WO2005045512A1

WO2005045512A1 - Optical modulator and communication system

Info

Publication number: WO2005045512A1
Application number: PCT/JP2004/015824
Authority: WO
Inventors: Akira Enokihara; Hiroyuki Furuya
Original assignee: Matsushita Electric Industrial Co., Ltd.
Priority date: 2003-11-10
Filing date: 2004-10-26
Publication date: 2005-05-19

Abstract

Disclosed is an optical modulator which comprises an optical waveguide (2) having at least two branched total reflection optical waveguides (2a, 2b). At least a part of the optical waveguide (2) is formed on or in a substrate (1) which is composed of an electro-optic material, and the region (gap portion (6)) between the two total reflection optical waveguides (2a, 2b) has a photonic crystal structure.

Description

Specification

Optical modulation element and communication system

Technical field

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

Background art

[0002] 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 that can operate at an ultra-high speed will increase in the future. Since it is difficult to cope with ultra-high-speed optical modulation by the conventional direct modulation using a semiconductor laser, development of an external modulation type element capable of high-speed operation has recently been urgently required. In particular, the so-called electro-optic light modulator using a dielectric crystal having a large Pockels effect can operate at a very high speed, and the optical signal phase disturbance due to light modulation is small! It is very suitable for high-speed information transmission and long-distance optical fiber communication. Furthermore, if an optical waveguide structure is used, miniaturization and high efficiency may be realized at once.

[0003] In general, an electro-optic light modulation element includes a transmission line provided as a modulation electrode on an electro-optic crystal for transmitting a modulation signal, and an optical waveguide formed near the transmission line. . In other words, it utilizes the phenomenon that when the refractive index of the optical waveguide changes according to the electric field induced around the modulation electrode, the phase of the light wave propagating through the optical waveguide changes with the modulation signal.

[0004] In an electro-optic light modulator, the electro-optic coefficient that is the basis of light modulation is relatively small in a normal crystal. Therefore, it is important for the optical modulation element of this system to efficiently apply an electric field to the optical waveguide in order to realize high modulation efficiency.

FIG. 5 is a simplified perspective view showing an example of a conventional light modulation element described in Non-Patent Document 1. In this light modulation element, an optical waveguide 2 is formed on the surface of a substrate 1 made of a material having an electro-optic effect. The optical waveguide 2 is composed of a region in which the refractive index of the substrate 1 is slightly increased from that of the other parts, and is formed by total reflection. Light waves can be guided. The refractive index is increased by, for example, thermally diffusing a metal into a part of the substrate 1. On both the left and right sides of the optical waveguide 2, on the upper surface of the substrate 1, modulation electrodes 3 made of a metal film such as aluminum or gold are provided. The modulation electrode 3 is composed of two lines 3a and 3b parallel to each other, and has a coplanar single line structure.

[0006] The optical waveguide 2 branches into two branch optical waveguides 2a and 2b at two branch points 7a and 7b, and input light input from the entrance-side optical waveguide 2c is split at one branch point 7a. After branching and passing through the two branch optical waveguides 2a and 2b, the other branching point 7b is configured to travel along the common exit side optical waveguide 2d.

[0007] On the substrate 1, a modulation electrode 3 having a coplanar single-line structure including two lines 3a and 3b extending along the branch 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 immediately 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. The other end is connected to a terminating resistor 5.

[0008] Input light is introduced from the entrance-side optical waveguide 2c, and undergoes an optical modulation action as described below when passing through each of the branch optical waveguides 2a and 2b.

When a high-frequency signal is supplied from the high-frequency signal source 4, the high-frequency signal propagates through the modulation electrode 3 in the same direction as light, and an electric field is generated in the gap 6. Then, due to the electro-optical effect, the refractive index of the material forming the branch optical waveguides 2a and 2b changes according to the electric field intensity. In the present embodiment, since the electric fields in the directions opposite to each other are applied to the branch optical waveguide 2a and the branch optical waveguide 2b, when the substrate 1 is made of, for example, a z-cut lithium niobate crystal, the two components are separated. Light passing through the branch optical waveguides 2a and 2b is given phase changes opposite to each other. Therefore, in the exit side optical waveguide 2d, interference between the two lights passing through the branch optical waveguides 2a and 2b occurs, and the intensity of the output light changes due to the interference. Act as a controller.

Non-Patent Document 1: IEEE Journal of Quantum Electronics. Vol.QE-13, no.4, pp287-290, 1977

Disclosure of the invention Problems the invention is trying to solve

[0010] A conventional electro-optic light modulation element uses an optical waveguide that confine light using a difference in refractive index. In such an optical modulation element, if the width of the gap 6 is reduced and the optical waveguides 2a and 2b are brought closer to each other, the intensity of the electric field formed in the gap 6 increases even when the same voltage is applied. Efficiency can be improved. If the width of the gap 6 is reduced while applying force, there is a problem that the influence of light wave coupling generated between the optical waveguides 2a and 2b increases. For this reason, it is considered that the width of the gap 2 cannot be generally reduced to about 20 to 30 m or less. This is one of the factors that hinder miniaturization and high efficiency of the device.

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

[0012] An optical modulation element according to the present invention is an optical modulation element having an optical waveguide structure having at least two branched total reflection optical waveguides, wherein at least a part of the total reflection optical waveguide is electrically operated. The substrate is formed on or in a substrate formed of a material exhibiting an optical effect, and a region between the two total reflection optical waveguides has a photonic crystal structure.

[0013] Preferably, in an embodiment, the photonic crystal structure is formed on the substrate.

[0014] In a preferred embodiment, the photonic crystal structure is realized by a periodic arrangement of concave portions and Z or convex portions provided on a main surface of the substrate.

[0015] In a preferred embodiment, the photonic crystal structure is realized by a periodic arrangement of concave portions provided on a main surface of the substrate, and the total reflection optical waveguide is formed on the main surface of the substrate. And the depth of the concave portion is / J, which is smaller than the thickness of the total reflection optical waveguide.

[0016] In a preferred embodiment, the photonic crystal structure is realized by a periodic arrangement of grooves provided on a main surface of the substrate and parallel to the total reflection optical waveguide.

In a preferred embodiment, the total reflection optical waveguide is provided on a main surface of the substrate, and a depth of the groove is smaller than a thickness of the total reflection optical waveguide. Preferably, in the embodiment, the total reflection optical waveguide is formed from a modified layer formed on a main surface side of the substrate.

[0019] In a preferred embodiment, an electrode for applying a high-frequency signal for optical modulation to light propagating through the total reflection optical waveguide is provided on the substrate.

[0020] Preferably, in the embodiment, the photonic crystal structure has a photo-band gap for a light wave to be propagated in one direction of the two branched total reflection optical waveguides and the other direction. Has a one-dimensional or two-dimensional photonic crystal structure.

[0021] In a preferred embodiment, the two total reflection optical waveguides form a Mach-Zehnder interferometer.

In a preferred embodiment, the photonic crystal structure is selectively formed only in a region between the two total reflection optical waveguides.

In a preferred embodiment, the distance between the two total reflection optical waveguides is 5 μm or less.

[0024] In a preferred embodiment, a core portion having a relatively high refractive index and a cladding portion having a relatively low refractive index are provided.

[0025] A communication system according to the present invention is a communication system including an optical modulation element for converting an electric signal into an optical signal, wherein the optical modulation element is any one of the above-described optical modulation elements. The invention's effect

According to the light modulation device of the present invention, the interval between the branch optical waveguides can be reduced, so that the modulation efficiency can be increased and the size of the light modulation device can be reduced. By using this optical modulation element for a communication system, communication using a high-frequency signal at the millimeter-wave level becomes possible.

BEST MODE FOR CARRYING OUT THE INVENTION

In recent years, research and development of various optical devices using “photonic crystals” have been promoted. “Photonic crystal” is a new optical material that forms a refractive index distribution with a period substantially equal to the wavelength of light. In a normal solid crystal, a band structure of electron energy is formed by a periodic arrangement of atoms. In a photonic crystal, a band structure of light energy is formed. With the formation of such a band structure, "photonic crystal" A "band gap" may be formed. Light having energy corresponding to the photonic band gap cannot propagate through a photonic crystal having a photonic band gap. In a photonic crystal having a band structure in which a photonic band gap does not exist, the propagation of light in a specific wavelength band is not completely blocked, but the propagation of light can be suppressed.

[0028] In the present invention, a photonic crystal structure is formed on a material having an electro-optic effect to realize a practical light modulation element.

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

(Embodiment 1)

First, a first embodiment of an optical modulation device according to the present invention will be described with reference to FIGS. 1 (a) and 1 (b). FIG. 1A is a plan view of the light modulation element of the present embodiment, and FIG. 1B is a cross-sectional view along the line AA ′.

As shown in FIG. 1A, the light modulation device of the present embodiment has a substrate 1 formed of a material exhibiting an electro-optic effect, and an optical waveguide 2 provided in the substrate 1. are doing.

[0032] The substrate 1 is made of lithium tantalate (LiTaO) single crystal, lithium niobate (LiNbO) single crystal.

It is made of a crystal material having an electro-optic effect such as 33 crystal. The optical waveguide 2 is suitably formed on the surface of the substrate 1 by using a proton exchange method using benzoic acid, a thermal diffusion method of a metal film, or the like.

The optical waveguide 2 of the present embodiment is branched into two total reflection optical waveguides 2a and 2b at two branch points 7a and 7b so as to operate as a Mach-Zehnder interferometer. Hereinafter, for the sake of simplicity, each of the branched total reflection optical waveguides is simply referred to as a “branch optical waveguide”. 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 exits from the other branch point 7b to the common exit-side optical waveguide 2d. Proceed to interfere.

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

[0035] The input light is introduced from the entrance-side optical waveguide 2c, and undergoes an optical modulation action as described below when passing through the branch optical waveguides 2a and 2b.

First, when a modulation signal is applied between the lines 3 a and 3 b of the modulation electrode, an electric field is generated in the gap 6. Then, due to the electro-optical effect of the substrate 1, the refractive index of the material forming the branch optical waveguides 2a and 2b changes according to the electric field intensity. In the present embodiment, since the electric fields in the directions opposite to each other are applied to the branch optical waveguide 2a and the branch optical waveguide 2b, when the substrate 1 is made of, for example, a z-cut lithium niobate crystal, Light passing through the two branched optical waveguides 2a and 2b is given opposite phase changes. Therefore, in the exit-side optical waveguide 2d, interference between the two lights passing through the branch optical waveguides 2a and 2b occurs, and the interference changes the intensity of the output light. Since the intensity of the output light can be modulated according to the modulation signal applied to the modulation electrode, the light modulation element of the present embodiment operates as a light intensity modulator.

The optical waveguide 2 in the present embodiment is provided with a refractive index difference not only in the vertical direction but also in the horizontal direction so that light can be confined in a direction parallel to the main surface of the substrate 1. That is, by using the mask pattern, a region having a higher refractive index than other portions is formed in the region of the substrate 1 that becomes the optical waveguide 2. The structure itself of the optical waveguide 2 is known.

A feature of the present 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. As a result, a photonic band gap occurs in a two-dimensional direction (a direction parallel to the main surface of the substrate 1). The role of the pits 9 in the present embodiment is to cut off light coupling and interference between the two branch optical waveguides 2a and 2b.

According to the present embodiment, a photonic band gap is generated in a two-dimensional direction (in-plane direction of the substrate 1) in the gap 6 between the optical waveguide 2a and the optical waveguide 2b. Cannot be propagated in the in-plane direction. This specific frequency propagates through the optical waveguide 2. It corresponds to the wavelength of the optical signal. By employing such a configuration, even when the optical waveguides 2a and 2b are arranged close to each other, it is possible to suppress the coupling of light waves between these optical waveguides. As a result, the optical waveguide 2a and the optical waveguide 2b can be made closer to each other as compared with the case where the photonic crystal structure is not provided. Specifically, according to the conventional optical modulator, the interval between the two optical waveguides needs to be about 20 to 30 m. According to the optical modulator of the present embodiment, the interval is about 5 m. Or it can be reduced to less. When the distance between the branched optical waveguides 2a and 2b is reduced, a strong electric field is generated in the gap 6 even when the same voltage is applied between the modulation electrode lines 3a and 3b. The amount of phase change given to the light wave increases, and the modulation efficiency improves.

[0040] Also, narrowing the interval between the optical waveguides 2a and 2b is useful for shortening the length (size in the light propagation direction) of the light modulation element. This will be described with reference to FIGS. 2 (a) and 2 (b).

FIGS. 2A and 2B show portions where two optical waveguides 2a and 2b are branched from the optical waveguide 2c, respectively. As can be seen from the comparison between FIGS. 2 (a) and 2 (b), when the distance between the branch optical waveguides 2a and 2b is reduced, the area required for branching can be shortened (L 1> L2 ), The size of the light modulation element can be reduced as a whole. As described above, when the interval between the branch optical waveguides 2a and 2b is reduced, the optical modulator can be sufficiently reduced in size even if the branch angle is not increased. It is difficult to increase the branch angle as long as a refractive index guided optical waveguide structure is adopted. For this reason, shortening the interval between the branch optical waveguides 2a and 2b greatly contributes to downsizing of the optical modulator.

[0042] The effect of the photonic crystal structure provided between the optical waveguides 2a and 2b also affects the propagation characteristics of light waves propagating through the optical waveguides 2a and 2b. In many cases, the group velocity of the light wave propagating through the optical waveguides 2a and 2b can be reduced by the presence of the photonic crystal structure. When the propagation speed of the light wave decreases, the energy of the light wave stored in the optical waveguides 2a and 2b increases, so that the modulation efficiency also improves.

The depth of the pit 9 is sufficient if the depth of the pit 9 is such that an electromagnetic field of a light wave propagating through the optical waveguide 2 is present (usually about 5 m or less). However, even if the pit 9 is relatively shallow, setting the refractive index of the optical waveguide 2 high will enhance the vertical confinement effect of the light wave. Therefore, the same effect as that obtained when the pit 9 is deep can be obtained. The diameter of the pits 9 is preferably set to about 1Z4 of the wavelength of the light wave in the substrate 1, and the arrangement period of the pits 9 is preferably set to about 1Z2 of the wavelength.

The wavelength of the light corresponding to the photonic band gap largely depends on the pit 9 arrangement period. Therefore, if the wavelength of the light wave to be propagated through the optical waveguide 2 is given, the arrangement period of the pits 9 is determined so as to prevent the propagation of the wavelength.

In FIG. 1A, on the main surface of the substrate 1, pits 9 are formed in the entire force gap 6 in which the pits 9 are arranged in the entire area of the gap 6 between the optical waveguides 2 a and 2 b. No need. Since the pits 9 can form a photonic crystal structure even with about several rows, if multiple rows of pits 9 are formed near the optical waveguide, the area other than the optical waveguide on the main surface of the substrate 1 is formed. It is not necessary to form pits 9 throughout.

[0046] In FIG. 1 (b), the inside of the pit 9 is described as being hollow and filled with air, but the inside of the pit 9 is filled with a material different from the material of the substrate 1. May be. For example, the main surface of 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 that of the optical waveguide 2 formed on the substrate 1.

The formation of the pit 9 can be performed, for example, as follows. That is, after a photosensitive resist is formed on the main surface of the substrate 1 by the photolithography technique, the photosensitive resist is exposed and developed using a photomask that defines an arrangement pattern of the pits 9. Next, the exposed portion of the substrate 1 may be selectively etched using the photosensitive resist patterned as described above as an etching mask. The optical waveguide 2 is formed on the main surface of the substrate 1 before the pit 9 is formed. If the etching for the pit 9 can be performed under the condition that the optical waveguide 2 is preferentially etched compared to other parts of the substrate 1, the pit 9 having a depth corresponding to the thickness of the optical waveguide 2 is reproduced. It becomes easy to form well.

When the substrate 1 is formed of a material having an electro-optic effect such as LiNbO,

Three

The etching for forming the array of the gates 9 can be performed by fluorine gas plasma RIE (reactive ion etching) or ICP (inductively coupled plasma). In the case of using ICP, if a gas such as CF, BC1, or CF with strong reducing properties is used, the base rate will be 0.5 mZ.

4 3 4 8

Plate 1 can be etched. In this method, the selectivity ratio for photosensitive resist is 1 realizable. The fact that LiNbO _x can be etched by ICP is described in the 63rd Applied Physics-related Lecture Meeting Preprints 26a-D-20.

It is also effective to use a ridge structure waveguide formed by etching or the like as the optical waveguide 2. In this case, first, a layer having a high refractive index is formed on the entire surface of the substrate 1 by a proton exchange method or a thermal diffusion method of a metal film. Then, a ridge structure is formed by etching the surface portion other than the waveguide. When the thickness of the layer having a higher refractive index is larger than the height of the ridge, it may be called a rib structure waveguide. It is sufficient that the height of the ridge is approximately the same as the wavelength of the light wave or approximately several times the wavelength.

[0050] The ridge-structured waveguide requires more steps for manufacturing as compared with the proton exchange waveguide and the heat diffusion waveguide, but has an advantage that the confinement of the light wave in the lateral direction can be sufficiently performed. . Further, the etching required for forming the ridge waveguide structure and the etching required for forming the pits 9 can be performed in the same step. By doing so, the manufacturing process can be simplified.

(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 the present embodiment, and FIG. 3B is a cross-sectional view along the line AA ′.

The light modulation element of the present embodiment has the same configuration as the light modulation element of the first embodiment except for the form of the photonic crystal structure provided in the gap 6 between the branch optical waveguides 2a and 2b. are doing. More specifically, a one-dimensional photonic crystal structure is employed in the present embodiment in order to suppress and cut off optical interference between the branch optical waveguides 2a and 2b.

Hereinafter, features of the present embodiment will be described.

In the light modulation device of the present embodiment, as shown in FIGS. 3A and 3B, a row of grooves 10 extending in the light propagation direction is formed in the gap 6 of the substrate 1. By proper selection of the width, spacing, and depth of the grooves 10, a photonic band gap is created across the grooves 10.

It is preferable to set the width and the cycle of the grooves 10 to be approximately 1Z4 and 1Z2, respectively, of the wavelength of the light wave in the substrate. In this case, the light wave propagates across the groove 10. Since it is impossible, the coupling between the optical waveguides 2a and 2b can be suppressed as in the second embodiment. Therefore, it is possible to bring the optical waveguide 2a and the optical waveguide 2b closer. Accordingly, by shortening the distance between the optical waveguides 2a and 2b, the light modulation efficiency can be improved for the above-described reason.

According to the present embodiment, as compared with the case where the arrangement of the pits 9 shown in FIGS. 1A and 1B is used, even if the depth of the concave portion (the groove 10) formed in the substrate 1 is small, The same effect can be achieved. For this reason, although the area of the region to be etched becomes large, the etching for forming the groove 10 is easy.

In the light modulation element in each of the above-described embodiments, an array of pits 9 or grooves 10 is formed so as to generate a photonic band gap, thereby suppressing the coupling between the branched optical waveguides. The band structure of the nick crystal does not necessarily need to have a photonic band gap.

[0058] Even in the case where the photonic crystal arranged between the branching optical waveguides does not have a photonic band gap, the light modulation element of the present invention can exert its effect. More specifically, if the alignment position of the bit 9 or the groove 10 also shifts the target position force due to a manufacturing error or the like, the alignment periodicity is slightly broken, so that a photonic band gap is not formed in the band structure of the photonic crystal. There are cases. Even in such a case, the coupling between the branched optical waveguides can be sufficiently suppressed.

When the vertical size (depth) of the pit 9 or the groove 10 is smaller than the thickness of the branch optical waveguide, the space (the gap 6) between the two side-by-side branch optical waveguides is set. In), a region having a photonic crystal structure and a region having no photonic crystal structure are present in layers above and below. In such a case, coupling may occur between the two branch optical waveguides via a region having no photonic crystal structure.However, the presence of the region having the photonic crystal structure causes the degree of the coupling. Can be made sufficiently small.

As described above, it is not necessary for the photonic crystal structure disposed in the gap 6 of the branch optical waveguide to completely block the coupling of the branch optical waveguides located on both sides. Even when complete interruption does not occur, the propagation constant of the light wave in the gap 6 can be sufficiently controlled by the arrangement of the photonic crystal. When the depth of the pit 9 or the groove 10 is smaller than the thickness of the branch optical waveguide, in order to sufficiently suppress the coupling between the branch optical waveguides, the depth of the pit 9 or the groove 10 must be changed. It is preferable to set the size to 5% or more of the wavelength of the light wave in the wave path. If the error of the periodicity of the pits 9 and the grooves 10 is within 50% of the period of the perfectly regular arrangement, the coupling between the branched optical waveguides can be sufficiently suppressed.

As described above, it is not necessary that the photonic crystal structure in the light modulation device of the present invention has a band gap, because the total reflection type waveguide is provided in each of the branched optical waveguide portions. This is because the structure is provided. That is, in the present invention, the formation of the optical waveguide itself is not intended to be realized by the band gap of the photonic crystal structure. Therefore, the function of confining light in the region where light is to be propagated (waveguiding function) does not need to be performed by utilizing the light confinement effect of the photo-band gap, and the degree of freedom in fabricating a photonic crystal structure is large. improves.

(Embodiment 3)

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

[0064] The fiber wireless system 50 of the present embodiment includes the optical modulator / demodulator 51 incorporating the optical modulation element of the first and second embodiments. The antenna 53 enables communication with a normal data communication network such as the Internet or a portable terminal, or reception of a CATV-powered signal or the like directly using, for example, a millimeter-wave carrier. The optical modulator / demodulator 51 incorporates an optical demodulator (for example, a photodiode) together with the optical modulator.

[0065] On the other hand, high-frequency radio signals such as millimeter waves are difficult to transmit over long distances, and are susceptible to signal interruption 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 that case, an optical modulator / demodulator 55 connected to the fiber wireless communication system 50 via 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 optical modulator / demodulator 55 includes an optical demodulation element ( (For example, a photodiode). When performing long-distance transmission or when transmitting indoors partitioned by a wall or the like, it is effective to transmit an optical signal modulated by a wireless signal such as a millimeter wave through the optical fiber 70.

As described above, according to the light modulation device of the present invention, the interval between the branch optical waveguides can be reduced, so that the modulation efficiency can be increased and the size of the light modulation device can be reduced. . By using this optical modulation element in a communication system, communication using a millimeter-wave-level high-frequency signal becomes possible.

Industrial applicability

[0067] The optical modulation element of the present invention is suitably used for high-speed optical communication, optical signal processing systems, and the like. In particular, it is suitable for high-speed information transmission and long-distance optical fiber communication, etc., because the phase of optical signals due to optical modulation is small.

Brief Description of Drawings

FIG. 1 (a) is a plan view of a first embodiment of a light modulation device according to the present invention, and FIG. 1 (b) is a cross-sectional view along the line AA ′.

2] (a) and (b) are plan views each showing an example of the shape and size of a branch waveguide in an optical modulation device. [FIG.

FIG. 3 (a) is a plan view of a second embodiment of the light modulation device according to the present invention, and FIG. 3 (b) is a cross-sectional view along the line AA ′.

FIG. 4 is a diagram showing an embodiment of a communication system according to the present invention.

FIG. 5 is a perspective view showing a conventional example of a light modulation element.

Explanation of symbols

[0069] 1

2 Optical waveguide

2a Branch optical waveguide

2b Branch optical waveguide

2c entrance side optical waveguide

2d exit side optical waveguide

3 Modulation electrode a Line b Line

Signal source Terminating resistor Gap a Branch point b Branch point Bent part Pit 0 Groove

Claims

The scope of the claims

[1] An optical modulator having an optical waveguide structure having at least two branched total reflection optical waveguides,

At least a part of the total reflection optical waveguide is formed on or in a substrate formed of a material exhibiting an electro-optic effect,

A light modulation element in which a region between the two total reflection optical waveguides has a photonic crystal structure.

2. The light modulation device according to claim 1, wherein the photonic crystal structure is formed on the substrate.

3. The light modulation device according to claim 2, wherein the photonic crystal structure is realized by a periodic arrangement of concave portions and Z or convex portions provided on a main surface of the substrate.

[4] The photonic crystal structure is realized by a periodic arrangement of concave portions provided on the main surface of the substrate, and

The total reflection optical waveguide is provided on a main surface of the substrate,

3. The light modulation device according to claim 2, wherein a depth of the concave portion is smaller than a thickness of the total reflection optical waveguide.

5. The light modulation device according to claim 3, wherein the photonic crystal structure is realized by a periodic arrangement of grooves provided on a main surface of the substrate and parallel to the total reflection optical waveguide.

6. The light modulation element according to claim 5, wherein the total reflection optical waveguide is provided on a main surface of the substrate, and a depth of the groove is smaller than a thickness of the total reflection optical waveguide. .

7. The light modulation device according to claim 1, wherein the optical waveguide is formed from a modified layer formed on a main surface side of the substrate.

[8] The light modulation element according to claim 1, wherein an electrode for applying a high-frequency signal for light modulation to light propagating through the optical waveguide is provided on the substrate.

[9] The photonic crystal structure has a one-dimensional or two-dimensional photonic band gap having a photonic band gap for a light wave to be propagated from one of the two total reflection optical waveguides to the other. 2. The light modulation device according to claim 1, which has a crystal structure.

[10] The optical modulator according to [], wherein the two total reflection optical waveguides form a Mach-Zehnder interferometer.

11. The light modulation device according to claim 1, wherein the photonic crystal structure is selectively formed only in a region between the two total reflection optical waveguides.

12. The light modulation device according to claim 1, wherein a distance between the two total reflection optical waveguides is 5 μm or less.

13. The optical modulation device according to claim 1, wherein each total reflection optical waveguide comprises a core portion having a relatively high refractive index and a cladding portion having a relatively low refractive index.

[14] A communication system including an optical modulation element for converting an electric signal into an optical signal, wherein the optical modulation element is the optical modulation element according to any one of claims 1 to 13. system.