JPH02222847A - Magnetic field sensor - Google Patents

Abstract

PURPOSE:To enable realization of a highly accurate sensor with variations in a transmission loss compensated by comparing intensities of light with two linearly polarized components contained in a polarized wave to allow measurement of an intensity of a magnetic field applied to a Faraday rotor based on an angle of rotation of a polarization plane therewith. CONSTITUTION:A polarization separation element 24 separates an incident light wave depending on a difference in a polarization plane. A Faraday rotor 18 as non-reciprocal polarization plane rotor arranged in a magnetic field to be measured makes a magnetic field detecting section. As for a reciprocal type polarization plane rotation element 25, the rotation of the polarization plane by 45 deg. is optimum. Moreover, a polarization separating light element 28 separates a light wave from a light waveguide loop separated with a beam splitter 27 according the polarization plane from each other and photo detectors 22 and 23 detect amplitudes of the light waves different in the polarization plane. Then, detection output signals of the detectors are introduced to a signal analyzer 29 to analyze, thereby calculating an intensity of the magnetic field detected with the Faraday rotor 18.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a loop-type magnetic field sensor that is sufficiently temperature-compensated using an optical waveguide loop.

(Prior art and its problems to be solved by the invention) As shown in FIG. The linearly polarized light beam that has passed is guided to the photodetector 22, and the magnitude of the applied magnetic field is determined based on the change in the amount of passed light corresponding to the rotation angle of the linearly polarized light plane according to the magnetic field applied to the Faraday rotator 18. It is something to know. In an actual device, for example, as shown in the figure, a polarizer 3
45° or π with respect to the polarization plane of the incident light that has passed through
The incident light that has passed through the Feraday rotator 18 is guided to the analyzer 35 whose plane of polarization has been rotated by /4 radian, and from the change in the output light amount, the angle of rotation of the plane of polarization by the Faraday rotator 18, and therefore the angle of rotation of the plane of polarization by the Faraday rotator 18. Find the strength of the applied magnetic field. Now, let the incident light intensity be D2, and the rotation angle of the polarization plane by the Faraday rotator 18 be θ. Then, the analyzer output is proportional to the following equation (1).

D"cos" (π/4-θ.) = (D"/2)10cos (2(g/4-θ.))
]=(D”/2) (1+5in2θ, )
(1) Therefore, from the above equation (1), the rotation angle θ. is determined, and furthermore, the applied magnetic field is measured. however,
In such magnetic field measurement, if the intensity of the incident light or the amount of loss in each part fluctuates, the amount of fluctuation directly causes a measurement error.
In particular, when an optical fiber is used for the human output optical path, changes in its transmission loss also appear in the output and may be confused with changes in the strength of the magnetic field to be measured. There is a problem in that it is difficult to remove the effects of fluctuations from the measurement results.

An object of the present invention is to provide a magnetic field sensor that can be used in combination with a non-reciprocal transducer and a temperature-compensated high-frequency bias for highly accurate magnetic field measurement or as a gyro.

Still another object of the present invention is to provide an optical magnetic field sensor in which measurement errors due to fluctuations in transmission loss and transmission constant of a polarization-maintaining optical fiber are eliminated.

(Means for Solving the Problems) The magnetic field sensor according to the present invention includes a polarization separation/synthesis means for separating and combining two linearly polarized light waves orthogonal to each other, and a polarization separation/synthesis means for separating and combining two linearly polarized light waves orthogonal to each other, and an optical waveguide loop that allows two light waves to enter the polarization separation/synthesis means at both ends, and to enter the polarization separation/synthesis means from the other end;
A Faraday rotator and a reciprocal rotator are provided to rotate the polarization planes of two mutually orthogonal linearly polarized light waves incident from both ends of the optical waveguide loop, respectively, at different angles, and the polarized light waves are rotated from both ends of the optical waveguide loop. The strength of the magnetic field applied to the Faraday rotator based on the rotation angle of the plane of polarization by the Faraday rotator, which is calculated by comparing the light intensities of two linearly polarized light components included in the polarized waves incident on the separation and combination means and combined. This method is characterized by being able to measure the degree of strength.

(Function) According to the present invention, it is possible to realize a magnetic field sensor that compensates for fluctuations in transmission loss in a transmission line and has highly accurate performance.

(Example) The present invention will be described in detail below with reference to the drawings.

A first configuration example of a dual polarization type stabilized magnetic field sensor according to the present invention is described below.
As shown in the figure. In the illustrated configuration, the output polarized light waves of a magnetic field sensor using an optical waveguide loop are separated by the plane of polarization to be detected stably and reliably, and the strength of the magnetic field to be measured can be accurately analyzed. That is, among the components located in the optical waveguide loop, 24 is a polarization separation element such as a polarization separation prism, which separates incident light waves depending on the difference in their polarization planes. In addition, 2 and 3 are reflecting mirrors, and 18
t is a non-reciprocal polarization plane rotator such as a Faraday rotator placed in the magnetic field to be measured, and serves as a magnetic field detection section.

Furthermore, 25 is a reciprocal polarization plane rotation element, which is optimally rotated by 45 degrees.

On the other hand, the polarization-maintaining optical fiber 26 that introduces and guides the input and output light waves to the polarization separation element 24 is not an essential component of a magnetic field sensor, but as described above, it changes the polarization plane in response to the magnetic field to be measured. It is useful for transmitting while maintaining the polarization plane of input and output polarized light waves to be rotated, and for increasing the accuracy and stabilization of rotation angle measurement, and is particularly useful for remote measurement. Note that a polarization-maintaining optical fiber is an optical fiber configured to be non-axially symmetrical so that only a single polarized wave can be propagated, and the polarized wave can be propagated while maintaining that polarized wave as it is.

Further, 27 is a beam splitter for separating input and output light waves, and 28 is a polarization separation optical element that separates light waves from the optical waveguide loop separated by the beam splitter 27 from each other according to the plane of polarization. Further, reference numerals 22 and 23 denote photodetectors, which detect the amplitudes of light waves with different planes of polarization, and guide their detection output signals to a signal analyzer 29 for analysis. Calculate the strength of the measured magnetic field.

In the magnetic field sensor having the configuration shown in FIG. 10 described above,
Now, when two mutually orthogonal linearly polarized light waves A and B enter the polarization separation element 24 via the polarization-maintaining optical fiber 26, these two light waves A and B are separated from each other and the leading wave loop is reversed. Proceed around. While the two light waves are traveling, the light wave A is rotated clockwise in the direction of travel by the reciprocal polarization plane rotation element 25 so that the light wave A is rotated by θ. A non-reciprocal Faraday rotator 1 that undergoes rotation of the plane of polarization by radians.
8 by θ. The plane of polarization undergoes rotation by radians, and the total (
θ. →−θ. ) radian, the light wave B undergoes rotation of the polarization plane by θ0 radians in the same clockwise direction in its traveling direction by the reciprocal polarization plane rotation element 25, and also undergoes non-reciprocal Faraday rotation. One θ due to child 18. The plane of polarization is rotated by radians,
This results in a total polarization plane rotation of (θ.-θ.) radians. Moreover, since light wave B is traveling in the opposite direction to light wave A, the plane of polarization is rotated clockwise by (θ.-θl,) radians in the direction of travel of light wave A. This means that

When the light wave A that has undergone the rotation of the polarization plane as described above enters the polarization separation element 24 again, it is the polarization component orthogonal to the polarization plane of the light wave A that can pass through the polarization separation element 24. ,
Its amplitude is sin (θ. + θ.) of the incident light wave A. Similarly, when the light wave B enters the polarization separation element 24 again, only the incident light wave can pass through the polarization separation element 24. It is sin (θ.-〇,) times the amplitude of B. These light waves that have passed through the polarization separation element 24 return to the transmission source side via the polarization-maintaining optical fiber 26, but because they return with a polarization plane different from that at the time of incidence, both the light waves A and B have different polarization planes on their outgoing and return paths. The light passes through the same optical path,
Differences in transmission characteristics such as transmission loss and phase constant for two different polarized waves of the optical fiber are canceled out between the outgoing path and the returning path.

The two light waves A and B that have returned via the polarization maintaining optical fiber 26 are split by the beam splitter 27 and guided to the polarization separation element 28, and the two light waves A and B having different polarization planes are separated. When the light beams are separated from each other and guided to photodetectors 22 and 23 to detect their respective light intensities, the detected light intensities are expressed by the following equations (2) and (3), respectively.

A"stn" (θ.+θ, ) (2)B
“sin” (θ. −θ, )
(3) Therefore, light wave A and light wave B at the time of incidence
The Faraday rotator 1 can be made equal in intensity to each other, or by adjusting the sensitivity of the photodetectors 22 and
If the levels of both detection output signals when no magnetic field is applied to 8 are made equal to each other, A2-B2 can be obtained. Therefore, the difference Δ in the rain detection outputs can be expressed as in the following equation (4).

Δ■＾”rtn” (θ, +θm) B”stn”
(θ.-〇,)-8” (-cos (2(θ, 10θ
. ) ) +cos (2(θ, -〇.)) 1,000^-Ln
2θ*w ・5in2θ, , , (4
) From this equation (4), the rotation angle θ of the polarization plane by the non-reciprocal Faraday rotator 18 is obtained. It is obtained as shown in the following equation (5).

θ, =H/2)sln-'(Δ/(A"5in2θ
a)) (5) According to this equation (5), the magnetic field to be measured applied to the Faraday rotator 18 is weak and the polarization plane rotation angle θ. In order to be able to easily and reliably detect a small value, it is desirable that A''5in2θ on the right side of equation (5) be large. Therefore, the polarization plane rotation angle θ by the reciprocal phase rotation element 25 should be 45° or (π/4
) is best set in radians. Note that the photodetector 22
The sum SUh of the rain detection outputs of and 23 is expressed by the following equation (6).

StlM-^”(1-cos2θ.4・Co11θ,v
) (6) Sum S of rain detection outputs according to equation (6)
The ratio r between IIM and the difference Δ due to (4) 2 is expressed as the following equation (7).

]/2 5in2θ. (When θ, v-45°) (7) This (
From equation 7), Faraday rotation angle θ is also obtained. can be calculated. In this case, even if the above-mentioned sum SKIM and difference Δ vary due to variations in the intensity of the human-powered light wave and transmission loss in the optical fiber or optical waveguide loop, the sum SIIM and the difference Δ of the rain detection outputs will change. The ratio ``is constant, and the Faraday rotation angle θ can be measured extremely stably.

On the other hand, the Faraday rotation angle θ. Since is proportional to the strength of the axial magnetic field applied to the Faraday rotor 18,
The Faraday rotation angle θ is determined as described above. By determining , the strength of the applied magnetic field can be calculated. In addition,
In addition to using an optically active medium such as glass as the reciprocal polarization plane rotation element 25, a conventionally known half-wave plate may be rotated at an appropriate angle and used as the reciprocal phase rotation element 25.

(Effects of the Invention) As is clear from the above description, according to the present invention, it is possible to realize a highly accurate magnetic field sensor that is temperature compensated by combining non-reciprocal polarization plane rotation elements.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing an example of the configuration of a magnetic field sensor to which a loop-type optical modulator is applied according to the present invention, and FIG. 2 is a configuration layout diagram showing the basic configuration of a conventional magnetic field sensor. 2... Reflector 18... Faraday rotator 22, 23
... Photodetector 24.28 ... Polarization separation element 2
5... Reciprocal polarization plane rotation element 26... Polarization maintaining optical fiber

Claims (1)

[Claims] 1. A polarization separation/synthesis means for separating and combining two linearly polarized light waves orthogonal to each other, and for making the two linearly polarized light waves separated by the polarization separation/synthesis means incident on both ends, respectively. an optical waveguide loop that enters the polarization separation/synthesis means from the other end; a Farreide rotator that rotates the polarization planes of two mutually orthogonal linearly polarized light waves that are incident from both ends of the optical waveguide loop at different angles; and a reciprocal rotator, wherein the Faraday rotator is calculated by comparing the light intensities of two linearly polarized components included in polarized waves synthesized by entering the polarization separation/synthesis means from both ends of the optical waveguide loop. A magnetic field sensor characterized in that the strength of the magnetic field applied to the Faraday rotator can be measured based on the rotation angle of the plane of polarization. 2. In the magnetic field sensor according to claim 1,
Introducing the two mutually orthogonal linearly polarized light waves to the polarization separation/synthesis means via a polarization maintaining fiber,
A magnetic field sensor characterized in that two linearly polarized light components included in the combined polarized light wave are derived through the polarization maintaining optical fiber.