JP2007212787A - Optical control element, optical switching unit, and optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical control element which has a core of an electro-optical effect crystal thin film and is small and operating on a low voltage changing its refractivity to use in an optical switching unit and an optical modulator, and to provide an optical switching unit and an optical modulator. <P>SOLUTION: The core formed by partly using an optical crystal thin film, having electro-optical effect is enclosed with a cladding. This clad is made of a dielectric materials, having a refractivity smaller than the core, and an optical waveguide is formed in a shape of a channel by using the core and the cladding. Furthermore, at least, a pair of electrodes are arranged, facing each other with the optical waveguide in between outside the cladding and a voltage is applied to the electrodes. Thus, an effective electrical field is produced inside the optical waveguide. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a light control element, a light switching unit, and a light modulator, and in particular, a light control element such as a light switch or a light intensity modulator applicable to various fields such as light transmission, light measurement, light memory, light writing, etc. The present invention relates to an optical switching unit and an optical modulator.

  Electrooptic crystals such as lithium niobate (LN) and lithium tantalate (LT) undergo a refractive index change proportional to the electric field formed in the crystal from the outside due to the so-called Pockels effect (primary electrooptic effect). Since the electro-optic effect of this crystal is a very high-speed response, it is applied to various light control elements such as an optical modulator and an optical switch for optical communication.

  In an optical modulator or the like, an optical waveguide type structure is generally used to modulate the refractive index of an optical crystal at high speed. Drive the optical control element at high speed by reducing the drive power and capacitance of the light control element by forming the optical waveguide structure by increasing the refractive index of the specific region of the electro-optic crystal and confining the light in the region where the refractive index is increased. Is possible.

As a general method for creating a waveguide structure, there are a method of diffusing Ti in a core portion and an ion exchange method for converting Li ions into protons. Also, a ridge shape is created on the surface of an optical crystal by etching. There is also a method of using an optical waveguide. Furthermore, as in the inventions disclosed in Patent Document 1 and Patent Document 2, a method of forming an electric field in the core region of the waveguide is proposed for the optical waveguide type optical modulation and switching element.
Japanese Patent No. 2996389 Japanese Patent Publication No. 7-050265

  However, conventionally, an optical waveguide structure has been created by providing a region having a slightly higher refractive index than the surroundings by Ti diffusion or the like in the vicinity of the surface of the bulk crystal. 12 to 15 are diagrams schematically showing a conventional optical waveguide. For example, as shown in FIGS. 12, 13, and 15, conventionally, electrodes are loaded on the surface of a substrate and a voltage is applied to form an electric field in the optical waveguide.

  Therefore, it is difficult to form an electric field completely in a vertical direction or a horizontal direction with respect to the core formation region of the optical waveguide, and there are problems such as an increase in operating voltage and an increase in electrode area. Conventionally, since the thickness of the electro-optic crystal can only be reduced to about 100 microns, as shown in FIG. 14, when a voltage is applied from above and below the substrate, the distance between the electrodes becomes 100 microns or more. It is difficult to modulate the refractive index at a low voltage.

  On the other hand, a method of forming a thin film from a bulk optical crystal has been proposed. For example, an LN thin film having a thickness of 1 micron or less is manufactured by using an optical crystal thin film forming method called an ion slice method. can do.

  The optical crystal has a relatively high refractive index. For example, the above-described LN has a refractive index of about 2.2. Therefore, the optical crystal thin film is channelized, that is, processed into a thin line shape and used for the core, and further, a low refractive index cladding material is formed around the optical waveguide, thereby producing an optical waveguide having a large refractive index difference between the core and the cladding. Can be formed.

  This allows light to be confined and propagated strongly in the optical waveguide, and since the optical waveguide can be bent with a minute curvature on the order of microns, compared to conventional waveguide-type light control elements, It is possible to dramatically reduce the element size.

  In view of such a problem, an object of the present invention is to provide a light control element, an optical switching unit, and an optical modulator in which electrodes are disposed to face an optical waveguide having an optical crystal thin film as a core.

  In order to achieve the above object, an invention according to claim 1 is an optical control element having an optical waveguide provided with a core and a clad, and an electrode for forming an electric field in a region where the core is disposed. Formed of a part of an optical crystal thin film having an electro-optic effect, and having a channel shape, and at least a pair of electrodes are arranged so as to face each other with the core sandwiched therebetween, and the clad is interposed between the core and the electrode It is formed.

  According to a second aspect of the present invention, in the light control element according to the first aspect, the clad is formed of a material different from that of the core, and the material of the clad is a dielectric having a refractive index lower than that of the core. To do.

  According to a third aspect of the present invention, in the light control element according to the first or second aspect, the optical waveguide is formed on the substrate, and the electrode sandwiches the core from the upper and lower portions of the core arrangement region. It is arranged.

  According to a fourth aspect of the present invention, in the light control element according to the first or second aspect, the optical waveguide is formed on the substrate, and the electrode sandwiches the core from the left part and the right part of the core arrangement region. It is arranged as follows.

  According to a fifth aspect of the present invention, in the light control element according to any one of the first to fourth aspects, the refractive index of the clad including at least two clads having different refractive indexes and sandwiching the core from the left and right is The refractive index of the clad sandwiching the core from above and below is lower.

  According to a sixth aspect of the present invention, in the light control element according to any one of the first to fifth aspects, the electrode has at least one of the pair of electrodes when the core is sandwiched between the pair of electrodes from the top and bottom or the left and right. The width or thickness of any one of the electrodes is equal to the width or thickness of the core.

  According to a seventh aspect of the present invention, in the light control device according to any one of the first to sixth aspects, the core is formed of lithium niobate or lithium tantalate.

  The invention according to claim 8 includes an optical waveguide having a Y-shaped branch path, and voltage applying means for forming an electric field by applying a voltage in the vicinity of the optical waveguide, and after being branched at the branch path An optical switching unit comprising the light control element according to any one of claims 1 to 7 in a part of the optical waveguide.

  The invention according to claim 9 includes a directional coupler including at least two or more optical waveguides, and voltage applying means for forming an electric field by applying a voltage in the vicinity of the optical waveguides. It is an optical switching unit provided with the light control element of any one of Claims 1-7 in a part.

  The invention according to claim 10 is an annular optical waveguide, a linear optical waveguide disposed close to the annular optical waveguide, and a voltage that forms an electric field by applying a voltage in the vicinity of the annular optical waveguide. An optical switching unit comprising an optical control element according to any one of claims 1 to 7 in a part of an annular optical waveguide.

  The invention according to claim 11 includes a Mach-Zehnder interferometer having at least two or more optical waveguides, and voltage applying means for applying an electric voltage in the vicinity of the optical waveguides to form an electric field. A part of the light control element according to any one of claims 1 to 7 is provided.

  As described above, according to the light control element, the optical switching unit, and the optical modulator of the present invention, the electrode can be disposed to face the optical waveguide having the optical crystal thin film as a core.

Hereinafter, the light control element, the optical switching unit, and the optical modulator of this embodiment will be described with reference to the drawings. In addition, this embodiment is not limited to what is described below, A various change is possible in the range which does not deviate from the meaning.
FIG. 1 is a cross-sectional view showing the configuration of the optical control element of this embodiment.

  As shown in FIG. 1, the optical control element of this embodiment includes a core 101, a clad 102, an upper electrode 103, and a lower electrode 104. The core 101 is an optical waveguide core made of an optical crystal thin film, and is surrounded by a clad 102. Further, the clad 102 is disposed so as to be sandwiched from above and below by the upper electrode 103 and the lower electrode 104.

  The core 101 is made of an optical crystal thin film having an electro-optic effect, and the refractive index changes when an electric field is formed in the core. The thickness of the optical crystal thin film constituting the core is designed so as to satisfy the condition that the optical waveguide operates in a single mode. Specifically, the thickness is about 1 micron or less.

  The electric field formed in the core 101 can be formed in a direction perpendicular to the optical waveguide by applying a voltage with a potential difference between the upper electrode 103 and the lower electrode 104. Further, as the electro-optic material, inorganic crystals such as lithium niobate (LN), lithium tantalate (LT), and KTP, and ceramics such as PZT and PLZT can be used.

  As the cladding 102 of the optical waveguide, a material having a lower refractive index than the core 101 is used to surround the core 101. As a result, a channel-shaped optical waveguide can be formed and light can be confined in the core 101 and in the vicinity thereof.

Here, the channel shape refers to an optical waveguide having a core surrounded by a clad material different from the core material. The material for forming the cladding is a dielectric material having a refractive index lower than that of the core. For example, a glass material such as SiO ₂ or SiON, a polymer, or the like is preferably used. Therefore, the amount of change in refractive index due to the electro-optic effect of the clad 102 of this embodiment is very small compared to the core 101.

In the channel-shaped optical waveguide structure according to the present embodiment, the refractive index difference between the core 101 and the clad 102 can be freely controlled by appropriately selecting the clad material. In the optical waveguide of the present embodiment, when the index of refractive index difference between the core 101 and the clad 102 is a relative refractive index difference Δ, the refractive index of the core is n ₁ , and the refractive index of the clad is n ₂ ,
Δ = (n ₁ ² −n ₂ ² ) / 2n ₂ ²
It is also possible to increase the relative refractive index difference Δ defined by (1) to about 40%.

  Moreover, although the cross-sectional shape of the core 101 is shown as a rectangle in FIG. 1, it goes without saying that a triangle, a trapezoid, a circle, an ellipse, and similar shapes are also included. In the present embodiment, the thinned optical crystal can be processed into a ridge shape to form an optical waveguide structure in which clads formed of a low refractive index material are added above and below the thin film. Thus, the channel-shaped optical waveguide in the present embodiment is a novel waveguide structure that is different from the conventional optical waveguide using an optical crystal material.

  For example, the refractive index in a specific region of the bulk optical crystal is slightly reduced by the method of diffusing impurities in the crystal or the method of exchanging part of the crystal ions with other ions as in the conventional example described above. In the optical waveguide formed by changing the material, the materials constituting the core and the clad are almost equal. Further, the relative refractive index difference of these conventional examples can be increased only to about 1% or less.

  In addition, the refractive index of both the core and the clad changes due to voltage application, but unlike the conventional example, the channel-shaped optical waveguide in this embodiment uses a dielectric material as the clad material around the core. Surrounding. Therefore, the refractive index difference of the optical waveguide can be increased, and the refractive index of the core greatly changes compared to the refractive index of the cladding due to the formation of the electric field.

  Also, the ridge-shaped optical waveguide as shown in FIG. 15 is processed on an optical crystal bulk substrate having a thickness of 100 microns or more so that only the light propagation region is relatively thick. The optical waveguide is made of a single material. On the other hand, the channel-shaped optical waveguide in this embodiment forms a core by an optical crystal thin film obtained by thinning an optical crystal to 1 micron or less, and the clad disposed above and below the core has a refractive index lower than that of the core material. This is an optical waveguide structure formed of a dielectric material. Therefore, the structure is completely different from the ridge-shaped optical waveguide.

  Further, according to the channel-shaped optical waveguide of the present embodiment, it is possible to confine the light strongly in the channel optical waveguide having a minute cross section by increasing the relative refractive index difference Δ, and the optical waveguide can be made minute. It becomes possible to bend with an appropriate curvature. In order to enable bending with a micron-order curvature, the value of Δ is preferably 5% or more, more preferably 15% or more.

  As described above, the relative refractive index difference Δ in the conventional example is 1% or less at most, and the micro bending on the micron order as described above cannot be performed. On the other hand, according to the channel-shaped optical waveguide in the present embodiment, Δ of 15% or more can be achieved by using a material having a low refractive index for the cladding material.

Further, in the present embodiment, the upper electrode 103 and the lower electrode 104 that form an electric field in the core 101 of the optical waveguide face each other so as to sandwich the core 101 and the clad 102 and are disposed in the vicinity of the optical waveguide. If the electrode interval between the upper electrode 103 and the lower electrode 104 is close, the applied voltage can be reduced. Therefore, it is preferable to reduce the distance between the electrodes, but the electromagnetic field distribution of the propagating light propagating in the optical waveguide is calculated, and the electrodes are formed apart from the core by a distance equal to or more than 10 ⁻² of the peak value of the distribution. ing. This makes it possible to form an electric field in the optical waveguide with almost no excessive loss for the light propagating through the optical waveguide.

Next, a specific configuration example of the light control element is shown.
For example, when LN is used as the material of the core 101, its refractive index is about 2.2. In order for the optical waveguide to operate in a single mode, the width and thickness of the core 101 are about 1 micron. The following is desirable. More specifically, assuming that the cross-sectional structure of the core 101 is a square and the surrounding cladding 102 is made of SiO ₂ having a refractive index of 1.45, the dimension of the core 101 is a square of about 340 nm × 340 nm. Thus, the optical waveguide operates in a single mode at a wavelength of 850 nm.

  In this case, the value of the relative refractive index difference Δ of the optical waveguide can be extremely increased to about 28%, and the optical power is strongly confined inside the core made of the electro-optic material. The electromagnetic field distribution confined in the optical waveguide is localized in a circle having a radius of about 1.5 microns from the center of the core, and the magnitude of the photoelectric magnetic field in the outer region is negligibly small.

  As described above, by arranging the electrode in the region where the photoelectric magnetic field does not exist, it is possible to form an electric field in the optical waveguide without giving a loss to the light propagating through the optical waveguide. In the example described above, the distance between the upper electrode 103 and the lower electrode 104 can be reduced to about 3 microns. Furthermore, since the electrode pair of the upper electrode 103 and the lower electrode 104 is opposed to each other so as to sandwich the core 101 from above and below, the direction of the electric field formed in the core region of the optical waveguide is compared with the conventional example. And can be stabilized. Therefore, even when a refractive index change equivalent to that of the conventional example is obtained, a refractive index change by applying a lower voltage is possible.

  As a material constituting the upper electrode 103 and the lower electrode 104, a transparent metal material such as ITO (Indium Tin Oxide) or IZO (Indium Zinc Oxide) may be used in addition to general metal thin films such as Cr, Au, and Al. it can. In addition, there is no problem even if the material which comprises the upper electrode 103 and the lower electrode 104 of this embodiment is the same material or a different material.

  Thus, according to the optical control element of the present embodiment, an effective electric field can be formed in the vertical or horizontal direction of the waveguide with respect to the optical waveguide whose core material is an electro-optic crystal. Therefore, it is possible to operate a light control element such as a small and high-speed optical modulator or optical switch with low power consumption.

  Next, FIG. 2 is a diagram schematically illustrating another example of the optical control element of the present embodiment.

  The optical control element shown in FIG. 2 includes a core 201, a clad 202, an upper electrode 203, a lower electrode 204, and a substrate 205. The core 201 and the clad 202 that encloses the core 201 are stacked on the substrate 205. Furthermore, the optical waveguide is disposed so as to be sandwiched between the upper electrode 203 and the lower electrode 204 from above and below. The lower electrode 204 is disposed between the clad 202 and the substrate 205.

  The thickness of the substrate 205 is about 100 to 1000 microns, and the material thereof may be an optical crystal that is the same material as the core 201 of the optical waveguide, or may be a different material such as a cheaper silicon substrate or quartz substrate. May be. In the present embodiment, in the channel optical waveguide having the core 201 made of an optical crystal, light is confined in a minute region in the channel optical waveguide, so that the distance between the upper electrode 203 and the lower electrode 204 can be shortened.

  Thus, an electric field is formed in the direction perpendicular to the substrate surface by the configuration shown in the present embodiment. For example, when an LN Z-cut plate is used, an effective electric field can be formed with respect to the core of the optical waveguide, which is effective for refractive index modulation of the optical waveguide. Light control elements such as modulators and optical switches can be operated with low power consumption.

  Next, FIG. 3 is a diagram showing a case where electrodes are arranged on the left and right in the optical control element of the present embodiment.

  The optical control element shown in FIG. 3 includes a core 301, a clad 302, a left electrode 303, a right electrode 304, and a substrate 305. The core 301 and the clad 302 that encloses the core 301 are stacked on the substrate 305. Further, a left electrode 303 and a right electrode 304 are provided on the left and right sides of the optical waveguide, and the optical waveguide is disposed between the left electrode 303 and the right electrode 304.

  The thickness of the substrate 305 is about 100 to 1000 microns, and the material thereof may be an optical crystal that is the same material as the core 301 of the optical waveguide, or may be a different material such as a cheaper silicon substrate or quartz substrate. May be. Here, the width of the clad 302 as the clad layer is wider than the width of the core 301 of the optical waveguide.

  Further, the width of the clad 302 may be set so that the optical power confined in the optical waveguide is sufficiently attenuated in the clad 302, that is, it is only required to be about 1 micron wider than the width of the core 301. . Further, a left electrode 303 and a right electrode 304 are provided so as to sandwich the clad 302 and the core 301 configured as described above from the left and right.

  By applying a voltage between the left electrode 303 and the right electrode 304, an electric field can be formed in a direction parallel to the surface of the substrate 305. The refractive index of the core 301 formed of the electro-optic material thin film changes when an electric field is formed. In the present embodiment, an electric field is formed in a direction parallel to the optical waveguide, which is effective for refractive index modulation when an LN X-cut plate is used, for example.

  Further, according to the present embodiment, the distance between the left electrode 303 and the right electrode 304 can be set to several microns. Furthermore, since the electric field is formed only near the core of the optical waveguide, the refractive index can be modulated by a low voltage. Therefore, according to the present embodiment, an electric field can be effectively formed in the core material made of an electro-optic crystal, and a light control element such as a small and high-speed optical modulator or optical switch can be provided. .

  Next, FIG. 4 is a diagram showing a case where the refractive index of the clad surrounding the core is made different between the upper and lower sides and the left and right sides in the optical control element of the present embodiment.

  The optical control element shown in FIG. 4 includes a core 401, claddings 402, 403, 404, and 405, an upper electrode 406, a lower electrode 407, and a substrate 408. The core 401 is surrounded by an upper cladding 402, a lower cladding 403, a left cladding 404, and a right cladding 405. The core 401 and the clads 402 to 405 that enclose the core 401 are stacked on the substrate 408. Further, an upper electrode 406 and a lower electrode 407 are provided in the vicinity of the optical waveguide, and are arranged so that the clad is sandwiched between the two electrodes.

  Here, each of the clads 402 to 405 has a refractive index of the left clad 404 and the right clad 405 arranged on the left and right of the core 401 so that the upper clad 402 and the lower clad 403 arranged on the upper and lower sides of the core 401 The clad is made of a different material so as to be smaller than the refractive index.

  In the configuration of FIG. 4A, the height of the upper clad 402 and the lower clad 403 is arranged to be equal to the height of the core 401. However, as shown in FIG. The upper clad 412 and the lower clad 413 sandwiched from above and below may have the same width as that of the core 411.

  In the optical waveguide, the radius of curvature that can bend without causing loss to propagating light is mainly determined by the difference between the refractive index of the clad disposed on the left and right of the core and the refractive index of the core. Therefore, by using a low refractive index material for the left cladding 404 and the right cladding 405 as in this embodiment, the radius of curvature can be reduced and the light control element can be miniaturized. .

Specifically, it is preferable to use a material such as air, low refractive index glass such as SiO ₂ , polymer, or the like as the material constituting the left cladding 404, 414 and the right cladding 405, 415. In the present embodiment, for example, when the cores 401 and 411 are LN and the clad is air, the value of Δ can be extremely high as about 40%. On the other hand, the materials of the upper clad 402 and 412 and the lower clad 403 and 413 disposed above and below the cores 401 and 411 are effective in consideration of adhesion to the substrate, the core material, and the electrode material. A material suitable for electric field formation can be selected.

  Here, in the configuration of FIG. 4, the widths of the upper electrodes 406 and 416 and the lower electrodes 407 and 417 are made wider than the widths of the cores 401 and 411 of the optical waveguide. Thereby, when a voltage is applied between the electrodes, an electric field can be formed in a direction perpendicular to the substrate.

  This is because the electric field is not formed in the direction completely perpendicular to the substrate near both ends of the electrode, but the direction of the formed electric field is perpendicular to the substrate surface near the center of the electrode. is there. Therefore, as in this embodiment, the electrode is made wider than the core width of the optical waveguide, and the optical waveguide is present only near the center of the electrode, so that a voltage is applied to the waveguide. In this case, the core region is not affected by the end of the electrode, that is, it is possible to form a uniform electric field in the vertical direction of the optical waveguide.

Next, a specific design example of the light control element will be described.
For example, assuming that the core 401 in FIG. 4A is LN, the upper clad 402 and the lower clad 403 are SiO ₂ , and the left clad 404 and the right clad 405 are air, the light wavelength is 850 nm. If the dimension is a square of about 360 nm × 360 nm, the optical waveguide operates in a single mode.

  The electromagnetic field of light propagating in the optical waveguide is confined within a circle having a radius of about 1 micron from the center of the core 401, and the photoelectric magnetic field is sufficiently attenuated in the outer region. Therefore, if the upper electrode 406 and the lower electrode 407 are arranged in the outer region, the electrode structure does not cause a loss in the propagation light. That is, it is sufficient if the distance between the electrodes is about 2 microns, and the distance between the electrodes can be reduced to 1/5 or less of the conventional structure. In addition, the electric field can be formed almost completely perpendicular to the optical waveguide, and more effective refractive index modulation is possible.

  Moreover, as a material which comprises the upper electrode 406 and the lower electrode 407, what is necessary is just a general metal material, for example, Cr, Au, Al, etc. are preferable. The substrate 408 serving as a support substrate can be selected in consideration of adhesion with an electrode material, and silicon, quartz, or the like is preferable. Of course, LN may be used as the support substrate.

  The optical waveguide having the above configuration can bend light without loss if the radius of curvature is 5 microns or more. By using it as a basic wiring element of the light control element, the element size of the light control element can be dramatically reduced. Miniaturization is possible.

  Heretofore, the electrode sandwiching the optical waveguide has been described as a configuration in which the upper electrode 406, 416 and the lower electrode 407, 417 are sandwiched from above and below. However, the present embodiment is not limited to this configuration. For example, as described with reference to FIG. 3, the electrodes may be disposed so as to be sandwiched from the left and right of the optical waveguide. Is obtained. In this case, an effective electric field is formed in a substantially horizontal direction with respect to the optical waveguide, and refractive index modulation can be performed at a low voltage.

  As described above, according to the present embodiment, an optical waveguide having an electro-optic crystal as a core can be bent in a minute region, and an electric field can be effectively formed on the waveguide material. It is possible to provide a light control element such as a small and high-speed optical modulator or optical switch.

  Next, FIG. 5 is a diagram showing a case where the widths of the core and the electrode are equal in the optical control element of the present embodiment.

  The optical control element shown in FIG. 5A includes a core 501, an upper clad 502, a lower clad 503, an upper electrode 504, a lower electrode 505, and a substrate 506. The core 501 is sandwiched between an upper clad 502 and a lower clad 503. The core 501, the upper clad 502 and the lower clad 503 are further sandwiched between the upper electrode 504 and the lower electrode 505.

  That is, in FIG. 5A, the width of the upper clad 502, the lower clad 503, the upper electrode 504, and the lower electrode 505 in the configuration shown in FIG. By applying a voltage between the upper electrode 504 and the lower electrode 505, an electric field is formed only in the vicinity of the core material made of an electro-optic crystal.

  Further, as shown in FIG. 5B, only the width of the upper electrode 514 may be equal to the width of the core 511 of the optical waveguide. Although it is described here that the widths are equal, it is not required to configure the waveguide core 511 and the upper electrode 514 so that the width is strictly equal. Due to problems in the manufacture of the device, a difference of about 10% is expected to appear between the electrode width and the core width, but the effect of this embodiment is not lost even if there is such a difference.

  Thus, according to the present embodiment, an effective electric field can be formed only in the vicinity of the core material, that is, only the desired optical waveguide can change the propagation characteristics by refractive index modulation. Therefore, it becomes possible to integrate optical waveguides whose propagation characteristics can be individually controlled with high density, and to provide light control elements such as small and high-speed optical modulators and optical switches. Become.

  In addition, although this embodiment mentioned above is effective with respect to all the optical waveguides which used the optical crystal which has an electrooptic effect as a core of an optical waveguide, as a more concrete material, for example, lithium niobate ( LN) and lithium tantalate (LT) are preferred.

  In forming a channel-shaped waveguide, it is necessary to make these materials, such as LN and LT, thinner to 1 micron or less. However, when forming a thin film by crystal growth, it is necessary to make the lattice constant of the substrate equal to the lattice constant of the thin film material, so a structure in which an electrode and a clad layer are sandwiched between the substrate and the optical crystal thin film layer Is difficult to form. In addition, the electro-optic constant of a thin film actually formed by growth may not be as large as the bulk. Therefore, it is preferable that the optical crystal thin film serving as the core is formed by thinning from a bulk crystal.

  As one of the methods for forming a thin film of 1 micron or less from a bulk optical crystal, an ion slice method can be applied. The ion slicing method is a method of forming a thin film from a bulk crystal by the following steps.

  First, hydrogen ions or helium ions having a small mass number are implanted into the optical crystal, and the implanted region is modified. As a result, the thermal expansion coefficient and the selectivity in wet etching can be made different from those of other crystal regions. Thereafter, for example, by heating the bulk crystal subjected to ion implantation, the flakes can be separated from the crystal on the bulk with the implanted region as a boundary.

  Further, the film thickness of the thin film obtained by such an ion slicing method is determined by the ion implantation depth from the surface of the bulk substrate, and this depth can be controlled by the ion implantation energy. Specifically, a lithium niobate thin film having a thickness of about 700 nm is obtained by injecting helium ions having an energy of 200 keV into a lithium niobate substrate. Thus, by controlling the implantation energy, it is possible to form an LN thin film having a film thickness of 1 micron or less with extremely high film thickness accuracy.

Here, a method for manufacturing the optical control element of this embodiment will be described.
FIG. 10 is a diagram schematically showing an optical control element for each process to be manufactured in the method for manufacturing an optical control element of the present embodiment. FIG. 11 is a flowchart showing a method for manufacturing the optical control element of this embodiment. The light control element of the present embodiment is manufactured by the following procedure.

  First, ions are implanted into the optical crystal substrate (step S101). On the other hand, a lower electrode layer and a lower cladding layer are formed on the support substrate (step S102). The optical crystal substrate thus formed after ion implantation is bonded to the support substrate on which the lower electrode layer and the lower cladding layer are formed (step S103).

  Next, the bonded substrate subjected to the bonding process is heated, and the optical crystal substrate is peeled off to form a thin film (step S104). Then, an upper clad layer and an upper electrode layer are formed on the thinned optical crystal thin film (step S105), and the electrode is patterned (step S106). Finally, the patterned electrode is masked to etch the core and the clad layer to form a channel waveguide (step S107). In this way, the optical control element of this embodiment is formed.

As the substrate material, lithium niobate (LN) or lithium tantalate (LT) which is the same material as the crystal to be thinned is preferable. During the heat treatment included in the thin film formation process by the ion slicing method, thermal expansion of the crystal occurs. By using the same material for the substrate material and the thinned crystal material, the influence of the thermal expansion of the crystal can be reduced. it can. Alternatively, a cheaper substrate such as silicon or quartz, which is a different material from the crystal to be thinned, may be used. Further, as a material for the clad formed between the substrate and the core and on the core, a glass material such as SiO ₂ or SiON having a refractive index of 2.1 or less, or a polymer material is suitable.

  In addition, film formation methods such as vacuum deposition, sputtering, CVD, and spin coating can be used for forming the electrode layer and the clad layer. For patterning the electrodes, a photolithography technique used in a normal semiconductor process can be used. Further, in etching, a dry etching process such as reactive ion etching (RIE) is effective.

  Thus, according to this embodiment, an optical control element structure as shown in FIG. 5B can be manufactured. It is of course possible to manufacture the light control element of the present embodiment other than the light control element shown in FIG. 5B by changing or expanding a part of the process steps.

  Next, FIG. 6 is a diagram illustrating a case where a Y-branch light control element is used as the optical control element of the present embodiment.

  As shown in FIG. 6, the optical control element of the present embodiment is a Y-branch type light control element, and includes an input waveguide 601, output waveguides 602 and 603, and refractive index modulation units 604 and 605. Yes. The refractive index modulators 604 and 605 are provided in part of the output waveguides 602 and 603, respectively. And it has the electric field formation means by this embodiment mentioned above, By forming an electric field with respect to the site | part of a desired optical waveguide, the material refractive index of the site | part can be changed.

  When no voltage is applied to any of the refractive index modulation units in the refractive index modulation units 604 and 605, the light input from the input waveguide 601 is divided into two equal parts of the optical power from the refractive index modulation units 604 and 605. Is output.

  On the other hand, for example, the refractive index of the waveguide core can be lowered by applying a voltage to the refractive index modulation unit 605 including electrodes to form an electric field. In this case, the optical power is output only from the output waveguide 602 having a relatively high refractive index of the core. That is, it is possible to provide a high-speed optical switching function to the light control element shown in FIG. 6 by switching the voltage application portion at high speed with the refractive index modulation units 604 and 605.

Next, FIG. 7 is a diagram schematically illustrating still another example of the optical control element of the present embodiment.
FIG. 7A is generally called a directional coupler, and includes a direct waveguide group including an input waveguide 701, 703, an output waveguide 702, 704, and an optical coupling waveguide 705, 706, and a refractive index. The modulation unit 707 is configured.

  The input waveguide 701 and the input waveguide 703, and the output waveguide 702 and the output waveguide 704 are separated from each other by a distance that does not cause optical coupling with each other, that is, a distance that is about 10 times the wavelength of light. . On the other hand, the optical coupling waveguide 705 and the optical coupling waveguide 706 are close to each other to the wavelength of light, and optical coupling occurs at this short distance (interval).

  For example, an optical signal input to the input waveguide 701 can be output from the output waveguide 704 via the optical coupling waveguides 705 and 706. The optical waveguide length of the optical coupling waveguides 705 and 706 at this time is called a beat length. This beat length depends on the change in the optical path length of the optical coupling waveguides 705 and 706. Therefore, by changing the refractive index in the waveguide material constituting the optical coupling portion, it is possible to control which of the output waveguide 702 and the output waveguide 704 outputs the optical signal. .

  When the core of the optical waveguide is composed of an electro-optic crystal thin film, the relative refractive index difference Δ of the optical waveguide can be increased, and the size of the light control element can be reduced. However, on the other hand, the distance between the optical waveguides in the coupling portion of the directional coupler is very small. Therefore, in the conventional electric field forming method, it has been difficult to independently provide effective refractive index modulation to each optical waveguide. However, according to the present embodiment, this problem can be solved.

  Further, since the refractive index modulation unit 707 includes the electric field forming unit of the present embodiment described above, for example, a configuration as shown in FIG. 7B is possible. FIG. 7B is a cross-sectional view of the optical waveguide at 708 in FIG.

  In FIG. 7B, the widths of the upper electrodes 717 and 719 and the lower electrodes 718 and 720 are substantially equal to the widths of the cores 711 and 712 of the optical waveguide. By configuring in this way, it is possible to individually form electric fields for the two optical waveguides. With the configuration shown in FIG. 7B, it is possible to form an electric field in a direction perpendicular to the substrate. Further, by reversing the direction of the applied voltage in the two optical waveguides, the refractive index It is also possible to reverse the sign of the change. Therefore, the effect of refractive index modulation can be further enhanced.

FIG. 7C shows an example of forming an electric field for the coupling portion of the directional coupler shown in FIG.
In FIG. 7C, an electrode 736 is provided between the core 731 and the core 732 of the optical waveguide. In order to prevent light absorption by the electrode during the transition of optical power at the optical coupling portion, the material constituting the electrode 736 is preferably transparent in the wavelength band to be used.

  With the configuration of FIG. 7C, it becomes possible to form an electric field in a direction parallel to the substrate. Furthermore, in the two optical waveguides, the direction of the applied voltage is reversed, thereby changing the refractive index. It is also possible to reverse the sign. Therefore, the effect of refractive index modulation can be further enhanced.

  As described above, according to this embodiment, it is possible to strongly confine optical power in the optical waveguide by using the electro-optic crystal thin film having a high refractive index for the core of the optical waveguide. Therefore, it is possible to reduce the size of the light control element, and to provide an ultra-high-speed small optical switching element by using high-speed refractive index modulation.

  Next, FIG. 8 is a diagram showing a case where a ring resonance type light control element is used as the optical control element of the present embodiment.

  The optical control element shown in FIG. 8 includes straight waveguide type optical waveguides 801, 802, 803, and 804, a ring type optical waveguide 805, and refractive index modulation units 806 and 807. The ring-shaped optical waveguide 805 is inserted between the linear waveguide optical waveguide 801-802 and the linear waveguide optical waveguide 803-804.

  The ring-shaped optical waveguide 805 functions as a wavelength selection element. Each of the linear optical waveguides 801, 802, 803, and 804 is responsible for transferring optical power to and from the ring optical waveguide 805. For example, when optical signals of various wavelengths are input from the linear optical waveguide 801, only the wavelength signal that resonates in the ring optical waveguide 805 is output to the linear optical waveguide 803 via the ring optical waveguide 805. On the other hand, optical signals of other wavelengths are output from the linear optical waveguide 802.

  The wavelength that resonates in the ring-shaped optical waveguide 805 depends on the optical path length of the ring-shaped optical waveguide 805. Therefore, the resonant wavelength can be changed by changing the refractive index of the material constituting the ring-shaped optical waveguide 805. It is.

  In the light control element of the present embodiment, the refractive index modulators 806 and 807 are provided in part of the ring-shaped optical waveguide 805. The refractive index modulators 806 and 807 have electric field forming means unique to the present embodiment described above. That is, the wavelength output from the linear optical waveguide 803 can be changed by effectively changing the refractive index only at a specific portion of the ring-shaped optical waveguide 805.

  Further, by forming the core material of the optical waveguide from an electro-optic crystal, it is possible to change the refractive index at an ultra high speed. Therefore, the light control element shown in FIG. 8 can function as a wavelength-selective high-speed optical switching element.

  Furthermore, if the core of the ring-shaped optical waveguide is an electro-optic crystal, the relative refractive index difference Δ can be increased to about 40%, and the waveguide can be bent sharply. Can be reduced to 10 microns or less. Therefore, the configuration of the present embodiment makes it possible to provide an ultra-high speed small optical switching element.

  Next, FIG. 9 is a diagram showing a case where a Mach-Zehnder interferometer type light control element is used as the optical control element of the present embodiment.

  The optical control element shown in FIG. 9 includes an input waveguide 901, an output waveguide 902, an optical branching unit 903, an optical coupling unit 904, optical waveguides 905 and 906 for phase modulation, and refractive index modulation units 907 and 908. And.

  The light control element as shown in FIG. 9 is generally called a Mach-Zehnder interferometer, and includes an optical branching unit 903 and an optical coupling unit 904 between an input waveguide 901 and an output waveguide 902. The optical power input to the input waveguide 901 is divided into two by the optical branching unit 903 and propagates through the phase modulation optical waveguides 905 and 906, respectively.

  In addition, a refractive index modulation unit configured by the light control element of the present embodiment described above is formed in both or one of the waveguides of the phase modulation optical waveguides 905 and 906. In FIG. 9, a refractive index modulation section 907 is formed in the phase modulation optical waveguide 905, and a refractive index modulation section 908 is formed in the phase modulation optical waveguide 906.

  When the refractive index modulation units 907 and 908 have the same structure, the light input through the input waveguide 901 is optically coupled by the optical coupling unit 904 even if it is branched by the optical branching unit 903. The phases of light are aligned and coupled. Therefore, the light input through the input waveguide 901 is output from the output waveguide 902 as it is.

  Here, for example, a voltage is applied to both or one of the refractive index modulators 907 and 908 to change the refractive index of the core in the optical waveguide. Then, at this time, the light propagated through the phase modulation optical waveguides 905 and 906 is multiplexed by the optical coupling unit 904 in a state of being out of phase due to a change in the refractive index of the core. If this shift is combined when the phase is shifted by π, the lights cancel each other and no light is output from the output waveguide 902.

  As in the present embodiment, in the refractive index adjusting unit configured by the electro-optic crystal waveguide, the intensity of the output light is changed at high speed by changing the refractive index of the core at high speed using the electro-optic effect. It is possible to provide an optical modulator that can be used. Here, the optical branching unit 903 and the optical coupling unit 904 are configured by a Y-branch optical waveguide, a directional coupler, a branching element using interference between higher-order modes, and the like.

  Further, by using an electro-optic crystal thin film having a high refractive index for the core of the optical waveguide, the optical power can be confined strongly in the optical waveguide having a minute cross section. In addition, the element size of the optical modulator can be reduced, and an electric field can be effectively formed in the core region of the optical modulator, so that the voltage required for optical modulation can be reduced. Therefore, the configuration of the present embodiment can provide a small and low power consumption optical modulator.

  As described above, according to the light control element, the optical switching unit, and the optical modulator of the present embodiment, an effective electric field is formed in the vertical or horizontal direction of the waveguide with respect to the optical waveguide using the electro-optic crystal as the core material. Therefore, it is possible to provide a light control element having an electric field forming means capable of modulating the refractive index with low voltage driving.

  Further, it is possible to provide an optical control element having an electric field forming means that enables an optical waveguide having an electro-optic crystal thin film as a core to be bent with a small curvature and enables refractive index modulation with low voltage driving. it can.

  In addition, the optical waveguide having an electro-optic crystal thin film as a core can effectively form an electric field only in the vicinity of the core region, and has an electric field forming means that enables refractive index modulation at a low voltage. A light control element can be provided.

  In addition, it is possible to provide a light control element that can be driven with a low voltage and that can perform path switching or light modulation at high speed.

  Furthermore, an effective electric field can be formed for an optical waveguide having an electro-optic crystal thin film as a core, and a minute light control element such as an optical modulator or a high-speed optical switch driven at a low voltage can be provided.

It is sectional drawing which shows the structure of the optical control element of this embodiment. It is a figure which shows typically another example of the optical control element of this embodiment. It is a figure which shows the case where the electrode is arrange | positioned on either side in the optical control element of this embodiment. In the optical control element of this embodiment, it is a figure which shows the case where the refractive index of the clad surrounding the circumference | surroundings of a core is varied with the upper and lower sides and right and left. In the optical control element of this embodiment, it is a figure which shows the case where the width | variety of a core and an electrode is made equal. It is a figure which shows the case where a Y branch type light control element is used for the optical control element of this embodiment. It is a figure which shows typically another example of the optical control element of this embodiment. It is a figure which shows the case where a ring resonance type light control element is used for the optical control element of this embodiment. It is a figure which shows the case where a Mach-Zehnder interferometer type light control element is used for the optical control element of this embodiment. It is a figure which shows typically the optical control element for every process manufactured in the manufacturing method of the optical control element of this embodiment. It is a flowchart which shows the manufacturing method of the optical control element of this embodiment. It is a figure which shows the structure of the conventional optical control element. It is a figure which shows the structure of the conventional optical control element. It is a figure which shows the structure of the conventional optical control element. It is a figure which shows the structure of the conventional optical control element.

Explanation of symbols

101, 201, 301, 401, 411, 501, 511, 711, 712, 731, 732 Core 102, 202, 302, 733, 734, Cladding 103, 406, 416, 504, 514, 717, 719 Upper electrode 104, 407, 417, 505, 515, 718, 720, lower electrode 205, 305, 408, 418, 506, 516, 721, 738, substrate 303 left electrode 304 right electrode 402, 412, 502, 512, 713, 715 , Upper clad 403, 413, 503, 513, 714, 716, lower clad 404, 414, left clad 405, 415, right clad 601, 701, 703, 901 input waveguide 602, 603, 702, 704, 902 Output waveguides 604, 605, 707, 06, 807, 907, 908 Refractive index modulator 705, 706, Optical coupling waveguide 735, 736, 737, 1205, 1206, 1303, 1304, 1403, 1404, 1503 Electrode 801, 802, 803, 804 Linear waveguide type Optical waveguide 805 Ring-shaped optical waveguide 903 Optical branching portion 904 Optical coupling portion 905, 906 Phase modulation optical waveguide 1201, 1202, 1301, 1401 Cores 1203, 1204, 1402, 1502 Buffer layers 1207, 1305 formed in the bulk crystal , 1405, 1504 Optical crystal 1501 Light propagation region

Claims (11)

An optical control element having an optical waveguide including a core and a clad, and an electrode for forming an electric field in an arrangement region of the core,
The core is formed of a part of an optical crystal thin film having an electro-optic effect, and has a channel shape,
The electrodes are arranged in at least a pair so as to face each other with the core interposed therebetween,
The light control element, wherein the clad is formed between the core and the electrode. The cladding is formed of a material different from the core;
2. The light control element according to claim 1, wherein the clad material is a dielectric having a lower refractive index than the core. The optical waveguide is formed on a substrate;
The light control element according to claim 1, wherein the electrode is disposed so as to sandwich the core from above and below the arrangement region of the core. The optical waveguide is formed on a substrate;
The light control element according to claim 1, wherein the electrode is arranged so as to sandwich the core from a left part and a right part of the arrangement region of the core. Comprising at least two or more claddings having different refractive indices;
5. The light control according to claim 1, wherein the refractive index of the cladding sandwiching the core from the left and right is lower than the refractive index of the cladding sandwiching the core from above and below. element.   The electrode has a width or thickness of at least one of the pair of electrodes equal to the width or thickness of the core when the core is sandwiched between the pair of electrodes from above and below or from the left and right. The light control device according to claim 1, wherein the light control device is a light control device.   The light control element according to claim 1, wherein the core is made of lithium niobate or lithium tantalate. An optical waveguide having a Y-shaped branch path, and voltage applying means for applying an electric voltage in the vicinity of the optical waveguide to form an electric field;
An optical switching unit comprising the light control element according to claim 1 in a part of the optical waveguide after being branched by the branch path. A directional coupler including at least two or more optical waveguides, and voltage applying means for applying an electric voltage in the vicinity of the optical waveguides to form an electric field,
An optical switching unit comprising the light control element according to claim 1 in a part of the optical waveguide. An annular optical waveguide, a linear optical waveguide disposed close to the annular optical waveguide, and a voltage applying means for forming an electric field by applying a voltage in the vicinity of the annular optical waveguide,
An optical switching unit comprising the light control element according to claim 1 in a part of the annular optical waveguide. A Mach-Zehnder interferometer having at least two or more optical waveguides, and voltage applying means for applying an electric voltage in the vicinity of the optical waveguides to form an electric field,
An optical modulator comprising the light control element according to claim 1 at a part of the optical waveguide.

Images