JPH08219947A - Optical fiber inspection equipment - Google Patents

Abstract

(57) [Abstract] [Purpose] The light applied to the DUT is made to have low coherence so as to prevent the interference of reflected light in the vicinity. [Structure] Sweep oscillator and intensity modulated E
/ O converter, the output light is incident on the object to be measured through the optical coupler, and the light reflected in the object to be measured is converted into an electric signal by the O / E converter, and this electric signal and the sweep oscillator An optical fiber that is configured to detect the distance to the reflection point and the amount of reflection in the object to be measured by mixing the output with a mixer to obtain a signal with a difference frequency and analyzing this difference frequency with a frequency analyzer. In the inspection device,
A means for expanding the light to be given to the object to be measured to be wider than the spectral line width of the light of the original E / O converter and preventing the interference of the reflected lights from the reflection points in the object to be measured is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inspection device for optical fibers and the like.

[0002]

2. Description of the Related Art In addition to a well-known OTDR (Optical Time Domain Reflectmeter), an optical fiber inspection apparatus includes an OFDR (Optical Frequency Domain Reflectme
ter). Figure 3 OFDR with fiber interferometer
The block diagram of is shown.

In FIG. 3, an electric / optical converter (hereinafter referred to as E /
The light (for example, laser light) emitted from the O converter 3 is supplied to the sweep oscillator 1 through the distributor 2.
It is driven by the output of the output light and is controlled so that the frequency of the output light is swept. The output light passes through the optical coupler 4 and is measured by an optical fiber (hereinafter referred to as DUT (Device Under Tes).
t) 5 is entered and the obstacle point in DUT 5 (eg fracture surface)
And so on. The reflected light enters the optical / electrical converter (O / E converter) 6 through the optical coupler 4, is converted into an electric signal, and enters the mixer 7.

The output of the sweep oscillator 1 is also input to the mixer 7 via the distributor 2 and outputs a signal having a difference frequency between the two input signals. This signal is input to the frequency analyzer 8 and its difference frequency is analyzed.

Here, the sweep oscillator 1 is set to a unit time (Δ
The sweep is performed linearly so that the frequency changes by Δf per t). The difference between the two input frequencies of the mixer 7 is proportional to the delay time difference. Since the delay time difference is proportional to the distance to the fault point in the DUT 5, the distance to the fault point can be known by performing frequency analysis, and the amount of reflection at the fault point, that is, the fault point can be determined from the signal size. You can know the size of.

For example, the output signal of the sweep oscillator 1 is 1
It is assumed that the sweep is performed at a rate of GHz / sec. D
If the UT 5 is an optical fiber with a refractive index of n = 1.5, the time t during which light travels back and forth through the fiber for 1 m is: t = 2 × n / C≈1 × 10 ⁻⁸ (sec) where C is in vacuum Is the speed of light. From the sweep rate of the sweep oscillator 1, this time t
Causes a frequency difference of 10 Hz. That is, by analyzing the difference frequency with the frequency analyzer 8, the position of the fault point can be detected at 1 m per 10 Hz.

Now, the coherence length of light will be considered as follows. The coherence length L is simply L = C / Δν, where Δν is the spectral line width (half-value width). Then, assuming that the spectral line width Δν is 100 MHz, L = 3 (m). Since this is a one-way distance in a vacuum, it corresponds to 1 m when converted in consideration of the round trip in the fiber and the refractive index (n = 1.5).

[0008]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
If multiple obstacle points in 5 are close and if E / O with high coherence is used (laser light etc.), the reflected lights from the adjacent obstacle points interfere with each other and become a noise component. There was a problem that it would end up.

It is an object of the present invention to provide an optical fiber inspection apparatus which solves the above problems by making the light applied to the DUT in the intensity modulation type OFDR have low coherence.

[0010]

In order to achieve such an object, according to the present invention, the E /
The output light of the O converter is intensity-modulated, the output light is incident on the object to be measured through the optical coupler, and the light reflected within the object to be measured is converted into an electric signal by the O / E converter. The signal of the difference frequency is obtained by mixing the signal and the output of the sweep oscillator with a mixer, and by analyzing this difference frequency with a frequency analyzer, the distance to the reflection point in the object to be measured and the reflection amount are detected. In the configured optical fiber inspection device, the light to be given to the object to be measured is expanded more than the spectral line width of the light of the original E / O converter, and the interference between the reflected lights from the reflection points in the object to be measured is caused. It is characterized in that it is provided with a means for preventing.

[0011]

A means for ensuring that the light incident on the DUT has low coherence is provided, and the spectral line width is set to the original E / O.
By making it larger than that of the converter, interference between reflected lights that are close to each other is prevented.

[0012]

The present invention will be described in detail below. FIG. 1 is a block diagram showing an embodiment of the optical fiber inspection apparatus according to the present invention, in which the light given to the DUT 5 is expanded more than the spectral line width of the light of the original E / O converter, and the reflected light from the fault point is shown. As a means for preventing mutual interference, a high frequency signal generator for superimposing a high frequency on the E / O drive signal and a synthesizer are used. The same parts as those in FIG. 3 are designated by the same reference numerals, and the description thereof will be omitted. In the figure, 9 is a high-frequency signal generator that generates a high-frequency signal (waveform is, for example, a sine wave), 10
Is a combiner that superimposes the output of the high-frequency signal generator 9 on the output of the distributor 2.

In such a configuration, the E / O converter 3
Is the output signal of the sweep oscillator 1 and the high frequency signal generator 9
It is driven by a signal obtained by combining the output signal of In addition,
In this case, a laser diode is used as the E / O converter 3. This allows the laser diode to easily generate laser light with an expanded spectral line width,
Even if there are multiple points of failure in the DUT 5
It is possible to prevent interference between reflected lights from the obstacle point.
The frequency of the output signal of the high frequency signal generator 9 is preferably different from the frequency of the output signal of the sweep oscillator 1.

FIG. 2 shows another embodiment of the present invention, which is a DUT.
As a means for preventing the interference of the reflected light from the fault point with each other by expanding the light to be given to 5 to the spectral line width of the light of the original E / O converter, a highly coherent laser beam (spectral line width 100 MHz The following) and an optical frequency shifter are used. In the figure, 11 is a frequency shifter for shifting the frequency of the output light of the E / O converter 3 and expanding the spectral line width. As the E / O converter 3, laser light having a high coherence (spectrum line width 100 MHz or less) can also be used. As a result, it is possible to prevent the interference of the reflected lights from the obstacle points, as in the above-described embodiment.

The present invention is not limited to the above embodiment. For example, a laser beam having a spectral line width of 100 MHz or more may be used as the E / O converter 3 without using the high frequency signal generator and the synthesizer or the frequency shifter 11, and the same effect can be obtained. . Also, E /
A light emitting diode (LED) or a high brightness diode (SLD) may be used as the O converter 3. Further, although it is not a method of directly changing the output light of the E / O converter 3, it is also possible to use Fourier transform in the frequency analysis method to find a fault point in the vicinity.

An optical amplifier (fiber type) may be provided for applications such as when the transmission loss of light is large or when the amount of light reflection at the fault point is small. Furthermore, when measuring the operating optical fiber, a wavelength different from the operating light wavelength is used as the E / O converter, and if necessary, in the preceding stage of the O / E converter 6. A wavelength discriminator that removes unnecessary wavelengths is used. As the wavelength discriminator in this case, an optical filter, an interference filter, or substance absorption can be used. For example, to inspect an optical fiber used at 1.55 μm, an E / O converter that emits light at 1.65 μm is used.
A wavelength discriminator that removes is used.

[0017]

As described above, the present invention has the following effects. Although the intensity modulation type OFDR can greatly extend the measurable distance (optical frequency modulation type OFDR
In DR, it is limited by the coherence length of the light source), 2
If there are obstacle points close to each other, the reflected lights interfere with each other and become a noise component. Therefore, E / O
A light source with a short coherence length is required as a converter. According to the present invention, the coherence length is short (including equivalent cases).
By using the light source, it is possible to realize an optical fiber inspection apparatus capable of detecting a fault point with high resolution (<1 m) even when there are two fault points close to each other.

[Brief description of drawings]

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

FIG. 2 is a block diagram of another embodiment of the present invention.

FIG. 3 is a configuration diagram showing an example of a conventional optical fiber inspection apparatus.

[Explanation of symbols]

 1 Sweep Oscillator 2 Divider 3 E / O Converter 4 Optical Coupler 5 DUT 6 O / E Converter 7 Mixer 8 Frequency Analyzer 9 High Frequency Signal Generator 10 Combiner 11 Frequency Shifter

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshihiko Tachikawa 2-3-9 Nakamachi, Musashino-shi, Tokyo Yokogawa Electric Co., Ltd.

Claims (4)

[Claims] 1. An output light of an E / O converter is intensity-modulated by an output of a sweep oscillator, and the output light is made incident on an object to be measured through an optical coupler and the light reflected inside the object to be measured is O / O.
The signal of the difference frequency is obtained by converting the electric signal with the E converter and mixing the electric signal with the output of the sweep oscillator with a mixer, and analyzing the difference frequency with a frequency analyzer In an optical fiber inspection device configured to detect a distance to a point and an amount of reflection, the light to be given to the object to be measured is expanded more than the spectral line width of the light of the original E / O converter, An optical fiber inspection apparatus comprising means for preventing interference between reflected lights from a reflection point in an object. 2. A laser beam having high coherence is used as the E / O converter, and the output light of the E / O converter is used as a means for preventing interference between reflected lights from the reflection points. 2. The optical fiber inspection apparatus according to claim 1, wherein an optical frequency shifter for shifting the frequency is used, and the apparent spectral line width is made wider than that of the original E / O converter. 3. A means for preventing interference between reflected lights from the reflection point comprises a high frequency signal generator for generating a high frequency signal, and a combiner for superimposing the high frequency signal on the output of the sweep oscillator. With the output of this synthesizer, the E / O
2. A converter is driven so as to drive the converter.
The optical fiber inspection device described. 4. The optical fiber inspection apparatus according to claim 1, wherein the frequency for performing the high frequency superposition is set to a frequency other than the frequency of the output signal of the sweep oscillator.