JPH0618365A - Light pulse tester - Google Patents

Abstract

PURPOSE:To conduct waveform observation of back scattering light just after fresnel reflection which has been impossible to be measured due to a trailing phenomenon caused by a step response characteristic of a filter and surely find a connection point in which an abnormality occurs and automatically conduct highly precise measurement even in the case where a plurality of the connection points exist. CONSTITUTION:Reflection light transmitted from an optical fiber to be measured accompanying supply of probe pulse light is synthesized with coherent local light whose polarization state is variably controlled in order to conduct coherent detection. And, when a polarization component of fresnel reflection light is shifted from another polarization component of local light and they do not interfere with each other, a level after detection of the fresnel reflection light lowers and back scattering just after the fresnel reflection impossible to be measured due a traveling phenomenon caused by step response characteristic of a filter, can be measured. In addition, at this time the level after the detection does not almost change because the polarization state changes at random and the back scattering is averaged.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention supplies a coherent optical pulse to an optical fiber to be measured and, together with this, backscattered light and reflected light returning from the optical fiber to be measured, and local light (local light). ) Is combined with coherent detection, and predetermined signal processing is performed to measure an optical loss or a failure point of the optical fiber to be measured.

[0002]

2. Description of the Related Art An optical pulse is supplied to an optical fiber to be measured,
Coherent detection is performed by combining the backscattered light and reflected light returning from the optical fiber under measurement with the supply of this optical pulse, and the local light, and performing calculations such as square addition on the detected signal. 2. Description of the Related Art An optical pulse tester is known as a device that measures loss or a failure point of an optical fiber under measurement by performing signal processing.

FIG. 4 is a block diagram of an optical pulse tester based on this type of coherent detection method. The optical pulse tester of the coherent detection system is based on the coherent light source 2
1, optical switch 22, optical pulse drive circuit 23, reference trigger generation circuit 24, optical branching / coupling device 25, O / E converter 26, AC amplifier 27, filter 28, A / D converter 29, square adder 30, The logarithmic converter 31 and the display 32 are provided.

In this optical pulse tester, when the reference trigger generated by the reference trigger generation circuit 24 is input to the optical pulse drive circuit 23 that drives the optical switch 22, the continuous light output from the coherent light source 21 is output. It is converted into an optical pulse by the optical switch 22 and is incident on the measured optical fiber 33 as probe pulse light. With the supply of the probe pulse light, the measured optical fiber 33
The backscattered light and the Fresnel reflected light are returned from, and the returned light is input to the optical branching / coupling device 25 via the optical switch 22. The optical branching / combining device 25 includes a coherent light source 21.
Local light is further input, and the beat signal output from the optical branching / coupling device 25 by mixing the local light and the return light is converted into an electric signal by the O / E converter 26. This electric signal is input to the A / D converter 27 after only the signal having the beat signal frequency passes through the filter 28 and is AC amplified by the AC amplifier 27. A / D
The converter 27 samples the input electric signal at a predetermined sampling period, squares and adds each sampled data, performs logarithmic conversion, and performs waveform processing, and the result is displayed on the display screen of the display 32. .

Here, the Fresnel reflected light from the measured optical fiber 33 accompanying the supply of the probe pulse light is generated by the difference in refractive index between air and glass when there is an optical connector coupling in the optical fiber line. The level depends on the reflectance of the end surface of the coupling portion and is higher than that of the inclined portion showing the backscattered light as shown in FIG. 5B. That is, when the input power of the measured optical fiber 33 is 0 dBm, the Fresnel reflected light at the mouth 33a of the measured optical fiber 33 is about −14 dBm, and the backscattered light generated immediately after this Fresnel reflected light is about -54 dBm
(When the pulse width is 1,55 μm and the pulse width is 1 μm), the difference is as large as about 40 dB.

Also, in this type of optical pulse tester,
When, for example, an A / O switch is used as the optical switch 22, the probe pulse light has a drive frequency f of the A / O switch.
The frequency is shifted by f0 according to 0 (usually about 100 to 150 MHz), and when a mechanical switch such as an optical directional coupler or a fiber coupler or an E / O switch is used, the frequency f0 becomes 0 Hz.

Therefore, the intermediate frequency of the coherently detected signal becomes the frequency of f0, and in order to pass the signal of this frequency f0, the intermediate frequency f0 is used as the filter 28.
I was using a bandpass filter. Also, especially f0
In the case of low frequency (including 0 Hz), a low pass filter was used.

[0008]

By the way, when the Fresnel reflected light having a higher level than the backscattered light is coherently detected, the detected signal becomes a pulse-shaped rectangular wave (see FIG. 6 (a)). When a bandpass filter is used as the filter 28 and a signal is transmitted at an increased response speed, a sootki phenomenon occurs due to the step response of the filter as shown in FIG. 6B. As shown in FIG. 5 (b), there is a problem that it is not possible to observe the waveform of the backscattered light from the location where the Fresnel reflection occurs.

On the other hand, when a low-pass filter having a slow response speed is used as the filter 28, the rising edge of the signal is delayed and both edges are rounded as shown in FIG. 6 (c), which reduces the dynamic range. For,
Normally, the signal is designed to pass through to a high frequency that can reproduce the coherently detected Fresnel reflected light. Therefore, even when a low-pass filter is used, a sootki phenomenon occurs similarly to the bandpass filter, and the waveform of the backscattered light cannot be observed during the dead zone while the sootoki phenomenon occurs.

That is, the Fresnel reflected light retains its polarization state, but the backscattered light has a random polarization state,
When coherent detection of local light and reflected light from the optical fiber, if the polarization state of both lights is changed, the output of the detection signal of the Fresnel reflection part changes, but the output of the detection signal of the backscattered light part It does not change. Therefore, depending on the polarization state, the level of Fresnel reflected light is high, and the above problem occurs.

Therefore, the present invention has been made in view of the above problems, and an object thereof is to observe the waveform of backscattered light immediately after Fresnel reflection, which could not be measured due to the Sosohiki phenomenon due to the step response characteristic of the filter. An object of the present invention is to provide an optical pulse tester capable of reliably performing.

[0012]

In order to achieve the above object, the optical pulse tester according to the present invention of claim 1 is coherent with the reflected light from the optical fiber to be measured accompanying the supply of the coherent probe pulse light. The signal coherently detected by the local light is subjected to signal processing by passing through a filter for extracting a predetermined frequency component, and an optical pulse tester for measuring an optical loss or a fault point of the optical fiber under measurement is used. A polarization control means for changing the polarization state of at least one of the light and the reflected light is provided. In the optical pulse tester according to the invention of claim 2, storage means in which data indicating a plurality of polarization states different for each predetermined distance are stored in advance, and the local light or the reflected light is stored based on the data of the storage means. And a polarization control means for automatically changing the polarization state of at least one of the above.

[0013]

According to the first aspect of the present invention, when the probe pulse light generated by the switching drive of the optical switch is supplied to the optical fiber to be measured, the optical pulse to be measured is reflected by the optical fiber to be measured as the probe pulse light is supplied. Backscattered light and Fresnel reflected light return to the optical switch side. This reflected light is combined with coherent local light whose polarization state is changed by the polarization control means, and is coherently detected. Then, by changing the polarization state of the local light, when the polarization component of the Fresnel reflected light deviates from the polarization component of the local light and no longer interferes, the level of the Fresnel reflected light decreases and the step response characteristic of the filter causes The backscattered light immediately after Fresnel reflection, which could not be measured, is measured. At this time, the backscattered light from the optical fiber to be measured has its polarization state randomly changed and averaged, so that the level after detection hardly changes. Further, the polarization control means in the invention of claim 2 automatically and variably controls the polarization state of the local light for each predetermined distance based on the data indicating a plurality of polarization states which are different for each predetermined distance in the storage means. The polarization state of the reflected light from the measured optical fiber may be variably controlled. Further, the polarization states of both the local light and the reflected light may be variably controlled by the polarization control means.

[0014]

1 is a block diagram showing an embodiment of an optical pulse tester according to the present invention. The optical pulse tester according to this embodiment comprises a coherent light source 1, a reference trigger generation circuit 2, an optical pulse drive circuit 3, an optical switch 4, a polarization controller (polarization control means) 5, an optical branching / coupling device 6, and O / E conversion. Device 7, AC amplifier 8, filter 9, A / D converter 10,
It is configured by including a square adder 11, a logarithmic converter 12, and a display 13.

The coherent light source 1 is an optical fiber 1 to be measured.
Continuously emitting coherent light of a certain wavelength on the 4 side,
Polarization controller 5 using a part of coherent light as local light
Is being supplied to.

The reference trigger generation circuit 2 generates a reference trigger at a predetermined interval (for example, 500 μsec) and outputs it to the optical pulse drive circuit 3. Further, the reference trigger generation circuit 2 uses a sampling start trigger for sampling a signal mixed by the optical branching / coupling device 6 and converted into an electric signal by the O / E converter 7 at the same time as the reference trigger.
It is output to the / D converter 10.

The optical pulse drive circuit 3 receives the optical pulse generation trigger from the reference trigger generation circuit 2 and outputs a switch drive signal for switching the optical switch 4 to the optical switch 4.

The optical switch 4 performs switching operation of the coherent light continuously emitted from the coherent light source 1 by a switch drive signal supplied from the optical pulse drive circuit 2. Then, the switching operation converts the coherent light into pulsed light (for example, 1 μsec coherent light pulse), and supplies the coherent probe pulsed light to the optical fiber 14 to be measured.

The polarization controller 5 is, for example, one in which an optical fiber is wound a plurality of times on a concentric circle and the entire wound portion is moved to control the polarization state, and one in which the optical fiber is bent to control the polarization state depending on the degree. , Optical element λ / 4
A plate and a λ / 2 plate are combined to rotate one of them to control the polarization state, and a waveguide for controlling the polarization state by applying a voltage to an electrode to change the vertical component or the horizontal component of the polarization and combining them. It is configured by using an optical element of the shape. Then, the polarization controller 5 variably controls the polarization state of the local light supplied from the coherent light source 1 in correspondence with the position of the Fresnel reflected light that is the return light from the optical fiber 14 to be measured. More specifically, in this embodiment, the Fresnel reflected light generated in each optical fiber coupling portion does not necessarily have the same polarization state, so that the Fresnel reflected position of each Fresnel reflected light is observed while observing the waveform displayed on the display 13. The polarization state of the local light is adjusted to variably control according to.

The optical splitter / coupler 6 changes the polarization state by the polarization controller 5 and the reflected light (backscattered light and Fresnel reflected light) returning from the optical fiber 14 to be measured with the supply of the coherent light pulse. The local light is mixed and output to the O / E converter 7.

The O / E converter 7 converts the light mixed in the optical branching / coupling device 6 into an electric signal and outputs it as an IF signal to the AC amplifier 8.

The AC amplifier 8 is an IF from the O / E converter 7.
The signal is AC-amplified and output to the filter 9 with sufficient gain.

The filter 9 is composed of a bandpass filter having a center frequency f0 which is the intermediate frequency of the coherently detected signal, and passes only the IF signal from the AC amplifier 8 and outputs it to the A / D converter 10. ing. In addition,
The filter 9 is composed of a low-pass filter when the center frequency f0 is 0 Hz.

The A / D converter 10 is a reference trigger generating circuit 2
The signal from the filter 9 is sampled at a predetermined sampling period in accordance with the sampling start trigger from, and converted into digital signal measurement data and output to the square adder 11.

The square adder 11 square-adds and averages the sampling data from the A / D converter 10, and outputs the average value data to the logarithmic converter 12.

The logarithmic converter 12 performs signal processing by logarithmically converting the average value data from the square adder 11 within an idle time until the next probe pulse light is supplied.

The display 13 displays the waveform data based on the signal processing in the logarithmic converter 12 on the display screen.

Next, the operation of the optical pulse tester configured as described above will be described. Optical pulse drive circuit 3
When the optical switch 4 is driven to switch by
The coherent light continuously emitted from the coherent light source 1 is supplied to the measured optical fiber 14 as probe pulse light.

When the probe pulse light is supplied to the measured optical fiber 14, the backscattered light and Fresnel reflected light are returned from the measured optical fiber 14 to the optical switch 4 side. This reflected light is input to the optical branching / coupling device 6 via the optical switch 4.

In the optical branching / coupling device 6, the optical fiber 14 under test is measured.
And the local light supplied from the coherent light source 1 without operating the polarization controller 5, and outputs the mixed light to the O / E converter 7. At this time, since the polarization components of the backscattered light are randomly changed and averaged, the O / E conversion output after being mixed by the optical branching coupler is almost constant without depending on the polarization state of the local light. However, since the polarization component of the Fresnel reflected light is maintained, the O / E after mixing is performed depending on the polarization state of the local light.
The conversion output changes, and the output becomes maximum when the polarization state matches the local light.

Next, the O / E converter 7 converts the mixed light into an electric signal (IF signal) and outputs the electric signal to the AC amplifier 8. The AC amplifier 8 outputs the IF signal from the O / E converter 7. AC-amplified and output to the filter 9.

The filter 9 passes a signal of a predetermined frequency component (driving frequency of the optical switch 4) of the AC amplified IF signal and outputs it to the A / D converter 10.

The A / D converter 10 samples the signal that has passed through the filter 9 at a predetermined sampling period in accordance with the timing of the measurement start trigger from the reference trigger generation circuit 2, and outputs this sampling data to the square adder 11.

The square adder 11 squares and adds the data sampled by the A / D converter 10 and averages them, and outputs the average value data to the logarithmic converter 12.

In the logarithmic converter 12, the measured optical fiber 14
Meanwhile, the average value data from the square adder 11 is logarithmically converted until the next probe pulse light is supplied. The waveform data based on this conversion is displayed on the display unit 13.
And is displayed on the display screen (see FIG. 2B).

Next, while observing the waveform of the display 13,
The polarization controller 5 is operated by observing the target level of the Fresnel reflected light and the polarization controller 5 is operated to variably control the polarization state of the local light. As a result, when the Fresnel reflected light from the optical fiber 14 to be measured changes the polarization state of the local light so that the polarized components of the Fresnel reflected light deviate from each other and do not interfere with the local light, as shown in FIG. The level of the Fresnel reflected light afterward is reduced.

Since the polarization component of the backscattered light changes randomly and is averaged, the level after detection in this portion hardly changes, and the O / E converter 7
Subsequent processing is performed in the same manner as above.

Therefore, according to the above-described embodiment, the polarization controller 5 is provided for variably controlling the polarization state of the reflected light from the optical fiber 14 to be measured and the mixed local light for each target Fresnel reflected light. With this configuration, it is possible to perform measurement by observing the waveform of the backscattered light immediately after the Fresnel reflected light, which could not be measured due to the step response characteristic of the filter. As a result, for example, in a star type network in which a plurality of terminals are connected to a single device connected in series by one transmission line, an abnormality such as disconnection or failure occurs at any of a plurality of connection points. However, it is possible to reliably specify the location and perform highly accurate measurement.

Next, FIG. 3 is a block diagram showing another embodiment of the optical pulse tester according to the present invention. The same components as those of the optical pulse tester of FIG. 1 except the polarization controller are omitted.

The optical pulse tester of this embodiment has a configuration in which the polarization state of the local light is variably controlled by selecting the position for each Fresnel reflection in the above-mentioned embodiment.
In addition to the polarization controller 5, a polarization data storage unit 1 in which data indicating a plurality of polarization states different for each predetermined distance is stored in advance.
5 and a polarization drive unit 16 that drives the polarization controller 5 at predetermined intervals based on the data in the polarization data storage unit 15 to variably control the polarization state of the local light.

According to this structure, the level of the Fresnel reflected light after detection is automatically lowered without observing the waveform on the display screen of the display 13, and the state immediately after each Fresnel reflection is measured in a short time. can do. Further, even if the connection points in the star-shaped network are simultaneously disconnected or an abnormality such as a failure occurs, the location can be specified and the measurement can be performed, and the measurement accuracy can be further improved.

By the way, in the above-mentioned embodiment, the polarization controller 5 is provided between the coherent light source 1 and the optical branching / coupling device 6 to variably control the polarization state of the local light. Since the polarization components of the backscattered light reflected from 14 are randomly changed and averaged and do not depend on the polarization state of the local light, the polarization controller 5 is placed between the optical switch 4 and the optical branching / coupling device 6. The polarization state of the reflected light from the optical fiber 14 to be measured may be variably controlled so that the polarized components of the local light and the Fresnel reflected light do not shift and interfere with each other. Further, the polarization controller 5 may be provided both between the coherent light source 1 and the optical branching / coupling device 6 and between the optical switch 4 and the optical branching / coupling device 6.

[0043]

As described above, according to the optical pulse tester of the present invention, the waveform of the backscattered light immediately after Fresnel reflection, which cannot be measured due to the Sosohiki phenomenon due to the step response characteristic of the filter, can be observed. Therefore, even if the optical fiber to be measured has a plurality of connection points, it is possible to reliably specify the connection point where an abnormality such as disconnection or failure has occurred and perform high-precision measurement. Moreover, in particular, according to the invention of claim 2, a plurality of points can be automatically measured, and
The level of the Fresnel reflected light after detection is automatically lowered, and the state immediately after each Fresnel reflection can be measured in a short time, and the measurement accuracy can be further improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an optical pulse tester according to the present invention.

FIG. 2A is a waveform diagram of Fresnel reflected light after changing the polarization state of local light. FIG. 2B is a waveform diagram of Fresnel reflected light before changing the polarization state of local light.

FIG. 3 is a block diagram showing another embodiment of the optical pulse tester.

FIG. 4 is a block diagram showing an example of a conventional optical pulse tester.

FIG. 5A is a diagram showing a connection state between an optical pulse tester and an optical fiber to be measured. FIG. 5B is a waveform diagram in the connection state.

6A is a diagram showing a signal after coherent detection of Fresnel reflected light, FIG. 6B is a diagram showing a signal of the detected signal after passing through a bandpass filter, and FIG. 6C is a diagram showing a signal of the detected signal after passing through a lowpass filter. Figure

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Coherent light source, 4 ... Optical switch, 5 ... Polarization control device (polarization control means), 6 ... Optical branching / coupling device, 9 ... Filter, 14 ... Optical fiber to be measured, 15 ... Polarization data storage section (storage means), 16 ... Polarization drive section (polarization control means).

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinichi Furukawa 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (2)

[Claims] 1. A signal coherently detected by reflected light from an optical fiber to be measured accompanying supply of coherent probe pulse light and coherent local light is passed through a filter for extracting a predetermined frequency component, and then the signal is passed. In the optical pulse tester for processing and measuring the optical loss and the fault point of the optical fiber to be measured, a polarization control means for varying the polarization state of at least one of the local light and the reflected light is provided. Optical pulse tester. 2. A signal coherently detected by reflected light from an optical fiber to be measured accompanying supply of coherent probe pulse light and coherent local light is passed through a filter for extracting a predetermined frequency component, and then the signal is passed. In the optical pulse tester for processing and measuring the optical loss or the fault point of the optical fiber to be measured, storage means in which data indicating a plurality of polarization states different for each predetermined distance are stored in advance, and data of the storage means And a polarization control means for automatically changing the polarization state of at least one of the local light and the reflected light based on the above.