JPH04138332A - Optical pulse tester - Google Patents

Abstract

PURPOSE:To measure the loss characteristic of a light transmitting path by using the output value of the optical pulse from a variable-wavelength light source, and correcting the level of the reflected light from an optical fiber to be measured. CONSTITUTION:In a light source control means 2, an angle theta of a diffraction grating 3A with respect to an intended wavelength and a pulse current IL from a semiconductor laser 3B are read out of a light-source control memory 1. The grating 3A is moved by a specified angle by the angle theta. Optical pulses having the specified wavelength is oscillated in a variable-wavelength light source 3. The optical pulses outputted from the light source 3 are split in a second light splitting means 4. One is transformed into a level correcting signal through a second photodetector 5. The optical pulses passed through the means 4 are outputted into an optical fiber to be measured 8 through ports A and B of a first light splitting means 6 and a connecting terminal 7. The reflected light from the optical fiber 8 is corrected by the signal from the photodetector 5 in a level correcting means 11 through the ports B and C of the means 6, a first photodetector 9 and amplifier 10. The result is stored in a measured data memory 12 for every oscillated wavelength. In an operating means 13, computation in the intended wavelength region is performed, and the distance to loss characteristic is displayed on a display device 14.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an optical pulse tester for measuring the characteristics of an optical transmission line such as an optical fiber, and particularly relates to an optical pulse tester using a variable wavelength light source. be.

[Conventional technology]

Optical fibers are used in optical LAN systems (Local Area Network System) that take advantage of their large capacity.
It is widely applied to long-distance transmission such as em) and submarine cables that take advantage of its low loss property. In recent years, research has been conducted on wavelength multiplexing to increase communication capacity and optical fiber amplifiers to extend transmission distance.

A multiplexer/demultiplexer is inserted in the wavelength division multiplexing communication line.
The transmission wavelength is determined from one input port to one output port. Furthermore, optical fiber amplifiers also have a narrow band with respect to the center wavelength of several nanometers, and wavelength restrictions are naturally imposed on transmission systems that include optical fiber amplifiers.

Normally, transmission loss, failure points, etc. are measured in optical communication systems. For this measurement, an optical pulse tester (Optica
l Time Domain Reflectomet
er, hereinafter referred to as 0TDR. ), but most of these current measuring instruments use Fapley-Perot semiconductor lasers with a fixed oscillation wavelength, making them unsuitable for transmission systems with wavelength restrictions such as those mentioned above. . As a means to solve this problem, there is the 0TDR, which has a built-in light source with multiple different wavelengths, and the method that generates Raman scattering in the fiber, which is shown in Japanese Patent Application Laid-Open No. 62-21035, "Optical Fiber Testing Device," to discretely change wavelengths. 0TDR with selectable light sources
and 0TDR using a light source whose wavelength can be continuously varied by changing the temperature of a semiconductor laser, as shown in Japanese Patent Application Laid-Open No. 2-151742 "Loss distribution measurement device for optical line including optical switch". .

[Problem to be solved by the invention]

0TDR, which allows wavelengths to be varied discretely, has many restrictions on measurement because it is not always possible to obtain data at a desired wavelength. In particular, a system that incorporates a plurality of light sources with different wavelengths is extremely expensive. 0TDR, which can continuously vary wavelength, does not have such restrictions, but conventional continuously variable wavelength 0TDR has the disadvantage that although it can measure loss characteristics at a fixed wavelength, it is not possible to compare loss characteristics between each wavelength. was there. For example, it is not possible to measure the wavelength dependence of loss at a fixed position.

This is because in a variable wavelength light source, when the wavelength is changed, the oscillation output changes.

[Purpose of the invention]

SUMMARY OF THE INVENTION In view of the above-mentioned circumstances, it is an object of the present invention to provide an optical pulse tester that can measure loss characteristics at two desired wavelength positions of an optical transmission line.

[Means to solve the problem]

In order to achieve this objective, the present invention obtains the value of the optical pulse output oscillated by the variable wavelength light source, and corrects the data of the reflected light from the optical fiber under test using that data, thereby making it possible to Perform level correction. One method for obtaining the value of the optical pulse output is to branch the output of the variable wavelength light source using an optical branching means, and to obtain it by applying the branched output to a photodetector. Alternatively, the value of the output of the variable wavelength light source may be stored in a memory as a wavelength versus output characteristic, and read out according to the oscillation wavelength.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a first embodiment of the present invention. An external cavity semiconductor laser using a diffraction grating is used as the variable wavelength light source. The principle diagram of this semiconductor laser is shown in Figures 2(a) and 2(b).
). The oscillation wavelength is controlled by an external resonator 3A consisting of a diffraction grating. The oscillation wavelength can be changed by changing the angle θ of the diffraction grating. As a method of obtaining the value of the magnitude of the optical pulse, a second
A light branching means 4 and a second photodetector 5 were provided.

Next, the operation in this embodiment will be explained. The desired wavelength to be oscillated is determined by keystrokes from the keyboard 15 or by a program built into the calculation means 13. The light source control memory 1 stores data for controlling the oscillation wavelength of the variable wavelength light source 3 according to the type of the variable wavelength light source 3. For example, if the variable wavelength light source 3 is an external cavity type semiconductor laser 3B using a diffraction grating as in this example, the angle θ of the diffraction grating with respect to the oscillation wavelength and the pulse current IL of the semiconductor laser 3B are stored. (e.g. 13
(Table 1, page). The light source control means 2 reads out the angle θ of the diffraction grating for a desired wavelength and the pulse current IL of the semiconductor laser 3B from the light source control memory 1.

θ is converted into a current of a 3A driving motor (not shown) for an external resonator (diffraction grating), and moves the diffraction grating to a predetermined angle. The pulse current IL flows through the semiconductor laser 3B, causing the variable wavelength light source 3 to oscillate a light pulse of a predetermined wavelength. A part of the energy of the optical pulse outputted from the variable wavelength light source 3 is branched by the second optical branching means 4 . This second optical branching means 4 is composed of, for example, an optical coupler. The branched light is converted into an electrical signal by the second photodetector 5, and becomes a level correction signal. The first light passes through the second light branching means 4.
The optical pulses outputted to the optical branching means 6 side pass from port A to port B of the first optical branching means 6 and are output to the optical fiber to be measured 8 connected to the connection end 7. Examples of the first optical branching means 6 include an optical coupler or an optical switch. The reflected light from the optical fiber 8 to be measured is guided from port B to port C of the first optical branching means 6 to the first photodetector 9, where it is converted into an electrical signal. This signal is amplified by the amplifier 10 and then corrected by the signal from the second photodetector 5 in the level correction means 11. The corrected signal is stored as measurement data in measurement data memory 1 for each oscillation wavelength.
2 is stored. Once the measurement in the desired wavelength range is completed,
The calculation process is performed by the calculation means 13, and the distance vs. loss characteristic measured in a normal 0TDR is displayed on the display 14, for example, as shown in FIG. In response to instructions from the keyboard 15, the measurement data can be converted into a desired form by the calculation means 13, and the loss versus wavelength characteristic at a specific position as shown in FIG. 3 can be displayed on the display 14, for example. In particular, if a three-dimensional display of distance, wavelength, and loss is performed, wavelength-dependent losses caused by abnormal bends or misalignment of the optical transmission line can be detected at a glance, which is convenient for maintenance.

FIG. 4 shows a second embodiment. As in the first embodiment, the variable wavelength light source 3 uses an external cavity type semiconductor laser using a diffraction grating. As a method for obtaining the value of the optical pulse output, the output level of the optical pulse is stored in advance in the light source control memory 1 along with the oscillation wavelength.

Next, the operation in this embodiment will be explained. The determination of the wavelength to oscillate is the same as in the first embodiment. The light source control memory 1 stores the angle θ of the diffraction grating, the pulse current IL of the semiconductor laser 3B, and the oscillation output P0 at that time, as in the first embodiment.
is memorized. The light source control means 2 reads out the angle θ of the diffraction grating at a desired wavelength and the pulse current IL of the semiconductor laser 3B from the light source control scale 1, and causes the variable wavelength light source 3 to oscillate. This light pulse is sent to the first light branching means 6
The signal is output from port A to port B to the optical fiber to be measured 8 connected to the connection end 7. The reflected light from the optical fiber 8 to be measured is guided from port B to port C of the first optical branching means 6 to the first photodetector 9, where it is converted into an electrical signal. This signal is amplified by amplifier 10.

On the other hand, the light source output data reading means 16 reads the output value of the optical pulse of the variable wavelength light source 3 from the light source control memory 1 and sends it to the level correction means 11.

The level correction means 11 corrects the output signal from the first photodetector 9 according to this signal. The subsequent operations are the same as in the first embodiment.

In this way, if the light source control memory 1 stores the output level of the optical pulse in advance, the second optical branching means 4 and the second photodetector 5 in the first embodiment shown in FIG. 1 are unnecessary. It is characterized by a simple structure.

On the other hand, since the first embodiment detects actual pulsed light, it is characterized by higher accuracy.

A third embodiment is shown in FIG. The variable wavelength light source 3 has a sixth
A two-electrode type D F B semiconductor laser shown in the figure is used. This semiconductor laser can continuously vary the oscillation wavelength by appropriately determining the currents 1+ and It flowing through the two electrodes. As a method of obtaining the value of the optical pulse output, the second photodetector 5 is placed behind the laser, taking advantage of the fact that this semiconductor laser also emits light backwards, and the second light detector 5 is placed behind the laser. The branching means was omitted. Furthermore, in this third embodiment, the first
As the optical branching means, an acousto-optic effect optical switch (hereinafter referred to as AO optical switch) was used.

The operation in this third embodiment will be explained. The determination of the wavelength to oscillate is the same as in the first embodiment.

The light source control memory 1 has currents 1+ flowing through the two electrodes corresponding to the oscillation wavelengths as shown in Table 2 (listed on page 14);
The relationship of It is stored.

The light source control means 2 reads the stored current 1+, Ig from the light source control memory 1, and causes the variable wavelength light source 3 to oscillate a light pulse of a predetermined wavelength.

The light source control memory 1 includes the above-mentioned wavelength control data as well as an AO optical switch 6a corresponding to the oscillation wavelength described later.
The drive frequency of is memorized.

The optical pulse from the variable wavelength light source 3 is transmitted to the AO optical switch 6a.
It enters port A of.

Here, the AO optical switch will be described. This AO optical switch utilizes the fact that when ultrasonic waves are applied to a medium, the refractive index of the medium changes periodically, causing diffraction. The relationship between the diffraction angle θ and the wavelength λ in an AO optical switch is as follows: where n is the refractive index of the medium that has an acousto-optic effect, ■ is the propagation speed of the ultrasonic wave in the medium that has an acousto-optic effect, and is the driving frequency f.
It can be expressed as the following equation (1).

θ-5in-'(fλ/2nV) (1) Here, assuming that the refractive index of the medium having an acousto-optic effect is the same over the entire range of wavelengths that can be oscillated by the variable wavelength light source, from equation (1), As is clear, in order to keep the diffraction angle θ constant, the product λ of the wavelength λ and the driving frequency f is adjusted according to the wavelength λ.
It can be seen that it is sufficient to change the drive frequency f so that f remains constant.

In this example, port A and port B of the AO optical switch are connected during the time period when optical pulses are generated, and port B and port C are connected during the time period when the reflected light from the optical fiber under test returns.
The drive frequency f at that time is determined using equation (1) from the oscillation possible wavelength range and stored in the light source control memory l. This drive frequency data is used by the AO optical switch control means 17 to generate an AO optical switch drive signal of a predetermined frequency by a signal generating means (not shown) built in the AO optical switch control means 17, and then generates an AO optical switch drive signal of a predetermined frequency. The operation of is controlled as described above.

The reflected light from the optical fiber 8 to be measured is transmitted through the AO optical switch 6a.
The signal enters the first photodetector 9 by switching, and is converted into an electrical signal. The subsequent operations are the same as in the first embodiment.

In this embodiment as well, the oscillation wavelength is stored in the light source control memory.
In addition to the driving frequency data of the AO optical switch, it is also possible to store the output level of the optical pulse.

〔Effect of the invention〕

The optical pulse tester of the present invention uses a variable wavelength light source as a light source, measures the level of the output pulse of the light source, or records the level, and uses it for necessary correction and control of the optical system. By using a pulse tester, in addition to the conventional distance vs. loss characteristics, it is possible to measure the loss characteristics of optical components installed in the middle of the line versus the wavelength without removing the optical components.

In particular, if distance, wavelength, and loss are displayed three-dimensionally, wavelength-dependent losses caused by abnormal bends or misalignment of the optical transmission line can be discovered at a glance, which is convenient for maintenance. Even if the optical transmission line is branched by a multiplexer/demultiplexer, the characteristics of only the desired line can be obtained by specifying the wavelength to be branched.

No. table No. table =14

[Brief explanation of drawings]

Figure 1 shows the first embodiment of the present invention, Figure 2 shows an example of an external cavity type semiconductor laser, Figure 3 shows an example of measurement results, and Figure 4 shows an example of the measurement results.
The figure shows the second embodiment of the present invention, Figure 5 shows the third embodiment of the present invention, Figure 6 shows the structure and characteristic example of a two-electrode DFB semiconductor laser, and Table 1 shows the external cavity. Table 2 shows an example of the data in the light source control memory when a type semiconductor laser is used, and Table 2 shows an example of the data in the light source control memory when a two-electrode type semiconductor laser is used. In the figure, 1 is a light source control memory, 2 is a light source control means, 3 is a variable wavelength light source, 4 is a second light branching means, and 5 is a second light source control means.
, 6 is a first optical branching means, 7 is a connection end, 8 is an optical fiber to be measured, 9 is a first photodetector, 10 is an amplifier, 11 is a level correction means, 12 is for measurement data memory,
Reference numeral 13 indicates a calculation means, and reference numeral 14 indicates a display device. Patent applicant: Anritsu Corporation Representative: Patent attorney: Ryutabe Koike Figure 6

Claims (1)

[Scope of Claims] An optical pulse tester for measuring characteristics of an optical transmission line, which includes a light source, an optical branching means, and a photodetector, comprising: a variable wavelength light source (3); and an optical pulse of the variable wavelength light source. Means (4, 5) for obtaining the output value, and means (1) for correcting the level of reflected light from the optical fiber to be measured (8) using the value.
1) An optical pulse tester characterized by comprising: