JP3078632B2 - Optical pulse tester - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention belongs to the field of optical communication systems. 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 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. For that purpose, it is necessary to measure and test the characteristics of the optical fiber line over a short distance with a high-performance tester. Optical pulse tester (hereinafter referred to as O
TDR (Optical Time Domain Reflectometer) transmits an optical pulse to the optical fiber under test, receives the reflected light and backscattered light from the optical fiber, and analyzes this to analyze the characteristics such as optical loss to a CRT or the like. This is a very useful tester because it is a display device that can test 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 such as heterodyne or homodyne reception is being studied. A conventional OTDR using the coherent detection technique will be described with reference to FIG.
In FIG. 7, the test signal light and the local signal light are generated by the same light source. 1 is a light source unit that generates light having a narrow line width spectrum, 2 is a first multiplexer / demultiplexer that branches the light emitted from the light source unit 1 into a test signal light a and a local signal light b, and 3 is a branched test signal. A first acousto-optic switch (hereinafter, referred to as an AO switch) 4 for pulsating light at a constant period and modulating the optical frequency, and 4 enters the pulsed test signal light into the optical fiber 5 to be tested and from the optical fiber 5. Is a second branching device that guides the reflected light and the backscattered light c to the tester. Note that the branching device 4 may be a second AO switch synchronized with the AO switch 3. Reference numeral 6 denotes a third multiplexer / demultiplexer that multiplexes the reflected light and backscattered light c guided into the tester and the local signal light b, and 7 denotes a multiplexed reflected light and backscattered light. A photodetector for optically / electrically converting the beat signal light d with the local signal light, and 8 for converting the beat signal to a baseband signal (however, in the case of a homodyne detection method in which the beat signal from the photodetector 7 is directly obtained as a baseband signal) Does not perform this conversion but proceeds to the next A / D conversion process), and then performs a A / D conversion and a square conversion. A signal converter 9 adds the square-converted signal of a fixed period to improve the SN ratio. An addition processor 10 is a logarithmic converter that performs logarithmic conversion of the added signal, and 11 is a longitudinal distribution (hereinafter referred to as O) of respective intensities of the reflected light and the backscattered light c.
A CRT 12 displaying a TDR waveform) is a timing generator for pulsing the test signal light a at a constant period or performing an addition process. 8 to 11 for electrical processing system 13
Is configured.

An OTDR using such a coherent detection technique (hereinafter referred to as a coherent detection OTDR)
In order to perform coherent detection, a light source unit that generates light having a narrow line width spectrum of several KHz is indispensable. In order to realize this, an output of a distributed feedback semiconductor laser (hereinafter referred to as a DFB-LD) or the like is required. An optical fiber having a length of about 1 km is fusion-spliced to an end, and a line width is reduced using backscattered light from the optical fiber.
FIG. 7 shows this example. The light source section 1 is composed of a semiconductor laser 14 or the like, an optical fiber 15 for narrowing the line width, and an optical isolator 19.

In the coherent detection OTDR, a beat signal light d obtained by combining a local signal light b having a relatively large intensity with a weak backscattered light c is received and detected. The intensity Ad of the beat signal light is represented by Ac
Assuming that the local signal light intensity is Ab, the local signal light intensity is proportional to the square root of the product of Ac and Ab. Therefore, the local signal light intensity A
By making b relatively large, even weaker backscattered light can be received, and the dynamic range as the OTDR can be expanded.

However, in the above-mentioned conventional optical pulse tester (OTDR), the dynamic range can be expanded by using the coherent detection technique. However, since the light source unit 1 having a narrow line width spectrum is used, the fading on the OTDR waveform is performed. There is a disadvantage of generating noise called noise. FIG. 8 shows a coherent detection OTD having the configuration of FIG.
An OTDR waveform example observed at R is shown. FIG. 8 is 10 km
This is an OTDR waveform when an optical pulse having a wavelength of 1.55 μm, a pulse width of 1 μs, a pulse period of 1 ms, and a peak intensity of −10 dBm is incident on a long optical fiber under test. OTDR
The number of additions for improving the SN ratio of the waveform is 2.6 × 10 ⁵ times. The addition processing time was about 260 seconds. OT
The DR waveform should be substantially straight, but there is a fluctuation of about ± 0.2 dB due to the fading noise. Due to this noise, it was difficult to determine a connection point in the middle of the optical fiber under test or a step due to a change in loss. For example, a step of about 0.1 dB cannot be discriminated as a step at all. Therefore, it is necessary to reduce fading noise of the coherent detection OTDR.

The fading noise is caused by interference between scattered lights scattered in a minute section corresponding to an optical pulse width in an optical fiber, and that the polarization dependence of backscattered light changes in the longitudinal direction of the optical fiber under test. caused by. The magnitude of the fluctuation on the OTDR waveform due to the fading noise is proportional to the reciprocal of the square root of the number of statistically independent backscattering waveforms. Therefore, the number of independent backscattering waveforms can be reduced by performing addition processing.

Therefore, conventionally, the optical frequency of the light source unit for generating the test signal light and the local signal light is changed, and the polarization state of the test signal light or the local signal light to be multiplexed is controlled so as to be independent. In order to reduce the fluctuation due to fading noise of the coherent detection OTDR, a method has been adopted in which the number of backscattering waveforms is increased. FIG. 9 is a block diagram of a conventional example of this fading noise reduction method. Reference numeral 14 denotes a semiconductor laser such as a DFB-LD capable of controlling the optical frequency, and reference numeral 16 denotes an optical frequency control unit, which constitutes a light source unit 22 capable of changing the optical frequency together with the optical fiber 15 for narrowing the line width and the optical isolator 19. I have. 17 is a polarization state controller, 18 is an optical frequency controller 16, and a polarization state controller 17
And a main control unit for controlling the timing generator 12. When the temperature of the semiconductor laser 14 is changed between 15 ° C. and 28 ° C., the emitted light intensity hardly changes, and the optical frequency of the light source unit 22 can be changed by about 140 GHz. The polarization state on the test signal light side can be controlled by inserting a polarization controller using elements such as a rotating phase plate, a rotating fiber crank, an electro-optic crystal, and a Faraday rotator. Here, a rotating phase plate type in which two phase plates of a (1/4) wavelength plate and a (1/2) wavelength plate are connected in series is used.

The light frequency of the light emitted from the light source unit 22 whose light frequency can be controlled can be changed by the light frequency control unit 16 while keeping its light output substantially constant. The spectral line width of the emitted light is reduced by the optical fiber 15 for narrowing the line width.
It has a sufficient line width to perform coherent detection. The emitted light is split by the first splitter 2 into a test signal light a and a local signal light b. Test signal light a branched
The polarization state of the optical fiber under test 5 is controlled by a polarization state controller 17, pulsed at a constant period by an AO switch 3 and optically frequency-modulated, and passed 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. In the third multiplexer / demultiplexer 6, the local signal light b and the reflected light and the backscattered light c are multiplexed. The multiplexed beat signal light d
Electric conversion. The beat signal is converted to a baseband signal by a signal converter 8, A / D-converted, and then square-converted, and then added by an adder 9 to improve the SN ratio.
After that, logarithmic conversion is performed by the logarithmic converter 10 and CRT is performed.
11 is displayed as an OTDR waveform. The timing generator 12 supplies a signal for pulsing the test signal light to the drive unit of the AO switch 3 and supplies an addition processing timing signal to the signal converter 8 and the addition processor 9.

The main controller 18 controls the optical frequency controller 16, the polarization state controller 17, and the timing generator 12,
Finally, the optical frequency is changed in accordance with the timing of the optical pulse transmitted to the optical fiber under test 5 to control the state of polarization. Therefore, the number of independent backscattering waveforms returning from the optical fiber under test increases, and
Fluctuation due to fading noise on the TDR waveform is reduced. FIG. 10 shows an example of an OTDR waveform measured by the coherent detection OTDR having the configuration shown in FIG. FIG.
Is a wavelength of 1.55 μm on an optical fiber under test having a length of 10 km.
m, pulse width 1 μs, pulse period 1 ms, peak intensity-
It is an OTDR waveform when an optical pulse of 10 dBm is incident. The number of additions for improving the SN ratio of the OTDR waveform is
Performed 2.6 × 10 ⁵ times. The addition time was about 260 seconds. The optical frequency was changed back and forth from the center optical frequency by ± 70 GHz. One round trip time is about 130 seconds. The polarization state was changed in four ways every 0.65 × 10 ⁵ additions.

[0011]

As a result, the fluctuation on the OTDR waveform due to fading noise is reduced to ± 0.1 dB as compared with the conventional example of FIG.
It is difficult to determine the step of dB. Therefore, it is necessary to further reduce the fluctuation due to the fading noise. The reason that this fluctuation cannot be sufficiently reduced is that, when a semiconductor laser such as a DFB-LD that changes the optical frequency by temperature control or the like is used for the light source unit 22 in the optical frequency variable unit, the optical frequency becomes linear with time. Does not change, but changes discretely at a certain time interval (hereinafter referred to as a hopping cycle). Since this hopping cycle is long, the optical frequency cannot be changed every predetermined cycle, and the number of independent backscattering waveforms This is because a sufficiently large amount cannot be added.

FIG. 5 schematically shows a time change of the optical frequency when the temperature of the semiconductor laser is changed. The hopping period was about 100 ms when varied between 15 ° C and 28 ° C. Therefore, the pulse period is 1
In the configuration of the coherent OTDR shown in FIG. 9 having the ms, the optical frequency changes only about once every 100 times, and a sufficient independent backscattering waveform cannot be obtained. In order to effectively reduce the fluctuation on the OTDR waveform due to the fading noise, it is necessary to shorten the hopping cycle to about the pulse cycle. However, in a semiconductor laser such as a DFB-LD that changes the optical frequency by the above-described temperature control or the like, there is a problem that the hopping cycle can be reduced to only about 100 ms. In addition, while using a semiconductor laser such as a DFB-LD that changes the optical frequency by temperature control or the like, the O.D.
In order to reduce the fluctuation on the TDR waveform, the number of independent backscattered waveforms must be increased by increasing the pulse period. However, this method has a problem that the addition processing time, that is, the measurement time is long, and is not suitable for practical use as an optical line test technique.

In view of the above problems, an object of the present invention is to provide an OTDR using a coherent detection method.
An object of the present invention is to provide an optical pulse tester in which fluctuation due to fading noise on a waveform is small and measurement time is short.

[0014]

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, wherein the test signal light is pulsed at a constant period. An optical pulse generating means for repeatedly transmitting the optical signal to the optical fiber under test every predetermined period, and optically multiplexing the reflected light and the backscattered light repeatedly returned from the optical fiber under test and the local signal light. Optical multiplexing means for obtaining a beat signal light, an opto-electrical conversion means for receiving the beat signal light and converting it to an electric signal, addition processing means for adding the electric signal, and In an optical pulse tester having display means for displaying the waveforms of the reflected light and the backscattered light, the signal light generating means for generating the test signal light and the local signal light is connected to a light source and the light source. Has an optical fiber, to propose an optical pulse tester, characterized by comprising the change making means for adding the variation such as the fiber parameter or length or Rayleigh scattering coefficient of the refractive index or the like of the optical fiber.

According to a second aspect of the present invention, in the optical pulse tester of the first aspect, the variation applying means applies variation to the optical fiber at each of the predetermined periods, and changes the optical fiber parameter or length or Rayleigh scattering. We propose an optical pulse tester characterized by varying the coefficient. According to a third aspect of the present invention, in the optical pulse tester according to the first or second aspect, the fluctuation applying means arranges a vibration generator in contact with or close to a part or the whole of the optical fiber. An optical pulse tester is proposed.

According to claim 4, claim 1, 2, or 3
In the optical pulse tester of (1), the fluctuation applying means arranges a temperature control unit in contact with or close to a part or the entirety of the optical fiber. According to a fifth aspect of the present invention, in the optical pulse tester of the first, second, third or fourth aspect, an optical frequency varying means for changing an optical frequency of both or any one of the test signal light and the local signal light is provided. An optical pulse tester characterized by this is proposed.

In claim 6, claims 1, 2, 3,
In the optical pulse tester of (4) or (5), the signal light generating means has a semiconductor laser, and by changing the temperature of the semiconductor laser, the optical frequency of both or one of the test signal light and the local signal light is changed. We propose an optical pulse tester characterized by changing. According to a seventh aspect of the present invention, in the optical pulse tester of the first, second, third, fourth or fifth aspect, the signal light generating means includes a semiconductor laser having one or more electrodes, and a current flowing through the electrodes is used. An optical pulse tester characterized by changing the optical frequency of both or either of the test signal light and the local signal light by changing the optical signal is proposed.

[0018]

According to the first aspect of the present invention, the test signal light and the local signal light are generated by the signal generating means. The test signal light is pulsed by an optical pulse generating means,
The light is repeatedly transmitted to the optical fiber under test every predetermined period.
The optical frequency of at least one of the test signal light and the local signal light is changed by the fluctuation applying unit. Further, the reflected light and the backscattered light that are repeatedly returned from the optical fiber under test are combined with the local signal light to be beat signal light. Here, for example, when a variation is given to the optical fiber connected to the light source by the variation giving unit, the optical frequency changes, and the interference characteristics of the reflected light, the backscattered light, the local light, and the beat signal light change. For,
An independent backscatter waveform is obtained. By performing the addition processing, the fluctuation of the OTDR waveform due to the fading noise is reduced. Based on the result of the addition processing, the display unit displays the waveforms of the reflected light and the backscattered light.

According to the second aspect, the optical frequency of the test signal light and / or the local signal light can be changed at predetermined intervals by the fluctuation applying means. According to the third aspect, the optical frequency of both or one of the test signal light and the local signal light is caused by the vibration of a vibration generator arranged in contact with or close to a part or the whole of the optical fiber. By applying vibration to the optical fiber, it can be changed at predetermined intervals.

According to a fourth aspect of the present invention, an optical frequency of at least one of the test signal light and the local signal light is in contact with or close to a part or the whole of the optical fiber. By either changing the temperature of the optical fiber and applying one or both of the vibration to the optical fiber by the vibration of a vibration generator disposed in contact with or close to a part or the whole of the optical fiber, It can be changed every predetermined period.

According to a fifth aspect of the present invention, the optical frequency of the test signal light and / or the local signal light is changed at predetermined intervals by the optical frequency varying means and the variation giving means. Can be. According to claim 6, the optical frequency of the test signal light and / or the local signal light has a semiconductor laser in the signal light generating means, and changes the temperature of the semiconductor laser. The change can be made at predetermined intervals by the change giving means.

According to a seventh aspect of the present invention, the optical frequency of both or one of the test signal light and the local signal light is such that the signal light generating means has a semiconductor laser having one or more electrodes, By changing the current flowing through the electrode and the variation applying means, the current can be changed at predetermined intervals.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides an OTDR waveform based on fading noise by changing the optical frequency of a light source at predetermined intervals by using only a variation applying means or by using an optical frequency varying means and a variation applying means. To reduce the fluctuation of Hereinafter, an embodiment in which the optical frequency variable unit and the variation applying unit are used will be described.

As a light source having an optical frequency variable means,
The following two types were tested. The first method is DFB-L
This is a method of changing the temperature of D. The DFB-LD mounted on the Peltier element keeps the DFB-LD drive current constant and when the temperature is changed from 15 ° C. to 28 ° C., the optical output is almost constant and the optical frequency is about 140 GHz.
Could be changed. As a second method, in the case of a DFB-LD having multiple electrodes, the optical frequency was able to be changed while the optical output was kept substantially constant by controlling the current flowing through the plurality of electrodes.

Further, an optical fiber for narrowing the line width was connected to the DFB-LD or the DFB-LD having multiple electrodes, and the optical frequency was changed by changing the optical fiber. The variation applying means includes a method of applying a vibration by a vibration generator arranged in contact with or in proximity to a part or the whole of the optical fiber, and a method of controlling a temperature in which the part or the entirety is arranged in contact with or in proximity to the optical fiber. There is a method of changing the temperature of the optical fiber by a part, and a method of changing the temperature by simultaneously using the method of applying the vibration and the method of changing the temperature.
By using such an optical frequency variable unit and a variation imparting unit, the optical frequency of the optical pulse incident on the optical fiber under test can be changed at predetermined intervals. FIG. 6 schematically shows a time change of the optical frequency when only the temperature of the DFB-LD is changed, and a time change of the optical frequency when the DFB-LD temperature change and the vibration to the optical fiber for narrowing the line width are applied. Is shown. The polarization state on the test signal light side is controlled using a polarization state controller. As a result, the beat signal light after being multiplexed with the backscattered light and the local signal light returned from the optical fiber under test has different interference characteristics and polarization characteristics at predetermined intervals. Therefore, the number of independent backscattered waveforms increases, and fluctuation of the OTDR waveform due to fading noise can be reduced by the addition processing.

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 FIG. 1, the same components as those of the above-described conventional example 1 are denoted by the same reference numerals, and description thereof will be omitted. Reference numeral 14 denotes a semiconductor laser such as a DFB-LD capable of controlling an optical frequency;
Reference numeral 6 denotes an optical frequency control unit, and reference numeral 20 denotes a vibration generator that applies vibration to the optical fiber 15 for narrowing the line width, and a light source whose optical frequency is variable together with the optical fiber 15 for narrowing the line width and the optical isolator 19. The part 23 is constituted. 17 is a polarization state controller,
Reference numeral 18 denotes a main control unit that controls the optical frequency control unit 16, the polarization state controller 17, the vibration generator 20, and the timing generator 12. The polarization state controller 17 is of a rotary phase plate type in which two phase plates of a (1/4) wavelength plate and a (1/2) wavelength plate are connected in series.

The configuration and operation of the coherent detection OTDR of the present invention will be described. Optical frequency controllable DFB-
The optical frequency of an optical pulse incident on the optical fiber under test is changed at a predetermined pulse period by using a vibration generator 20 as a variation applying means on an optical fiber 15 for narrowing a line width connected to a semiconductor laser 14 such as an LD. The light source unit 23 that can be used is configured. The spectral line width of the light source unit 23 has a sufficient line width for performing coherent detection by the optical fiber 15 for narrowing the line width. The output and line width were almost constant.

The light emitted from the light source 23 is split by the first splitter 2 into a test signal light a and a local signal light b. The split test signal light a is supplied to the polarization state controller 17.
The polarization state is controlled by the AO switch 3, the pulse is pulsed at a constant period by the AO switch 3, the optical frequency is modulated, and the light is incident on the optical fiber 5 to be tested via the second coupler 4. 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. In the third multiplexer / demultiplexer 6, the local signal light b and
The reflected light and the backscattered light c are multiplexed. The combined beat signal light d is optically / electrically converted by the photodetector 7. The beat signal is converted to a baseband signal by a signal converter 8, A / D-converted, and then square-converted.
Is added to improve the SN ratio. Thereafter, the logarithmic converter 10 performs logarithmic conversion and displays the OTDR waveform on the CRT 11. The timing generator 12 supplies a signal for pulsing the test signal light to the drive unit of the AO switch 3 and supplies an addition processing timing signal to the signal converter 8 and the addition processor 9. The main controller 18 controls the optical frequency controller 16, the polarization state controller 17, the vibration generator 20 and the timing generator 12, and finally responds to the timing of the optical pulse transmitted to the optical fiber 5 under test. The optical frequency is changed to control the polarization state.

With such a configuration and operation, the OTDR of the present invention can change the optical frequency of the optical pulse incident on the optical fiber under test at predetermined intervals. Therefore, the number of independent backscattered waveforms returned from the optical fiber under test is larger than that in the conventional example 2, and the fluctuation due to the fading noise on the OTDR waveform is further reduced by the addition processing.

The result of experimentally confirming the effect of the present invention will be described. Coherent detection OTDR shown in FIG.
FIG. 2 shows an OTDR waveform obtained by measuring the optical fiber under test having a length of 10 km. The optical pulse transmitted to the optical fiber under test 5 has a wavelength of 1.55 μm and a pulse width of 1 μm.
s, the pulse period is 1 ms, the peak intensity is −10 dBm, the spectral line width of the light source 23 is 3 kHz or less, and the local signal light intensity input to the third coupler 6 is almost 0 dB.
Bm. 2.6 × 10 ⁵ additions for improving the SN ratio of the backscattered waveform were performed. The addition time is about 260 seconds. The polarization state was changed in four ways every 0.65 × 10 ⁵ additions. The above measurement conditions are the same as the measurement conditions in FIG. The temperature of the semiconductor laser 14 is increased from 15 ° C. to 2
The optical frequency of the light source section 23 was reciprocated ± 70 GHz from the central optical frequency by changing the temperature between 8 ° C. One round trip time is about 130 seconds. Further, this optical frequency is changed at predetermined intervals by the fluctuation applying means. At this time, the vibration generator 20 is composed of, for example, a signal generation source and a piezo element, and generates vibration when a predetermined electric signal is applied to the piezo element. In the first embodiment, vibration having an amplitude of several μm and a frequency of several hundred Hz to several kHz was generated.

As a result, independent backscattered waveforms were obtained at predetermined intervals, and by adding them, fluctuations on the OTDR waveform due to fading noise could be reduced in a short time. The fading noise is much smaller than that of the conventional example shown in FIGS. 8 and 10 and is 0.06 dB or less.
It can be seen that the waveform is substantially linear, which confirms the effectiveness of the present invention.

Next, a second embodiment of the present invention is shown in FIG. The light source unit 24 capable of changing the optical frequency of the optical pulse incident on the optical fiber under test at a predetermined cycle is replaced with a semiconductor laser 14 such as a DFB-LD capable of controlling the optical frequency and the temperature control unit 21 for narrowing the line width. An experiment system shown in FIG. 1 was constructed by using the fluctuation imparting means provided on the optical fiber 15, and an experiment was performed under the same conditions as described above. A Peltier device was used as the temperature control unit 21. As a result, a result similar to that of FIG. 2 was obtained.
The effectiveness of the present invention was confirmed also in the configuration of FIG.

Next, a third embodiment of the present invention is shown in FIG. A light source unit 25 capable of changing the optical frequency of an optical pulse incident on the optical fiber under test at a predetermined period; a semiconductor laser 14 such as a DFB-LD capable of controlling the optical frequency;
The vibration generator 20 and the temperature control unit 21 were constituted by fluctuation applying means provided in the optical fiber 15 for narrowing the line width, and the experiment system shown in FIG. 1 was constructed, and an experiment was performed under the same conditions as described above. A Peltier device was used as the temperature control unit 21. As a result, the second
The results similar to those in the figure were obtained, and the effectiveness of the present invention was confirmed also in the configuration of FIG.

In the first, second, and third embodiments of the present invention, the same effect can be obtained by using a two-electrode DFB-LD as the semiconductor laser 14 such as a DFB-LD capable of controlling the optical frequency. Obtained. Further, in the above, the embodiment in which the optical frequency varying means and the variation giving means are used has been described, but almost the same effect can be obtained when only the variation giving means is used.

[0035]

As described above, the light frequency variable light source and the fluctuation imparting means are used to shorten the optical frequency popping period and reduce the optical frequency of the optical pulse incident on the optical fiber under test. It has become possible to change it at predetermined intervals. The beat signal light after being multiplexed with the backscattered light and the local signal light returned from the optical fiber under test,
The interference characteristic differs every predetermined period, and as a result, an independent backscattering waveform is obtained every predetermined period. By adding these, fluctuations on the OTDR waveform due to fading noise could be reduced in a short time.

Therefore, by using the optical pulse tester of the present invention, the OTDR caused by fading noise, which is the biggest problem of the OTDR using the coherent detection technique, is described.
Fluctuations on the waveform were reduced in a short time, and it was possible to determine a connection point in the middle of the optical fiber under test and a step due to a change in loss. That is, the coherent detection OTDR can be put to practical use as a tester for constructing a highly reliable optical line.

[Brief description of the drawings]

FIG. 1 shows a first embodiment of the present invention.

FIG. 2 is an example of a backscattered waveform by the OTDR of the present invention.

FIG. 3 shows a second embodiment of the present invention.

FIG. 4 shows a third embodiment of the present invention.

FIG. 5 is a schematic diagram of a time change of an optical frequency when an optical frequency variable unit is used.

FIG. 6 is a schematic diagram of a time change of an optical frequency when using an optical frequency variable unit and a variation giving unit.

FIG. 7 is a configuration example of a conventional coherent detection OTDR.

8 is an example of a backscattered waveform by the coherent detection OTDR of FIG. 7;

FIG. 9 is a configuration example in which fading noise is reduced in a conventional coherent detection OTDR.

10 is an example of a backscattered waveform by the coherent detection OTDR of FIG. 9;

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 light source unit 2 first multiplexer / demultiplexer 3 AO switch 4 second multiplexer / demultiplexer 5 optical fiber under test 6 third multiplexer / demultiplexer 7 light receiver 8 signal converter 9 addition processor 10 logarithmic converter 11 CRT 12 Timing generator 13 Electric processing system 14 Semiconductor laser 15 Optical fiber for narrowing line width 16 Optical frequency control unit 17 Polarization state controller 18 Main control unit 19 Optical isolator 20 Vibration generator 21 Temperature control unit 22, 23, 24, 25 Optical frequency variable light source

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Izumi Mikawa Nippon Telegraph and Telephone Corporation, 1-6, Uchisaiwai-cho, Chiyoda-ku, Tokyo (56) References JP-A-61-201133 (JP, A) JP-A Sho 63-313030 (JP, A) JP-A-2-79034 (JP, A) JP-A-64-75932 (JP, A) JP-A-2-165116 (JP, A) JP-A 64-75910 (JP, A) A) JP-A-63-250514 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01M 11/00-11/02 JICST file (JOIS) Practical file (PATOLIS) Patent file ( PATOLIS)

Claims (7)

(57) [Claims] A signal light generating means for generating a test signal light and a local signal light; an optical pulse generating means for pulsing the test signal light at a predetermined cycle and repeatedly sending the test signal light to an optical fiber under test at a predetermined cycle; Optical multiplexing means for optically multiplexing the reflected light and the backscattered light returning from the optical fiber under test and the local signal light to obtain a beat signal light; and receiving the beat signal light and converting it into an electric signal. An optical pulse tester comprising: a photoelectric conversion means for converting; an addition processing means for adding the electric signal; and a display means for displaying the waveforms of the reflected light and the backscattered light based on the result of the addition processing. In the above, the signal light generating means for generating the test signal light and the local signal light has a light source and an optical fiber connected to the light source, and optical fiber parameters such as a refractive index of the optical fiber. Properly the OTDR characterized by comprising the change making means for applying variations such as a length or Rayleigh scattering coefficient. 2. The apparatus according to claim 1, wherein the variation applying means applies a variation to the optical fiber at each of the predetermined periods, and varies the optical fiber parameter, length, Rayleigh scattering coefficient, or the like. Light pulse tester. 3. The pulse tester according to claim 1, wherein said fluctuation applying means arranges a vibration generator in contact with or close to a part or the whole of said optical fiber. 4. The optical pulse tester according to claim 1, wherein the variation applying means arranges a temperature controller in contact with or close to a part or the whole of the optical fiber. 5. An apparatus according to claim 1, further comprising an optical frequency varying means for changing an optical frequency of at least one of the test signal light and the local signal light.
The optical pulse tester according to 3 or 4. 6. The signal light generating means has a semiconductor laser, and by changing a temperature of the semiconductor laser,
6. The optical pulse tester according to claim 1, wherein the optical frequency of at least one of the test signal light and the local signal light is changed. 7. The signal light generating means includes a semiconductor laser having one or more electrodes, and by changing a current flowing through the electrodes, an optical frequency of at least one of the test signal light and the local communication light is changed. The optical pulse tester according to claim 1, 2, 3, 4, or 5, wherein