JP2019052896A - Optical time domain reflection measurement method and optical time domain reflection measuring device - Google Patents

Abstract

To provide an optical time domain reflection measurement method and device with which it is possible to accurately measure a phase state even in measuring the distance exceeding the coherency of a light source in a phase OTDR.SOLUTION: An optical time domain reflection measurement method and an optical time domain reflection measuring device pertaining to the present invention branch a probe pulse used in measurement, cause it to pass through a ring mechanism to create artificial continuous light, and use it as local light of a phase OTDR. The continuous light is light of a phase equal to the probe pulse, so that it is possible to reduce the influence of phase noise.SELECTED DRAWING: Figure 1

Description

  The present disclosure relates to an optical time domain reflection measurement method and apparatus capable of measuring the state of a measurement object from one side by observing the intensity and phase of reflected light or backscattered light in an optical component or an optical transmission line. is there.

  As a method capable of measuring reflected light and backscattered light from an optical component or an optical transmission line, there is an optical time domain reflection measurement method (C-OTDR) using a coherent detection technique. C-OTDR bifurcates the continuous light output from the light source, one is used as local light, the other is pulsed, and incident on the measurement object as a probe pulse to generate reflected light and backscattered light from the measurement object In addition, the loss distribution and the failure point of the measurement target can be specified by observing the intensity of the interference beat signal obtained by coherent detection of the local light and the local light on the time axis.

  In recent years, as described in Non-Patent Document 1, by applying a high coherent light source to C-OTDR and observing not only the intensity of the interference beat signal of the obtained backscattered light but also the phase information, the object to be measured A technique for detecting a dynamic strain applied to the film, that is, a vibration has been studied. In this method, a beat signal generated by causing backscattered light from each point inside the measurement target to interfere with local light is received, and vibration is detected from the time and the phase change of the scattered light. Such a method of measuring the phase of scattered light using C-OTDR to which a highly coherent light source is applied is called phase OTDR.

Y. Lu, et al, “Distributed vibration sensor based on coherent detection of phase-OTDR”, IEEE Journal of Lightwave Technology, vol. 28, no. 22, November, 2010

  In the phase OTDR, in order to correctly measure the phase change of the scattered light, it is necessary to interfere with the scattered light and the local light in a coherent state. For example, if the time until the probe pulse is incident and returns to the incident end as scattered light, that is, the round-trip propagation time is longer than the coherence time of the light source, the scattered light and local light are no longer coherent, so the received interference The phase of the beat signal becomes random due to the phase noise of the light source, and it becomes impossible to detect the change in phase information experienced by the probe pulse, that is, vibration. Therefore, the distance over which vibration can be detected (proportional to the round-trip propagation time) is limited by the coherence time of the light source. To perform measurements over long distances, a highly coherent light source with very low source phase noise and a narrow spectral linewidth is required. It is necessary to use it. However, even such a highly coherent light source has a finite coherence time, and there is a limit to the distance over which vibration can be measured. Further, even when the round-trip propagation time is within the coherence time of the light source, the phase fluctuation itself due to the phase noise of the light source exists, so that the accuracy is deteriorated when analyzing the detailed phase change.

  That is, when the phase change is measured by the phase OTDR, there is a problem that the measurable distance is limited by an increase in the coherence time of the light source, that is, the fluctuation of the phase noise. Therefore, an object of the present invention is to provide an optical time domain reflection measurement method and an optical time domain reflection measurement apparatus capable of accurately measuring a phase state even in measurement of a distance exceeding the coherency of a light source in phase OTDR.

  In order to solve the above-described problems, the present invention reduces the influence of phase noise by branching a probe pulse used for measurement, passing it through a ring configuration, and using it as pseudo continuous light and as phase OTDR local light. It was decided to.

Specifically, the optical time domain reflectometry method according to the present invention is:
An incident procedure for making one pulsed light obtained by pulsing light from a light source incident on a measured fiber as a probe pulse,
Continuous light is generated by using the other pulsed light obtained by pulsing the light from the light source and delaying a part branched from the other pulsed light and placing it between the pulses of the other pulsed light. A light generation procedure;
An interference procedure for obtaining an interference beat signal by causing the continuous light generated in the continuous light generation procedure to interfere with return light from the measured fiber as local light;
An analysis procedure for analyzing the intensity and phase of the interference beat signal acquired in the interference procedure;
It is characterized by performing.

In addition, the optical time domain reflection measurement apparatus according to the present invention,
Incident means for making one pulsed light obtained by pulsing the light from the light source incident on the measured fiber as a probe pulse;
Continuous light is generated by using the other pulsed light obtained by pulsing the light from the light source and delaying a part branched from the other pulsed light and placing it between the pulses of the other pulsed light. Light generating means;
Interfering means for acquiring an interference beat signal by causing the continuous light generated by the continuous light generating means to interfere with return light from the measured fiber as local light,
Analyzing means for analyzing the intensity and phase of the interference beat signal acquired by the interference means;
It is characterized by providing.

  In the present invention, by generating pseudo continuous light using a probe pulse, light having the same phase as that of the probe pulse can be obtained. By using this continuous light as local light of phase OTDR, phase noise of the light source can be obtained. The components can be offset. Therefore, the present invention can provide an optical time domain reflection measurement method and an optical time domain reflection measurement apparatus capable of accurately measuring the phase state even in the measurement of the distance exceeding the coherency of the light source in the phase OTDR.

  In the optical time domain reflection measurement method and apparatus according to the present invention, it is preferable that a delay time for delaying a part branched from the other pulsed light is equal to a pulse width of the other pulsed light. It can be completely continuous light.

  In the optical time domain reflection measurement method and apparatus according to the present invention, it is preferable to adjust a part of the light intensity branched from the other pulsed light. The average light intensity of the continuous light formed can be adjusted.

  The optical time-domain reflection measurement method and apparatus according to the present invention provides the next probe pulse from the time when the probe pulse reflected and returned from the farthest end of the measured fiber interferes with the local light. It is preferable that a part of the light intensity branched from the other pulsed light is attenuated until it enters the light. In the OTDR measurement, a plurality of measurements are required to obtain final information by adding and averaging the measurement results and comparing the data between the measurements. Therefore, it is necessary to block the continuous light for each measurement. By attenuating the delayed pulse light, the average light intensity of the continuous light to be formed can be lowered, and a continuous light blocking state can be formed.

  The present invention can provide an optical time domain reflection measurement method and an optical time domain reflection measurement apparatus capable of accurately measuring a phase state even in measurement of a distance exceeding the coherency of a light source in phase OTDR.

It is a figure explaining the optical time domain reflection measuring apparatus which concerns on this invention. It is a figure explaining the structure which produces the pseudo | simulation continuous light which has a phase equal to a probe pulse in the optical time domain reflection measuring apparatus which concerns on this invention.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

FIG. 1 is a diagram illustrating an optical time domain reflection measurement apparatus 301 according to the present embodiment. The optical time domain reflection measurement device 301 includes:
An incident means 101 for making one pulsed light obtained by pulsing light from the light source 10 incident on the measured fiber 50 as a probe pulse;
Continuous light is generated by using the other pulsed light obtained by pulsing the light from the light source 10 and delaying a part branched from the other pulsed light and placing it between the pulses of the other pulsed light. Light generating means 18;
An interference unit 102 that obtains an interference beat signal by causing the continuous light generated by the continuous light generation unit 18 to interfere with the return light from the measured fiber 50 as local light;
Analyzing means 103 for analyzing the intensity and phase of the interference beat signal acquired by the interference means 102;
It is characterized by providing.

Output light from the light source 10 is input to the pulsing unit 12 to create a probe pulse. Pulsing unit 12 has a function of shifting the optical frequency of the probe pulse by f _a. The pulsing unit 12 can be realized by an acoustooptic device or an LN modulator. It is also possible to configure the pulsing unit 12 with a semiconductor optical amplifier element and a frequency shift unit. The frequency shift unit can be realized by an acoustooptic device or an LN modulator. In this case, the frequency shift unit may be installed on the local light emission path.

  The probe pulse generated by the pulsing unit 12 is bifurcated by the coupler 14, and one is incident on the measured fiber 50 via the optical circulator 15. Reflected light and scattered light generated by the probe pulse incident on the fiber to be measured 50 pass through the optical circulator 15 again and are combined with local light by the optical coupler 16.

The other of the two probe pulses branched by the coupler 14 is incident on a local light emission path having a ring structure (continuous light generation means 18). The ring structure of the continuous light generating means 18 includes an optical coupler 21, an optical attenuator 22, an optical amplifier 23, and a band pass filter 24 installed in the local light emission path as shown in FIG. It is desirable that the propagation delay time given by passing through the ring structure of the continuous light generating means 18 is the same as the probe pulse width. As shown in FIG. 2, the probe pulse is divided into a component that enters the ring structure and a component that does not enter the ring structure. Then, since the propagation delay time of the ring structure is the same as the probe pulse width, the probe pulses incident on the ring structure are continuously arranged after the probe pulses that have not entered the ring structure after circulating around the ring structure. It will be. By repeating this, pseudo continuous light having a phase θ ′ (t ₀ ) equal to the probe pulse can be created.

  Note that the ring structure of the continuous light generation means 18 amplifies the probe pulse by the optical amplifier 23 and the band pass filter 24 to compensate for the loss that occurs when the probe pulse circulates. It can be avoided that the average light intensity of the continuous light produced by the pulse connection is attenuated.

  In addition, in OTDR measurement, a plurality of measurements are required in order to obtain final information by addition average of measurement results or data comparison between each measurement. Therefore, when moving to the next measurement, it is necessary to cut off the continuous light once created by pulse connection, and to create a new continuous light made by pulse connection with the probe pulse used for the next measurement. . The ring structure of the continuous light generation means 18 blocks the probe pulse that circulates by the optical attenuator 22 arranged on the path, so that the continuous light is formed by pulse connection to the extent that the optical receiver 17 cannot detect optical interference. Reducing the average light intensity.

  The timing at which the ring structure of the continuous light generating means 18 cuts off the probe pulse is such that the probe pulse reflected from the farthest end of the measured fiber 50 is combined with the local light and then the next probe pulse is measured fiber. 50 until it is incident on 50.

  The ring structure is an example, and the continuous light generating means 18 may have another structure as long as a part of the probe pulse can be delayed.

  The continuous light produced by the pulse connection of the continuous light generating means 18 is combined with the reflected light and scattered light generated by the probe pulse as local light by the optical coupler 16 to generate an interference beat signal, which is received by the optical receiver 17.

ここで、ωは光源１０の光周波数、θ（ｔ）はランダムな光の位相である。 The above will be described using mathematical formulas.
The electric field of the output light from the light source 10 is expressed as follows.

Here, ω is the optical frequency of the light source 10, and θ (t) is the phase of random light.

ここで、ｔ_０は光源１０からの出力光がパルス化された時間である。またプローブパルスは有限の幅を持つため、パルスの先頭が後方散乱されパルス内の各地点における位相と重なった後に観測されることを考慮し、θ’（ｔ_０）はプローブパルス内の瞬時的な位相が重ね合わされたものとする。 Field when the output optical probe pulse was only frequency shift f _a is the phenomenon scatter or backscatter undergoing phase change θ _{FUT (t)} caused by the vibration applied to the fiber to be measured 50 is as follows It is expressed in

Here, t ₀ is the time when the output light from the light source 10 is pulsed. Since the probe pulse has a finite width, θ ′ (t ₀ ) is an instantaneous value in the probe pulse, considering that the head of the pulse is backscattered and observed after overlapping with the phase at each point in the pulse. It is assumed that various phases are superimposed.

ここでＷはパルス幅、Ｎはｔ／Ｗ以下の最大の整数である。θ（ｔ_０＋ｔ−ＮＷ）はパルス連結で作られた連続光の位相を表す。 The electric field of continuous light by pulse connection used as local light is expressed as follows.

Here, W is a pulse width and N is a maximum integer of t / W or less. θ (t ₀ + t−NW) represents the phase of continuous light generated by pulse connection.

The output current value from the optical receiver when these are combined to generate an interference beat signal is expressed as follows.

  The analysis unit 103 calculates the phase of the interference beat signal measurement data of the interference means 102 obtained in this way. This can be calculated, for example, by performing a Hilbert transform on each measurement data to obtain a sine component, calculating a tan component from the original data that is a cos component, and further applying an inverse tan function.

The phase Φ obtained by the interference means 102 is expressed as follows.

Here, θ ′ (t ₀ ) −θ (t ₀ + t−NW) is a term representing phase noise. If the delay time difference between the two becomes larger than the coherence time of the light source, the fluctuation of the phase noise increases and the phase information is obtained. You can't get it. The delay time difference of this phase noise term depends on t since the conventional phase OTDR is | t ₀ −t |. On the other hand, the delay time difference of the phase noise term in the optical time domain reflectometry apparatus 301 is t−NW (<W) as shown in Equation 6, so that the pulse width W is the maximum. This is because the optical time domain reflection measurement apparatus 301 does not increase t, that is, the light source phase noise increases according to the measurement distance, unlike the conventional phase OTDR, and the light source has a coherence time of about the pulse width. This indicates that the influence of the phase noise term is negligible. As described above, the influence of the phase noise is removed, and only the phase change θ _FUT (t) applied to the measured fiber 50 that is the object to be measured can be obtained.

  Here, in order to avoid disturbances such as vibrations, that is, phase noise due to disturbances, from being added to the continuous light generating means 18, it is effective to install a local light emission path and a ring in a soundproofed box. .

Further, as the coherence time required for the light source, the conventional phase OTDR requires more than the round-trip propagation time of the probe pulse to the farthest end to be measured, whereas in the optical time domain reflection measurement apparatus 301, the coherence of the light source 10 is required. The time may be longer than the pulse width. In the measurement of the conventional phase OTDR, there ratio about 10 ^second round trip propagation time and the pulse width, optical time domain reflectometry apparatus 301, the use of economical light source 10 can be relaxed coherence required for the more light source Is possible.

  As described above, the probe light is incident on the local light emission path including the continuous light generation unit 18 to generate local light, thereby reducing the influence of the light source phase noise and limiting the measurable distance by the coherence of the light source in the conventional phase OTDR Can be overcome.

(Effect of the invention)
The optical time domain reflection measurement method and apparatus according to the present invention can accurately measure the phase state even at a distance exceeding the coherency of the light source in the phase OTDR. Moreover, since the optical time domain reflection measurement method and apparatus according to the present invention do not require the use of a highly coherent light source, an economical apparatus configuration is possible.

  The present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. Various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the above embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiment examples may be appropriately combined.

10: light source 12: pulsing unit 14: coupler 15: optical circulator 16: coupler 17: optical receiver 18: continuous light generating means 21: coupler 22: optical attenuator 23: optical amplifier 24: bandpass filter 50: measured Fiber 101: Incident means 102: Interference means 103: Analysis unit 301: Optical time domain reflection measurement device

Claims (8)

An optical time domain reflection measurement method comprising:
An incident procedure for making one pulsed light obtained by pulsing light from a light source incident on a measured fiber as a probe pulse,
Continuous light is generated by using the other pulsed light obtained by pulsing the light from the light source and delaying a part branched from the other pulsed light and placing it between the pulses of the other pulsed light. A light generation procedure;
An interference procedure for obtaining an interference beat signal by causing the continuous light generated in the continuous light generation procedure to interfere with return light from the measured fiber as local light;
An analysis procedure for analyzing the intensity and phase of the interference beat signal acquired in the interference procedure;
An optical time domain reflection measurement method characterized in that:   2. The optical time domain reflection measurement according to claim 1, wherein, in the continuous light generation procedure, a delay time for delaying a part branched from the other pulsed light is equal to a pulse width of the other pulsed light. Method.   3. The optical time domain reflectometry method according to claim 1, wherein in the continuous light generation procedure, a part of the light intensity branched from the other pulsed light is adjusted. In the continuous light generation procedure,
In the interference procedure, from when the probe pulse reflected and returned at the farthest end of the measured fiber interferes with the local light, until the next probe pulse enters the measured fiber. 4. The optical time domain reflectometry method according to claim 3, wherein a part of the light intensity branched from the other pulsed light is attenuated. An optical time domain reflection measurement device,
Incident means for making one pulsed light obtained by pulsing the light from the light source incident on the measured fiber as a probe pulse;
Continuous light is generated by using the other pulsed light obtained by pulsing the light from the light source and delaying a part branched from the other pulsed light and placing it between the pulses of the other pulsed light. Light generating means;
Interfering means for acquiring an interference beat signal by causing the continuous light generated by the continuous light generating means to interfere with return light from the measured fiber as local light,
Analyzing means for analyzing the intensity and phase of the interference beat signal acquired by the interference means;
An optical time domain reflection measurement apparatus comprising:   6. The optical time domain reflection measurement according to claim 5, wherein the continuous light generating means has a delay time for delaying a part branched from the other pulsed light equal to a pulse width of the other pulsed light. apparatus.   The optical time domain reflectometry apparatus according to claim 5 or 6, wherein the continuous light generation means adjusts a part of the light intensity branched from the other pulsed light. The continuous light generation means includes
In the interference means, from when the probe pulse reflected and returned at the farthest end of the measured fiber interferes with the local light, until the next probe pulse enters the measured fiber. 8. The optical time domain reflectometry apparatus according to claim 7, wherein a part of the light intensity branched from the other pulsed light is attenuated.

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