JPH0249667B2 - JIKIKOGAKUSOCHI - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Technical Field of the Invention The present invention relates to a heterodyne-type magnetic field measuring device, and particularly to a device that measures a magnetic field by the heterodyne-type using light.

(b) Background of the technology In recent years, magnetic field measurement methods using small solid-state transducers such as Hall elements or magnetoresistive elements have been put into practical use. In addition, SQUID (Superconducting Quantum) using the Josephson effect
Devices that can measure magnetic fields with high precision, such as interference devices, have also been developed.

(c) Conventional technology and problems Hall elements and magnetoresistive elements detect changes in the movement of current carriers inside them due to magnetic fields as changes in electromotive force or current, and the measurement accuracy that can be obtained is generally limited. It's not expensive.

On the other hand, SQUID can detect weak magnetic flux of about one flux quantum, but the measurement element needs to be cooled to an extremely low temperature and the device is large-scale, so its application is limited to special purposes.

(d) Purpose of the Invention The purpose of the present invention is to provide a novel magneto-optical device that is relatively small and capable of highly accurate measurement.

(e) Structure of the Invention The present invention provides a semiconductor laser having an active layer that outputs two linearly polarized lights whose vibration planes are orthogonal to each other;
a first reflecting mirror provided on one light emitting end side of the semiconductor laser, and a main axis between the one light emitting end and the first reflecting mirror with respect to the active layer of the semiconductor laser; a first λ for converting the two linearly polarized lights that are provided at an angle of +45 degrees and emitted from the one light exit end into circularly polarized lights that rotate in opposite directions in their traveling directions; /4 plate, a second reflecting mirror provided on the other light emitting end side of the semiconductor laser, and a second reflecting mirror provided on the other light emitting end side of the semiconductor laser;
and a reflecting mirror such that its main axis forms an angle of −45 degrees with respect to the active layer of the semiconductor laser, and the two linearly polarized lights emitted from the other light emitting end are A second λ/4 for converting into circularly polarized light that rotates in opposite directions in the direction of travel.
plate, and a Faraday rotator provided between the second λ/4 plate and the second reflecting mirror and for solving degeneracy of the mutually opposite circularly polarized light, One form of this includes a case where the light emitting end of the semiconductor laser and the λ/4 plate are connected by a polarization maintaining fiber.

(f) Embodiments of the invention Examples of the invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, in which a semiconductor laser 1 has a reflecting surface (reflecting mirror) forming an external cavity.
2 and 3 are provided. Further, a λ/4 plate 4 is provided in front of the reflecting surface 2, and a Faraday rotator 5 and a λ/4 plate 6 are provided in front of the reflecting surface 3. In the figure, 7 is a type of optical fiber called a polarization maintaining fiber.

Generally, the resonator constituted by the semiconductor laser 1 includes:
There are TE waves and TM waves whose polarization planes are orthogonal to each other. However, in the system with the configuration shown in FIG. 1, for example, the TE wave emitted from the semiconductor laser 1 to the left side in the figure becomes, for example, clockwise circularly polarized light by passing through the λ/4 plate 4. , this is reflected by the reflecting surface 2 and passes through the λ/4 plate 4 again, thereby becoming linearly polarized light whose plane of polarization has been rotated by π/2, that is, a TM wave. Similarly, the TM wave emitted from the semiconductor laser 1 to the right in the figure is λ/
By passing through the 4-plate 6, it becomes, for example, counterclockwise circularly polarized light, and after being reflected by the reflecting surface 3, it passes through the λ/4 plate 6 again and becomes a TE wave.
In other words, the light emitted as a TE wave is reflected by the reflecting surface 2.
After making one round trip between and 3, it returns to the TE wave. On the contrary, the light emitted as a TM wave, after making one round trip between the reflecting surfaces 2 and 3,
Return to TM waves.

In the above system, two types of light pass through the Faraday rotator 5, that is, the semiconductor laser 1 in the figure.
The TE wave and TM wave emitted to the right side of the beam are each circularly polarized in opposite directions. Under a magnetic field, the Faraday rotator 5 exhibits different refractive indices for clockwise and counterclockwise circularly polarized light. Therefore, there will be a difference in the time required for each circularly polarized light to pass through the Faraday rotator 5 having a constant thickness. As a result, the distances between the reflecting surfaces 2 and 3 are apparently different for the two types of light, and these lights have different resonance frequencies.

Here, if a Faraday rotator is not provided, or if a magnetic field is not applied even if it is provided, the clockwise and counterclockwise circularly polarized light are in a time-reversal relationship with each other, so they are degenerate. . When a magnetic field is applied to the Faraday rotator, the degeneracy is resolved, and each mode of light has a frequency of ω + Δω.
and ω-Δω. However, ω is the center frequency.

Therefore, by providing means for controlling and changing the applied magnetic field, an optical frequency modulator can be constructed.

On the other hand, the difference in refractive index exhibited by the Faraday rotator for each of the right-handed and left-handed circularly polarized lights is proportional to the magnetic field strength. That is, nr~nl∞H where nr and nl are the refractive indexes for clockwise and counterclockwise circularly polarized light, respectively, and H is the magnetic field strength. Therefore, the above difference frequency Δω is the magnetic field strength H
By detecting the Δω, the magnetic field strength can be determined, and a magnetic field measuring device can also be constructed.

The detection of Δω can be performed by detecting a beat signal obtained by interfering the two lights of frequencies ω+Δω and ω−Δω generated as described above. According to this method, measurement accuracy is several orders of magnitude higher than that of conventional magnetic field measurement methods, which detect changes in the rotation angle of the plane of polarization that occur when linearly polarized light passes through a Faraday rotator. becomes possible.

In the illustrated measurement system, a polarization maintaining fiber 7 is provided. The polarization-maintaining fiber is a commercially available optical fiber having the property of preserving the polarization mode of light passing through it. When a semiconductor laser is used as a light source, TE waves and TM waves whose polarization planes are orthogonal to each other can be emitted as described above, but inside a polarization maintaining fiber, the TE waves and TM waves are
Waves pass independently of each other, just as they do when propagating through space. Therefore, by using the polarization maintaining fiber, the optical axis of the left half (semiconductor laser 1, λ/4 plate 4, reflective surface 2) of the system shown in FIG. There is no need for a spatial arrangement in which the optical axes of the four plates 6, Faraday rotator 5, and reflection surface 3 coincide with each other. That is, by installing only the right half of the above system in the magnetic field to be measured,
This makes measurement possible. As a result, it is possible to configure the right half of the system as a small probe and use it to perform highly accurate magnetic field measurements.
In this measuring device, since the probe and the main body of the device including the laser light source are coupled by an optical fiber, there is of course an advantage that accuracy does not deteriorate due to externally induced noise.

Note that in the semiconductor laser used in the present invention, it is desirable to apply an antireflection coating to the light emitting end facet.

(g) Effects of the Invention According to the present invention, it is possible to measure a magnetic field in a microscopic space with high precision without requiring cooling and without being affected by externally induced noise, and it is also possible to modulate an optical frequency. It has the effect of

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of the configuration of a magnetic field measuring device according to the present invention. In the figure, 1 is a semiconductor laser, 2 and 3 are reflective surfaces, 4 and 6 are λ/4 plates, 5 is a Faraday rotator, and 7 is a polarization-maintaining fiber.

Claims (1)

[Scope of Claims] 1. A semiconductor laser having an active layer that outputs two linearly polarized lights whose vibration planes are orthogonal to each other; a first reflecting mirror provided on one light emitting end side of the semiconductor laser; The light is provided between one light emitting end and the first reflecting mirror so that its main axis forms an angle of +45 degrees with respect to the active layer of the semiconductor laser, and the light is emitted from the one light emitting end. a first λ/4 plate for converting the two linearly polarized lights into oppositely circularly polarized lights in their traveling directions; and a second λ/4 plate provided on the other light emitting end side of the semiconductor laser. A reflecting mirror is provided between the other light emitting end and the second reflecting mirror so that its main axis forms an angle of −45 degrees with respect to the active layer of the semiconductor laser, and a second λ/4 plate for converting the two linearly polarized lights emitted from the exit end into circularly polarized lights in opposite directions in their traveling directions; 1. A magneto-optical device comprising: a Faraday rotator provided between the second reflecting mirror and a Faraday rotator for solving degeneracy of the mutually opposite circularly polarized light. 2. The magneto-optical device according to claim 1, wherein the other light emitting end of the semiconductor laser and the second λ/4 plate are optically connected by a polarization maintaining fiber.