JPH0534239A - Light pulse tester - Google Patents

Abstract

PURPOSE:To reduce fading noise on backscattered waveforms and identify a connection portion and a loss change portion by slightly changing optical frequency of test signal light and local signal light by an LD humidity control part at least per period of a light pulse. CONSTITUTION:An LD humidity control part 22 is used to control temperature in the vicinity of a DFB-LD 21 to slightly change optical frequency while an outgoing light output from a light source part 23 is maintained approximately constant. The outgoing light is divided 24 into test signal light (a) and local signal light (b), and the pulsed 3 light (a) enters an optical fiber 5 to be tested. Reflected light and backscattered light (c) of the fiber 5 is combined 26 with the light (b) to be beat signal light (d), which is photoelectrically converted 27 and added 9 for improvement of SN and displayed 11. Since the frequency of the light (a),(b) slightly changes at least per period of light pulses, coherent characteristics of the light (c) slightly differs for each period. Therefore coherent components of the light (c) are averaged to reduce fading noise on backscattered waveforms.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to an optical pulse tester,
In particular, the present invention relates to an optical pulse tester that tests characteristics such as optical loss of an optical fiber and an optical fiber line which are transmission media of optical signals.

[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 long distance in a short time with a highly reliable tester. Optical pulse tester (hereinafter,
OTDR (Optical Time Domain Reflectometer) is a technique that sends an optical pulse to the optical fiber under test, receives the reflected light and backscattered light from the optical fiber, analyzes this, and outputs the characteristics such as optical loss to a CRT. It is a display device, and is a very useful tool because it can be tested by inputting and outputting light from one end of an optical fiber. Therefore, research and development for expanding the measurable distance of OTDR (this is referred to as a dynamic range) has been conventionally performed.

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

Conventional OT using coherent detection technology
The 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 figure, 1 is a light source section for generating light with a narrow line width spectrum, 2 is a first branching / branching device for branching the light emitted from the light source section 1 into a test signal light a and a local signal light b, 3 Pulsed the branched test signal light a at a constant cycle, and
A first acousto-optic switch for optical frequency modulation (hereinafter referred to as AO
(Referred to as a switch) 4 is a second multiplexer / demultiplexer which makes pulsed test signal light a incident on the optical fiber under test 5 and guides reflected light and backscattered light c from the optical fiber under test 5 to the tester. This is the second AO switch.

Further, 6 is a third for multiplexing the reflected light and the backscattered light c guided into the tester and the local signal light b.
, A light receiver 7 for converting the beat signal light d of the reflected light and the backscattered light c and the local signal light b, which are combined, into an electric signal, and 8 is output from the light receiver 7. A / A
A signal converter for D-conversion and further square-conversion, 9 is an addition processor for adding square-converted signals of a constant cycle for improving the SN ratio, 10 is a logarithmic converter for logarithmically converting the added signals, 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 cycle or for performing addition processing. Electrical signal processing system 13 by 8 to 11
Is configured.

O using such a coherent detection technique
The TDR is for receiving and detecting the beat signal light d which is a combination of the weak reflected light and the backscattered light c with the local signal light b having a relatively large intensity. The intensity Ad of the beat signal light d is proportional to the square root (Ac · Ab) ^1/2 of these products, where the intensity of the reflected light and the backscattered light c is Ac 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 reflection ports and backscattered light c.
The dynamic range of 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 achieve this, a distributed feedback semiconductor laser (hereinafter,
A single mode optical fiber 15 having a length of about 1 km is fusion-spliced to the output end of the DFB-LD 14) or the like, and the backscattered light from the optical fiber 15 is used to narrow the line width. Has been.

[0008]

However, in the above-mentioned conventional optical pulse tester (OTDR), although the dynamic range can be expanded by using the coherent detection technique, the light source unit 1 having a narrow line width spectrum is used. There is a drawback that noise called fading noise is generated on the backscattering waveform.

The cause of this is that the polarization dependence of the reflection port and the backscattered light c differs in the longitudinal direction of the optical fiber under test 5, and the good coherency of the light source section 1 causes the optical pulse width in the fiber. This is because scattered light from a considerable minute section interferes. FIG. 3 shows an example of the backscattering waveform observed by the conventional coherent detection OTDR. FIG. 3 shows a test optical fiber 5 having a length of 10 km, a wavelength of 1.55 μm and a pulse width of 1
This is a backscattering waveform when an optical pulse having a pulse duration of μs, a pulse period of 8 ms and a peak intensity of −10 dBm is incident.

The number of additions for improving the SN ratio of the backscattered waveform is 2.6 × 10 ⁵ . The waveform should be almost straight if there is no fading noise, but fluctuations of about ± 0.2 dB are observed due to fading noise. Such noise is caused by a connection part or a part where the loss changes in the middle of the optical fiber 5 under test.
Originally, when a step should be seen on the backscattering waveform, there is a possibility that the step cannot be discriminated. Original step is 0.1
If it is in the order of dB, the result is that the step difference cannot be determined at all. Therefore, fading noise on the coherent detection OTDR waveform is not preferable for use as a tool when constructing a highly reliable optical line.

In view of the above problems, an object of the present invention is to provide an optical pulse tester (OTDR) using a coherent detection method.
In order to provide an optical pulse tester with a small fading noise on the backscatter waveform.

[0012]

In order to achieve the above-mentioned object, the present invention provides a signal light generating means for the test signal light and the local signal light and a pulse for the test signal light. Optical pulse generating means for repeatedly sending to the optical fiber under test at predetermined intervals, and reflected light and backscattered light repeatedly returned from the optical fiber under test are received and combined with the local signal light to produce a visual signal. -Optical multiplexing means for generating a beat signal light, opto-electric conversion means for converting the beat signal light into an electric signal, and addition processing means for performing an addition process on the electric signal,
In an optical pulse tester including a display unit that displays the waveforms of the reflected light and the backscattered light based on the result of the addition processing, the optical frequencies of the test signal light and the local signal light are set to the optical pulse. Proposes an optical pulse tester provided with an optical frequency varying means for slightly changing it at least every one cycle.

According to a second aspect of the present invention, in the optical pulse tester according to the first aspect, the optical frequency varying means proposes an optical pulse tester that linearly changes the optical frequency in response to a change in time. .

According to a third aspect of the present invention, in the optical pulse tester according to the first aspect, the optical frequency varying means is at least from the optical pulse sending time of the signal light generating means,
An optical pulse tester is proposed which keeps the optical frequency constant until the backscattered light from the far end of the optical fiber under test due to the optical pulse is received by the optical combining means.

Further, in claim 4, claim 1, 2 or 3
In the optical pulse tester described above, the signal light generating means includes a semiconductor laser that generates the test signal light and the local signal light, and the optical frequency changing means changes the temperature of the semiconductor laser. Then, an optical pulse tester for changing the optical frequencies of the test signal light and the local signal light is proposed.

Further, in claim 5, in the optical pulse tester according to claim 1, 2 or 3, the signal light generating means has a semiconductor laser for generating the test signal light and the local signal light. Then, the optical frequency changing means proposes an optical pulse tester for changing the optical frequency of the test signal light and the local signal light by changing the current passing through the semiconductor laser.

[0017]

According to the first aspect of 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 to be tested at predetermined intervals. It is repeatedly sent to the optical fiber. The optical frequencies of the test signal light and the local signal light are slightly changed by the optical frequency changing means at least every one cycle of the optical pulse. The reflected light and the backscattered light repeatedly returned from the optical fiber under test are received by the light combining means and combined with the local signal light to be beat signal light. Here, for example, when the optical frequencies of the test signal light and the local signal light are changed little by little by the optical frequency changing means, the interference characteristics of the reflected light, the backscattered light, and the beat signal light are It will be different for at least one cycle of the light pulse. These beat signal lights are converted into electric signals by the photoelectric conversion means. Further, the beat signals having slightly different interference characteristics and the like are subjected to addition processing by addition processing means to average the interference components of the backscattered light, and based on the result of the addition processing, the reflected light is displayed by the display means. And the waveform of the backscattered light is displayed.

According to a second aspect of the present invention, the optical frequencies of the test signal light and the local signal light are linearly changed by the optical frequency changing means in response to the change of time.

According to a third aspect of the present invention, the optical frequencies of the test signal light and the local signal light are determined by the optical frequency varying means, at least from the optical pulse sending time by the signal light generating means, by the optical pulse. It is kept constant until the backscattered light from the far end of the optical fiber under test is received by the light combining means.

According to the present invention, the signal light generating means has a semiconductor laser, and the temperature of the semiconductor laser is changed by the optical frequency changing means. The optical frequencies of the test signal light and the local signal light generated from the semiconductor laser are changed corresponding to the change of the temperature.

Further, according to a fifth aspect of the present invention, the signal light generating means has a semiconductor laser, and the energizing current to the semiconductor laser is changed by the optical frequency changing means. The optical frequencies of the test signal light and the local signal light generated from the semiconductor laser are changed corresponding to the change of the energizing current.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention reduces fading noise on a backscattered waveform by varying the optical frequency of a light source uniformly or randomly and at least for every at least one pulse of repeatedly emitted optical pulses. Is what you are trying to do. However, the light output from the light source must be kept substantially constant.

As a method of changing the optical frequency while keeping the light output from the light source substantially constant, a method of changing the temperature of the DFB-LD is known. Figure 4 shows the DFB when the temperature of the DFB-LD mounted on the Peltier device is changed.
-Change of wavelength (optical frequency) λ with respect to temperature change of LD,
And the change P of the light output. In the figure,
The horizontal axis represents the temperature of the DFB-LD, and the vertical axis represents changes in wavelength and optical frequency and changes in optical output. Here, the central wavelength is 1551 nm, and the temperature of the DFB-LD is changed between 15 ° C and 28 ° C. Also, D
Since the drive current of the FB-LD is constant, the light output is almost constant even if the temperature is changed. On the other hand, the wavelength is about 1.5
nm has changed.

DFB-LDs having multiple electrodes are
By controlling the current flowing through the plurality of electrodes, it is possible to change the optical frequency while keeping the optical output substantially constant. FIG. 5 shows a change λ in the wavelength and the optical frequency and a change P in the optical output with respect to the sum of the drive currents of the two electrodes when the currents applied to the respective electrodes of the two-electrode DFB-LD are changed. In the figure, the horizontal axis represents the sum of drive currents of the two electrodes, and the vertical axis represents changes in wavelength and optical frequency and changes in optical output. Where the change in light output is ± 0.5
A region of not more than dB and a wavelength change width of about 0.6 nm can be sufficiently used in the present invention.

By using such a light source, the optical pulse of the test signal light and the wavelength of the local signal light, that is, the optical frequency, incident on the optical fiber under test are slightly changed at least every one pulse. As a result, the reflected light and the backscattered light returning from the optical fiber under test, and the beat signal light obtained by combining the reflected light and the backscattered light with the local signal light are generated at least every one pulse. The interference characteristics are slightly different. When these slightly different beat signal lights are detected and the addition processing is performed by the electrical processing system, the interference component of the backscattered light from the optical fiber under test is averaged, so that fading noise on the backscattered waveform can be reduced.

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a first embodiment of the present invention. In the figure, the same components as those in the conventional example described above are designated by the same reference numerals, and the description thereof will be omitted. Reference numeral 21 is a temperature-controllable DFB-LD, 22 is a Peltier element or the like, and is an LD temperature control unit for controlling the temperature of the DFB-LD 21. The light source 23 has a variable frequency.
24 to 26 are optical fiber couplers for coupling and branching (hereinafter, referred to as optical fiber couplers), 27 is a double balance type PIN-FET light receiver (hereinafter, referred to as light receiver),
A main control unit 28 controls the operations of the LD temperature control unit 22 and the timing generator 12, and is composed of a CPU or the like. The light source unit 23, the optical fiber couplers 24 to 26, and the light receiver 27 are used in place of the light source unit 1, the first to third couplers 2, 4, 6 and the light receiver 7 in the conventional example.

Next, the operation of the coherent detection OTDR of the first embodiment of the present invention having the above-mentioned configuration will be described. The temperature controllable DFB-LD 21 controls the ambient temperature as shown in FIG. 4 under the control of the LD temperature controller 22, and as a result, the optical frequency is changed while the optical output is kept substantially constant. .

In this way, the light emitted from the light source section 23 whose optical frequency is variable is split into the test signal light a and the local signal light b by the optical fiber coupler 24. The branched test signal light a is pulsed at a constant cycle by the AO switch 3 and is subjected to optical frequency modulation, and is incident on the optical fiber 5 under test via the optical fiber coupler 25. Optical fiber under test 5
The reflected light and the backscattered light c from the optical fiber coupler 25
Through the optical fiber coupler 26. The local signal light b is also guided to the optical fiber coupler 26, and the reflected light and the backscattered light c and the local signal light b are combined to generate a beat signal light d.

The beat signal light d generated by the optical fiber coupler 26 is optically / electrically converted by the light receiver 27.
The light receiver 27 electrically converts the beat signal light d output from the two terminals of the optical fiber coupler 26, and at the same time, the light receiver 2
It is used to reduce the noise generated in 7.
The beat signal optically / electrically converted by the light receiver 27 is
The signal converter 8 performs A / D conversion and square conversion,
Addition is performed by the addition processor 9 for improving SN. Furthermore, after this, logarithmic conversion is performed by the logarithmic converter 10, and it is displayed on the CRT 11 as a temporal change of the signal, that is, a distribution in the longitudinal direction. The timing generator 12 supplies a signal for pulsing the test signal light a to the drive unit of the AO switch 3 and also supplies an addition processing timing signal to the addition processor 9.

The main control section 28 controls the operation of the LD temperature control section 22 and the timing generator 12, and as a result, controls the optical frequency in accordance with the timing of the optical pulse sent to the optical fiber 5 under test. is there.

FIGS. 6A and 6B are views conceptually showing the timing of optical pulse transmission and the change in optical frequency. In the figure, the horizontal axis represents time and the vertical axis represents optical frequency. Fig. 6 (a) shows the optical frequency changed linearly with time. Fig. 6 (b) shows that the optical frequency is changed before the optical pulse is sent and the optical pulse is sent. The optical frequency is kept constant until the light pulse is transmitted. Figure 6 (a)
The method shown in 1 is characterized by easy control. On the other hand, the method shown in FIG. 6B makes control complicated. further,
It is necessary to speed up the response when changing the optical frequency,
There is a feature that the optical frequency change for each optical pulse can be arbitrarily set.

Next, the coherent detection O of the first embodiment is performed.
The effects of the present invention will be described based on the experimental results using TDR. The optical fiber 5 under test has a length of 10 km, and the optical pulse sent to the optical fiber 5 under test has a wavelength of 1.5 μm.
m, pulse width 1 μs, pulse period 8 ms, peak intensity-
It is 10 dBm, which is the same as the conventional example shown in FIG. The spectral line width is set to 3 KHz or less by the single mode optical fiber 15 for narrowing the line width of 1 km, and
The intensity of the local signal light b input to the optical fiber coupler 26 is approximately 0 dBm, which is set to be the same as in the case of the conventional example. Further, the number of additions for improving the SN of the backscattered waveform was 2.6 × 10 ⁵ times as in the conventional example.
Therefore, the addition time is 2080 seconds. The change of the optical frequency for 2080 seconds was linearly changed as shown in FIG. 6 (a), and the change width was reciprocated between 140 GHz. One round trip time is about 60 seconds.

The backscattering waveform obtained by the experiment under the above-mentioned conditions is shown in FIG. In the figure, the horizontal axis represents the length of the optical fiber under test 5, and the vertical axis represents the intensity of the backscattered light c. It can be seen that the fading noise is reduced to 0.5 dB or less as compared with the experimental result of the conventional example shown in FIG. 3, and the waveform is almost linear.

As described above, when the wavelength of the optical pulse incident on the optical fiber under test 5, that is, the optical frequency is slightly changed for each pulse or for every plural pulses, the reflected light returning from the optical fiber under test 5 is returned. The backscattered light c and the beat signal light d obtained by combining the reflected light and the backscattered light c with the local signal light b have slightly different interference characteristics and the like for each one pulse or a plurality of pulses. When this slightly different beat signal light d is detected and the addition processing is performed by the electric signal processing system 13, the backscattered light c from the optical fiber under test 5 is detected.
Since the interference components of 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. 8 is a block diagram showing the second embodiment. In the figure, the same components as those in the first embodiment described above are designated by the same reference numerals and the description thereof will be omitted. Reference numeral 31 is a semiconductor laser having a plurality of electrodes, for example, a DFB-LD or DBR-LD having a few electrodes, 32 is a drive current controller corresponding to the plurality of electrodes, 33 is an AO switch, and 34 is a drive current control. Unit 32 and timing generator 1
2 is a main control unit for controlling the control unit 2. Semiconductor laser 31, drive current controller 32, AO switch 33, and main controller 34
Of the DFB- in the first embodiment described above.
LD 21, LD temperature controller 22, optical fiber coupler 2
5, used in place of the main control unit 28.

The semiconductor laser 31 is controlled by a drive current controller 32 capable of driving a current of a number corresponding to a plurality of electrodes, and the ratio of the currents flowing through the plurality of electrodes is appropriately set, so that the semiconductor laser 31 shown in FIG. As described above, the optical frequency can be changed while keeping the fluctuation of the optical output small.

Next, the operation of the second embodiment having the above construction will be described. The output light of the semiconductor laser 31 has its spectral line width reduced by the single-mode optical fiber 15 for narrowing the line width, and is branched by the optical fiber coupler 24 into the test signal light a and the local signal light b. The test signal light a is pulsed by the AO switch 3, and then enters the optical fiber 5 under test through the AO switch 33. The reflected light and the backscattered light c from the optical fiber under test 5 are guided to the optical fiber coupler 26 via the AO switch 33.
The local signal light b is also guided to the optical fiber coupler 26, and the reflected light and the backscattered light c and the local signal light b are combined to generate a beat signal light d. After that, the beat signal light d generated by the optical fiber coupler 26 is
It is the same as in the first embodiment described above in that it is photoelectrically converted by the light receiver 27 and is electrically processed by the electrical signal processing system 13 for display.

However, the second embodiment is characterized in that the AO switch 33 is used for inputting / outputting light to / from the optical fiber 5 under test. The AO switch 33 operates based on the control signal from the timing generator 12, and is connected to the light source section 23 side during the time when the optical pulse is sent to the optical fiber under test 5, and the other time during the optical fiber under test. The switch operation is performed so that the backscattered light c from 5 is guided to the light receiver 27 side. Therefore, by slightly adjusting the connection start time to the light receiver 27 side, even if there is a large reflected light at the incident end of the optical fiber 5 under test, this large reflected light is prevented from entering the light receiver 27. Therefore, there is an advantage that the backscattered waveform is not distorted and observed due to the saturation of the light receiver 27 and the electric system amplifier.

Further, the second embodiment is a semiconductor laser 31.
Since the drive current is controlled, there is an advantage that the response of the optical frequency change is fast. As a result, it becomes possible to control the optical frequency change as shown in FIG. 6 (b).

The method shown in FIG. 6 (a) is easy to control, but if the amount of change in the optical frequency is increased, the optical frequency of the backscattered light c returning after sending one pulse and the local signal light b The difference between the optical frequencies becomes too large to be outside the reception band of the electric signal processing system 13, and finally the backscattering waveform cannot be obtained. Therefore, it is necessary to reduce the change amount of the optical frequency and increase the number of additions to some extent.

On the other hand, in the method shown in FIG. 6 (b),
After the optical pulse is sent, the optical frequency is kept constant until the backscattered light c from the entire length of the fiber under test 5 is received, so that the backscattered waveform cannot be observed even if the change amount of the optical frequency is increased. There is no. Therefore, since the optical frequency can be largely changed within a small number of additions, a smooth backscattering waveform can be obtained in a short time.

In the concept shown in FIG. 6 (b), the optical frequency is 1
Although it changes for every pulse, even if it changes every time a plurality of pulses is sent, it does not affect the fading noise reduction effect. However, it only increases the time required to reduce fading noise.

As described above, the first and second aspects of the present invention
In the embodiment of the present invention, the optical frequency of the light pulse incident on the optical fiber 5 to be tested is slightly changed at least every one pulse by using the light source unit 23 with variable optical frequency, and the beat signal light having slightly different interference characteristics and the like. By adding d,
The interference component of the backscattered light c from the optical fiber under test 5 is averaged to reduce fading noise on the backscattered waveform.

Therefore, by using the optical pulse tester of the present invention, fading noise on the backscattering waveform, which is the biggest problem of the OTDR using the coherent detection technique, can be reduced, so that the optical fiber 5 to be tested can be provided in the middle thereof. If there is a connection point or a point where the loss changes, it can be identified as a step on the backscattering waveform. That is, as a tool for constructing a highly reliable optical line, coherent detection OTDR
Can be put to practical use.

[0045]

As described above, according to claim 1 of the present invention, the optical frequencies of the test signal light and the local signal light are slightly changed at least every one cycle of the optical pulse.
The interference characteristics of the backscattered light and the beat signal light from the optical fiber under test are slightly different for each at least one cycle of the light pulse, and the interference component of the backscattered light is averaged to obtain the backscattered light. Fading noise on the 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 becomes possible to identify it as a step on the backscattering waveform, and as a tool for constructing a highly reliable optical line, coherent detection It has a very excellent effect that the pulse tester can be put to practical use.

According to the second aspect, in addition to the above effects, the optical frequencies of the test signal light and the local signal light can be changed by simple control.

According to the third aspect, in addition to the above effects, the backscattering waveform cannot be observed even if the amount of change in the optical frequency is increased, and the smooth backscattering is achieved in a short time. Waveforms can be obtained.

According to claim 4 or 5, in addition to the above effects, by using a semiconductor laser as a light source,
It has an advantage that the optical frequency of the output light can be easily changed.

Further, according to the fifth aspect, in addition to the above effect, since the drive frequency of the semiconductor laser is controlled to change the optical frequency, there is an advantage that the response of the optical frequency change is fast.

[Brief description of 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 relationship between a temperature change of a semiconductor laser and an optical frequency change.

FIG. 5 is a diagram showing a relationship between a change in total value of drive currents of a multi-electrode laser and a change in optical frequency.

FIG. 6 is a diagram showing changes in optical frequency with respect to optical pulse transmission.

FIG. 7 is a diagram showing a backscattering waveform measured using the first embodiment of the present invention.

FIG. 8 is a configuration diagram showing a coherent detection type optical pulse tester of a second embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Light source part, 2, 4, 6 ... Combiner / brancher, 3 ... AO switch, 5 ... Optical fiber under test, 7 ... Photoreceiver, 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 ... DFB-LD, 22 ... LD temperature control part, 23 ... Light source part, 24-26 ... Combined / branching optical fiber coupler, 27 ... Photoreceiver, 28 ... Main control part, 31 ... Semiconductor laser, 32 ... Drive Current control unit, 33 ... AO switch, 34 ... Main control unit.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Izumi Mikawa             1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo             Inside Telegraph and Telephone Corporation

Claims (5)

[Claims] 1. A signal light generating means for the test signal light and the local signal light, an optical pulse generating means for pulsing the test signal light and repeatedly sending the pulsed test signal light to an optical fiber under test at predetermined intervals, and the under test. Optical combining means for receiving reflected light and backscattered light repeatedly returning from the optical fiber and combining with the local signal light to generate beat signal light, and the beam combining means.
Optical-electrical conversion means for converting the signal light into an electric signal, an addition processing means for adding the electric signals, and a 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 slightly changing the optical frequencies of the test signal light and the local signal light at least every one cycle of the optical pulse. Optical pulse tester. 2. The optical pulse tester according to claim 1, wherein the optical frequency changing unit linearly changes the optical frequency in response to a change in time. 3. The optical frequency variable means at least from the optical pulse transmission time by the signal light generating means until the backscattered light from the far end of the optical fiber under test due to the optical pulse is received by the optical multiplexing means. The optical pulse tester according to claim 1, wherein the optical frequency is constant during the period. 4. The signal light generating means includes a semiconductor laser for generating the test signal light and the local signal light, and the optical frequency changing means changes the temperature of the semiconductor laser to change the temperature of the semiconductor laser. The optical pulse tester according to claim 1, 2 or 3, wherein the optical frequencies of the test signal light and the local signal light are changed. 5. The signal light generating means has a semiconductor laser for generating the test signal light and the local signal light, and the optical frequency varying means changes a current passing through the semiconductor laser. The optical pulse tester according to claim 1, 2 or 3, wherein the optical frequencies of the test signal light and the local signal light are changed.