JP3088031B2 - Optical pulse tester - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pulse tester,
In particular, the present invention relates to an optical pulse tester for testing characteristics such as optical loss of an optical fiber and an optical fiber line as a transmission medium of an optical signal.

[0002]

2. Description of the Related Art Conventionally, in order to realize a highly reliable and economical optical communication system, it is important to construct a highly reliable and economical optical fiber line. Must be measured and tested over a short distance with a highly reliable tester. Optical pulse tester (hereinafter referred to as
An OTDR (Optical Time Domain Reflectometer) transmits an optical pulse to an optical fiber under test, receives reflected light and backscattered light from the optical fiber, analyzes the light and analyzes characteristics such as optical loss to a CRT or the like. This is a very useful tool because it is a display device and can be tested by inputting and outputting light from one end of an optical fiber. For this reason, research and development for increasing the OTDR measurable distance (this is called a dynamic range) has been conventionally performed.

In order to expand the dynamic range, there are mainly employed a method of increasing the intensity of a pulse transmitted to the optical fiber under test and a method of improving the receiving sensitivity of backscattered light and the like. As one method for improving reception sensitivity, application of a coherent detection technique called heterodyne or homodyne reception is being studied.

Conventional OT using coherent detection technology
DR will be described with reference to FIG. In FIG. 2, the test signal light and the local signal light are generated by the same light source. In the drawing, reference numeral 1 denotes a light source unit for generating light having a narrow line width spectrum, and 2 denotes a first multiplexer / demultiplexer for splitting the light emitted from the light source unit 1 into a test signal light a and a local signal light b. Pulsates the branched test signal light a with a constant period,
A first acousto-optic switch (hereinafter referred to as AO) for optical frequency modulation
4) A second multiplexer / demultiplexer or 4 for inputting the pulsed test signal light a to the optical fiber under test 5 and guiding the reflected light and the backscattered light c from the optical fiber 5 to be tested to the tester. This is the second AO switch.

Reference numeral 6 denotes a third signal for multiplexing the reflected light and the backscattered light c guided into the tester with the local signal light b.
7 is a photodetector for performing optical-to-electric conversion of the combined reflected light and backscattered light c and the bit signal light d of the local signal light b, and 8 is output from the photodetector 7. A /
A signal converter for D-conversion and further square conversion, 9 is an addition processor for adding the square-converted signal of a fixed period for improving the SN ratio, 10 is a logarithmic converter for logarithmically converting the added signal, and 11 is reflected light. A CRT 12 for displaying the longitudinal distribution of the intensities of the backscattered light c and the backscattered light c is a timing generator for pulsing the test signal light a at a constant period or performing an addition process. 8 to 11, the electric signal processing system 13
Is configured.

An O using such a coherent detection technique
The TDR receives and detects a beat signal light d obtained by combining a local signal light b having a relatively large intensity with a weak reflected light and a backscattered light c. The intensity Ad of the beat signal light d is proportional to the square root (Ac · Ab) ^1/2 of the product of the reflected light and the backscattered light c, where Ac is the intensity and the intensity of the local signal light b is Ab. Therefore, by making the local signal light intensity Ab relatively large, it is possible to receive even weaker reflected light and backscattered light c,
The dynamic range as the OTDR can be expanded.

Further, in order to perform coherent detection in the coherent detection OTDR, the light source unit 1 for generating light having a narrow line width spectrum of several KHz is indispensable.
To realize this, a distributed feedback semiconductor laser (hereinafter, referred to as
A single-mode optical fiber 15 having a length of about 1 km is fusion-spliced to an output end of a DFB-LD 14 or the like, and a line width is reduced by using backscattered light from the optical fiber 15. Have been.

[0008]

However, in the above-mentioned conventional optical pulse tester (OTDR), the dynamic range can be expanded by using the coherent detection technique. There is a disadvantage that noise called fading noise is generated on the backscattered waveform.

The reason for this is that the polarization dependence of the reflected light and the backscattered light c differs in the longitudinal direction of the optical fiber under test 5 and the light pulse width in the fiber due to the good coherency of the light source 1. Scattered light from a considerable minute section interferes.

FIG. 3 shows an example of a backscattered waveform observed by the conventional coherent detection OTDR. FIG. 3 shows that an optical fiber 5 under test having a length of 10 km has a wavelength of 1.55 μm and a pulse width of 1.
This is a backscattered waveform when an optical pulse having μs, a pulse period of 1 ms, and a peak intensity of −10 dBm is incident.

The number of additions for improving the SN ratio of the backscattered waveform is 3.9 × 10 ⁵ times. If there is no fading noise, the waveform should be substantially straight, but a fluctuation of about ± 0.2 dB is observed due to the fading noise. Such noise is caused by a connection point or a point where loss changes in the middle of the optical fiber under test 5.
Originally, when a step should be seen on the backscattered waveform, the step may not be discriminated. The original step is 0.1
If it is on the order of dB, the result is that no level difference can be determined. Therefore, fading noise on the coherent detection OTDR waveform is not preferable when used as a tool when constructing a highly reliable optical line.

An object of the present invention is to provide an optical pulse tester (OTDR) using a coherent detection method.
It is an object of the present invention to provide an optical pulse tester in which fading noise on a backscattered waveform is small.

[0013]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a signal light generating means for generating test signal light and local signal light, and pulsating the test signal light so that the test signal light is pulsed at predetermined intervals. An optical pulse generation means for repeatedly sending out to the test optical fiber, and receiving reflected light and backscattered light repeatedly returned from the optical fiber under test, and multiplexing with the local signal light to form a beat signal light. An optical multiplexing means for generating the beam;
Photoelectric conversion means for converting the signal light into an electric signal, addition processing means for adding the electric signal, and display means for displaying the waveforms of the reflected light and the backscattered light based on the result of the addition processing. An optical pulse tester comprising: an optical frequency varying means for changing the optical frequency of the test signal light and the local signal light at predetermined intervals of the optical pulse; and the test signal light or the local signal. The polarization state of light
An optical pulse tester provided with a polarization state control means for changing the optical pulse at every predetermined cycle is proposed.

[0014]

According to the present invention, the test signal light and the local signal light are generated by the signal light generating means, and the test signal light is pulsed by the optical pulse generating means and is repeatedly transmitted to the optical fiber under test every predetermined period. Sent out. The optical frequencies of the test signal light and the local signal light are changed by one cycle of the optical pulse or by a plurality of cycles by the optical frequency variable means. Further, the polarization state of the test signal light or the local signal light is changed by one period or plural periods of the optical pulse by the polarization state control means. The reflected light and the backscattered light that are repeatedly returned from the optical fiber under test are received by the optical multiplexing means and multiplexed with the local signal light to form a beat signal light. Here, for example, when the optical frequencies of the test signal light and the local signal light are slightly changed by the optical frequency varying means, the interference characteristics of the reflected light, the backscattered light, and the beat signal light are changed. It will be different for each predetermined period of the light pulse. Further, when the polarization state of the test signal light or the local signal light is changed little by little by the polarization state control means, the polarization characteristics of the reflected light, the backscattered light and the beat signal light are changed. It will be different for each predetermined period of the light pulse. These beat signal lights are converted into electric signals by photoelectric conversion means. Further, the beat signals having slightly different interference characteristics and polarization characteristics are subjected to addition processing by addition processing means to average out noise due to interference on backscattered light and noise components due to polarization dependence, and Based on the result of the addition processing, the display means displays the waveforms of the reflected light and the backscattered light.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to at least one of an optical pulse that repeatedly transmits the optical frequency of a light source that emits test signal light and local signal light, and the polarization state of the test signal light or local signal light. It is intended to reduce fading noise on the backscattered waveform by making a slight change for each pulse. However, the light output from the light source must be kept almost constant.

As a method of changing the optical frequency while keeping the light output from the light source almost constant, a method of changing the temperature of the DFB-LD is known. For example, using a 1.5 μm band DFB-LD mounted on a Peltier device,
As a result of keeping the D drive current constant and changing the LD temperature between 15 ° C. and 28 ° C., the optical output could be changed by ± 70 GHz with little variation of ± 0.5 dB or less.

A DFB-LD having multiple electrodes is
By controlling the current flowing through the plurality of electrodes, it is possible to change the optical frequency while keeping the optical output almost constant. For example, using a two-electrode DFB-LD in the 1.5 μm band and changing the current flowing through each electrode,
The optical output was ± 0.5 dB or less with little change, and the optical frequency could be changed by ± 30 GHz.

Using such a light source, the wavelength of an optical pulse incident on the optical fiber under test, that is, the optical frequency is slightly changed at least for each pulse, and the polarization of the test signal light or the local signal light is changed. The state is changed slightly at least every one pulse. As a result, the backscattered light returning from the optical fiber under test and the beat signal light obtained by multiplexing the backscattered light and the local signal light have an interference characteristic and a polarization slightly at least every one pulse. Different characteristics.
If this slightly different beat signal light is detected and added by the electric signal processing system, noise due to interference on the backscattered light from the optical fiber under test and noise components due to polarization dependence are averaged. Fading noise on the scattering waveform can be reduced.

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram showing a first embodiment of the present invention. In the figure, the same components as those of the above-described conventional example are denoted by the same reference numerals, and description thereof will be omitted. Reference numeral 21 denotes a semiconductor laser such as a DFB-LD capable of controlling the temperature, and 22 denotes a temperature control unit which is constituted by a Peltier element or the like and controls the temperature of the semiconductor laser 21. A light source unit 23 whose optical frequency is variable together with the optical fiber 15 is formed. Reference numeral 24 denotes a polarization state controller, for example, (1 /
4) A rotary phase plate system in which two phase plates, a wave plate and a (1/2) wave plate, are connected in series, or a Faraday in which two fiber type Faraday rotators are connected in series via a fiber coil. -A rotor type or the like is used. 2
Reference numeral 5 denotes a main control unit including a CPU and the like, which controls operations of the temperature control unit 22, the polarization state controller 24, and the timing generator 12. The light source unit 23 is used instead of the light source unit 1 in the conventional example. The polarization state controller 24 is interposed between the AO switch 3 and the multiplexer / demultiplexer 4.

Next, the operation of the coherent detection OTDR according to the first embodiment of the present invention having the above-described configuration will be described. As described above, the temperature of the semiconductor laser 21 whose temperature can be controlled is controlled by the temperature controller 22. As a result, the optical frequency is changed while the optical output is kept substantially constant. Further, the spectral line width of the laser light emitted from the semiconductor laser 21 is narrowed by the narrowing optical fiber 15 to a line width sufficient for performing coherent detection.

As described above, the light emitted from the light source unit 23 whose optical frequency is variable is split by the first splitter 2 into a test signal light a and a local signal light b. Test signal light a branched
Is pulsed at a fixed cycle by the AO switch 3 and is optically frequency-modulated, and its polarization state is controlled by a polarization state controller 24. The optical fiber 5 under test is transmitted through a second multiplexer / demultiplexer 4. Is incident on. The reflected light and the backscattered light c from the optical fiber under test 5 are guided to the third coupler 6 via the second coupler 4. The local signal light b is also guided to the third coupler 6 and the reflected light and the backscattered light c
And the local signal light b are multiplexed to generate a beat signal light d.

The beat signal light d generated by the third coupler 6 is subjected to optical / electrical conversion by a light receiver 7 such as a double-balanced PIN-FET. The beat signal optically / electrically converted by the light receiver 7 is A / D-converted and square-converted by the signal converter 8 and added by the addition processor 9 for SN improvement. Thereafter, the logarithmic converter 10 performs logarithmic conversion and displays the signal on the CRT 11 as a temporal change, that is, a distribution in the longitudinal direction. Timing generator 12
Supplies a signal for pulsing the test signal light a to the drive unit of the AO switch 3 and supplies an addition processing timing signal to the addition processor 9.

The main control unit 25 controls the operation of the temperature control unit 22, the polarization state controller 24, and the timing generator 12, and as a result, the test is performed in accordance with the timing of the optical pulse transmitted to the optical fiber 5 under test. The optical frequency of the signal light a and the local signal light b are changed, and the polarization state of the test signal light a is controlled. The control of the optical frequency and the polarization state is performed by changing the optical frequency and the polarization state, for example, before transmitting the optical pulse, and after transmitting the optical pulse, maintaining the optical frequency and the polarization state constant until before transmitting the next optical pulse. Alternatively, it is performed so as to change linearly in response to time regardless of the transmission timing of the optical pulse.

Next, the coherent detection O of the first embodiment is described.
The effects of the present invention will be described based on experimental results using TDR. In this experiment, a polarization state controller 24 of a rotating phase plate type was used. The length of the optical fiber under test 5 is 10 km, and the light pulse transmitted to the optical fiber 5 under test has a wavelength of 1.55 μm, a pulse width of 1 μs, and a pulse period of 1 μm.
ms, and the peak intensity is −10 dBm, which is the same as the conventional example shown in FIG. Further, the spectral line width is reduced to 3 by the single mode optical fiber 15 for narrowing the line width of 1 km.
KHz or less, and the intensity of the local signal light b input to the third multiplexer / demultiplexer 6 is almost 0 dBm, which is set to be the same as in the conventional example. The number of additions for improving the SN of the backscattered waveform is 3.9 as in the conventional example.
× 10 ⁵ times. Therefore, the addition time is 390 seconds.

FIGS. 4A to 4C show backscattering waveforms obtained by experiments. In each figure, the horizontal axis represents the length of the optical fiber under test 5, and the vertical axis represents the backscattered light c.
Represents the intensity of

FIG. 4A is a diagram showing a backscattering waveform when the optical frequency is changed during the addition of 3.9 × 10 ⁵ times while maintaining the polarization state of the test signal light a constant. is there. here,
The change in the optical frequency was made to reciprocate between ± 70 GHz substantially linearly in response to time irrespective of the light pulse transmission timing, and one round trip time was set to 65 seconds. It can be seen that the fading noise is smaller than the experimental result of the conventional example shown in FIG.

FIG. 4 (b) shows that the optical frequencies of the test signal light a and the local signal light b are kept constant, and 0.65 × 10 ⁵ additions are performed during 3.9 × 10 ⁵ additions. FIG. 7 is a diagram illustrating a backscattering waveform when the polarization state of the test signal light a is changed for each number of times. This also shows that the fading noise is smaller than the experimental result of the conventional example shown in FIG.
FIG. 4C is a diagram showing a backscattering waveform when the optical frequencies of the test signal light a and the local signal light b are changed and the polarization state of the test signal light a is changed. Here, the change of the optical frequency is substantially linearly reciprocated between ± 70 GHz corresponding to the time irrespective of the optical pulse transmission timing, and one round trip time is 65 seconds, and 3.9 × 10 ⁵ additions are performed. During this time, the polarization state of the test signal light a was changed every 0.65 × 10 ⁵ times. The fading noise is much smaller than 0.05 dB as compared with the experimental result of the conventional example shown in FIG. 3, and it can be seen that the waveform is almost straight. FIG. 4 shows only the optical frequency changed.
The fading noise is smaller than that of the backscattered waveform of FIG. 4B in which only the polarization state is changed, and the effectiveness of the present invention can be confirmed.

As described above, the optical frequency of the light pulse incident on the optical fiber under test 5 is slightly changed at least every one pulse, and the polarization state of the test signal light a is slightly changed every at least one pulse. Then, the back scattered light c returning from the optical fiber under test 5 and the beat signal light d obtained by multiplexing the back scattered light c and the local signal light b.
Has a slightly different interference characteristic and polarization characteristic at least for each pulse. If this slightly different beat signal light d is detected and added by the electric signal processing system, noise due to interference on the backscattered light from the optical fiber 5 to be tested and noise due to polarization dependence are averaged. Fading noise on the backscattering waveform can be reduced.

Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 5 is a configuration diagram showing the second embodiment. In the figure, the same components as those in the first embodiment described above are denoted by the same reference numerals, and description thereof will be omitted. Reference numeral 31 denotes a semiconductor laser having a plurality of electrodes, for example, a DFB-LD having two electrodes; 32, a drive current control unit corresponding to the plurality of electrodes of the semiconductor laser 31; 33, a main control unit; 32, a main control unit that controls the polarization state controller 24 and the timing generator 12. Semiconductor laser 3
1. Each of the drive current control unit 32 and the main control unit 33 includes the semiconductor laser 21 of the first embodiment described above,
It is used in place of the temperature control unit 22 and the main control unit 25. In the second embodiment, the output optical pulse from the AO switch 3 is directly incident on the second branching device 4, and
The polarization state controller 24 is interposed between the first multiplexer / demultiplexer 2 and the third multiplexer / demultiplexer 6 to change the polarization state of the local signal light b.

The semiconductor laser 31 is controlled by a drive current control unit 32 capable of driving a number of currents corresponding to a plurality of electrodes. The optical frequency can be varied while keeping small.

Next, the operation of the second embodiment having the above configuration will be described. The output light of the semiconductor laser 31 is reduced in spectral line width by the single-mode optical fiber 15 for narrowing the line width, and is split by the first splitter 2 into test signal light a and local signal light b. . After the test signal light a is pulsed by the AO switch 3, the second
Through the optical fiber 5 under test. The reflected light and the backscattered light c from the optical fiber under test 5 are guided to the third coupler 6 via the second coupler 4. Further, the polarization state of the local signal light b is controlled by the polarization state controller 24, and then guided to the third coupler 6.
In the third multiplexer / demultiplexer 6, the reflected light and the backscattered light c and the local signal light b are multiplexed to generate a beat signal light d. Thereafter, the beat signal light d generated by the third coupler 6 is optically / electrically converted by the light receiver 7 and further subjected to electric processing by the electric signal processing system 13 to be displayed. This is the same as in the first embodiment.

Further, the coherent detection O of the second embodiment
In the experiment using the TDR, a backscattering waveform similar to the result of the first experiment shown in FIG. 4 was obtained. That is, in this experiment, similarly to the experiment of the first embodiment described above, the polarization state controller 24 of the rotating phase plate type is used, the length of the optical fiber 5 under test is 10 km, and the length of the optical fiber 5 under test is
The light pulse to be transmitted to the device has a wavelength of 1.55 μm and a pulse width of 1.
μs, a pulse period of 1 ms, and a peak intensity of −10 dBm. The conditions for changing the optical frequency and the polarization state were the same as in the experiment of the first embodiment.

As a result, when only the optical frequencies of the test signal light a and the local signal light b are changed, a backscattering waveform similar to that shown in FIG. 4A is obtained, and the polarization of the local signal light b is obtained. When only the wave state was changed, a backscattering waveform similar to that of FIG. 4B was obtained. Further, the test signal light a and the local signal light b
4C, and the backscattering waveform when the polarization state of the local signal light b was changed was the same as that in FIG.

As described above, the optical frequency of the optical pulse incident on the optical fiber under test 5 is slightly changed at least every one pulse, and the polarization state of the local signal light b is slightly changed every at least one pulse. The back scattered light c returning from the optical fiber under test 5 and the beat signal light d obtained by combining the back scattered light c and the local signal light b are changed at least every one pulse. The interference characteristics and the polarization characteristics are slightly different. This slightly different beat signal light d
Is detected and added by the electric signal processing system, noise due to interference on the backscattered light from the optical fiber under test 5 and noise components due to polarization dependence are averaged, so that fading noise on the backscattered waveform is obtained. Can be reduced.

As described above, the first and second embodiments of the present invention
In this embodiment, the light frequency of the test signal light a and the local signal light b is set to at least 1
The pulse signal is slightly changed for each pulse, and the polarization state of the test signal light a or the local signal light b is slightly changed for at least one pulse, so that the beat signal light d having slightly different interference characteristics and polarization characteristics. Is added to reduce the fading noise on the backscattered light c from the optical fiber 5 under test.

Therefore, by using the optical pulse tester of the present invention, fading noise on the backscattered waveform, which is the biggest problem of the OTDR using the coherent detection technique, is reduced. If there is a connection point or a point where the loss changes, it can be determined as a step on the backscattered waveform. That is, coherent detection OTDR is used as a tool for constructing a highly reliable optical line.
Can be put to practical use.

[0037]

As described above, according to the present invention, the optical frequencies of the test signal light and the local signal light are changed every predetermined period of the optical pulse, and the test signal light or the local signal light is further changed. Since the polarization state of the light is changed, the interference characteristics and the polarization characteristics of the backscattered light from the optical fiber under test and the beat signal light are different for each predetermined period of the light pulse, and The noise due to the interference of the scattered light and the noise component due to the polarization dependence are averaged, and the fading noise on the backscattered waveform is reduced. As a result, if there is a connection point or a point where the loss changes in the middle of the optical fiber under test, it can be identified as a step on the backscattered waveform, and the coherent detection light is used as a tool for constructing a highly reliable optical line. This is a very excellent effect that the pulse tester can be put to practical use.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a coherent detection type optical pulse tester according to a first embodiment of the present invention.

FIG. 2 is a block diagram showing a conventional coherent detection type optical pulse tester.

FIG. 3 is a diagram showing a backscattering waveform measured using a conventional example.

FIG. 4 is a diagram showing a backscattering waveform measured using the first embodiment of the present invention.

FIG. 5 is a configuration diagram showing a coherent detection type optical pulse tester according to a second embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Light source part, 2, 4, 6 ... Branching device, 3 ... AO switch, 5 ... Optical fiber under test, 7 ... Light receiver, 8 ... Signal converter, 9 ... Addition processor, 10 ... Logarithmic converter, 11 ... CR
T, 12: timing generator, 13: electric signal processing system,
14: DFB-LD, 15: Optical fiber for narrowing line width, 2
DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser, 22 ... Temperature control part, 23 ... Light source part
24: polarization state controller, 25: main control unit, 31: semiconductor laser, 32: drive current control unit, 33: main control unit.

Continuation of the front page (72) Inventor Izumi Mikawa 1-6-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-2-79034 (JP, A) JP-A-62-231129 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01M 11/00-11/02 H04B 9/00 Practical file (PATOLIS) Patent file (PATOLIS)

Claims (1)

(57) [Claims] A signal light generating means for generating a test signal light and a local signal light; an optical pulse generating means for pulsating the test signal light and repeatedly transmitting the test signal light to an optical fiber under test at predetermined intervals; An optical multiplexing means for receiving reflected light and backscattered light repeatedly returned from the optical fiber, and multiplexing with the local signal light to generate a beat signal light;
Photoelectric conversion means for converting the signal light into an electric signal, addition processing means for adding the electric signal, and display means for displaying the waveforms of the reflected light and the backscattered light based on the result of the addition processing. An optical pulse tester comprising: an optical frequency variable means for changing an optical frequency of the test signal light and the local signal light at predetermined intervals of the optical pulse; and the test signal light or the local signal. An optical pulse tester, comprising: a polarization state control unit that changes a polarization state of light at predetermined intervals of the optical pulse.