JP3064136B2 - Measurement method of two-dimensional magnetic field distribution - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring a two-dimensional magnetic field distribution used for flaw detection of a steel sheet, evaluation of a superconducting material, detection of residual strain of stainless steel, and the like.

[0002]

2. Description of the Related Art Conventionally, two-dimensional magnetic field distribution is measured by scanning a magneto-electric conversion element such as a minute Hall element or a magneto-resistance element on an object to be measured, or spreading the magneto-electric conversion element in two dimensions. I was going. However, although small, the size of the magnetoelectric conversion element is about 0.5 mm square even if it is small, and spatial resolution was not satisfactory.

In recent years, a method of measuring a two-dimensional magnetic field distribution using a magnetic garnet film (hereinafter, referred to as an RIG film) has attracted attention. FIG. 1 shows an outline of this measuring method. The light emitted from the light source 1 becomes linearly polarized light after passing through the polarizer 2,
The light impinges on the RIG film 4 disposed immediately above the object 3 to be measured.
After that, the light passes through the inside of the RIG film 4, is reflected by the reflection film 5, passes through the RIG film 4 again, passes through the polarizer 6, enters the photodetector 7, and is converted into an electric signal.

In general, the relative angle between the polarizer 2 and the polarizer 6 is set to 45 ° at which the signal level becomes maximum. If the magnetic field intensity from the measured object 3 is spatially (two-dimensionally) modulated, the RIG film 4
The principle is that the light that has passed through is subjected to an optical rotation action and is intensity-modulated when passing through the polarizer 6. This method uses RIG
A spatial resolution corresponding to the magnetic domain width of the film 4 (several tens of μm or less) can be obtained, and a large area can be measured according to the area of the RIG film 4 by scanning the RIG film 4 with light incident on the RIG film 4. Becomes possible.

[0005]

However, although the reason is not clear in this method, even when there is no modulation in the magnetic field intensity from the object to be measured, it depends on the position in the plane of the RIG film irradiated with the incident light. There is a phenomenon that the intensity of light returning from the RIG film, that is, the intensity of light incident on the photodetector 7 in FIG.
There has been a problem that this abnormal noise causes a decrease in S / N (preferably 15 dB or more) when the modulation of the magnetic field intensity from the object to be measured is small.

Therefore, as a result of examining the cause of the abnormal noise, the state of multiple reflection of light inside the RIG film changes at a position within the RIG film surface due to unevenness in the thickness of the RIG film. It turned out to be because. Further, it was found that this phenomenon appeared even when the thickness of the RIG film was non-uniform about 0.1 μm. However, the current thickness control by polishing the RIG film is 1 μm, and it was difficult to measure when the magnetic field modulation was small.

SUMMARY OF THE INVENTION In view of such circumstances, an object of the present invention is to provide a method for measuring a two-dimensional magnetic field distribution with a good S / N even when the modulation of the magnetic field intensity from the object to be measured is small.

[0008]

According to the method of measuring a two-dimensional magnetic field distribution according to the present invention, linearly polarized light is made incident on a magnetic garnet film having a reflective film formed on a surface facing an object to be measured.
In the method of measuring the two-dimensional magnetic field distribution by intensity-modulating the light reflected from the reflective film and emitted from the magnetic garnet film through a polarizer, the polarizer may be arranged such that the light is incident on the magnetic garnet film with respect to the polarization plane of the light. In an arrangement of 65 ° to 75 °.

[0009]

The present inventors have conducted investigations and have found that even when there is no modulation in the magnetic field intensity from the object to be measured, the photodetector in FIG. the size of the abnormal noise intensity of the incident light is referred to vary to 7, the relative angle or the polarization plane of the incident light to the RIG film and polarization of the polarizer 6 and the polarizer 6 and the polarizer 2 in FIG. 1
It has been found that it depends on the angle formed by the plane (hereinafter referred to as a polarizer relative angle). Therefore, S / N can be improved by setting the relative angle of the polarizer to an optimum value.

[0010]

First, the relationship between the magnitude of the abnormal noise and the relative angle of the polarizer was examined in detail. Abnormal noise was measured with the configuration shown in FIG. The light source 1 has a laser diode for emitting light having a wavelength of 0.78 μm, the polarizers 2 and 6 have a polarizer beam splitter, and the RIG film 4 has a thickness of 15 μm.
And a rectangular (GdBi) ₃ (FeAl) ₅ O ₁₂ of 5 mm × 50 mm, and a Si photodiode as the photodetector 7. Also, RIG film 4
Was provided with an Al reflective film 5 by a vacuum evaporation method.

In this case, the object to be measured was not arranged because only the magnitude of the abnormal noise was measured. The measurement is performed by moving the rectangular RIG film 4 in parallel along the long side and recording the output of the photodetector while changing the light irradiation position, and further changing the relative angle of the polarizer. This measurement was performed. The light incident on the RIG film 4 was a 1 mmφ spot light.

FIG. 2 shows the measurement results of the magnitude of the abnormal noise when the relative angle of the polarizer is changed, with the fluctuation of the output of the photodetector as the magnitude of the abnormal noise. Here, since through the polarizer 6 and changing the polarizer relative angle also changes the absolute amount of light incident on the light detector 7, an average value of the output of the photodetector when varying polarizer relative angle Standardized, and
When the relative angle of the polarizer is 45 °, the magnitude of abnormal noise is 1
And From the results in FIG. 2, the magnitude of the abnormal noise greatly depends on the relative angle of the polarizer, and the relative angle of the polarizer is 70 °.
It can be seen that the abnormal noise is minimized in the vicinity. The thickness of the RIG film was changed, but the tendency in FIG. 2 was hardly changed.

According to FIG. 2, the polarizer relative angle at which anomalous noise is minimized is around 70 °, which is 45 ° which is the polarizer relative angle at which the signal level due to the magnetic field modulation becomes the largest.
And different. Then, the object to be measured is placed next as shown in FIG. 1, and it is assumed that the magnetic field intensity is modulated.
The polarizer relative angle of 15 dB or more was determined. A steel plate provided with artificial scratches was used as the object to be measured. That is, the magnetic field leaking from the artificial scratch on the steel plate modulates the magnetic field intensity.

The measurement is the same as the measurement of the magnitude of the above-mentioned abnormal noise, except that a steel plate as an object to be measured is arranged. The results are shown in FIG. The signal due to the leakage magnetic field from the scratch on the steel plate becomes maximum when the polarizer relative angle is 45 °, and becomes almost zero at 0 ° and 90 °. On the other hand, the abnormal noise becomes minimum around 70 ° reflecting the result of FIG. S / N is only 5d at 45 ° which is the conventional polarizer relative angle.
While it is about B, it becomes 18 dB or more around 70 °. Also, when the polarizer relative angle is 65 ° to 75 °, the S / N is 15 dB.
As described above, the measurement of the signal by the leakage magnetic field from the scratch on the steel plate was successfully performed.

[0015]

As described above, according to the method of measuring a two-dimensional magnetic field distribution according to the present invention, it is possible to measure a slight modulation of the magnetic field intensity distributed two-dimensionally with good S / N.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a method for measuring a two-dimensional magnetic field distribution according to the present invention using a magnetic garnet film.

FIG. 2 is a diagram showing the relationship between the magnitude of abnormal noise and the relative angle of a polarizer.

FIG. 3 is a diagram illustrating a relationship between S / N and a relative angle of a polarizer.

[Description of Signs] 1 Light source 2, 6 Polarizer 3 Object to be measured 4 Magnetic garnet film (RIG film) 5 Reflective film 7 Photodetector

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kazuyuki Haruna 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. (56) References JP-A-5-333124 (JP, A) ( 58) Field surveyed (Int.Cl. ⁷ , DB name) G01R 33/00-33/18

Claims (1)

(57) [Claims] 1. A linearly-polarized light is made incident on a magnetic garnet film having a reflective film provided on a surface facing an object to be measured, and light reflected by the reflective film and emitted from the magnetic garnet film is passed through a polarizer. In a method for measuring a two-dimensional magnetic field distribution by intensity modulation, the two-dimensional magnetic field distribution is characterized in that the polarizer is arranged at 65 ° to 75 ° with respect to a polarization plane of light incident on the magnetic garnet film. Measurement method.