JPH05341243A - Mode converter and optical filter - Google Patents

Abstract

PURPOSE:To provide the mode converter being suitable for constituting the optical filter. CONSTITUTION:The mode converter 10 is constituted of a waveguide 16, and a waveguide 18 of a radio wave. The waveguide 16 is formed by arranging alternately and periodically an inverting part 12 and a spacer part 14 along the waveguide direction P of light L, and the waveguide 18 guides a radio wave D along the optical waveguide direction P. Also, each inverting part 12 is formed by arranging alternately and periodically electro-optical crystals 12a and 12b along the optical waveguide direction P. The electro-optical crystals 12a and 12b are arranged in a position which an electric field of the radio wave D reaches. Moreover, the Z axis direction of the electro-optical crystals 12a and 12b is allowed to be orthogonal to the optical waveguide direction P, and also, the X axis directions of the electro-optical crystals 12a and 12b are set to the directions being opposite to each other, and also, made parallel to the direction R of an electric field of the radio wave D. As for the mode converter 10, its mode conversion efficiency is high, and also, its operating speed is high, therefore, the purpose can be attained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mode converter, particularly a mode converter suitable for use in constructing an optical filter, and an optical filter for separating light by using mode conversion.

[0002]

2. Description of the Related Art Conventionally, there has been known an optical filter which separates light by forming a periodical change in refractive index using a sound wave. In this optical filter, a sound wave is incident on the acousto-optic material to form a periodic refractive index change by the acousto-optic effect. Then, the first polarized light, such as ordinary light, is made incident on the acousto-optic material, and the first polarized light is mode-converted into the second polarized light, which is orthogonal to the polarized light, such as extraordinary light, by the periodic change in the refractive index. Mode conversion occurs between first and second polarizations of equal wavelength. The mode conversion efficiency is maximized when the propagation constant difference between the first and second polarized waves having the same wavelength and the period of the refractive index change, that is, the wavelength of the sound wave satisfy a specific relationship, and when the specific relationship is not satisfied, the mode is changed. The conversion rate is small or becomes substantially zero. Moreover, the difference in the propagation constants of these polarized lights has different values depending on the wavelength. Therefore, by changing the wavelength of the sound wave, it is possible to arbitrarily select the wavelengths of the first and second polarized lights that maximize conversion efficiency. The wavelength of the sound wave is
It changes if the frequency of the sound wave is changed. Therefore, by making the output light of the acousto-optic material incident on the polarizer, it is possible to separate light having a desired wavelength.

Further, there is an optical filter configured to form a periodical refractive index change by using radio waves. Reference: JP-A-2-250
It is disclosed in Japanese Patent Publication No. 041. In this optical filter,
An electric field of a radio wave is applied to the electro-optic material to form a periodic change in the refractive index due to the electro-optic effect. Then, the light is made incident on the phase grating formed by this change in the refractive index, whereby the light is diffracted and spatially separated.

[0004]

However, in the conventional optical filter utilizing sound waves, the speed of the sound waves propagating through the acousto-optic material is slow, so that the change in the refractive index of the period depending on the wavelength of the light to be separated is changed. The time (switching time) required to form the film in the predetermined area becomes about microseconds, which is delayed.

Further, in the conventional optical filter utilizing radio waves, since the refractive index distribution of the electro-optical material is controlled only by the radio waves, the diffraction efficiency cannot be increased sufficiently and therefore the intensity of the separated light is not always sufficient. It cannot be raised.

In order to solve the above-mentioned conventional problems, an object of the present invention is to provide a mode converter and an optical filter which can form a periodic refractive index change and can perform mode conversion efficiently and which has a high operation speed. Especially.

[0007]

In order to achieve this object, a mode converter according to the first aspect of the present invention is provided with an optical waveguide and an optical waveguide having inversion portions and spacer portions which are alternately and periodically arranged along the optical waveguide direction. A first and a second electrical device in which each inversion part is alternately and periodically arranged along the optical waveguide direction and the Z-axis direction is orthogonal to the optical waveguide direction. Having an optical crystal, arranging the first and second electro-optical crystals at positions where the electric field of the radio wave extends, and making the X-axis direction or the Y-axis direction of these optical crystals opposite to each other and parallel to the electric field direction of the radio wave. It is characterized by being formed.

The mode converter of the second aspect of the invention comprises an optical waveguide having an inversion portion and a spacer portion, which are alternately and periodically arranged along the optical waveguide direction, and a radio wave guide for guiding a radio wave along the optical waveguide direction. And each inversion part has first and second magneto-optical crystals arranged alternately and periodically along the optical waveguide direction and having the Z-axis direction orthogonal to the optical waveguide direction. The crystal is arranged at a position where the magnetic field of the radio wave extends, and the X-axis direction or the Y-axis direction of these optical crystals are opposite to each other and parallel to the direction of the magnetic field of the radio wave.

Further, the optical filter of the third invention comprises a directional coupler formed by arranging first and second optical waveguides having different equivalent refractive indices in close proximity to each other and a radio wave guide for guiding a radio wave along the optical waveguide direction. At least one of the first and second optical waveguides has an inversion section, and the inversion section is alternately and periodically arranged along the optical waveguide direction, and the Z-axis direction is orthogonal to the optical waveguide direction. Two electro-optic crystals, the first and second electro-optic crystals are arranged at positions where the electric field of the radio wave extends, and the Z-axis directions of these optical crystals are opposite to each other and parallel to the direction of the electric field of the radio wave. It is characterized by

The optical filter according to the fourth aspect of the present invention forms a sinusoidally changing electric field along the optical waveguide direction with a directional coupler formed by arranging first and second optical waveguides having different equivalent refractive indices in close proximity. At least one of the first and second optical waveguides has an inversion part, and the inversion part is alternately and periodically arranged along the optical waveguide direction so that the Z-axis direction is orthogonal to the optical waveguide direction. It has first and second electro-optical crystals, and the first and second electro-optical crystals are arranged at positions where an electric field formed by the electrodes extends, and the Z-axis directions of these optical crystals are opposite to each other and the electrodes are formed. It is characterized in that it is formed parallel to the direction of the electric field of the electric field.

[0011]

According to the first invention, the Z-axis directions of the first and second electro-optic crystals are made orthogonal to the optical waveguide direction, and the X-axis or Y-axis directions of these electro-optic crystals are made opposite to each other and the radio wave It is parallel to the direction of the electric field.

Therefore, by applying an electric field of a radio wave to the first and second electro-optic crystals of each inversion portion, it is possible to form a refractive index change that periodically changes along the optical waveguide direction. Moreover, since this change in the refractive index is formed with the X-axis or Y-axis directions of the first and second electro-optic crystals being opposite to each other,
Due to this change in the refractive index, two orthogonal polarizations (for example, T
One of the M mode light and the TE mode light) can be converted to the other.

When the wavelength of the radio wave is changed, the wavelength of the polarized light that is mode-converted changes accordingly. Furthermore, as the length of the inversion part in the optical waveguide direction is shortened and / or the birefringence of the spacer part is lowered, the wavelength range of polarized light capable of mode conversion becomes wider. Therefore, it is most preferable to use a material having no birefringence as the material for forming the spacer portion.

According to the second invention, the Z-axis directions of the first and second magneto-optical crystals are made orthogonal to the optical waveguide direction, and
The X-axis and Y-axis directions of these magneto-optical crystals are made opposite to each other and parallel to the direction of the magnetic field of radio waves.

Therefore, by applying a magnetic field of a radio wave to the first and second magneto-optical crystals of each inversion portion, it is possible to form a refractive index change that periodically changes along the optical waveguide direction. Moreover, since this change in the refractive index is formed with the X-axis or Y-axis directions of the first and second magneto-optical crystals being opposite to each other,
Due to this change in the refractive index, two orthogonal polarizations (for example, T
One of the M mode light and the TE mode light) can be converted to the other.

When the wavelength of the radio wave is changed, the wavelength of the polarized light that is mode-converted changes accordingly. Furthermore, as the length of the inversion part in the optical waveguide direction is shortened and / or the birefringence of the spacer part is lowered, the wavelength range of polarized light capable of mode conversion becomes wider. Therefore, it is most preferable to use a material having no birefringence as the material for forming the spacer portion.

Further, according to the third invention, the Z-axis directions of the first and second electro-optic crystals are made orthogonal to the optical waveguide direction, the Z-axis directions of these electro-optic crystals are made opposite to each other, and the electric field of the radio wave is Set parallel to the direction.

Therefore, by applying an electric field of a radio wave to the first and second electro-optic crystals of the inversion portion, it is possible to form a refractive index change that periodically changes along the optical waveguide direction. Moreover, since the change in the refractive index is formed such that the Z-axis directions of the first and second electro-optic crystals are opposite to each other, the change in the refractive index causes symmetric mode light (even mode light) and antisymmetric mode light. One of (odd mode light) can be converted to the other. When the wavelength of the radio wave is changed, the wavelength of the light whose mode is converted changes accordingly.

On the other hand, the first and second optical waveguides form a directional coupler. Moreover, the symmetric mode light guided through the directional coupler is from one of the first and second optical waveguides, and the antisymmetric mode light guided through the directional coupler is from the other of the first and second optical waveguides. , Selectively emit. Therefore, by providing an inverting portion in at least one of the first and second optical waveguides and performing mode conversion, light can be selectively separated according to the wavelength.

According to the fourth invention, the Z-axis direction of the first and second electro-optic crystals is made orthogonal to the optical waveguide direction, and
The Z-axis directions of these electro-optic crystals are opposite to each other and parallel to the direction of the electric field of the electric field formed by the electrodes.

Therefore, by exerting an electric field formed by the electrodes on the first and second electro-optic crystals of the inversion portion, it is possible to form a refractive index change that periodically changes along the optical waveguide direction. Moreover, since the change in the refractive index is formed such that the Z-axis directions of the first and second electro-optic crystals are opposite to each other, the change in the refractive index causes symmetric mode light (even mode light) and antisymmetric mode light. One of (odd mode light) can be converted to the other. Moreover, when the wavelength of the radio wave is changed, the wavelength of the light whose mode is converted changes accordingly.

On the other hand, the first and second optical waveguides form a directional coupler. Moreover, the symmetric mode light guided through the directional coupler is from one of the first and second optical waveguides, and the antisymmetric mode light guided through the directional coupler is from the other of the first and second optical waveguides. , Selectively emit. Therefore, by providing an inverting portion in at least one of the first and second optical waveguides and performing mode conversion, light can be selectively separated according to the wavelength.

[0023]

Embodiments of the present invention will be described below with reference to the drawings. It should be noted that the drawings are only schematically shown to the extent that the present invention can be understood, and therefore the embodiments are not limited to the illustrated examples.

FIG. 1 is an overall perspective view schematically showing the construction of the embodiment of the first invention, showing a state in which an optical filter is constructed by combining a mode converter and a polarizer of this embodiment.

As shown in the figure, the mode converter 10 of this embodiment has an optical waveguide 16 having an inversion portion 12 and a spacer portion 14 which are alternately and periodically arranged along the waveguide direction P of the light L. , A radio wave guide 18 for guiding the radio wave D along the optical waveguide direction P. In the figure, the wave guide direction of the radio wave D is shown by an arrow with a reference sign Q, and the radio wave guide 18 is shown by a chain line. Each inversion section 12 has first and second electro-optic crystals 12a and 12b arranged alternately and periodically along the optical waveguide direction P and having the Z-axis direction orthogonal to the optical waveguide direction P.

The first and second electro-optic crystals 12a and 12b are arranged at positions where the electric field of the radio wave D extends, and
The X-axis directions of the optical crystals 12a and 12b are made opposite to each other and parallel to the direction R of the electric field of the radio wave D.

In this embodiment, the first and second electro-optic crystals 12a and 12b are made of a birefringent crystal such as Li.
NbO ₃ is used, and the thicknesses of the crystals 12a and 12b in the optical waveguide direction P are set to the same thickness t ₀ . Then, the crystals 12a and 12b are brought into close contact with each other and arranged alternately along the optical waveguide direction P. Each inversion section 1 of the optical waveguide 16
These crystals 12a at 2, the arranged order and distribution設個number 12b is the same, the thickness in the optical waveguide direction P of the inversion section and the thickness t ₁ equal to each other.

The spacer portion 14 is made of a material having no birefringence. Then the respective thickness equal T ₀ thickness in the optical waveguide direction P of each spacer portion 14 having the optical waveguide 16, is brought into close contact with the inverted portion 12 and the spacer portion 14 arranged alternately along the optical waveguide direction P. Inversion unit 1
2 is arranged at a constant interval T ₁ (T
₁ = T ₀ + t ₁ ). Further, the radio wave guide 18 is used as a waveguide, and the optical waveguide 16 is arranged in the radio wave guide 18 so that the electric field of the radio wave D guided through the radio wave guide 18 reaches the optical waveguide 16. The waveguide direction Q of the radio wave D is made parallel to the optical waveguide direction P. In the illustrated example, the waveguide directions P and Q are opposite to each other, but may be the same direction.

FIGS. 2A and 2B are diagrams schematically showing the relationship between the Z-axis direction and the optical waveguide direction of the first and second electro-optic crystals. As shown in the figure, the optical axis (c-axis) of the first and second electro-optic crystals 12a and 12b is the Z-axis, and the Z of the first and second electro-optic crystals 12a and 12b forming the inversion unit 12 is Z. The axial direction is orthogonal to the optical waveguide direction P.

3 (A) and 3 (B) are diagrams schematically showing the relationship between the X-axis direction of the first and second electro-optic crystals and the direction of the electric field of radio waves. As shown in FIG.
First and second electro-optic crystals 12a and 12 which constitute 2
The X-axis directions of b are opposite to each other and parallel to the electric field direction R of the radio wave D.

In the examples shown in these figures, the Z-axis directions of the first and second electro-optic crystals 12a and 12b constituting the reversing section 12 are the same and the Y-axis directions are opposite to each other. The axial direction and the Y-axis direction are not limited to the illustrated example.

The mode converter 10 of this embodiment converts the mode of one of ordinary light (polarization having a plane of polarization perpendicular to the Z axis) and extraordinary light (polarization having a plane of polarization parallel to the Z axis) to the other. And in the Z-axis direction and the X-axis direction as described above.
By determining the axial direction, the component of the electro-optic coefficient that contributes to the mode conversion between these ordinary and extraordinary rays (hereinafter,
(Also referred to as a mode conversion contributing component) is the first electro-optic crystal 1
2a has a positive value and second electro-optic crystal 12b has a negative value. Alternatively, the first electro-optic crystal 12a has a negative value and the second electro-optic crystal 12a has a negative value.
In b, take a positive value.

Thus, the value of the mode conversion contribution component is positive,
When each inversion unit 12 of the optical waveguide 16 is configured to have a negative and opposite value and an electric field of the radio wave D is applied to each inversion unit 12, the coupling coefficient between the ordinary light and the extraordinary light in the optical waveguide 16 is changed. The value changes sinusoidally with a beat with respect to the distance along the optical waveguide direction P. By changing the coupling coefficient in this way, it is possible to carry out mode conversion of the ordinary light of the specific wavelength corresponding to the wavelength of the radio wave D into the abnormal light or the abnormal light of the specific wavelength into the ordinary light.

By the way, the applicant of the present application has proposed the technique disclosed in Japanese Patent Laid-Open No. 4-109216 as another invention related to the present invention. According to the disclosed technique, a plurality of mode conversion elements are provided in the optical waveguide, and each mode conversion element is configured by finger electrodes for control and ground alternately arranged along the optical waveguide, and has a sine wave with a beat. The value of the voltage applied to these finger electrodes is sinusoidally changed along the optical waveguide so that a wavelike electric field distribution is formed along the optical waveguide. With such an electric field distribution formed,
The ordinary light of a specific wavelength corresponding to the period of the sinusoidal voltage value applied to the finger electrodes can be mode-converted into abnormal light or the abnormal light of a specific wavelength can be converted into ordinary light.

The mode conversion in the present invention is an application of the mode conversion in the above-disclosed technique, the inversion unit 12 in the embodiment of the present invention corresponds to the mode conversion element in the above-disclosed technique, and The electric field of the radio wave D in the example corresponds to the voltage applied to each finger electrode in the above-described disclosed technology.

FIGS. 4A and 4B are views for explaining the function of the spacer portion. In FIG. 4A, the distribution of the electric field intensity d1 of the electric field formed at a certain time by the radio wave guided in the optical waveguide 16 is conceptually shown in an overlapping manner on the optical waveguide 16, and the inversion section 14 is indicated by dots. Is attached. Figure 4
In FIG. 4B, only the reversal portion 12 except the spacer portion 14 of the optical waveguide 16 is arranged in close contact along the propagation axis S of the radio wave D, and the distribution of the electric field strength d1 shown in FIG. It is shown conceptually overlaid on the part 12. Further, in these figures, the j-th inversion section 12 and the spacer section 14 which are sequentially counted in the wave guide direction Q of the radio wave are denoted by reference numerals 12 _j and 14 _j.
Is attached.

Since the spacer portion 14 does not have birefringence, only the electric field intensity d1 acting on the inversion portion 12 contributes to mode conversion between ordinary light and extraordinary light. Therefore, the reversing unit 1
Considering only the electric field intensity d1 that acts on 2, the case where a radio wave of wavelength λ ₁ is guided through the optical waveguide 16 (see FIG.
(A)), effectively, a wavelength λ shorter than the wavelength λ _1.
2 (λ ₂ = (t ₁ / T ₁ ) · λ ₁ ) radio wave inverting section 12
Is equivalent to acting on.

Therefore, even if the wavelength λ ₁ of the radio wave guided through the optical waveguide 16 is made longer than, for example, several mm (for example, a millimeter wave or a microwave is used as the radio wave), the generation and control of the radio wave can be facilitated. Since the wavelength λ _{2 of} the radio wave that effectively acts on the inverting section 12 can be shortened to, for example, 1 mm or less, the wavelength range in which mode conversion can be performed between ordinary light and extraordinary light can be widened.

FIG. 5 shows the mode conversion characteristics of this embodiment obtained by numerical analysis. In FIG. 5, the vertical axis represents the mode conversion rate and the horizontal axis represents the wavelength of light.

In this embodiment, the peak A in the wavelength band centered on the wavelength λ _R and the wavelength band centered on the wavelength λ _L (λ ₀ -λ _R = λ _L -λ ₀ = Δλ, λ, not shown). _{At 0} and the peak B of the reference wavelength, a mode conversion rate that is practically satisfactory can be obtained. The half-width Δλ _F of the peaks A and B is inversely proportional to the thickness t ₁ of the inverting portion 12 and the refractive index difference Δn between the ordinary light and the extraordinary light in the inverting portion 12. However, Δn is the inversion unit 12
The refractive index difference in the state where the refractive index of is not electrically changed is represented and takes a positive value. Further, when the wavelength of the radio wave D is changed, Δλ is changed, and accordingly, the central wavelengths λ _R and λ _{L of the} peaks A and B are changed. Maximum variable amount of Δλ Δλ
_max is a ratio of the inversion period t ₂ (t ₂ = 2 · t ₀ , see FIG. 1) of the crystal axes of the first and second electro-optic crystals 12a and 12b to the thickness t ₁ of the inversion portion 12, that is, t _2. Proportional to / t ₁ . These points are the same as the technique disclosed in Japanese Patent Laid-Open No. 4-109216. According to this embodiment, the maximum variable amount Δλ _max can be set to about 20 to 30 nm.

The operating speed (switching time) of the mode converter 10 of this embodiment is given by the time required for the radio wave D to reach the end of the optical waveguide 16. If the length from the start end to the end of the optical waveguide 16 is 10 cm or less, it is possible to realize an operating speed on the order of 10 ⁻⁹ seconds.

Further, the disposition period T ₁ of the inverting section 12 is T ₁ = m
-It is given by λ ₀ / Δn (m = 1, 2, 3, ...).
The material forming the inversion part 12 is LiNbO ₃ and the reference wavelength λ _0.
= 1.55 μm, the arrangement period T ₁ is 18 μm
It should be about.

An optical wavelength filter is constituted by the mode converter 10 of this embodiment, a polarizer 20 which transmits one polarization of ordinary light and extraordinary light, and a polarizer 22 which transmits the other polarization of ordinary light and extraordinary light. Yes (see Figure 1). In this case, one polarized light is made incident on the optical waveguide 16 via the polarizer 20,
In the optical waveguide 16, one polarization having the wavelength λ _R and / or λ _L is mode-converted into the other polarization, and then the wavelength λ
Polarizer 2 which selectively selects the other polarized light having _R and / or λ _L
2 should be transmitted.

FIG. 6 is a perspective view schematically showing the overall construction of the second embodiment of the first invention. The mode converter 24 of this embodiment includes an optical waveguide 30 having an inversion section 26 and a spacer section 28, and a radio wave guide 32, and the inversion section 26 has first and second electro-optic crystals 26a and 26b.

In this embodiment, the optical waveguide 30 is made of LiNb.
A ridge-type waveguide made of O ₃ is used, and the inversion portion 26 is a domain inversion region formed in the optical waveguide 30. For example, a Z-cut LiNbO ₃ substrate 34 is provided with a LiNbO ₃ optical waveguide layer 36.
Are laminated by an epitaxial growth method, and then the optical waveguide layer 3
Ti is selectively diffused into the first electro-optical crystal 26a forming region of No. 6 in FIG. Ti is not diffused into a region of the optical waveguide layer 36 other than the region where the first electro-optic crystal 26a is formed. Next, heat treatment is performed to reverse the X, Y, and Z axis directions of the Ti diffusion region. The X, Y, and Z axis directions of the region in which Ti is not diffused are not inverted. As a result, the first and electro-optic crystals 26a and 26b are formed. Next, a ridge is formed on the optical waveguide layer 36. The optical waveguide 30 is composed of this ridge portion.

The radio wave guide 32 is composed of electrodes 32a and 32b forming a microstrip line. These electrodes 32a and 32b are provided close to one and the other side portions of the optical waveguide 30 and are provided on the optical waveguide layer 36.
The radio waves are guided along the optical waveguide direction P by extending a and 32b in the optical waveguide direction P.

FIGS. 7A and 7B are views showing an example of each axial direction of the first and second electro-optic crystals. As described above, when Ti is selectively diffused into the LiNbO ₃ optical waveguide layer 36 to form the first and second electro-optic crystals 26a and 26b, the first, electro-optic crystal 26a and the X-, Y-, and Z-axis directions are different from each other. The X, Y and Z axis directions of the second electro-optic crystal 26b are opposite to each other as shown in the figure, and the domains 26a and 26b form a domain inversion region. The radio wave guided through the radio wave guide 32 is X of these crystals 26a and 26b.
The radio wave guide 32 is formed so as to form an electric field parallel to the axial direction.
Make up.

As described above, Ti is selectively used for the optical waveguide layer 36.
To diffuse the first and second electro-optic crystals 26a and 26b.
In the case of forming, the spacer portion 28 having birefringence is formed. According to the calculation simulation when the spacer portion 28 has the birefringence, the maximum variable amount Δλ _max (see FIG. 5) becomes smaller than that in the first embodiment. in this case,
The magnitude of the maximum variable amount Δλ _max depends on the ratio of the lengths t ₁ and T ₀ of the inversion portion 26 and the spacer portion 28 in the optical waveguide direction P, that is, t ₁ / T ₀ , and the maximum length decreases as the length T ₀ decreases. The variable amount Δλ _max can be increased.

In addition, H ⁺ ions are selectively diffused into the spacer portion 28 by the ion exchange method to form the spacer portion 2.
8 birefringence is reduced, which results in a maximum variable amount Δλ _max
Can also be increased.

Also in this embodiment, the arrangement period T ₁ of the inverting section 14 is given by T ₁ = m · λ ₀ / Δn.

FIG. 8 is a front view schematically showing the configuration of the third embodiment of the first invention. The same components as those of the second embodiment are designated by the same reference numerals.

The third embodiment has the same structure as the second embodiment except that the structure of the radio wave guide 38 is different.

The radio wave guide 38 of this embodiment is a dielectric 38.
a and a ground electrode 38b. The dielectric 38a is provided on the optical waveguide 30 by extending in the optical waveguide direction P, and the ground electrode 38b is provided in the optical waveguide direction P on the surface of the substrate 34 opposite to the side where the optical waveguide layer 36 is provided. Set up.

In the above-mentioned first to third embodiments of the first invention, the X-axis directions of the first and second electro-optic crystals are made opposite to each other and parallel to the direction R of the electric field of the radio wave D. Besides, the Y-axis directions of the first and second electro-optic crystals may be made opposite to each other and parallel to the direction R of the electric field of the radio wave D.

Next, an embodiment of the second invention will be described. The second invention uses the magneto-optical effect to perform mode conversion between ordinary light and extraordinary light, and is the same as the first invention in principle. Therefore, the embodiment of the second invention can be constructed similarly to the embodiment of the first invention.

FIG. 9 is an overall perspective view schematically showing the construction of the second embodiment of the present invention, showing a state in which an optical filter is constructed by combining the mode converter and the polarizer of this embodiment. As shown in the figure, the mode converter 40 of this embodiment has an optical waveguide 46 having an inversion portion 42 and a spacer portion 44, which are alternately and periodically arranged along the light guiding direction P of the light L.
And a radio wave guide 48 for guiding the radio wave D along the optical waveguide direction P. Each inversion unit 42 has first and second magneto-optical crystals 42a and 42b arranged alternately and periodically along the optical waveguide direction P, and the Z-axis direction is orthogonal to the optical waveguide direction P.

Then, the first and second magneto-optical crystals 42a and 42b are arranged at positions where the magnetic field of the radio wave D extends, and
The X-axis directions of the optical crystals 42a and 42b are made opposite to each other and parallel to the direction r of the magnetic field of the radio wave D.

In this embodiment, the first and second magneto-optical crystals 42a and 42b are birefringent crystals, and the crystals 42a and 42b have the same thickness t ₀ in the optical waveguide direction P. And these crystals 42a and 4
2b are closely attached and arranged alternately along the optical waveguide direction P. The arranging order and the number of these crystals 42a and 42b are the same in each inversion portion 42 of the optical waveguide 46, and the thickness of each inversion portion in the optical waveguide direction P is the same thickness t ₁ .

The spacer portion 44 is formed of a material having no birefringence. The spacers 44 of the optical waveguide 46 have the same thickness T ₀ in the optical waveguide direction P, and the reversal portions 42 and the spacers 44 are closely attached and arranged alternately along the optical waveguide direction P. Inversion unit 4
The arrangement interval of the two in the optical waveguide direction P is a constant interval T ₁ . Further, the radio wave guide 48 is used as a waveguide, and the optical waveguide 46 is arranged in the radio wave guide 48 so that the magnetic field of the radio wave D guided by the radio wave guide 48 reaches the optical waveguide 46. The waveguide direction Q of the radio wave D is made parallel to the optical waveguide direction P. In the illustrated example, the waveguide directions P and Q are opposite to each other, but may be the same direction.

FIGS. 10A and 10B are diagrams schematically showing the relationship between the Z-axis direction and the optical waveguide direction of the first and second magneto-optical crystals. As shown in the figure, the optical axis (c-axis) of the first and second magneto-optical crystals 42a and 42b is the Z-axis,
First and second magneto-optical crystals 42a constituting the reversing part 42
The Z-axis directions of and 42b are orthogonal to the optical waveguide direction P.

Further, FIGS. 11A and 11B are diagrams schematically showing the relationship between the X-axis direction of the first and second magneto-optical crystals and the direction of the magnetic field of radio waves. As shown in the same figure, the first and second magneto-optical crystals 42a and 4 constituting the reversing part 42 are also provided.
The X-axis directions of 2b are opposite to each other and parallel to the direction r of the magnetic field of the radio wave D.

In the examples shown in these figures, the Z-axis directions of the first and second magneto-optical crystals 42a and 42b constituting the reversing section 42 are the same and the Y-axis directions are opposite to each other. The direction and the Y-axis direction are not limited to the illustrated example.

The mode converter 40 of this embodiment converts the mode of one of ordinary light (polarized light having a polarization plane perpendicular to the Z axis) and extraordinary light (polarized light having a polarization plane parallel to the Z axis) to the other. And in the Z-axis direction and the X-axis direction as described above.
By determining the axial direction, the component of the magneto-optical coefficient that contributes to the mode conversion between these ordinary and extraordinary rays (hereinafter,
(Also referred to as a mode conversion contribution component), the first magneto-optical crystal 4
2a has a positive value and the second magneto-optical crystal 42b has a negative value. Alternatively, the first magneto-optical crystal 42a has a negative value and the second magneto-optical crystal 42a
In b, take a positive value.

Thus, the value of the mode conversion contribution component is positive,
When each inversion unit 42 of the optical waveguide 46 is configured to have a negative and opposite value and the magnetic field of the radio wave D is applied to each inversion unit 42, the coupling coefficient between the ordinary light and the extraordinary light in the optical waveguide 46 is changed. The value changes sinusoidally with a beat with respect to the distance along the optical waveguide direction P. By changing the coupling coefficient in this way, it is possible to carry out mode conversion of the ordinary light of the specific wavelength corresponding to the wavelength of the radio wave D into the abnormal light or the abnormal light of the specific wavelength into the ordinary light.

FIGS. 12A and 12B are views for explaining the function of the spacer portion. In FIG. 12 (A),
The distribution of the magnetic field strength d2 of the magnetic field formed by the radio wave guided in the optical waveguide 46 at a certain time is conceptually shown in an overlapping manner on the optical waveguide 46, and the spacer of the optical waveguide 46 is shown in FIG. Except for the part 44, only the reversing part 42 receives radio waves D
12A and 12B are arranged in close contact with each other along the propagation axis S, and the distribution of the magnetic field strength d2 shown in FIG. Further, in these figures, the j-th inversion portion 42 and the spacer portion 44, which are sequentially counted in the wave guiding direction Q of the radio wave, are shown by reference numerals 42 _j and 44 _j .

Since the spacer portion 44 does not have birefringence, only the magnetic field intensity d2 acting on the inversion portion 42 contributes to the mode conversion between ordinary light and extraordinary light. Therefore, the reversing unit 4
Considering only the magnetic field strength d2 that acts on 2, the radio wave of wavelength λ ₁ is guided in the optical waveguide 46 (see FIG. 4).
(A)), effectively, a wavelength λ shorter than the wavelength λ _1.
2 (λ ₂ = (t ₁ / T ₁ ) · λ ₁ ) radio wave inverting section 42
Is equivalent to acting on.

Therefore, even if the wavelength λ ₁ of the radio wave guided through the optical waveguide 46 is made longer than, for example, several millimeters to facilitate the generation and control of the radio wave, the wavelength λ of the radio wave that effectively acts on the inverting section 12. _{Since 2} can be shortened to, for example, 1 mm or less, the wavelength range in which mode conversion can be performed between ordinary light and extraordinary light can be widened.

Also in this embodiment, the mode conversion characteristics similar to those of the first invention can be obtained (see FIG. 5). If the length from the start end to the end of the optical waveguide 46 is set to 10 cm or less, the operating speed of the mode converter 40 can be on the order of 10 ⁻⁹ seconds. The reversing unit 4
The arrangement period T _{1 of 2} is given by T ₁ = m · λ ₀ / Δn.

Further, also in this embodiment, as in the case of the first invention, an optical wavelength filter can be constructed by the mode converter 40 of this embodiment and the polarizers 20 and 22.

The second invention is not limited to the embodiment shown in FIG. 9, and the same embodiments as the second and third embodiments of the first invention can be realized in the second invention.

FIG. 13 is a perspective view schematically showing the overall construction of the first embodiment of the third invention. As shown in the figure, the optical wavelength filter 50 of this embodiment has a directional coupler 56 in which first and second optical waveguides 52 and 54 having different equivalent refractive indices are arranged in proximity to each other and a directional coupler 56 along the optical waveguide direction. And a radio wave guide 58 that guides radio waves.

The first waveguide 52 has the inverting portions 60, and the inverting portions 60 are alternately and periodically arranged along the optical waveguide direction P and Z
It has first and second electro-optic crystals 60a and 60b whose axial direction is orthogonal to the optical waveguide direction P. In the figure, the first electro-optic crystal 60a is shown with dots.

The optical crystals 60a and 60b are arranged at positions where the electric field of the radio wave extends, and the Z-axis directions of the optical crystals 60a and 60b are made opposite to each other and parallel to the electric field direction R of the radio wave D. ..

In this embodiment, the first and second waveguides 52 and 54 are formed by diffusing Ti into a substrate 62 having an electro-optical effect (or Pockels effect), such as a LiNbO ₃ or KTP substrate. .. The waveguides 52 and 54 are arranged close to each other so as to cause the interaction of light, and the directional coupler 56 is configured. Together with these waveguides 52
And 54 by varying the width of the waveguides, or by loading a film on the waveguides 52 and or 54, or by adding a refractive index controlling substance to the waveguides 52 and 54.
And 54 have different equivalent refractive indices.

The first and second electro-optical crystals 60a and 60b have birefringence, and these optical crystals 60a and 6b
0b is formed in the first waveguide 52 with the Z-axis (c-axis) directions being opposite to each other. The lengths of the optical crystals 60a and 60b in the optical waveguide direction P are set to be equal to t ₀ , and the optical crystals 60a and 60b are closely contacted and arranged alternately along the optical waveguide direction P. These optical crystals 6
0a and 60b constitute an inversion unit 60 having a domain inversion structure.

Further, the radio wave guide 58 is connected to the electrodes 58a and 5a.
8b. The electrode 58a is disposed on the side portion of the first waveguide 52 and extends along the first waveguide 52 to form the substrate 6
Provided in 2. Further, the electrode 58b is provided along the first waveguide 52 and provided on the inversion portion 60 of the first waveguide 52. The radio wave D is guided in parallel with the optical waveguide direction P by these electrodes 58a and 58b. In the illustrated example, the light guiding direction P and the wave guiding direction Q of the radio wave D are opposite to each other, but the directions P and Q may be the same direction.

FIGS. 14A and 14B are diagrams schematically showing the relationship between the Z-axis direction of the first and second electro-optic crystals and the optical waveguide direction and the electric field direction of radio waves. As shown in the figure, the optical axis (c-axis) of the first and second electro-optic crystals 60a and 60b is the Z-axis, and the Z of the first and second electro-optic crystals 60a and 60b forming the reversal unit 60 is Z. The axial direction is orthogonal to the optical waveguide direction P. Along with this, the Z-axis directions of the first and second electro-optic crystals 60a and 60b forming the reversing unit 60 are made opposite to each other and parallel to the direction R of the electric field of the radio wave D.

In the example shown in the figure, the X-axis directions of the first and second electro-optic crystals 60a and 60b constituting the reversing section 60 are opposite and the Y-axis direction is opposite. The directions and the Y-axis directions are not limited to the illustrated examples.

Next, separation of light by the optical filter 50 will be described. Here, one of the symmetric mode (even mode) and the antisymmetric mode (odd mode) is referred to as mode A, and the other of the symmetric and antisymmetric modes is referred to as mode B. When the equivalent refractive index of the first waveguide 52 is lower than the equivalent refractive index of the second waveguide 54, the mode A is an antisymmetric mode and the mode B is a symmetric mode. When the index is higher than the equivalent refractive index of the second waveguide 54, the mode A is a symmetric mode and the mode B is an antisymmetric mode.

The first and second waveguides 52 and 54 form one waveguide system U. First waveguide 52 to waveguide system U
When light is incident from the light source, the light is guided in the waveguide system U in the mode A. The peak of the field distribution of the light guided in the waveguide system U in the mode A is almost in the first waveguide 52. When light is incident on the waveguide system U from the second waveguide 52, the light is guided in the waveguide system U in mode B. The peak of the field distribution of the light guided in the waveguide system U in the mode B is almost in the second waveguide 54.

Further, in the region where the inversion part 60 of the waveguide system U is provided, the mode A light of the specific wavelength is mode-converted into the mode B light or the mode B light of the specific wavelength is converted into the mode A light.

Therefore, when wavelength-multiplexed light is made incident on the waveguide system U from the first waveguide 52, the light of the specific wavelength is changed to the mode B.
Then, the light having the other wavelength can be emitted from the first waveguide 52 in the mode A, and the light can be separated according to the wavelength. When wavelength-multiplexed light is made incident on the waveguide system U from the second waveguide 54, light of a specific wavelength is emitted from the first waveguide 52 in mode A and light of other wavelengths is made to be second in mode B. It can be emitted from the waveguide 54.

The wavelengths of the light whose modes are converted are λ _R and λ _L
(Λ _R −λ ₀ = λ ₀ −λ _L , λ ₀ : center wavelength).

The inversion period t ₂ of the crystal axis of the inversion unit 60 (see FIG. 1)
3) is defined so as to satisfy the following expression (1). Δn _io / λ ₀ = 1 / t ₂ (1) Δn _io is given by the following equation (2). Δn _io = Δβ / (2 · π / λ ₀ ) ... (2) where Δβ is a propagation constant difference between the symmetric and antisymmetric modes, and is an equivalent refractive index difference between the first and second waveguides 52 and 54. Δ
It can be obtained from n _e .

Next, the principle of mode conversion will be described. The principle of mode conversion in the third invention can be considered by making the symmetric mode and the antisymmetric mode correspond to the TM and TE modes in the technique disclosed in Japanese Patent Laid-Open No. 4-109216.

FIG. 15 is a diagram for explaining the mode conversion. FIG. 15A shows a distribution state of the electric field intensity d3 formed in the first optical waveguide 52 at a certain time by the electric wave D propagating through the radio wave guide, and the electric field intensity d3 is represented by the first and second electro-optic crystals 60a. And 60b are conceptually shown in association with the positions in the optical waveguide direction P. In FIG. 15B, the distribution state of the refractive index change amount Δn generated in the first and second electro-optic crystals 60a and 60b by the electric field intensity d3 shown in FIG. 15A is associated with the distribution state of the electric field intensity d3. Showed. The refractive index change amount Δn is the difference between the refractive index of the electro-optical crystals 60a and 60b when not electrically changed and the refractive index of the electro-optical crystals 60a and 60b when electrically changed. In these figures, S represents the propagation axis of the radio wave D.

As shown in FIG. 15A, the reversing unit 60 is also provided.
The direction V of the spontaneous polarization (or electric dipole when the inversion part 60 is formed of a ferroelectric crystal) is periodically inverted for each of the first and second electro-optic crystals 60a and 60b.

When the electric field strength d3 reaches the inversion portion 60 with the distribution shown in FIG. 15A, the refractive index change amount Δn occurs with the distribution shown in FIG. 15B. The value of the refractive index change amount Δn of the first electro-optic crystal 60a and the value of the refractive index change amount Δn of the second electro-optic crystal 60b correspond to the relationship between the electric field direction R and the spontaneous polarization direction V, Positive and negative are opposite values. The magnitude of the refractive index change amount Δn of each of the optical crystals 60a and 60b is proportional to the magnitude of the electric field strength d3.

Therefore, the refractive index change amount Δn is determined by two periodic functions exp {i · (2 · π / t ₂ ) · z ₀ } and exp.
It is represented by the product of {i · (2 · π / λ _m ) · z ₀ }. Here, z ₀ represents the position in the optical waveguide direction P, and λ _m represents the wavelength of the radio wave D.

The coupling coefficient K between the symmetric and antisymmetric modes is an amount proportional to the refractive index change amount Δn and can be expressed by the following equation (3).

[0091]

[Equation 1]

However, K ₀ is the first and second electro-optic crystals 6
The coupling coefficient between the symmetric and antisymmetric modes in the state where the refractive indexes of 0a and 60b are not changed electrically is shown.

As can be understood from the expression (3), exp
Mode conversion between antisymmetric mode occurs with a phase component of {i · (Δβ / 2) · z 0} symmetric mode and exp having the phase component of {-i · (Δβ / 2) · z 0} ..

When the following equation (4) or (5) is established, the difference between the phase components of the symmetric and antisymmetric modes and the phase component of the coupling coefficient K become zero regardless of the value of z ₀ , and as a result, Effective mode conversion occurs between symmetric and antisymmetric modes. 2 · π · (1 / t ₂ + 1 / λ _m ) = Δβ …… (4) 2 · π · (1 / t ₂ −1 / λ _m ) = Δβ …… (5) Δβ = (2 · π / Since λ ₀ ) · Δn _io , mode conversion occurs at a wavelength λ that satisfies the following expression (6). Wavelength λ
Corresponds to λ _R and λ _L described above. λ = Δn _io · (t ₂ · λ _m ) / (t ₂ ± λ _m ) (6) When the inversion period t ₂ satisfies the equation (1), the following equation (7) is obtained from the equation (6). can get. λ = λ ₀ · (n _io · λ _m ) / (λ ₀ ± Δn _io · λ _m ) ... (7) If λ ₀ << Δn _io · λ _m , the wavelength λ is approximately expressed by the following equation (8) ). λ≈λ ₀ · {1 ± (λ ₀ / Δn _io · λ _m ) = λ ₀ · (1 ± t ₂ / λ _m ) ... (8) Therefore, as can be understood from the equation (8), By changing the wavelength λ _m , the wavelength λ of the mode-converted light can be arbitrarily changed.

FIG. 16 is a perspective view schematically showing the overall structure of the fourth embodiment of the invention. The components corresponding to those of the third embodiment of the invention are designated by the same reference numerals, and detailed description of the same points as those of the third embodiment will be omitted.

The optical filter 64 of this embodiment has a directional coupler 56 formed by arranging first and second optical waveguides 52 and 54 having different equivalent refractive indexes in close proximity to each other, and a sine wave along the optical waveguide direction P. And an electrode 66 that forms an electric field that changes to.

The first optical waveguide 52 has inverting portions 60, and the inverting portions 60 are alternately and periodically arranged along the optical waveguide direction P, and the first and second Z-axis directions are orthogonal to the optical waveguide direction. It has electro-optic crystals 60a and 60b. The first and second electro-optical crystals 60a and 60b are arranged at positions where the electric field formed by the electrode 66 extends, and the Z-axis directions of the optical crystals 60a and 60b are opposite to each other and the electric field formed by the electrode 66 is formed. Parallel to the direction of the electric field of.

In this embodiment, the electrode 66 is replaced by the electrode member 66.
a and 66b. The electrode member 66a is disposed on the side portion of the first waveguide 52, extends along the optical waveguide direction P, and is provided on the substrate 62. In addition, the plurality of electrode members 66b are arranged along the optical waveguide direction P at predetermined intervals so as to be separated from each other by the inversion section 6.
0 above. These electrode members 66a and 66b form a distribution of the electric field strength that changes sinusoidally along the optical waveguide direction P, as in the case of the electric field strength d3 (see FIG. 15) in the third embodiment of the invention. Other than that, the configuration is similar to that of the third embodiment.

Also in this embodiment, light can be separated by the same principle as in the case of the third invention.

The present invention is not limited to the above-mentioned embodiments, and therefore, the forming material, the disposing position, the shape, the direction, and the like of each constituent can be arbitrarily changed.

[0101]

As is apparent from the above description, according to the first aspect of the invention, since the mode conversion can be performed by using the radio wave, the operating speed can be increased. Moreover, the change in the refractive index that contributes to the mode conversion can be controlled by the arrangement period of the inversion section and the arrangement period of the first and second electro-optic crystals in addition to the wavelength of the radio wave, so that the mode conversion can be performed efficiently. A refractive index change can be formed. Therefore, when an optical filter is constructed using the mode converter of the present invention, the mode conversion efficiency can be increased and the intensity of the separated light can be increased.

Further, according to the second aspect of the invention, since the mode conversion can be performed by using radio waves, the operating speed can be increased. Moreover, since the change in the refractive index that contributes to the mode conversion can be controlled by the arrangement period of the inversion section and the arrangement period of the first and second magneto-optical crystals in addition to the wavelength of the radio wave, the mode conversion can be performed efficiently. A refractive index change can be formed. Therefore, when an optical filter is constructed using the mode converter of the present invention, the mode conversion efficiency can be increased and the intensity of the separated light can be increased.

Further, according to the third aspect of the invention, since the mode conversion can be performed by using radio waves, the operating speed can be increased. Moreover, the change in the refractive index that contributes to the mode conversion can be controlled by the arrangement period of the inversion section and the arrangement period of the first and second electro-optic crystals in addition to the wavelength of the radio wave, so that the mode conversion can be performed efficiently. A refractive index change can be formed. As a result, the intensity of the separated light can be increased.

Further, according to the third aspect of the invention, mode conversion can be performed by controlling the voltage applied to the electrodes, so that the operating speed can be increased. Moreover, since the change in the refractive index that contributes to the mode conversion can be controlled by the voltage applied to the electrodes, the disposition period of the inversion section, and the disposition period of the first and second electro-optic crystals, the mode conversion can be performed efficiently. A refractive index change can be formed. As a result, the intensity of the separated light can be increased.

[Brief description of drawings]

FIG. 1 is a perspective view schematically showing an overall configuration of a first embodiment of the first invention.

2A and 2B are diagrams showing the Z-axis directions of the first and second electro-optic crystals.

3A and 3B are diagrams showing the X-axis directions of the first and second electro-optic crystals.

4A and 4B are diagrams for explaining the function of a spacer portion.

FIG. 5 is a diagram showing mode conversion characteristics in the embodiment of the first invention.

FIG. 6 is a perspective view schematically showing an overall configuration of a second embodiment of the first invention.

7A and 7B are diagrams showing the axial directions of the first and second electro-optic crystals.

FIG. 8 is a front view schematically showing a configuration of a third embodiment of the first invention.

FIG. 9 is a perspective view schematically showing an overall configuration of a first embodiment of the second invention.

10A and 10B are views showing the Z-axis directions of the first and second magneto-optical crystals.

11A and 11B are diagrams showing the X-axis direction of the first and second magneto-optical crystals.

12A and 12B are diagrams for explaining the function of the spacer portion.

FIG. 13 is a perspective view schematically showing the overall configuration of an embodiment of the third invention.

14A and 14B are diagrams showing the Z-axis directions of the first and second electro-optic crystals.

15A and 15B are diagrams for explaining mode conversion.

FIG. 16 is a perspective view schematically showing an overall configuration of an embodiment of the fourth invention.

[Explanation of symbols]

 10, 24, 40: Mode converter 12, 26, 42, 60: Inversion part 12a, 26a, 60a: First electro-optic crystal 12b, 26b, 60b: Second electro-optic crystal 14, 28, 44: Spacer part 16 , 30, 46: Optical waveguides 18, 32, 48, 58: Radio wave guide 42a: First magneto-optical crystal 42b: Second magneto-optical crystal 50: Optical filter 52: First waveguide 54: Second waveguide 56: Direction Sex coupler

Claims (4)

[Claims] 1. An optical waveguide having an inversion section and a spacer section, which are alternately and periodically arranged along an optical waveguide direction, and a radio wave guide for guiding an electric wave along the optical waveguide direction. , Having first and second electro-optical crystals arranged alternately and periodically along the optical waveguide direction and having the Z-axis direction orthogonal to the optical waveguide direction. A mode converter characterized in that the optical crystal is arranged in such a range that the X-axis direction and the Y-axis direction of these optical crystals are opposite to each other and parallel to the direction of the electric field of the radio wave. 2. An optical waveguide having an inversion portion and a spacer portion, which are alternately and periodically arranged along the optical waveguide direction, and a radio wave guide for guiding an electric wave along the optical waveguide direction, each inversion portion being provided. , Having first and second magneto-optical crystals arranged alternately and periodically along the optical waveguide direction and having a Z-axis direction orthogonal to the optical waveguide direction, A mode converter characterized in that the optical crystal is arranged in a range where the X-axis direction and the Y-axis direction of these optical crystals are opposite to each other and parallel to the direction of the magnetic field of the radio wave. 3. A directional coupler formed by arranging first and second optical waveguides having different equivalent refractive indices in proximity to each other, and a radio wave guide for guiding a radio wave along an optical waveguide direction. At least one of the two optical waveguides has an inversion part, and the inversion part is alternately and periodically arranged along the optical waveguide direction.
It has first and second electro-optic crystals whose axial direction is orthogonal to the optical waveguide direction, and the first and second electro-optic crystals are arranged at positions where the electric field of radio waves extends, and the Z-axis direction of these optical crystals is set. , An optical filter characterized in that they are opposite to each other and parallel to the direction of the electric field of radio waves. 4. A directional coupler formed by arranging first and second optical waveguides having different equivalent refractive indices in close proximity to each other, and an electrode forming an electric field that changes sinusoidally along the optical waveguide direction, At least one of the first and second optical waveguides has an inversion portion, and the inversion portion is arranged alternately and periodically along the optical waveguide direction.
It has first and second electro-optical crystals whose axial direction is orthogonal to the optical waveguide direction, and the first and second electro-optical crystals are arranged at positions where an electric field formed by the electrodes reaches and Z of these optical crystals is arranged. An optical filter, wherein the axial directions are opposite to each other and are parallel to the direction of the electric field of the electric field formed by the electrodes.