Magnetic sensor with faraday element

Details Magnetic sensor with faraday element Magnetic sensor with faraday element

2001-06-06

2002-10-08

Lefkowitz, Edward (Department: 2862)

Electricity: measuring and testing

Magnetic

Magnetometers

C324S096000, C359S484010, C385S012000

Reexamination Certificate

active

06462539

ABSTRACT:

BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a magnetic sensor utilizing a phenomenon in which a plane of polarization of light traveling through a Faraday element rotates in proportion to the strength of a magnetic field. More particularly, the invention relates to a technique, used in measuring a remote magnetic field, for eliminating influences of variations in the plane of polarization caused by disturbances acting on the light transmitted through optical fibers over a long distance.
(2) Description of the Related Art
A conventional magnetic sensor of this type employs a Faraday element for compactness and high sensitivity. Specifically, the strength of a magnetic field is detected by determining a rotating angle of a plane of polarization applied when polarized light passes through the Faraday element.
Generally, it is adequate to transmit light from a light source by using an optical fiber (single mode optical fiber) and pass the light through a Faraday element with no variations occurring with the plane of polarization. However, the light transmitted through the optical fiber undergoes variations in polarization caused, for example, by phase differences due to disturbances such as environmental conditions encountered during transmission. In such a case, the following techniques are employed to eliminate influences of the variations in polarization due to disturbances and the like.
In a first technique, a polarizer is disposed between the outlet of the optical fiber and the Faraday element to allow only linearly polarized light having a plane of polarization in a particular orientation to pass through the Faraday element.
In a second technique, an optical fiber that maintains a plane of polarization is employed so that light from a light source does not easily undergo variations in the plane of polarization due to disturbances occurring during transmission. As shown in
FIG. 1
, for example, in order that rays of light R be transmitted with a plane of polarization maintained perpendicular to X-axis, materials
21
are arranged symmetrically across Y-axis to exert a fixed pressure.
However, the conventional constructions noted above have the following drawbacks.
In the first technique having a polarizer disposed between the outlet of the optical fiber and the Faraday element, incident linearly polarized light changes into elliptically polarized light. This is because the light from the light source undergoes indefinite variations in the plane of polarization during transmission through the optical fiber, under the influence of disturbances due to environmental conditions such as temperature and pressure. It is therefore impossible to determine a transmission angle of the polarizer as desired. That is, where the light transmitted through the optical fiber becomes linearly polarized light with a plane of polarization at right angles (90°) to the polarizer, for example, the light is totally blocked by the polarizer and does not enter the Faraday element.
In the second technique employing an optical fiber that maintains a plane of polarization, where the optical fiber is used over a long distance, a discrepancy of polarization occurs from an initial misalignment (Y-axis in
FIG. 1
) between the plane of polarization maintaining optical fiber and the plane of polarization of incident light. The discrepancy brings about variations in the plane of polarization due to environmental conditions occurring during transmission. The plane of polarization maintaining optical fiber produces crosstalk which is an intrinsic property thereof. This results in extra polarized components precluding accurate detection results.
With detection results acquired under the foregoing conditions, it is difficult to determine whether the rotation of the plane of polarization is due to a magnetic field or to disturbances such as changes in environmental conditions as noted above.
SUMMARY OF THE INVENTION
This invention has been made having regard to the state of the art noted above, and its object is to provide a magnetic sensor of high sensitivity for eliminating influences of disturbance.
The above object is fulfilled, according to this invention, by a magnetic sensor utilizing a phenomenon in which a plane of polarization of light traveling through a Faraday element rotates in proportion to the strength of a magnetic field, the magnetic sensor comprising:
a light output device;
a light branching device connected to the light output device through a first light transmitting device;
a sensor head connected to the light branching device through a second light transmitting device;
a light detecting device connected to the light branching device through a third light transmitting device; and
a computing device for receiving detected signals from the light detecting device;
the sensor head including an optical device, a first birefringent plate, a first Faraday element and a reflecting device arranged in series from an end of the sensor head connected to the second light transmitting device;
the light output device outputting light;
the light branching device receiving the light transmitted from the light output device through the first light transmitting device, and emitting the light to the second light transmitting device;
the optical device of the sensor head converting the light transmitted from the light branching device through the second light transmitting device into parallel light;
the first birefringent plate of the sensor head separating the parallel light received from the optical device into two polarized rays having planes of polarization orthogonal to each other with respect to an optical axis of the first birefringent plate;
the first Faraday element of the sensor head transmitting the two polarized rays from the first birefringent plate, and converting the strength of a magnetic field to be detected into a rotating angle of the planes of polarization of the two polarized rays;
the reflecting device of the sensor head reflecting the two polarized rays transmitted through the first Faraday element, back into the first Faraday element, such that each of the two polarized rays reciprocates along the same optical path;
the first birefringent plate of the sensor head separating each of the two polarized rays returned from the first Faraday element into two polarized rays (four polarized rays in total) orthogonal to each other and having an amplitude level corresponding to the rotating angle of the respective polarized rays;
the optical device of the sensor head selectively transmitting two orthogonal polarized rays returning along optical paths substantially the same as incidence optical paths, among the four polarized rays emitted from the first birefringent plate;
the light branching device branching the two polarized rays transmitted from the optical device through the second light transmitting device, to the third light transmitting device:
the light detecting device detecting light intensities of the two polarized rays transmitted from the light branching device through the third light transmitting device; and
the computing device deriving the strength of the magnetic field from the light intensities detected by the light detecting device.
Specifically, linear light outputted from the light output device may become elliptically polarized light indefinite in both axis and ellipticity due to disturbances occurring in the course of transmission to the sensor head. Such polarized light is passed through the first birefringent plate disposed in the sensor head, whereby the polarized light may be used as separated into two polarized rays orthogonal to each other based on the crystallographic axis of the birefringent plate, while retaining a total energy of light intensity, regardless of rotation of the planes of polarization.
The two polarized rays have the planes of polarization rotated by magnetic field strength in the course of reciprocation through the first Faraday element. These two polarized rays are transmitted through the first birefringent plate aga

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