JP2731320B2 - 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 (hereinafter referred to as an OTDR (Optical Time Tester) for testing characteristics such as optical loss of an optical fiber and an optical fiber line as a transmission medium of an optical communication signal. Domain Reflectomete
r)).

[0002]

2. Description of the Related Art An OTDR is a device for transmitting an optical pulse to an optical fiber under test, receiving and analyzing reflected light and backscattered light from the optical fiber under test, and displaying characteristics such as optical loss on a CRT or the like. is there. This is a very useful tool because the characteristics of the optical fiber under test can be tested from one end. Therefore, research and development for expanding the measurable range of the OTDR (this is called a dynamic range) has been conventionally performed. In order to expand the dynamic range, a method of increasing the intensity of a pulse transmitted to the optical fiber under test and a method of improving the receiving sensitivity of backscattered light or the like are mainly used. As one method for improving reception sensitivity, application of a coherent detection technique such as heterodyne or homodyne reception is being studied.

Conventional OT using coherent detection technology
DR will be described with reference to FIG. In FIG. 4, the test signal light and the local signal light are generated by the same light source, and the test signal light side is optically frequency-modulated to perform heterodyne reception. In FIG. 4, reference numeral 1 denotes a narrow line width light source that generates a test signal light and a local signal light having a narrow line width spectrum, and 2 denotes a first light source that branches the light emitted from the light source 1 into a test signal light a and a local signal light b. 3 is an acousto-optic switch (hereinafter referred to as an AO switch) for pulsating the branched test signal light at a constant period and modulating the optical frequency, and 4 is an optical fiber under test 5 for the pulsed test signal light. And a second branching device 6 for guiding the reflected light and the backscattered light c from the optical fiber to the tester, and the reflected light and the backscattered light c guided into the tester and the local signal light b. A light receiving device 7 for optically-to-electrically converting the combined reflected light and beat signal light d of the backscattered light and the local signal light; 8, an electric signal amplifier; 9 is the amplified electric signal and local oscillation Mixer for mixing the local electrical signals generated by the 10, 11
, An electric filter for blocking high-frequency signals; 12, a signal converter for analog-to-digital conversion;
Reference numeral 4 denotes a CRT for displaying the longitudinal distribution of reflected light and backscattered light intensity, and reference numeral 15 denotes a timing generator necessary for pulsing the test signal light at a constant period or performing addition processing. Reference numerals 8 to 13 constitute an electric processing system.

In the OTDR using such a coherent detection technique, a beat signal light d obtained by combining a local signal light b having a relatively large intensity with a weak backscattered light c is received and detected. The intensity Ad of the beat signal light is
Assuming that the backscattered light intensity is Ac and the local signal light intensity is Ab, it is proportional to the square root of the product of Ac and Ab. Therefore, by making the local signal light intensity Ab relatively large, even weaker backscattered light can be received,
The dynamic range as the OTDR can be expanded.

Usually, a wavelength of 1.3 μm used for optical communication,
At 1.55 μm, it is necessary to drive the AO switch at a frequency of about 100 MHz in order to use the AO switch as an optical frequency modulator. Therefore, the optical frequency shifts by about 100 MHz (here, it is just 100 MHz). As a result, the beat signal light d obtained by multiplexing the backscattered light and the reflected light with the local signal light in the third multiplexer / demultiplexer is subjected to photoelectric conversion in the light receiver 7 and then its spectrum is observed. An intermediate frequency signal (IF signal) having a sideband component around 100 MHz. In order to convert this IF signal into a baseband signal at an electric level and process the signal, and finally display a backscattered waveform or the like on a CRT,
First, the IF signal is amplified by the amplifier 8 and mixed with the local electric signal from the local oscillator 10 by the mixer 9. Furthermore, the high frequency component of the baseband signal is cut off by the electric filter 11 and input to the signal converter 12.

The sideband of the beat signal is 1 / W when the optical pulse width of the test signal light is W, and is 1 MHz when the optical pulse width is 1 μs, for example. Therefore,
100.1MHz when the light pulse width is 1μs
Alternatively, it is set to about 99.9 MHz, and an electric filter that cuts off 1 MHz or more is used. With such a configuration, the amplifier 8 needs to have a large amplification performance up to a high frequency band of 100 MHz band and low noise.

[0007]

SUMMARY OF THE INVENTION Such a 100 MH
It is more difficult to produce an amplifier with low noise and high gain characteristics up to the band of z or more than the low frequency amplifier, and it is difficult to achieve the shot noise limit of the coherent detection method due to the thermal noise generated by the amplifier. There was a problem. As a result, it has been an obstacle to increase the dynamic range of the OTDR. Further, the local oscillator and the mixer also need to have good characteristics at a high frequency of the 100 MHz band, and such a device has a disadvantage that it is expensive.

An object of the present invention is to reduce the beat signal frequency at the optical level in an OTDR using a coherent detection method, thereby facilitating the design of an electric system including an amplifier and realizing an OTDR having a large dynamic range. Things.

[0009]

According to the present invention, there is provided an OTDR using a coherent detection system, wherein a plurality of optical frequency modulators are provided for a test signal light side, a local signal light side, or both light sides. The individual optical frequency shifts are made positive and negative, the frequency of the beat signal light obtained by multiplexing the backscattered light and the reflected light with the local signal light is lowered, and the required band of the amplifier used after the photoelectric conversion is reduced. It is to narrow. When a local oscillator or a mixer is used, these required bands are also narrowed.

When the AO switch is used as an optical frequency modulator, the sign of the optical frequency shift can be easily realized by changing the arrangement of the piezoelectric element and the ultrasonic absorber in the AO switch. As a configuration of the OTDR using the coherent detection method, two AO switches are arranged in tandem on the test signal light side, and the first one performs optical frequency modulation and pulsing, and the second one performs optical frequency modulation. In this case, the driving frequency of the first AO switch is 100 HMz, the shift is positive, and the driving frequency of the second AO switch is 10
Assuming that the shift is 0.1 MHz and the shift is negative, the backscattered light and the reflected light from the test optical fiber are shifted by 0.1 MHz negative. When the backscattered light and the reflected light are combined with the local signal light, the beat signal has a beat frequency of 0.1.
MHz.

On the other hand, when an optical switch having no optical frequency modulation function is provided on the test signal light side and two AO switches are provided in tandem on the local signal light side, the first switch on the local signal light side is used. By making the frequency shift of the second AO switch positive and negative, the beat signal frequency can be made smaller than in the case of a single AO switch.

Further, when the AO optical switch is provided on the test signal light side and the local signal light side, by using ones having the same optical frequency shift code and slightly different shift amounts, for example, at 100 MHz and 100.1 MHz, The beat signal frequency can be set to 0.1 MHz by using both of them that shift positively.

[0013]

When a plurality of optical frequency modulation means are provided only on the test signal light side or the local signal light side, the frequency shift is made positive and negative, and when a plurality of optical frequency modulation means are provided on the test signal light side and the local signal light side. By setting the frequency shift to positive or negative, the frequency of the beat signal obtained by multiplexing the backscattered light and the reflected light with the local signal light can be reduced. Therefore, a circuit that detects and amplifies beat signal light only needs to operate at a low frequency.

[0014]

FIG. 1 is a diagram for explaining a first embodiment of the present invention, in which 21 is a first AO switch, and 22 is a second AO switch.
The switch 23 is an amplifier operating in a several MHz band. The optical frequency shift of each AO switch is positive in the first AO switch 21 and negative in the second AO switch 22.

The configuration and operation of the coherent detection OTDR of the present invention will be described. By connecting an optical fiber to a semiconductor laser such as a DFB-LD or the like, the light emitted from the narrow linewidth light source 1 whose spectral linewidth has been narrowed is converted into a test signal light a and a local signal light by a first coupler 2. b. The branched test signal light a is pulse-pulsed at a fixed cycle by the first AO switch 21 and optically frequency-modulated to the positive side. Further, the second AO switch 22 is optically frequency-modulated to the negative side. The light enters the optical fiber under test 5 via the branching device 4.

The reflected light and the backscattered light c from the optical fiber under test 5 are guided to the third coupler 6 via the second coupler 4. The local signal light b is also guided to the third multiplexer / demultiplexer 6, and is multiplexed with the reflected light and the backscattered light c. The multiplexed beat signal light d is a double-balanced PI
The light-to-electric conversion is performed by a light receiver 7 such as an N-FET. The electrical converted beat signal is input to the signal converter 12 after being amplified by the amplifier 23.

The signal converter 12 performs analog-digital conversion and further square conversion, adds the signals to improve the SN in the signal processor 13, performs logarithmic conversion, and displays the signals on the CRT 14 as a temporal change, that is, a distribution in the longitudinal direction. You. The timing generator 15 supplies a trigger signal for pulsing the test signal light to the drive unit of the first AO switch 21 and supplies a timing signal to the signal converter 12 and the signal processor 13.

As the first AO switch 21, a piezoelectric element and an ultrasonic absorber arranged so as to generate a positive optical frequency shift are used. As the second AO switch 22, a piezoelectric element and an ultrasonic absorber are used. Are used, the arrangement of which causes a negative optical frequency shift. The driving frequency of the AO switch is, for example, 1
Set to 00 MHz and 100.1 MHz. Then, the test signal pulse incident on the optical fiber under test has an optical frequency shifted to the negative side by 0.1 MHz as compared with the emitted light from the narrow line light source.

The backscattered light and reflected light from the optical fiber under test also have an optical frequency shift to the negative side of 0.1 MHz.
Therefore, the optical frequency of the beat signal light obtained by combining the backscattered light and reflected light c from the optical fiber under test and the local signal light b is shifted to the negative side by 0.1 MHz.
That is, the absolute value of the optical frequency of the beat signal light is 100 M of the absolute value of the optical frequency shift amount of the first AO switch 21.
Hz, and the absolute value of the optical frequency shift amount of the second AO switch 22 is smaller than 100.1 MHz.

The beat signal obtained by subjecting this beat signal light to light-to-electric conversion has a center frequency of 0.1 MHz.
When the optical pulse width is 1 μs, the sideband of the beat signal (= spectral width) is 1 MHz as described above, so that the bandwidth of the amplifier 23 of about 1 MHz is sufficient.
In the conventional configuration shown in FIG. 4, since the frequency of the beat signal is 100 MHz, an amplifier for amplifying up to the 100 MHz band is required. However, such an amplifier is not required in the present invention. Is sufficient. The design and manufacture of amplifiers with high dynamic range, good linearity and low noise are more difficult and expensive at higher frequencies. Further, a mixer, a local oscillator, and an electric filter which are conventionally used become unnecessary.

Therefore, according to the present invention, there is an advantage that the design and manufacture of the OTDR are facilitated and the cost can be reduced. Further, since the noise generated in the amplifier can be reduced, there is an advantage that the dynamic range of the OTDR can be increased. FIG. 2 shows a second embodiment of the present invention. Second AO switch 2
2 is used as a branching device with the optical fiber 5 to be tested and as an optical frequency modulator for reflected light and backscattered light from the optical fiber 5 to be tested. The sign and magnitude of the optical frequency shift are the same as in the first embodiment.

The 0th-order light side (the side connected when the drive signal is off) of the second AO switch 22 is connected to the first AO switch 2.
1, and the primary light side (the side connected when the drive signal is on) is connected to the third coupler 6. The drive signal for the second AO switch 22 is supplied from the timing generator 15, and the time during which the test signal light pulse passes through the second AO switch is off, and the other times are on. By doing so, the reflected light and the backscatter from the optical fiber under test 5 are guided to the third coupler 6 and the optical frequency thereof is modulated.

In the present embodiment, the optical frequency of the test signal light pulse incident on the optical fiber under test 5 and the reflected light and backscattered light generated by the optical fiber under test are shifted to 100 MHz positive. The reflected light and the backscattered light guided to the third branching device 6 via the AO switch 22 are re-modulated and have a frequency shifted to the negative side of 0.1 MHz. Accordingly, similarly to the first embodiment, since the beat signal is obtained around 0.1 MHz, the amplifier 23
Band of several MHz is sufficient. Further, as in the first embodiment, a mixer, a local oscillator, and an electric filter are not required. Further, in the present embodiment, the second multiplexer / demultiplexer is not required, so that the configuration is further simplified.

FIG. 3 shows a third embodiment of the present invention. Second
AO switch 22 is disposed on the local signal light side. The signs of the optical frequency shifts are both positive, and the shift amount is an example where the first AO switch is 100 MHz and the second AO switch is 105 MHz. Optical fiber under test 5
The reflected light and the backscattered light from the optical signal are shifted optically to the positive side of 100 MHz, and the local signal light is shifted optically to the positive side of 105 MHz, so that the beat signal multiplexed by the third coupler 6 is 5 MHz. IF signal. Therefore, in the electric system, the signal is dropped to a baseband signal by the mixer 24 and the local oscillator 25. The configuration of the electric system is the same as that of the conventional example, but since the IF frequency is low, the amplifier 23,
The operating frequencies of the mixer 24 and the local oscillator 25 can be reduced.

As a result, an amplifier, a mixer, a local oscillator,
The design and manufacture of the electric filter becomes easier than before. Also,
Since the noise generated in the amplifier can be reduced, there is an advantage that the dynamic range of the OTDR can be increased.
Also in the present embodiment, the absolute value of the optical frequency of the beat signal light is smaller than the absolute value of the optical frequency shift amount of the first AO switch 21 of 100 MHz, and the second AO switch 2
The absolute value of the optical frequency shift amount of No. 2 is smaller than 105 MHz.

Of the above-described embodiments, in the first and second embodiments, the optical frequency modulation means is provided only on the test signal light side. In the third embodiment, the optical frequency modulation means is provided. Is shown on the test signal light side and the local signal light side. However, even when the optical frequency modulation means is provided only on the local signal light side, the absolute value of the optical frequency of the beat signal light is The optical frequency shift amount of each optical modulator can be made smaller than the absolute value, and the same effects as those of the first to third embodiments can be obtained.

[0027]

As described above, according to the present invention, in the OTDR using the coherent detection method, a plurality of optical frequency modulators are provided for the test signal light side, the local signal light side, or both light sides. And the individual optical frequency shifts are made positive and negative, and the frequency of the beat signal light obtained by multiplexing the backscattered light and the reflected light with the local signal light is reduced, so that it is used after photoelectric conversion. The required band of the amplifier can be narrowed. When a local oscillator or a mixer is used, these required bands can be narrowed.

By using the present invention, 100 MHz
An amplifier that amplifies the OTDR is not required, and an amplifier up to a few MHz band is sufficient, and a mixer, a local oscillator, and an electric filter can be eliminated, so that the design and manufacture of the OTDR are easy and inexpensive. Also, since the noise generated in the amplifier can be reduced, there is an advantage that the dynamic range of the OTDR can be increased.

[Brief description of the drawings]

FIG. 1 shows a first embodiment of the present invention.

FIG. 2 shows a second embodiment of the present invention.

FIG. 3 shows a third embodiment of the present invention.

FIG. 4 shows an example of a conventional coherent detection OTDR configuration

[Explanation of symbols]

 1: Narrow line width light source that generates light having a narrow line width spectrum 2: First branching device 3: Acousto-optic switch (AO switch) 4: Second branching device 5: Optical fiber under test 6: Third 7: optical receiver 8: amplifier 9: mixer 10: local oscillator 11: electric filter 12: signal converter 13: signal processor 14: CRT 15: timing generator 21: first AO switch 22: first 2 AO switch 23: Amplifier 24: Mixer 25: Local oscillator

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Izumi Mikawa 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-4-279834 (JP, A)

Claims (1)

(57) [Claims] 1. A means for generating test signal light and local signal light, means for pulsating the test signal light at a fixed period and repeatedly sending it to the optical fiber under test, and repeatedly returning from the optical fiber under test. Means for multiplexing the reflected light and the backscattered light with the local signal light, light detecting means for detecting the combined repetitive beat signal light and converting it to an electric signal, and processing the repeatedly obtained electric signal to process the electric signal. An electrical signal processing system for displaying the waveform of the reflected light or the backscattered light from the optical fiber under test; timing generation means for performing repetitive processing; and either or both of the test signal light side and the local signal light side An optical pulse tester comprising: a plurality of optical modulators having different optical frequency shift amounts; The number shift is positive or negative, OTDR, wherein the absolute value of the optical frequency of the beat signal light is smaller than the absolute value of the optical frequency shift amount of each of the optical modulator.