JP2005141156A - Optical modulator and communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small-sized and highly efficient optical modulator which can be incorporated in a communication system and the like. <P>SOLUTION: The optical modulator has an optical waveguide 2 extending along a certain surface, a first optical guide part (the array of pits 9) regulating propagation of an optical wave in a direction parallel to the surface and a second optical guide part (a part other than a high refractive index layer 11 in a substrate 1) regulating the propagation of the optical wave in a direction vertical to the surface. The first optical guide part has a photonic crystal disposed at the external part of the optical waveguide 2, the second optical guide part has a region having a refractive index lower than that of the optical waveguide 2, and both the first and the second optical guide parts are provided in the substrate formed of a material exhibiting an electrooptical effect. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical modulation element used in an optical communication system and an optical signal processing system, and a communication system having the optical modulation element.

  An optical modulation element is a basic element in high-speed optical communication, an optical signal processing system, and the like, and it is considered that the need for an optical modulation element capable of operating at an ultra-high speed will increase in the future. Since direct modulation using a semiconductor laser that has been conventionally used is difficult to cope with ultra-high-speed optical modulation, the development of an external modulation type element capable of high-speed operation has been urgent recently. In particular, a so-called electro-optic light modulation element using a dielectric crystal having a large Pockels effect is capable of ultra-high-speed operation, and there is little disturbance in the phase of the optical signal accompanying light modulation. It is very suitable for information transmission and long-distance optical fiber communication. Furthermore, if an optical waveguide structure is used, there is a possibility that both miniaturization and high efficiency can be realized at once.

  In general, an electro-optic light modulation element includes a transmission line that propagates a modulation signal provided as a modulation electrode on an electro-optic crystal, and an optical waveguide formed in the vicinity of the transmission line. That is, a phenomenon is utilized in which when the refractive index of the optical waveguide portion changes according to the electric field induced around the modulation electrode, the phase of the light wave propagating in the optical waveguide changes with the modulation signal.

  By the way, in the electro-optic light modulation element, the electro-optic coefficient that is the basis of the light modulation is relatively small in a normal crystal. Therefore, in this type of light modulation element, it is important to efficiently apply an electric field to the optical waveguide in order to achieve high modulation efficiency.

  FIG. 6 is a simplified perspective view showing an example of a conventional light modulation element described in Non-Patent Document 1. In FIG. In this light modulation element, an optical waveguide 2 is formed on a surface portion of a substrate 1 made of a material having an electro-optic effect. The optical waveguide 2 is composed of a region where the refractive index of the substrate 1 is slightly increased as compared with other portions, and can guide light waves by total reflection. The refractive index is increased by thermally diffusing metal into a part of the substrate 1. On both the left and right sides of the optical waveguide 2, a modulation electrode 3 made of a metal film such as aluminum or gold is provided on the upper surface of the substrate 1. The modulation electrode 3 is composed of two lines 3a and 3b which are parallel to each other, and has a coplanar line structure.

  The optical waveguide 2 is branched into two branched optical waveguides 2a and 2b at two branch points 7a and 7b, and the input light input from the entrance-side optical waveguide 2c is branched at one branch point 7a. After passing through the two branch optical waveguides 2a and 2b, the other branch point 7b is configured to travel along the common exit-side optical waveguide 2d.

  On the substrate 1, a modulation electrode 3 having a coplanar line structure including two lines 3a and 3b extending along the branched optical waveguides 2a and 2b of the optical waveguide 2 is provided. Each inner end of each line 3a, 3b is formed so as to be located almost directly above the center of each branch optical waveguide 2a, 2b, and a high frequency signal source 4 is connected to one end of the modulation electrode 3, A termination resistor 5 is connected to the other end.

  The input light is introduced from the entrance-side optical waveguide 2c and undergoes a light modulation action as follows when passing through the branch optical waveguides 2a and 2b.

When a high frequency signal is supplied from the high frequency signal source 4, it propagates through the modulation electrode 3 in the same direction as the light, and an electric field is generated in the gap 6. Then, due to the electro-optic effect, the refractive index of the material constituting the branched optical waveguides 2a and 2b changes according to the electric field strength. In this embodiment, since electric fields in opposite directions are applied to the branched optical waveguide 2a and the branched optical waveguide 2b, when the substrate 1 is made of, for example, a z-cut lithium niobate crystal, Opposite phase changes are given to the light passing through the optical waveguides 2a and 2b. Therefore, in the exit-side optical waveguide 2d, interference between the two lights that have passed through the branch optical waveguides 2a and 2b occurs, and the intensity of the output light changes due to this interference, so that the light modulation element of this embodiment is a light intensity modulator. Works as.
IEEE Journal of Quantum Electronics. Vol. QE-13, no. 4, pp287-290, 1977)

  A conventional electro-optic light modulator uses an optical waveguide that confines light using a refractive index difference. In this case, if a sharp bend is made in the bent portion 8, a light wave is emitted outside the optical waveguide and causes a loss. Therefore, the bend angle (usually about 1 °) needs to be very small. In addition, in the branch portions 7a and 7b, if the branch angle is increased, the light wave is not transferred to the branch optical waveguide but is similarly emitted outside the optical waveguide to cause loss. The angle needs to be very small. Therefore, there is a problem that the element size is particularly large in the longitudinal direction.

  Further, by reducing the width of the gap 6 and bringing the optical waveguides 2a and 2b closer, the generated electric field strength can be increased for the same voltage, so that the modulation efficiency can be improved. Because of the influence of the coupling of light waves, it is usually not possible to make it below 20-30 μm. This is also one of the factors that hinder the miniaturization and high efficiency of the element.

  The present invention has been made in view of the above circumstances, and a main object thereof is to provide a small and highly efficient light modulation element that can be incorporated into an optical communication system or the like.

An optical modulation element according to the present invention includes an optical waveguide extending along a certain surface, a first light guide portion for restricting light wave propagation in a direction parallel to the surface, and the light wave propagation in a direction perpendicular to the surface. A light modulation element having a second light guide portion to be regulated, wherein the first light guide portion has a photonic crystal disposed outside the optical waveguide, and the second light guide portion has the second light guide portion. A region having a refractive index lower than the refractive index of the optical waveguide is included, and both the first light guide portion and the second light guide portion are formed in a substrate formed of a material exhibiting an electro-optic effect. And the second light guide portion has a crystal structure substantially the same as the crystal structure of the optical waveguide. In a preferred embodiment, the optical waveguide is on the substrate or the substrate. Is formed inside.

  In a preferred embodiment, the substrate has a main surface parallel to the surface, and the optical waveguide and the first light guide portion are formed on the main surface side of the substrate, and are below the optical waveguide. positioned.

  In a preferred embodiment, the first light guide portion has a periodic arrangement structure of concave portions and / or convex portions provided on the main surface of the substrate.

  In a preferred embodiment, the optical waveguide is composed of a modified layer formed on the main surface side of the substrate.

  In a preferred embodiment, an electrode for applying a high-frequency signal for light modulation to light propagating through the optical waveguide is provided on the substrate.

  In a preferred embodiment, the first light guide portion has a two-dimensional photonic crystal structure having a photonic band gap with respect to a light wave that is to propagate in a direction parallel to the surface.

  In a preferred embodiment, at least a part of the optical waveguide is in contact with the photonic crystal structure of the first light guide portion.

  In a preferred embodiment, the optical waveguide has at least two branch optical waveguides, thereby forming a Mach-Zehnder interferometer.

  In a preferred embodiment, a region between the two branch optical waveguides has a photonic crystal structure.

  In a preferred embodiment, the branch angle of the two branch optical waveguides is 5 ° or more.

  In a preferred embodiment, the interval between the two branch optical waveguides is 10 μm or less.

  The communication system according to the present invention is a communication system including an optical modulation element for converting an electrical signal into an optical signal, and the optical modulation element is the optical modulation element according to any one of claims 1 to 13. It is.

  According to the light modulation element of the present invention, the light modulation efficiency is improved and the element can be miniaturized. By using this light modulation element in a communication system, communication using a high-frequency signal of a millimeter wave level becomes possible.

  In recent years, research and development of various optical devices using “photonic crystals” have been promoted. A “photonic crystal” is a new optical material in which refractive index media are arranged with a period similar to the wavelength of light. In a normal solid crystal, a band structure of electron energy is formed by a periodic arrangement of atoms, whereas in a photonic crystal, a band structure of light energy is formed. Along with the formation of the band structure, a “photonic band gap” is formed in the photonic crystal. Light having energy corresponding to the photonic band gap cannot propagate through the photonic crystal having the photonic band gap.

  In the present invention, a practical light modulation element is realized by forming a photonic crystal structure in a material having an electro-optic effect.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1)
First, a first embodiment of a light modulation element according to the present invention will be described with reference to FIGS. 1 (a) and 1 (b). Fig.1 (a) is a top view of the light modulation element of this embodiment, FIG.1 (b) is the sectional view on the AA 'line.

  As shown in FIG. 1A, the light modulation element of the present embodiment includes a substrate 1 formed of a material exhibiting an electro-optic effect, and an optical waveguide provided in the substrate 1. The confinement of the light wave in the optical waveguide is performed by two different means. That is, the light modulation element includes a first light guide portion that restricts light wave propagation in a direction parallel to the main surface of the substrate, and a second light guide portion that restricts light wave propagation in a direction perpendicular to the main surface of the substrate. Have.

The first light guide portion has a photonic crystal disposed outside the optical waveguide, and the second light guide portion has a region having a refractive index lower than that of the optical waveguide. The first light guide portion and the second light guide portion are both provided in the substrate, and the second light guide portion has a crystal structure substantially the same as the crystal structure of the optical waveguide. The substrate 1 is made of a crystalline material having an electro-optic effect, such as a lithium tantalate (LiTaO ₃ ) single crystal or a lithium niobate (LiNbO ₃ ) single crystal. In the main surface of the substrate 1, a large number of recesses (pits) 9 formed by etching are arranged in a region functioning as the first light guide portion. Of the main surface of the substrate 1, the pit 9 is not formed in the region where the optical waveguide 2 is provided, and the planar layout of the optical waveguide 2 is the region where the pit 9 is formed (first light guide portion). It is prescribed by.

  By appropriately setting the shape, size and arrangement of the pits 9, a so-called two-dimensional photonic crystal structure can be configured. In the photonic crystal structure portion thus formed, a photonic band gap is generated in a two-dimensional direction (in-plane direction parallel to the main surface of the substrate 1). When a photonic band gap occurs, it is possible to prevent a light wave having a certain frequency from propagating in the in-plane direction. In FIG. 1, the pit 9 is described as having a cylindrical shape, but the shape of the pit 9 is not limited to a cylindrical shape.

  In this embodiment, a two-dimensional photonic crystal structure is used to form a two-dimensional guide structure of light waves having a frequency corresponding to the photonic band gap. It is possible to propagate a light wave in a direction perpendicular to the principal plane of the. Therefore, in the light modulation element of the present embodiment, a refractive index difference is formed between the optical waveguide and the lower region so that the light wave does not leak downward from the optical waveguide. As described above, one of the feature points of the present embodiment is that the lateral light confinement is performed by the photonic crystal structure, and the vertical light confinement is realized by the difference in refractive index. More specifically, as shown in FIG. 1B, a portion (high refractive index layer 11) having a slightly higher refractive index than the other regions is formed in the surface region of the substrate 1. The high refractive index layer 11 can be easily formed, for example, by performing a process such as metal thermal diffusion or proton exchange on a crystal having an electro-optic effect. The basic crystal structure of the portion where the high refractive index layer 11 is formed is substantially the same as the crystal structure of the substrate 1, but the refractive index is higher than that of the other portions. The “second guide portion” described above corresponds to a portion other than the high refractive index layer 11 inside the substrate 1 (a portion adjacent to the high refractive index layer 11).

Note that the propagation control of the light wave in the depth direction can be realized by, for example, the configuration shown in FIGS. 2A and 2B (configuration based on a simple combination of known techniques). In the configuration shown in FIGS. 2A and 2B, the substrate 1 supports the waveguide portion so as to float in the air. Such a structure is manufactured as follows, for example. First, the Si layer 110 is formed on the substrate 1 made of SiO ₂ . After preparing a two-dimensional photonic crystal structure by forming an array of small openings in the Si layer 110, supplies the SiO ₂ the chemical etching, the substrate 1 of SiO ₂ via the opening To do. By doing so, etching proceeds from a region of the substrate 1 that is in contact with the opening. By appropriately selecting the etching time, a space having a predetermined thickness can be formed below the waveguide portion.

However, when such a configuration is adopted, the cavity cannot be formed with good reproducibility depending on the state of the chemical used for partial etching of the substrate 1 and the etching time, and the manufacturing yield is lowered. In addition, when an electro-optic crystal typified by LiNbO ₃ is used for the substrate, such a manufacturing method cannot be applied. Therefore, the configuration shown in FIGS. 2A and 2B is realized using the electro-optic crystal. I can't.

  On the other hand, in this embodiment, in order to restrict the propagation of light waves in the depth direction, instead of performing processing such as etching on the substrate, surface modification is performed on the main surface side of the substrate 1, A refractive index layer 11 is formed. The light wave is confined in a region having a relatively high refractive index, and light having a wavelength corresponding to the photonic band gap does not enter the region having the photonic crystal structure. For this reason, according to the light modulation element of the present embodiment, the light wave propagates only through the portion where the pit 9 does not exist (the portion of the optical waveguide 2) in the high refractive index layer 11 formed on the substrate surface. .

  Note that the size of the photonic band largely depends on the arrangement period of the pits 9. For this reason, when the wavelength of the light wave to be propagated through the optical waveguide is given, the arrangement period of the pits 9 is determined so as to prevent the propagation of the wavelength.

  In FIG. 1A, the pits 9 are arranged in the entire region other than the optical waveguide in the main surface of the substrate 1, but in reality, it is necessary to arrange the pits 9 at a position away from the optical waveguide. Absent. The pits 9 can also form a photonic crystal structure with several rows. For this reason, if a plurality of rows of pits 9 are formed in the vicinity of the optical waveguide, the pits 9 do not need to be formed in the entire region other than the optical waveguide on the main surface of the substrate 1.

  Therefore, the array of pits 9 may be formed only in the vicinity of a portion that is largely bent in the optical waveguide. That is, various types of conventionally known optical waveguides and the optical waveguide of the present invention may be used in combination on the same substrate.

  In FIG. 1B, the inside of the pit 9 is described as being hollow and filled with air, but the inside of the pit 9 may be filled with a material different from the material of the substrate 1. Good. For example, the main surface of the substrate 1 may be covered with an insulating film. In this case, the refractive index of the insulating film needs to have a value different from the refractive index of the high refractive index layer 11 formed on the substrate 1.

  The pit 9 can be formed as follows, for example. That is, after a photosensitive resist is formed on the main surface of the substrate 1 by a photolithography technique, this photosensitive resist is exposed and developed using a photomask that defines the arrangement pattern of the pits 9. Next, the exposed portion of the substrate 1 may be selectively etched using the photosensitive resist thus patterned as an etching mask. The high refractive index layer 11 is formed on the main surface of the substrate 1 before the pits 9 are formed. If etching for the pits 9 can be performed under the condition of preferentially etching the high refractive index layer 11 as compared with other parts of the substrate 1, the pits have a depth corresponding to the thickness of the high refractive index layer 11. 9 is easily formed with good reproducibility.

When the substrate 1 is formed of a material having an electro-optic effect such as LiNbO ₃ , etching for forming an array of pits 9 is performed by fluorine-based gas plasma RIE (reactive ion etching) or ICP (inductively coupled plasma). Can be done. In the case of ICP, the substrate 1 can be etched at a rate of 0.5 μm / min by using a highly reducing gas such as CF ₄ , BCl ₃ , or C ₄ F ₈ . In this method, a selection ratio of 1 with respect to the photosensitive resist can be realized. In addition, the fact that LiNbO _x can be etched by ICP is described in the 63rd Applied Physics Related Conference Lecture Proceedings 26a-D-20.

  The optical waveguide 2 of this embodiment is branched into two branch optical waveguides 2a and 2b at two branch points 7a and 7b so as to operate as a Mach-Zehnder interferometer. The input light input from the entrance-side optical waveguide 2c branches at one branch point 7a, passes through the two branch optical waveguides 2a and 2b, and then travels along the common exit-side optical waveguide 2d at the other branch point 7b. To do. A dotted arrow in FIG. 1A indicates a light propagation path.

  On the main surface side of the substrate 1, a modulation electrode 3 composed of two lines 3 a and 3 b is provided along the branch optical waveguides 2 a and 2 b of the optical waveguide 2. Each inner end of each line 3a, 3b is formed so as to be located immediately above the center of each branch optical waveguide 2a, 2b. Each of the lines 3a and 3b of the modulation electrode 3 is constituted by a metal film such as aluminum or gold formed by using a process such as vacuum deposition, photolithography, and etching.

  The input light is introduced from the entrance-side optical waveguide 2c and undergoes a light modulation action as follows when passing through the branch optical waveguides 2a and 2b.

  When a modulation signal is applied between the lines 3 a and 3 b of the modulation electrode 3, an electric field is generated in the gap 6. Then, due to the electro-optic effect, the refractive index of the material constituting the branched optical waveguides 2a and 2b changes according to the electric field strength. In the present embodiment, since electric fields in opposite directions are applied to the branched optical waveguide 2a and the branched optical waveguide 2b, when the substrate 1 is made of, for example, a z-cut lithium niobate crystal, Opposite phase changes are given to the light passing through the branched optical waveguides 2a and 2b. Therefore, in the exit-side optical waveguide 2d, interference between the two lights that have passed through the branch optical waveguides 2a and 2b occurs, and the intensity of the output light changes due to this interference, so that the light modulation element of the present embodiment performs light intensity modulation. Operates as a vessel.

  In the region of the substrate 1 where the pits 9 are formed, a photonic band gap is generated in a two-dimensional direction (in-plane direction of the substrate 1), so that light waves having a frequency corresponding to the band gap are formed by the pits 9. Cannot propagate to the area where For this reason, even if the angle at the bent portion 8 and the branched portion 7 is very large, the loss due to light wave radiation at these portions is very small. Therefore, the size of the light modulation element of this embodiment can be significantly reduced particularly in the longitudinal direction as compared with a light modulation element using a conventional optical waveguide.

  Hereinafter, this point will be described in detail with reference to FIG.

FIGS. 7A to 7C show portions where two optical waveguides 2a and 2b branch from the optical waveguide 2c, respectively. As can be seen from a comparison between FIG. 7A and FIG. 7B, when the branch angle between the optical waveguide 2a and the optical waveguide 2b is increased, the area required for branching is reduced, and the light modulation element is reduced. The size can be reduced (L1> L2). However, an optical modulation element that confines light in the in-plane direction of the substrate using a total reflection type waveguide (a waveguide formed by a Ti diffusion method of a LiNbO ₃ crystal substrate) that is used in a standard in the optical modulation element Then, the branch angle between the optical waveguide 2a and the optical waveguide 2b cannot be increased to, for example, 5 ° or more. For this reason, the branch angle is set to about 0.1 to 2 °. In the known light modulation element structure, the distance between the optical waveguides at the portion where the optical waveguides 2a and 2b run in parallel is about 20 to 30 μm. For this reason, length L1 will be a magnitude | size about 1-20 mm. For this reason, in order to reduce the size of the light modulation element, it is strongly required to shorten the length L1.

  On the other hand, as can be seen from FIG. 7C, when the distance between the optical waveguides 2a and 2b in the region where the optical waveguide 2a and the optical waveguide 2b run in parallel is shortened, the light modulation element can be downsized even when the branch angle is small. Yes (L2≈L3). However, if the interval between the optical waveguides 2a and 2b is shortened, there arises a problem that light propagating through the optical waveguides 2a and 2b interferes with each other. For this reason, in the conventional total reflection waveguide, it is difficult to bring the optical waveguides 2a and 2b closer to 10 μm or less.

  On the other hand, in the light modulation element of this embodiment, since the light is guided by the photonic crystal structure, an optical waveguide bent at a steep angle can be realized as shown by the broken line in FIG. It is also possible to suppress interference between the optical waveguides.

  Furthermore, according to the light modulation element of the present embodiment, since the photonic crystal structure is present in the vicinity of the optical waveguides 2a and 2b, an effect of reducing the group velocity of the light waves propagating through the optical waveguides 2a and 2b can be obtained. . When the propagation speed of the light wave is lowered in this way, the energy of the light wave accumulated in the optical waveguides 2a and 2b increases. This effect also improves the modulation efficiency.

  The depth of the pits 9 is sufficient if it is deep enough to contain an electromagnetic field of light waves propagating in the optical waveguide 2 (usually about 5 μm or less). However, even if the pit 9 is relatively shallow, if the refractive index of the high refractive index layer 11 is set high, the effect of confining the light wave in the vertical direction is strengthened, and thus the same effect as that obtained when the pit 9 is deep. The effect of can be obtained. The diameter of the pits 9 is preferably set to about ¼ of the wavelength of the light wave in the substrate 1, and the arrangement period of the pits 9 is preferably set to about ½ of the wavelength.

  In the present embodiment, as shown in FIG. 1B, the high refractive index layer 11 is also present in the region where the photonic crystal structure is formed on the main surface of the substrate 1. Since the guide in the horizontal direction of the light wave is realized by a photonic crystal structure, it is not necessary to form the high refractive index layer 11 only in the optical waveguide portion. For this reason, a mask may be used for processing such as metal diffusion and proton exchange necessary for forming the high refractive index layer 11 on the substrate surface, and the high refractive index layer 11 may be formed only in a selected region. .

(Embodiment 2)
Hereinafter, a second embodiment of the light modulation device according to the present invention will be described with reference to FIGS. 3 (a) and 3 (b). FIG. 3A is a plan view of the light modulation element of this embodiment, and FIG. 3B is a cross-sectional view taken along the line AA ′.

  In the light modulation element of this embodiment, as shown in FIG. 3A, a proton exchange method using benzoic acid, a thermal diffusion method of a metal film, or the like is used on the surface portion of the substrate 1 having an electro-optic effect. An optical waveguide 2 formed in this manner is provided. The substrate 1 is formed from the same material as that of the substrate 1 used in the first embodiment.

  The optical waveguide 2 in this embodiment is provided with a refractive index difference not only in the vertical direction but also in the lateral direction so that light can be confined in a direction parallel to the main surface of the substrate 1. That is, in the first embodiment, the high refractive index layer 11 is formed on the main surface of the substrate 1 without a mask. However, in this embodiment, by using a mask pattern, other regions other than the region that becomes the optical waveguide are used. A region having a high refractive index is also formed by this portion.

  The optical waveguide 2 is branched into two branch optical waveguides 2a and 2b at two branch points 7a and 7b. The light input from the entrance-side optical waveguide 2c branches at one branch point 7a, passes through the two branch optical waveguides 2a and 2b, and then travels along the common exit-side optical waveguide 2d at the other branch point 7b. And operates as a Mach-Zehnder interferometer using an optical waveguide.

  A characteristic point of this embodiment is that a large number of pits 9 formed by etching are formed between the optical waveguides 2a and 2b. By appropriately setting the shape, size, and arrangement of the pits 9, a so-called two-dimensional photonic crystal structure is formed. Thereby, a photonic band gap is generated in a two-dimensional direction (a direction parallel to the main surface of the substrate 1). The role of the pit 9 in this embodiment is to block light coupling / interference between the two branch optical waveguides 2a and 2b.

  On the substrate 1, a modulation electrode 3 composed of two lines 3 a and 3 b is provided along the branched optical waveguides 2 a and 2 b of the optical waveguide 2. Each inner end of each line 3a, 3b is formed so as to be located immediately above the center of each branch optical waveguide 2a, 2b. Each of the lines 3a and 3b of the modulation electrode 3 is constituted by a metal film such as aluminum or gold formed by using a process such as vacuum deposition, photolithography, and etching.

  The input light is introduced from the entrance-side optical waveguide 2c and undergoes a light modulation action as follows when passing through the branch optical waveguides 2a and 2b.

  When a high frequency signal is input from one end of the modulation electrode 3 and the modulation signal propagates to the lines 3 a and 3 b of the modulation electrode 3, an electric field is generated in the gap 6. Then, due to the electro-optic effect, the refractive index of the material constituting the branched optical waveguides 2a and 2b changes according to the electric field strength. In the present embodiment, since electric fields in opposite directions are applied to the branched optical waveguide 2a and the branched optical waveguide 2b, when the substrate 1 is made of, for example, a z-cut lithium niobate crystal, Opposite phase changes are given to the light passing through the branched optical waveguides 2a and 2b. Therefore, in the exit-side optical waveguide 2d, interference between the two lights that have passed through the branch optical waveguides 2a and 2b occurs, and the intensity of the output light changes due to this interference, so that the light modulation element of the present embodiment performs light intensity modulation. Operates as a vessel.

  According to the present embodiment, a photonic band gap is generated in the two-dimensional direction (in-plane direction of the substrate 1) in the gap 6 between the optical waveguide 2a and the optical waveguide 2b. By adopting such a configuration that cannot be propagated to the optical waveguide, even when the optical waveguide 2a and the optical waveguide 2b are arranged close to each other, coupling of light waves between these optical waveguides can be suppressed. As a result, the optical waveguide 2a and the optical waveguide 2b can be brought closer to each other compared to the case where no photonic crystal structure is provided. Specifically, according to the conventional light modulation element, the distance between the two optical waveguides needs to be about 20 to 30 μm, but according to the light modulation element of the present embodiment, this distance is about 10 μm or less. It is possible to reduce it. When the distance between the branched optical waveguides is reduced in this way, a strong electric field is generated in the gap 6 even when the same voltage is applied between the modulation electrodes 3a and 3b, so that the phase change applied to the light waves in the optical waveguides 2a and 2b. The amount is increased and the modulation efficiency is improved.

  Further, the influence of the photonic crystal structure of the optical waveguides 2a and 2b also affects the propagation characteristics of the light wave propagating through the optical waveguides 2a and 2b. In many cases, the presence of the photonic crystal structure can reduce the group velocity of the light waves propagating through the optical waveguides 2a and 2b, thereby reducing the propagation speed of the light waves and entering the optical waveguides 2a and 2b. Light wave energy is stored. This effect can also improve the modulation efficiency.

(Embodiment 3)
Hereinafter, a third embodiment of the light modulation device according to the present invention will be described with reference to FIGS. 4 (a) and 4 (b). FIG. 4A is a plan view of the light modulation element of this embodiment, and FIG. 4B is a cross-sectional view taken along line AA ′.

  The light modulation element of this embodiment has the same configuration as that of the light modulation element of Embodiment 2 except for the photonic crystal structure provided between the branched optical waveguides. That is, in the present embodiment, a one-dimensional photonic crystal structure is adopted in order to suppress / block optical interference of the branched optical waveguide.

  As shown in FIGS. 4A and 4B, a row of grooves 10 extending in the light propagation direction is formed in the gap portion 6. Even when a so-called one-dimensional photonic crystal structure in which a photonic band gap is generated in a direction crossing the groove 10 by appropriately selecting the width, interval, and depth of the groove 10 is similarly effective. In addition, it is preferable to set the width | variety and the period of a row | line | column of the groove | channel 10 to about 1/4 and 1/2 of the wavelength in the board | substrate of a light wave, respectively. In this case, since the light wave cannot propagate in the direction across the groove 10, the coupling between the optical waveguides 2a and 2b can be suppressed as in the second embodiment. For this reason, the optical waveguide 2a and the optical waveguide 2b can be brought close to each other. Thereby, by shortening the interval between the optical waveguides 2a and 2b, the light modulation efficiency can be improved for the reason described above.

  According to this embodiment, compared with the case where the arrangement of the pits 9 shown in FIG. 3 is used, the same effect can be exhibited even if the depth of the recess (groove 10) formed in the substrate 1 is shallow. For this reason, although the area of the area | region range which should be etched becomes large, the etching for forming the groove | channel 10 is easy.

(Embodiment 3)
Next, an embodiment of a fiber radio system according to the present invention will be described with reference to FIG.

  A fiber radio system 50 according to the present embodiment includes an optical modulator / demodulator 51 incorporating the light modulation element according to the first and second embodiments. The antenna 53 can directly perform communication with a normal data communication network such as the Internet, communication with a portable terminal, reception of a signal from a CATV, or the like using, for example, a millimeter wave carrier wave. The optical modulator / demodulator 51 includes a light demodulating element (for example, a photodiode) in addition to the light modulating element.

  On the other hand, a radio signal having a high frequency such as millimeter wave is difficult to transmit over a long distance, and is susceptible to signal blocking by an object. Therefore, communication with the data communication network 61, the CATV 62, and the mobile phone system 63 can be performed using the wireless device 60 and the antenna 64 attached to the wireless device. In this case, an optical modulator / demodulator 55 connected via the fiber radio communication system 50 and the optical fiber 70 and an antenna 54 attached thereto are further provided. Then, signals can be exchanged with the wireless device 60 via the antennas 54 and 64 and the optical modulator / demodulator 55. The light modulator / demodulator 55 includes a light demodulating element (for example, a photodiode) in addition to the light modulating element. When long-distance transmission is desired or when transmission is performed indoors that is partitioned by a wall or the like, it is effective to transmit an optical signal modulated with a radio signal such as a millimeter wave through the optical fiber 70. .

  The light modulation element of the present invention is suitably used for high-speed optical communication, an optical signal processing system, and the like. In particular, since there is little disturbance in the phase of the optical signal accompanying optical modulation, it is suitable for high-speed information transmission and long-distance optical fiber communication.

(A) is a top view of 1st Embodiment of the light modulation element by this invention, (b) is the A-A 'sectional view taken on the line. These are a top view and sectional drawing of the principal part. (A) is a top view of the hypothetical light modulation element at the time of combining a well-known technique in the case where a light modulation element is comprised with the waveguide which has a photonic crystal structure, (b) is the A- It is A 'sectional view. (A) is a top view of 2nd Embodiment of the light modulation element by this invention, (b) is the A-A 'sectional view taken on the line. (A) is a top view of 3rd Embodiment of the light modulation element by this invention, (b) is the A-A 'sectional view taken on the line. 1 is a diagram showing an embodiment of a communication system according to the present invention. It is a perspective view which shows the prior art example of a light modulation element. (A) to (c) are all plan views showing examples of the shape and size of the branching waveguide in the light modulation element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2 Optical waveguide 2a Branch optical waveguide 2b Branch optical waveguide 2c Inlet side optical waveguide 2d Outlet side optical waveguide 3 Modulation electrode 3a Line 3b Line 4 Signal source 5 Termination resistor 6 Gap 7a Branch point 7b Branch point 8 Curved portion 9 Pit 10 groove 11 high refractive index layer

Claims (13)

An optical waveguide extending along a certain surface; a first light guide portion for restricting light wave propagation in a direction parallel to the surface; and a second light guide portion for restricting light wave propagation in a direction perpendicular to the surface. A light modulation element comprising:
The first light guide portion includes a photonic crystal disposed outside the optical waveguide, and the second light guide portion includes a region having a refractive index lower than that of the optical waveguide. And
Each of the first light guide portion and the second light guide portion is provided in a substrate formed of a material exhibiting an electro-optic effect, and the second light guide portion is formed of the optical waveguide. An optical modulation element having a crystal structure substantially the same as the crystal structure.   The light modulation element according to claim 1, wherein the optical waveguide is formed on or in the substrate. The substrate has a main surface parallel to the surface;
The optical waveguide and the first light guide portion are formed on the main surface side of the substrate,
The light modulation element according to claim 1, wherein the light modulation element is located below the optical waveguide.   4. The light modulation element according to claim 3, wherein the first light guide portion has a periodic arrangement structure of concave portions and / or convex portions provided on a main surface of the substrate.   5. The light modulation element according to claim 3, wherein the optical waveguide includes a modified layer formed on a main surface side of the substrate.   The light modulation element according to claim 1, further comprising: an electrode for applying a light modulation high-frequency signal to light propagating through the optical waveguide on the substrate.   The light according to claim 1, wherein the first light guide portion has a two-dimensional photonic crystal structure having a photonic band gap with respect to a light wave that is to propagate in a direction parallel to the surface. Modulation element.   The light modulation element according to claim 1, wherein at least a part of the optical waveguide is in contact with a photonic crystal structure of the first light guide portion.   The light modulation element according to claim 1, wherein the optical waveguide has at least two branched optical waveguides, thereby forming a Mach-Zehnder interferometer.   The light modulation element according to claim 9, wherein a region between the two branch optical waveguides has a photonic crystal structure.   The light modulation element according to claim 9 or 10, wherein a branch angle of the two branch optical waveguides is 5 ° or more.   The light modulation element according to claim 9, wherein an interval between the two branch optical waveguides is 10 μm or less. A communication system including an optical modulation element for converting an electrical signal into an optical signal,
The communication system, wherein the light modulation element is the light modulation element according to claim 1.

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