JPH0829457A - Electric field sensor device - Google Patents

Abstract

PURPOSE:To provide a high sensitivity device by deforming a base plate with the application of a stress for changing the length of an optical waveguide so as to produce a phase difference in the light wave, so that the light bias control of a light modulator can be made possible without applying a DC bias. CONSTITUTION:When a stress is applied to a base plate 4 by means of a stress application device 14, the base plate 4 whose one end has been fixed to a casing 8 by a fixing device 15 is deflected. In this case, when two optical waveguides 5a, 5b lie in the position symmetrical with regard to the enter line of the base plate 4, tension is applied to one side of the optical waveguides 5a, 5b, and compression is applied to the other side thereof, so that the one side is extended, while the other side is contracted. Accordingly, the phase of the light wave on one side propagating through the optical waveguide 5a, 5b is advanced, while the phase of the light wave on the other side is delayed, so that a phase difference in the light waves in both the optical waveguides is caused, and a strongly modulated optical signal can be obtained. Thus, even in the state where a voltage is not applied and the DC bias is zero, the light bias control of a light modulator is made possible, and by using this, a device having high sensitibily and large in the dynamic range can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric field sensor device using an optical waveguide type optical modulator, and is particularly effective when applied to a high speed optical communication system using an analog optical signal such as CATV, electromagnetic field measurement and the like. Technology.

[0002]

2. Description of the Related Art Generally, an optical modulator using an optical waveguide is used in the fields of high-speed optical communication systems using analog optical signals such as CATV and electromagnetic field measurement.

FIG. 9 shows the configuration of an electric field sensor device used for electromagnetic field measurement. In FIG. 9, 1 is a light source for sending unmodulated optical power to an optical modulator, 2 is a relationship between the polarization of a light wave propagating in the polarization maintaining fiber and the polarization of the light source, and linear polarization A polarization compensator for propagating a light wave in the fiber, a polarization maintaining fiber 3 for propagating the optical power of the light source to the optical modulator, and an optical modulator 4 formed thereon. A substrate made of a material having an electro-optical effect, 5 an optical waveguide, 6 an electrode of an optical modulator, 7 a metal rod of an electric field sensor device, 8 a housing accommodating an optical modulator of an electric field sensor device, 9 Is a single mode optical fiber connecting an optical modulator and a photodetector, 10 is a photodetector, and 11 is a level meter for measuring the electric signal level converted by the photodetector.

In the field of electromagnetic field measurement, especially in the field of EMC (electromagnetic environment), as typified by a runaway accident of an automatic vehicle, a digital processing circuit is erroneously operated by a disturbance wave emitted from a switch of an electric lamp in recent years. The phenomenon is becoming a problem, and an antenna for measuring such impulsive disturbance wave level is required.

At present, a small dipole antenna, a conical antenna or a horn antenna is used as an antenna for measuring an impulsive electromagnetic field. However, these antennas use a coaxial cable to connect between the antenna and the signal measuring instrument, so the measured electromagnetic field characteristics are affected by the cable, and the measurement level varies depending on the cable layout. Will end up. Therefore, an electric field sensor device in which a sensor unit for detecting an electromagnetic field and a measuring instrument are connected by an optical fiber is under study.

In particular, the electric field sensor device shown in FIG. 9 does not need to have a built-in battery and can obtain a constant sensitivity characteristic over a wide frequency band. Therefore, it is possible to measure a pulsed interference wave or an interference wave of an information device. It is effective for the characteristic evaluation of the test equipment for testing the proof stress against.

This electric field sensor device uses a laser diode having a wavelength of 1.3 μm as a light source and lithium niobate as a crystal having an electro-optical effect. The light wave emitted from the laser diode propagates through the polarization maintaining fiber 3 and reaches the substrate 4, the optical waveguide 5, the electrode 6 of the optical modulator, the metal rod 7 of the electric field sensor device, and the housing 8.

The structure of the optical modulator is shown in FIG. In FIG. 10, 12 and 13 are optical waveguides. The optical modulator shown in FIG. 10 uses a Matsuhachender optical interferometer, and the incident light is once separated into two optical waveguides 12 and 13 and recombined, and the intensity is increased by the phase difference of the light passing through the two waveguides. A modulated optical signal is obtained.

Now, when the metal rod 7 is placed in an electric field, a voltage is induced between the electrodes 6 by electromagnetic induction, and the electric field is applied as shown in FIG. At this time, as shown in FIG. 11, a downward electric field is applied to the waveguide 12 and an upward electric field is applied to the waveguide 13. When an electric field is applied, the refractive index of the waveguide changes due to the electro-optic effect. In this case, since the directions in which the electric field is applied are the opposite directions in the waveguide 12 and the waveguide 13, the phase of the light wave traveling through one of the waveguides is advanced, and the phase of the light wave of the one waveguide is delayed. Therefore, an intensity-modulated optical signal proportional to the phase difference of the light waves propagating through these waveguides, that is, the applied voltage can be obtained.

The optical signal propagates through the single mode optical fiber 9 and is sent to the photodetector 10. The electric field strength can be obtained by measuring the optical signal level converted into an electric signal by the photodetector 10 with a level meter.

At this time, the relationship between the electric field strength E and the output level V of the photodetector is expressed by the equation (1).

[0012]

## EQU1 ## V _r = 0.5 ηl _out * [1 + cos {((Eh _e / (1 + jωC _m Z _a )) / V ₀ ) π + φ}] (1) In Expression 1, V ₀ : Half of the optical modulator wavelength voltage l _out: maximum optical power output level of the optical modulator eta: conversion efficiency h _e photodetectors: effective length of the dipole elements composed of two metal rods C _m: input capacitance of the optical modulator Z _a : Drive point impedance of dipole element composed of two metal rods φ: Phase difference (optical bias angle) of light waves propagating in the waveguides 12 and 13 when no electric field is applied.

FIG. 12 shows the relationship between the voltage applied to the electrodes of the electric field sensor device and the optical power output level of the electric field sensor device. In the measurement, a triangular wave was applied to the element of the electric field sensor device, and the output level V _r change of the photodetector was measured. From the equation (1), the relationship between the voltage V _x applied between the electrodes and the output level V _r of the photodetector is given by the equation (2).

[0014]

V _r = 0.5 ηl _out * [1 + cos {(V _x / V ₀ ) π + φ}] (2) Since the optical power output level is proportional to the output level V _r of the photodetector, the optical detection By measuring the output level of the lamp, the fluctuation of the optical power output level can be measured.
As shown in Expression (2), when the applied voltage V _x is changed, a sine curve as shown in FIG. 12 is drawn. When the applied voltage is zero, equation (2) becomes equation (3).

[0015]

## EQU00003 ## V _r = 0.5 ηl _out * [1 + cos {φ}] (3) As shown in FIG. 9, the optical modulator used in the electric field sensor device does not apply a DC bias. Operate. As shown in Expression (3), the optical power level at this time is determined by the optical bias angle (φ).

[0016]

By the way, in the case of the Matsuhachender optical interferometer, the optical waveguides 12 and 13 are usually manufactured so that the lengths thereof are the same. This is because with the current technology, it is difficult to control the individual lengths of the waveguides in consideration of the phase of light. In that case,
In the state where no voltage is applied (when the DC bias is zero), the phase difference between the light waves propagating through the two waveguides is zero (the optical bias angle is zero), and the optical power output level becomes maximum from the formula (3). In FIG. 12, when the applied voltage is zero, the optical power output level is almost maximum, and it can be seen that the conventional modulator is used with the optical bias angle being almost zero.

FIG. 13 shows the relationship between the input level of the optical modulator and the modulated optical signal level in this case. As shown in FIG. 13, when the optical bias angle approaches zero (the point where the value of the optical power in the figure is the maximum), the slope of the curve representing the relationship between the optical power output level and the applied voltage decreases (the optical bias angle It can be seen that when is zero, the slope of the curve is zero) and the modulation efficiency becomes poor (that is, the sensitivity of the electric field sensor device decreases). It can also be seen that at this position, the straight line portion of the curve is short and the dynamic range is small.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a waveguide type optical modulator capable of controlling an optical bias without applying a DC bias. It is intended to provide a technique capable of realizing an electric field sensor device having high sensitivity and a large dynamic range by using the electric field sensor device.

The above and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.

[0020]

The outline of the representative one of the inventions disclosed in the present application will be briefly described as follows.

(1) Two metal rods arranged in series,
An electric field sensor device comprising an air gap formed between the metal rods and an optical modulator provided in the air gap, wherein the optical modulator has at least two optical waveguides on a substrate having an electro-optical effect. And a control electrode for controlling a light wave propagating through these optical waveguides, and a means for producing a difference in length between the optical waveguides on the substrate.

(2) Two metal rods arranged in series,
An electric field sensor device comprising an air gap formed between the metal rods and an optical modulator provided in the air gap, wherein the optical modulator has at least two optical waveguides on a substrate having an electro-optical effect. A control electrode for controlling a light wave propagating through these optical waveguides, and a surface other than a surface which is perpendicular to the propagation direction of the light wave of each optical waveguide of the substrate and on which the waveguide is formed and its opposing surface. It is provided with a means for applying a stress from a point to deform the substrate to change the phase difference of the light waves propagating through the respective optical waveguides.

(3) In the electric field sensor device according to the above (1) or (2), the optical modulator is a Matsuhachender type optical modulator using the interference of two optical waveguides.

(4) In the electric field sensor device according to any one of (1) to (3), the electro-optical crystal forming the substrate of the optical modulator is lithium niobate.

[0025]

According to the above-mentioned means, since the relationship between the applied voltage and the optical power output is represented by a sine curve, it is possible to adjust the optical power output level to the half of the maximum value (optical bias angle φ is 90 degrees). If so, the dynamic range and sensitivity can be maximized. FIG. 8 shows the relationship between the applied voltage level and the optical modulation signal output level at this optical bias angle. As shown in FIG. 8, it can be seen that the slope of the curve at this optical bias angle is the largest and the sensitivity is also the largest at this optical bias angle.

[0026]

Embodiments of the present invention will be described below in detail with reference to the drawings.

In all the drawings for explaining the embodiments, those having the same function are designated by the same reference numerals, and the repeated description thereof will be omitted.

FIG. 1 is a plan view showing a schematic structure of an electric field sensor device according to an embodiment of the present invention, FIG. 2 is an enlarged view of a portion indicated by a circle in FIG. 1, and FIG. 3 is a line AA in FIG. Cross section,
FIG. 4 is a cross-sectional view taken along the line BB of FIG. 1, and FIG. 5 is a diagram showing a model used for analyzing the amount of deformation of the optical waveguide of this embodiment.

In FIGS. 1 to 5, 3 is a polarization-maintaining fiber for propagating the optical power of the light source to the optical modulator, and 4 is a material having an electro-optical effect on which the optical modulator is formed. The fabricated substrates, 5a and 5b are optical waveguides, 6 is an electrode of an optical modulator, 7 is a metal rod (metal rod), 8 is a housing containing the optical modulator of the electric field sensor device, and 9 is an optical modulator. A single mode optical fiber connecting the photodetectors, 14 is a stress applying device for applying an external force to the substrate on which the optical modulator is formed to apply stress (for example, a nylon screw for stress application is used), Reference numeral 15 is a fixing device for fixing the substrate 4, and 16 is a nylon nut.

The substrate 4 has a length of 55 mm and a width of 1
A lithium niobate crystal having a thickness of 0.5 mm and a thickness of 0.5 mm is used, and titanium is diffused on the crystal to form a Matsuhachender optical interferometer. The electrode 6 has a length of 40 mm, a half-wave voltage of about 3 V, and an inter-electrode capacitance of about 10 pF.

Further, as shown in FIG. 3, the housing 8 is made of acrylic, and a nylon stress applying device (stress applying nylon screw) 14 is attached to the side surface thereof. A nylon nut 17 is interposed so that the nylon stress applying device 14 does not loosen. In this way, by attaching the nylon stress applying device 14 to the acrylic housing 8, stress is applied in the lateral direction of the substrate 4.

As shown in FIGS. 1, 2, 3, and 4, the electric field sensor device of this embodiment is provided with two optical waveguides 5a and 5b provided on the substrate 4, and these two optical waveguides 5a and 5b are provided. Stress is applied by a stress applying device 14 made of nylon from a surface that is perpendicular to the longitudinal direction of the optical waveguides 5a and 5b and is not a surface on which the optical waveguides are formed or a surface that opposes the optical waveguides, and the substrate 4 is deformed. An optical modulator is used which controls the optical bias angle by changing the phase difference between the light waves propagating through the two optical waveguides 5a and 5b by changing the lengths of 5a and 5b.

In FIGS. 1, 2, 3 and 4, when stress is applied to the substrate 4 by the stress applying device 14, the substrate 4
The other end of is fixed to the housing 8 by the fixing device 15 and therefore bends as shown in FIG. In this case, if the two optical waveguides 5a and 5b are located symmetrically with respect to the center line of the substrate 4, a tensile stress is applied to one optical waveguide 5a and a compressive stress is applied to the other optical waveguide 5b. Therefore, the length of the one waveguide 5a extends and the length of the other waveguide 5b contracts.

This analytical model is shown in FIG. In such a case, the substrate 4 is considered as an elastic body having one end fixed, and when the other end is deformed, the substrate 4 draws a part of an arc of a circle having a radius r determined by the displacement amount. Therefore, the lengths L ₁ and L ₂ of the two optical waveguides 5a and 5b are approximately represented by equations (4) and (5) using the analytical model shown in FIG.

[0035]

[Number 4] _{L 1 = 2 * (r +} △ r / 2) α + k △ L 0/2 ... (4)

[0036]

[Number 5] _{L 2 = 2 * (r- △} r / 2) α-k △ L 0/2 ... (5) here

[0037]

[Equation 6] r = d / (2 * sin ² α) (6)

[0038]

## EQU7 ## α = Tan to ¹ (d / a) (7) d: Deformation amount of substrate 4 due to stress application (see FIG. 5) a: Distance between fixed part of substrate 4 and stress application part (FIG. 5) Δr: Distance between the two optical waveguides 5a and 5b ΔL ₀ : Difference in length of optical waveguide when stress is not applied k: Coefficient ( ₁ when L ₁ is longer, L ₂ is longer Long time-
1) is.

Therefore, the difference (ΔL) in the length of the waveguide when stress is applied is given by equation (8).

[0040]

[Formula 8] ΔL = L ₁ −L ₂ = 2 * Δrα + kΔL ₀ (8) Therefore, the phase difference (Δβ) is given by the formula (9).

[0041]

Δβ = β (L ₁ −L ₂ ) = βΔL (9) β: phase constant of light wave

[0042]

Β = 2π / λ _g = 2π / (λ / n) (10) λ: wavelength of light wave in free space n: refractive index of waveguide

[0043]

Δβ = 2π (n / λ) ΔL = 4π (n / λ)
* Δrα + 2π (n / λ) kΔL ₀ (11)

In equation (11), the first item is a phase change caused by applying stress to deform the substrate 4,
The second item is the phase change that occurs during the manufacturing process. With the current technology, it is difficult to control the length of the optical waveguide. The present invention also has a feature that an error generated in such a manufacturing process can be corrected by applying stress.

FIG. 6 shows a measurement example of the relationship between the amount of deformation of the substrate 4 and the optical bias angle. In the measurement, a substrate made of lithium niobate (thickness 0.5 mm, width 1 mm, length 55 mm) 4
A stress is applied in the width direction of the substrate 4 at a position of 50 mm from the fixed portion by using the above-mentioned Matsuha Chenda type optical modulator,
The relationship between the amount of deformation of the substrate 4 and the optical bias angle in the applying section was measured.

The straight line shown in FIG. 6 is a theoretical value obtained by using the equation (11). The value of kΔL was obtained from the optical bias angle when no stress was applied. The wavelength of the light wave was 1.3 μm used in the measurement, and the refractive index n of the optical waveguide was 2.2 as the average value of the substrate 4 made of lithium niobate.

As shown in FIG. 6, the theoretical value and the measured value are in good agreement, and it can be seen that this phase change is caused by the change in the length of the optical waveguide.

Lithium niobate has a characteristic that the optical constant changes by applying stress, but as shown in FIG. 6, the deformation amount is very small as 0.2 mm, and the applied stress is 4 g or less. The change of the optical constant due to the stress has little effect on the optical bias angle. For example, nylon (a linear expansion coefficient of 1.0 × 10 ~
⁴ / ° C.), the change in the optical bias angle per 1 ° C. is 0.001 deg / ° C. or less.

It is also conceivable that the optical constant of the substrate is changed by applying stress to control the optical bias angle, but in order to do so, it is necessary to apply large stress and the temperature is changed. In such a case, there is a problem that the applied stress changes greatly due to a slight expansion / contraction of the application jig, and stable control cannot be performed.

However, in the method of the present invention, since the substrate 4 is only deformed, the deformation of the substrate 4 is gentle even with the temperature deformation of the stress applying jig, and the optical bias angle changes greatly. do not do. Therefore, it is possible to provide an optical modulator that is stable against ambient temperature fluctuations.

The optical modulator for the electric field sensor device described so far has been applied with a DC bias not only to control the electric field sensor device but also to control the optical modulator to the position with the best linearity. The present invention can also be applied to the case of using for optical communication using analog optical signals such as CATV.
In particular, when a DC bias is applied, a phenomenon called DC drift occurs, and even if a constant DC voltage is applied, there is a problem that the optical bias angle cannot be maintained at a constant value. An optical communication system that solves this problem There is a feature that can be realized.

Further, in order to apply stress to the substrate 4 and apply asymmetrical strain to the optical waveguide, it is desirable that the width of the substrate 4 be as narrow as possible, but if it is too narrow, the mechanical strength will drop, so it is from about 10 mm to 1 mm. The degree is a practical value.

The measurement result of the half-wave voltage of this embodiment is shown in FIG. As shown in FIG. 7, in this embodiment, since the optical bias angle adjusting mechanism is provided, the optical bias angle is adjusted to the position where the dynamic range is the largest (the position where the optical power level is half the maximum level). can do.

Since it is necessary to measure a large electric field strength in the measurement of the electromagnetic pulse, when the optical bias angle is set to the optimum position, the dynamic range becomes large, and the stronger pulse waveform can be measured more accurately. And the effect is very large.

As can be seen from the above description, according to the present embodiment, the relationship between the applied voltage and the optical power output is represented by a sine curve, so that the optical power output level is at a position half the maximum value (optical bias angle). If φ is adjusted to 90 degrees, the dynamic range can be maximized. FIG. 8 shows the relationship between the applied voltage level and the optical modulation signal output level at this optical bias angle. As shown in FIG. 8, it can be seen that the slope of the curve at this optical bias angle is the largest and the sensitivity is also the largest at this optical bias angle.

Although the present invention has been specifically described based on the above embodiments, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the invention. Is.

[0057]

The effects obtained by the typical one of the inventions disclosed in the present application will be briefly described. A stress is applied to a substrate on which an optical modulator using an optical waveguide as described below is formed. Then, by deforming the substrate to change the length of the optical waveguide and changing the phase difference of the optical wave between the optical waveguides, an optical modulator capable of adjusting the optical bias angle to an optimum value can be obtained. As a result, an electric field sensor device having high sensitivity and a large dynamic range can be realized.

[Brief description of drawings]

FIG. 1 is a plan view showing a schematic configuration of an electric field sensor device according to an embodiment of the present invention.

FIG. 2 is an enlarged view of a portion indicated by a circle in FIG.

3 is a sectional view taken along line AA of FIG.

FIG. 4 is a cross-sectional view taken along the line BB of FIG.

FIG. 5 is a diagram showing a model used for analyzing the amount of deformation of the optical waveguide of the present embodiment.

FIG. 6 is a diagram showing a relationship between a substrate deformation amount and an optical bias angle in the present embodiment.

FIG. 7 is a diagram showing the relationship between the inter-electrode voltage and the optical power output level in the present embodiment.

FIG. 8 is a diagram showing a relationship between an optical bias angle and an optical modulation signal output level in the present embodiment.

FIG. 9 is a diagram showing a basic overall schematic configuration of a conventional electric field sensor device.

FIG. 10 is a diagram showing a schematic configuration of a conventional optical modulator.

11 is a diagram showing an electric field applied to an optical waveguide of the optical modulator shown in FIG.

FIG. 12 is a diagram showing an example of measurement of an optical bias angle of a conventional electric field sensor device.

FIG. 13 is a diagram showing a relationship between an optical bias angle and an optical modulation signal output level of a conventional electric field sensor device.

[Explanation of symbols]

1 ... Light source, 2 ... Polarization compensator, 3 ... Polarization maintaining fiber,
4 ... Substrate, 5, 5a, 5b ... Optical waveguide, 6 ... Electrode of optical modulator, 7 ... Metal rod (metal rod), 8 ... Housing, 9 ... Single-mode optical fiber, 10 ... Photodetector, 11 ... Level meter, 12, 13 ... Optical waveguide, 14 ... Stress applying device,
15 ... Board fixing device, 16 ... Nylon nut.

Front page continuation (72) Inventor Toru Sugamata, Sumitomo Cement Co., Ltd., 1 Kanda Mitoshiro-cho, Chiyoda-ku, Tokyo (72) Inventor Junichiro Minowa 1 Kami-Mishiro-cho, Chiyoda-ku, Tokyo Sumitomo Cement Co., Ltd.

Claims (4)

[Claims] 1. An electric field sensor device comprising two metal rods arranged in series, a void formed between the metal rods, and an optical modulator provided in the void, wherein the optical modulator A device for providing at least two optical waveguides on a substrate having an electro-optical effect, a control electrode for controlling a light wave propagating through these optical waveguides, and means for causing a difference in length between the optical waveguides on the substrate. An electric field sensor device characterized by being provided. 2. An electric field sensor device having two metal rods arranged in series, a void formed between the metal rods, and an optical modulator provided in the void, wherein the optical modulator A container having at least two optical waveguides on a substrate having an electro-optical effect, a control electrode for controlling light waves propagating through these optical waveguides, and a propagation direction of the light waves of the respective optical waveguides on the substrate at right angles and A means is provided for applying stress from one point of a surface other than the surface on which the waveguide is formed and the surface opposite to the waveguide to deform the substrate and change the phase difference of the light waves propagating through the optical waveguides. A characteristic electric field sensor device. 3. The electric field sensor device according to claim 1, wherein the optical modulator is a Matsuhachender type optical modulator using interference of two optical waveguides. 4. The electric field sensor device according to claim 1, wherein the electro-optic crystal forming the substrate of the optical modulator is lithium niobate.