JPH0431068B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an optical magnetic field measuring device that utilizes the Faraday effect.

[Technical background of the invention and its problems]

The Faraday effect is a phenomenon in which the plane of polarization of a magneto-optical material placed in a magnetic field rotates in proportion to the strength of the magnetic field. Now, assuming that the strength of the magnetic field is H and the length of the magneto-optical material in the direction of the magnetic field is L, the rotation angle θ of the plane of polarization is θ=VHL (1). Here, V is called the Verdet constant and is determined by the material of the magneto-optical substance.

In Figure 4, which shows a conventional magnetic field measuring device using the Faraday effect, 1 is a light source that emits linearly polarized light, 2 is a magneto-optical material that has a Faraday effect (hereinafter referred to as a Faraday rotator), and 3 is a polarized light source for polarizing incident light. Wollaston prism that separates into two linearly polarized lights whose wavefronts are orthogonal; 4a and 4b are photoelectric converters that convert optical signals into electrical signals; and 5 is a rotation of the plane of polarization from the output signals of the two photoelectric converters 4a and 4b. This is an arithmetic circuit for determining the angle θ.

Now, as shown in Fig. 5, linearly polarized light E _i whose polarization plane is 45 degrees with respect to the Rotated linearly polarized light E ₀ is emitted.
When this linearly polarized light E _i is separated by the Wollaston prism 3 into two linearly polarized lights E _x and E _y whose polarization planes are orthogonal, E _x and E _y become as clear from Fig. 5.
It is expressed by equation (2).

E _x = E ₀ cos (45° + θ) E _y = E ₀ sin (45° + θ) ...(2) The photoelectric conversion devices 4a and 4b generate linearly polarized light E _x , E _y
is an electrical signal proportional to the light intensity |E _x | ² , |E _y | ²
Convert to _x , _y . _x and _y are shown by equation (3).

The arithmetic circuit 5 calculates the rotation angle of the plane of polarization from _x and _y , so that _y − _x / _y + _x = sin2θ (4). That is, in the configuration shown in Figure 4, the detected amount is
The form is sin2θ, and to directly find the rotation angle θ,
This method had the disadvantage of requiring a division mechanism using equation (4). Furthermore, if the drift and gain of the photodiodes (not shown) used in the photoelectric converters 4a and 4b are not the same, errors may occur.

[Purpose of the invention]

An object of the present invention is to provide a highly accurate magnetic field measuring device that does not require a complicated arithmetic circuit such as a dividing mechanism and is not affected by optical transmission loss and drift of a photoelectric converter.

[Summary of the invention]

The magnetic field measurement device according to the present invention includes a light emitting source that generates linearly polarized waves of 0°, 60°, and 120°, and a three-phase signal having a phase difference of 120° electrical angle, and a signal for each phase. consists of a drive circuit that excites the light so that it repeats on and off every 1/2 cycle, and a drive circuit that passes each linearly polarized light wave through a Faraday rotator and an analyzer and enters a photoelectric converter, and outputs the output signal of the photoelectric converter. The present invention is characterized by comprising a phase detection circuit that detects the phase of the phase modulated wave obtained by the above.

[Embodiments of the invention]

The present invention will be described below with reference to an embodiment shown in FIG. In FIG. 1, the same parts as in FIG. 4 are given the same reference numerals. In Figure 1, 6a, 6
b and 6c are three-phase light emitting sources made of single-mode linearly polarized semiconductor lasers, and as shown in Figure 2, linearly polarized light of ₀ °, 60° of linearly polarized light of ₆₀ °, and linearly polarized light of 120° are emitted.
₁₂₀ , these linearly polarized lights pass through Faraday rotator 2 and analyzer 7, and only the component in the direction of the principal axis of analyzer 7 (in this example, the x-axis is the principal axis) is synthesized into photodiode 8. incident.

Now to Faraday rotator 2 the above ₀ , I ₁₆₀ ,
₁₂₀ and a magnetic field H is applied, the linearly polarized light on the output side is proportional to the strength of the magnetic field, and as shown in Figure 2, θ
The linearly polarized light is rotated by ₀ ′, ₆₀ ′, and ₁₂₀ ′.
Now, when the polarization angle of the linearly polarized light wave incident on the analyzer 7 is offset from the main axis of the analyzer by θ, and the light intensity of the incident wave is taken as the light intensity of the output wave, the light intensity ′ of the output wave is ′=
It is known that cos ² θ=/2(1+cos2θ).

Therefore, if the light intensities of the light sources 6a, 6b, and 6c are respectively ₀ , ₆₀ , and ₁₂₀ , and the light intensities of the respective output waves of the analyzer 7 are ₀ ', ₆₀ ', and ₁₂₀ ', then the following (5) is obtained.
The formula holds true.

Photodiode 8 receives these lights ₀ ′, ₆₀ ′,
₁₂₀ ' and converts it into an electrical signal proportional to the combined light. Therefore, the combined incident light intensity I _s of the photodiode 8 is expressed by equation (6).

I _s = ₀ ′ ₆₀ ′ + ₁₂₀ ′ ……(6) On the other hand, ₀ , ₆₀ , ₁₂₀ are as shown in Figure 3.
₀ emits light with light intensity A in the range of ωt = 0 to π,
The process of reducing the light intensity to 0 in the range of ωt=π to 2π can be repeated. For ₆₀ and ₁₂₀ , each from ₀ ,
Turns on and off with a phase delay of 120° and 240°. This ON−
The OFF timing is controlled by the drive circuit 11.

Therefore, the rectangular wave shown in FIG. 3 is expanded into a Fourier series as shown in equation (7).

Substitute equation (5) into equation (6).

I _s = 2A/π {3/2 sin (ωt-2θ) + sin3ωt + 3/10 sin (5ωt + 2θ) +...] ...(8), and an electrical signal proportional to the above equation (8) is obtained as the output of photodiode 8. It will be done.

When only the frequency ω component is extracted from this output by a low-pass filter 9, the output of the low-pass filter 9 becomes a signal proportional to 3A/πsin(ωt−2θ). Therefore, when the magnetic field H changes and θ changes, the waveform becomes phase modulated by 2θ. The phase detector 10 compares the phase modulation waveform with the reference phase of the drive circuit 11 to obtain an output signal proportional to θ.

Therefore, if the ON-OFF ratio per cycle of each light of ₀ , ₆₀ , and ₁₂₀ is not equal as shown in Figure 3, that is, if it is non-equal, the second harmonic component will be included in the Fourier series expansion. appear. Then, the low-pass filter 9 is required to transmit the fundamental wave component and block the second harmonic component and above. In contrast, in the configuration of the present invention, as can be seen from equation (8), the fundamental wave component may be transmitted and the third harmonic component may be blocked. It is well known that a low-pass filter blocks higher harmonic components. Therefore, it goes without saying that the structure of the filter is simpler if it does not include the second harmonic.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to detect the angle θ with high accuracy without error due to photodiode drift and sensitivity changes, and it is also possible to detect the angle θ with high accuracy without any error, and also without the need for complicated arithmetic circuits such as conventional dividing mechanisms. No longer needed.
Therefore, a highly reliable magnetic field measuring device can be obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an example of the magnetic field measuring device according to the present invention, FIG. 2 is a vector diagram illustrating that the polarization angle rotates due to the Faraday effect of the magnetic field measuring device according to the present invention, and FIG. A timing diagram of the light intensity of the light emitting source in the present invention; FIG. 4 is a block configuration diagram showing an example of a conventional magnetic field measuring device;
FIG. 5 is a vector diagram illustrating the rotation of the polarization angle due to the Faraday effect in a conventional magnetic field measuring device. 1... Light source, 2... Faraday rotator, 3... Wollaston prism, 4a, 4b... Photoelectric converter,
5... Arithmetic circuit, 6a, 6b, 6c... Light emitting source, 7...
Analyzer, 8... Photodiode, 9... Low pass filter, 10... Phase detector, 11... Drive circuit.

Claims (1)

[Claims] 1. A light emitting source that generates each linearly polarized light wave whose polarization plane has an angle of 0°, 60°, and 120°, and each of the linearly polarized waves is a three-phase signal with a phase difference of 120° electrical angle, and each phase The signal consists of a drive circuit that excites the signal so that it repeats on and off every 1/2 period, a Faraday rotator and an analyzer through which each linearly polarized light wave passes, and an input of the linearly polarized light wave that has passed through this analyzer. What is claimed is: 1. A magnetic field measurement device comprising: a photoelectric converter that receives the signal and emits an electrical output signal; and a phase detection circuit that detects the phase of a phase modulated wave obtained from the output signal of the photoelectric converter.