JPH11344416A - Computing method of reference value in beam line test and beam-line testing apparatus using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a computing method in which a reference value is computed with high accuracy regarding return light from an optical fiber, to be measured, in a beam line test and in which the level of the return light can be evaluated properly. SOLUTION: Prescribed optical pulses are made incident on an optical fiber 3, to be measured, via a directional coupler 2 from a light source part 1. Their return light is received by a light receiving part 4, it is passed through a digital conversion operation or the like in a digital conversion part 5, and it is then supplied to a CPU 7 so as to be stored in a RAM 8 as linear waveform data on the return light. After that, the CPU 7 computes moving average data in which the linear waveform data is averaged in every prescribed distance interval, and it computes its difference data. Then, an attenuation constant in which a return light level is expressed by an exponential function expression is computed on the basis of the difference data. An attenuation constant which uses a temporary reference value is computed on the basis of the moving average data. A correct reference value which corresponds to the attenuation constant is found on the basis of it.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for calculating a reference value in an optical line test for injecting light into an optical fiber to be measured and measuring the return light thereof, and an optical line test apparatus using the same.

[0002]

2. Description of the Related Art OTDR (Optical Time Domain Reflec)
In optical pulse testers such as tometers, by measuring the return light consisting of backscattered light and Fresnel reflected light when an optical pulse is incident on the optical fiber to be measured, loss of the optical fiber can be determined and fault points can be determined. Conduct an optical line test for detection. In such an optical line test, the absolute intensity of the return light varies depending on the characteristics of the measured optical fiber (light level of the backscattered light), measurement conditions, and the like. It is customary to calculate the value and evaluate the level of the return light by the value obtained by subtracting the reference value from the measured intensity.

Conventionally, for such a reference value,
The calculation is performed by calculating a data numerical value for a noise portion in a region exceeding the measurement range. FIG. 10 shows a conventional procedure for calculating a reference value and subsequently evaluating a return light level. FIG.
First, an optical pulse is incident on the optical fiber to be measured, and the linear waveform data of the return light is measured (step ST
10) Then, a reference value is calculated by averaging the data in the area of the linear waveform data that exceeds the measurement range and canceling out noise components (step ST11).

For example, if linear waveform data at a measurement distance of 0 km to 160 km as shown in FIG. 11 is measured in step ST10, a part of this (area beyond the measurement range; here, the measurement distance of 128 km to 144 km) is enlarged. A noise fluctuation range as shown in FIG. 12 appears. The data in the region exceeding the measurement range indicates a waveform in which noise is superimposed on the reference value, and is data of a waveform in which only the noise component fluctuates.

Therefore, in step ST11 of FIG.
The average value of the data in this area is calculated as follows,
This is set as a reference value Ref. Ref = [y (k + 1) + y (k + 2) + y (k + 3) + ... + y (k + 1000)]
/ 1000 Here, y (k + 1), y (k + 2),..., Y (k + 1000) are digital data (raw data, that is, logarithmic conversion) converted from the level of the return light to the OTDR. Linear data before processing). K is an index corresponding to a distance parameter corresponding to an area beyond the measurement range, and y (k + 1) to y
(k + 1000) is set to be digital data in the area beyond the measurement range.

Next, in step ST12 of FIG. 10, using the calculated value of the reference value Ref, the linear data within the measurement range is logarithmically converted as follows. z (j) = 5 · log (y (j) −Ref) Here, y (j) is return light linear data, and j is an index corresponding to a distance parameter. Therefore, "y
(J) -Ref "represents the absolute value of the light level of the backscattered light included in the measured return light, and z (j) is log data of the backscattered light. Here, j is the OTDR specification. Although it varies depending on the like, each value is approximately in the range of about 1 to 10,000.

The log data z (i)
Is performed (step ST13), and predetermined processing such as measurement of a loss or a failure point of the optical fiber to be measured and various evaluations are performed.

[0008]

By the way, as described above, in the optical line test, since the absolute intensity of the return light varies depending on the characteristics of the optical fiber to be measured and the like, the reference value is also the same as that of the optical fiber (backscattered light). Light level), measurement conditions, and the like. Therefore, it is necessary to calculate a reference value each time the return light is measured.

However, the conventional procedure for calculating a reference value is based on a simple calculation of calculating an average value for a part of a region where only noise is present. Return light linearity (log) from the far end of the optical fiber
Data waveform linearity) may not be maintained. That is, there is a possibility that an error may be included in the calculated reference value, so that the logarithmically converted log data waveform may be bent up and down.

For example, when the linear waveform data as shown in FIG. 11 is measured, a normal waveform when logarithmic conversion is performed using an accurate reference value corresponding to the characteristics of the optical fiber to be measured is shown in FIG. As shown in FIG. 14 and FIG. 15, waveforms obtained when logarithmic conversion is performed using an incorrect reference value are shown. FIG. 14 shows an abnormal waveform in which the log data waveform absorbs the fact that the plus error has entered the reference value and bends downward, and FIG. 15 shows the logarithmic waveform in which the minus error has entered the reference value. The data waveform shows an abnormal waveform that is bent upward and floated.

When the linearity of the return light from the far end of the measured optical fiber is impaired, the display range of the logarithmically converted waveform is shortened from the range in which the measurement was performed. For example, as shown in FIG.
Even if the data of the return light is acquired in the measurement range of 0 km, the logarithmically converted waveform can be obtained only in the range of the measurement distance of 0 km to 100 km as shown in FIGS.

As described above, in the conventional optical line test, an accurate reference value of the return light cannot be obtained, and the level of the measured return light cannot be appropriately evaluated in some cases. there were.

The present invention has been made in view of such circumstances, and it is possible to calculate a highly accurate reference value for return light from an optical fiber to be measured in an optical line test, and to appropriately evaluate a return light level. It is an object of the present invention to provide a method for calculating a reference value in an optical line test that enables the above, and an optical line test apparatus using the same.

[0014]

According to the first aspect of the present invention,
In a method of calculating a reference value as a reference value of a measured return light level in an optical line test for measuring return light with respect to light incident on an optical fiber to be measured, an exponential function term and a constant term using a measurement distance as a variable And calculating the coefficient of the measured distance in the exponential function term when expressing the return light level by the sum of the measured return light level and the measured return light level, using the exponential function In the term, the constant term when expressed using the coefficient is obtained, and the constant term is used as the reference value.

According to a second aspect of the present invention, in the method for calculating a reference value in the optical line test according to the first aspect,
For each fixed interval in the measurement distance, a first step of obtaining an average value of the measured return light levels, a second step of obtaining a difference between the average values, and, based on the difference, A third step of identifying a position where the level of the backscattered light is equal to the noise level; and a fourth step of determining the coefficient based on the difference determined for a predetermined range before the position. It is characterized by:

According to a third aspect of the present invention, in the method of calculating a reference value in the optical line test according to the second aspect,
The third step is characterized in that the position where the sign of the difference changes is detected to identify the position.

According to a fourth aspect of the present invention, in the method for calculating a reference value in the optical line test according to the second or third aspect, the first temporary reference value is determined based on the average value in a predetermined range far from the position. A fifth step of obtaining the coefficient, and calculating the coefficient when expressing the measured return light level by using the first temporary reference value as the constant term,
A sixth step of calculating the distance coefficient as a distance coefficient of the first reference value;
A seventh step of obtaining the coefficient as a second distance coefficient when the temporary reference value is expressed as the constant term, and the coefficient, the first and second temporary reference values, and the first and second And an eighth step of obtaining the constant term based on the distance coefficient of (8).

According to a fifth aspect of the present invention, there is provided an optical line test apparatus for measuring return light with respect to light incident on an optical fiber to be measured. It is characterized by having arithmetic processing means for calculating a reference value by a calculation method.

According to a sixth aspect of the present invention, in the optical line test apparatus according to the fifth aspect, the arithmetic processing means calculates a reference value calculated with respect to an average value of the measured return light levels at regular intervals in the measurement distance. , And performing logarithmic conversion.

According to a seventh aspect of the present invention, in the optical line test apparatus according to the sixth aspect, there is further provided a display means for displaying the average value after logarithmic conversion by the arithmetic processing means.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS <Structure> An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a configuration of an optical line test apparatus according to one embodiment of the present invention. In FIG. 1, solid lines indicate connection lines for transmitting analog electric signals, solid white lines indicate connection lines for transmitting light, and broken lines indicate control signals or digital signals. Connection lines are shown.

In FIG. 1, reference numeral 1 denotes a light source unit for generating an optical pulse, and supplies a predetermined optical pulse to an optical fiber 3 to be measured via a directional coupler 2. Directional coupler 2
Is a method in which an optical pulse input from the light source unit 1 is incident on the measured optical fiber 3 and return light from the measured optical fiber 3 (backscattered light, Fresnel reflected light when there is a connection point, noise, etc.) To the light receiving unit 4. The measured optical fiber 3 is an optical fiber that forms an optical path for measuring a return light and performing a test.

The light receiving unit 4 is a PD (photo diode)
The return light from the optical fiber under test 3 sent from the directional coupler 2 is received, converted into an electric signal, and supplied to the digital conversion unit 5. The digital conversion unit 5 appropriately amplifies the electric signal supplied from the light receiving unit 4, converts the electric signal into a digital signal, and supplies the digital signal to the waveform processing unit 6. The waveform processing unit 6 averages or logarithmically converts the data of the return light digitally converted by the digital conversion unit 5 as necessary.

Reference numeral 7 denotes a CPU (central processing unit) which controls the operations of the light source unit 1, the light receiving unit 4, the digital conversion unit 5, the waveform processing unit 6 and the like. Using the processed data and the like, various arithmetic processes and instructions for storage or display of data are performed. In addition, this CPU
The details of the control operation, arithmetic processing, and the like of 7 will be described later.

Reference numeral 8 denotes a RAM (random access memory) used as a general-purpose storage area for arithmetic processing of the CPU 7, which is mainly composed of a digital conversion unit 5 and a waveform processing unit 6.
The waveform data processed in step is received from the CPU 7 and stored. Reference numeral 9 denotes a ROM (read only memory) in which processing procedures in each section of the optical line test apparatus are stored.
The CPU 7 reads out and executes each processing procedure stored in the ROM 9.

Reference numeral 10 denotes an external storage device such as an FDD (Floppy Disk Drive), which is controlled by the CPU 7 and stores necessary waveform data and the like in a predetermined storage medium.

Reference numeral 11 denotes a display unit constituted by a CRT (cathode ray tube) display or the like, which receives the processed waveform data from the CPU 7 and displays it in a list. Reference numeral 12 denotes a key panel including a plurality of keys for receiving an instruction input or the like of various operations in the optical line test apparatus. In this optical line test apparatus, the above-described components except for the optical fiber 3 to be measured are housed in a predetermined housing, and the display unit 1
1 and the key panel 12 are arranged at predetermined positions on the front surface of the housing.

<Operation> (1) Theory of Calculating Reference Value Next, the operation of the optical line test apparatus having the above-described configuration will be described. The following describes the theory under which the reference value is calculated.

When an optical pulse is incident on the optical fiber to be measured, the linear data of the return light to be measured is noise-carrying, but the change in the data substantially matches the exponential function. Therefore, the linear data including the reference value can be represented by the following equation.

Y (i) = exp (−a × x (i) + b) + ref (1) where, y (i): the optical power (linear power) corresponding to the distance x (i)
Data) x (i): distance from the optical pulse incident side of the optical fiber to be measured i: parameter for representing the same distance (measurement point number) a: attenuation constant b: optical power constant ref: reference value

Next, when the equation (1) is differentiated, the following equation (2) is obtained. y ′ (i) = − a · exp (−a · x (i) + b) (2) From this equation (2), a constant can be obtained by performing logarithmic conversion on data −y ′ (i) and performing approximate linear calculation. It can be seen that a and b can be calculated. That is, if an approximate straight line representing the logarithmically converted data -y '(i) is obtained, the attenuation constant a and the optical power constant b can be calculated from the constant representing the straight line.

Then, based on the calculated attenuation constant a, a reference value ref is calculated such that the data y (i) represented by the above equation (1) is fitted to the measured return light linear data, Find the optimal reference value. Hereinafter, the processing procedure for obtaining the reference value will be specifically described together with the operation of the entire optical line test apparatus.

(2) Specific Operation Next, a specific operation of the optical line test apparatus will be described. FIG. 2 is a flowchart showing a procedure of automatic calculation of a high-precision reference value performed by the CPU 7 or the like in the operation, and a program defining the calculation procedure of this flowchart is stored in the ROM 9 as a control program. In the following, description will be made assuming an embodiment in which the present invention is applied to an OTDR (optical pulse tester) capable of measuring data up to 10,000 points as the optical line test apparatus. The measurement distance is 160 km, and one point corresponds to 16 m.

First, in the present optical line test apparatus, when an instruction command for measuring the return light is input from the key panel 12,
The CPU 7 having received this performs a process of controlling other components to measure linear waveform data. That is, the CPU 7
Upon receiving the instruction command, the light source unit 1 first controls the light source unit 1 to generate a predetermined optical pulse, and causes the optical pulse to enter the optical fiber 3 to be measured via the directional coupler 2. Thereafter, the light receiving unit 4 is controlled to detect the return light arriving from the optical fiber 3 to be measured and convert it into an electric signal. Then, the digital conversion unit 5 is controlled to convert the electric signal into an OT
A DR linear data waveform is used. Then, the waveform processing unit 6
Is processed to process the OTDR linear data waveform, and the light receiving level y at each point i for each fixed distance (16 m)
(I) Linear waveform data y (1), y indicating [dB]
(2), ..., y (10000).

The linear waveform data y (i) of each point i obtained in this way is supplied from the waveform processing section 6 to the CPU 7, and the CPU 7 executes the linear waveform data y (i).
(I) is stored in the RAM 8. Thus, the measurement of the linear waveform data, which is the pre-processing in the reference value calculation of FIG. 2, is completed. Here, the linear waveform data y (i) stored in the RAM 8 is the same waveform data as that shown in FIG.

Subsequently, when the CPU 7 receives a measurement start command from the key panel 12, the CPU 7 interprets this and reads out a control program from the ROM 9 to execute. As a result, the CPU 7 proceeds to step ST shown in FIG.
The process proceeds to 1.

In step ST1, the linear waveform data stored in the RAM 8 is read out, and for each fixed interval dkm in the measurement area, the average value of the linear waveform data at points included in each interval dkm is calculated. Here, as an example, d = 3.2, and the measurement distance is 0 km to 160 km.
The average value of the linear waveform data is calculated at intervals of 3.2 km. In the following,
The average at regular intervals determined in this way is called a “moving average”
And the same fixed interval is called a “moving interval”.

Thus, moving average data as shown in FIG. 3 is obtained from the linear waveform data as shown in FIG. The moving average of FIG.
It shows the average value for every 3.2 km of the movement interval in the km measurement area.

Next, the process proceeds to the process of step ST2, and the difference of the moving average data obtained in step ST1 is calculated. This difference is the difference between the moving average data and the moving average data at the next (distant) moving interval adjacent to the moving interval, and is calculated by the following equation. D (1) = Y (1) -Y (2) D (2) = Y (2) -Y (3): D (j) = Y (j) -Y (j + 1): where Y (j ) Is the moving average data at the j-th moving interval, and D (j) is the difference data for the moving average data Y (j).

In step ST2, difference data D (j) is calculated for each moving average data Y (j) in this way, and difference data as shown in FIG. 4 is obtained from the moving average data shown in FIG. Note that FIG. 4 corresponds to FIG. 3 in a measurement area of a measurement distance of 64 km to 160 km corresponding to FIG.
The difference data obtained from the average value at intervals of 2 km is shown. Since the difference data D (j) calculated here is the difference between the moving average data Y (i) and the moving average data Y (i + 1) in the far adjacent movement interval, the linear waveform data (the above equation (1)) ) Is obtained by inverting the sign of the differential (formula (2)), that is, the above-mentioned data −y ′ (i)
It corresponds to.

Subsequently, at step ST3, the difference data D
Data in which the sign of (j) is negative is detected as a boundary point. In the example shown in FIG. 4, from the origin in the figure (from the left side)
The eleventh difference is detected as this boundary point.

Since this boundary point corresponds to the moving interval at which the difference between the moving average and the distant point becomes negative for the first time, the point at which the exponential decay of the measured linear waveform data substantially ends, that is, , Corresponds to a boundary point between a region (measurement range) where the contribution of the exponential function term (backscattered light) is larger than the noise level and a region where the noise level is larger than the contribution. Therefore, for the region before this point, the measured linear waveform data can be considered to be represented by equation (1), and near this point, the contribution of the exponential function term in equation (1) is determined by the noise level. In the area after this point, the contribution from noise is dominant, and the level obtained by subtracting the reference value from the measured linear waveform data is almost equal to the noise level.

Next, the process proceeds to step ST4, from the movement interval a predetermined distance Ekm before the position of the movement interval corresponding to the boundary point to the movement interval immediately before the boundary point (the linear waveform data is expressed by the equation (1)). , A logarithmic transformation is performed on each difference data in a predetermined range in the area represented by, and a slope of an approximate straight line representing the difference data after the logarithmic conversion is obtained, and an attenuation constant a is obtained from the value of the slope. Here, the predetermined distance Ek
m is a parameter for determining the range of the difference data for obtaining the approximate straight line, and a value suitable for obtaining the approximate straight line is appropriately set according to the state of the difference data before the boundary point. or,
In this case, the approximate straight line represents data obtained by logarithmically converting the difference (≒ −y ′ (i)) of the moving average data in the area represented by the equation (1) from the measured linear waveform data. Therefore, the attenuation constant a obtained here corresponds to the attenuation constant a in Expressions (1) and (2).

In this embodiment, as an example, the predetermined distance Ekm is set to 32 km (movement interval: 10), and the difference data from the position of [boundary point movement interval position -32 km] to the movement interval position immediately before the boundary point is set. Is logarithmically converted to obtain the slope of the approximate straight line, and the attenuation constant a is determined from the slope. That is, first, the differences D (T−10), D (T
-9),..., D (T−1) are logarithmically converted. Where T is a parameter index corresponding to the boundary point,
The value indicates the eleventh difference in FIG. 4, and the differences D (T−10) to D (T−1) correspond to the ten difference data from the origin to immediately before the boundary point in FIG. It has become corresponding.

The logarithmically converted log [D (T-1
0)], log [D (T-9)],..., Log [D (T
-1)], the slope A of the approximate straight line is obtained, and the attenuation constant a is calculated as “a = A”.

Thereafter, the process proceeds to step ST5, where an average value is calculated for the moving average data (distant) by a predetermined distance from the boundary point, and that value is used as a temporary reference value ref1 used in the subsequent step. That is, the moving average data of the moving interval between the predetermined distance Fkm and the predetermined distance Gkm from the moving interval position of the boundary point is averaged, and the average value is set as the temporary reference value ref1.

Here, the predetermined distances Fkm and Gkm are distance parameters for determining the range of the moving average data for obtaining the temporary reference value ref1, and are appropriately set according to the state of the moving average data after the boundary point. For example, the range determined by these distance parameters is set to be a region including only noise components. In this embodiment,
These predetermined distances are defined as F = 22 km (moving intervals of 7),
G = 32 km (10 movement intervals), and moving average data Y (T + 7) and Y (T +) in a range from the position of [boundary point movement interval position + 22 km] to 32 km away.
8),..., Y (T + 17), and calculate an average value of the average value to obtain a temporary reference value ref1. This corresponds to 17 in FIG.
This is equivalent to obtaining the average value of the moving average data of the twenty-seventh to the twenty-seventh.

Subsequently, the process proceeds to step ST6, in which the same range as the range in which the attenuation constant a was calculated in step ST4 (from the position of [movement interval position of boundary point-32 km] to the movement interval position immediately before the boundary point. The range is referred to as a “predetermined range before the boundary point”.
The logarithmic conversion is performed by subtracting the temporary reference value ref1 from (i), and the slope of an approximate straight line representing the data after the logarithmic conversion is obtained.

Here, the value obtained by subtracting the provisional reference value ref1 from the moving average data Y (i) means that the absolute value of the light level of the backscattered light contained in the measured return light is determined by the provisional reference value ref1. Therefore, in the above equation (1), it corresponds to a value obtained by subtracting the reference value ref from the optical power y (i). Therefore, the attenuation constant a1, which is the slope of the approximate line of the logarithmically converted data, is calculated by adding the temporary reference value re to the reference value ref.
This corresponds to the attenuation constant a in the equation (1) when f1 is used to represent the linear waveform data.

In the process of obtaining the attenuation constant a1, specifically, first, the absolute value Z () of the light level of the backscattered light in the moving average data Y (i) is used using the temporary reference value ref1 calculated in step ST5. i) (hereinafter,
It is called “moving average absolute value Z (i)”. ) Is calculated by the following equation. Z (i) = Y (i) -ref1 Here, as for the moving average absolute value Z (i), a value for each moving interval in a predetermined range before the boundary point is calculated. ) To (T-1) are calculated by the above equations.

The moving average absolute value Z thus obtained
(T-10) to Z (T-1) are logarithmically converted to (Z
(I)-> log [Z (i)]), log [Z
(I)], the slope of the approximate straight line is determined, and the slope is defined as an attenuation constant a1. At this time, the intercept of the approximate straight line is also obtained. This intercept corresponds to the optical power constant b in equation (1) when the linear waveform data is represented by using the temporary reference value ref1 as the reference value ref (hereinafter, this is represented by the symbol “b1”). .

Next, the tentative reference value is changed, and the attenuation constant a2 when linear waveform data is represented using the tentative reference value after the change is calculated (step ST7). In the present embodiment, the temporary reference value r
The attenuation constant a2 is calculated using the following temporary reference value ref2 obtained by changing ef1 by dref. ref2 = ref1-dref

Here, dref is a parameter for changing the provisional reference value, and is calculated as dref = exp (-a1.x1 + b1) using the exponential function term in equation (1). X1 in the expression is a distance from the boundary point to a position behind the predetermined distance Fkm (here, 22km). Therefore, dref calculated by this equation is
In the approximate expression of the linear waveform data using the temporary reference value ref1, the attenuation constant a1, and the optical power constant b1,
It corresponds to the value of the exponential function term at a position where the contribution of the exponential function term is small. Therefore, the value of dref is smaller than the temporary reference value ref1, and the temporary reference value ref2 is a value obtained by slightly changing the temporary reference value ref1.

The relationship between the reference value ref and the attenuation constant a in the equation (1) is represented by a curve (symbol C) as shown in FIG.
It has become. Then, (temporary reference value ref1,
The attenuation constant a1) is one point on the curve C, and the reference value ref corresponding to the attenuation constant a on the curve C is a correct reference value. Therefore, the provisional reference value r
ef1 is changed by the parameter dref as described above, and the attenuation constant a2 corresponding to the provisional reference value ref2 after the change is obtained, thereby obtaining the point (r
ef1, a1) and the curve C near the point (ref, a)
The other point (ref2, a2) above is determined.

The procedure for calculating the damping constant a2 is as described in the above step S.
Same as T6. That is, the moving average absolute value Z (i)
Is calculated for each movement interval in a predetermined range before the boundary point (Z (i) = Y (i) -ref2, i = (T-10)
~ (T-1)), and the data (log)
For [Z (T-10)] to log [Z (T-1)]), the slope of the approximate straight line is obtained, and the slope is set as an attenuation constant a2.

Then, the process proceeds to step ST8, where a final correct reference value ref is calculated. The provisional reference value ref1 and the attenuation constant a1 and the provisional reference value ref2 and the attenuation constant a2 calculated in the above-described processing are approximate values based on the measured linear waveform data, and therefore are close to the correct reference value ref and the attenuation constant a. ing. Therefore, in the curve C of FIG. 5, the point (ref,
a) to point (ref2, a2) and point (ref1,
The deviation of a1) is extremely small, and a first-order approximation can be made for the range where these points are located. That is,
"(Ref-ref1) / (a-a1) = (ref1-
ref2) / (a1-a2) = dref / (a1-a)
2) "can be considered to hold.

Therefore, a correct reference value ref is calculated by the following equation derived from this relationship. ref = ref1- (a1-a) .dref / (a1-
a2) Note that dref = ref1-ref2.

By the above processing, an accurate reference value ref for fitting the measured linear waveform data to the equation (1) is calculated. In addition, a, ref1, a1, dref, and a2 calculated by actually performing the above-described processes.
FIG. 6 shows an example of the calculated values of and ref.

In this optical line test apparatus, C
The PU 7 calculates a reference value ref, performs various processes on the linear waveform data using the reference value ref, and performs predetermined processes such as measurement of a loss or a failure point of the measured optical fiber 3 and various evaluations. For example, the linear waveform data stored in the RAM 8 is read out, and the respective linear waveform data y
Calculate z (i) = 5 · log (y (i) −ref) for (i). As a result, the measured return light data is used as log data of the level of the backscattered light (the backscattered light and the Fresnel reflected light when there is a connection point or the like), and the log data is supplied to the display unit 11 for display. .

In this case, since the above-described accurate reference value is used, the waveform displayed in the log on the display unit 11 is a normal waveform maintaining linearity as shown in FIG. The boundary point is located at a position indicated by an arrow in FIG.

Further, in this optical line test apparatus, the moving average data Y (i) is stored in the RAM 8 and the level of the backscattered light and the like included in the moving average data is evaluated using the reference value ref. You may do it. That is, the reference value ref is subtracted from the moving average data Y (i) to obtain a moving average absolute value Z (i), which is logarithmically converted and displayed on the display unit 11.

FIGS. 8 and 9 show an example of a state in which moving average data is displayed in logarithm by this processing. FIG.
Is a log display of the waveform of the absolute value (moving average absolute value) of the moving average data in the range of 0 km to 160 km (waveform of log [Z (i)]).
The log display in a range of 160 km is shown enlarged.

Here, the moving average data is obtained by averaging a plurality of linear waveform data as described above. Therefore, even in a distant region where the noise level is larger than the contribution of the exponential function term and is usually out of the measurement range, the moving average in a region where the noise level is not much larger than the contribution of the exponential function term As for the data, the noise level is canceled and the transition mode in which the waveform is represented by the equation (1) is maintained.

Then, such moving average data is converted into an absolute value of backscattered light or the like by an accurate reference value,
Since logarithmic conversion is performed, according to the log display of the moving average data by the present optical line test apparatus, the level of the backscattered light or the like can be appropriately adjusted even for the waveform in the far region as described above, which is usually outside the measurement range. Can be shown. This is also shown in the log displays of FIGS. 8 and 9, and when these displays are compared with the display of FIG. 7, the displays of FIG. 8 and FIG. You can see that it is done.

Therefore, according to the present optical line test apparatus, the measurable distance can be extended, and the dynamic range can be improved. In the example shown in FIGS. 8 and 9, the measurement distance is extended by several tens of kilometers, and the dynamic range is greatly improved.

The display is not limited to the display as shown in FIGS. 7 to 9 and various data calculated during the above processing may be displayed on the display unit 11 as needed. The data may be stored in a predetermined storage medium in the storage device 10.

Although the embodiment of the present invention has been described above, the specific method of calculating the reference value and the specific configuration of the optical line test apparatus according to the present invention are not limited to the above embodiment. For example, in the above embodiment, the linear waveform data is fitted to the equation (1) by first-order approximation using a plurality of temporary reference values and an attenuation constant. The reference value may be obtained by fitting the linear waveform data to equation (1) using an iterative calculation method of sequentially calculating data on the curve C moving toward the reference point ref, a).

[0068]

As described above, according to the present invention, when the return light level is represented by an exponential function term and a constant term, the coefficient of the measurement distance in the exponential function term is calculated based on the derivative of the measured return light level. Since the reference value in the optical line test is calculated by calculating the constant term for representing the measured return light level based on the coefficient, the reference value corresponding to the accurate exponential function term must be obtained. Can be. As a result, an effect is obtained that a highly accurate reference value can be accurately calculated for the return light from the measured optical fiber in the optical line test.

According to the second aspect of the present invention, a difference is obtained for the average value of the measured return light levels at regular intervals, and the difference is obtained before the position where the level of the backscattered light is equal to the level of the noise. Since the coefficient is determined based on the difference in the predetermined range, the derivative of the measured return light level is considered by the difference between the average values, and the coefficient is accurately calculated in a range where the contribution of the exponential function term is large. You can ask. According to the third aspect of the present invention, the position where the level of the backscattered light is equal to the level of the noise in this case is identified by the sign change of the difference.

According to the fourth aspect of the present invention, the first value based on the average value in a predetermined range far from the position.
The coefficient when using the temporary reference value is determined as a first distance coefficient, and the coefficient when using a second temporary reference value different from the first temporary reference value is determined as a second distance coefficient, Since the constant term is determined based on the coefficient, the first and second temporary reference values, and the first and second distance coefficients, the relationship between the measured distance coefficient and the reference value is first-order approximated. The reference value corresponding to the coefficient is obtained. As a result, the reference value can be obtained quickly,
By using the constant term and a value close to the coefficient as the temporary reference value and the distance coefficient, a highly accurate reference value can be accurately and quickly calculated.

According to a fifth aspect of the present invention, there is provided an arithmetic processing unit for calculating a reference value by the method for calculating a reference value in the optical line test according to any one of the first to fourth aspects in the optical line test apparatus. Therefore, it is possible to realize an optical line test apparatus using a high-precision reference value as described above. As a result, it is possible to appropriately evaluate the return light level, and it is possible to obtain an effect that linearity when the return high level is expressed by logarithmic conversion is also maintained.

Further, according to the invention of claim 6, the logarithmic conversion is performed by subtracting the reference value from the average value of the measured return light levels at regular intervals in the measurement distance. Can be obtained by performing logarithmic conversion of the average value in which is canceled by a high-precision reference value. As a result, an appropriate logarithmic conversion value can be obtained even in a distant region that is normally outside the measurement range, so that the measurement distance can be extended and the dynamic range can be improved.

According to the seventh aspect of the present invention, there is further provided a display means for displaying the average value after the logarithmic conversion, so that the logarithmic conversion display in which the measurement distance is increased and the dynamic range is improved. Can be realized.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an optical line test apparatus according to an embodiment of the present invention.

FIG. 2 is a flowchart showing a procedure of automatic calculation of a highly accurate reference value performed by a CPU 7 or the like in the operation of the optical line test apparatus.

FIG. 3 shows a moving interval of measured linear waveform data.
Moving average calculated every 2 km (measured distance 64 km to 16 km
0 km).

FIG. 4 is a diagram showing a difference (measured distance of 64 km to 160 km) calculated from the moving average of FIG.

FIG. 5 is a diagram illustrating a relationship between a reference value and an attenuation constant when a return light level is represented by an exponential function expression.

6 is a diagram illustrating an example of calculated values such as an attenuation constant and a reference value calculated by actually performing each process of FIG. 2;

FIG. 7 is a waveform after logarithmic conversion of backscattered light or the like displayed on the display unit 11 in the optical line test apparatus (measurement distance 0 k
m to 160 km).

FIG. 8 is a view showing a waveform (measured distance: 0 km to 160 km) after logarithmic conversion obtained from the moving average data displayed on the display unit 11 in the optical line test apparatus.

FIG. 9 shows a part of the waveform of FIG. 8 (measured distance: 64 km to 160 k) displayed on the display unit 11 in the optical line test apparatus.
m) An enlarged view.

FIG. 10 is a diagram showing a conventional procedure of calculating a reference value and subsequently evaluating a return light level.

FIG. 11 is a diagram showing a state in which return light (measurement distance: 0 km to 160 km) measured in an optical line test is linearly displayed.

12 is a part of FIG. 11 (measuring distance 128 km to 1
It is a figure which shows the fluctuation range of the noise which expanded 44km).

FIG. 13 shows a normal waveform when the return light is logarithmically converted using an accurate reference value (measurement distance: 0 km to 100 k)
It is a figure which shows m).

FIG. 14 shows an abnormal waveform when the return light is logarithmically converted using a reference value containing a plus error (measurement distance 0 k
m to 100 km).

FIG. 15 shows an abnormal waveform when the return light is logarithmically converted using a reference value having a negative error (measurement distance 0
km to 100 km).

[Explanation of symbols]

 Reference Signs List 1 light source unit 3 optical fiber to be measured 4 light receiving unit 7 CPU 8 RAM 9 ROM 11 display unit

Claims (7)

[Claims] 1. A method for calculating a reference value as a reference value of a measured return light level in an optical line test for measuring return light with respect to light incident on an optical fiber to be measured, comprising: When the return light level is represented by the sum of a function term and a constant term, the coefficient of the measurement distance in the exponential function term is obtained based on the derivative of the measured return light level with respect to the measurement distance, and the measured return light level is obtained. Is calculated using the coefficient in the exponential function term, and the constant term is used as the reference value. 2. The method for calculating a reference value in an optical line test according to claim 1, wherein a first step of obtaining an average value of the measured return light levels at regular intervals in a measurement distance; A second step of obtaining a difference, a third step of identifying a position where the level of the backscattered light is equal to the level of the noise in the measured return light level based on the difference, and a predetermined step before the position. And a fourth step of obtaining the coefficient based on the difference obtained for the range. 3. The method for calculating a reference value in an optical line test according to claim 2, wherein the third step detects a position where the sign of the difference changes and identifies the position. Calculation method of reference value in optical line test. 4. The method for calculating a reference value in an optical line test according to claim 2, wherein a fifth step of obtaining a first temporary reference value based on the average value in a predetermined range far from the position. A sixth step of obtaining the measured return light level as the first distance coefficient when the coefficient when representing the first temporary reference value by using the constant term as a first distance coefficient; A seventh step of obtaining the coefficient as a second distance coefficient when a second temporary reference value different from the first temporary reference value is represented by using the constant term; and the coefficient, the first and second coefficients. An eighth step of obtaining said constant term based on said temporary reference value and said first and second distance coefficients. 5. An optical line test apparatus for measuring return light with respect to light incident on an optical fiber to be measured, wherein the reference value is calculated by the method for calculating a reference value in an optical line test according to claim 1. An optical line test apparatus comprising an arithmetic processing means for calculating. 6. The optical line test apparatus according to claim 5, wherein the arithmetic processing unit performs logarithmic conversion by subtracting the calculated reference value from the average value of the measured return light levels at regular intervals in the measurement distance. An optical line test apparatus characterized by performing: 7. The optical line test apparatus according to claim 6, further comprising display means for displaying the average value after logarithmic conversion by the arithmetic processing means.