JPH0961294A - Optical fiber tester - Google Patents

Abstract

PROBLEM TO BE SOLVED: To test optical fiber with weak reflection light by making output light from an intensity modulating light source incident to an optical fiber, and making its reflection light to a wave detecting means to obtain differential frequency between an intensity modulating photoelectric signal and an output signal of the wave detecting means. SOLUTION: When output light from an intensity modulating light source 50 for modulating intensity base don an electric signal with frequency swept is incident to an object 6 (DUT) such as an optical fiber, reflection occurs at a fault point existing in the DUT 6. The reflection light is reflected by a beam splitter 4 of an optical means 5 to interfere with a wave detecting means 52a while being combined with output light from a light source 10 by the beam splitter 11 to be detected by a light detector 7. Then the output from the detector 7 and the output from an oscillator 1 branched by a distributor 2 are combined by a mixer 8, and the DUT 6 is tested by a frequency analyzer 9 based on differential frequency of the two input signals. Thus since the intensity modulating signal of several to several tens of times can be obtained, the optical fiber can be tested even when the reflection signal is weak.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber inspection apparatus, and more particularly to an optical fiber inspection apparatus having higher sensitivity.

[0002]

2. Description of the Related Art OTDR (Optical Time Domain Refle) is generally known as a conventional optical fiber inspection apparatus.
ctmeter) and OFDR (Optical Frequency Domain Reflec)
tmeter). The OFDR converts an electric signal whose frequency has been swept into an optical signal and then enters the optical fiber to detect reflected light from a failure point in the optical fiber and identify the failure point from the frequency information.

FIG. 4 is a block diagram showing an example of such a conventional intensity modulation type OFDR. In FIG. 4, 1 is an oscillator for sweeping the output frequency, 2 is a distributor,
3 is a light source capable of controlling output light by an external signal such as a semiconductor laser, 4 is a beam splitter, 5 is a lens, 6 is a DUT (Device Under Test) such as an optical fiber, 7 is a photodetector,
Reference numeral 8 is a mixer, and 9 is a frequency analyzer.

Further, 1 to 3 are intensity modulation light sources 50, 4 and 5 are optical means 51, 7 is detection means 52, and 8 and 9
Respectively constitute the analysis means 53.

The output of the oscillator 1 is connected to the distributor 2, one output of the distributor 2 is connected to the light source 3, and the other output is connected to one input terminal of the mixer 8. The output light of the light source 3 passes through the beam splitter 4 and the lens 5 and is transmitted to the DUT.
6 is incident.

The reflected light from the DUT 6 enters the beam splitter 4 through the lens 5, is reflected by the beam splitter 4 and enters the photodetector 7. The output of the photodetector 7 is connected to the other input terminal of the mixer 8, and the output of the mixer 8 is connected to the frequency analyzer 9.

The operation of the conventional example shown in FIG. 4 will be described. The output of the oscillator 1 is divided into two by the distributor 2, and the light source 3 is driven by one of the outputs. At this time, the output light of the light source 3 is intensity-modulated by the output of the oscillator 1.

The intensity-modulated light is incident on the DUT 6, and is reflected at a fault point existing in the DUT 6. This reflected light is detected by the photodetector 7, and is combined with the other output of the distributor 2, that is, the output of the oscillator 1, in the mixer 8.

The frequency analyzer 9 inspects the DUT 6 such as an optical fiber based on the difference frequency between the two input signals which are the outputs of the mixer 8.

That is, when the oscillator 1 is swept linearly with a constant frequency change per unit time, the difference frequency between the two input signals connected to the mixer 8 is proportional to the delay time difference, and this delay time difference is within the DUT 6. Since it is proportional to the distance to the obstacle point, the distance from the difference frequency to the obstacle point in the DUT 6 can be obtained, and the size of the obstacle point can be obtained from the signal strength of the difference frequency, so that the optical fiber can be inspected.

For example, the oscillator 1 is set to "1 GHz / sec"
When the DUT 6 is an optical fiber having a group index of refraction “n = 1.5” and the speed of light in vacuum is “C”, the time “t” for light to travel back and forth through the optical fiber “1 m” is , T = 1 × 2 × n / C≈1 × 10 ⁻⁸ [sec] (1)

Further, from the sweep rate, 1 × 10 ⁹ × 1 × 10 ^-8 = 10 ⁹ × 10 ^-8 = 10 [Hz] (2), and a difference frequency of "10 Hz" is generated.

In other words, if the distance to the fault point in the DUT 6 is "1 m", "10 Hz", "10 m", "100 Hz", and "100 m", "1 KH".
The frequency output called "z" is obtained as the frequency output of the mixer 8.

As a result, in this example, the difference frequency "10 Hz" is the distance "1" to the fault point in the DUT 6.
Since it corresponds to m ″, by analyzing the difference frequency in the frequency analyzer 9, the distance to the fault point in the DUT 6 can be obtained.

[0015]

However, if the intensity of the reflected light becomes weak due to a long distance to the obstacle point in the DUT 6 or a small obstacle point, the reflected light will be affected by the sensitivity of the photodetector 7 and the like. There is a problem that it cannot be detected and the difference frequency cannot be obtained. Therefore, the problem to be solved by the present invention is to realize an optical fiber inspection apparatus capable of inspecting even weak reflected light.

[0016]

In order to solve such a problem, according to the first aspect of the present invention, a frequency-swept electric signal is converted into an optical signal and then is incident on an optical fiber to cause an obstacle in the optical fiber. In an optical fiber inspection device that detects the reflected light from a point and identifies the fault point from the frequency information,
An intensity-modulated light source that outputs light intensity-modulated based on an electric signal whose frequency is swept, and a light source, and the reflected light from a fault point in the optical fiber interferes with the output light of the light source to generate an electric signal. Detecting means for converting to, an optical means for making the output light of the intensity-modulated light source incident on the optical fiber and for making the reflected light from the optical fiber incident on the detecting means, the frequency-swept electrical signal and the And an analysis unit for inspecting the optical fiber based on a frequency difference from the output signal of the detection unit.

According to a second aspect of the present invention, there is provided an intensity-modulated light source for intensity-modulating and outputting the output light of the light source included in the detecting means according to the first aspect of the present invention based on an electric signal whose frequency is swept. It is a thing.

[0018]

The reflected light generated at the fault point existing in the DUT and the output light of the light source are caused to interfere with each other and detected by the photodetector, so that the power of the directly reflected light is several times higher than the conventional example. It is possible to obtain an intensity-modulated signal several tens of times,
Even weakly reflected light can be inspected.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings. FIG. 1 is a configuration block diagram showing an embodiment of an optical fiber inspection apparatus according to the present invention. In FIG.
Is a light source that outputs continuous light, and 11 is a beam splitter. Further, here, 1 to 9, 50, 51 and 53 are shown in FIG.
7 and 10 and 11 constitute the detecting means 52a.

The connection relationship is almost the same as that of the conventional example shown in FIG. 4, except that the reflected light of the beam splitter 4 is reflected by the beam splitter 11 and is incident on the photodetector 7 and the output light of the light source 10. Is the beam splitter 11
Is a point which is transmitted through and is incident on the photodetector 7.

The operation of the embodiment shown in FIG. 1 will now be described. The output light of the intensity modulation light source 50 is incident on the DUT 6,
Reflections occur at the points of failure present within the DUT 6. This reflected light is combined with the output light of the light source 10 and interferes, and is detected by the photodetector 7.

The mixer 8 synthesizes the output of the photodetector 7 and the other output of the distributor 2, in other words, the output of the oscillator 1, and the frequency analyzer 9 is based on the difference frequency between the two input signals which is the output of the mixer 8. Inspect the DUT 6 such as an optical fiber.

For example, if the power of the reflected light from the DUT 6 is "E1" and the power of the output light of the light source 10 is "E2", then the relationship of E1 <E2 (3) holds, and the two interfere with each other to detect light. The power "E" detected by the device 7 is the difference in the optical frequencies of the two that interfere with each other by "Δ".
f _opt ”, E≈E1 + E2 + (E1 · E2) ^1/2 · cos (Δf _opt ) (4)

Here, "E1 = 10 μW", "E2 = 1"
If mW ”, then from the third term of equation (4), (E1 · E2) ^1/2 · cos (Δf _opt ) = (10 × 10 ⁻⁶ · 10 ⁻³ ) ^1/2 · cos (Δf _opt ) = 100 × 10 ⁻⁶ · cos (Δf _opt ) (5), which is the conventional method for detecting the power “E1” of the direct reflected light. It is possible to obtain an intensity-modulated signal that is about 10 times that of the example.

That is, even if the power "E1" of the reflected light is weak, the power "E2" of the output light of the light source 10 is increased to detect the third term in the equation (4) by the photodetector 7. Thus, it is possible to obtain the required intensity modulation signal.

On the other hand, regarding the frequency output of the mixer 8, the optical frequency difference "Δf" between the two lights interfered by the detection means 52a.
_opt ”will be added as an offset component.

That is, when "Δf _opt = 10 GHz" is set, "10 GHz + 10 Hz" if the distance to the fault point in the DUT 6 is "1 m", and "1" if it is "10 m".
0GHz + 100Hz ", if it is" 100m ", it is" 10 "
A frequency output such as “GHz + 1 KHz” is obtained as the frequency output of the mixer 8.

As a result, the reflected light generated at the fault point existing in the DUT 6 and the output light of the light source 10 are caused to interfere with each other and detected by the photodetector 7, so that an optical fiber inspection apparatus capable of inspecting even weak reflected light. Can be realized.

FIG. 2 is a block diagram showing the configuration of the second embodiment of the optical fiber inspection device according to the present invention. FIG.
Reference numeral 12 is a modulator. Also, here, 1, 2,
Reference numerals 4 to 11, 51, 52a and 53 are assigned the same reference numerals as those in FIG. 1, and reference numerals 1, 2 and 12 constitute the intensity modulation light source 50a.

The connection relationship is almost the same as that of the embodiment shown in FIG. 1, except that the output of the distributor 2 is connected to the modulator 12 and the output light of the light source 10 is incident on the modulator 12.
This is the point where the output light of the modulator 12 is incident on the DUT 6 via the beam splitter 4 and the lens 5 as intensity modulated light.

The operation of the second embodiment shown in FIG. 2 will be described here. The operation of the modulator 12 is the same as that of the embodiment shown in FIG. 1 except that the output light of the light source 10 is intensity-modulated based on the output of the distributor 2 and is incident on the DUT 6.

However, since the same light source is used in the second embodiment, the optical frequency difference "Δf _opt " described in equations (4) and (5) is "0".

That is, the offset component becomes "0" and D
If the distance to the obstacle point in the UT6 is "1m", "10"
Hz "," 10m ", then" 100Hz "," 100
If m ", a frequency output of" 1 KHz "is obtained as the frequency output of the mixer 8.

FIG. 3 is a block diagram showing the configuration of a third embodiment of the optical fiber inspection device according to the present invention. FIG.
13 is a frequency shifter. Also, where
1, 2, 4 to 12, 50a, 51 and 53 are designated by the same reference numerals as those in FIG. 2, and 7, 10, 11 and 13 constitute the detecting means 52b.

The connection relationship is almost the same as that of the embodiment shown in FIG. 2, and the output light of the light source 10 is incident on the beam splitter 11 via the frequency shifter 13.

The operation of the third embodiment shown in FIG. 3 will be described here. This operation is also the same as that of the second embodiment shown in FIG. 2, except that the interference light is shifted by the frequency shifter 13.

However, for the frequency output of the mixer 8, therefore, the frequency component shifted by the frequency shifter 13 is added as an offset component.

Further, although the frequency shifter 13 is provided between the light source 10 and the beam splitter 11 in FIG.
A frequency shifter 13 may be provided between the light source 10 and the modulator 12 or between the beam splitter 4 and the modulator 12. The operation in this case is similar to that of the third embodiment shown in FIG.

[0039]

As is apparent from the above description,
The present invention has the following effects. By interfering the reflected light generated at the fault existing in the DUT with the output light of the light source and detecting the light with the photodetector, it is possible to realize an optical fiber inspection device capable of inspecting even weak reflected light.

[Brief description of drawings]

FIG. 1 is a configuration block diagram showing an embodiment of an optical fiber inspection apparatus according to the present invention.

FIG. 2 is a configuration block diagram showing a second embodiment of the optical fiber inspection apparatus according to the present invention.

FIG. 3 is a configuration block diagram showing a third embodiment of the optical fiber inspection device according to the present invention.

FIG. 4 is a configuration block diagram showing an example of a conventional intensity modulation type OFDR.

[Explanation of symbols]

 1 Oscillator 2 Distributor 3,10 Light source 4,11 Beam splitter 5 Lens 6 DUT 7 Photodetector 8 Mixer 9 Frequency analyzer 12 Modulator 13 Frequency shifter 50, 50a Intensity modulation light source 51 Optical means 52, 52a, 52b Detection means 53 Analytical means

Claims (2)

[Claims] 1. An optical fiber which converts an electric signal whose frequency is swept into an optical signal and then enters the optical fiber to detect reflected light from a fault point in the optical fiber to identify the fault point from the frequency information. In an inspection device, an intensity-modulated light source that outputs light intensity-modulated based on an electric signal whose frequency is swept, and a light source, and interferes the reflected light from a fault point in the optical fiber with the output light of the light source. Detecting means for converting the output light of the intensity-modulated light source into the optical fiber and the reflected light from the optical fiber into the detecting means; An optical fiber inspection device comprising: an analysis unit that inspects the optical fiber based on a difference frequency between an electric signal and an output signal of the detection unit. 2. An optical fiber according to claim 1, further comprising an intensity-modulated light source for intensity-modulating the output light of the light source of the detecting means based on an electric signal whose frequency is swept, and outputting the intensity-modulated light source. Inspection device.