JP3223942B2 - Optical fiber inspection equipment - Google Patents

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 return loss measurement accuracy.

[0002]

2. Description of the Related Art A conventional optical fiber inspection apparatus using optical interference (OFDR: Optical Frequency Domain Reflectmet)
er) is to determine the distance between the reflection points of the optical fiber to be inspected and the amount of reflection from the reflection points, and there is a method 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 analyzer such as an FFT. . Here, 2 to 4 constitute the Michelson interferometer 50.

The output light of the variable wavelength light source 1 is incident on a beam splitter 2, which reflects a part of the incident light and impinges on a mirror 3, transmits the remaining incident light and passes through a lens 4. The light enters 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 also enters the beam splitter 2 via the lens 4 and is combined with the reflected light from the mirror 3.

[0006] The interference light resulting from the multiplexing is applied to the photodetector 6.
And is converted into an electric signal. The output of the photodetector 6 is connected to a frequency analyzer 7, and a control signal from the frequency analyzer 7 is connected to the variable wavelength light source 1.

Here, 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 light detector 6 and analyzed by the frequency analyzer 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 the interference intensity at a certain optical frequency. This optical frequency is equal to the beam splitter 2, mirror 3,
The distance 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 analyzer 7.

For example, the frequency detected by the photodetector 6 is “fm”, the difference between the distance between the beam splitter 2 and the mirror 3 and the distance between the reflection points of the beam splitter 2 and the optical fiber 5 is “L”, and the distance difference If the wave number of the light within “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”, "F
m ″ is fm = dN / dt = (2 · n · L / c ₀ ) · df / dt (1)

If 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 “df / dt” is also constant since the wavelength sweep is constant in terms of optical frequency. Accordingly, 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 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.

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

The sweep condition for 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, the smaller the spectral line width. Long distance measurement is possible.

From equation (2), the length resolution “ΔL” is given by ΔL = Δfm / {(2 · n / c ₀ ) · df / dt} (4)

[0018]

However, when frequency components are analyzed by FFT or the like, "Δf" in equation (4) is used.
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”.

Here, increasing "df" corresponds to increasing the sweep width of the optical frequency. in this case,
Since the sweep rate “df / dt” is no longer linear or constant, and it is difficult to maintain “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 be degraded and sufficient resolution may not be obtained. FIG.
FIG. 9 to FIG. 9 are characteristic curve diagrams showing such specific examples.

FIG. 6 shows a case where the interference light frequency and the analysis frequency match, FIG. 7 shows a case where the interference light frequency and the analysis frequency are shifted by ４ of the frequency resolution, and FIG. FIG. 9 shows a case where the analysis frequency is shifted by の of the frequency resolution, and FIG. 9 shows a case where FIGS.

If the interference light frequency and the analysis frequency deviate from each other, the spectrum is spread and the length resolution is reduced, and the peak value is changed by 1 dB or more, so that the reflection loss measurement accuracy is reduced.

When a plurality of reflection points are present on the optical fiber 5 to be inspected, reflected lights at the respective reflection points may interfere with each other, and a ghost corresponding to a 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 and control means 51, and "f1-f"
A ghost corresponding 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 frequencies "f1-f2", it is assumed that the reflection point exists at the position "L1-L2". May be done. Therefore, an object of the present invention is to realize an optical fiber inspection apparatus capable of improving ghost resolution by improving the length resolution and return loss measurement accuracy.

[0026]

In order to achieve the above object, according to the present invention, the distance between the reflection points of an optical fiber to be inspected and the amount of reflection from the reflection points are determined using light interference. In an optical fiber inspection apparatus, a variable wavelength light source capable of sweeping an output light frequency, and the output light of the variable wavelength light source is made incident on a movable mirror and the optical fiber to interfere with reflected light from the movable mirror and the optical fiber. A Michelson interferometer, a photodetector for detecting the output light of the Michelson interferometer, and the position of the movable mirror are changed.
The variable wavelength light source at each position of the movable mirror.
And a calculation control means for obtaining the distance and the reflection amount of the reflection point based on the output of the photodetector obtained by sweeping the wavelength of the output light .

[0027]

The interference light is measured by changing the optical path of the reference light, and the measurement results are superimposed, so that the length resolution and the return loss measurement accuracy are improved. The ghost can be recognized by measuring the interference light by changing the optical path of the reference light and detecting the presence or absence of a frequency shift at this time.

[0028]

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. Here, 1, 2, and 4 to 7 are denoted by the same reference numerals as in FIG. In FIG. 1, 3a is a movable mirror, and 8 is a control device.

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

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 to enter the movable mirror 3a, transmits the remaining incident light, and enters the optical fiber 5 via the lens 4.

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

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

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

Here, the operation of the embodiment shown in FIG. 1 will be described. The basic operation is the same as that of 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 The point is that the position of the movable mirror 3a is controlled based on this.

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 with respect to the distance difference "L".

When the optical path of the reference light is changed by “L _3a ” by the movable mirror 3a, the equation (2) becomes as follows: L = L _3a + fm / {(2 · n / c ₀ ) · df / dt} (5) ). This “L _3a ” is 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 flowchart showing the processing in the arithmetic control means 51 of the embodiment shown in FIG. Further, the optical path “L _3a ” of the reference light is set by the movable mirror 3a.
Are changed by “ΔL / k” (k is an integer).

First, initialization is performed in (a),
In (b), the wavelength of the output light of the variable wavelength light source 1 is swept.
The interference light is measured Te. In (c), the frequency analysis device 7
To obtain a frequency spectrum. In (d) and (e), the movable mirror 3a changes the optical path of the reference light to “ΔL /
k) is changed, and if the moving amount is equal to or less than “ΔL”, (b) and (c) are repeated.

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

After changing the value of “L _3a ” by “ΔL”, “fm” in equation (5) in FIG.
m ”is changed, and if the changed value is equal to or less than the maximum value of fm, the steps (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 flowchart 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 return loss measurement accuracy is improved.

As a result, the interference light is measured by changing the optical path of the reference light, and the measurement results are superimposed, whereby the length resolution and the return loss measurement accuracy are improved.

Also, by changing the optical path of the reference light, the peak of interference becomes uneven around the skirt.
It is better to change the optical path of the reference light only within a range of about 2ΔL with the interference peak as the center and superimpose the measurement results.

Next, a method for recognizing a ghost will be described. Here, normal measurement is performed with the movable mirror 3a positioned at the distance "D". Frequency under the conditions of the conventional example "
f1 "," f2 "and" f1-f2 "are the arithmetic control means 5
1 is detected.

From the equation (1), the frequencies "f1" and "f2" are expressed as follows: f1 = (2 · n · L1 / c ₀ ) · df / dt (6) f2 = (2 · n · L2 / c ₀ ) Df / dt (7)

Next, when the movable mirror 3a is moved by the distance "d" and measured again, 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.

Assuming that the second term of the equations (8) and (9) is “f0”, the frequency detected by the arithmetic control means 51 is f1 ′ = f1 + f0 (10) f2 ′ = f2 + f0 (11) Becomes

At this time, similarly to the above, the frequency "f1 '"
Ghost “f1′−f” generated by interference between “f1” and “f2 ′”
2 ′ ″ becomes f1′−f2 = (f1 + f0) − (f2 + f0) = f1−f2 (12), and is not affected by moving the movable mirror 3a by the distance “d”.

As a result, the arithmetic and control means 51 can move the movable mirror 3a by the distance "d" from the measurement position, and can determine that the mirror which has not been shifted in frequency is a ghost. Therefore, the arithmetic and 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 a ghost without using the movable mirror 3a and 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 interference of the reflected lights at the reflection points, the arithmetic control unit 51 can recognize and process this frequency.

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

Although the ghost recognition is described using two reflection points, three or more reflection points may be used.

[0053]

As is apparent from the above description,
According to the present invention, the following effects can be obtained. By measuring the interference light by changing the optical path of the reference light and superimposing the measurement results, it is possible to realize an optical fiber inspection apparatus capable of improving the length resolution and the reflection loss measurement accuracy.

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

[Brief description of the drawings]

FIG. 1 is a configuration block diagram showing one 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 device.

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 analyzer 8 Controller 50, 50a Michelson interferometer 51 Operation control means

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-5-203412 (JP, A) JP-A-59-54907 (JP, A) JP-A-62-157502 (JP, A) (58) Investigation Field (Int.Cl. ⁷ , DB name) G01M 11/00-11/02 G01B 9/00-9/10

Claims (1)

(57) [Claims] An optical fiber inspection apparatus for determining a distance between a reflection point of an optical fiber to be inspected and an amount of reflection from the reflection point using interference of light, comprising: a variable wavelength light source capable of sweeping an output optical frequency; A Michelson interferometer that causes the output light of the variable wavelength light source to enter a movable mirror and the optical fiber and interferes with reflected light from the movable mirror and the optical fiber; and a light that detects the output light of the Michelson interferometer. A detector, the position of the movable mirror being changed , and the movable mirror being changed.
Sweep the wavelength of the output light of the variable wavelength light source at each position of
An optical fiber inspection apparatus, comprising: an arithmetic control unit that obtains the distance and the reflection amount of the reflection point based on the output of the photodetector obtained by the above.