JPH07190886A - Inspection apparatus of optical fiber - Google Patents

Abstract

PURPOSE:To recognize a ghost by a method wherein the optical path of reference light is changed, interference light is measured, measured results are overlapped, length resolution and reflection-loss measuring accuracy are enhanced and the existence of the shift of a frequency is detected. CONSTITUTION:The wavelength of output light is swept by a variable- wavelength light source 1. Reflected light by a moving mirror 3a is made to interfere with reflected light from the reflection point of an optical fiber 5. Interference light is converted into an electric signal by a photodetector 6, and the electric signal is frequency-analyzed 7. Thereby, the distance between a beam splitter 2 and the reflection point of the optical fiber 5 is found. At this time, the moving mirror 3a is moved by an arithmetic ask control means 51, the optical path length of reference light is changed by La within the range of resolution L, the interference light is measured, a spectrum is found, and the intensity of interference with reference to a distance difference L is displayed. Thereby, the spectrum is narrowed, length resolution is enhanced, a peak value is increased, and reflection-loss measuring accuracy is enhanced. In addition, when the existence of the shift of a frequency is detected, a ghost can be recognized.

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 using optical interference, and more particularly to an optical fiber inspection apparatus having improved length resolution and reflection loss measurement accuracy.

[0002]

2. Description of the Related Art A conventional optical fiber inspection device (OFDR: Optical Frequency Domain Reflectmet) using optical interference.
er) is for obtaining the distance of the reflection point of the optical fiber to be inspected and the reflection amount from the reflection point, and there is one using a Michelson interferometer. FIG. 4 is a configuration block diagram showing an example of such a conventional optical fiber inspection apparatus.

In FIG. 4, 1 is a variable wavelength light source, 2 is a beam splitter, 3 is a mirror, 4 is a lens, 5 is an optical fiber to be inspected, 6 is a photodetector, and 7 is a frequency analysis device such as FFT. . Here, 2 to 4 compose a Michelson interferometer 50.

The output light of the variable wavelength light source 1 is incident on a beam splitter 2, the beam splitter 2 reflects a part of the incident light and makes it incident on a mirror 3, and transmits the remaining incident light through a lens 4. It is incident on the optical fiber 5.

The light reflected by the mirror 3 enters the beam splitter 2 again. On the other hand, the reflected light from the reflection point of the optical fiber 5 is also incident on the beam splitter 2 via the lens 4, and is combined with the reflected light from the mirror 3.

The interference light generated as a result of the multiplexing is the photodetector 6
And is converted into an electric signal. The output of the photodetector 6 is connected to the frequency analysis device 7, and the control signal from the frequency analysis device 7 is connected to the variable wavelength light source 1.

The operation of the conventional example shown in FIG. 4 will be described. The wavelength of the output light is swept by the variable wavelength light source 1, and the reflected light from the mirror 3 and the reflected light from the reflection point of the optical fiber 5 interfere with each other. This interference light is detected by the photodetector 6 and analyzed by the frequency analysis device 7.

When the sweep speed of the wavelength is constant in terms of optical frequency, the interference light of the reflected light of the mirror 3 and the reflected light from the reflection point of the optical fiber 5 has a peak of interference intensity at a certain optical frequency. This optical frequency is beam splitter 2 and mirror 3
Is proportional to the difference in distance between the beam splitter 2 and the reflection point of the optical fiber 5.

That is, since the distance between the beam splitter 2 and the mirror 3 is known, the distance between the beam splitter 2 and the reflection point of the optical fiber 5 can be obtained by frequency analysis in the frequency analysis device 7.

For example, the frequency detected by the photodetector 6 is "fm", the difference in distance between the beam splitter 2 and the mirror 3 and between the beam splitter 2 and the reflection point of the optical fiber 5 is "L", and the distance difference is If the wave number of light in “L” is “N”, the refractive index of the medium is “n”, the speed of light in vacuum is “c ₀ ”, and the frequency of the output light of the variable wavelength light source 1 is “f”, then the frequency is "F
m ″ is fm = dN / dt = (2 · n · L / c ₀ ) · df / dt (1).

When the equation (1) is modified with respect to the distance difference "L", L = fm / {(2n / c ₀ ) df / dt} (2)

In equation (2), "n" and "c ₀ " are constants, and since the wavelength sweep is constant in terms of optical frequency, "df / dt" is also constant. Therefore, it is possible to specify the distance difference "L" from the frequency "fm" detected by the photodetector 6.

Further, as described above, the beam splitter 2
Since the distance between the mirror 3 and the mirror 3 is known, the distance between the beam splitter 2 and the reflection point of the optical fiber 5 can be obtained.

FIG. 5 is a characteristic curve diagram showing a measurement example according to the conventional example, in which the optical fiber 5 is 30 m, 20 c.
The optical fibers of m and 60 cm are connected, and the reflection from each connection point is measured. In FIG. 5, "a", "b", and "c" indicate reflections from each connection point.

The sweep condition of the measurement shown in FIG. 5 is df / dt = 71 GHz / s = 2.27 GHz / 32 ms (3).

Since the interference intensity on the vertical axis has an attenuation of "-4.3 dB" per coherent length of the variable wavelength light source 1, the longer the coherent length, in other words, the narrower the spectral line width of the variable wavelength light source 1 is. It enables long-distance measurement.

Further, the length resolution "ΔL" is given by the equation (2) as follows: ΔL = Δfm / {(2n / c ₀ ) df / dt} (4)

[0018]

However, when the frequency component is analyzed by FFT or the like, "Δf" in equation (4) is
Since “m” is determined by the data time width and “n” and “c ₀ ” are constant, “df” must be increased in order to improve the length resolution “ΔL”.

Increasing "df" corresponds to increasing the sweep width of the optical frequency. in this case,
Since the sweep rate “df / dt” is not linear or constant and it is difficult to keep “df / dt” constant, it is difficult to improve the length resolution.

In the FFT, the frequency of the interference light and the FFT
If the analysis frequencies do not match, the analyzed spectrum may deteriorate and sufficient resolution may not be obtained. Figure 6
9 is a characteristic curve diagram showing such a specific example.

FIG. 6 shows the case where the interference light frequency and the analysis frequency match, FIG. 7 shows the case where the interference light frequency and the analysis frequency deviate by 1/4 of the frequency resolution, and FIG. 8 shows the interference light frequency. FIG. 9 shows the case where the analysis frequency is shifted by ½ of the frequency resolution, and FIG. 9 shows the case where FIGS.

When the interference light frequency and the analysis frequency are deviated, the spectrum spreads and the length resolution deteriorates, and the peak value changes by 1 dB or more, and the reflection loss measurement accuracy decreases.

When the optical fiber 5 to be inspected has a plurality of reflection points, the reflected lights at the reflection points may interfere with each other and a ghost corresponding to the difference frequency may be detected.

For example, if there are two reflection points "L1" and "L2", two frequencies "f1" and "f2" are detected by the arithmetic control means 51, and further "f1-f".
A ghost equivalent to 2 "is also detected at the same time.

Since the arithmetic control means 51 cannot discriminate the difference between the frequencies "f1" and "f2" and the frequency "f1-f2", it is observed that the reflection point exists at the position "L1-L2". May be Therefore, an object of the present invention is to realize an optical fiber inspection device capable of recognizing a ghost by improving the length resolution and the reflection loss measurement accuracy.

[0026]

In order to achieve such an object, according to the present invention, the distance of a reflection point of an optical fiber to be inspected and the reflection amount from the reflection point are obtained by using light interference. In an optical fiber inspection device, a variable wavelength light source capable of sweeping an output light frequency, and output light of the variable wavelength light source is incident on a movable mirror and the optical fiber, and reflected light from the movable mirror and the optical fiber is interfered with each other. A Michelson interferometer, a photodetector that detects the output light of this Michelson interferometer, and the position of the movable mirror are controlled, and the distance and the reflection amount of the reflection point are based on the output of the photodetector. And a calculation control means for obtaining

[0027]

By measuring the interference light by changing the optical path of the reference light and superposing the measurement results, the length resolution and the reflection loss measurement accuracy are improved. The ghost can be recognized by changing the optical path of the reference light, measuring the interference light, and detecting the presence or absence of the frequency shift at this time.

[0028]

The present invention will be described in detail below 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. Here, 1, 2, and 4 to 7 are assigned the same reference numerals as in FIG. In FIG. 1, 3a is a movable mirror, and 8 is a control device.

Reference numerals 2, 3a and 4 constitute Michelson interferometer 50a, and 7 and 8 constitute arithmetic control means 51, respectively.

The output light of the variable wavelength light source 1 is incident on the beam splitter 2. The beam splitter 2 reflects a part of the incident light and makes it enter the movable mirror 3 a, and transmits the remaining incident light and makes it enter the optical fiber 5 via the lens 4.

The light reflected by the movable mirror 3a is incident on the beam splitter 2 again. On the other hand, the reflected light from the reflection point of the optical fiber 5 is also incident on the beam splitter 2 via the lens 4, and is combined with the reflected light from the movable mirror 3a.

The interference light generated as a result of the multiplexing is the photodetector 6
And is converted into an electric signal. The output of the photodetector 6 is connected to the frequency analysis device 7, and the control signal from the frequency analysis device 7 is connected to the variable wavelength light source 1.

Further, the output of the frequency analysis device 7 is connected to the control device 8, and the control signal of the control device 8 is transmitted to the movable mirror 3a.
Connected to.

The operation of the embodiment shown in FIG. 1 will be described here. The basic operation is the same as in the conventional example shown in FIG. 4, except that the spectrum from the frequency analysis device 7 and the position information of the movable mirror 3a are given to the control device 8, and the control device 8 uses this position information. The point is to control the position of the movable mirror 3a based on the above.

The arithmetic control means 51 changes the optical path of the reference light by "L _3a " by the movable mirror 3a, measures the interference light to obtain a spectrum, and displays the interference intensity for the distance difference "L".

Further, when the optical path of the reference light is changed by "L _3a " by the movable mirror 3a, the equation (2) becomes L = L _3a + fm / {(2 · n / c ₀ ) · df / dt} (5 ) Is transformed. This "L _3a " can be changed within the range of the length resolution "ΔL".

Further, the processing in the arithmetic control means 51 will be described with reference to a flowchart. FIG. 2 is a flow chart showing the processing in the arithmetic control means 51 of the embodiment shown in FIG. Further, the movable mirror 3a allows the optical path "L _3a " of the reference light
Is changed by “ΔL / k” (k is an integer).

First, in (a), initialization and the like are performed,
In (b), the interference light is measured. In (c), the frequency analyzer 7 obtains a frequency spectrum. (D)
In (e) and (e), the optical path of the reference light is changed by “ΔL / k” by the movable mirror 3a, and if the movement amount is “ΔL” or less, (b) and (c) are repeated.

Next, in (f), the distance difference "L" is obtained by using the equation (5). In (g), the interference intensity is the vertical axis,
The distance difference “L” is plotted on the horizontal axis. In (h) and (i), the value of “L _3a ” in the equation (5) is set to “ΔL / k”.
If the changed value is equal to or less than “ΔL”, (f) and (g) are repeated.

After changing the value of "L _3a " by "ΔL", "fm" in equation (5) in (j) is changed to "Δf".
m ”is changed, and if the changed value is less than or equal to the maximum value of fm, (f) to (i) are repeated.

FIG. 3 is a characteristic curve diagram showing the characteristic of the interference intensity with respect to the distance difference "L" obtained by the flow chart shown in FIG. As can be seen from FIG. 3, the spectrum is narrowed, the length resolution is improved, the peak value is increased, and the reflection loss measurement accuracy is improved.

As a result, by changing the optical path of the reference light to measure the interference light and superposing the measurement results, the length resolution and the reflection loss measurement accuracy are improved.

Further, by changing the optical path of the reference light, the peak of interference becomes uneven around the hem,
It is better to change the optical path of the reference light only in the range of about "2ΔL" around the interference peak and superimpose the measurement results.

A method of recognizing a ghost will be described. Here, the movable mirror 3a is set at the position of the distance "D", and normal measurement is performed. If it is the condition of the conventional example, frequency ”
f1 "," f2 "and" f1-f2 "are the operation control means 5
Detected in 1.

[0045] The frequency "f1" and "f2" with the formula (1), f1 = (2 · n · L1 / c 0) · df / dt (6) f2 = (2 · n · L2 / c 0)・ It becomes df / dt (7).

Next, as measured again by moving the movable mirror 3a distance "d" only, f1 '= {2 · n · (L1 + d) / c 0} · df / dt = (2 · n · L1 / c ₀ ) .df / dt + (2.n.d / c ₀ ) .df / dt (8) f2 '= {2.n. (L2 + d) / c ₀ } .df / dt = (2.n.L2 / C ₀ ) · df / dt + (2 · n · d / c ₀ ) · df / dt (9) is detected.

If the second terms of equations (8) and (9) are respectively set to "f0", the frequency detected by the arithmetic control means 51 is f1 '= f1 + f0 (10) f2' = f2 + f0 (11) Becomes

At this time, the frequency "f1 '" is the same as described above.
"F1'-f caused by the interference between" f2 '"and"f2'"
2 ′ ″ becomes f1′−f2 = (f1 + f0) − (f2 + f0) = f1−f2 (12), and is not affected by moving the moving mirror 3a by the distance “d”.

As a result, the arithmetic control means 51 can move the movable mirror 3a from the measurement position by the distance "d", and judge that the object having no frequency shift is a ghost. Therefore, the arithmetic control unit 51 can control the display to delete the display of the ghost or display the ghost separately from the reflected light from the reflection point.

It is also possible to identify the ghost without using the movable mirror 3a in a state where the reference light does not return. That is, since the frequency detected in this state is only the ghost due to the mutual interference of the reflected lights at the reflection points, the arithmetic control unit 51 can recognize and process this frequency.

Although the optical fiber 5 is used as the inspection target in the embodiment shown in FIG. 1, the invention is not limited to this.

Further, although recognition of a ghost has been described with two reflection points, it may be three or more.

[0053]

As is apparent from the above description,
The present invention has the following effects. By changing the optical path of the reference light, measuring the interference light, and superposing the measurement results, it is possible to realize an optical fiber inspection device capable of improving the length resolution and the reflection loss measurement accuracy.

Further, by changing the optical path of the reference light to measure the interference light and detecting the presence or absence of the frequency shift at this time, an optical fiber inspection apparatus capable of recognizing the ghost can be realized.

[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 flowchart showing a process in an arithmetic control unit of the embodiment.

FIG. 3 is a characteristic curve diagram showing characteristics of interference intensity.

FIG. 4 is a configuration block diagram showing an example of a conventional optical fiber inspection apparatus.

FIG. 5 is a characteristic curve diagram showing a measurement example according to a conventional example.

FIG. 6 is a characteristic curve diagram showing a specific example.

FIG. 7 is a characteristic curve diagram showing a specific example.

FIG. 8 is a characteristic curve diagram showing a specific example.

FIG. 9 is a characteristic curve diagram showing a specific example.

[Explanation of symbols]

1 variable wavelength light source 2 beam splitter 3 mirror 3a movable mirror 4 lens 5 optical fiber 6 photodetector 7 frequency analysis device 8 control device 50, 50a Michelson interferometer 51 calculation control means

Front page continued (72) Inventor Mamoru Arihara 2-932 Nakamachi, Musashino City, Tokyo Yokogawa Electric Co., Ltd.

Claims (1)

[Claims] 1. A variable wavelength light source capable of sweeping an output optical frequency in an optical fiber inspection device for obtaining a distance of a reflection point of an optical fiber to be inspected and a reflection amount from the reflection point by using light interference, A Michelson interferometer that causes the output light of the variable wavelength light source to enter the movable mirror and the optical fiber and causes the reflected light from the movable mirror and the optical fiber to interfere, and a light that detects the output light of the Michelson interferometer. An optical fiber inspection apparatus comprising: a detector; and an arithmetic control unit for controlling the position of the movable mirror and obtaining the distance and the reflection amount of the reflection point based on the output of the photodetector. .