coupler in multiple stages.
The 1-input 2-output optical fiber coupler constituting the 1-input N-output branch element is formed by removing the coating of the two optical fibers, placing the two optical fibers in close contact in parallel, and heating the two fibers. This optical fiber is fused and drawn over a predetermined length in the longitudinal direction to produce a two-input two-output optical fiber coupler, and only one of the input terminals of the obtained two-input two-output optical fiber coupler is connected. It can be realized by using it, but by controlling the fusion length and the elongation ratio, it is possible to realize a 1-input 2-output optical fiber coupler having various characteristics.

In such a one-input two-output optical fiber coupler, the branching ratio changes sinusoidally with wavelength.
For example, as shown in FIG.
One-input / two-output optical fiber coupler having a branching ratio of 1: 1 at 55 μm, ie, a wavelength of 1.31 μm and a wavelength of 1.5
At 5 μm, it is possible to realize an optical fiber coupler that evenly distributes the optical power incident from the input terminal to the two output terminals.

On the other hand, by controlling the fusion length and the extension length of the optical fiber coupler, for example, as shown in FIG. 3, an optical fiber coupler having a wavelength of 1.31 μm, 1.53 μm and a branching ratio of 1: 1. 1.31μ as shown in Fig. 4
It is also possible to obtain an optical fiber coupler having a branching ratio of 1: 1 at m and a wavelength of 1.59 μm. That is, to realize an optical fiber coupler in which the branching ratio at the communication light wavelength of 1.31 μm is maintained at 1: 1 and the branching ratio near the test light wavelength of 1.55 μm is freely changed. Is possible.

As shown in FIGS. 2 to 4, 1.55 μm
The wavelength dependence of the branching ratio in the 1.55 μm band differs between optical fiber couplers having different wavelengths at which the branching ratio in the band is 1: 1.

FIG. 5A shows a method of configuring the branching element 20 having the structure of FIG. 1 according to the present invention with a one-input two-output optical fiber coupler. FIG. 5 shows seven 1-input / 2-output optical fiber couplers 51 and 52, each of which is a branching element for branching an optical fiber before branching into an optical fiber after branching into eight optical fibers.
The optical fiber coupler 51 is 1560 nm, 52 is 1540 nm, 53 is 1580 nm, and 54 is 1530 n.
m and 55 are 1550 nm, 56 is 1570 nm, and 57 is 1590 nm, and the branching ratio is 1: 1. Also,
The optical fiber couplers 51 to 57 have a branch ratio of 1: 1 at 1.31 μm.

FIG. 5B shows a calculation result of the branch ratio of the optical power to each branch line corresponding to each test wavelength when the branch element of FIG. 5A is used. FIG. 5 (B)
In the upper row, the wavelength of the test light is shown in the upper row, and the right column shows the core number of the branched optical fiber. The test light wavelength is 15
25 nm, 1535 nm, 1545 nm, 1555n
m, 1565 nm, 1575 nm, 1585 nm, 15
95 nm. The numerical value described in the column of the intersection of the row corresponding to the specific wavelength in the upper row and the row corresponding to the core number of the specific post-branch optical fiber in the right column is distributed to the post-branch optical fiber at the wavelength. Of the optical power, that is, the branching ratio. The sum of the optical power distributed to all the branched optical fibers at a specific wavelength is 1.

The method of obtaining the loss distribution of the branch optical line from the numerical values shown in FIG. 5B is as follows. Each wavelength of the test light is λ _i (i = 1 to N), and the measurement result of the reflected light from the optical fiber after branching when the test light of λ _i is used is D (λ _i ) and the test of λ _i If the branching ratio of the light to the k-th branched optical fiber is R _k (λ _i ), and the reflected light indicating the longitudinal loss distribution state of the k-th branched optical fiber is W _k , D (λ _i ) = Σ _{k = 1toN} R _k (λ _i ) * W _k holds, and N simultaneous linear equations hold by changing the wavelength of the test light from λ ₁ to λ _N. The reflected light W _k (K = 1 to N) indicating the loss distribution state in the direction can be obtained as a solution. Here, Σ _{k =} 1 to N indicates the sum total from 1 to N. R _k (λ _i ) * W _k indicates the product of R _k (λ _i ) and W _k . The arithmetic unit for calculating W _k (K = 1 to N) from D (λ _i ) (i = 1 to N) based on the above equation is based on the OTD shown in FIG.
OTDR test equipment 11 even if installed in R test equipment 11
Any of them may be installed separately.

In order to uniquely obtain the solution of the simultaneous linear equation, the number of simultaneous equations, that is, the number of wavelengths of the test light may be the same as the number of optical fibers after branching. If the number is larger than the number of cores of the branch optical line, no particular problem occurs. On the other hand, if the number of wavelengths of the test light is smaller than the number of cores of the post-branch optical fiber, the solution of the simultaneous linear equation is not uniquely determined, so the loss distribution of the specific post-branch optical fiber of the branch optical line is obtained. I can't do that.
The one-input two-output optical fiber coupler constituting the branch element used in FIG. 5 has a wavelength of 1.31 μm, which is the wavelength of communication light.
Since the branching ratio is 1: 1 in such a case, the branching elements configured in a tree shape have a function of equally distributing the communication light to the optical fiber after branching.

[0023]

As described above, in the method for testing a branch optical line composed of a single-fiber pre-branch optical fiber and a plurality of post-branch optical fibers connected through a branch element, the single-fiber pre-branch optical fiber is provided with: A pulsed light transmitting wavelength variable light source is coupled, and the test light is incident on the optical fiber after branching through the branching element. At this time, the branching ratio of the branching element is different at each wavelength of the test light. Select a plurality of wavelengths of the pulsed test light to be sent to the pre-branch optical fiber, detect reflected light from the post-branch optical fiber for the plurality of wavelengths of test light, and observe the waveform on a CRT. Based on the branching ratio, the system calculates the reflected light from each post-branch optical fiber by solving a system of linear equations to estimate the loss distribution in the longitudinal direction of each of the post-branch optical fibers. It becomes possible. Further, the loss distribution in the longitudinal direction of the post-branch optical fiber can be estimated without using an expensive element such as AWG and having a characteristic of a branching ratio that is sensitive to a change in environmental temperature as a branching element constituting the branching optical line. Becomes possible.

In order to uniquely determine the solution of the simultaneous linear equations, the number of the branch optical lines and the number of the wavelengths of the test light need only match, so that the loss of the N branched optical fibers can be estimated. The N-ary simultaneous linear equation may be solved based on the result of measurement using N wavelengths, and the number of test light wavelengths required for the measurement is equal to the number of test light wavelengths required in the prior art. The same as the prior art shown in FIG.
There is no problem that the number of wavelengths of the test light increases and the measurement time increases. It is to be noted that since a technique for obtaining required characteristics is relatively easy for a 1-input 2-output optical fiber coupler, it is necessary to form a 1-input N-output branch element by combining these in multiple stages to reliably obtain the required characteristics. It is effective for. In addition, 1 which constitutes a 1-input N-output branch element
In the optical fiber coupler having two inputs and two outputs, it is desirable that the branching ratio at the communication wavelength of 1.31 μm is 1: 1 in order to realize the equal distribution of the signal light.

If the wavelength dependence of the branching ratio of the plurality of one-input two-output optical fiber couplers constituting the branching element does not differ from each other, the wavelength of the test light is changed to reflect the light from the optical fiber after each branching. When the sum of light is measured and the simultaneous linear equation is solved to obtain the loss distribution of each branch optical fiber, the equations constituting the simultaneous linear equation may not be independent from each other. Since the solution of the above-mentioned simultaneous linear equations cannot be uniquely determined, the loss distribution of each branch line cannot be estimated. Therefore,
In the case where the branching element is configured by arranging a one-input two-output optical fiber coupler in a tree shape, the wavelength at which the branching ratio is 1: 1 near 1.55 μm of each one-input two-output optical fiber coupler is considered. Desirably, they are different from each other.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a test method of a branch optical line according to the present invention.

FIG. 2 is a diagram illustrating characteristics of an optical fiber coupler.

FIG. 3 is a diagram illustrating characteristics of an optical fiber coupler used in a method for testing a branched optical line according to the present invention.

FIG. 4 is a diagram illustrating characteristics of an optical fiber coupler used in a method for testing a branched optical line according to the present invention.

FIG. 5A is a diagram showing a configuration of a branch element of a branch optical line according to the present invention. (B) is a diagram showing a branching ratio at the wavelength of each test light of the branching element.

FIG. 6 is a diagram showing a test method of a branch optical line according to the related art.

[Explanation of symbols]

11: OTDR test apparatus 11a: terminal 12: light receiver 13: light source 13a: emission end face 13b: end face opposite to the emission end face 14: optical fiber coupler 15: fiber selector 15b: output end 16: fiber Bragg grating 17: Fiber selector 17a: Optical fiber 18: Optical fiber coupler 19: Optical fiber before branching 20: Branching element 21: Optical fiber after branching 22: Optical fiber 51, 52, 53, 54, 55, 56, 57: Optical fiber coupler 61: Tunable OTDR 62: Optical fiber before branching 63: Optical fiber after branching 64: Wavelength selective optical branching circuit

Claims (7)

[Claims] 1. A 1-input / N-output branch element, a pre-branch optical fiber connected to an input-side end of the 1-input / N-output branch element, and an output side of the 1-input / N-output branch element. In a branch optical line composed of a post-branch optical fiber connected to each end, a pulse-like test light is transmitted to the pre-branch optical fiber to detect reflected light from each point in the longitudinal direction of the post-branch optical fiber. A method for testing a branched optical line, which estimates a loss distribution in the longitudinal direction of the post-branch optical fiber, wherein the pulse-like test light has a variable wavelength,
A plurality of wavelengths of the pulse-like test light to be transmitted to the pre-branch optical fiber such that the respective branch ratios from the pre-branch optical fiber of the branch element to the post-branch optical fibers are different for each wavelength of the test light. Selecting, detecting the sum of the respective reflected lights from the respective post-branch optical fibers for each of the test light beams of the plurality of wavelengths, and calculating the reflected light from the respective post-branch optical fibers based on the branch ratio. And a method for estimating the loss distribution of each of the branched optical fibers. 2. The method according to claim 1, wherein the number of selected wavelengths of the pulsed test light to be transmitted to the pre-branch optical fiber is equal to the number of cores of the post-branch optical fiber forming the branch optical line. Inspection method for a branched optical line according to 1. 3. A one-input N-output branch element, a pre-branch optical fiber connected to an input-side end of the one-input N-output branch element, and an output side of the one-input N-output branch element. An OTDR test device connected to the post-branch optical fiber and the pre-branch optical fiber respectively connected to the ends, transmitting pulse-like test light having different wavelengths, and receiving reflected light from the post-branch optical fibers, The measurement results of the reflected light from the post-branch optical fiber at each test wavelength of the OTDR test apparatus and the branch ratio of the branch element to each post-branch optical fiber at each test wavelength are used to determine the respective post-branch optical fibers. A branching optical line with a test device, characterized by comprising an arithmetic device for determining a loss distribution in the longitudinal direction. 4. The branch optical line with a test device according to claim 3, wherein an optical fiber coupler is used as the branch element. 5. The branch optical line with a test apparatus according to claim 4, wherein the branch element is configured by connecting one-input two-output optical fiber couplers in multiple stages. 6. The branch optical line with a test apparatus according to claim 5, wherein the branch element has a branch ratio of 1: 1 at a wavelength of communication light. 7. The branch optical line according to claim 6, wherein the wavelength of the communication light is 1.31 μm, and the wavelength of the test light is in a 1.55 μm band.