This application is a National Stage Entry of PCT/JP2019/017581 filed on Apr. 25, 2019, which claims priority from Japanese Patent Application 2018-161342 filed on Aug. 30, 2018, the contents of all of which are incorporated herein by reference, in their entirety.

TECHNICAL FIELD

The present invention relates to an optical time domain reflectometer, a test method of an optical transmission line, and a test system of an optical transmission line.

BACKGROUND ART

An optical time domain reflectometer referred to as an OTDR is generally used to detect breakage of an optical transmission line consisting of an optical fiber or the like. The optical time domain reflectometer outputs an optical pulse, which is a monitoring light, to the optical transmission line, and monitors a return time and intensity of a backscattered light (so-called a return light) from each part of the optical transmission line to detect a breakage position. Since the return light does not return from the breakage position and thereafter, the breakage position of the optical transmission line can be determined by detecting a timing at which the intensity of the return light decreases.

For example, it has been proposed that an OTDR waveform determination method capable of selecting not only the breakage of the optical transmission line but also an abnormal part due to a wound state of a fiber constituting the optical transmission line (PTL1).

Further, as described above, there are known various methods of detecting a position of a failure position by outputting a pulse light (Monitoring light) to an optical transmission line and measuring a time at which a reflected light from the failure position returns (PTL2 and PTL3).

CITATION LIST

Patent Literature

PTL1: Japanese Unexamined Patent Application Publication No. 2011-38785PTL2: Japanese Unexamined Patent Application Publication No. H4-132931PTL3: Japanese Unexamined Patent Application Publication No. 2008-3068

SUMMARY OF INVENTION

Technical Problem

The general optical time domain reflectometer described above is used to determine the breakage position when breakage has occurred in the optical transmission line. In this case, the monitoring light is output to the optical transmission line, a curve indicating an intensity change of the return light is acquired, and a user determines a timing at which the intensity of the return light decreases, thereby determining the breakage position. However, in order to maintain the optical transmission line more reliably, it is desirable to automatically detect the breakage of the optical transmission line and notify the user of the detection result. Therefore, it is required to automatically detect the breakage of the optical transmission line and to detect the breakage position with high accuracy.

The present invention has been made in view of the aforementioned circumstances and aims to accurately and automatically detect a breakage position of an optical transmission line.

Solution to Problem

An aspect of the present disclosure is an optical time domain reflectometer including: a light source configured to output a monitoring light; an optical detection unit configured to detect a return light from an optical transmission line and output a detection signal indicating an intensity of the return light; an optical multiplexer/demultiplexer configured to output the monitoring light input from the light source to the optical transmission line, and output the return light input from the optical transmission line to the optical detection unit; a processing unit configured to detect a first timing at which a value of the detection signal becomes less than a first threshold value, detect a second timing at which the value of the detection signal becomes less than a second threshold value different from the first threshold value, and calculate a first change rate which is a change rate of the detection signal in a period between the first timing and the second timing, in which the processing unit: calculates the first change rates for a plurality of periods by changing the first threshold value and the second threshold value; and, when a second change rate that is a change rate between the first change rate in a first period and the first change rate in a second period adjacent to the first period is greater than a change rate threshold value, detects a position corresponding to either the first timing or the second timing in the first period as a breakage position of the optical transmission line.

An aspect of the present disclosure is a test method of an optical transmission line including: outputting a monitoring light to an optical transmission line; detecting a return light from the optical transmission line, and outputting a detection signal indicating an intensity of the return light; and detecting a first timing at which a value of the detection signal becomes less than a first threshold value, detecting a second timing at which the value of the detection signal becomes less than a second threshold value different from the first threshold value, and calculating a first change rate which is a change rate of the detection signal in a period between the first timing and the second timing, in which the first change rates for a plurality of periods are calculated by changing the first threshold value and the second threshold value; and, when a second change rate that is a change rate between the first change rate in a first period and the first change rate in a second period adjacent to the first period is greater than a change rate threshold value, a position corresponding to either the first timing or the second timing in the first period is detected as a breakage position of the optical transmission line.

An aspect of the present disclosure is a test system of an optical transmission line including: a first optical transmission apparatus configured to transmit and receive optical signals; a second optical transmission apparatus configured to transmit and receive the optical signals; an optical transmission line connecting the first optical transmission apparatus and the second optical transmission apparatus; and an optical time domain reflectometer connected to the first optical transmission apparatus and configured to test the optical transmission line; in which the optical time domain reflectometer includes: a light source configured to output a monitoring light; an optical detection unit configured to detect a return light from the optical transmission line and output a detection signal indicating an intensity of the return light; an optical multiplexer/demultiplexer configured to output the monitoring light input from the light source to the optical transmission line, and output the return light input from the optical transmission line to the optical detection unit; a processing unit configured to detect a first timing at which a value of the detection signal becomes less than a first threshold value, detect a second timing at which the value of the detection signal becomes less than a second threshold value different from the first threshold value, and calculate a first change rate which is a change rate of the detection signal in a period between the first timing and the second timing, in which the processing unit: calculates the first change rates for a plurality of periods by changing the first threshold value and the second threshold value; and, when a second change rate that is a change rate between the first change rate in a first period and the first change rate in a second period adjacent to the first period is greater than a change rate threshold value, detects a position corresponding to either the first timing or the second timing in the first period as a breakage position of the optical transmission line.

Advantageous Effects of Invention

According to the present invention, it is possible to accurately and automatically detect a breakage position of an optical transmission line.

