JPS6154421A - Light pulse testing instrument - Google Patents

Abstract

PURPOSE:To cause output light which has large peak light power and short pulse width to strike an optical fiber to be tested, and to obtain high resolution by providing a switch driving means which turns on an optical switch in a short time synchronously with the light emission of a light source. CONSTITUTION:The light source 8 is a Q switch Nd:YAG laser, whose output light has large peak light power, an optical switch 9 is inserted into a passage of its output light, and its output light is coupled with one terminal of the optical fiber 3 to the tested through an optical directional coupler 2. The optical switch 9 is a semiconductor laser optical switch in this case, and when a driving current is supplied to its drive terminal, the light passage is turned on to supply the driving current to a driving circuit 10, which is driven by a synchronizing circuit 11 in synchronism with the light emission of the light source 8. The level of this driving current is set to a value which is a little bit smaller than the current value at which laser light emission is started and the optical switch 9 turns on the output light of the light source 8 only when a current exceeding the driving current is supplied. Consequently, an optical signal which has large peak light power and short pulse width appears at the output of the optical switch 9.

Description

[Detailed description of the invention] [Industrial application field] The apparatus of the present invention is used for testing communication optical fibers.

The present invention is for detecting a failure point by inputting a light pulse from one end of an optical fiber and observing its reflected light or scattered light at the rear 11, or for measuring loss of one optical member and splice loss of an optical fiber. The present invention relates to an optical pulse testing device.

[Conventional technology]

Figure 2 shows a schematic diagram of a typical conventional optical pulse tester. The optical pulse signal output from the pulse light source 1 passes through the optical directional coupler 2 and is input into the optical fiber 3 under test. Inside the optical fiber 3 under test, backscattered light is generated due to minute fluctuations in the refractive index, or Fresnel repulsion light is generated at break points, connection points, etc. These are the optical fiber 3 to be measured.
The signal travels in the opposite direction, passes through an optical directional coupler 2, is sent to a photodetector 4, and is converted into an electrical signal. This signal is amplified by an amplifier 5, processed by a waveform average calculation process 2 (:6), and displayed on a display 7.

Optical power PbS scattered in the optical fiber and returned back again
If the optical pulse width is t8 and the peak optical power is P, then pb, ocp-j, ・−・−・・
-(1). Further, the resolvable distance for reflection position detection is proportional to the optical pulse width L8 of the pulsed light source. For example, when t is 10 nS, the resolution is 1 m, and when t is 100 nS, the resolution is 10 m. Therefore, in order to improve the distance resolution, it is necessary to narrow the optical pulse width t8,
In this case, the reflected optical power decreases for the same peak optical power, making it impossible to provide a long optical fiber length that allows measurement of optical loss, detection of break points, and the like.

For example, when the optical pulse width t1 is 10 nS, the signal-to-noise ratio improvement amount by averaging processing is 30 dB, and the optical loss of the optical fiber under test is 5 dB (one way), the return loss of backscattered light from the optical fiber is about 73 dB, and the lowest detection level of the photodetector is 145 dBm, so the required peak optical power P is estimated to be about +23 dBm.

[Problem that the invention seeks to solve]

When using a semiconductor laser as a pulsed light source, it is easy to achieve a short pulse width of about 10 nS by modulating the drive current, but it is currently difficult to obtain a large optical power as described above. It is. On the other hand, if a high-output laser such as a Q-switched Nd:YAG laser is used as a pulsed light source, the desired peak optical power can be obtained, but the pulse width is as wide as about 200 nS, making it impossible to obtain high distance resolution. there were.

The present invention solves this problem by providing a device that can input output light with a large peak optical power and a short pulse width into an optical fiber under test, and can obtain a high value of 5i'A u'is. The purpose is to

[Means for solving problems]

The present invention includes a light source that generates pulsed light, a coupling means that makes the output light of the light source enter one end of an optical fiber under test,
A visual observation means for observing reflected light and/or backscattered light generated by the output light in the optical fiber under test at one end of the optical fiber (Ii+7) is installed in the output optical path of the light source. It is characterized by comprising an inserted optical switch and switch driving means for driving the optical switch to conduct for a short time in response to the light emission from the light source.

[For production]

As a light source, a light source whose output light pulse has a large peak optical power and a long pulse width is used, and an optical switch is used in the output optical path to cut out the output light whose peak optical power is large and the pulse width is short.

〔Example〕

FIG. 1 is a block diagram of an apparatus according to a first embodiment of the present invention. The light source 8 is a Q-switched Nd: YAG laser (output light wavelength 1.32 μm) with a high peak optical power of output light.
It is. An optical switch 9 is inserted into the path of the output light,
The output light is coupled to one end of the optical fiber 3 under test by the optical directional coupler 2 . Backscattered light or reflected light generated inside the optical fiber 3 under test enters the photodetector 4 via the optical directional coupler 2 and is converted into an electrical signal.
The signal is amplified by an amplifier 5, processed by a waveform average calculation processor 6, and displayed on a display 7.

In this example, the optical switch 9 is a semiconductor laser optical switch (L
(D optical switch), which makes the optical path conductive when a drive current is applied to its drive terminal.

This optical switch 9 is supplied with a drive current by a drive circuit 10. This driving circuit 10 is connected to the light source 8 by a synchronizing circuit 11.
It is driven in synchronization with the light emission of the

FIG. 3 is an explanatory diagram of the operation of this light source 8 and optical switch 9. As shown in FIG. 3 (al), the light 728 rises gradually with the passage of time t, reaches a high peak optical power, and falls gradually. The vertical axis P indicates the output optical power. As shown in FIG. 3 (1+1), it is driven by 1.

The vertical axis (■) indicates the drive current of the optical switch 9. The level of this drive current is set to a value slightly lower than the current value Itl that starts laser emission. Only during the period when a current exceeding the drive current ILh of the optical switch 9 is applied, four output lights from the light source 8 are allowed to pass through. Therefore, as shown in FIG. 3(C), the output of the optical switch 9 is an optical signal having a large peak optical power and a short pulse width.

As a result of the test, the pulse width is large. was 10 nS, and an optical signal with rise and fall times of 1 nS could be obtained. The test results are shown in FIG. Fig. 4 (al is the output light waveform of the light source 8, Fig. 4 fbl is the drive current waveform of the optical switch 9, and Fig. 4 fc) is the waveform of the optical signal that has passed through the optical path 1° Sochi 9. be.

By selecting a semi-dedicated laser optical switch 9 whose characteristic laser oscillation wavelength is close to the output light wavelength of the light source 8, the I! One loss can be reduced.

FIG. 5 is a diagram showing an example of test results of the above-mentioned embodiment device. As shown in Figure 5 (al), the test was conducted by providing connection points at a point approximately 30 m and approximately 40 m from the input end of the optical fiber under test. In this test, the level of backscattered light was observed north of the time axis.In this case, the pulse width of the incident light was approximately 200 nS, and the distance was 10 m.
It is not possible to distinguish between two connection points with one title. Figure 5fcl is the result of testing with the configuration shown in Figure 1, and the level of backscattered light decreases stepwise at the two connection points, and the two connection points separated by 10m can be distinguished. Indicates that it can be identified.

FIG. 6 is a block diagram of an apparatus according to a second embodiment of the present invention. This embodiment is characterized by an optical switch 12 inserted into the output optical path of the light source 8. However, in this embodiment, an optical switch 12 for switching the direction of the optical path is used to connect the light source and observation means to the optical fiber under test without using an optical directionality detector.

This optical switch 12 applies the output light of the light source 8 to one end of the optical fiber 3 under test for a short time in a first mode, and then sends the optical signal appearing at one end of the optical fiber 3 under test to a photodetector in a second mode. It acts to lead to 4.

FIG. 7 is a structural diagram of this optical switch 12. It has a structure in which an electrophonic transducer 14 is joined to an ultrasonic light deflection element 13. A is a boat that combines with light source 8;
B is a boat coupled to the optical fiber 3 under test, and C is a boat coupled to the photodetector 4. By applying a high frequency voltage from the drive circuit 10 to the transducer 14, light diffraction occurs within the ultrasonic optical deflection element 13. The light incident from boat B is diffracted within the ultrasonic deflection element 13 and output to boat C. When no high frequency voltage is applied to the transducer 14, the light incident from the boat 1-A is output to the boat B.

FIG. 8 is an operation time chart of the apparatus of this embodiment.

FIG. 8 (al is an incident light pulse from light 6Q8 which enters port A of the optical switch 12. Using the synchronization circuit 11 and drive circuit 10, the high frequency voltage is transduced at the timing shown in FIG. 8(b). When applied to the user 14, light diffraction occurs.Therefore, the optical pulse shown in FIG. The light generated at the end of the test optical fiber 3 and propagating backward enters the boat B again.FIG. 8+dl shows the waveform of this backward propagating light.

The Fresnel reflected light at the input end of the optical fiber 3 under test corresponds to the rising portion of the waveform in FIG. 8(d) where the light intensity is high. When the light enters from the boat 1-B of the optical switch 12, the Fresnel reflected light is not limited to the boat C because a driving voltage is applied thereto. Subsequently, the backscattered light is guided to boat C by turning the optical switch 12.

FIG. 8 (el) shows the optical output waveform from boat C.

The optical output from boat C is input to photodetector 4 and converted into an electrical signal. This signal is processed by an amplifier 5 and a waveform averaging device 6 and displayed on a display 7.

This embodiment has advantages such as being able to eliminate one-member loss in coupling between the optical switch and optical direction detection 2); and being able to eliminate Fresnel reflections at the input end of the optical fiber under test. There is.

FIG. 9 shows another example of the structure of the optical switch. The electrode 1 is placed between the polarizer 15 and the analyzer 17, which are orthogonal to each other.
It is constituted by an electro-optical element 17 with a numeral 8. 1. to the electro-optical element 17; Using iNb03, an electric field is applied in the y direction in two directions in which the light travels.

FIG. 10 is a diagram showing the relationship between applied voltage and transmission intensity of an optical switch having this structure. When the distance between the electrodes is 1 w+m and the length in two directions is 30 mm, the voltage Vπ at which the transmitted light intensity is maximum at a wavelength of 0.633 μm is 31.5 V. Therefore, when the voltage applied to the electro-optical element is zero, the transmitted light output of this electro-optic modulator is zero, and when the applied voltage is Vπ, the transmitted light output is maximum. The present invention can be implemented using this optical switch in the device shown in FIG.

The present invention can be practiced with optical switches having structures other than those described in the above examples.

〔Effect of the invention〕

As explained above, according to the present invention, the output light of a light source having a high peak optical power is extracted using a high-speed response optical switch, converted into pulsed light having a large peak optical power and a narrow pulse width, and this is utilized. Since the pulse test is performed using the same method, an optical pulse tester with high distance resolution and a long measurable optical fiber length of 1M can be used.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an apparatus according to a first embodiment of the present invention. FIG. 2 is a block diagram of a conventional device. FIG. 3 is an explanatory diagram of the operation of the apparatus according to the embodiment of the present invention. FIG. 4 is a diagram showing the test results of the apparatus according to the present invention. FIG. 5 is a diagram showing the test results of the apparatus according to the present invention. FIG. 6 is a block diagram of an apparatus according to a second embodiment of the present invention. FIG. 7 is a structural diagram of an optical switch used in this second embodiment device. FIG. 8 is an explanatory diagram of the operation of this second embodiment device. FIG. 9 is a diagram showing another structure of the optical switch. FIG. 10 is an explanatory diagram of the operation of this optical switch. DESCRIPTION OF SYMBOLS 1... Light source, 2... Optical directional coupler, 3... Optical fiber under test, 4... Photodetector, 5... Amplifier, 6
... Waveform average calculation circuit, 7... Display, 8... Light source, 9... Optical switch, 10... Drive circuit, 11.
... Synchronous circuit, 12... Optical switch, 13... Ultrasonic optical deflection element, 14... Transducer, 15...
- Polarizer, 16... Electro-optical element, 17... Analyzer, 18... Electrode.

Claims (3)

[Claims] (1) A light source that generates pulsed light; A coupling means that inputs the output light of this light source into one end of the optical fiber under test; and An optical pulse testing device comprising an observation means for observing at one end of the optical fiber, an optical switch inserted into the output optical path of the light source, and the optical switch being made conductive for a short time in synchronization with the light emission of the light source. 1. An optical pulse testing device characterized by comprising: a switch driving means for driving the optical pulse test device as shown in FIG. (2) The optical pulse testing device according to claim (1), wherein the coupling means includes an optical directional coupler. (3) The optical switch operates between a first mode in which the output light of the light source is briefly applied to one end of the optical fiber to be measured, and a second mode in which the optical signal appearing at one end of the optical fiber to be measured is guided to the observation means. The optical pulse testing device according to claim (1), which is a changeover switch having: