JP3208050B2 - 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 remotely and accurately testing the optical loss and other characteristics of an optical fiber 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, the reliability and economics of an optical fiber line serving as an optical transmission line are important issues. for that reason,
It is necessary to accurately measure section loss, connection loss, and return loss of an optical fiber without affecting communication.

An optical pulse tester that meets such a requirement transmits an optical pulse to an optical fiber under test, receives reflected light and backscattered light from the optical fiber under test, analyzes the reflected light and backscattered light, and analyzes the reflected light and backscattered light. This device displays the optical loss and other characteristics of an optical fiber on a screen. Conventionally, a method shown in FIG. 11 has been used to perform an optical line monitoring test without affecting a communication signal (Japanese Patent Application No. 6-127678).

In FIG. 11, an optical transmitter 1 and an optical receiver 2 are connected to both ends of an optical fiber 4 under test. A communication signal from the optical transmitter 1 is sent to the optical receiver 2 via the optical fiber under test 4, converted into an electric signal, passed through the bandpass filter 6, and input to the receiver 7. The optical pulse transmitted from the optical pulse tester 40 to test the optical fiber 4 under test is incident on the optical fiber 4 under test via the optical directional coupler 3. The optical pulse is incident on the optical receiver 2 together with the communication signal. The optical pulse is shaped so that the frequency component of the received signal of the optical pulse does not occur in the communication signal band.

For example, if a pulse having a wide pulse width is used, its frequency component is limited to a low frequency, so that the optical pulse of the optical pulse tester does not affect a communication signal using a high frequency band. However, the distance resolution of the optical pulse tester deteriorates in proportion to the pulse width. Therefore, it has been proposed to generate a pulse waveform into a special waveform instead of a rectangular wave (Japanese Patent Application No. Hei 6-127678).

In this case, for example, when the pulse waveform has a raised cosine waveform with a pulse width of 100 ns, the spectral component of the pulse signal in the frequency band of the communication signal (for example, near 100 MHz) is a rectangular pulse having the same pulse width of 100 ns. In comparison, it is about 40 dB smaller. Therefore, by adopting such a special pulse waveform, it is possible to suppress the deterioration of the communication signal due to the optical pulse of the optical pulse tester being incident on the optical receiver 2.

FIG. 12 is a block diagram showing a conventional optical pulse tester 40. As shown in FIG. The reflected light and the backscattered light returning from the optical fiber under test 4 enter the light receiver 22 via the optical directional coupler 3, are converted into electric signals, and are input to the A / D conversion circuit 23. The output of the A / D conversion circuit 23 is sequentially input to the addition processing circuit 24. The display 25 displays a waveform obtained by logarithmically converting the addition signal in the addition processing circuit 24. The timing generation circuit 26 sends a synchronization signal of a predetermined cycle to the addition processing circuit 24 and the pulse driver 42 for generating an optical pulse waveform. The pulse driver 42 for generating an optical pulse waveform receives the synchronization signal and sends a pulse to the test light source 41.

The conventional optical pulse tester has been described above. However, in a test using such a conventional optical pulse tester, it is difficult to perform measurement with an ultra-high distance resolution.
The reason will be described below. In order to make the distance resolution of the optical pulse tester extremely high, it is necessary to narrow the pulse width of the transmitted optical pulse. However, when the pulse width of the pulse waveform as shown in FIG. 13 is reduced, the frequency spectrum of the optical pulse is broadened in proportion to the reciprocal of the pulse width. A frequency component of the received signal of the test light pulse is generated, which deteriorates transmission quality.

Therefore, at present, when a monitoring test is performed on an optical line such as a subscriber optical line having a short wiring length and densely connected points, the communication is not affected and the communication signal light within the optical line is not affected. The development of an optical pulse tester that measures the loss and the like at ultra-high altitude distance resolution is an issue.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to accurately determine section loss, connection loss, return loss, and the like of a communication signal in an optical fiber under test with an ultra-high distance resolution without affecting communication. It is to provide a device for grasping.

[0011]

SUMMARY OF THE INVENTION The present invention provides an optical pulse generating means for generating an optical pulse having a signal waveform that can be cut off by an electric frequency filter built in an optical communication receiving terminal device, and receiving the generated optical pulse. Means for sending to the test optical fiber, means for extracting reflected light and backscattered light returning from the optical fiber under test, opto-electrical conversion means for receiving the extracted reflected light and backscattered light and converting them into electric signals,
An electric signal processing unit that performs processing such as addition of an electric signal, and an optical pulse tester including a unit that displays a waveform of reflected light and backscattered light of the optical pulse based on a result of processing the electric signal, The optical pulse generation means includes means for generating an optical pulse obtained by time-division multiplexing test light and dummy light having different wavelengths (or optical frequencies) from each other, and further comprising an optical pulse obtained by time-division multiplexing the test light and the dummy light. Selective receiving means for transmitting a signal to the optical fiber under test and blocking a signal by the dummy light among signals by the reflected light and the backscattered light returning from the optical fiber to be tested and selectively receiving only a signal by the test light. Equipped.

The time-division multiplexed pulse transmitted from the optical pulse tester according to the present invention is sufficiently removed by the bandpass filter built in the demodulator because the pulse width is wide, and the optical line is monitored without affecting communication. Testing can be performed. Further, the wavelength (or frequency) -dependent selective receiving means built in the optical pulse tester can extract only the test signal light with a narrow pulse width from the time-division multiplexed pulse with a wide pulse width, so that an ultra-high distance resolution can be obtained. It is possible to perform a monitoring test on the optical line.

[0013]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of an optical line monitoring test using the optical pulse tester of the present invention. In the figure, parts other than the optical pulse tester 20 of the present invention are the same as those of the conventional optical line monitoring test system shown in FIG.

FIGS. 2 to 9 are block diagrams showing an embodiment of the optical pulse tester 20 of the present invention. In the figure, a test light source 27-1 (wavelength λm) and a dummy light source 27-2 (wavelength λd
≠ λm), pulse driver 28 for generating test optical pulse waveform
-1, Dummy optical pulse waveform generation pulse driver 28-2,
Except for the optical coupler 21 and the wavelength-dependent filter 29, the other parts are the same as those of the conventional optical pulse tester, and the same reference numerals are given and the description is omitted. The wavelength-dependent optical filter 29 is not used in the above-mentioned conventional example, but is a conventionally known means.

A pulse driver for generating a test optical pulse waveform
28-1 is to perform measurements with ultra-high altitude resolution,
Like pulse 100 (shaded area) shown in FIG.
This is a waveform generator for generating a pulse having a sufficiently narrow pulse width T1. The dummy optical pulse waveform generating pulse driver 28-2 is a waveform generating device for generating dummy light (portions 101 and 102 in FIG. 10) for time division multiplexing with a test signal pulse. FIG. 10 is a diagram showing a state in which the test light pulse and the dummy light pulse are time-division multiplexed, where T1 is the pulse width of the test light pulse having the wavelength λm, and T2 is the pulse width of T1 and the dummy light pulse having the wavelength λd. And the time width added.

The test light generated by the two pulse drivers is time-division multiplexed to generate a synthesized pulse, which is transmitted to the optical fiber under test 4. In the case of the optical signals shown in FIGS. 10A and 10B, the pulse width T2 of the time-division multiplex pulse is wide, so that the high frequency component of the received signal is very small. Therefore, even if a test is performed by the optical pulse tester 20 when a high-frequency communication signal is being transmitted to the optical fiber 4 under test, the communication signal power to noise power ratio of the receiver 7 does not deteriorate. On the other hand, from the reflected light and the backscattered light of the time-division multiplexed optical signal returning to the optical pulse tester 20, only the signal light having a narrow pulse width of the wavelength λm is extracted by the wavelength-dependent optical filter 29. The test of the optical fiber under test can be performed with a high distance resolution.

The pulse shown in FIG. 10A (a rectangular pulse having a pulse width T2) and the pulse shown in FIG.
d cosine waveform), both have the same pulse width T
As described above, even in the case of 2, the high-frequency component of the pulse shown in FIG. 10 (b) is significantly lower, and therefore the latter can be said to have better performance. In FIG. 10 (a), the test light pulse having the wavelength .lambda.m is shown at the edge of the pulse having the pulse width T2.
It is needless to say that it can be anywhere within the light pulse.

The raised cosine waveform is represented by the following expression of c (t). c (t) = (A / 2) (1 + cos (π · B T -t)), - T2 ≦ t ≦ T2 c (t) = 0, | T2 | <t where, A is the peak value, B _T = 1 / T2, T2 is the pulse width, and t is the time.

In FIG. 10B, the test light pulse having the wavelength λm is positioned at the center of the raised cosine waveform light pulse having the pulse width T2. At this time, the power of the test light pulse having the wavelength λm is maximized, and the test accuracy is also maximized. Of course, the position of the test light pulse may be changed within this light pulse within a range where there is no problem from the viewpoint of accuracy or the like. At this time, the shape of the test light pulse having the wavelength λm is shaped so as to constitute a part of the raised cosine waveform. Here, the use of the combination of the test light pulse having the wavelength λm and the dummy light pulse having the wavelength λd is called time division multiplexing.

Next, an embodiment shown in FIGS. 3 to 5 will be described.
Pulse drivers 28-1 and 28-2 and external modulators 30-1 and 30 in FIG.
-2, switching optical switch 31 in FIG. 4, high-speed tunable light source in FIG.
The other parts except 32 are the same as those in the configuration of the optical pulse tester in FIG.

In the embodiment shown in FIG. 2, the light sources 27-1 and 27-2 are directly modulated. However, in the embodiment shown in FIG. 3, the test signal external modulator 30-1 and the dummy signal external modulator 30-1 are externally provided. A modulator 30-2 is provided to perform external modulation to obtain a test light pulse and a dummy light pulse. In the embodiment shown in FIG. 4, after the test light and the dummy light having different wavelengths λm and λd have been respectively time-division multiplexed by the switching optical switch 31, continuous light is generated by the external modulator 30 from the continuous light. I do. At this time, the pulse driver 28 for generating an optical pulse waveform operates upon receiving a timing signal from the timing generation circuit 26 so that the optical pulse includes both the test light and the dummy light.

In the embodiment shown in FIG.
A continuous light whose wavelength is temporally changed is obtained from a DFB or DBR laser (this is referred to as frequency shift keying). Generate a pulse. FIGS. 3 to 5
Other operations in the embodiment shown in FIG. 7 are the same as those in the embodiment shown in FIG.

Next, an embodiment shown in FIGS. 6 to 9 will be described.
A light source with good coherence is used for the light source 27-1 in FIGS. 6 to 8 and the high-speed wavelength tunable light source 32 in FIG. 9, and a part of the output is used as reference light. By mixing and receiving the reflected light or backscattered light of the light and selectively receiving the beat signal by an electric filter, the signal by the dummy light can be cut off and the signal by the test light can be selectively received. Can be. That is, the signal by the dummy light is blocked by the heterodyne detection or the homodyne detection, and only the signal by the signal light is selectively received. In this case, the wavelength-dependent optical filter 29 becomes unnecessary.

Other operations of the embodiment shown in FIGS. 6, 7, 8 and 9 are the same as those of the embodiment shown in FIGS. 2, 3, 4 and 5, respectively. In the above description, the wavelength of the time-division multiplexed light is two wavelengths λm and λd, but it goes without saying that more wavelengths may be used.

[0025]

As described above, by using the optical pulse tester of the present invention, when performing a monitoring test on an optical line, the optical signal section loss and connection of the communication signal can be obtained without affecting communication. Loss, return loss and the like can be accurately grasped with high distance resolution.

[Brief description of the drawings]

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

FIG. 2 is a block diagram showing details of an example of an optical pulse tester of the present invention.

FIG. 3 is a block diagram showing details of an example of an optical pulse tester according to the present invention.

FIG. 4 is a block diagram showing details of an example of an optical pulse tester according to the present invention.

FIG. 5 is a block diagram showing details of an example of an optical pulse tester according to the present invention.

FIG. 6 is a block diagram showing details of an example of an optical pulse tester according to the present invention.

FIG. 7 is a block diagram showing details of an example of an optical pulse tester according to the present invention.

FIG. 8 is a block diagram showing details of an example of an optical pulse tester according to the present invention.

FIG. 9 is a block diagram showing details of an example of an optical pulse tester according to the present invention.

FIG. 10 is a diagram showing an example of a waveform of a pulse transmitted from the optical pulse tester of the present invention.

FIG. 11 is a block diagram showing a conventional optical line monitoring test method.

FIG. 12 is a block diagram showing details of a conventional optical pulse tester.

FIG. 13 is a diagram illustrating an example of a waveform of a pulse transmitted from a conventional optical pulse tester.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 In-station apparatus 2 Optical receiver 3 Optical directional coupler 4 Optical fiber under test 5 Demodulator 6 Bandpass filter 7 Receiver 20 Optical pulse tester 21 Optical coupler 22 Optical receiver 22 A / D conversion circuit 24 Addition processing Circuit 25 Display 26 Timing generator 27-1 Test light source 27-2 Dummy light source 28-1 Pulse driver for generating test optical pulse waveform 28-2 Pulse driver for generating dummy optical pulse waveform 29 Wavelength dependent optical filter REFERENCE SIGNS LIST 30 external modulator 30-1 external modulator for signal light 30-2 external modulator for dummy light 31 optical switch 32 high-speed tunable light source 40 conventional optical pulse tester 41 conventional test light source 42 conventional optical pulse generation Pulse driver for

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-151532 (JP, A) JP-A-7-87019 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04B 17/00 H04B 10/08

Claims (1)

(57) [Claims] 1. An optical pulse generating means for generating an optical pulse having a signal waveform which can be cut off by an electric frequency filter built in an optical communication receiving terminal device, a means for transmitting the generated optical pulse to an optical fiber under test, Means for extracting the reflected light and backscattered light returning from the optical fiber under test, photoelectric conversion means for receiving the extracted reflected light and backscattered light and converting them into electric signals, and processing such as addition of electric signals. An electrical signal processing unit for performing, and an optical pulse tester comprising a unit for displaying a waveform of the reflected light and the backscattered light of the optical pulse based on a result of processing the electrical signal, wherein the optical pulse generating units have wavelengths of each other. (Or optical frequency)
Means for generating an optical pulse obtained by time-division multiplexing test light and dummy light different from each other, further transmitting an optical pulse obtained by time-division multiplexing the test light and dummy light to the optical fiber under test, An optical pulse tester comprising: selective receiving means for blocking a signal by a dummy light among signals by a reflected light and a backscattered light returning from a light source and selectively receiving only a signal by a test light.