JPH06294705A - Optical pulse tester - Google Patents

Abstract

PURPOSE:To provide an otical pulse tester having wide dynamic range in which the test signal light incident on an optical amplification relayless line has an intensity substantially equal to that of a communication signal light and the fluctuation of intensity is suppressed. CONSTITUTION:A second signal light (c) emitted from a second light source section 2 and having a wavelength different from that of a test signal light is multiplexed on an optical test signal pulse (a) generated from a first light source section 1 by means of an optical multiplexer 6. This constitution realizes an optical pulse tester having wide dynamic range in which the test signal light has an intensity substatially equal to that of a communication signal light and the fluctuation of intensity is suppressed.

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 testing characteristics such as optical loss of an optical fiber which is a transmission medium of an optical signal and an optical fiber line, and more particularly to a repeater optical line including an optical amplifier. It is applied as a test technique.

[0002]

In order to realize an economical optical communication system, a repeaterless long-distance optical communication system has been studied.
In this system, an optical amplifier is arranged between communication optical fiber lines to optically amplify communication light attenuated while propagating through an optical fiber, thereby extending the communicable distance. Usually, in such an optical communication system, a plurality of optical amplifiers are arranged between optical fiber lines for communication. As an optical amplifier, an erbium-doped optical amplifier (hereinafter, EDFA (Erbium-dope
d fiber amplifier) is often used.
Hereinafter, a repeaterless optical fiber line including a plurality of such optical amplifiers is referred to as an optical amplification repeaterless line.

In order to construct a highly reliable and economical optical amplifier repeater line, it is necessary to measure and test the characteristics of this line with a highly reliable tester.

Optical pulse tester (hereinafter referred to as OTDR (Optical
(Time Domain Reflectometer)) sends an optical pulse to the optical fiber under test, receives the reflected light and backscattered light from the optical fiber, analyzes this, and displays the characteristics such as optical loss on a CRT or the like. It is a device and can be tested by inputting / outputting light from one end of an optical fiber, and is regarded as a very useful tester. 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,
Mainly used are a method of increasing the intensity of a light pulse transmitted to the optical fiber under test and a method of improving the reception sensitivity of backscattered light. An optical amplification technique using an EDFA is used as a method for increasing the intensity of the transmitted light pulse. As a method for improving the reception sensitivity, coherent detection technology such as heterodyne or homodyne reception is used. An OTDR that combines these optical amplification technology and coherent detection technology (hereinafter, referred to as optical amplification coherent OTDR) has a dynamic range of 35 dB or more. This is a normal OT using the direct detection method.
Compared to DR, it has a large dynamic range of 15 dB or more.

When the optical amplification repeater line is tested by using the OTDR, the optical pulse transmitted by the OTDR is also amplified by the optical amplifier. Therefore, the minimum required dynamic range for OTDR corresponds to the optical fiber line loss between optical amplifiers. Normally, the repeater interval of optical amplifier is about 100km
Therefore, a dynamic range of about 21 dB or more, which is equivalent to the loss, is required. Therefore, application of the optical amplification coherent OTDR can be considered.

[0007]

In the optical amplification non-repeatered line, a part of the output intensity in the EDFA is measured by the photodetector, and the output intensity of the excitation light source of the EDFA is controlled by using the feedback circuit to be amplified. The output intensity of the signal light for the subsequent communication is kept constant. Therefore, when a light significantly higher than the output light intensity for communication enters the EDFA, the photodetector measuring a part of the intensity of the amplified output light is saturated, and the output intensity of the EDFA by the feedback circuit is saturated. There was a problem in that there was an abnormality in control and the light receiver was destroyed. Also, E
When an optical pulse with a pulse width W and a pulse period T is incident on the DFA, the amplification degree of the optical pulse is about T / W times larger than that of continuous light, and a part of the output light of the EDFA is measured. There was a problem that the existing light receiver was destroyed. Therefore, it is difficult to apply a tester that outputs a high-intensity optical pulse such as the optically amplified coherent OTDR to the optically amplified repeater line.

From the above, as the test technique for the optical amplification non-repeatered line, the intensity fluctuation of the test signal light incident on the optical amplification non-repeatered line is small, and its intensity is almost the same as the signal light intensity for communication. Moreover, an optical pulse tester with a wide dynamic range is required.

In view of the above problems, an object of the present invention is that the intensity fluctuation of the test signal light incident on the optical amplification repeater line is small, and its intensity is almost the same as the signal light intensity for communication, and It is to provide an optical pulse tester having a wide dynamic range.

[0010]

In order to achieve the above object, the present invention provides, in claim 1, a signal light generating means for the test signal light and the local signal light, and the test signal light is pulsed at predetermined intervals. Optical pulse generating means for transmitting the test signal light pulse to the optical fiber under test, the reflected light and the backscattered light repeatedly returning from the optical fiber under test, and the local signal light. Optically combining means for optically combining and the beat signal light, opto-electric converting means for receiving the beat signal light and converting it to an electric signal, and electric signal processing for performing addition processing of the electric signal In the optical pulse tester comprising means and display means for displaying the waveforms of the reflected light and the backscattered light based on the result of the electric signal processing, a signal light having a wavelength different from that of the test signal light is supplied. Optical superimposing means for generating a second signal light generating means, and superimposing the second signal light generated by the second signal light generating means on the test signal light pulse generated by the optical pulse generating means. To have. Further, in claim 2, the second signal light generating means is the second signal light generating means.
The optical output intensity control means for controlling the output intensity of the signal light is used. Further, in claim 3, the optical pulse generating means is composed of an acousto-optic switch, and the acousto-optic switch simultaneously serves as the light superimposing means. Further, in claim 4, the optical amplification means for amplifying the signal light from the optical superposition means is provided.

[0011]

According to the first aspect of the present invention, since the second signal light having a wavelength different from that of the test signal light is superimposed on the light pulse of the light pulse generation means by the light superimposing means, the intensity fluctuation of the test signal light is caused. Is small, and its intensity can be adjusted to almost the same level as the signal light intensity for communication. Therefore, the ED of the optical amplification unrepeatered line
The test can be performed without any abnormality in the FA output intensity control.

Further, according to the second aspect, since the output intensity of the second signal light can be adjusted by the optical output intensity control means, the intensity fluctuation of the test signal light can be set to a predetermined amount.

According to the third aspect of the invention, the acousto-optic switch can serve both as the light pulse generating means and the light superimposing means, so that the device can be made smaller.

According to the present invention, the test signal light can be amplified by the optical amplifying means. Therefore, even if the signal light intensity for communication is large, the intensity can be adjusted to the same level by adjusting the amplification degree. it can.

[0015]

EXAMPLE A first example of the present invention will be described with reference to FIG.

Reference numeral 1 is a first light source section. In order to perform coherent detection, a light source that emits light with a narrow linewidth spectrum of several KHz is indispensable. To realize this,
The light source unit 1 is a distributed feedback semiconductor laser 21 (hereinafter referred to as DFB
-LD), an optical fiber 22 for narrowing the line width, and an optical isolator 23. At the output end of the DFB-LD 21, an optical fiber 22 for narrowing the line width of about 1 km is fusion-spliced, and the backscattered light from the optical fiber 22 is used to narrow the line width. The line width of is less than 10KHz. Further, the DFB-LD 21 can change the wavelength of the output light of the light source unit 1 at a predetermined cycle by the wavelength controller 24. Hereinafter, the wavelength of the output light from the light source unit 1 is λ1 (f1 in the optical frequency range), and the wavelength tunable amount is Δλ1.
([Delta] f1 in optical frequency domain).

Reference numeral 2 is a second light source section. The output light from the light source unit 2 outputs a light c having a wavelength different from the wavelength of the output light from the first light source unit 1. This light source unit 2 is a DFB-LD
It is composed of 31, a drive circuit 32 and a wavelength controller 33. The output intensity from the light source unit 2 is adjusted by the drive circuit 32, and the wavelength of the output light is adjusted by the wavelength controller 33. Hereinafter, the wavelength of the output light from the light source unit 2 will be λ2
It is written as (f2 in the optical frequency domain).

In the present invention, the output light from the first light source section 1 is divided into two and used as test signal light and local signal light. A first optical multiplexer / demultiplexer 3 branches the output light of the first light source unit 1 into a test signal light a and a local signal light b.
Reference numeral 4 denotes an acousto-optical switch (hereinafter, referred to as AO) for pulsating the branched test signal light a at a constant cycle and modulating the optical frequency.
It is called a switch). The optical modulation frequency of the AO switch at this time is 120 MHz. Reference numeral 5 is a rotary phase plate type polarization state controller in which two phase plates, a half wave plate and a quarter wave plate, are connected in series. By rotating each wave plate, an arbitrary polarization state can be obtained. Can output light.

Reference numeral 6 denotes an optical superimposing device, which is used for the second light source section 2
The output light c from the above is superposed on the test signal light pulse a.
The superimposed test signal light (a + c) is incident on the test optical fiber 8 by the second optical multiplexer / demultiplexer 7. The reflected light and the backscattered light d from the optical fiber 8 under test are
It is guided to the third optical coupler / splitter 9 via the second optical coupler / splitter 7. The optical multiplexer / demultiplexer 9 multiplexes the reflected light and the backscattered light d with the local signal light b. Reference numeral 10 denotes a photodetector for converting the beat signal light e of the reflected light and the backscattered light, which are combined, and the local signal light, into electric signals.

Reference numeral 11 converts a beat signal into a baseband signal, and a low frequency band transmission electric filter (hereinafter, L).
A signal converter including a PF). L at this time
When the cutoff frequency of the PF is B, the reception band of the optical pulse tester is B. Reference numeral 12 denotes a square addition processor that performs A / D conversion in a predetermined cycle, then performs square conversion, and adds the square-converted signals in a predetermined cycle to improve the SN ratio. Thirteen
Is a signal processor that subtracts the offset power value from the added signal, performs logarithmic conversion, and displays the longitudinal distribution of the intensities of the reflected light and the backscattered light d (hereinafter referred to as the OTDR waveform). Reference numeral 14 is a timing generator for pulsing the test signal light a at a constant cycle and for performing addition processing. The main controller 15 controls the wavelength controller 24, the drive circuit 32, the wavelength controller 33, the polarization state controller 5, and the timing generator 14.

Next, the operation of the present invention will be described.

FIG. 2 shows a test signal light (a + c) obtained by combining the output lights of the first light source unit 1 and the second light source unit 2 described above by the second optical multiplexer / demultiplexer 6. It is assumed that the pulse width of the AO switch 4 is W, and the predetermined period for outputting the pulse is T. Repeated at the cycle T, and the wavelength λ1
And the light of two wavelengths λ2 are output, and only the light of the wavelength λ2 is output during the other time (time region 2). The output intensity of the test signal light is adjusted by adjusting the intensity of the second light source unit 2 to suppress the variation of the intensity in the time region 1 and the time region 2.

The wavelength of the output light a of the first light source section 1 can be changed by the wavelength controller 24 at every predetermined cycle, and the polarization state of the test signal light pulse a is the polarization state. The controller 5 can adjust to any polarization state.
By using these, by changing the wavelength (that is, the optical frequency) and the polarization state of the test signal light during the addition process, it is possible to remove fading noise that occurs when coherent detection is performed using a narrow linewidth light source. (For details, see Japanese Patent Application No. 3-195317 and Japanese Patent Application No. 3-196491).

Considering in the wavelength region, the test signal light (a + c) is the wavelength-division multiplexed test light pulse of wavelength λ1 and continuous light of wavelength λ2. Therefore, when this test signal light (a + c) is incident on the optical fiber 8 under test, the backscattered light of wavelength λ1 and wavelength λ2 returns. Since the test signal light of wavelength λ1 is a pulse, if only the backscattered light of wavelength λ1 is separated and detected, an OTDR waveform representing the distribution of loss and reflection of the optical fiber 8 under test can be obtained. However, since the test signal light of the wavelength λ2 is continuous light, its backscattered light indicates the loss of the optical fiber 8 under test, etc.
It does not have a waveform. Since the intensity of the backscattered light is about T / W times larger, if the backscattered light of the wavelength λ2 is received without being separated, the sensitivity of receiving the wavelength λ1 is deteriorated and the dynamic range of the OTDR is reduced.

Therefore, in the present invention, the optical frequency f2 of the output light of the second light source unit 2 is adjusted so as to satisfy the conditional expression (1) represented by | f1-f2 | + Δf1> 2B (1). At this time, the backscattered light of wavelength λ2 (optical frequency f2) is removed by the LPF as a high frequency component when converted into a baseband signal by the signal converter 11. That is, the backscattered light of wavelength λ2 is out of the reception band of the receiver, and only the backscattered light of wavelength λ1 is received.

The results of experimentally confirming the effects of the present invention will be described.

The optical pulse tester of the present invention shown in FIG. 1 was constructed to measure the optical fiber 8 under test. The wavelength λ1 of the output light of the first light source unit 1 used here is 1.551 μm (f
1 = 193.426THz), the wavelength variation Δλ1 is about 2 nm (Δf1 = 0.2
50 THz). The wavelength λ2 of the output light of the second light source unit 2 is 1.
It is 554 μm (f2 = 193.050THz). Therefore, conditional expression (1)
Is fully satisfied. The pulse width W of the AO switch 4 is 1 μs and the pulse period is 4 ms. The intensity of the test signal light to the optical fiber under test 8 is about -5 dBm. The number of additions is 2 of 18
It is a ride. In order to reduce fading noise, λ1 was changed from 1.552 μm to 1.550 μm during the addition process, and the polarization state was changed in four ways every 2 16th addition. Ten
As a result of measuring the optical fiber 8 under test at 0 km, a dynamic range of about 24 dB was obtained. From the above, it is possible to obtain an optical pulse tester having a small fluctuation in the intensity of the test signal light between the time domain 1 and the time domain 2 and a large dynamic range.
The effectiveness of the present invention was confirmed.

The configuration of the second embodiment is shown in FIG. This embodiment has the same configuration, function and effect as those of the first embodiment except the points described below. The output light c from the second light source unit 2 enters from the 0th-order side input port of the AO switch 4, and is combined with the output light a from the first light source unit 1. At this time, the fluctuation of the light intensity of the test signal between time region 1 and time region 2 is about 0.3 dB, which is sufficiently small as the fluctuation of the intensity of the test signal light incident on the optical unrepeatered line.

As a result of measuring the optical fiber 8 under test of 100 km under the conditions shown in Example 1, the dynamic range was about 24.5 dB. In this embodiment, since the AO switch 4 also serves as the optical superimposing device 6 in FIG. 1, the number of parts is reduced and the structure is simplified. The configuration of the third embodiment is shown in FIG.
Shown in. In this embodiment, after the polarization state controller 5 of the second embodiment, an erbium-doped optical fiber amplifier (hereinafter,
41 referred to as EDFA). Other configurations are
This is similar to the above-mentioned embodiment. The amplification degree of the EDFA 41 was adjusted so that the test signal light intensity was about 6 dBm, which was almost the same as the communication light intensity of the optical repeater line. At this time, the fluctuation of the intensity of the test signal light between time domain 1 and time domain 2 is about 0.3d.
It was B.

In FIG. 5, under the conditions shown in the first embodiment,
The OTDR waveform which measured the optical fiber 8 under test of 140 km is shown. It can be seen from FIG. 5 that a dynamic range of about 30 dB was obtained.

From the above, an optical pulse tester with a small variation in the intensity of the test signal light and a large dynamic range was obtained, and the effectiveness of the present invention was confirmed.

The optical pulse tester of the present invention can be used as a conventional optical amplification coherent OTDR when the drive circuit 32 of the second light source section 2 is turned off.
Has a dynamic range of 35 dB or more.

[0033]

As described above, the claims 1 and 2 of the present invention are as follows.
According to 2 or 4, it is possible to provide an optical pulse tester in which the fluctuation of the intensity of the test signal light is small, the intensity is almost the same as the signal light intensity for communication, and the dynamic range is large. Further, according to claim 3, in addition to the above effects, there is an advantage that the number of parts is reduced and the configuration is simplified.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a test signal light of the present invention.

FIG. 3 is a block diagram showing a second embodiment of the present invention.

FIG. 4 is a block diagram showing a third embodiment of the present invention.

FIG. 5 is a diagram showing an OTDR waveform according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... 1st light source part, 2 ... 2nd light source part, 3 ... 1st optical multiplexer / demultiplexer, 4 ... Acousto-optic switch, 5 ... Polarization state controller,
6 ... Optical superimposing device, 7 ... 2nd optical combining / branching device, 8 ... Optical fiber under test, 9 ... 3rd combining / branching device, 10 ... Light receiving device, 11 ... Signal converter, 12 ... Square addition processing device, 13 ... signal processor,
14 ... Timing generator, 15 ... Main control unit, 21 ... Distributed feedback semiconductor laser, 22 ... Optical fiber for narrowing line width, 2
3 ... Optical isolator, 24 ... Wavelength controller, 31 ... DFB
-LD, 32 ... Driving circuit, 33 ... Wavelength controller, 41 ... E
DFA.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yahei Koyamada 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo Inside Nippon Telegraph and Telephone Corporation (72) Fumihiko Yamamoto 1-1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation

Claims (4)

[Claims] 1. A signal light generating means for the test signal light and the local signal light, and a pulse of the test signal light for each predetermined period to generate a test signal light pulse, and the test signal light pulse is applied to the optical fiber under test. Optical pulse generation means for sending to the optical fiber, an optical multiplexing means for optically multiplexing reflected light and backscattered light repeatedly returning from the optical fiber under test with the local signal light to obtain beat signal light, and the beat Opto-electric conversion means for receiving the signal light and converting it into an electric signal, electric signal processing means for performing addition processing of the electric signal, and waveforms of the reflected light and the backscattered light based on the result of the electric signal processing. An optical pulse tester provided with a display unit for displaying a second signal light having a wavelength different from that of the test signal light.
And a light superimposing means for superimposing the second signal light generated by the second signal light generating means on the test signal light pulse generated by the light pulse generating means. Characteristic optical pulse tester. 2. The second signal light generating means is the second signal light generating means.
2. The optical pulse tester according to claim 1, further comprising an optical output intensity control means for controlling the output intensity of the signal light. 3. The optical pulse tester according to claim 1, wherein the optical pulse generation means is composed of an acousto-optic switch, and the acousto-optic switch also serves as the optical superposition means at the same time. 4. The optical pulse tester according to claim 1, further comprising an optical amplification means for amplifying the signal light from the optical superposition means.