JPH02226035A - Method of locating fault point of optical fiber and apparatus therefor - Google Patents

Abstract

PURPOSE:To improve resolution of a distance to a fault point by a method whereby, while measuring pulse lights are repeatedly projected, the timing of a sampling pulse light is sequentially shifted to sample lights having the sum or difference frequency, and a monitoring waveform similar to the change in the strength of a returning light after the lapse of time is obtained. CONSTITUTION:As a signal processing circuit 10 controls a measuring pulse light source 1 so that a measuring pulse light PIN is output at every constant interval, it shifts the timing sequentially every time the measuring pulse light PIN is output, thereby to control a sampling pulse light source so that a sampling pulse light is output. When the sampling pulse light is projected at a timing being shifted by constant timing, lights of the total or difference frequency can be obtained at a nonlinear optical element 6. Accordingly, a monitoring waveform similar to the change of a returning light POUT changing with time can be obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an optical fiber failure point searching method for optically searching for a failure point in an optical fiber, and an apparatus using the same (OTDR).
:0pt1cal Time Dosaln R
(eflectoseter).

[Conventional technology]

An optical fiber fault detection device called an optical fiber analyzer inputs pulsed light for measurement into the optical fiber under test and detects temporal changes in the intensity of the returned light caused by Rayleigh scattered light (backscattered light) or Fresnel reflected light. This is used to detect failure points. The time resolution of such 0TDR is limited by the time width (pulse width) of the measurement pulse light, the response speed of the photodetector, the sampling time of the A/D converter that converts the detected analog signal into a digital signal, etc. , the most important of these is the response speed of the photodetector. Therefore, avalanche photodiodes (A, P D ) and the like, which have a fast response speed, have been used as photodetectors.

On the other hand, if there is a fault in the optical fiber to be measured, strong Fresnel reflected light will be generated at the fault point, and this will be incident on the photodetector as return light. As a result, so-called ``striping'' occurred in the response waveforms of the photodetector and the downstream amplifier, and an unobservable region appeared in the optical fiber section further behind the failure point (particularly immediately after it). Therefore, in order to eliminate this drawback, a masking function using an optical deflector has been conventionally added.

[Problem to be solved by the invention]

However, in conventional devices that use APD as a photodetector, the response speed of APD is about 100 psec at maximum, so the time resolution cannot be increased beyond this, and therefore the distance resolution in the optical fiber under test cannot be increased. , 5
It was not possible to raise the level higher than (to) degree. Furthermore, even if an optical deflector is used to remove the "striping", due to limitations such as the response speed of the optical deflector itself and the bandwidth of the drive circuit, the above unobservable region cannot be kept below approximately 30cII+. could not.

Therefore, the present invention can improve the distance resolution of the failure point by improving the time resolution of the returned light intensity,
Moreover, it is an object of the present invention to provide an optical fiber failure point searching method that can prevent the occurrence of an unviewable area immediately after the failure point, and a device using the same.

[Means to solve the problem]

The optical fiber fault point searching method and device according to the present invention include:
This method is used to search for fault points in an optical fiber under test based on temporal changes in the intensity of the return light when a measurement pulse light is input into the optical fiber under test. The feature is that the observed waveform is obtained by sampling by performing sum or difference frequency mixing with the sampling pulse light.

That is, the method of the present invention combines sampling pulsed light (pump light) with a sufficiently short time width (pulse width) and different wavelengths compared to the temporal change in the intensity of the returned light, and performs sum or difference frequency mixing. By generating a sum or difference frequency light by repeating the incidence of the measurement pulse light and sequentially shifting the timing of the sampling pulse light to sample the sum or difference frequency light, it is possible to generate a sum or difference frequency light similar to the temporal change in the returned light intensity. It is characterized by obtaining 11 PJ waveforms.

Further, the device of the present invention includes a sampling pulse light source that emits sampling pulse light as the pump light, an optical coupling means that couples the returned light and the sampling pulse light, and an optical coupling means that inputs the coupled light and sums it. or a nonlinear optical element that generates sum or difference frequency light by difference frequency mixing;
A light detection means for detecting the intensity of the sum or difference frequency light, a measurement pulse light is emitted at a predetermined time interval, and a sampling pulse light is emitted with the timing sequentially shifted each time the measurement pulse light is emitted repeatedly. An analysis to obtain an observed waveform similar to the temporal change in the intensity of the returned light by sampling the detection output of the light source control means that controls the above two light sources and the light detection means at each emission timing of the sampling pulse light. It is characterized by comprising means.

Here, a display means for displaying the observed waveform obtained by the analysis means may be provided.

[Effect]

According to the optical fiber fault detection method of the present invention, the returned light is summed or difference frequency mixed with sampling pulse lights having different frequencies, thereby being sampled with the time width and timing of the sampling pulse lights. Therefore, by extracting the sum or difference frequency light components while shifting the timing of the sampling pulse light and combining them, it is possible to obtain an observed waveform similar to the temporal change in the return intensity.

Furthermore, according to the device of the present invention, the above points i11 and 11 can be achieved simply by combining a sampling pulse light source that outputs ultrashort optical pulses, a nonlinear optical element, or a light detection means used in conventional 0TDR. Waveforms can be obtained. Furthermore, if a display means such as a CRT is provided, the observed waveform can be displayed on an image.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a configuration diagram of an apparatus to which an optical fiber fault detection method according to an embodiment of the present invention is applied, FIG. 2 is a timing diagram for emitting one measurement pulse light, and FIG. FIG. 3 is a timing diagram showing how measurement pulse light is emitted and sampled multiple times. As shown in FIG. 1, measurement pulse light P (wavelength λ1) from an n1 constant pulse light source 1 composed of, for example, a semiconductor laser is incident on a target 7711 constant optical fiber 3 via a first optical coupler 2. be done. The measurement pulse light PIN is attenuated by Rayleigh scattering and Fresnel reflection during the process of propagating through the optical fiber 3 to be measured, and the solid line in the figure
This will be shown as a pulse waveform. On the other hand, the return light P 2 becomes as shown by a pulse waveform indicated by a dotted line.

OUT Therefore, as shown in the timing chart in Fig. 2,
When the measurement pulsed light PIN as shown in a) is incident on the optical fiber 3 to be measured, the temporal change in the intensity Iλ1 of the return light P is as shown in FIG. 2(b).

Here, the point UT where the intensity of the returned light P 2 changes rapidly indicates a failure point of the optical fiber 3 to be measured or a connection point by an optical connector, and a gradual change indicates Rayleigh scattering in the fiber.

The returned light P enters the optical coupler 4 of the second UT via the first optical coupler 2, where it is combined (optically coupled) with the sampling pulse light as the pump light emitted from the sampling pulse light source 5. . Here, the sampling pulse light source 5 is composed of, for example, a semiconductor laser, and the wavelength λ of the output sampling pulse light is the return light P
It must be different from the wavelength λ1 of 2 OUT , and its time width (pulse width) must be sufficiently short compared to the temporal change of the return light P as shown in Figure 2 (C). Must be OUT.

When such return light P and sampling pulse UT light are combined by the second optical coupler 4 and input into the nonlinear optical element 6, the wavelength becomes λ3 due to sum or difference frequency mixing.
A sum or difference frequency light is generated.

Here, the nonlinear optical element 6 is formed of an anisotropic crystal such as LiNbO2, and the light generated by sum or difference frequency mixing by this nonlinear optical element 6 is sum frequency mixed light (intensity: Iλ, vibration To give an example of the number: ω −2π・C/λ3), let the intensities of the incident light be Iλ, Iλ2,
If the vibration frequency is ω1-2π・C/λ, ω−2π・C/λ (C is the speed of light), then it becomes λ I λ ω3−+1 +″)2.

Therefore, when the light emitted from the nonlinear optical element 6 is incident on the spectroscopic means 7 and only the sum or difference frequency light of the wavelength λ3 is extracted, the photodetector 8 produces light as shown by hatching in FIG. 2(d). Intensity is detected.

In this case, an APD or the like can be used as the photodetector 8, but if the light is weak, a photomultiplier tube (PMT) can be used.
You can use The output of the photodetector 8 is amplified by an amplifier 9 and sent to a signal processing circuit 10.

The signal processing circuit 10 outputs all constant pulsed light P at regular time intervals.
While controlling the 811 constant pulse source 1 so that the IIFj constant pulse light PIN is output, the sampling pulse light source 5 sequentially shifts the timing so that the sampling pulse light is output every time the IIFj constant pulse light PIN is output. control. This will be explained with reference to Figure 3 (J
As shown in 1), the measurement pulsed light P is transmitted at time points t, t2, . ...IN
1t7. ...and Δ
p1 Emitted from constant pulse light source 1 and returned light P
The temporal change of is repeated 00 times as shown in FIG. On the other hand, the sampling pulse light is at time t+Δti as shown in FIG.

t2+Δt2. ......, t7+Δt7.・・・・・・
...is emitted repeatedly.

Here, Δt −Δ1 −Δt −Δt 2− ”'・
・1Δt −Δta”... Therefore, it can be seen that the sampling pulse light is sequentially emitted with the timing shifted by a certain period of time.Then, in the nonlinear optical element 6, the timing t +Δt2...・・・・t
+Δt, . . . causes sum or difference frequency mixing, and sum or difference frequency light as shown by hatching in FIG. 3(d) is obtained. Therefore, as shown by the dashed line in the figure (d), an observed waveform similar to the time tlT change of the return light P is obtained. The processing results are recorded and averaged, and sent from the signal processing circuit 10 to the display device 11, where the observed waveform is displayed on the screen.

The time resolution of this I11 waveform can be improved by shortening the pulse width of the sampling pulse light, and since it is easier to obtain such ultra-short optical pulses than to improve the response speed of APD, etc., the distance It is possible to significantly improve resolution. Furthermore, since a sampling method is employed, it is possible to prevent unobservable areas from occurring on the observed waveform due to "striping" when there is strong return light.

Next, the above embodiment will be explained in more detail using numerical values.

First, the measurement pulse light source 1 has a wavelength (λl-1, 3 μm) used in normal optical communication.

A semiconductor laser with a diameter of 1.55 μm) can be used.

The pulse width of the measurement pulse light PIN is usually several 1 Or+se
However, in order to improve the distance resolution, it is desirable to use ultra-short pulse light with a pulse width of about 30 psec. Such ultra-short pulsed light can be easily obtained by causing a semiconductor laser to emit light with a drive pulse current having a pulse width of about 100 psec and using the first pulse of relaxation oscillation.

A semiconductor laser can also be used for the sampling pulse light source 5 that outputs sampling pulse light as pump light, but in order to increase the distance resolution, ultra-short pulse light with a pulse width of about 30 psec is used as the API constant pulse light P1. - It is desirable to output using a similar method. Here, the wavelength λ of the sampling pulse light must be different from the wavelength λ of the measurement pulse light PIN, for example, λ
The wavelength is set to 2-850 nm. Therefore, the measurement pulse light P
If the r-wavelength λ1 is 1.55 μm, the wavelength λ3 of the sum frequency light is about 549 nm (visible light).

An optical directional coupler or the like can be used as the first optical coupler 2 and the second optical coupler 4, and a narrow band optical filter or a spectrometer can be used as the spectroscopic means 7. In addition, since the present invention samples the UT temporal change of the return light P using the sampling pulse light as the pump light, the photodetector 8
, the amplifier 9, the A/D converter in the signal processing circuit 10, etc. do not require particularly high-speed response. When it is desired to obtain high conversion efficiency in the nonlinear optical element 6, a waveguide structure may be used.

〔Effect of the invention〕

As explained above in detail, according to the optical fiber fault detection method of the present invention, the return light intensity is summed or difference frequency mixed with sampling pulse lights having different frequencies, thereby increasing the time width of the sampling pulse light. and sampled at the timing. Therefore, by extracting the sum or difference frequency light while shifting the timing of the sampling pulse light and combining them, it is possible to obtain an observed waveform similar to the temporal change in the return intensity.

Further, according to the apparatus of the present invention, the above observed waveform can be obtained simply by combining a sampling pulse light source that outputs ultrashort optical pulses, a nonlinear optical element, a normal high-speed photodetector, and the like. Furthermore, if a display means such as a CRT is provided, the observed waveform can be displayed on an image.

By adopting a sampling method, the present invention improves the time resolution of the returned light intensity and greatly improves the distance resolution of the fault point, and also substantially eliminates the unobservable area immediately after the fault point. , it is possible to realize highly accurate failure point search.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a device to which an optical fiber fault detection method according to an embodiment of the present invention is applied, and FIG. 2 shows the operation of the embodiment.
FIG. 3 is a timing diagram showing the sampling of the returned light in the embodiment. 1... n1 constant pulse light source, 2... first optical coupler,
3... Optical fiber to be measured, 4... Second optical coupler,
5... Sampling pulse light source, 6... Nonlinear optical element, 7... Spectroscopic means, 8... Photodetector, 9...
Amplifier, 10... Signal processing circuit, 11... Display device, PHJ... Measurement pulse light, P... Return light P0 UT Patent applicant Hamamatsu Photonics Co., Ltd. Agent Patent attorney
Yoshiki Hase Procedural Amendment Form

Claims (1)

[Scope of Claims] 1. An optical fiber fault point searching method for searching for a fault point in the optical fiber to be measured based on the temporal change in the intensity of the returned light when the measurement pulse light is input to the optical fiber to be measured, Sampling pulsed light having a sufficiently short time width and different wavelengths compared to the temporal change in the returned light intensity is combined with the returned light to generate sum or difference frequency light by sum or difference frequency mixing, and the measurement pulse By sequentially shifting the timing of the sampling pulse light while repeating the incidence of light at predetermined time intervals and sampling the sum or difference frequency light, an observed waveform similar to the temporal change in the intensity of the returned light is obtained. An optical fiber fault point search method characterized by: 2. From the temporal change in the intensity of the returned light when the measurement pulse light from the measurement pulse light source is incident on the optical fiber to be measured,
The optical fiber fault point search device for searching for a fault point in the optical fiber under test includes a sampling pulse light source that emits sampling pulse light having a sufficiently short time width and a different wavelength compared to the temporal change in the return light intensity; , an optical coupling means that couples the returned light and the sampling pulse light; a nonlinear optical element that receives the light from the optical coupling means and generates sum or difference frequency light by sum or difference frequency mixing; a light detection means for detecting the intensity of the difference frequency light, the measurement pulse light is emitted at predetermined time intervals, and the sampling pulse light is emitted with the timing sequentially shifted each time the measurement pulse light is emitted repeatedly. a light source control means for controlling the measurement pulse light source and the sampling pulse light source so as to control the measurement pulse light source and the sampling pulse light source, and sampling the detection output of the light detection means at each emission timing of the sampling pulse light,
An optical fiber failure point searching device comprising: an analysis means for obtaining an observed waveform similar to the temporal change in the intensity of the returned light. 3. The optical fiber fault point searching device according to claim 2, further comprising display means for displaying the observed waveform obtained by the analysis means. 4. The optical fiber failure point searching device according to claim 2 or 3, further comprising spectroscopic means for extracting sum or difference frequency light between the nonlinear optical means and the light detection means.