JPH0663908B2 - Optical pulse test equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention uses an optical fiber type amplifier to search for a failure point in a long optical fiber under measurement with high efficiency in a small size.
The present invention relates to a device for detecting a break point, measuring a loss, and the like.

[Prior Art] Various methods such as an optical pulse method, an autocorrelation method by M-sequence random pulse modulation, a photon counting method, a frequency sweep method, and a heterodyne method are available as methods for searching for a failure point and detecting a break point of an optical fiber. Proposed and realized.

The optical pulse method is the simplest method among them,
It is often used because a semiconductor laser can be used as a light source to search for a fault point in a long optical fiber of 50 km or more. The device used to carry out this optical pulse method is an optical pulse tester (Optical Time Domain Reflectometer, hereinafter O).
It is called TDR), and is configured as shown in FIG. 4, for example.

In FIG. 4, 1 is an electric pulse generator, 2 is an optical pulse light source, 3 is an optical directional coupler, 4 is an optical fiber to be measured, 5
Is a photodetector, 6 is an electric signal amplifier, 7 is a signal processing unit for averaging, and 8 is a display unit.

The optical pulse light source 2 is excited by the electrical pulse from the electrical pulse generator 1, and the obtained optical pulse is incident on the optical fiber 4 to be measured via the optical directional coupler 3. As the light pulse propagates in the optical fiber 4, very weak back Rayleigh scattering occurs. This backward Rayleigh scattering returns inside the optical fiber 4 and is guided to the photodetector 5 via the directional coupler 3. Since the level of the optical signal based on the backward Rayleigh scattering is a weak signal by about -50 dB with respect to the incident light, the output of the photodetector 5 is amplified by the amplifier 6. Further, the amplified output is supplied to the averaging processing unit 7, where the S / N ratio is improved,
Finally, it is displayed on the display unit 8.

FIG. 5 shows how the backscattering signal is displayed on the display unit 8 at the connection point between the two optical fibers 4A and 4B and the breaking point of the optical fiber 4B. In this way, the connection point can be broken by the step, and the break point can be judged from the sudden drop in level.

Here, as the directional coupler 3, (a) one using the polarization characteristics of calcite, (b) one using an optical switch with an ultrasonic optical polarizer, (c) a method using an optical fiber coupler, etc. are proposed. ing. These methods (a), (b) and (c)
Are shown in FIGS. 6 (a), (b) and (c), respectively. Method (a) uses a 3 dB coupler for a naturally polarized input, so the total of -3 dB at the input and -3 dB at the reception of scattered light
There is a 6 dB loss. The method (b) becomes an effective switch by coupling the ultrasonic polarizer to the optical fiber 4 to be measured when it is OFF and receiving scattered light when it is ON. In the method (c), the same 3 dB coupler as that in (a) is constructed by fusing two optical fibers.

[Problems to be Solved by the Invention] In the optical pulse method described above, when the optical pulse from the optical pulse light source 2 is incident on the single mode optical fiber 4 to be measured, the optical level is lowered by at least about 3 dB. Existed. Moreover, the same thing also exists when a weak backward Rayleigh scattering signal is received at the photodetector 5,
It was unavoidable to remove the coupling loss. Therefore, in order to expand the dynamic range, it is necessary to use a high output light source.

Therefore, an object of the present invention is to send an optical pulse to an optical fiber and perform various tests on the optical fiber.
In this method, the optical loss generated when the optical pulse is coupled to the optical fiber is compensated by the optical amplification in the optical fiber, and the backscattered light from the optical fiber is optically amplified to expand the dynamic range of the OTDR method. Therefore, it is an object of the present invention to provide an optical pulse test apparatus capable of searching for a fault point of a long optical fiber, detecting a break point, measuring loss, and the like.

[Means for Solving the Problems] In order to achieve such an object, the present invention supplies an optical pulse to an optical fiber to be measured through an optical directional coupler to cause an obstacle to the optical fiber to be measured. In an optical pulse tester that performs point search, break point detection, loss measurement, etc., a rare earth element is placed between the optical directional coupler and the fiber under test from the directional coupler side to the optical fiber type coupler and the core of the optical fiber. It is characterized in that optical fiber amplifiers configured by doping elements are sequentially arranged.

[Operation] In the present invention, the rare earth element Er (erbium) or Nd
An optical fiber amplifier with (Neodymium) doped in the core of the optical fiber is attached to the front stage of the measured fiber.
In the prior art, such an optical fiber amplifier is
None of them have been adopted as part of DR.

In the present invention, even if there is a coupling loss, the optical fiber amplifier can amplify about 20 dB, so that the incident optical pulse can be increased in power and guided to the fiber under measurement. Since both the optical fiber amplifier and the fiber under test are ordinary single-mode fibers, the coupling loss can generally be suppressed to 0.5 dB or less. Further, regarding the reception of the backward Rayleigh scattering, the Rayleigh scattering level is originally smaller than the incident level of about -50 dB, so that it is difficult to detect, but in the present invention, the S / N is used by using the amplification characteristic of the optical fiber optical amplifier. Since the ratio can be improved, the dynamic range can be expanded, and as a result, the diagnostic distance at the break point can be dramatically increased.

Embodiments Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows a first embodiment of the present invention, in which 1 is an electric pulse generator, 2 is a laser light source, and 3 is a direction using the ultrasonic optical deflector shown in FIG. 6 (b). A sex coupler, 4 is a fiber to be measured which is a target of a fault point search, 5 is a photodetector, 6 is an electric signal amplifier, 7 is a signal processing section, and 8 is a display section. The above configuration is the same as the conventional example shown in FIG. Reference numeral 9 is a drive circuit for the ultrasonic light deflection type directional coupler 9. In this embodiment, an optical amplification unit 14 having the following configuration is arranged between the directional coupler 3 and the optical fiber 4. That is,
In the optical amplification section 14, 10 is an optical fiber type coupler, 11 is an optical fiber amplifier having a core portion doped with a rare earth element (Er or Nd), and 12 is a light source for exciting the optical fiber amplifier 11. That is, the optical pulse from the directional coupler 3 and the excitation light from the light source 12 are guided to the optical fiber amplifier 11 via the coupler 10, and the amplified optical pulse is guided to the optical fiber 4.

Here, first, the light source 2 is excited by the electric pulse from the electric pulse generator 1 to generate an optical pulse. This optical pulse passes through the optical fiber 13, passes through the ultrasonic wave type optical directional coupler 3, and enters the optical fiber type coupler 10. That is,
When the ultrasonic switch is OFF, the laser light source 2 is connected to the coupler 10 via the directional coupler 3. Conventionally, at the time of coupling an optical pulse from the light source 2 to the coupler 10, an optical loss of at least 3 dB was involved, but in the present embodiment, the loss is compensated by the optical amplification section 14 shown by a broken line block, and The amplified pulse is made incident on the measured fiber 4. A light source 12 is used for pumping the optical fiber amplifier 11, and the pumping light is supplied to the light source 2 by the optical fiber coupler 10.
It is multiplexed with the optical pulse from and guided to the optical fiber amplifier 11.

As a result, a gain of about 20 dB can be easily obtained.
Considering the 2 dB / km optical fiber fault point search,
20 / (2 ・ 0.2) ＝ 50km It is possible to extend the search distance, which is a very good method.

In particular, the light source 2 is a low-power laser (several
In the case of mW output), the optical pulse amplifier 14 shown by the broken line is effective, and the fiber coupling output for the optical fiber 4 is several tens.
It can be increased up to 0 mW. As the optical pulse thus amplified by the optical pulse amplifier 14 propagates through the fiber 4 to be measured, Rayleigh scattering occurs, and the component propagating backward (backward Rayleigh scattering) is the incident end of the optical fiber 4. Come back to. The backscattered light is very weak, but the optical fiber amplifier 14 plays a major role here as well. That is, with the gain of about 20 dB of the optical fiber amplifier 11, the Rayleigh scattered light is amplified like the incident pulse,
The dynamic range can be expanded.

The backward Rayleigh scattered light thus amplified is incident on the ultrasonic wave directional coupler 3. At that time, when the directional coupler drive circuit 9 is turned on, the light is guided to the photodetector 5 by Bragg diffraction. If the gain in the optical fiber amplifier 14 is insufficient, the electric signal amplifier 6 further amplifies the gain, and the averaging processor 7 improves the S / N ratio. The resulting output is displayed on the display unit 8.

Next, the characteristic of the amplification degree of the optical fiber amplifier 11 is shown in FIG. In this case, the wavelength of the light pulse light source 2 is set to 1.535 μm, the excitation light source 12 is an Ar laser (wavelength 514 nm),
The excitation input was 2.5W. It can be seen from FIG. 2 that when the length of the optical fiber amplifier 11 is 2 m, a gain of about 20 dB is obtained. In this optical fiber amplifier 11, Er, which is a rare earth, is doped in the 800 ppm core portion,
Optical amplification occurs at a wavelength of 1.535 μm.

FIG. 3 shows a time chart of the operation of the apparatus shown in FIG. Here, (a) is the waveform of the backward Rayleigh scattering from the optical fiber 4 immediately before the ultrasonic optical directional coupler 3,
(b) guides the backscattering signal to the photodetector 5 by the operation of turning the optical directional coupler 3 from OFF to ON and turning it OFF, and (c) shows
The backscattering signal obtained by the operation of (b) is shown. In this way, the optical pulse test apparatus having the optical fiber amplifier 14 can be constructed.

[Effects of the Invention] As described above, according to the present invention, by adding an optical fiber amplifier to an optical pulse tester, it is possible to directly amplify an incident optical pulse at an optical level, and yet to reduce the intensity. Since the backscattered signal can also be amplified, there is an advantage that the dynamic range of the optical pulse tester can be greatly expanded by 20 dB or more. Therefore, the present invention is extremely effective in searching for a fault point and measuring loss in a long optical fiber.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the device of the present invention, FIG. 2 is a characteristic diagram showing the amplification characteristics of the optical fiber amplifier of the present invention, FIG. 3 is an operation time chart of the device of the present invention, and FIG. FIG. 5 is a configuration diagram showing an example of a conventional general optical pulse test device, FIG. 5 is an explanatory diagram of a backscattering signal obtained in FIG. 4, and FIGS. 6 (a), (b) and (c) are conventional diagrams. As an optical directional coupler for the optical pulse tester used, an example using calcite type birefringence, an example of an ultrasonic optical deflector type configuration, and an example of an optical fiber type coupler Is. 1 ... Electric pulse generator, 2 ... Laser light source, 3 ... Optical directional coupler, 4,4A, 4B ... Measured optical fiber, 5 ... Photodetector, 6 ... Amplifier, 7 ... Signal processing (averaging processing) unit, 8 ... Display unit, 9 ... Optical directional coupler driving unit, 10 ... Optical fiber type coupler, 11 ... Optical fiber amplifier, 12 ... Optical fiber amplifier pumping Light source, 13 ... Optical fiber, 14 ... Optical fiber amplifier.

Claims (1)

[Claims] 1. An optical pulse test apparatus for supplying an optical pulse to an optical fiber to be measured through an optical directional coupler to search for a fault point, detect a break point, measure loss of the optical fiber to be measured. , An optical fiber amplifier formed by doping a core portion of the optical fiber with a rare earth element from the directional coupler side between the optical directional coupler and the fiber to be measured. An optical pulse test device characterized by the above.