JP3114104B2 - Electric field sensor device using electro-optic effect - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode other than an electrode for detecting an electric field for the purpose of accurately measuring the electric field intensity at a certain point in a space so as not to disturb the surrounding electromagnetic field distribution by inserting a sensor. Relates to an electric field sensor composed entirely of non-metal.

[0002]

2. Description of the Related Art In recent years, as typified by the runaway problem of automatic vehicles, a phenomenon in which a digital processing circuit malfunctions due to an interference wave or the like radiated from a switch of an electric light has become a problem in recent years. To cope with this problem, an antenna for measuring the level of such an impulse interference wave is required.

At present, a small dipole antenna, a conical antenna and a horn antenna are used as an antenna for measuring an impulsive electromagnetic field. However, these antennas use a coaxial cable to connect the antenna and the signal receiver, so the characteristics of the received electromagnetic field are affected by the cable, and the measurement level may not be as high as that in the case of cable routing. Because of the change, measurement with excellent reproducibility was difficult. Therefore, an electric field sensor that connects a sensor unit for detecting an electromagnetic field and a receiver with an optical fiber has been studied. This type of electric field sensor has a built-in light emitting element such as a laser diode inside the sensor due to its mechanism,
It is categorized as a type incorporating a light modulator using a crystal having an electro-optic effect such as LiNbO ₃ .

[0004] Of these two types of antennas, the latter is:
Although the sensitivity is lower than the former, it is effective for long-term observation of a strong electromagnetic field pulse which may cause a malfunction of a digital device because it does not require a built-in battery. FIG. 5 shows the basic overall configuration of the latter sensor device.

In FIG. 5, 1 is a light source, 2 is a graded index lens, 3-1 is a polarization maintaining optical fiber,
2 is a single mode fiber, 4 is a graded index lens (for input), 5 is a polarizer, 6 is a sensor rod, 7 is a crystal having an electro-optic effect, 8 is a Babinet-Souille phase compensator, 9 is an analyzer, and 10 is an analyzer. A graded index lens (for output), 11 is a photodetector, and 12 is a receiver.

This sensor device has a wavelength of 1.3 μm as a light source.
As the crystal 7 having the electro-optical effect, two 1 mm × 1 mm × 10 mm LiNbO ₃ are used for the laser diode of m. The light wave exiting the laser diode 1 propagates through the polarization maintaining fiber 3-1 and reaches the sensors 4 to 10. After the light wave reaching the sensor is converted into a parallel light beam by the graded index lens 4, the light wave is converted into only a linearly polarized light component by the polarizer 5. Since the light incident on the crystal 7 in the state of linearly polarized light changes in the birefringence of the crystal axis in the same direction as the electric field vector, the propagation speed of the light wave changes only in the polarization component in the same direction as the direction of the applied electric field vector. I do. Therefore, the polarization of the light wave exiting the crystal is elliptically polarized. The light wave emitted from the crystal is phase-compensated by the Babinet-Soureil phase compensator 8 so that the light wave becomes circularly polarized light when no electric field is applied at the exit end of the crystal. Since the shape of the elliptically polarized light of this light wave changes depending on the strength of the applied electric field, if only a specific polarization plane is extracted using the analyzer 9, an intensity-modulated optical signal is obtained.

At this time, the electric field E applied to 7 and the modulation signal V
The relationship of m is represented by equation (1). _{Vm = αI 0 [1-cos} (δ B + δ 0 + πβE / V 0)] (1) δ 0 = (2π / λ) (n 0 -n e) (L 1 -L 2) V 0 = λd / ( ^{_{n e 3 γ c L 1)}} γ c = γ 33 - (n 0 / n e) 3 γ 13 where, V _0: half-wave voltage I _0: light signal amplitude [delta] _B: Babinesoureiyu phase compensator according to the phase correction the value d: thickness of the crystal n _e: refractive index in a direction electric field is applied n _0: direction perpendicular to the direction of the refractive index gamma ₃₃ an electric field is applied, gamma _13: electro-optic constant L _1, L _2: crystalline Is the wavelength of light.

FIG. 6 shows the structure of the sensor portion. In FIG. 6, reference numeral 13 denotes a base for mounting the optical modulator, and reference numeral 14 denotes a prism for reflecting light. As shown in the figure, a modulator is installed at the center of two metal rods (length 15 cm). With this sensor, DC to about 700MHz
The electric field strength of 1 V / m can be observed in the frequency range of

FIG. 7 shows the temperature dependence of the optical power passing through the sensor (the ratio of the optical power incident on the optical fiber 3-1 to the optical power emitted from the optical fiber 3-2). In the measurement, the sensor was placed in a thermostat, a stabilizing light source was placed outside the thermostat, optical power was input to 3-1 of the sensor, and the output level coming out of 3-2 was measured with an optical power meter. FIG. 7 shows the variation of the passing optical power when the ambient temperature of the electric field sensor is changed. As shown in FIG.
It can be seen that when the angle is changed by 40 degrees, the passing optical power fluctuates by about 25 dB at the maximum. As shown in equation (1),
Since the sensitivity of the electric field sensor is proportional to the transmitted light power (optical signal amplitude), it can be seen that this electric field sensor has a disadvantage that the sensitivity variation due to the ambient temperature is extremely large.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to solve these problems and to realize an electric field sensor device using the electro-optic effect, which has little sensitivity fluctuation due to ambient temperature.

[0011]

SUMMARY OF THE INVENTION A feature of the present invention is that a pair of conductor rods (6) are arranged with a gap, a polarizer, an optical crystal (7) having an electro-optic effect, and a Babinet-Sourille phase between the gaps. A light intensity modulator (100) having a compensator is arranged, a modulation electric field is applied between the pair of conductor bars (6), and the light intensity modulator (100) is applied to the light intensity modulator (100). In the electric field sensor device for measuring the electric field intensity based on the intensity of the output light from the optical crystal, the light of the optical crystal (7) passes.
Make sure there are no gaps between the four sides
A surrounding member having a substantially constant linear expansion coefficient is provided,
The surrounding member is a rectangular parallelepiped member having a length L, a thickness c, and a width c + a.
Material and a rectangular parallelepiped material 2 of length L, thickness c, width c + b
The present invention is directed to an electrolytic sensor device using an electro-optic effect having a book .

[0012]

[Function] As a cause of the temperature fluctuation shown in FIG. 7, the refractive index and the electro-optical constant of a crystal having an electro-optical effect such as LiNbO ₃ fluctuate with temperature. Fluctuate. Therefore, it was examined which of the factors caused the fluctuation of the transmitted light power.

FIG. 8 shows a change in transmitted light power due to a change in temperature of the refractive index and the electro-optic constant of LiNbO ₃ . In FIG. 8, the black dots are LiNb.
This is a measured value when the measurement is performed in a state where temperature distortion is not applied to the O ₃ crystal. The solid line shows the temperature fluctuation of the transmitted light power calculated based on the temperature dependence of the refractive index and the electro-optic constant reported in the literature. As shown in the figure, the theoretical value and the measured value are almost the same and the transmitted light power fluctuation due to temperature fluctuation is ± 0.2 dB or less. It can be seen that this is not the cause of the optical power fluctuation. Therefore, it is understood that the factors of the fluctuation of the transmitted light power are the fluctuation of the refractive index and the electro-optic constant due to the temperature distortion applied to the crystal.

FIGS. 1 and 9 show the difference between the features of the present invention and the prior art. In these figures, 15 is a table on which a crystal having an electro-optical effect is mounted, and 16 is a table on which an optical system constituting an optical modulator is assembled. A feature of the present invention is that, while only one surface of the crystal 7 is conventionally fixed to the pedestal 15 with an adhesive as shown in FIG. 9, as shown in FIG. That is, the crystal 7 having the effect is surrounded by a uniform material 17 having a small linear expansion coefficient. With such a configuration, since the crystal is surrounded by a material having the same linear expansion coefficient, there is an advantage that the temperature distortion applied to the crystal becomes uniform and the temperature dependence of the electric field sensor sensitivity can be reduced.

From the above results, the present invention has improved the temperature dependency of the sensitivity, which is a drawback of the sensor which has been reported so far. Therefore, even if the ambient temperature fluctuates, the fluctuation of the sensitivity is small and stable and practical. There is an advantage that an electric field sensor can be realized.

[0016]

FIG. 2 shows a specific embodiment of the present invention. FIG.
Numeral 17 denotes a crystal having an electro-optic effect surrounded by a material having the same linear expansion coefficient as shown in FIG. 1, and numeral 18 (a) denotes a lead wire connecting the antenna rod and the crystal 7. As shown in FIG. 3, in this embodiment, the length is 10 mm, the width is 1 mm, and the thickness is 1 m.
m two crystals, the optical axes of which are perpendicular to each other,
The circumference is 20mm long, 3.5mm wide and 2.5m thick
m quartz glass. In FIG. 3, reference numeral 18 denotes an electrode of the optical modulator. In this embodiment, a Cr-Au alloy is deposited on this portion.

Here, in the structure shown in FIG. 3, the relationship between the size of the crystal having the electro-optic effect and the size of the material surrounding the crystal and having a small linear expansion coefficient will be described. For example, when the crystal having the electro-optic effect is a rectangular parallelepiped having a length L, a thickness a, and a width b, two materials having a small linear expansion coefficient having a length L, a thickness c, and a width c + a, and a length L, a thickness If two materials having a small linear expansion coefficient c and a width c + b are combined as shown in FIG. 3, as shown in FIG. Can be.

Next, the operation of this embodiment will be described with reference to FIG. The light wave incident from the polarization maintaining fiber 3-1 is converted into a parallel light beam by the graded index lens 4, and the light wave is shifted by 45 degrees with respect to the crystal optical axis of LiNbO ₃ by the polarizer 5.
It is converted to a linearly polarized light component inclined at an angle. In this embodiment, the polarizer 5
The size of the optical modulator is reduced by using a Ramipole. Further, the optical connector, the graded index lens, and the polarizing plate are integrated to simplify the assembly as compared with FIG. The light that has passed through the polarizing plate passes through the LiNbO ₃ crystal 7, is phase-compensated by a Babinet-Sourille phase compensator 8 so as to be circularly polarized with no voltage applied, passes through the analyzer 9, and passes through the graded index lens 10. The light enters the single mode fiber 3-2.

[0019]

FIG. 4 shows the temperature dependence of the passing optical power in this embodiment. FIG. 4 shows the fluctuation of the optical power passing through the electric field sensor when the ambient temperature of the electric field sensor is changed. As shown in the figure, when the ambient temperature is changed from 0 degree to 40 degrees, the fluctuation of the passing light power is about 1 dB at the maximum, and the temperature characteristic is significantly improved as compared with the conventional product shown in FIG. I understand.

Since the passing power fluctuation is proportional to the sensitivity fluctuation,
It can be seen that the electric field sensor according to the present embodiment has a much smaller sensitivity variation than the conventional product even when the ambient temperature changes. Based on the above results, the present invention has improved the temperature dependence of sensitivity, which is a drawback of the conventionally reported sensors, and has a stable and practical electric field sensor with small fluctuations in sensitivity even when the ambient temperature fluctuates. There are benefits that can be realized.

[Brief description of the drawings]

FIG. 1 is a view showing a mounting structure of a crystal according to the present invention.

FIG. 2 is a diagram showing a specific example of the present invention.

FIG. 3 is a diagram showing a state where an electro-optical crystal is surrounded by an enclosing member.

FIG. 4 is a diagram showing temperature dependency in the present invention.

FIG. 5 shows a configuration of a conventional sensor device.

FIG. 6 shows a sensor member in a conventional device.

FIG. 7 shows the temperature dependence of the output light power in the conventional technique.

FIG. 8 shows the temperature dependence of the emitted light power when only the temperature dependence of the refractive index and the electro-optic constant of the crystal are taken into account.

FIG. 9 shows a conventional technique of a crystal mounting structure.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 light source 2 graded index lens 3 polarization plane maintaining optical fiber 3 ′ single mode optical fiber 4 graded index lens (for input) 5 polarizer 6 sensor electrode 7 crystal having electro-optical effect 8 Babinet-Souille phase compensator 9 analyzer 10 Graded index lens (for output) 11 Photodetector 12 Receiver 13 Substrate for mounting optical modulator 14 Prism 15 Crystal mounting base 16 Base on which optical system constituting optical modulator is assembled 17 Crystal Assembly surrounded by surrounding member 18 Electrode of light modulator 100 Light intensity modulator assembly

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-184772 (JP, A) JP-A-60-210770 (JP, A) JP-A 1-163875 (JP, U) (58) Survey Field (Int.Cl. ⁷ , DB name) G01R 29/08 G01R 15/24

Claims (2)

(57) [Claims] An optical intensity modulator (100) having a pair of conductor rods (6) arranged with a gap, a polarizer, an optical crystal (7) having an electro-optic effect, and a Babinet-Souille phase compensator between the gaps. ) Is arranged, an electric field for modulation is applied between the pair of conductor bars (6), and the electric field intensity is determined by the intensity of the output light from the light intensity modulator (100) output from the modulator. In the electric field sensor device to be measured, four sides of the optical crystal (7) through which light does not pass are separated by gaps.
The linear expansion coefficient is substantially
A constant surrounding member is provided, and the surrounding member has a length L and a thickness.
Two rectangular parallelepiped materials of length c and width c + a, length L and thickness
c, and two rectangular parallelepiped materials having a width of c + b . 2. The electric field sensor device according to claim 1, wherein said surrounding member is made of quartz glass .