JP2901106B2 - 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 for measuring characteristics such as optical loss and reflection of an optical fiber and an optical fiber line.

[0002]

2. Description of the Related Art An optical pulse tester is used for transmitting an optical fiber or an optical fiber line formed by connecting a plurality of optical fibers (hereinafter, an optical fiber and an optical fiber line are collectively referred to as an optical fiber). This is a device that transmits a pulse, receives reflected light and backscattered light from an optical fiber or the like, breaks it, and measures characteristics such as optical loss and reflection of the optical fiber or the like. The optical pulse tester is the most important measuring device for the construction and maintenance of an optical line, and is widely used all over the world.

The magnitude of optical loss of an optical fiber or the like that can be measured by an optical pulse tester (hereinafter referred to as a dynamic range) is the most important performance of an optical pulse tester. The dynamic range is determined by the intensity of an optical pulse transmitted from the optical pulse tester to the optical fiber under test and the like, and the receiving sensitivity to reflected light and backscattered light. In order to expand the dynamic range, studies on increasing the light pulse intensity and improving the receiving sensitivity have been actively conducted.

As an effective method for improving the receiving sensitivity, application of a coherent detection technique such as heterodyne detection or homodyne detection is being studied.

A conventional optical pulse tester using the coherent detection technique will be described with reference to FIG. In FIG. 9, reference numeral 1 denotes a light source unit that generates light having a narrow line width spectrum, and 2 denotes a light emitted from the light source unit 1 as a test signal light a and a local signal light b.
A first optical multiplexer / demultiplexer, 3 is an optical switch for pulsating the branched test signal light a at a constant period, 4 is a pulsed test signal light a which is incident on the optical fiber 5 to be tested, etc. , A second optical coupler for guiding the reflected light and the backscattered light c returning from the optical fiber under test 5 to the tester.
Reference numeral 6 denotes a third unit that combines the reflected light and the backscattered light c guided into the tester with the local signal light b to generate a combined light d.
7, a light receiver for converting the multiplexed light d into light / electricity, 8 a signal processing unit for processing the electric signal output from the light receiver 7 and generating waveform data of reflected light and backscattered light, 9 is a display unit for displaying a waveform. 10 of the light source unit 1 is a semiconductor laser, 11 is an optical fiber used as an external resonator to narrow the transmission spectrum of the semiconductor laser 10, and 12 is a drive for supplying a DC current to the semiconductor laser 10. Circuit. Further, 13 of the signal processing unit 8 passes a frequency component including a beat signal of the reflected light and the backscattered light c and the local signal light b in the electric signal output from the light receiver 7, and other unnecessary signals. A filter for removing a frequency component.
A signal converter for converting and further squaring the signal, 15 is an addition processor for adding a signal that is repeatedly input at a fixed period to improve the SN ratio, and 16 is a logarithm after subtracting an offset power value from the added signal. Log converter to convert, 17
Is a timing generator for generating a timing signal for adjusting the timing of pulsing of the signal light a by the optical switch 3 and A / D conversion, square conversion, and addition processing; 18 is a main control unit; 18 is a display unit 9; The converter 16 and the timing generator 17 are controlled.

In an optical pulse tester using coherent detection technology, weak reflected light and backscattered light c are combined with strong local signal light b to receive both beat signal components. The signal component contained in the backscattered light c is amplified and received, and the receiving sensitivity is improved. FIG. 10 shows a comparison between the reception sensitivity of the optical pulse tester of the coherent detection method and the reception sensitivity of the optical pulse tester of the direct detection method without using the coherent technique. The receiving sensitivity of the optical receiver is represented by the lowest receivable optical power, and the lower the value, the higher the receiving sensitivity. As can be seen from FIG. 10, the reception sensitivity is such that the reception bandwidth (the bandwidth of the filter 13) is 1 MHz.
The coherent detection method is higher than the direct detection method, about 20 dB in the case of 10 MHz and about 15 dB in the case of 10 MHz. in this way,
Since the receiving sensitivity can be improved by using the coherent detection technique, the dynamic range of the optical pulse tester is expanded.

However, in order to perform coherent detection,
The light source unit 1 that generates light having a spectral line width sufficiently smaller than the bandwidth of the filter 13 is required. The bandwidth of the filter 13 is set so that the optical loss of the optical fiber under test 5 or the like can be reduced by a distance accuracy of 10
Set to about 1MHz when measuring at 0m, and to about 10MHz when measuring at a distance accuracy of 10m. Therefore, the spectral line width of the light source unit 1 must be sufficiently narrower than this. It is difficult to realize such a narrow line width by using the semiconductor laser 10 alone. Therefore, the length of the output end of the semiconductor laser 10 is
A method of connecting an optical fiber 11 of about 100 m to 1 km and narrowing the oscillation spectrum of the semiconductor laser 10 by the backscattered light from the optical fiber 11 is adopted.

[0008]

As described above, the reception sensitivity can be improved by using the coherent detection technique. However, in the optical pulse tester using the conventional coherent detection technique, fading noise is generated on the backscattered waveform. There is a drawback that causes fluctuation. The fading noise has a narrow spectral line width of the test signal light a,
Interference occurs between the backscattered lights returning from the respective points of the optical fiber under test 5 and the like. The interference occurs when they are superimposed in the same phase, and are weakened when they are superimposed in the opposite phase.

FIG. 11 shows an example of a backscattered waveform observed by a conventional optical pulse tester using the coherent detection technique.
FIG. 11 shows an optical fiber under test 5 having a length of 10 km and a wavelength of 1.5 km.
[mu] m, is obtained backscatter waveform light pulse obtained by repeatedly incident at 1ms intervals of the pulse width 1 .mu.s 2 ¹⁶ times addition on average. The waveform should be almost straight if there is no fading noise, but there is a fluctuation of about ± 0.3 dB due to the fading noise. Such a fluctuation may make it impossible to determine the step when there is a step in the backscattered waveform when there is a connection part or a part where the loss changes in the middle of the optical fiber under test 5 or the like. If the original step is about 0.1 dB, the step cannot be discriminated at all.

As described above, from the backscattered waveform observed by the conventional optical pulse tester using the coherent detection technique, the fluctuation of the fading noise is large, so that it is possible to accurately measure the optical loss of an optical fiber or the like. Can not.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide an optical pulse tester which realizes high reception sensitivity using a coherent detection technique and has low fading noise.

[0012]

In order to achieve the above object, according to the present invention, there is provided a signal light having a semiconductor laser which generates at least a test signal light and a local signal light. A generating means, an optical pulse generating means for pulsating the test signal light at predetermined intervals and repeatedly sending the test signal light to an optical fiber under test or the like, and the reflected light and the backscattered light repeatedly returning from the optical fiber under test or the like. An optical pulse tester comprising: optical multiplexing means for generating multiplexed light by multiplexing with local signal light; opto-electric conversion means for converting the multiplexed light into an electric signal; and signal processing means for processing the electric signal In, the optical frequency of the test signal light and the local signal light,
DC current or changes with time to the semiconductor laser
The semiconductor laser is oscillated by applying a current
And at the same time as the pulsing cycle of the test signal light.
The DC current or a pulse current immediately prior to the pulsed and
Superimposed on the current that changes with time to the semiconductor laser
We propose an optical pulse tester provided with an optical frequency changing means that changes by energizing .

[0013]

According to a second aspect of the present invention, there is provided an optical pulse tester according to the first aspect , wherein the temperature of the semiconductor laser is changed with time.

[0015]

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.
Sent repeatedly to the optical fiber under test etc. at predetermined intervals
However, the signal light generating means includes a test signal light and a local signal.
A semiconductor laser that emits light
Apply a DC current or a current that changes with time.
Causes the semiconductor laser to oscillate,
Pulse current synchronized with the pulse
Changes with the DC current or time just before
By energizing the semiconductor laser superimposed on the current
Inducing hopping of the oscillation mode of the semiconductor laser,
Test the optical frequency of the test signal light and the local signal light
Synchronous with the pulsing cycle of the signal light and immediately before pulsing
For changing the optical frequency of the test signal light and the local signal light is changed immediately before the synchronization with and pulsed cycle of the pulse of the test signal light. Since the shape of the fluctuation due to the fading noise of the backscattered light changes according to the optical frequency of the test signal light, in this case, the reflected light and the backscattered light that are repeatedly returned from the optical fiber under test or the like are separated by the optical multiplexing means. The signal is multiplexed with the local signal light, converted into an electric signal by photoelectric conversion means, and further subjected to A / D conversion, square conversion, addition processing and logarithmic conversion by signal processing means.
Waveform data of reflected light and backscattered light is generated. By inputting this waveform data to an appropriate display device, the waveforms of the reflected light and the backscattered light can be displayed. In this way, the fluctuation on the backscattered waveform finally obtained by adding a large number of backscattered lights having different fluctuation shapes is much larger than that measured by an optical pulse tester using the conventional coherent detection technology. To be reduced.

[0016]

According to claim ( 2 ), hopping of the oscillation mode of the semiconductor laser is easily caused by changing the temperature of the semiconductor laser according to claim ( 1 ), and the optical frequency of the hopping is reduced. The change is increased and the waveform fluctuation is further reduced.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An important point of the present invention lies in the signal light generating means. First, an embodiment of the signal light generating means according to the present invention will be described.

In an optical pulse tester using a coherent detection technique, a signal light generated by a signal light generating means is provided between a time when an optical pulse is transmitted to an optical fiber under test and a time when reflected light and backscattered light return. The change in optical frequency must be kept below about 100KHz. By the way, the time required from sending an optical pulse to the optical fiber under test having a length of 50 km until all the reflected light and the backscattered light return is 0.5 ms.

On the other hand, in order to suppress the fluctuation of the backscattered waveform due to fading noise, it is necessary to change the optical frequency for each optical pulse to be transmitted.

Therefore, according to the present invention, the optical frequency jumps immediately before the generation of the optical pulse, and the change in the optical frequency from sending the optical pulse to returning all the reflected light and the backscattered light is about 100 KHz. An optical pulse tester is configured using the following signal light generating means.

Conventionally, there has been no such signal light generating means. In a conventional coherent detection type optical pulse tester, signal light generating means for oscillating a semiconductor laser by applying a direct current to the semiconductor laser using an optical fiber as an external resonator is used. FIG. 12 shows an example of a time change of the optical frequency of the conventional signal light generating means. The change of the optical frequency of the signal light emitted from the conventional signal light generating means is stabilized at several kHz or less for several hundred ms, and jumps at an average interval of about 200 to 300 ms. When the measurement is performed using such signal light generating means, since the measurement is performed 200 to 300 times at the same optical frequency, the backscattered light having the same waveform fluctuation shape is generated.
It will be captured 200 to 300 times. Therefore, even if the addition processing is performed, the waveform fluctuation due to fading noise cannot be effectively reduced.

FIG. 1 shows a first embodiment of the signal generating means according to the present invention. In FIG. 1, reference numeral 19 denotes a light source unit which provides signal light generating means according to the present invention, 20 denotes a semiconductor laser, 21 denotes an optical fiber used as an external resonator, 22
Is a drive circuit for supplying a current to the semiconductor laser 20. The drive circuit 22 supplies a superimposed current of a DC current and a pulse current to the semiconductor laser 20 as shown in FIG. The cycle of the pulse current is synchronized with the cycle of pulsing the test signal light, and is sent immediately before pulsing. Semiconductor laser 20
When such a current is applied, hopping of the oscillation mode occurs due to the impact of the pulse current, and the optical frequency jumps in synchronization with the pulse current as shown in FIG. On the other hand, while the pulse current does not flow, the optical frequency is stabilized by the optical fiber 21, and its change is as small as several kHz or less. In the case of the present embodiment, the optical frequency changes by mode hopping within a range of about several hundred MHz.

FIG. 3 shows a second embodiment of the signal light generating means according to the present invention. In FIG. 3, reference numeral 23 denotes a light source unit that provides signal light generating means, 24 denotes a semiconductor laser, 25 denotes an optical fiber used as an external resonator, and 26 denotes a drive circuit that supplies a current to the semiconductor laser 24. The drive circuit 26 supplies the semiconductor laser 24 with a current that changes slowly and linearly, for example, a current that changes by 0 to 60 mA in 1 to 2 seconds and a pulse current as shown in FIG. The cycle of the pulse current is synchronized with the cycle of pulsing the test signal light and flows immediately before pulsing. Semiconductor laser 24
When such an electric current is applied, the optical frequency changes stepwise as shown in FIG. In this case, the range of the optical frequency changed by mode hopping is as large as about 50 GHz. The reason why the optical frequency changes in this way is that the oscillation frequency of the semiconductor laser changes according to the magnitude of the current flowing. The change in the optical frequency between hoppings is as small as several KHz or less.

FIG. 5 shows a third embodiment of the signal light generating means according to the present invention. In FIG. 5, reference numeral 27 denotes a light source unit for providing signal light generating means, 28 denotes a semiconductor laser, 29 denotes an optical fiber used as an external resonator, 30 denotes a drive circuit for supplying a current to the semiconductor laser 28, and 31 denotes a semiconductor. The temperature controller controls the temperature of the laser 28.
The drive circuit 30 supplies a superimposed current of a DC current and a pulse current to the semiconductor laser 28 as shown in FIG. The cycle of the pulse current is synchronized with the cycle of pulsing the test signal light and flows immediately before pulsing. Temperature controller 31
As shown in FIG. 6, the temperature of the semiconductor laser 28 is slowly changed, for example, 0 to 15 ° C. in 1 to 2 seconds. When the superimposed current of the DC current and the pulse current is applied while changing the temperature in this way, the optical frequency changes stepwise as shown in FIG. In this case, the range of the optical frequency changed by mode hopping is 150G.
It is as large as around Hz. The reason why the optical frequency changes in this way is that the oscillation frequency of the semiconductor laser greatly changes depending on the temperature. The change in the optical frequency between hoppings is as small as several KHz or less.

Next, an embodiment of the optical pulse tester according to the present invention will be described.

FIG. 7 is a block diagram showing an embodiment of an optical pulse tester according to the present invention. In the present embodiment, an optical pulse tester is configured using the signal light generating means shown in FIG.
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 27 denotes a light source unit that provides signal light generating means, and 28 denotes a semiconductor laser,
9 is an optical fiber used as an external resonator, 30 is a drive circuit for supplying a current to the semiconductor laser 28, and 31 is a temperature controller for controlling the temperature of the semiconductor laser.
The drive circuit 30 supplies a superimposed current of a DC current and a pulse current to the semiconductor laser 28. The energization of the pulse current
The drive circuit 30 receives the timing signal from the timing generator 17, and performs the operation immediately before the optical switch 3 pulses the test signal a. The temperature controller 31 is a main controller 1
8 to control the temperature of the semiconductor laser 28.

Next, the operation of the optical pulse tester of the present invention having the above configuration will be described. A signal light is generated by the light source unit 27, and the signal light is split into a test signal light a and a local signal light b by the first optical multiplexer / demultiplexer 2. The test signal light a is repeatedly pulsed by the optical switch 3 and transmitted to the optical fiber under test 5 or the like 5 via the second optical multiplexer / demultiplexer 4. Here, the optical frequency of the signal light generated by the light source unit 27 is synchronized with the pulse period, and
It changes stepwise as shown in FIG. The jump in optical frequency occurs immediately before pulsing. Optical fiber under test 5
The reflected light and the back-scattered light c that are repeatedly returned from the optical fiber are guided to the tester via the second optical coupler / demultiplexer 4 and multiplexed with the local signal light b by the third optical coupler / demultiplexer 6 to be combined. Wave light d is obtained. The combined light d is converted into an electric signal by the light receiver 7. After the filter 13 removes unnecessary frequency components from the electric signal, the electric signal is A / D-converted by the signal converter 14 and further square-converted. Adder 15
Improves the signal-to-noise ratio by adding the square-transformed signal that repeatedly enters. The logarithmic converter 16 subtracts the offset power value from the signal subjected to the addition processing, and then performs logarithmic conversion to generate waveform data of the reflected light and the backscattered light c. The display unit 9 receives the waveform data and displays a waveform. The timing generator 17 generates a timing signal to generate a pulse current from the drive circuit 30, A / D conversion and square conversion of an electric signal by the pulse signal converter 14 of the test signal light a by the optical switch 3, The timing of the addition process by the addition processor 15 is adjusted. The main control unit 18 controls the temperature controller 31, the timing generator 17, the logarithmic converter 16, and the display unit 9.

Next, the effects of the present invention will be described based on experimental results using the optical pulse tester of the present embodiment.

FIG. 8 shows an example of a backscattering waveform measured using the optical pulse tester of this embodiment. Fig. 8 shows the backscattered light obtained by repeatedly injecting an optical pulse with a wavelength of 1.5 µm and a pulse width of 1 µs into a 10 km long optical fiber under test at 1 ms intervals.
It is obtained by averaging ¹⁶ times, and the measurement conditions are shown in Fig. 1.
This is the same as the conventional example shown in FIG. In FIG. 8, the waveform fluctuation due to fading noise is approximately ± 0.03 dB,
It is reduced to about one tenth of the conventional example shown in FIG.

As described above, in the optical pulse tester of this embodiment, since the optical frequency of the optical pulse transmitted to the optical fiber under test is changed for each pulse, the fluctuation shape due to the fading noise of the backscattered light changes for each pulse. In contrast, the fluctuation on the backscattered waveform finally obtained by adding them is significantly reduced as compared with the case where the fluctuation is measured by an optical pulse tester using a conventional coherent detection technique.

[0032]

According to the present invention, as described above,
According to the method, by changing the optical frequency of an optical pulse transmitted from an optical pulse tester using a coherent detection technique to an optical fiber under test or the like for each pulse by using hopping of an oscillation mode of a semiconductor laser, high reception is achieved. A backscattered waveform with small fading noise can be obtained while achieving sensitivity, and high-precision measurement can be performed in a wide dynamic range.

[0033]

According to claim ( 2 ), the change of the optical frequency for each pulse can be increased, and the waveform fluctuation can be reduced more effectively.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of signal light generating means according to the present invention.

FIG. 2 is a diagram showing light pulse transmission, current flowing to a semiconductor laser, changes in semiconductor laser temperature and optical frequency according to a first embodiment of the signal light generating means according to the present invention.

FIG. 3 is a block diagram showing a second embodiment of the signal light generating means according to the present invention.

FIG. 4 is a diagram showing light pulse transmission, a change in a current supplied to a semiconductor laser, and a change in an optical frequency according to a second embodiment of the signal light generating means according to the present invention.

FIG. 5 is a block diagram showing a third embodiment of the signal light generating means according to the present invention.

FIG. 6 is a diagram showing an optical pulse transmission, a change in an electric current supplied to a semiconductor laser and an optical pulse frequency according to a third embodiment of the signal light generating means according to the present invention.

FIG. 7 is a configuration diagram of an embodiment of an optical pulse tester according to the present invention.

FIG. 8 is a diagram showing an example of a backscattering waveform observed by the optical pulse tester according to the present invention.

FIG. 9 is a configuration diagram of a conventional coherent detection optical pulse tester.

FIG. 10 is a comparison diagram of reception sensitivity between a coherent detection method and a direct detection method.

FIG. 11 is a diagram showing an example of a backscattered waveform observed by a conventional coherent detection optical pulse tester.

FIG. 12 is a diagram showing transmission of an optical pulse and a change in an optical frequency by a conventional signal light generating means.

[Explanation of symbols]

1, 19, 23, 27 light source unit, 2, 4, 6 optical coupler / demultiplexer, 3 optical switch, 5 optical fiber under test, etc., 7 optical receiver, 8 signal processing unit, 9 display unit, 10,20,2
4, 28 semiconductor laser, 11, 21, 25, 29 optical fiber, 12, 22, 26, 30 drive circuit, 1
3 filter, 14 signal converter, 15 addition processor,
16 logarithmic converter, 17 timing generator, 18 main controller, 31 temperature controller.

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Izumi Mikawa 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-4-370732 (JP, A) JP-A Heisei 3-13835 (JP, A) JP-A-3-111732 (JP, A) JP-A-2-251729 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G01M 11/02 G01M 11/00

Claims (2)

(57) [Claims] A signal light generating means having a semiconductor laser for generating at least a test signal light and a local signal light;
An optical pulse generating means for pulsating the test signal light at predetermined intervals and repeatedly sending out the test signal light to the optical fiber under test and the like, and reflecting light and backscattered light repeatedly returning from the optical fiber under test and the like as the local signal light An optical pulse tester comprising: an optical multiplexing unit that generates multiplexed light by multiplexing; a photoelectric conversion unit that converts the multiplexed light into an electric signal; and a signal processing unit that processes the electric signal. the optical frequency of the signal light and the local signal light, the
Direct current or time-varying current to a semiconductor laser
Oscillates the semiconductor laser by energizing the current
Together, Pa which in synchronization with the period of the pulse of the test signal light
The DC current or time immediately before the pulse current
To the semiconductor laser by superimposing it on the current that changes
An optical pulse tester characterized by comprising an optical frequency changing means for changing the frequency. 2. The temperature of the semiconductor laser changes with time.
OTDR according to claim 1, wherein the to of.