JP3327416B2 - 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 testing characteristics such as optical loss of an optical fiber and an optical fiber line as a transmission medium of an optical signal, and more particularly, to a repeater optical line including an optical amplifier. It is applied as a test technique.

[0002]

2. Description of the Related Art In order to realize an economical optical communication system, a repeaterless long distance optical communication system is being studied.
This is to dispose an optical amplifier between communication optical fiber lines to optically amplify communication light attenuated while propagating through an optical fiber, thereby extending a 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 referred to as an EDFA (Erbium-dope) having a large amplification degree in a communication wavelength band.
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.

[0003] In order to construct a highly reliable and economical optical amplification repeaterless line, it is necessary to measure and test the characteristics of this line using a highly reliable tester for all lines.

An optical pulse tester (hereinafter referred to as OTDR (Optical
A Time Domain Reflectometer) transmits an optical pulse to the optical fiber under test, receives reflected light and backscattered light from the optical fiber, analyzes the light, and displays characteristics such as optical loss on a CRT or the like. This device is a very useful tester because it can be tested by inputting and outputting light from one end of an optical fiber. For this reason, research and development for increasing the OTDR measurable distance (this is called a dynamic range) has been conventionally performed.

[0005] To expand the dynamic range,
A method of increasing the intensity of the transmitted light pulse to the optical fiber under test and a method of improving the receiving sensitivity of backscattered light or the like are mainly used. As a technique for increasing the transmitted light pulse intensity, an optical amplification technique using an EDFA or the like is used. As a method for improving the reception sensitivity, a coherent detection technique such as heterodyne or homodyne reception is used. An OTDR that combines these optical amplification techniques and coherent detection techniques (hereinafter referred to as optically amplified coherent OTDR) has a dynamic range of 35 dB or more. This is a normal OT using the direct detection method.
The dynamic range is larger than DR by 15dB or more.

When testing an optical amplification repeaterless line using an OTDR, an optical amplifier also amplifies an optical pulse transmitted from the OTDR. Therefore, the minimum required dynamic range for OTDR corresponds to the optical fiber line loss between optical amplifiers. Normally, the optical amplifier repeat interval is about 100km
Therefore, a dynamic range of about 21 dB or more corresponding to the loss is required. Then, application of the optical amplification coherent OTDR is considered.

[0007]

In an optically amplifying non-repeated line, a part of the output intensity in an EDFA is measured by a photodetector, and the output intensity of an excitation light source of the EDFA is controlled using a feedback circuit. The output intensity of the signal light for later communication is kept constant. Therefore, when light that is significantly higher than the communication output light intensity enters the EDFA, the light receiver that measures a part of the amplified output light intensity is saturated, and the output intensity of the EDFA by the feedback circuit is saturated. There has been a problem that the control may be abnormal or the light receiver may be destroyed. Also, E
When an optical pulse having 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 the continuous light, and a part of the output light of the EDFA is measured. However, there is a problem that an existing light receiver is destroyed. Therefore, it is difficult to apply a tester that outputs high-intensity optical pulses, such as the optically amplified coherent OTDR, to an optically amplified non-repeater line.

As described above, as a test technique for an optical amplification repeaterless line, the intensity fluctuation of the test signal light incident on the optical amplification repeaterless line is small, and the intensity is almost equal to the signal light intensity for communication. An optical pulse tester with a large dynamic range is required.

SUMMARY OF THE INVENTION In view of the above problems, it is an object of the present invention to reduce the fluctuation of the intensity of a test signal light incident on an optically amplifying non-repeater line, the intensity of which is substantially the same as the signal light intensity for communication, and An object of the present invention is to provide an optical pulse tester having a large dynamic range.

[0010]

According to the present invention, in order to achieve the above object, a signal light generating means for generating test signal light and local signal light is provided. Optical pulse that is converted into a test signal optical pulse
Generating means and the beat signal the test signal light pulse multiplexes repeated returning reflected light and backscattered light from the tested optical fiber when delivered under test optical fiber and said local signal light optically Optical multiplexing means for obtaining light, photoelectric conversion means for receiving the beat signal light and converting it to an electric signal, electric signal processing means for adding this electric signal, and the like, based on the result of the electric signal processing A display means for displaying the waveforms of the reflected light and the backscattered light, comprising a second signal light generating means for generating signal light having a wavelength different from the test signal light, It has a light superimposing means for superimposing the continuous light serving as the second signal light generated by the second signal light generation means to said test signal pulse generated by the pulse generator means, the tested light off
Instead of simply sending a test signal light pulse to the fiber,
The test signal light pulse and the second signal related to the superposition
Light was sent . Further, in claim 2, the second
The signal light generating means includes light output intensity control means for controlling the output intensity of the second signal light. According to a third aspect of the present invention, the light pulse generating means is constituted by an acousto-optic switch, and the acousto-optic switch also serves as the light superimposing means at the same time. According to a fourth aspect of the present invention, there is provided an optical amplifying unit for amplifying the signal light from the optical superimposing unit.

[0011]

According to the first aspect of the present invention, since the second signal light having a different wavelength from the test signal light is superimposed on the light pulse of the light pulse generating means by the light superimposing means, the intensity fluctuation of the test signal light is changed. Is small, and the intensity can be adjusted to approximately the same level as the signal light intensity for communication. Therefore, the ED of the optical amplification non-repeated line
The test can be performed without causing an abnormality in the output intensity control of the FA.

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, since the acousto-optic switch can serve both as the light pulse generating means and the light superimposing means, the size of the device can be reduced.

According to the fourth aspect of 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 high, it can be adjusted to the same level by adjusting the amplification degree. it can.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described with reference to FIG.

Reference numeral 1 denotes a first light source unit. In order to perform coherent detection, a light source that generates light with a narrow linewidth spectrum of several KHz is indispensable.
The light source unit 1 includes a distributed feedback semiconductor laser 21 (hereinafter, DFB).
-LD), an optical fiber 22 for narrowing the line width, and an optical isolator 23. An optical fiber 22 for narrowing the line width of about 1 km is fusion-spliced to the output end of the DFB-LD 21, and the line width is narrowed by using the backscattered light from the optical fiber 22. Has a line width of 10 KHz or less. 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 domain), and the wavelength variable amount is Δλ1.
(Δf1 in the optical frequency domain).

Reference numeral 2 denotes a second light source unit. The output light from the light source unit 2 outputs 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 comprises a drive circuit 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 is λ2
(F2 in the optical frequency domain).

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

Reference numeral 6 denotes an optical superimposer, and the second light source unit 2
Is superimposed 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 under test 8 are:
The light is guided to the third optical splitter 9 via the second optical splitter 7. The optical coupler 9 combines the reflected light and the backscattered light d with the local signal light b. Reference numeral 10 denotes a light receiver for optically / electrically converting beat signal light e of the combined reflected light and backscattered light and the local signal light.

Numeral 11 converts a beat signal into a baseband signal and a low-frequency transmission electric filter (hereinafter referred to as L).
PF). L at this time
Assuming that 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 and then performs square conversion, and adds the square-converted signal in a predetermined cycle to improve the SN ratio. 13
Is a signal processor that subtracts the offset power value from the signal subjected to the addition processing, performs logarithmic conversion, and displays the longitudinal distribution (hereinafter, referred to as an OTDR waveform) of the intensity of each of the reflected light and the backscattered light d. Reference numeral 14 denotes a timing generator for pulsing the test signal light a at a constant period or performing an addition process. The main controller 15 controls the wavelength controller 24, the drive circuit 32 and 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 multiplexing the output lights of the first light source unit 1 and the second light source unit 2 by the second optical coupler 6. It is assumed that the pulse width of the AO switch 4 is W, and a predetermined period for outputting a pulse is T. The wavelength λ1 is repeated at a period T and only for the time of the pulse width W (time region 1).
At this time, only light of wavelength λ2 is output. The output intensity of the test signal light was adjusted by adjusting the intensity of the second light source unit 2 to suppress the fluctuation of the intensity in the time domain 1 and the time domain 2.

The wavelength of the output light a of the first light source unit 1 can be changed at predetermined intervals by the wavelength controller 24, and the polarization state of the test signal light pulse a is changed to the polarization state. An arbitrary polarization state can be adjusted by the controller 5.
By using these, by changing the wavelength (that is, the optical frequency) and the polarization state of the test signal light during the addition processing, it is possible to remove fading noise generated when coherent detection is performed using a narrow linewidth light source. (For details, see Japanese Patent Application Nos. 3-195317 and 3-96491).

The test signal light (a + c) is a wavelength multiplex of a test light pulse having a wavelength λ1 and a continuous light having a wavelength λ2 when considered in the wavelength region. Therefore, when the test signal light (a + c) is incident on the optical fiber under test 8, the backscattered light having the wavelengths λ1 and λ2 returns. Since the test signal light of the wavelength λ1 is a pulse, if only the backscattered light of the wavelength λ1 is separated and detected, an OTDR waveform representing the loss and reflection distribution of the optical fiber 8 to be tested can be obtained. However, since the test signal light having the wavelength λ2 is continuous light, the backscattered light thereof is an OTDR representing the loss of the optical fiber 8 under test.
It does not have a waveform. Since the backward scattered light intensity is about T / W times larger, if the backscattered light having the wavelength λ2 is received without being separated, the sensitivity for receiving the wavelength λ1 is degraded, and the dynamic range of the OTDR is reduced.

Therefore, in the present invention, the optical frequency f2 of the output light from 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, when the backscattered light having the wavelength λ2 (optical frequency f2) is converted into a baseband signal by the signal converter 11, it is removed by the LPF as a high frequency component. That is, the backscattered light having the wavelength λ2 is out of the receiving band of the receiver, and only the backscattered light having the wavelength λ1 is received.

The result of experimentally confirming the effect of the present invention will be described.

An optical pulse tester according to the present invention shown in FIG. 1 was constructed, and an optical fiber under test 8 was measured. The wavelength λ1 of the output light of the first light source unit 1 used here is 1.551 μm (f
1 = 193.426 THz) and the wavelength change amount Δλ1 is about 2 nm (Δf1 = 0.2
50THz). The wavelength λ2 of the output light of the second light source unit 2 is 1.
It is 554 μm (f2 = 193.050 THz). Therefore, conditional expression (1)
Satisfying enough. The pulse width W of the AO switch 4 is 1 μs and the pulse period is 4 ms. The test signal light intensity to the optical fiber under test 8 is about -5 dBm. 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 processing, and the polarization state was changed four times every 2 16 times of addition. Ten
As a result of measuring the optical fiber under test 8 of 0 km, a dynamic range of about 24 dB was obtained. As described above, an optical pulse tester having a small variation in the intensity of the test signal light between the time domain 1 and the time domain 2 and a large dynamic range can be obtained.
The effectiveness of the present invention was confirmed.

FIG. 3 shows the configuration of the second embodiment. This embodiment has the same configuration, operation and effect as those of the first embodiment except for the points described below. The output light c from the second light source unit 2 enters from the 0th-order 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 the time domain 1 and the time domain 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 non-repeater line.

The dynamic range was about 24.5 dB as a result of measuring the optical fiber under test 8 of 100 km under the conditions shown in the first embodiment. In this embodiment, since the AO switch 4 also serves as the optical superimposer 6 in FIG. 1, the number of components is reduced and the configuration is simplified. FIG. 4 shows the configuration of the third embodiment.
Shown in In this embodiment, after the polarization state controller 5 of the second embodiment, an erbium-doped optical fiber amplifier (hereinafter, referred to as an “erbium-doped fiber amplifier”) is used.
EDFA) 41 was inserted. Other configurations are
This is the same as the previous embodiment. By adjusting the amplification of the EDFA 41, the test signal light intensity was set to approximately 6 dBm, which is almost the same as the communication light intensity of the optical non-repeater line. At this time, the variation of the intensity of the test signal light between the time domain 1 and the time domain 2 is about 0.3 d.
B.

FIG. 5 shows the condition shown in the first embodiment.
5 shows an OTDR waveform obtained by measuring the optical fiber under test 8 of 140 km. FIG. 5 shows that a dynamic range of about 30 dB was obtained.

As described above, an optical pulse tester having a small fluctuation in the intensity of the test signal light and a large dynamic range was obtained, and the effectiveness of the present invention was confirmed.

When the driving circuit 32 of the second light source unit 2 is turned off, the optical pulse tester of the present invention can be used as a conventional optically amplified coherent OTDR,
Has a dynamic range of 35dB or more.

[0033]

As described above, according to the first aspect of the present invention,
According to (2) or (4), it is possible to provide an optical pulse tester in which the intensity fluctuation 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. According to the third aspect, in addition to the above-described effects, there is an advantage that the number of parts is reduced and the configuration is simplified.

[Brief description of the 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 according to 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 coupler / branch, 4 ... Acousto-optic switch, 5 ... Polarization state controller,
6 optical superimposing device, 7 second optical multiplexer / demultiplexer, 8 optical fiber under test, 9 third optical multiplexer / demultiplexer, 10 photoreceiver, 11 signal converter, 12 square addition processor, 13 ... signal processor,
14 timing generator, 15 main controller, 21 distributed feedback semiconductor laser, 22 optical fiber for narrowing line width, 2
3: optical isolator, 24: wavelength controller, 31: DFB
-LD, 32: drive circuit, 33: wavelength controller, 41: E
DFA.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Yahei Oyamada, Inventor 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Fumihiko Yamamoto 1-16-1 Uchisaiwaicho, Chiyoda-ku, Tokyo Japan (56) References JP-A-4-72540 (JP, A) JP-A-4-132932 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01M 11 / 02

Claims (4)

(57) [Claims] A signal light generating means for generating test signal light and local signal light; an optical pulse generating means for generating a test signal light pulse by pulsing the test signal light at predetermined intervals; When sent to the optical fiber under test
And optical multiplexing means for obtaining a beat signal light repeating returning the reflected light and backscattering light and the local signal light from the tested optical fiber optically multiplexed electrical signal by receiving the beat signal light A photoelectric conversion means for converting the electric signal into an electric signal, an electric signal processing means for adding the electric signal, and a display means for displaying the waveforms of the reflected light and the backscattered light based on the result of the electric signal processing. A second optical pulse tester that generates a signal light having a wavelength different from the test signal light.
And a light superimposing means for superimposing the second signal light, which is a continuous light, generated by the second signal light generating means on the test signal light pulse generated by the light pulse generating means. Yes, and simply sends the test signal light pulse the under test optical fiber
Alternatively, the test signal light pulse related to the superposition
And an optical pulse tester for transmitting a second signal light . 2. The method according to claim 1, wherein the second signal light generating means includes:
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. An optical pulse tester according to claim 1, wherein said optical pulse generating means comprises an acousto-optical switch, and said acousto-optical switch also serves as said optical superimposing means at the same time. 4. The optical pulse tester according to claim 1, further comprising an optical amplifier for amplifying the signal light from the optical superimposing unit.