JPH0587683A - Photo pulse tester - Google Patents

Abstract

PURPOSE:To reduce the distortion of the measuring waveform due to noises by branching the coherent light to a signal light and a locally emitted light, combining the back scattering light generated when a modulated pulse signal obtained by modulating the signal light enters an optical fiber with the locally emitted light, and A/D converting and averaging a photoelectrically converted light of the detected combined light. CONSTITUTION:This tester is provided with a branching filter 2 for branching the coherent light, from a light source 1 to a signal light 11 and a locally emitted light 12, a modulated signal generator 3 for generating a plurality of modulated signals 16, and an optical modulator 4 which modulates the signal light 11 to a modulated pulse signal 13. When the modulated signal 16 generated in the generator 3 is applied to the modulator 4, the intensity of the signal light 11 is modulated, and consequently a modulated light pulse signal 13 is output. There are provided a branching filter 5 which takes out a back scattering light 14 generated when the signal 13 enters an optical fiber 10, an optical multiplexer 6 which combines the scattering light 14 and the locally emitted light 12, and a photodetector 7 which detects the combined light. Accordingly, the interference between the back scattering lights is averaged in a signal processing part, whereby the distortion of the measuring waveform can be reduced.

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 loss and break position of an optical fiber by using optical homodyne light reception or optical heterodyne light reception.

[0002]

2. Description of the Related Art Next, the structure of a conventional optical pulse tester will be described with reference to FIG. The configuration of FIG. 5 is a configuration using homodyne light reception. In FIG. 5, 21 is a coherent light source, 2 is an optical splitter, 5 is an optical splitter, 6 is an optical multiplexer, 7 is a photodetector, 8 is a signal processing unit, 10 is an optical fiber, 24 is an optical pulse modulator, 25 is a timing generator. 5 is the same as FIG. 8 of Japanese Patent Application No. 3-175980.

The light source 21 continuously emits coherent light having a stable optical frequency, and the optical splitter 2 splits the light emitted from the light source 21 into a signal light 11 and a local light 12. The optical pulse modulator 24 performs optical pulse modulation on the signal light 11 with a signal from the timing generator 25, and emits it as an optical pulse signal 22. The optical branching device 5 branches the backscattered light 14 from the optical fiber 10 to the optical multiplexer 6. The optical multiplexer 6 combines the backscattered light 14 from the optical fiber 10 and the local light 12,
The photodetector 7 performs optical homodyne detection on the signal light multiplexed by the optical multiplexer 6. The signal processing unit 8 amplifies the signal photoelectrically converted by the photodetector 7, performs A / D conversion, and averages and displays.

[0004]

In FIG. 5, when the backscattered light 14 from the optical fiber 10 is measured using the light source 21 with good coherency, the intensity noise of the backscattered light 14 increases and the measured waveform becomes Because of the distortion, the accuracy and accuracy are not sufficient for measuring the loss of the optical fiber and the position of the break point.
4A and 4A are models of measured waveforms when a light source with good coherency is used.

The optical pulse signal 13 spreads in the optical fiber 10 by its pulse width and travels forward to generate scattered light at each point of the optical fiber 10. Scattered light generated at a certain position and time within the pulse width is combined with scattered light that is generated and returned at another position and time further ahead and returns to the optical fiber 10 backward, and at another position and time. It returns while being combined with the scattered light generated in. As a result of the combination of the scattered lights, that is, cancellation and strengthening occur due to interference, the light intensity after the combination depends on the phase and amplitude of each scattered light. The manner of this interference differs depending on the scattering region where the synthesis occurs. This is considered to be one of the main causes of the intensity noise of the backscattered light 14.

In the present invention, when the signal light 11 emitted from the coherent light source 1 is modulated by the optical modulator 4,
By inputting the modulated optical pulse signal 13 generated by the plurality of modulated signals 16 generated from the modulated signal generator 3 into the optical fiber 10, the phase or magnitude when the light scattered at each position is combined is changed. Then, by changing the combined light intensity for each modulated optical pulse signal 13,
An object of the present invention is to provide an optical pulse tester that averages the light intensity of the backscattered light 14 generated by coherent light and reduces the distortion of the measurement waveform due to intensity noise.

[0007]

In order to achieve this object, in the present invention, a light source 1 for emitting continuous coherent light is provided.
And the coherent light from the light source 1 to the signal light 11 and the local light 12
An optical branching device 2 for branching the optical signal, a modulation signal generator 3 for generating a plurality of modulated signals 16, and an optical modulator 4 for modulating the signal light 11 branched by the optical branching device 2 into a modulated optical pulse signal 13.
An optical branching device 5 for extracting a backscattered light 14 generated by the modulated optical pulse signal 13 entering the optical fiber 10, a backscattered light 14 branched by the optical branching device 5, and a light branching device 2. An optical multiplexer 6 for multiplexing the local light 12
The optical detector 6 includes a photodetector 7 for detecting the light multiplexed by the optical multiplexer 6, and a signal processing unit 8 for amplifying, A / D converting, and averaging the signals optoelectrically converted by the photodetector 7.

[0008]

1 is a block diagram of the optical pulse tester according to the present invention. In contrast to FIG. 5, the modulation signal generator 3 is used instead of the timing generator 25 in FIG. 5, two or more kinds of modulation signals 16 are provided, and the modulation signal 16 is connected to the optical modulator 4. Further, instead of the optical pulse modulator 24 of FIG. 5, the optical modulator 4 is used in FIG. 1 and has a function of optical intensity modulation and optical phase modulation. The other configuration of FIG. 1 is the same as that of FIG. FIG. 1 shows a case where homodyne light reception is used. When heterodyne light reception is used, a modulator such as an acousto-optical element that shifts the optical frequency is further used as the light modulator 4.

Next, an example of the structure of the light modulator 4 shown in FIG. 1 will be described with reference to FIG. FIG. 2A uses a light intensity modulator for the light modulator 4. By applying the modulation signal 16 generated by the modulation signal generator 3 to the light intensity modulator 4, the light intensity of the signal light 11 is modulated and the modulated light pulse signal 13 is output. For the light intensity modulator 4, for example, if a Mach-Zehnder interferometer type light intensity modulator applying the electro-optic effect of LiNbO ₃ is used, the transmittance for incident light can be changed by the applied voltage.

FIG. 2A shows that an optical phase modulator is used for the optical modulator 4. The modulated signal 16a generated by the modulated signal generator 3 is applied to the optical intensity modulator 4a, and the modulated signal 16b generated by the modulated signal generator 3 is applied to the optical phase modulator 4b, thereby inputting to the optical modulator 4. The signal light 11 thus obtained is modulated into an optical pulse, which is then subjected to phase modulation, and a modulated optical pulse signal 13 is output. The optical phase modulator 4b includes
For example, if a single waveguide type optical phase modulation element applying the electro-optic effect of LiNbO ₃ is used, it is possible to change the phase of incident light by the applied voltage and output.

Next, the relationship between the modulated signal 16 and the modulated optical pulse signal 13 according to the configuration example of FIG. 2 will be described with reference to FIG. FIG. 3A shows how the modulated light pulse signal 13 is emitted as light whose intensity is modulated by the modulated signal 16 of FIG. The horizontal axis shows time, and the horizontal axes of the two graphs show the same time. Here, an example in which three types of modulation are performed is shown.

FIG. 3A shows the modulated signals 16a and 16a of FIG.
6B shows that the modulated light pulse signal 13 is converted into a pulse by 6b and is emitted as phase-modulated light. The horizontal axis shows time, and the horizontal axes of the three graphs show the same time. Here, an example in which three types of modulation are performed is shown.

Next, it will be described with reference to FIG. 4 that the fluctuation of the measured waveform is reduced by averaging. FIG. 4A shows the measured waveform when the modulated signal is as shown in FIG. 3A, and FIG. 4A is the measured waveform when the modulated signal is as shown in FIG. 3B. The horizontal axis of FIG. 4 is the time after the modulated light pulse signal 13 is emitted, and corresponds to the distance to the region of the fiber where the scattered light is generated. 4A and FIG. 4A show different states of interference between scattered lights.
In the distance in which the strengthening occurs in A, the strengthening does not always occur in the same distance in FIG. On the contrary, at the distance where the cancellation occurs in FIG. 4A, the cancellation does not always occur even at the same distance in FIG. 4A. Therefore, collecting a lot of such data,
As the averaging is performed, the results of various interferences are averaged, so that a waveform with less distortion can be obtained as shown in FIG. 4C.

[0014]

According to the present invention, since two or more kinds of modulated light pulse signals are used, the intensity noise of the backscattered light generated due to the high coherency of the light source averages out various scattered light interferences. By doing so, it is possible to improve the accuracy and accuracy in the measurement of the loss and break point position of the optical fiber.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical pulse tester according to the present invention.

2 is a configuration diagram of an optical modulator 4 and a modulation signal generator 3 of FIG.

3 is a relational diagram between the modulated signal 16 and the modulated optical pulse signal 13 of FIG.

FIG. 4 is a measurement waveform diagram when the modulated optical pulse signal 13 is different.

FIG. 5 is a block diagram of a conventional optical pulse tester.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 light source 2 optical splitter 3 modulation signal generator 4 optical modulator 5 optical splitter 6 optical multiplexer 7 photodetector 8 signal processor 10 optical fiber 11 signal light 12 local light 13 modulated light pulse signal 14 backscattered light 16 Modulation signal

Claims (1)

[Claims] 1. A light source for emitting continuous coherent light
(1), a first optical branching device (2) that branches the coherent light of the light source (1) into a signal light (11) and a local light (12), and a modulation signal that generates a plurality of modulation signals (16) A generator (3), an optical modulator (4) that modulates the signal light (11) split by the first optical splitter (2) into a modulated optical pulse signal (13), and a modulated optical pulse signal The second optical branching device (5) for extracting the backscattered light (14) generated by making the (13) incident on the optical fiber (10), and the backscattering light branched by the second optical branching device (5) Light (14) and first
The optical multiplexer (6) that combines the local light (12) that has been split by the optical splitter (2) and the photodetector (7) that detects the light that has been multiplexed by the optical multiplexer (6).
And the signal photo-electrically converted by the photodetector (7) is amplified and A / D
An optical pulse tester comprising a signal processing unit (8) for converting and averaging.