JP2008020229A - Light pulse tester - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize a pulse light tester capable of setting a suitable distance range by detecting a far end position of the testing optical fiber even there is a variation of noise floor level. <P>SOLUTION: This tester is obtained by improving a light pulse tester provided with a plurality of distance range for measuring the optical fiber by detecting the return light of incident light on the measuring optical fiber. This tester is characteristically provided with the operation means for obtaining the noise floor level of the light receiving part under the condition of no emission of pulse light, and the range setting means for setting distance range by comparing the threshold value obtained by the threshold operation means and the signal level of the return light pulse. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical pulse tester having a plurality of distance ranges, emitting pulsed light to an optical fiber to be measured, and detecting the return light of the emitted pulsed light by a light receiving unit to measure the optical fiber to be measured. More specifically, the present invention relates to an optical pulse tester that detects the far end position of an optical fiber to be measured and sets an optimum distance range even if the noise floor level fluctuates.

  In an optical communication system that performs data communication using an optical signal, it is important to monitor an optical fiber that transmits the optical signal. An optical pulse tester (hereinafter abbreviated as OTDR (Optical Time Domain Reflectometer)) is used for laying and maintaining optical fibers. In OTDR, pulse light is repeatedly incident on the optical fiber under measurement from the measurement connector at the input / output end of the OTDR, and the levels of the reflected light and backscattered light from the optical fiber to be measured and the light reception time are measured. Measure the state of the measurement optical fiber such as breakage and loss.

  FIG. 4 is a diagram showing a configuration of a conventional OTDR. In FIG. 4, an optical fiber to be measured F1 is an optical fiber to be measured on which measurement of disconnection, loss, etc. is performed. The OTDR 100 has a measurement connector CN connected to the optical fiber F1 to be measured, and outputs pulsed light from the measurement connector CN to the optical fiber F1 to be measured, and return light (reflected light or backscattered light) of this pulsed light. Is input via the measurement connector CN.

  The OTDR 100 includes a light emitting unit 10, an optical coupler 20, a band pass filter (hereinafter abbreviated as BPF) 30, a light receiving unit 40, a signal processing unit 50, and a display unit 60. The light emitting unit 10 outputs pulsed light. The optical coupler 20 outputs the pulsed light from the light emitting unit 10 to the bandpass filter 30 and outputs the return light from the measured optical fiber F1 to the light receiving unit 40. The BPF 30 is provided between the optical coupler 20 and the measurement connector CN, and transmits only light in a predetermined wavelength band (including the wavelength of the pulsed light of the light emitting unit 10). The light receiving unit 40 receives light from the optical coupler 20. The signal processing unit 50 controls the output timing of the pulsed light from the light emitting unit 10, and an electric signal is input from the light receiving unit 40. The display unit 60 displays the processing result of the signal processing unit 50.

The operation of such an apparatus will be described.
The light emitting unit 10 outputs pulsed light at a predetermined timing in accordance with an instruction from the signal processing unit 50. Then, the pulsed light output from the light emitting unit 10 enters the measured optical fiber F1 through the optical coupler 20, the BPF 30, and the measurement connector CN. Rayleigh scattering occurs inside the optical fiber F1 to be measured, and a part thereof travels in the direction opposite to the traveling direction of the pulsed light and returns to the OTDR 100 as backscattered light. Further, the Fresnel reflected light generated at the connection point of the measured optical fiber F1 also returns to the OTDR 100.

  Then, the return light from the measured optical fiber F1 enters the light receiving unit 40 through the measurement connectors CN, BPF 30, and the optical coupler 20. Further, an OE conversion circuit (not shown) of the light receiving unit 40 converts the incident light into an electric signal (photocurrent) corresponding to the optical power of the incident light. Then, the IV conversion circuit (not shown) of the light receiving unit 40 converts the photocurrent into a voltage, and the amplification circuit (not shown) of the light receiving unit 40 amplifies the converted voltage to a desired level. Further, an AD conversion circuit (not shown) of the light receiving unit 40 converts an analog signal into a digital signal and outputs it to the signal processing unit 50.

  The signal processing unit 50 then measures the distance of the optical fiber F1 to be measured and the optical signal level of the return light from the time from when the electrical signal and pulsed light from the light receiving unit 40 are emitted to when the signal processing unit 50 enters the light receiving unit 40. The measurement result is displayed on the display unit 60 with the horizontal axis representing distance and the vertical axis representing the return light signal level.

  Further, since the signal level of the return light is very weak, the pulse light is repeatedly output to the optical fiber F1 to be measured, and the noise is reduced by averaging a plurality of measurement values. Here, FIG. 5 shows a measurement result obtained by measuring in the distance range 40 [km]. The fiber length of the optical fiber to be measured F1 is, for example, about 32 [km].

  As shown in the measurement waveform example shown in FIG. 5, the optical fiber F1 to be measured has a loss from the near end (left end of the horizontal axis in FIG. 5) on the measurement connector CN side of the OTDR 100 toward the far end. The waveform is shown. Further, the noise floor level of each circuit in the light receiving unit 40 of the OTDR 100 is displayed after the far end of the measured optical fiber F1.

JP-A-11-344416

  There are various lengths of the optical fiber F1 to be measured. Therefore, the OTDR 100 has a plurality of distance ranges (for example, a 10 [km] range, a 20 [km] range, a 40 [km] range, an 80 [km] range, and the like). And the user can set the distance range of a horizontal axis with the fiber length of the to-be-measured optical fiber F1. On the other hand, in order to reduce the work of the user, the signal processing unit 50 also has a function of detecting the far end position (also called the far end distance) of the optical fiber F1 to be measured and setting it to an appropriate distance range.

  Specifically, the signal processing unit 50 divides the horizontal axis at a predetermined distance interval, and obtains an average value of the signal level for each divided interval. And when the calculated | required average value becomes below a threshold value, the distance is determined to be a far end position, and it sets to the shortest distance range containing the distance. For example, if the far end position is determined to be about 32 [km], the distance range is set to the 40 [km] range.

  The fiber length of the optical fiber to be measured F1 measured by the user cannot be grasped in advance by the manufacturer of the OTDR 100, so the noise floor level was measured for each OTDR 100 before shipment or during calibration, and measured. The value obtained by adding a predetermined offset value to the noise floor level is stored in the OTDR 100 as a threshold value.

  Further, the return light to the light receiving unit 40 of the OTDR 100 is very weak. For this reason, the signal level of the electrical signal obtained by OE conversion of the return light by an OE conversion circuit (for example, an avalanche photodiode) is also very small. Therefore, an amplification circuit (not shown) composed of multi-stage amplifiers amplifies the OE-converted minute electric signal with a large amplification factor so that it can be processed by the signal processing unit 50 subsequent to the light receiving unit 40. . In this way, the signal processing unit 50 sets an optimum distance range from the electric signal amplified to a desired signal level.

  Further, in the actual operating environment of the OTDR 100, the ambient temperature of the OTDR 100 is often not constant. When the ambient temperature of the OTDR 100 changes, the magnitude of noise generated in the amplifier of the amplifier circuit changes, and the noise floor level generated in the entire light receiving unit 40 also changes. Therefore, if the threshold value is set to the same value as the noise floor level, the noise floor level increases when the temperature of the measured optical fiber F1 is higher than the temperature when the noise floor level is measured. It occurs that the far end of the measurement optical fiber F1 cannot be detected. Therefore, the threshold value for determining the noise floor level is obtained by adding a predetermined value to the noise level floor level as described above in consideration of the noise floor level fluctuation.

  However, when the noise floor level is not increased, or when the optical fiber F1 to be measured is long and the signal level at the far end is low, the optical fiber F1 to be measured actually exists. It is determined that the far end is at a position shorter than the far end. In addition, an inappropriate distance range is set, and there is a problem that erroneous detection may occur that display cannot be performed up to the actual far end of the measured optical fiber F1.

  Accordingly, an object of the present invention is to realize an optical pulse tester capable of detecting the far end position of an optical fiber to be measured and setting an optimum distance range even if the noise floor level fluctuates. .

The invention described in claim 1
In the optical pulse tester having a plurality of distance ranges, the light receiving unit detects the return light of the pulsed light emitted to the optical fiber to be measured and measures the optical fiber to be measured.
In a state where the pulsed light is not emitted, a threshold value calculation means for obtaining a noise floor level of the light receiving unit and obtaining a threshold value;
Range setting means for setting the distance range by comparing the threshold value obtained by the threshold value calculation means with the signal level of the return light of the pulsed light is provided.
The invention according to claim 2 is the invention according to claim 1,
The range setting means divides the signal from the light receiving unit that has detected the return light into a plurality of sections at predetermined intervals, obtains an average value of the signal level for each of the divided sections, and calculates the average value and the threshold value. Are compared.
The invention according to claim 3 is the invention according to claim 2,
The range setting means is characterized in that the far end distance of the optical fiber to be measured is calculated using a coefficient given according to the distance value obtained from the comparison result.
The invention according to claim 4 is the invention according to claim 3,
The measurement result of the optical fiber to be measured is displayed in a minimum distance range including the far end distance value.

  According to the present invention, the threshold value calculation means calculates the threshold value based on the data from the light receiving unit in the absence of pulsed light. Then, the range setting means emits pulsed light, and sets the distance range for the optical fiber to be measured from the measurement result of the return light of the pulsed light and the threshold value. As a result, even if the noise floor level of the light-receiving unit fluctuates due to external factors such as ambient temperature fluctuations of the optical pulse tester, the threshold value is obtained based on the noise floor level acquired immediately before the measurement. The far end position of the optical fiber can be properly detected, and measurement can be performed within the optimum distance range.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a block diagram showing an embodiment of the present invention. Here, the same components as those shown in FIG. In FIG. 1, a signal processing unit 70 is provided instead of the signal processing unit 50. The signal processing unit 70 includes a light emission unit control unit 71, a threshold value calculation unit 72, a threshold value storage unit 73, a range setting unit 74, and a measurement unit 75, and controls the timing at which the light emitted from the light emission unit 10 is output. To do. Then, the signal processing unit 70 measures the distance of the optical fiber F1 to be measured and returns from the time from when the digital electrical signal and pulsed light are emitted from the light receiving unit 40 to the light emitting unit 10 and when the return light enters the light receiving unit 40. The optical signal level of the light is measured, and the measurement result is displayed on the display unit 60 with the horizontal axis representing the distance and the vertical axis representing the return light signal level.

  The light emitting unit control unit 71 controls the output of pulsed light from the light emitting unit 10 in accordance with instructions from the threshold value calculating unit 72, the range setting unit 74, and the measuring unit 75. The threshold value calculation means 72 receives a digital electric signal from the light receiving unit 40. The threshold value storage unit 73 stores the threshold value obtained by the threshold value calculation unit 72. The range setting unit 74 reads the threshold value stored in the threshold value storage unit 73 and receives a digital electric signal from the light receiving unit 40. The measuring means 75 receives a digital electric signal from the light receiving unit 40 and performs measurement in the distance range set by the range setting means 74. Further, the measuring means 75 displays the measurement result on the display unit 60.

  The operation of such an apparatus will be described. FIG. 2 is a flowchart for explaining the operation of the apparatus shown in FIG. FIG. 3 is a diagram showing an example of measurement results, in which the horizontal axis represents distance and the vertical axis represents signal level.

  The threshold calculation unit 72 stops the output of the pulsed light from the light emitting unit 10 via the light emitting unit control unit 71. Then, an AD conversion circuit (not shown) converts data in a state where the light receiving unit 40 is not incident with light, that is, data including only noise generated in an amplifier circuit (not shown) in the light receiving unit 40. It is converted into digital data and output to the threshold value calculation means 72. Then, the threshold value calculation means 72 logarithmically converts the noise-only data and averages it to obtain the noise floor level (see the upper part of FIG. 3 (in the figure, the noise-only data is “REF data”). )). Further, the threshold value calculation means 72 adds a predetermined value (several [dB]) to the obtained noise floor level to obtain a threshold value for noise floor determination (S10). The value is stored in the value storage means 73 (S11).

  Next, the range setting unit 74 causes the light emitting unit 10 to output pulsed light via the light emitting unit control unit 71. As a result, the light emitting unit 10 emits pulsed light, and the emitted pulsed light enters the optical fiber F1 to be measured via the optical coupler 20, the BPF 30, and the measurement connector CN. Then, the return light from the optical fiber F1 to be measured is received and detected by the light receiving unit 40 via the measurement connector CN, BPF 30, and the optical coupler 20. Further, the light receiving unit 40 outputs digital data obtained by converting the return light into an electric signal to the range setting unit 74. Then, the range setting means 74 converts the time from when the pulse light is emitted until it is received by the light receiving unit 40 into a distance, and obtains the measurement result of the distance and the signal level of the return light. The pulsed light is emitted a plurality of times, and the measurement results obtained a plurality of times are averaged (S12).

  Then, the range setting unit 74 divides the horizontal axis into a plurality of sections at a predetermined interval (for example, 1 [km] interval) from the averaged measurement result, and obtains an average value of the signal level for each divided section, The average value is compared with the threshold value. In addition, the range setting means 74 measures the interval in which the average value crosses the threshold value, that is, the value obtained by adding the coefficient given according to the distance to the distance value where the average value is equal to or less than the threshold value. The far end distance of the optical fiber F1 is determined. Here, a predetermined coefficient is added to the distance value obtained from the threshold value (the correction by the coefficient may use not only addition but also multiplication). This is to display noise in about 10 to 20% of the length). In this way, by displaying the noise, it is possible to surely confirm whether or not the measurement is possible up to the far end of the measured optical fiber F1. Then, the shortest distance range including the far end distance (the distance including the noise portion) is set in the measuring means 75 (S13).

  And the measurement means 75 measures the to-be-measured optical fiber F1 based on the set distance range. In other words, the light emitting unit 10 is caused to repeatedly output pulsed light at a cycle that is optimal for the set distance range via the light emitting unit control means 71. Then, the measuring means 75 measures the distance of the measured optical fiber F1 and the optical signal level of the returned light from the return light data from the measured optical fiber F1 for each pulsed light and the timing at which the pulsed light is emitted. To do. Further, the measuring means 75 displays the measurement result on the display unit 60 with the horizontal axis as the distance and the vertical axis as the signal level of the return light. The horizontal axis of the measurement result displayed on the display unit 60 is displayed in the set distance range (that is, the minimum distance range including the far end distance value detected by the range setting unit 74) among the acquired measurement results. (See the lower part of FIG. 3) (S14).

  Thus, the threshold value calculation means 72 calculates the threshold value based on the data from the light receiving unit 40 in the absence of pulsed light. Then, the range setting means 74 emits pulsed light, obtains the far end distance value of the measured optical fiber F1 from the measurement result of the return light of the pulsed light and the threshold value, and sets the distance range. Thereby, even if the noise floor level of the light receiving unit 40 fluctuates due to external factors such as temperature fluctuation around the OTDR 100, the threshold value is obtained based on the noise floor level acquired immediately before the measurement. The far end position of the fiber F1 can be properly detected, and measurement can be performed within the optimum distance range.

The present invention is not limited to this, and may be as shown below.
The configuration in which the threshold value calculation means 72 measures the noise floor level in the non-light emitting state and obtains the threshold value immediately before the measurement of the optical fiber F1 to be measured is shown. Is within the time range that does not affect the detection of the distance range, the measuring means 75 ends the measurement of the return light from the optical fiber F1 to be measured, and then the threshold value calculating means 72 sets the threshold value. You may ask for it.

  The range setting means 74 determines the distance range. In order to determine the distance range, the pulse width of the pulsed light (time axis) for determining the fiber length for each application such as long distance and short distance is used. Measurement may be performed with the above changed. In this case, the range where the influence of reflection varies depending on the pulse width. Accordingly, the far end distance may be calculated by determining the section that is several times or more the distance width converted from the pulse width to the distance, and the position below the threshold as the fiber length.

  Further, the range setting means 74 has shown a configuration in which the far end distance is calculated using a coefficient given according to this distance to the distance value across the threshold. It may be the far end distance.

  And although the structure which provides BPF30 was shown, it is not necessary to provide BPF30. Thereby, the cost of the measurement system including the OTDR 100 can be suppressed, and the OTDR 100 can be downsized.

  Furthermore, a directional coupler or the like may be used in place of the optical coupler 20. In short, the pulse light from the light emitting unit 10 is output only to the measured optical fiber F1, and the return light from the measured optical fiber F1. May be output only to the light receiving unit 40.

It is the block diagram which showed one Example of this invention. It is a flowchart explaining operation | movement of the apparatus shown in FIG. It is the figure which showed an example of the measurement result of the noise floor level in the apparatus shown in FIG. 1, and the measurement result of the to-be-measured optical fiber F1. It is the figure which showed the structure which connected the to-be-measured optical fiber to the conventional OTDR. It is the figure which showed an example of the measurement result of the apparatus shown in FIG.

Explanation of symbols

40 Light Receiving Unit 72 Threshold Calculation Unit 74 Range Setting Unit 100 OTDR
F1 Optical fiber to be measured

Claims (4)

In the optical pulse tester having a plurality of distance ranges, the light receiving unit detects the return light of the pulsed light emitted to the optical fiber to be measured and measures the optical fiber to be measured.
In a state where the pulsed light is not emitted, a threshold value calculation means for obtaining a noise floor level of the light receiving unit and obtaining a threshold value;
An optical pulse tester characterized by comprising range setting means for setting the distance range by comparing the threshold value obtained by the threshold calculation means with the signal level of the return light of the pulsed light.   The range setting means divides the signal from the light receiving unit that has detected the return light into a plurality of sections at predetermined intervals, obtains an average value of the signal level for each of the divided sections, and calculates the average value and the threshold value. The optical pulse tester according to claim 1, wherein:   3. The optical pulse test according to claim 2, wherein the range setting means calculates the far end distance of the optical fiber under measurement using a coefficient given according to the distance value obtained from the comparison result. vessel. 4. The optical pulse tester according to claim 3, wherein the measurement result of the optical fiber to be measured is displayed in a minimum distance range including the far end distance value.

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