JPH02304328A - Method and device for searching fault point of optical fiber - Google Patents

Abstract

PURPOSE:To expand a dynamic range and to improve time resolving power by subjecting the return light from an optical fiber to be measured to a sum or differential frequency mixing with pumping light of the wavelength different from the wavelength of the return light and determining an observation waveform. CONSTITUTION:The return light POUT from the optical fiber 3 to be measured and the pump light of the wavelength different from the wavelength of the return light POUT from a pumping light source 5 are synthesized in an optical coupler 4 and the synthesized light is made incident on a nonlinear optical element 6 where the sum or differential frequency light of the wavelength lambda3 is formed by the sum or differential frequency mixing. If, therefore, the light emitted from the element 6 is made incident on a spectroscopic means 7 and only the sum frequency light of the wavelength lambda3 is extracted, the wavelength lambda3 of this light has the wavelength shorter than the wavelengths lambda1, lambda2 of the original light and the use of an Si-photodiode of a small dark current or the photomultiplier tube suitable when light is slight is possible as a photodetector 8 and, therefore, the waveform of the high sensitivity and low noise is obtainable.

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).
:0ptical Tiff1e Domain
Rcf'lectometer).

[Conventional technology]

An optical fiber fault detection device called an optical fiber analyzer inputs pulsed light for measurement into a constant optical fiber 1-1, and calculates the time of return light intensity caused by Rayleigh scattered light (backscattered light) or Fresnel reflected light. It is designed to detect physical changes and detect failure points based on this. In such 0TDR, an avalanche photodiode (APD), a high-speed photodiode, or the like has conventionally been used as a photodetector.

On the other hand, if there is a fault in the 111 constant optical fiber, strong Fresnel reflected light will be generated at the fault point, and this will enter the photodetector as return light I7. As a result, a so-called "flank" occurs in the response waveforms of the photodetector and the downstream amplifier, and an unobservable region appears in a time region further behind (especially immediately after) the failure point. Therefore, in order to eliminate this drawback, a masking function using an optical deflector or the like has been conventionally added.

[Problem to be solved by the invention]

However, conventional infrared light detection requires Gs and InS.
b P APDs and photodiodes are used, but the noise of these photodetecting elements is higher than that of visible-use St photodiodes, and the current amplification factor is also low.
It could not be used for point searching due to long-distance optical fiber failures. On the other hand, photomultiplier tubes and Si-AP as photodetectors
Although D has extremely low noise and high sensitivity, the sensitivity is extremely low in the long wavelength band. In particular, it has extremely low sensitivity to infrared light with a wavelength of 1.3 to 1.55 μm used in optical fiber communications, so it is rarely put into practical use.

On the other hand, even if an attempt is made to eliminate the "striping" by using an optical internal device, due to limitations such as the response speed of the optical deflector itself and the bandwidth of the drive circuit, the above-mentioned view a-1 cannot be achieved. 3
It was not possible to reduce the distance to approximately 0 m or less. Furthermore, if an optical deflector is used, a loss of 3 to 6 dB will occur.

SUMMARY OF THE INVENTION Therefore, the present invention provides a method for searching for a fiber failure point that can expand the dynamic range and improve the temporal resolution, and desirably prevent the occurrence of a ΔP1 impossible region immediately after the failure point, and an apparatus using the same. The purpose is to provide

[Means to solve the problem]

The optical fiber fault point searching method and device according to the present invention include:
It is applied to searching for fault points in a constant optical fiber under test based on the temporal change in the intensity of the returned light when a constant pulse light of δ or 1 is input into the optical fiber under test. It is characterized by obtaining an observed waveform by performing sum or difference frequency mixing of light and pump light.

That is, the method of the present invention combines pump light having a different wavelength from the return light from the optical fiber to be measured to the return light to generate sum or difference frequency light by sum or difference frequency mixing,
By observing the temporal change in the sum or difference frequency light intensity, an observed waveform similar to the temporal change in the returned light intensity is obtained.

The apparatus of the present invention also includes a pump light source that emits pump light having a wavelength different from that of the return light from the optical fiber to be measured, and a sum or difference frequency light that is generated by inputting the return light and the pump light and performing sum or difference frequency mixing. It is characterized by comprising a nonlinear optical element that generates the light and an observation means that observes temporal changes in the intensity of the sum or difference frequency light.

Here, the pump light source (for example, a semiconductor laser) may be controlled so that the pump light intensity becomes zero or a sufficiently low level at time intervals when the intensity of the returned light suddenly increases due to Fresnel reflection, etc. The means may be constructed using a photomultiplier tube, a counter that counts the output thereof, and a noise removal circuit that removes as noise the output of the photomultiplier tube that is below a predetermined level.

[Effect]

According to the optical fiber fault detection method of the present invention, return light is summed or difference frequency mixed with pump light having different frequencies, so that light with spectral sensitivity characteristics suitable for the wavelength of the sum or difference frequency light is produced. Using a detector, it is possible to obtain an observed waveform similar to the temporal change in return intensity.

Further, according to the device of the present invention, only a pump light source that outputs pump light, a nonlinear optical element that generates sum or difference frequency mixing, or a photodetection means used in conventional 0TDR etc. can be combined. The above observed waveform can be obtained. Furthermore, if the pump light is modulated at the peak of the returned light intensity, "flank" can be eliminated. Furthermore, by employing a photon counting method using a photomultiplier tube, the dynamic range can be dramatically improved. Furthermore, if a display means such as a CRT is provided, viewing
The lp1 waveform can also 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 block diagram of an apparatus to which an optical fiber fault detection method according to an embodiment of the present invention is applied, and FIG. 2 is a timing diagram showing the state of 7I-1 constant. As shown in Figure 1,
For example, all constant pulse light F composed of a semiconductor laser
The measurement pulsed light PIN (wavelength λ1) from 7.1 is the first
is input into the optical fiber 3 to be measured via the optical coupler 2 . 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 becomes as shown by a solid line pulse waveform in the figure. On the other hand, the return light P
is shown by the pulse waveform of the point UT line. Therefore, as shown in the timing chart of FIG. 2, when the measurement pulse light PIN as shown in FIG. 2(a) is incident on the fixed optical fiber 3,
When the intensity of the return light P is Iλ1, the quantitative change in UT is as shown in FIG. 2(b). Here, the return light P
A point where the intensity changes rapidly indicates a significant failure point in the UT optical fiber 3 under test or a connection point by an optical connector, etc., and a gradual change indicates Rayleigh scattering in the fiber.

The returned light P 2 is incident on the optical coupler 4 of the second UT 2 via the first optical coupler 2 , where it is combined (optically coupled) with the pump light emitted from the pump light source 5 .

Here, the pump light source 5 is composed of, for example, a semiconductor laser, but the wavelength λ2 of the output pump light must be different from the wavelength λ1 of the return light P. This is because the second harmonics (λ, λ) of the pump light and return light are generated from the nonlinear optical element.
This is because when the pump light and the return light have the same wavelength, they become the same wavelength as the sum frequency light and become a noise component.

When such return light P and the pump light are combined by the second light coupler 4 and incident on the nonlinear optical element 6, sum or difference frequency light having a wavelength of λ3 is generated by sum or difference frequency mixing. Ru. 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 the nonlinear optical element 6 is sum frequency mixed light (intensity: ■ λ, frequency: (
, 73M2π・C/λ3), the intensity of the incident light is Iλ, Iλ2, and the frequency is ω −2π・C
/l
If iλ ω −2π・C/λ2 (C is light 1'
2 speed), I λ ■ ■ λ fist I λω3−ω1+ω
It becomes 2. In addition to this, the second harmonic of the aforementioned return light and pump light! λ and ■λ5 are also generated.

Therefore, when the light emitted from the nonlinear optical element 6 enters the spectroscopic means 7 to extract only the sum frequency light of wavelength λ3, the wavelength λ3 of this light is shorter than the wavelength of the original light (λ, λ2). Therefore, as the photodetector 8, S! with less darkness of 71m! −
A photodiode or a photomultiplier tube (PMT) suitable for use when the light is weak can be used.

As described above, in this embodiment, as a feature of 'W41, the wavelength-converted light (for example, the end of a wave branch) by the pump light and the light to be measured is targeted for detection, so that the following characteristic effect is obtained.

That is, a nonlinear optical element as a wavelength conversion means is provided in front of the PMT to convert light in a long wavelength band to light in a towable range,
Photodetection can be performed with PMT. As a wavelength conversion means, one having a waveguide structure made of a nonlinear optical material such as LiNbO2 has been developed, and its conversion efficiency has reached 1% or more. The quantum efficiency when this spherical wavelength conversion means is combined with PMT is about 0.3%, which is 1/170 to 1/170 compared to the quantum efficiency (50 to 80%) of APD currently used in commercially available 0TDRs. It is about 1/270. However, APD has a multiplication factor of about 400 or less for Gc-APD, and I
n GaAs, it is low at about 40 or less, and noise due to dark current and shot noise during avalanche amplification are large.

Therefore, the minimum reception level is 40MHz in the 100MH2 band.
It is about nW. On the other hand, the combination of a nonlinear optical element and a PMT as a wavelength conversion means has a low overall quantum efficiency, but since the PMT as a detector has low noise and high gain, the minimum reception level is in the 100 MH2 band. It is about 8 nW, and has a better S/N than a device using an APD and a larger dynamic range.

The output of the photodetector 8 is amplified by an amplifier 9 and sent to a signal processing circuit 10. Here, an A/D converter (not shown) is provided at the input part of the signal processing circuit to digitize the signal.
A counter is provided in place of the D converter.

The signal processing circuit 10 controls the measurement pulse light source 1 so that the Δ-1 constant pulse light PIN is output at a predetermined timing, and detects a sudden rise in the level of the return light P. The pump light source 5 is controlled to have a low level. To explain this with reference to Fig. 2, as shown in Fig. 2 (a), the measurement pulse light P
When IN is emitted from n1 constant pulse light source 1, return light P
The temporal change in UT is as shown in Figure UT (b). Here, the times t, tz, and tz at which the light intensity peaks correspond to failure points, optical connectors, etc. in the optical transmission path.

When this is observed using conventional technology, a significant "heel pull" occurs at time 1, 12.13, as shown in FIG. 2(c). Therefore, in this embodiment, the pump light source 5 is controlled by a command from the signal processing circuit 10, and Δt in FIG.
, Δt, and Δt3, that is, when the intensity of the returned light reaches its peak, the pump light Iλ2 is not output (or the output is at a sufficiently low level). in particular,
The semiconductor laser constituting the pump light source 5 is capable of high-speed modulation of about 10 GH2, and since it oscillates as a DC laser, the drive current to the semiconductor laser is
Control is performed as shown in figure (f). Then, the nonlinear optical element 6
The level of the pump light Iλ2 incident on is shown in Fig. 2(d).
Even if the Fresnel reflected light returns at a higher level for a period of Δ11°105 times as shown in FIG. As a result, a δ-1 waveform corresponding to the time change UT of the return light P as shown in FIG. 2(e) is obtained. The result of this processing is
The signal is sent from the signal processing circuit 10 to the display device 11, and the kll 11?1 waveform is displayed on both sides.

The timings t, tz, and ta shown in FIG. 2 are obtained as follows. First, an incident pulse P is input into the optical fiber 3, and the return light P is a wave I N
Look at the OUT shape i'1llll
Then, the waveform shown in FIG. 2(C) is obtained.

Here, since the sudden rising point of the observed waveform corresponds to the above-mentioned timing t, tz, tz, this is stored in the signal processing circuit 10, and then the incident pulse PI is
N is input into the optical fiber 3, and the waveform of the sum frequency light is observed. At this time, timings t and tz are stored in the signal processing circuit 10.

Based on tz, the semiconductor laser constituting the pump light source 5 is modulated at high speed with the current shown in FIG. 2(f). Note that the timings 1, 12, and 13 may be determined by a separately provided photodetector, or may be manually controlled from the waveform on the display device.

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 ΔIIJ constant pulse light PIN is usually several tens of nanometers.
sec to several μsec, but 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 a tii short pulse destination can be easily achieved by causing the 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 pump light source 5 that outputs pump light. Here, the wavelength λ of the pump light
must be different from the wavelength λ1 of the measurement pulsed light PIN, and is set to λ22-850n, for example. Therefore,
If the wavelength λ1 of the n1 constant pulse light PIN is 1.55 μm, the wavelength λ3 of the sum frequency light is about 549 nm (visible light). Optical directional couplers etc. can be used as the first optical coupler 2 and the second optical coupler 4, and the spectroscopic means 7
A narrow band optical filter or a spectrometer can be used.

Next, a modification of the present invention will be explained with reference to FIGS. 3 and 4.

This variant is il? In order to improve the constant dynamic range, a so-called photon counting method is applied. In other words, when the light incident on a photomultiplier tube (PMT) becomes extremely weak, the output of the PMT becomes discrete and pulse-like.
It is known that the time interval between the pulses is inversely proportional to the amount of incident light. Therefore, by counting the pulse interval or the number of pulses within a predetermined time, the intensity of the incident light can be determined. The output signal of the PMT after passing through the amplifier is as shown in FIG. 3(a). A predetermined threshold value Vth is applied to this output.
By providing the following and cutting signals below ■th, it is possible to remove noise from the PMT and the amplification section (AMP).

Therefore, by counting the number of pulses within a predetermined time, it is possible to perform optical measurement with an extremely high S/N ratio by removing noise.

The output signal of the PMT after amplification is shown as a pulse waveform distribution as shown in FIG. 3(b). In the figure, the solid curve shows the sum of the signal pulse and the dark current pulse, and the hatching shows the dark current component. As shown in the figure, it can be seen that many of the noise components below the signal component can be removed by cutting below the threshold value vth.

Figure 4 shows the relationship between the intensity of the return light (light to be measured) from the optical fiber and the output (signal pulse) of the PMT.

Shown schematically. As shown, the frequency of pulse output from the P FvI T corresponds to the intensity of the returned light.

Therefore, by counting the number of output pulses for each of channels CHo to CH at equal intervals (Δt) and storing the count values in individual memories, the return light intensity can be determined for each channel CHo to CHo. Here, by repeating photon counting as shown in FIG. 4 a plurality of times and adding the count values for each channel CHo-C1n, it becomes possible to search for optical fiber fault points with high resolution and a high dynamic range.

Various modifications are possible to the present invention.

For example, for nonlinear optical elements, Aj! Those using GaAs, MBANP (2-methyl benzine-amino
5.Nitropyridine) is applicable.

Furthermore, if a Cerenkov radiation type waveguide structure is used, sensitivity and S/N can be improved.

〔Effect of the invention〕

As explained above in detail, according to the method of the present invention, the returned light is summed or difference frequency mixed with the pump light having different frequencies, and this is detected, so that the spectrometer suitable for the wavelength of the detected light is adjusted. A photodetector with sensitivity characteristics can be used. Therefore, a waveform with high sensitivity and low noise can be obtained.

Further, according to the device of the present invention, a pump light source that outputs pump light, a nonlinear optical element, or a conventional 0TDR
The above observed waveforms can be obtained simply by combining the photodetection means used in, etc. In addition, by modulating the pump light at the peak of the intensity of the return light due to Fresnel reflection, so-called "tail drag" can be eliminated. Furthermore, by employing a photon counting method using a photomultiplier tube, the dynamic range can be improved. Furthermore, if a display means such as a CRT is provided, the observed waveform can be displayed on an image.

As described above, the present invention greatly improves the distance resolution in the optical fiber under test, and also makes it possible to substantially eliminate the ΔF1 impossible region immediately after the failure point.
Highly accurate failure point search can be realized.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a device to which the optical fiber fault detection method according to the embodiment of the present invention is applied, Fig. 2 is a timing diagram showing the operation of the embodiment, and Figs. 3 and 4 are diagrams showing the photon counting method. It is a figure which shows the effect|action of the Example adopted. 1... Measurement pulse light source, 2... First optical coupler, 3
. . . aP1 constant optical fiber, 4. Second optical coupler, 5. Pump light source, 6. Nonlinear optical element, 7.
...spectroscopy means, 8...photodetector, 9...amplifier, 1
0...Signal processing circuit, 11...Display device, PIN・
・・a? 1 Constant pulse light, P...Return light. UT Representative Patent Attorney Yoshiki Hase Practical example of action photons and decimal method Figure 3

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, The pump light having a different wavelength from the returned light is combined with the returned light to generate sum or difference frequency light by sum or difference frequency mixing, and the temporal change in the intensity of the sum or difference frequency light is observed. An optical fiber failure point search method characterized by obtaining an observed waveform similar to the temporal change in return light intensity. 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 searching device that searches for a fault point in the optical fiber under test includes a pump light source that emits pump light having a wavelength different from that of the returned light; An optical fiber failure point searching device comprising: a nonlinear optical element that generates sum or difference frequency light by mixing; and observation means that observes temporal changes in the intensity of the sum or difference frequency light. 3. The optical fiber according to claim 2, further comprising a light source control means for controlling the pump light source so that the intensity of the pump light becomes zero or a sufficiently low level at a time interval when the intensity of the return light suddenly increases. Failure point search device. 4. The optical fiber fault point searching device according to claim 2 or 3, further comprising spectroscopy means for extracting sum or difference frequency light between the nonlinear optical means and the observation means. 5. The observation means includes a photomultiplier tube for detecting the sum or difference frequency light, a counter for counting the output thereof, and a noise for removing as noise the output of the photomultiplier tube that is below a predetermined level. 5. The optical fiber fault point searching device according to claim 2, further comprising a removal circuit.