JPH1020264A - Reflection type optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily enable the separation of only a signal light beam obtained by the simple constitution. SOLUTION: A part between the end face of a substrate 32 and a reflection film 74 is coated with an anisotropic refraction index film 72 by oblique vapor depositor as the reflection part 43 of this optical modulator and a sandwiched structure is formed. The anisotropic refraction index film 72 is deposited so that the optical axis is inclined by 45 deg. to the normal of the substrate and the thickness of the film is adjusted so that the phase difference between the polarization of light wave transmitted through the film in the direction of optical axis and the polarization in the direction orthogonal to the axis becomes ΠZ (function ing as a λ/4 plate). Consequently, the polarizing plane of reflected beams generated on the halfway boundary is not rotated and the only polarizing plane of the light beam from the optical modulator is rotated. Since the reflection part 43 is composed of the double layer structure of the anisotropic refraction index film 72 and the reflection film 74, the modulator converts beams to the reflection beam generated on the halfway boundary and a signal light beam having the different polarizing plane.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reflection type optical modulator, and more particularly, to a reflection type optical modulator which reflects light propagating toward one side in a direction opposite to the light and propagates the same. .

[0002]

2. Description of the Related Art A measuring device for measuring an electric field, a voltage, or the like as a physical quantity in the natural world is known. The measurement requires a device for measuring the electric field intensity without disturbing the electric field to be measured in the measurement field. Assuming that the electric field to be measured in the measurement field is not disturbed, a waveguide type optical modulator for modulating light propagating in the waveguide formed on the substrate is formed as a reflection type in which light is reflected inside. This makes it possible to form a reflection type optical modulator that does not disturb the electric field to be measured. As described above, a measuring device has been proposed in which a reflection type optical modulator that does not disturb the electric field to be measured is remotely disposed as a sensor, and the electric field and the voltage are measured using the modulation action of the reflection type optical modulator. (JP-A-4-172261
Reference).

FIG. 1 shows an electric field measuring apparatus 100 using a reflection type optical modulator for remote electric field measurement. The electric field measuring device 100 includes a measuring unit 52, a propagation unit 54, and a sensor unit 5.
6 is comprised. The measuring unit 52 is a CW such as a semiconductor laser.
A light source 10 used for oscillation is provided, and a collimating lens 20, a polarizing beam splitter (hereinafter, referred to as PBS) 12, a Faraday rotator (hereinafter, referred to as FR) 14, a lens 22, and a polarization-maintaining fiber are provided on the emission side of the light source 10. (Hereinafter referred to as PMF) 26 are arranged in order. The polarization plane of the light emitted from the light source 10 is PB
It is adjusted in a predetermined direction passing through S12 (the direction of arrow A in FIG. 1). On the reflection side of the PBS 12, a lens 24 and a photodetector 16 are sequentially arranged. The light detector 16 includes:
An amplifier 18 for amplifying a signal corresponding to the detected light intensity and outputting the amplified signal to a terminal Tg is connected.

[0004] Propagation unit 54 which is continuous with measurement unit 52 is provided with PM
F28 and F30, each of which is coupled so that the polarization plane is preserved and propagated, and one end of which is coupled to the end of the PMF 26 to which the light from the light source 10 reaches.

[0005] A sensor section 56 that is continuous with the propagation section 54 includes a substrate 32, and a waveguide 34 is provided on the substrate 32. One end of the waveguide 34 is in contact with the other end of the PMF 30. The other end of the waveguide 34 is connected to a branch multiplexing section 36.
The branching / multiplexing section 36 is connected to one ends of the waveguides 38 and 40 provided on the substrate 32. Waveguides 38, 4
The reflection part 42 is provided at the other end of 0. On the waveguides 38 and 40, an electrode 44 connected to a dipole antenna 48 and an electrode 46 connected to a dipole antenna 50 are loaded.

In the electric field measuring apparatus 100, light emitted from the light source 10 passes through the PBS 12 and the FR 14, and
The polarization plane is rotated by 45 degrees during transmission through R14 (the polarization plane in the direction of arrow B in FIG. 1). The light transmitted through the FR 14 is focused by the lens 22 and is incident on the PMF 26. This PMF
The light incident on 26 is transmitted to the sensor section 56 via the PMFs 28 and 30.

The light transmitted to the sensor unit 56 is
4. The light propagates through the branching / combining section 36 and the waveguides 38 and 40 and is reflected by the reflecting section 42. The reflected light propagates in the opposite direction and returns to the PMF 30. In this sensor unit 56,
Light propagating according to the surrounding electric field strength (detected by the dipole antenna) is intensity-modulated.

That is, the light transmitted through the waveguide 34 branches into the waveguides 38 and 40 in the branching / combining section 36. A voltage corresponding to the measured physical quantity (electric field) is applied to the electrodes 44 and 46 for modulation on the waveguides 38 and 40.
That is, the electric field to be measured is converted into a voltage by the dipole antennas 48 and 50 as transducers, and
4, 46. An electro-optic effect is caused by the applied voltage, and a phase difference is generated between the respective lights passing through the waveguides 38 and 40. Each light that has propagated through the waveguides 38 and 40 reaches an end face, and is reflected by the reflection unit 42. The reflected light propagates in the waveguides 38 and 40 in the opposite directions, reaches the branching / multiplexing unit 36, is multiplexed in the branching / multiplexing unit 36, and is returned to the PMF 30. Since each light propagated from the waveguides 38 and 40 has a phase difference, they are multiplexed and PM
The light returned to F30 becomes intensity-modulated light (hereinafter, signal light).

This signal light is transmitted through PMFs 30 and 28 to P
It reaches MF26. This modulated signal light is again transmitted to FR
14 is transmitted. At the time of transmission of this FR14, the polarization plane is 4
Since it is rotated by 5 degrees, it becomes a polarization plane (polarization plane in the direction of arrow C in FIG. 1) rotated 90 degrees with respect to the initial state (polarization plane in the direction of arrow A in FIG. 1). As a result, the modulated signal light is reflected by the PBS 12 and enters the photodetector 16.

As described above, the light (input light L1) between the PBS 12 and the PMF 28, that is, the light emitted from the light source 10
FR14 that rotates the plane of polarization in the portion where the signal light passes
Is provided, the PBS 12 can correctly separate the input light L1 and the signal light L2, and can reflect only the signal light L2. In addition, since the electric field is measured by remotely arranging the measuring unit 52, the electric field intensity can be measured without disturbing the electric field to be measured by converting the intensity of the modulated signal light into a voltage by using a photodetector. Can be.

However, when measuring such an optical integrated circuit as a reflection type optical modulator using a remote sensor,
There is a problem that interference occurs between light (reflected light) reflected by various optical components such as a lens and a beam splitter and light (signal light) that should be originally used for measurement, and noise is increased in the measured signal. .

More specifically, as shown in FIG. 1, an input end of the PMF, a fiber connector for coupling the PMF,
MF and optical integrated circuit (sensor part 5 which is a reflection type optical modulator)
Reflected light R1, R2, R3, and R4, which should be unnecessary, are generated at various interfaces such as a connection portion with 6). These reflected lights R1 to R4 have the same plane of polarization as the modulated signal light Sig (the plane of polarization in the direction of arrow B in FIG. 1), and the reflected lights R1 to R4 and the signal light Sig follow the same path. , Incident on the photodetector 16. That is, since the plane of polarization of the reflected light has the same angle as the original signal light returning from the reflection type optical modulator, the plane of polarization is rotated at FR 14 similarly to the signal light, and is reflected at PBS 12. Light detector 1
6 is reached. For this reason, the reflected light and the modulated signal light interfere with each other on the photodetector 16, causing large noise and light amount fluctuation, and the accuracy may be significantly reduced.

In order to solve this problem, there has been proposed a sensor for rotating the direction of polarization, that is, the plane of polarization in the sensor section (see Japanese Patent Application Laid-Open No. 61-256205). In this technique, as shown in FIG. 2, linearly polarized light emitted from the PMF 30 is collimated by a lens 60 and irradiated on a mirror 64 via a λ / 4 plate 62. The irradiated light is reflected by the mirror 64 and converged on the PMF 30 by the lens 60 via the λ / 4 plate 62. Accordingly, the linearly polarized light from the PMF 30 is converted into circularly polarized light when passing through the λ / 4 plate 62, and the light reflected by the mirror 64 is converted into linearly polarized light when transmitted through the λ / 4 plate 62 again. . As a result, the plane of polarization of the linearly polarized light from the PMF 30 differs from the plane of polarization of the linearly polarized light incident on the PMF 30 by the light reflected by the mirror 64 by π / 2. Therefore, the reflected light and the modulated signal light do not interfere with each other on the photodetector 16.

[0014]

However, the conventional sensor shown in FIG. 2 requires a bulk component such as a lens, so that it cannot be connected or applied to an optical integrated circuit no matter how finely processed. It was practical. As other methods, measures such as using a low coherence light source, applying an anti-reflection coating to various interfaces, or obliquely polishing and connecting various interfaces to prevent reflection are considered.
It is not practical because a complicated process is required and assembly is difficult.

SUMMARY OF THE INVENTION An object of the present invention is to provide a reflection type optical modulator having a simple configuration and capable of easily separating only the obtained signal light in consideration of the above fact.

[0016]

According to a first aspect of the present invention, there is provided a reflection type optical modulator comprising: a first waveguide portion for transmitting light; and an electrode for modulating light. A plurality of second waveguide portions provided and branched from the first waveguide portion; and a second waveguide portion provided at an end portion of the second waveguide portion branched from the first waveguide portion. A thin-film reflector comprising: a reflector for reflecting light from each of the wave guides; and a thin-film portion formed on the reflection side of the reflector and formed to function as a λ / 4 plate for transmitted light. And

According to a second aspect of the present invention, in the reflection type optical modulator according to the first aspect, the thin film portion of the thin film reflecting portion is formed of an anisotropic refractive index film.

According to a third aspect of the present invention, in the reflection type optical modulator according to the second aspect, the anisotropic refractive index film is formed by oblique deposition.

According to a fourth aspect of the present invention, in the reflective optical modulator according to the third aspect, the reflecting portion is formed by metal evaporation.

According to the first aspect of the present invention, the light that has propagated through the first waveguide section is branched and propagates through a plurality of second waveguides. The light propagating in one direction is reflected by the thin film reflector and returned as light propagating in the opposite direction to the light. The light traveling in the opposite direction propagates through the plurality of second waveguides in the opposite direction, is multiplexed, and propagates in the first waveguide portion in the opposite direction. In the thin film reflecting portion, the phase difference of light transmitted in the thin film portion functioning as a λ / 4 plate becomes π / 2, and the light is transmitted twice before and after reflection, so that light returned as light traveling in the opposite direction is returned. Is rotated by π / 2 from the polarization plane of light propagating in one direction. As described above, the respective polarization planes of the light propagating in one direction and the light propagating in the opposite direction are orthogonal to each other at π / 2, so that interference does not occur even if they are multiplexed. In addition, the polarization plane can be rotated without using a polarization plane rotation unit such as a Faraday rotator as in the related art, and the light can be completely separated only by the polarization beam splitter or the like.

The thin film portion of the thin film reflecting portion is formed of an anisotropic refractive index film as described in claim 2, whereby a λ / 4 plate is finely processed and bulk components such as lenses are used. There is no need to construct it, the structure is simple and λ /
A part functioning as four plates can be configured. This anisotropic refractive index film can be easily formed by oblique vapor deposition as described in claim 3. For example, the anisotropic refractive index film is made of Ta ₂ O.
It can be formed by oblique vapor deposition such as ₅ . The anisotropic refractive index film formed by the oblique deposition is deposited such that its optical axis is inclined by 45 degrees with respect to the normal line of the substrate of the reflection type optical modulator, and the film thickness is adjusted in the optical axis direction of the transmitted light. Is set so that the phase difference between the polarized light and the polarized light in the direction perpendicular thereto becomes π / 2, that is, functions as a λ / 4 plate. A configuration in which the wavefront rotates 90 degrees can be adopted.

According to a fourth aspect of the present invention, the reflecting portion of the thin film reflecting portion is formed by metal deposition.
The thin film reflecting portion is formed of a λ / 4 formed by an anisotropic refractive index film.
It can be formed with a simple two-layer structure composed of a plate and a reflective film. Therefore, the light propagating through the waveguide is reflected at the interface by the thin film having a simple two-layer structure (reflected light) without forming the λ / 4 plate finely and using a bulk component such as a lens. Can be converted into light whose polarization plane is easy to separate.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the present embodiment, the present invention is applied to an electric field measuring device having an electric field sensor using a reflection type optical modulator. In this embodiment, since the configuration is substantially the same as that of the electric field measuring device described in the above-described conventional technique, the same portions are denoted by the same reference numerals, and detailed description will be omitted. In addition, the polarization plane in the following description is defined on a yz plane orthogonal to the x-axis in a three-dimensional space in which the x-axis is the optical axis direction in which light propagates, the y-axis is the sagittal direction, and the z-axis is the meridional direction. The description will be made using the directions shown in FIGS.

As shown in FIG. 3, an electric field measuring apparatus 102 using the reflection type optical modulator of the present embodiment for an electric field sensor is:
It is composed of a measuring section 53, a propagation section 54, and a sensor section 57 composed of an optical integrated circuit. The measuring unit 53 is connected to the measuring unit 52 in the configuration of the electric field measuring device 100 in FIG.
4, and an optical isolator 70 is provided between the collimator lens 20 and the PBS 12. The optical isolator 70 is for preventing an influence such as a change in laser oscillation due to propagation of light in a direction opposite to the light emitted from the light source 10. In this optical isolator 70, the polarization plane of the light transmitted through the optical isolator 70 is P
It is adjusted in the direction passing through the BS 12 (the direction of the polarization plane in the direction of arrow A in FIG. 3). If the oscillation of the light source 10 is not affected by the propagation of the light in the opposite direction, the optical isolator 70 is not particularly necessary.

In this embodiment, the light source 10 is 1.3 μm.
A semiconductor laser that oscillates in the m-band wavelength region is used for CW oscillation. The light source 10 is not limited to a semiconductor laser that oscillates in a 1.3 μm wavelength band,
A semiconductor laser that oscillates in the band may be used.
A gas laser such as a Ne laser may be used, and another type of light source may be used. The photodetector 16 uses a photodiode.

The sensor section 57 is composed of a reflection type optical modulator, and is provided on the substrate 32 on which the waveguides 34, 38 and 40 are provided.
The reflector 43 is mounted. This substrate 32 is made of LiNb
It is made of O ₃ and the waveguide is made by Ti diffusion. The reflection section 43 includes an anisotropic refractive index film 72 and a light reflection film 74. Therefore, waveguide 3
Each light transmitted through 8 and 40 passes through the anisotropic refractive index film 72, is reflected by the light reflecting film 74, and is again transmitted to the anisotropic refractive index film 72.
, And is returned to the waveguides 38 and 40.

The anisotropic refractive index film 72 is formed by oblique deposition of, for example, Ta ₂ O ₅ as described later. The optic axis of the anisotropic refractive index film 72 formed by the oblique deposition is 45 degrees with respect to the normal to the substrate 32 of the optical modulator (optical integrated circuit).
It is deposited so as to tilt at an angle. Further, the anisotropic refractive index film 7
2 has a phase difference of π / 2 between the polarization in the optical axis direction and the polarization in the direction perpendicular to the optical axis direction of the light transmitted through the anisotropic refractive index film 72.
That is, it is set to function as a λ / 4 plate. The light reflecting film 74 is generally made of Au, A
It is formed of a reflective film of a metal thin film similar to the conventional one in which a metal thin film of g, Al or the like is deposited.

Next, the operation of the present embodiment will be described. The light emitted from the light source 10 passes through an optical isolator 70 for preventing the return light from affecting the laser oscillation, and
Irradiated to BS12. The light transmitted through the PBS 12 is focused by the lens 22 and propagates to the sensor unit 57 via the PMFs 26, 28, and 30. The light propagated to the waveguide of the optical integrated circuit as the sensor unit 57 is propagated while maintaining a predetermined polarization plane, for example, a polarization plane perpendicular to the substrate surface (the direction of arrow A in FIG. 3).

The light propagating through the waveguide 34 is transmitted to the branching / multiplexing unit 3.
At 6, the light is branched into waveguides 38 and 40. The waveguides 38 and 40 are loaded with electrodes 44 and 46 for modulation, and each of the electrodes 44 and 46 has a voltage corresponding to a physical quantity to be measured (electric field), that is, a dipole antenna 48 as a transducer. The voltage converted by 50 is applied. An electro-optic effect is generated by the applied voltage, and a phase difference is generated in each light transmitted through each of the waveguides 38 and 40. Each light transmitted through the waveguides 38 and 40 reaches the end face, and is reflected back by the reflecting portion 43 including the anisotropic refractive index film 72 and the light reflecting film 74.

When the light propagating in the optical waveguide is reflected by the two-layered film of the anisotropic refractive index film 72 and the light reflecting film 74 and returns, the light travels through the anisotropic refractive index film 72 functioning as a λ / 4 plate. And twice on the return trip. As a result, a phase difference of π is generated in total, and as a result, the plane of polarization is rotated by 90 degrees, and becomes a light of a plane of polarization parallel to the substrate surface (in the direction of arrow C in FIG. 3), which is the same path as the outward path. Will return. Waveguide 3
The light returning from each of the light beams 8 and 40 is multiplexed by the branching multiplexing unit 36, and the phase difference is converted into a change in intensity by mutual interference. In this way, the electric field to be measured is applied to the electrodes 44 and 46.
Is converted into a light intensity change by the applied voltage of the above and the phase difference of the light propagating through the waveguides 38 and 40. Therefore, this change in light intensity corresponds to a change in electric field. The multiplexed light becomes signal light that is intensity-modulated light.

This signal light is again transmitted through the waveguide 34 and the PMF
The light passes through 30, 28, and 26 and is returned to the PBS 12. Here, the polarization plane is rotated by 90 degrees with respect to the outward path, and the polarization plane is polarized in the horizontal direction (the direction of arrow C in FIG. 3), so that the signal light is reflected by the PBS 12. The reflected signal light is photoelectrically converted by the photodetector 16 to become an electric signal. By measuring this electric signal, the electric field at the place where the sensor unit 57 is placed is measured.

Here, as described in the section of the prior art, the light input / output end to the optical fiber, the optical fiber connector,
Reflected light is generated at various interfaces such as a connection portion between the optical fiber and the optical integrated circuit. That is, the input end of the PMF, P
At various interfaces such as a fiber connector coupling the MF and a connection portion between the PMF and the sensor unit 56, reflected light which is originally unnecessary is generated.

However, in the present embodiment, the plane of polarization of these reflected lights does not rotate. That is, the polarization plane of these reflected lights is the polarization plane in the direction passing through the PBS 12 (the direction of arrow A in FIG. 3). Therefore, these reflected lights are P
Even when the light reaches the BS 12, its polarization plane remains in the initial state, passes through the PBS 12 without being reflected by the PBS 12, and is removed by the optical isolator 70. Therefore, the light does not reach the photodetector 16.

On the other hand, the signal light Sig ′ from the sensor unit 57
Is a plane of polarization in a direction (direction of arrow C in FIG. 3) inclined by 90 degrees from the direction passing through the PBS 12 (direction of arrow A in FIG. 3), and is reflected by the PBS 12 and incident on the photodetector 16.

As described above, the reflected light and the signal light are completely separated from each other and interfere with each other on the surface of the photodetector 16 as in the prior art.
There is no fluctuation in light quantity, no noise or the like.

Also, because the extinction ratio of the PBS 12 is not sufficient or the adjustment of the optical system is incomplete, a part of the reflected light that is originally unnecessary is reflected by the PBS 12 and
, Since the planes of polarization of the reflected light and the signal light are orthogonal to each other, they do not interfere with each other, and no fluctuations in light quantity and noise occur as described above.

Next, the effectiveness of the reflection type optical modulator which is the sensor unit 57 used in the electric field measuring apparatus of the present embodiment will be described.

FIG. 4 shows the results of an experiment performed to evaluate the improvement of the dynamic range by suppressing the noise in the electric field measuring apparatus according to the present embodiment. The characteristic was shown. In the figure,
The horizontal axis indicates the voltage applied to the reflection type optical modulator, and the vertical axis indicates the intensity of light output from the optical modulator corresponding to the amount of light incident on the reflection type photodetector. Further, in the drawing, a curve 80 shown by a solid line in FIG. 4 shows the characteristic when there is no reflected light.
A curve 82 indicated by a dotted line shows the characteristic when there is reflected light. The light intensity when the applied voltage is substantially zero corresponds to the amount of bias light. If there is reflected light, as will be seen, the offset P _r to the original bias light quantity P _b
Are added to the apparent bias light amount P _b ′. Therefore, the characteristic of the optical modulator is a characteristic to which the offset _Pr is added.

As is well known, noise included in the output voltage of the photodetector is represented by shot noise of the output photocurrent and thermal noise of the load resistance. The former shot noise can be expressed by the following equation.

[0040] _{V O = 2 · R L ·} (e · r d · (P b + P r)) 1/2 where, V _O shot contained in the output of the photodetector noise, R _L photodetector load resistance, e is the electron charge, r _d is the amount of light / current conversion sensitivity of the photodetector, P _b is bias amount,
_Pr is the offset light amount due to the reflected light.

The detection limit of the sensor unit 57 using the reflection type optical modulator of this embodiment is determined by the noise of the photodetector. When there is reflected light as in the above equation, the generated noise increases by a half power of the amount of reflected light. For this reason, the detection limit is increased accordingly, and it becomes difficult to detect a weak signal. Therefore, if the optical modulator does not have a sufficiently low loss, the offset due to the reflected light substantially coincides with the amount of bias light, and a signal having a magnitude several times the theoretical detection limit may be detected. .

On the other hand, in the present embodiment, since the reflected light is not incident on the photodetector 16, the offset 80 does not occur, and the characteristic of the curve 80 shown by the solid line in FIG. Does not increase, so that the dynamic range can be expanded as compared with the conventional example.

As described above, in the present embodiment, the planes of polarization of the signal light and the reflected light at the various interfaces are orthogonal to each other. Even if these lights are multiplexed, no interference occurs, and the PB
Only signal light can be completely separated by S or the like. Therefore, fluctuation of output signal light intensity, noise, and drift due to interference with return light do not occur. Further, since only the signal light that does not include the reflected light is incident on the photodetector, shot noise is reduced and the dynamic range can be expanded.

Next, formation of an obliquely deposited film of the anisotropic refractive index film 72 used in the sensor section 57 of the present embodiment will be described.

The anisotropic refractive index film 72 is formed as a λ / 4 plate.
However, the vapor deposition material for forming the λ / 4 plate is
Any material that is transparent to light is required, and has a large refractive index.
Ta _TwoO_Five, WO_Three, CeO_TwoIt is preferable to use
No. There is no particular limitation on the vapor deposition method.
Sub beam heating, sputtering, or the like can be used. Steam
The wearing direction is in the plane B as shown in FIG.
Angle θ. In FIG. 5, plane A is opposite
A plane including a launch surface (yz plane) is shown, and a plane B is a traveling direction of a light wave.
Direction (x-axis) and makes 45 ° with the substrate normal (z-axis)
The straight line q indicates the intersection of the plane A and the plane B. Corner
The value of the degree θ is preferably about 30 ° to 80 °. Steam
The substrate temperature during deposition should be 1/3 or less of the melting point of the deposition material
It is preferable that the reaction be performed at room temperature.

The anisotropic refractive index film 72 of the present embodiment was formed as follows. First, Ta ₂ O ₅ was formed to a thickness of 7 μm by electron beam evaporation. Since this film was colored brown due to oxygen vacancies, it was heat-treated at 200 ° C. for 2 hours in an oxygen atmosphere to increase the transparency. As a result, a transparent λ / 4 plate was obtained. Next, Au was deposited to a thickness of 0.2 μm by electron beam evaporation.

As described above, the substrate 3 is used as the reflection section 43.
A two-layer reflecting film composed of an anisotropic refractive index film 72 and a light reflecting film 74 was formed on the end face of No. 2. The polarization plane of the signal light obtained by being reflected by the reflector 43 formed in this way is orthogonal to the polarization plane of the reflected light at various interfaces, and these lights are multiplexed. Even without interference, only signal light can be completely separated by PBS or the like. Therefore, fluctuation, noise, and drift of the obtained output signal do not occur due to interference between the reflected light and the signal light. In addition, since only the signal light without reflected light is incident on the photodetector, shot noise is reduced and the dynamic range can be expanded.

Therefore, the anisotropic refractive index film capable of separating only the signal light can be incorporated in an optical integrated circuit (optical modulator) and can be formed without requiring a complicated process. Since noise due to interference between reflected light and signal light at various interfaces can be suppressed, it is possible to easily improve the measurement accuracy of the sensor section, which is a reflective optical modulator, and expand the dynamic range. Therefore, the performance of the electric field measuring device can be improved.

According to the present embodiment, the end face reflection film of the reflection type optical modulator is formed of a λ / 4 plate formed by oblique deposition and a reflection film.
With the layered structure, the propagation mode of the signal light can be converted to a mode orthogonal to the reflected light, thereby eliminating any coherent noise and reducing the fluctuation.

The reflection type optical modulator which is the sensor unit of the electric field measuring apparatus of this embodiment is a reflection type optical modulation element and an optical fiber for measuring an electric field and a voltage without using a metal signal cable. It is suitable for use in a measuring device composed of In addition, the anisotropic refractive index film included in the reflection portion also functions as a means for suppressing noise generated by interference between reflected light from various connection surfaces and the like and signal light.

In the present embodiment, the case where the present invention is applied to an electric field measuring device having a reflection type optical modulator functioning as an electric field sensor has been described. However, the present invention is not limited to the electric field sensor. However, the configuration can be substantially the same when applied to other sensor applications such as a voltage sensor and a magnetic field sensor. For example, when applied to a voltage sensor, it can be realized only by replacing the dipole antenna connected to the electrode of the optical modulator in FIG. 3 with a lead wire or a contact probe connected to a voltage measurement point. When used as a magnetic field sensor, a loop antenna may be used instead of a dipole antenna. That is, a similar sensor can be configured by converting any physical quantity to be measured into a voltage using a predetermined transducer and connecting the voltage to the electrode of the optical modulator.

Although the photodetector 16 of this embodiment uses a photodiode, another photodetector such as a photomultiplier tube may be used. In addition, although the description has been given by using the branch interference type optical modulator as an example of the reflection type optical modulator which is an optical integrated circuit, other modulators such as a directional coupler type and a cross type optical modulator are similar. Is possible.

[0053]

As described above, according to the first aspect of the present invention, the reflecting portion for reflecting the light from each of the second waveguide portions and the transmitting light are provided at the end of the second waveguide portion. Is provided with a thin-film reflecting portion formed of a thin-film portion formed so as to function as a λ / 4 plate, so that the polarization plane of the light reflected by the thin-film reflecting portion and propagated in the opposite direction is reflected by the light before reflection. Whereas π
/ 2 are orthogonal and multiplexed, no interference occurs. Therefore, any coherent noise caused by the same polarization plane can be suppressed. This has the effect of reducing intensity fluctuations due to noise in the image.

According to the second aspect of the present invention, since the thin film portion is formed of an anisotropic refractive index film, the λ / 4 plate does not need to be formed by fine processing or the like, and has a simple structure and is easy. Thus, there is an effect that a light transmitting portion functioning as a λ / 4 plate can be formed.

According to the fourth aspect of the present invention, since the reflecting portion is formed of a reflecting film formed by metal deposition and the thin film reflecting portion is formed of an anisotropic refractive index film, the thin film reflecting portion can be simplified. It can be formed in a two-layer structure. Therefore, λ /
The polarization plane that can easily separate the light propagating through the waveguide from the light reflected at the interface is formed by a simple thin film with a two-layer structure without micro-processing the four plates and using bulk components such as lenses. There is an effect that the light can be converted to orthogonal light.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of a conventional electric field measuring device.

FIG. 2 is a block diagram showing a schematic configuration of a sensor unit for preventing light interference in a conventional measuring device.

FIG. 3 is a block diagram illustrating a schematic configuration of an electric field measurement device according to the present embodiment.

FIG. 4 is a diagram illustrating characteristics of applied voltage and light intensity of the reflection type optical modulator according to the present embodiment.

FIG. 5 is an image diagram for explaining a deposition direction of an anisotropic refractive index film.

[Explanation of symbols]

 57 Sensor unit 43 Reflecting unit 72 Anisotropic refractive index film 74 Light reflecting film

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tadashi Ichikawa 41 Toyota-Chuo Research Institute, Inc. No. 41, Changchun Yokomichi 1 Toyota Central Research Institute, Inc.

Claims (4)

[Claims] A first waveguide section for propagating light; a plurality of second waveguide sections each provided with an electrode for light modulation, and branched from the first waveguide section; A reflection portion provided at an end of the waveguide portion branched from the first waveguide portion and reflecting light from each of the second waveguide portions; and a reflection portion formed on the reflection side of the reflection portion and transmitting the light. And a thin film reflecting portion formed of a thin film portion formed to function as a λ / 4 plate with respect to light. 2. The reflection type optical modulator according to claim 1, wherein the thin film portion of the thin film reflection portion is formed of an anisotropic refractive index film. 3. The reflection type optical modulator according to claim 2, wherein the anisotropic refractive index film is formed by oblique vapor deposition. 4. The reflection type optical modulator according to claim 3, wherein the reflection section is formed by metal deposition.