JP2006178275A - Optical waveguide and optical modulating element, and communications system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical waveguide used in an optical modulating element which is appropriately used in an optical communications system and which has high modulation efficiency, and to provide a high-frequency circuit element which realizes desired frequency and attenuation with facilitated construction. <P>SOLUTION: In the optical waveguide 6, which has a substrate 1 composed of a material with an electro-optic effect and a ridge portion 2 on a surface of the substrate 1, the optical waveguide 6 has a periodical structure, composed of a plurality of grooves 3-1, or circular pits formed on the rear face of the substrate 1 to lower the group velocity of light propagating through the optical waveguide 6. By forming the grooves 3-1, or the circular pits on the rear face side of the substrate 1, periodical effective refractive index variation is imparted effectively to the waveguided light; and a significant effect is obtained, even with shallow grooves. Furthermore, high-efficiency optical modulation is realized, by constructing the optical modulating element using the waveguide. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

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

  An optical modulation element is a basic element in high-speed optical communication and an optical signal processing system, and it is considered that the need for an optical modulation element capable of operating at an ultrahigh speed will increase in the future.

  Conventionally, direct modulation by a semiconductor laser that has been used is difficult to cope with ultra-high speed optical modulation, and recently, an external modulation type element capable of high-speed operation has been urgently developed. In particular, electro-optic light modulators that use dielectric crystals with a large Pockels effect are capable of ultra-high-speed operation, and optical signal phase disturbance associated with light modulation is low, so high-speed information transmission is possible. It is very suitable for 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 conductor line that functions as a modulation electrode provided on an electro-optic crystal, and an optical waveguide formed in the vicinity of the transmission conductor line. When a high frequency signal for modulation is applied to the modulation electrode, 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.

  The electro-optic coefficient, which is one of the basic parameters that determine the efficiency of light modulation, is relatively small for ordinary crystals. Therefore, in order to achieve high modulation efficiency with an optical modulation element that uses the electro-optic effect, it is important to increase the interaction between the modulation electric field and the light wave in the optical waveguide.

  FIG. 11 is a perspective view showing a conventional light modulation element described in Non-Patent Document 1. FIG. This light modulation element includes an optical waveguide (6a to 6d) formed on the surface of a substrate 1 made of a crystal material having an electro-optic effect, and an electric signal for modulation (modulation) to light propagating through the optical waveguide (6a to 6d). And a modulation electrode 10 for applying a wave). The modulation electrode 10 has a coplanar conductor line structure constituted by two conductor lines 10a and 10b parallel to each other.

  The optical waveguides 6a to 6d include an entrance-side optical waveguide portion 2c into which light to be modulated (input light) is introduced, an exit-side optical waveguide portion 6d from which modulated light is output, and an entrance-side optical waveguide portion 6c and an exit. Two branching waveguide portions 14a and 14b for coupling the side optical waveguide portion 6d are provided.

  The optical waveguides 6a to 6d are branched into two branch waveguides 6a and 6b at two branch points 14a and 14b, and the input light input from the entrance-side optical waveguide 6c is branched at one branch point 14a. After passing through the two branching waveguides 6a and 6b, the other branching point 14b is configured to travel along the common exit-side optical waveguide 6d.

  The inner ends of the conductor lines 10a and 10b constituting the modulation electrode 10 are located immediately above the central portions of the branch waveguides 6a and 6b, and a high frequency wave for modulation is provided at one end of each of the conductor lines 10a and 10b. The signal source 11 is connected to the other end, and the terminating resistor 12 is connected to the other end.

When a high-frequency signal (modulated wave) is supplied from the signal source 11 to the modulation electrode 10, the modulated wave propagates on the modulation electrode 10 in the direction of light propagation and forms an electric field in the gap 13. For this reason, the refractive index of the material which comprises the branched waveguides 6a and 6b changes according to an electric field strength by the electrooptic effect. Since electric fields in opposite directions are applied to the branching waveguide 6a and the branching waveguide 6b, when the substrate 1 is made of, for example, a z-cut lithium niobate crystal, two branching waveguides 6a and 6b are used. The light passing through is subjected to opposite phase changes.
IEEE Journal of Quantum Electronics, Vol. QE-13, no. 4, pp287-290, 1977)

  According to the light modulation element shown in FIG. 11, the interaction between the light wave and the modulation signal wave is increased by causing the modulated wave propagating through the modulation electrode 10 and the light wave propagating through the optical waveguide 6 to travel in the same direction. The aim is to achieve high-efficiency optical modulation.

  However, since the electro-optic constant of an electro-optic crystal typified by lithium niobate is very small, even if the modulation electrode 10 is extended to have a length of about several centimeters, about several volts is required to obtain sufficient modulation. Must be applied to the electro-optic crystal. In order to reduce the size of the light modulation element and reduce the necessary modulation voltage, it is necessary to improve the interaction between the light wave and the modulation electric field.

  On the other hand, since the optical waveguide usually used has almost no group velocity dispersion, the propagation velocity (here, the group velocity of the light wave has a meaning) is almost determined by the refractive index of the waveguide. The refractive index of an electro-optic crystal typified by lithium niobate is about 2.1, whereas the dielectric constant for microwaves is as high as about 20 to 40, so the speed of light is about the speed of microwaves. As a result, there is a problem that even if a traveling wave type electrode is used, perfect speed matching between light and signal waves cannot be achieved in the light modulation element. Therefore, even if the modulation electrode is set long, appropriate modulation cannot be realized, and the modulation efficiency is not improved.

  Even if the modulation electrode structure is devised and the effective dielectric constant of the microwave is lowered to achieve speed matching to some extent, the group velocity of light waves is very fast (in the case of a lithium niobate substrate, It is necessary to make the electrode length very long (to increase the interaction length between the light wave and the modulation electric field) for efficient light modulation. In addition to being very large, there is a limit to improving modulation efficiency due to microwave propagation loss at the modulation electrode.

  The present invention has been made to solve the above-described problems, and a main object thereof is to provide an optical modulation element having high modulation efficiency that is suitably used in an optical communication system and an optical waveguide used therefor.

  Another object of the present invention is to provide a communication system including a small light modulation element capable of efficiently performing light modulation.

An optical waveguide having a substrate made of a material having an electro-optic effect and a ridge structure on the surface of the substrate,
An optical waveguide further comprising a periodic structure for reducing the group velocity of light propagating through the optical waveguide,
The periodic structure includes a plurality of grooves or circular pits formed on the back surface of the substrate.

Further, in a preferred embodiment, an optical waveguide having a substrate made of a material having an electro-optic effect and a ridge structure on the surface of the substrate,
An optical waveguide further comprising a periodic structure for reducing the group velocity of light propagating through the optical waveguide,
The periodic structure includes a plurality of grooves or circular pits formed on the surface of the substrate, and the ridge structure is formed on the plurality of grooves or circular pits.

Further, in a preferred embodiment, an optical waveguide having a substrate made of a material having an electro-optic effect and a ridge structure on the surface of the substrate,
An optical waveguide further comprising a periodic structure for reducing the group velocity of light propagating through the optical waveguide,
The periodic structure comprises a plurality of high refractive index portions formed on the surface of the ridge structure portion,
The substrate is made of a lithium niobate or lithium tantalate material,
The plurality of high refractive index portions are formed by a proton exchange method or a metal thermal diffusion method.

  Further, in a preferred embodiment, the periodic structure has a dimension in which the width and period of the groove or pit does not form a photonic band gap and reduces the group velocity of light waves.

  In a preferred embodiment, a light modulation element including a modulation electrode for applying an electro-optic light modulation electric field to the optical waveguide is configured.

  In a preferred embodiment, there is provided a communication system comprising: the light modulation element; an optical fiber that transmits the modulated light output from the light modulation element; and means for providing a modulation electrical signal to the light modulation element. It is composed.

  According to the optical waveguide of the present invention, a low group velocity characteristic can be obtained with a low loss. Therefore, a small and highly efficient characteristic can be realized, for example, by applying it to a waveguide portion of an optical modulation element. By using this optical modulation element, high-speed optical modulation is possible with a very small configuration and a low-power drive circuit, so that high performance and miniaturization of the high-speed optical fiber communication system can be expected. Particularly effective in optical fiber radio systems using millimeter wave signals.

  In recent years, research and development of various optical devices using the “photonic crystal structure” has been advanced. The “photonic crystal structure” is a structure in which refractive index changes having a period similar to the wavelength of light are periodically arranged. In a normal solid crystal, a band structure of electron energy is formed by periodic arrangement of atoms, whereas in a photonic crystal structure, a band structure of light energy is formed and light waves propagate due to anomalous dispersion effect. It is known that the characteristics are greatly influenced, or there is a wavelength region (photonic band gap) where the light wave cannot propagate depending on the case.

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

(Embodiment 1)
First, refer to FIG. The substrate 1 is made of an electro-optic material such as lithium tantalate (LiTaO ₃ ) single crystal or lithium niobate (LiNbO ₃ ) single crystal. The optical waveguide of this embodiment is formed on the substrate 1 by a ridge portion 2 formed by etching or the like.

  Further, grooves 3-1 arranged periodically as shown in the figure are formed on the back surface of the substrate 1 of the ridge-shaped optical waveguide to constitute a photonic crystal structure portion 4. Now, the photonic crystal structure will be described. The relationship between the wave number and the frequency of the light wave depends on the frequency of the light wave used, the refractive index of the substrate 1, and the size, period, and depth of the groove 3-1. It is possible to have unique characteristics such as A structure having such a unique characteristic is generally called a photonic crystal structure, and in this case, a one-dimensional photonic crystal structure is formed. Further, as shown in FIG. 2, there may be a wavelength region in which no light wave can exist, and this is called a photonic band gap 8. The slope of the curve in FIG. 2 corresponds to the group velocity of light waves. In a normal optical waveguide, the relationship between the wave number and the frequency is proportional, and is not a curve but a straight line (dotted line in the figure). Therefore, in a normal waveguide, the inclination is always constant, and the group velocity is also constant. However, in the case of the photonic crystal structure, the light velocity that passes through the photonic crystal structure portion is much smaller in the portion with the gentle inclination (for example, at the position of 9) than in the case without the photonic crystal structure. Become. What has been described here is a so-called one-dimensional photonic crystal structure whose refractive index changes periodically with respect to the light propagation direction. In this case, in the vicinity of the photonic band gap as shown in FIG. As shown in the figure, a region with a slow group velocity is obtained.

  Returning to the optical waveguide of this configuration in FIG. 1, since the light wave propagating through the optical waveguide portion 6 passes through the photonic crystal structure portion 4, the frequency of the passing light wave is set to the low group velocity state portion of FIG. Is set, the group velocity of the light wave is reduced by the principle of the photonic crystal as compared with the case where the groove 3-1 is not provided. Therefore, the waveguide having this structure operates as a waveguide having a small group velocity (having a slow wave structure).

  Here, in this ridge type optical waveguide, the confinement of the light wave in the vertical direction of the optical waveguide portion 6 is due to the refractive index difference between the upper and lower air, and the horizontal direction is the ridge portion 2 and the ridges on both sides thereof. It is confined by the effective refractive index difference of the light wave between the substrate portion and the non-substrate portion. Therefore, it is confirmed that the center of the distribution in the cross section of the guided light 5 does not fall within the ridge portion 2 but rather tends to enter the substrate 1. Since the center of the electric field distribution of the guided light 5 is located near the back surface of the substrate 1 rather than the top surface of the ridge portion 2, the groove 3-1 is formed on the substrate as shown in this configuration. By forming it on the back surface of 1, it is possible to exert a greater influence on the guided light 5 than when it is formed on the surface of the ridge portion 2. This is because when the groove 3-1 having the same shape is formed, the back surface of the substrate 1 can give a larger change in the effective refractive index to the guided light 5 than the surface of the ridge portion 2 as in the present configuration. Therefore, a greater reduction in county speed can be realized. In other words, in this configuration, a shallow groove is required to achieve the same decrease in the group speed, and the etching process can be simplified. In addition, since the formation of the ridge portion 2 and the formation of the groove 3-1 are performed on different substrate surfaces, higher processing accuracy can be realized than etching twice on the same surface.

  In general, processing by etching or the like of an electro-optic material such as lithium niobate is very difficult. Thus, a periodic structural change is provided in the optical waveguide, and the effective refractive index of the guided light 5 is periodically changed. In order to change the waveguide characteristics, the structure that changes the effective refractive index more effectively is very important.

  Actually, using a lithium niobate substrate, for example, the period 0.5λ of the groove 3-1 (λ is the wavelength of the light wave in the substrate: free space wavelength divided by the refractive index of the substrate), the groove 3-1. It is confirmed by calculation that there is a wavelength region in which the group velocity is small before and after the wavelength λ when the width of λ is 0.25λ. The group velocity can control the group velocity characteristics of the light wave propagating through the optical waveguide portion 6 by changing the width, period, and depth of the groove 3-1. Specifically, if the period of the groove 3-1 is in the range of 0.3λ to 0.7λ, the width is in the range of 0.1λ to 0.5λ, and the depth is in the range of 0.05λ or more, it is effective in reducing the group velocity, and effective in that range. You can adjust the group speed. The actual magnitude of the group velocity is determined by the frequency of the light wave and the width, depth, and period of the groove 3-1, but if the width and period are the same, the group velocity tends to decrease as the groove is deepened. It is in. With such a structure, if the group velocity is reduced to about 1/3, it can be matched with the high-frequency propagation velocity. Therefore, when used in a light modulation element using a traveling wave type modulation electrode, Since the interaction with the modulation signal works very effectively, it has a great effect.

  Further, if the width and period are the same, the groove speed is further deepened to further reduce the group velocity, and if the group velocity is reduced to 1/5 or less, the interaction effect between the light wave and the modulation signal becomes larger. Thus, the present invention is effective in a light modulation element using a relatively short electrode such as a resonator type electrode or a concentrated constant type electrode instead of a traveling wave type electrode. In particular, at a group speed of 1/100 or less, the interaction effect becomes extremely large, and the effect of the optical waveguide of this configuration is further exhibited. The modulation element can be realized. In particular, when a resonator-type electrode is used, the modulation efficiency is further improved by the resonance of the modulation signal in the electrode, so that there is an even greater effect by increasing the efficiency of the modulation element.

  The wavelength λ of the light wave to be used is not limited in order to exert the effect of the present invention. However, infrared light (1.55 μm or 1.3 μm, which is often used in the field of communication, etc.) Is very effective. In this case, the width of the groove 3-1 is in the range of about 0.1 to 1 μm, and it can be manufactured by the current microfabrication technology.

When the substrate 1 is formed of a material having an electro-optic effect such as LiNbO _3, the etching for forming the ridge portion 2 and the groove 3-1 is performed by fluorine-based gas plasma RIE (reactive ion etching) or ICP (reactive ion etching). Inductively coupled plasma). In the case of ICP, the substrate 1 can be etched at a rate of about 0.5 μm / min by using a highly reducing gas such as CF ₄ , BCl ₃ , C ₄ F ₈ . This method can realize a selection ratio of 1 with respect to a photosensitive resist for ultraviolet rays and electron beams. It should be noted that the fact that LiNbO ₃ can be etched by ICP is described in the 63rd Applied Physics Related Conference Lecture Collection 26a-D-20. It is sufficient that the height of the ridge portion 2 is approximately the same as the wavelength of the light wave or several times as high. On the other hand, the width dimension of the ridge portion 2 greatly affects the propagation characteristics of the light wave propagating in the waveguide. However, the single-mode waveguide is within the range of about 0.5 to 5 times the previous λ value. Since a waveguide is obtained, the effectiveness is high in the dimension range. In addition, the light wave can be propagated even in a width dimension larger than that, and the present invention is effective.

  In addition, as shown in FIG. 3, when the pits 3-2 having a circular shape or the like are formed on the back surface of the substrate 1 instead of the grooves, the group velocity of the guided light 5 can be reduced by the same principle. it can. In this case, a so-called two-dimensional photonic crystal structure having periodicity in the lateral direction is obtained. In this case, unlike the one-dimensional photonic crystal structure described above, a photonic band gap may or may not occur. In the configuration of the present invention, when a photonic band gap occurs, there is a low group velocity state portion 9 in which the group velocity decreases around the photonic band gap, as in the one-dimensional photonic crystal structure. Is set in the low group velocity state portion 9, the group velocity of the light wave is reduced by the principle of the photonic crystal as compared with the case without the pit 3-2. Furthermore, even when a photonic band gap does not occur as a two-dimensional photonic crystal, there is a frequency region that reduces the group velocity by appropriately selecting the frequency of the light wave and the diameter, period, and depth of the pits. It has been confirmed that the group velocity can be reduced as in the case where the band gap is generated. Therefore, the waveguide having this structure operates as a waveguide having a low group velocity (having a slow wave structure), and the effect described in the first embodiment can be similarly realized.

  Further, in this configuration, by forming the pit 3-2 in the optical waveguide 6 capable of propagating a plurality of modes, the propagation characteristics of each mode can be changed separately, and the degree of freedom in designing the waveguide is improved. In addition, if the propagation mode in which the group speed is more effectively reduced is selectively used, the group speed can be more effectively reduced. Further, when the pit 3-2 is used, higher processing accuracy than that of the groove 3-1 is required. However, since the etching area is small, the efficiency of the etching process can be improved.

  Actually, using a lithium niobate substrate, for example, the pit period is 0.43λ (λ is the wavelength of the light wave in the substrate: free space wavelength divided by the refractive index of the substrate), and the pit diameter is 0.17λ. In this case, it is confirmed by the calculation by the plane wave expansion method that the group velocity becomes small. The group velocity can control the group velocity characteristics of the light wave propagating through the optical waveguide portion 6 by changing the diameter, period, and depth of the pits 3-2. Specifically, if the pit period is in the range of 0.3λ to 0.7λ, the diameter is in the range of 0.1λ to 0.5λ, and the depth is in the range of 0.05λ or more, it is effective for reducing the group speed, and the group speed is effective in that range. Can be adjusted.

  In the above embodiment, the description has been given of the case where the groove 3-1 or the pit 3-2 is a cavity, but there is a difference between the refractive index of the substrate 1 in the groove 3-1 or the pit 3-2. The effect can be exhibited similarly. In this case, the larger the refractive index difference between the substrate 1 and the material in the groove 3-1 or the pit 3-2, the similar effect can be exhibited with a smaller number of grooves 3-1 or pits 3-2. Become. Specifically, the groove 3-1 or the pit 3-2 is filled with a material having a refractive index different from that of the substrate 1, or a normal optical waveguide manufacturing method such as diffusion of metal such as titanium or a proton exchange method. It is also effective to apply the above and periodically form a portion having a high refractive index. Actually, a favorable refractive index change can be created by using lithium niobate crystal for the substrate and thermally diffusing a 700 nm thick titanium pattern in an oxygen atmosphere at 1000 ° C. for 5 hours. In the proton exchange method, for example, a pattern of a chromium film is formed on the surface of a lithium tantalate substrate by removing only a portion where the refractive index is to be increased by the lift-off method, and the substrate is placed in benzoic acid at 240 ° C. for about 1 By soaking in time, the refractive index can be increased only at the desired location. In the proton exchange method, the refractive index change is limited only to the extraordinary refractive index, but there is an advantage that a large refractive index change can be obtained. In these cases, the amount of change in the refractive index is smaller than that of forming a groove by etching and making the inside hollow. However, since a processing process such as etching is not required, the process is simplified and the substrate surface is flat. This also has the advantage that the propagation loss of the light wave is reduced.

(Embodiment 2)
First, referring to FIG. The substrate 1 is made of an electro-optic material such as lithium tantalate (LiTaO ₃ ) single crystal or lithium niobate (LiNbO ₃ ) single crystal. The optical waveguide of this embodiment is constituted by a ridge portion 2 formed on a substrate 1 by a sputtering vapor deposition method and an etching method.

  Further, grooves 3-1 arranged periodically as shown in the figure are formed on the surface portion of the substrate 1 of the ridge-shaped optical waveguide, and constitute a photonic crystal structure portion 4. As shown in FIG. 4B, the fabrication sequence may be as follows. First, the groove 3-1 is formed on the substrate surface, and the ridge portion 2 is formed thereon. In this case, the groove 3-1 may be hollow, but when the refractive index of the ridge portion 2 is different from the refractive index of the substrate 1, the material constituting the ridge portion 2 is the groove 3-1 Even if it enters, the effect is demonstrated similarly. Also for this photonic crystal structure, as shown in FIG. 2, the frequency of the light wave used, the refractive index of the substrate 1, and the size, period, and depth of the groove 3-1, The relationship between the wave number and the frequency of the light wave can be given. Based on the same principle, the light wave passing through the photonic crystal structure part 4 has a very small group velocity compared to the case without the photonic crystal structure part 4. Become. Therefore, the waveguide having this structure operates as a waveguide having a low group velocity (having a slow wave structure), and the effect described in the first embodiment can be similarly realized.

  Here, also in this ridge type optical waveguide, the confinement of the light wave in the vertical direction of the optical waveguide portion 6 is due to the refractive index difference between the upper and lower air, and the horizontal direction is the ridge type optical waveguide. It is confined by the difference in effective refractive index of the light wave between the portion 2 and the substrate portion having no ridge on both sides thereof. Therefore, as described above, the center of the distribution in the cross section of the guided light 5 does not fall within the ridge portion 2 but tends to enter the substrate 1 as well. As shown in the configuration, when the groove 3-1 is formed in the boundary surface portion between the ridge portion 2 on the surface of the substrate 1 and the substrate 1, the groove 3-1 is formed at a position substantially coinciding with the center of the electric field of the guided light 5. Since it is formed, the waveguide light can be given a very large periodic refractive index change, resulting in a large periodic change in the effective refractive index. Therefore, it is possible to realize an effective decrease in the county speed. Compared with the configuration of the first embodiment, this configuration has a very large effect although the manufacturing process is complicated. From another point of view, this configuration requires a very shallow groove to achieve the same reduction in the group speed. This simplifies the etching process and pattern accuracy when forming the groove 3-1. Can be improved.

  In addition, as shown in FIG. 5, if a layer having a high refractive index is formed on the surface of the substrate 1 in advance by a proton exchange method or the like, the guided light is further applied to the surface portion of the substrate 1 where the grooves 3-1 exist. Because it concentrates, it is even more effective. In this case, since the light wave can be confined in the vertical direction by the high refractive index layer 7, the thickness of the substrate 1 is not limited in this configuration, and a sufficiently thick substrate can be used. It is.

  The high-refractive index layer 7 is not limited to such a proton exchange method. For example, even if a metal such as titanium is thermally diffused or another material having a high refractive index is formed by a sputter deposition method or the like. It is valid. Further, in this configuration, as in the first embodiment, it is effective to form pits instead of grooves.

(Embodiment 3)
First, referring to FIG. The substrate 1 is made of an electro-optic material such as lithium tantalate (LiTaO ₃ ) single crystal or lithium niobate (LiNbO ₃ ) single crystal. The optical waveguide of this embodiment is formed on the substrate 1 by a ridge portion 2 formed by etching or the like.

  Further, a high refractive index portion 3-3 arranged periodically as shown in the figure is formed on the surface of the ridge portion 2, and constitutes a photonic crystal structure portion 4. This photonic crystal structure portion 4 is formed in the same manner as described in the above embodiment, depending on the frequency of the light wave used, the refractive index of the substrate 1, and the size, period, and depth of the high refractive index portion 3-3. 2, the light wave passing through the photonic crystal structure portion 4 can be compared with the group velocity compared to the case without the photonic crystal structure portion 4 by the same principle. Becomes very small. Therefore, the waveguide having this structure operates as a waveguide having a low group velocity (having a slow wave structure), and the effect described in the first embodiment can be similarly realized. Actually, the method of forming the high refractive index portion 3-3 can be formed by the metal thermal diffusion method or the proton exchange method similar to the method described in the latter half of the first embodiment. Since the process is unnecessary, the process is simplified, and there is an advantage that the propagation loss of the light wave is reduced because the substrate surface remains flat.

  In the present configuration, the confinement of the light wave in the vertical direction of the guided light 5 is due to the refractive index difference between the high refractive index portion 3-3 and the air above and the substrate 1 below. Unlike the previous embodiment, it is confined by the refractive index difference between the ridge portion 2 and the air on both sides thereof. Therefore, a waveguide having strong lateral confinement and less loss due to bending can be realized, and the thickness of the substrate 1 is not limited, and a thick substrate can be used. Further, since the refractive index change of the high refractive index portion 3-3 and the surrounding refractive index is almost the periodic change of the effective refractive index of the guided light, a relatively large periodic change is brought about, and the effective group speed is reduced. A reduction can be realized. Further, even when a periodic refractive index change having a shape corresponding to the pit shown in FIG.

(Embodiment 4)
Please refer to FIG. An optical modulator configured using the optical waveguide described in the first to third embodiments will be described. The photonic crystal structure portion 5 of the waveguide described in the first embodiment is composed of modulation electrodes 10a and 10b made of a metal film formed by, for example, vacuum deposition and etching. Here, a vertical electric field is applied to the optical waveguide portion 6 by applying a voltage between the electrodes. Since the substrate 1 has an electro-optic effect, the refractive index of the optical waveguide portion 6 changes according to the electric field strength, causing a phase change in the light wave passing through the waveguide portion 6. Thereby, this element operates as an optical phase modulator. The electro-optic crystal represented by the lithium niobate crystal also has a large anisotropy in the electro-optic coefficient, and the largest electro-optic coefficient is γ ₃₃ , which is polarized in the c-axis direction of the crystal. Similarly, the refractive index received by the light wave is a coefficient that varies with the electric field in the c-axis direction. Therefore, in this configuration, the substrate 1 is a crystal substrate (z-cut lithium niobate crystal) cut so that the surface perpendicular to the c-axis is the surface, and light waves (TM) polarized in the vertical direction (TM The effectiveness becomes higher when the wave is used. Here, as described in the first embodiment, in this waveguide configuration, the group velocity of the guided light wave can be reduced, so that the time for receiving the refractive index change due to the electro-optic effect is also increased in inverse proportion to the time. As a result, the light modulation effect increases. Further, in order to expand the modulation frequency band of the light modulation element of FIG. 7, the modulation efficiency can be improved by using the modulation electrode 10 as a transmission line and propagating the modulation wave in the same direction as the light wave and modulating it. However, when such a traveling wave electrode is used for the modulation electrode 10 of FIG. 7, the group velocity of light is adjusted by adjusting the width, period, depth, etc. of the groove 3-1 of the photonic crystal structure portion 4. It is also possible to match the speed of the light and the modulated wave. As a result, a broadband and highly efficient optical modulator can be realized. In this configuration, when a voltage is applied between the modulation electrodes 10a and 10b, a strong electric field is generated in the optical waveguide portion 6 in the vertical direction, so that the substrate 1 (for example, z) in which a large refractive index change occurs with respect to the vertical electric field. -cut lithium niobate crystal) is effective.

(Embodiment 5)
The light modulation element of other embodiment is illustrated in FIG. The light modulation element of this embodiment also operates as an optical phase modulator. In this configuration, when a voltage is applied between the modulation electrodes 10a and 10b, a configuration in which a strong electric field is generated in the optical waveguide portion 6 in the lateral direction is used. Unlike the previous embodiment, in order to generate a large refractive index change with respect to the electric field in the transverse direction when using a normal electro-optic crystal, the c-axis is oriented laterally with respect to the waveguide. This configuration is very effective when the substrate 1 (for example, y-cut lithium niobate crystal) is used and the light wave (TE wave) polarized in the lateral direction is used.

(Embodiment 6)
Reference is now made to FIG. In this configuration, the optical modulator configuration described in the fourth or fifth embodiment is developed, and the Mach-Zehnder interferometer is configured by the optical waveguide portion 6. In this case, when a voltage is applied between the modulation electrodes 10a and 10b, electric fields in opposite directions are generated in the photonic crystal structure portions 5 of the two optical waveguide portions 6a and 6b, so that the optical waveguide portions 6a and 6b are connected to each other. The propagating light waves undergo phase changes opposite to each other, and when they are combined, they interfere with each other and are converted into light intensity changes corresponding to the amount of phase change. As a result, the optical modulator of this configuration can modulate the light intensity according to the voltage between the modulation electrodes. In this configuration, although there is a difference between phase modulation and intensity modulation with the optical modulator described in the fourth or fifth embodiment, high-efficiency optical modulation is possible in the same manner.

  In the fourth to sixth embodiments, only the mode in which the optical waveguide structure of the present invention described in the first to third embodiments is applied to an optical modulation element has been described. However, the optical waveguide structure of the present invention has low loss. Since it is possible to realize an optical waveguide with a small group velocity, it is not limited to this application, and a highly efficient nonlinear optical effect element, etc. is realized by increasing the interaction between the light wave and the substrate material by a small group velocity. Is also possible. Nonlinear optical effect elements can be expected to realize harmonic generation, wavelength conversion, and photoelectric conversion elements.

(Embodiment 7)
Next, an embodiment of a fiber radio system according to the present invention will be described with reference to FIG.

  The fiber radio system 50 of the present embodiment includes an optical modulator / demodulator 51 incorporating the light modulation element in the fourth to sixth 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. 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 an optical modulation signal that is faithful to the modulation signal and has little phase disturbance can be generated, it is suitable for high-speed information transmission and long-distance optical fiber communication.

(A) is sectional drawing of 1st Embodiment of the optical waveguide by this invention, (b) is a perspective view from a back surface Dispersion characteristic diagram of light wave in photonic crystal structure (A) is sectional drawing of 2nd Embodiment of the optical waveguide by this invention, (b) is a perspective view from a back surface (A) is sectional drawing of 3rd Embodiment of the optical waveguide by this invention, (b) is a perspective view Sectional drawing of 4th Embodiment of the optical waveguide by this invention (A) is sectional drawing of 3rd Embodiment of the optical waveguide by this invention, (b) is a perspective view (A) is a plan view of the first embodiment of the light modulation element according to the present invention, and (b) is a cross-sectional view taken along line A-A ′ thereof. (A) is a plan view of a second embodiment of the light modulation element according to the present invention, and (b) is a cross-sectional view taken along line A-A ′ thereof. (A) is a plan view of a third embodiment of the light modulation element according to the present invention, and (b) is a cross-sectional view taken along line A-A ′ thereof. The figure which shows embodiment of the communication system by this invention A perspective view showing a conventional example of a light modulation element

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2 Ridge part 3-1 Groove 3-2 Pit 3-3 High refractive index part 4 Photonic crystal structure part 5 Waveguide light 6 Optical waveguide part 6a Branched waveguide part 6b Branched waveguide part 6c Entrance side optical waveguide part 6d Exit side optical waveguide portion 7 High refractive index layer 8 Photonic band gap 9 Low group velocity state portion 10, 10a, 10b Modulation electrode 11 Signal source 12 Termination resistor 13 Gap portion 14a Branch point 14b Branch point

Claims (7)

An optical waveguide having a substrate made of a material having an electro-optic effect and a ridge structure on the surface of the substrate,
An optical waveguide further comprising a periodic structure for reducing the group velocity of light propagating through the optical waveguide,
An optical waveguide characterized in that the periodic structure comprises a plurality of grooves or circular pits formed on the back surface of the substrate. An optical waveguide having a substrate made of a material having an electro-optic effect and a ridge structure on the surface of the substrate,
An optical waveguide further comprising a periodic structure for reducing the group velocity of light propagating through the optical waveguide,
The periodic structure comprises a plurality of grooves or circular pits formed on the surface of the substrate, and the ridge structure is formed on the plurality of grooves or circular pits. Waveguide. An optical waveguide having a substrate made of a material having an electro-optic effect and a ridge structure on the surface of the substrate,
An optical waveguide further comprising a periodic structure for reducing the group velocity of light propagating through the optical waveguide,
The periodic structure comprises a plurality of high refractive index portions formed on the surface of the ridge structure portion,
The substrate is made of a lithium niobate or lithium tantalate material,
The optical waveguide, wherein the high refractive index portion is formed by a proton exchange method or a metal thermal diffusion method. 3. The optical waveguide according to claim 1, wherein the plurality of grooves or pits are hollow, or have a refractive index different from that of the substrate. 5. The periodic structure according to claim 1, wherein the width or period of the groove or pit is within a range in which a photonic band gap is not formed, and has a dimension that reduces a group velocity of light waves. Optical waveguide. The light modulation element according to any one of 1 to 5, further comprising a modulation electrode for applying an electro-optic light modulation electric field to the optical waveguide. A communication system comprising: the light modulation element according to claim 6; an optical fiber that transmits the modulated light output from the light modulation element; and means for providing an electric signal for modulation to the light modulation element.

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