JPH04350574A - Electric field sensor using electrooptical effect - Google Patents

Abstract

PURPOSE:To improve the temp. characteristics in an electric field sensor using an electro-optical crystal. CONSTITUTION:In an electric field sensor, a pair of conductor rods 6 are arranged so as to provide a gap therebetween anda light intensity modulator having a polarizer, an optical crystal 7 having electro-optical effect and a Babinet-Soleil compensator is arranged to the gap and a modulating electric field is applied across a pair of the conductor rods 6 and the intensity of an electric field is measured on the basis of the intensity of the output light from the modulator of the light applied to the light intensity modulator. An enclosing member 15 almost uniform in the coefficient of linear expansion is provided so as to surround the light non-transmitting side surface of the optical crystal 7.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] In order to accurately measure the electric field strength at a certain point in space, the present invention aims to prevent the surrounding electromagnetic field distribution from being disturbed by inserting a sensor. relates to an electric field sensor made entirely of non-metals.

0002

2. Description of the Related Art In recent years, as typified by the problem of automatic cars running out of control, the phenomenon of digital processing circuits malfunctioning due to interference waves emitted from light switches has become a problem. To address this problem, an antenna is needed to measure the level of such impulsive interference.

Currently, micro dipole antennas, conical antennas, and horn antennas are used as antennas for measuring impulsive electromagnetic fields. However, since these antennas use coaxial cables for the connection between the antenna and the signal receiver, the characteristics of the received electromagnetic field are affected by the cable, and the measured level may vary depending on the cable routing situation. Because of this change, it has been difficult to measure with excellent reproducibility. Therefore, an electric field sensor that connects a sensor unit that detects an electromagnetic field and a receiver using an optical fiber is being considered. Due to its mechanism, this type of electric field sensor has a built-in light emitting element such as a laser diode inside the sensor.
It is classified as having a built-in optical modulator using a crystal with an electro-optic effect such as LiNbO3.

[0004] Of these two types of antennas, the latter is
Although it is less sensitive than the former, since it does not require a built-in battery, it is effective for long-term observation of strong electromagnetic field pulses that can cause digital equipment to malfunction. The basic overall configuration of the latter sensor device is shown in FIG.

In FIG. 5, 1 is a light source, 2 is a graded index lens, 3-1 is a polarization-maintaining optical fiber, and 3-
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 with an electro-optic effect, 8 is a Babinet-Sourail phase compensator, 9 is an analyzer, and 10 is a A graded index lens (for output), 11 a photodetector, and 12 a receiver.

This sensor device uses a wavelength of 1.3μ as a light source.
Two 1 mm x 1 mm x 10 mm LiNbO3 crystals are used as crystals 7 having an electro-optic effect. The light wave exiting the laser diode 1 propagates through the polarization maintaining fiber 3-1 and reaches the sensors 4-10. The light waves that have reached the sensor are converted into parallel light beams by a graded index lens 4, and then converted into only linearly polarized light components by a polarizer 5. When light enters the crystal 7 in a linearly polarized state, the birefringence of the crystal axis in the same direction as the electric field vector changes, so the propagation speed of the light wave changes only for the polarized component in the same direction as the applied electric field vector. do. Therefore, the polarization of the light wave exiting the crystal becomes elliptically polarized light. The light wave emitted from the crystal is phase-compensated by the Babinet-Sourail phase compensator 8 so that it becomes circularly polarized light when no electric field is applied at the output end of the crystal. Since the shape of the elliptically polarized light wave changes depending on the strength of the applied electric field, if only a specific plane of polarization is extracted using the analyzer 9, an intensity-modulated optical signal can be obtained.

At this time, the electric field E applied to 7 and the modulation signal V
The relationship between m is expressed by equation (1). Vm=αI0 [1-cos (δB +
δ0 + πβE/V0 )] (1) δ
0 = (2π/λ) (n0 - ne ) (L1 - L2
)
V0 = λd/(ne3γc L1)

γc = γ33-(n
0 /ne )3 γ13
Here, V0: Half-wave voltage I0: Amplitude of optical signal δB: Phase correction value by Babinet-Souleille phase compensator d: Thickness of crystal ne: Refractive index in the direction in which the electric field is applied n0: In the direction in which the electric field is applied Vertical refractive index γ33, γ13: Electro-optic constants L1, L2: Crystal length λ: Wavelength of light.

FIG. 6 shows the structure of the sensor portion. In FIG. 6, 13 is a base for attaching the optical modulator, and 14 is a prism for reflecting light. As shown in the figure, the modulator is installed at the center of two metal rods (15 cm long). Using this sensor, DC ~ about 700MHz
An 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 this sensor (the ratio of the optical power entering the optical fiber 3-1 and the optical power exiting from the optical fiber 3-2). In the measurement, the sensor was placed in a thermostatic oven, a stabilized light source was placed outside the thermostatic oven, optical power was input to the sensor 3-1, and the output level output from the sensor 3-2 was measured using an optical power meter. FIG. 7 shows the fluctuation of the passing optical power when the ambient temperature of the electric field sensor is changed. As shown in the figure, the ambient temperature should be set between 0 degrees and 4 degrees.
It can be seen that when changing by 0 degrees, the transmitted optical power fluctuates by about 25 dB at maximum. As shown in Equation (1), the sensitivity of the electric field sensor is proportional to the passing optical power (optical signal amplitude), so it can be seen that this electric field sensor has the drawback of very large sensitivity fluctuations depending on the ambient temperature.

0010

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

[0011]

[Means for Solving the Problems] A feature of the present invention is that a pair of conductor rods (6) are arranged with a gap between them, and between the gap, a polarizer, an optical crystal (7) having an electro-optic effect, and a Babinet-Souleille phase A light intensity modulator (100) having a compensator is arranged, a modulating electric field is applied between the pair of conductor rods (6), and the light applied to the light intensity modulator (100) is modulated by the modulator. In an electric field sensor device that measures electric field intensity based on the intensity of output light from the optical crystal (7), an electro-optic effect is used in which an enclosing member having a substantially uniform coefficient of linear expansion that surrounds the side surface of the optical crystal (7) through which light does not pass is created. It is in the electrolytic sensor device that was used.

0012

[Effect] ■LiNb
It is conceivable that the refractive index and electro-optic constant of a crystal with an electro-optic effect, such as O3, fluctuate depending on the temperature. (2) The refractive index and electro-optic constant fluctuate due to temperature strain applied to the crystal. Therefore, we investigated which factors caused the fluctuations in transmitted light power.

FIG. 8 shows fluctuations in the power of transmitted light due to temperature fluctuations in the refractive index and electro-optic constant of LiNbO3. In FIG. 8, the black dots are measured values obtained when measurements were performed in a state where no temperature strain was applied to the LiNbO3 crystal. Moreover, the solid line is the temperature fluctuation of the transmitted optical power calculated based on the temperature dependence of the refractive index and electro-optic constant reported in the literature. As shown in the figure, the theoretical value and the measured value are almost in agreement, and the transmitted light power fluctuation due to temperature fluctuation is less than ±0.2 dB, so the temperature dependence of the refractive index and electro-optic constant of (■) is It can be seen that this is not a factor in the fluctuation of transmitted light power. Therefore, it can be seen that the cause of the power fluctuation of the transmitted light is the fluctuation of the refractive index and electro-optical constant due to the temperature strain applied to the crystal (2).

The features of the present invention and the differences from the prior art are shown in FIGS. 1 and 9. In these figures, 15 is a table on which a crystal having an electro-optic effect is placed, and 16 is a table on which an optical system constituting an optical modulator is assembled. The feature of the present invention is that, while conventionally only one side of the crystal 7 was fixed to the pedestal 15 with adhesive as shown in FIG. 9, as shown in FIG. This is because the effective crystal 7 is surrounded by a material 17 having a small and uniform coefficient of linear expansion. With this configuration, since the crystal is surrounded by a material having the same coefficient of linear expansion, the temperature strain applied to the crystal becomes uniform, and there is an advantage that the temperature dependence of the electric field sensor sensitivity can be reduced.

[0015] From the above results, the present invention improves the temperature dependence of sensitivity, which is a drawback of conventionally reported sensors. There are advantages that electric field sensors can achieve.

[0016]

[Embodiment] A specific embodiment of the present invention is shown in FIG. Figure 2
Reference numeral 17 is a crystal having an electro-optical effect surrounded by a material having an equal linear expansion coefficient as shown in FIG. 1, and 18(a) is a lead wire connecting the antenna rod and the crystal 7. As shown in Figure 3, in this example, the length is 10 mm, the width is 1 mm, and the thickness is 1 m.
Arrange two crystals of m so that their optical axes are perpendicular to each other,
The circumference is 20mm long, 3.5mm wide, and 2.5m thick.
It is covered with m quartz glass. In FIG. 3, reference numeral 18 indicates an electrode of the optical modulator, and in this embodiment, a Cr--Au alloy is deposited on this portion.

Here, in the structure shown in FIG. 3, the relationship between the dimensions of a crystal having an electro-optic effect and the dimensions of a material having a small coefficient of linear expansion surrounding the crystal will be described. For example, if a crystal having an electro-optic effect is a rectangular parallelepiped with length L, thickness a, and width b, two materials with small linear expansion coefficients of length L, thickness c, and width c+a and a material with a length L and thickness If two materials with small coefficients of linear expansion with width c and width c+b are combined as shown in Figure 3, they can be brought into close contact with each other so that there are no gaps between the four surfaces of the crystal through which light does not pass, as shown in Figure 3. Can be done.

Next, the operation of this embodiment will be explained using FIG. 2. A light wave incident from the polarization-maintaining fiber 3-1 is converted into a parallel light beam by a graded index lens 4, and converted into a linearly polarized light component tilted by 45 degrees with respect to the optical axis of the LiNbO3 crystal by a polarizer 5. In this embodiment, a lamipole is used as the polarizer 5 to reduce the size of the optical modulator. Furthermore, the optical connector part, graded index lens, and polarizing plate are integrated, making assembly easier than in FIG. 6. The light that passed through the polarizing plate is LiNb
The wave passes through the O3 crystal 7, is phase-compensated by the Babinet Soureil phase compensator 8 to become a circularly polarized wave with no voltage applied, passes through the analyzer 9, and enters the single mode fiber 3-2 through the graded index lens 10. be done.

[0019]

FIG. 4 shows the temperature dependence of the transmitted light power in this embodiment. FIG. 4 shows the variation in optical power passing through the electric field sensor when the ambient temperature around the electric field sensor is changed. As shown in the figure, when the ambient temperature is changed from 0 degrees to 40 degrees, the maximum variation in the transmitted light power is about 1 dB, and the temperature characteristics are significantly improved compared to the conventional product shown in Figure 7. I understand.

Since the passing power fluctuation is proportional to the sensitivity fluctuation,
It can be seen that the electric field sensor of this example exhibits extremely small sensitivity fluctuations compared to conventional products even when the ambient temperature changes. Based on the above results, the present invention has developed a stable and practical electric field sensor that improves the temperature dependence of sensitivity, which is a drawback of conventionally reported sensors, and has small fluctuations in sensitivity even when the ambient temperature fluctuates. There are benefits that can be achieved.

[Brief explanation of the drawing]

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

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

FIG. 3 is a diagram showing a state in which an electro-optic crystal is surrounded by a surrounding member.

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

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

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

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

FIG. 8 shows the temperature dependence of the output optical power when considering only the temperature dependence of the refractive index and electro-optic constant of the crystal.

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

[Explanation of symbols]

1 Light source 2 Graded index lens 3 Polarization maintaining optical fiber 3' Single mode optical fiber 4 Graded index lens (for input) 5
Polarizer 6 Sensor electrode 7 Crystal with electro-optic effect 8 Babinet Soureil phase compensator 9 Analyzer 10 Graded index lens (for output) 1
1 Photodetector 12 Receiver 13 Substrate 14 for mounting the optical modulator Prism 15 Table 16 on which the crystal is placed Table 17 on which the optical system constituting the optical modulator is assembled Assembly 18 in which the crystal is surrounded by a surrounding member Optical modulator electrode 100 Optical intensity modulator assembly

Claims (3)

[Claims] Claim 1: A light intensity modulator (100) in which a pair of conductor rods (6) are arranged with a gap between them, and between the gap there is a polarizer, an optical crystal (7) having an electro-optic effect, and a Babinet-Sourail phase compensator. ), a modulation electric field is applied between the pair of conductor rods (6), and the light intensity modulator (100)
In an electric field sensor device that measures electric field intensity based on the intensity of output light from the modulator, an enclosing member having a substantially uniform coefficient of linear expansion surrounds a side surface of the optical crystal (7) through which light does not pass. is characterized by being able to generate
Electric field sensor device using electro-optic effect. 2. In the electric field sensor device according to claim 1, a rectangular parallelepiped crystal having a length L, a thickness a, and a width b is used as a crystal having an electro-optic effect constituting the optical modulator. ，
The light of this crystal does not pass through two materials with almost uniform linear expansion coefficients of thickness c and width c + a and two materials with almost uniform linear expansion coefficients of length L, thickness c and width c + b. An electric field sensor device using an electro-optical effect, characterized in that four surfaces are brought into close contact with each other so that no gaps are formed. 3. The electric field sensor device according to claim 1, wherein quartz glass is used as a surrounding member that is brought into close contact with the crystal having the electro-optic effect. .