JP2010066557A - Optical waveguide-type rf optical converter, optical modulation element and optical filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical waveguide-type RF optical converter which is compact and can be manufactured at a low cost, and to provide an optical modulation element. <P>SOLUTION: The optical waveguide-type RF optical converter 1 includes a ring resonator 10 and a ferroelectric layer 13. The ring resonator 10 includes a ring-like optical waveguide 11 provided so as to be optically coupled to the optical waveguide. The ferroelectric layer 13 is provided so as to form a portion of the clad of the ring-like optical waveguide 11. When an electric field is applied from the outside to the ferroelectric layer 13, as the refractive index of the ferroelectric layer 13 is changed corresponding to the strength of the electric field, the effective refractive index of the ring-like optical waveguide 11 is changed as well. Since the filter characteristics of the ring resonator 10 are shifted to the short wavelength side or the long wavelength side by the change of the effective refractive index, the intensity of light is changed when the wavelength of light propagated through the optical waveguide-type RT optical converter 1 is in the shift range of the filter characteristics. In such a manner, RF signals are converted to light intensity signals. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical waveguide type RF optical converter that converts an RF (Radio Frequency) signal into an optical signal using an optical waveguide, an optical modulation element that modulates an optical signal by an RF signal, and a plurality of wavelengths of light. The present invention relates to an optical filter that selects light having a partial wavelength from the inside.

Conventionally, in the optical communication field and the like, an RF optical converter that converts an RF signal into an optical signal has been used (see Non-Patent Document 1). This RF optical converter is configured using a dielectric resonant antenna and a crystal (EO crystal) that produces an electro-optic (EO) effect. The dielectric resonant antenna is made of barium titanate or the like and has a cylindrical shape. The EO crystal is made of lithium niobate and constitutes a disk-type resonator. The EO crystal is disposed below the dielectric resonant antenna, and the refractive index changes according to the electric field generated by the RF signal received by the dielectric resonant antenna. The EO crystal is adjacent to a diamond prism through which light emitted from the laser light source is transmitted, and is optically coupled to the diamond prism. The RF optical converter modulates the intensity of light emitted from the laser light source in accordance with the RF signal received by the dielectric resonant antenna (in other words, converts the RF signal into a light intensity signal) (non-conversion). (See Patent Document 1).
Hsu et al., “All-dielectric photonic-assisted radio front-end technology”, nature photonics, 2007, volume 1, p. 535-538

  Since the EO crystal used in the RF optical converter is formed by polishing a bulk crystal of lithium niobate, it is difficult to integrate it with a waveguide that guides light emitted from the laser. It is. Further, the dielectric resonant antenna has a diameter of 11.25 mm and a height of 9 mm, and is large, so that it is difficult to integrate. For this reason, the RF optical converter using such an EO crystal and a dielectric resonant antenna has a large overall size and is restricted from being mounted on an application device. Further, since the EO crystal is made from a bulk crystal of lithium niobate, it is more expensive than, for example, a silicon substrate.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an optical waveguide type RF optical converter, an optical modulation element, and an optical filter that are small and can be manufactured at low cost. And

  An optical waveguide type RF optical converter of the present invention includes a ring resonator including a ring-shaped optical waveguide that is configured to include a core and a cladding and is optically coupled to the optical waveguide, and one of the claddings of the ring-shaped optical waveguide. And a dielectric layer whose refractive index changes according to the strength of the electric field applied from the outside. This optical waveguide type RF optical converter converts an intensity change of an external electric field (radio wave) into an intensity change of light passing through the optical waveguide.

  An optical modulation element of the present invention includes a ring resonator including an optical waveguide having a light incident end and a light output end, and a ring-shaped optical waveguide that includes a core and a clad and is provided so as to be optically coupled to the optical waveguide. And a dielectric layer that is provided so as to form a part of the clad of the ring-shaped optical waveguide and whose refractive index changes according to the strength of the electric field applied from the outside. This light modulation element modulates the intensity of light incident from the light incident end of the optical waveguide by an external electric field (radio wave) and emits the light from the light emitting end.

  An optical filter of the present invention includes an optical waveguide having a light incident end and a light emitting end, a ring resonator including a ring-shaped optical waveguide configured to include an optical waveguide and a core and a clad, and And a dielectric layer which is provided so as to form a part of the clad of the ring-shaped optical waveguide and whose refractive index changes according to the strength of the electric field applied from the outside. This optical filter selects light of a specific wavelength from among a plurality of wavelengths incident from the light incident end of the optical waveguide according to the electric field and emits the light from the light emitting end.

  In the optical waveguide type RF optical converter or optical modulation element of the present invention, the refractive index of the dielectric layer changes according to the external electric field generated by the RF signal, and as a result, the effective refractive index of the ring-shaped optical waveguide changes. Due to this change in the effective refractive index, the resonance condition of the ring resonator changes and the filter characteristics shift in the short wavelength direction or the long wavelength direction, so that the intensity of light propagating through the optical waveguide changes. Thereby, in the optical waveguide type RF optical converter of the present invention, the input RF signal is converted into a light intensity signal. In the light modulation element of the present invention, incident light having a constant intensity is intensity-modulated by an RF signal and output as emitted light.

  In the optical filter of the present invention, light of a plurality of wavelengths is incident from the light incident end of the optical waveguide, and the selected wavelength changes according to the strength of the external electric field due to the RF signal that changes with time from among the light of these wavelengths By utilizing this, light of a specific wavelength is selected, and the selected light is emitted from the light exit end. As a result, an optical filter is configured in which the wavelength of the output light changes from moment to moment.

  The optical waveguide type RF optical converter, optical modulation element or optical filter of the present invention preferably includes an antenna for concentrating an external electric field on the dielectric layer. The dielectric layer can be made of a ferroelectric material such as lithium niobate. The ring resonator and the dielectric layer can be formed by being integrated on a substrate such as a silicon substrate using a thin film process.

  According to the optical waveguide type RF optical converter, optical modulation element or optical filter of the present invention, since the dielectric layer is disposed so as to form a part of the cladding of the ring optical waveguide, the ring optical waveguide and the dielectric layer are arranged. Can be integrated and formed on a substrate such as a silicon substrate, which facilitates miniaturization. Furthermore, since the dielectric layer is formed as a thin film by a thin film process, the manufacturing process is easy and the amount of material used can be reduced, so that the material cost is reduced. Therefore, the manufacturing cost can be reduced.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a perspective configuration of an optical waveguide type RF optical converter according to an embodiment of the present invention. FIG. 2 shows a cross-sectional configuration taken along line AA in FIG.

  The optical waveguide type RF optical converter (hereinafter referred to as “RF optical converter”) 1 includes a ring resonator 10, a line-shaped optical waveguide 12, a ferroelectric layer 13, and an antenna 14.

  As shown in FIG. 1, the ring resonator 10 is a micro ring resonator in which a ring-shaped optical waveguide 11 through which light propagates is formed on a substrate. The ring-shaped optical waveguide 11 is mainly composed of a core 11a and a clad 11b surrounding the core 11a (FIG. 2), and its diameter is about several tens of μm. In the ring resonator 10, the resonance wavelength is determined mainly by the length of one circumference of the ring-shaped optical waveguide 11 and the effective refractive index of the ring-shaped optical waveguide 11. The ring-shaped optical waveguide 11 is optically coupled to the line-shaped optical waveguide 12 at the coupling portion 11c.

  The line-shaped optical waveguide 12 extends along the tangential direction of the ring-shaped optical waveguide 11 on a substrate 16 (FIG. 2) made of silicon or the like, for example, and overlaps the ring-shaped optical waveguide 11 at the coupling portion 11c. Located on the lower side. Light having a specific wavelength is incident on the light incident end 12i of the line-shaped optical waveguide 12, and light whose intensity is modulated by the ring resonator 10 is emitted from the light emitting end 12o. Here, the line-shaped optical waveguide 12 corresponds to a specific example of “optical waveguide” of the present invention.

  The ferroelectric layer 13 is a thin film provided so as to circumscribe the ring resonator 10. Specifically, the ferroelectric layer 13 covers the circumferential surface of the ring-shaped optical waveguide 11 thereon. The ferroelectric layer 13 is made of a material whose refractive index changes according to the intensity of an electric field applied from the outside, for example, a ferroelectric material such as lithium niobate single crystal. When the refractive index of the ferroelectric layer 13 changes according to the strength of the electric field, the effective refractive index of the ring-shaped optical waveguide 11 changes according to the change in the refractive index. The effect of the change in the refractive index of the ferroelectric layer 13 will be described in detail later. An adhesion improving film 15 described later is interposed between the ring-shaped optical waveguide 11 and the ferroelectric layer 13. The ferroelectric layer 13 and the adhesion improving film 15 constitute a part of the clad of the ring-shaped optical waveguide 11. Here, the ferroelectric layer 13 corresponds to a specific example of the “dielectric layer” of the present invention.

  The antenna 14 is provided on the ferroelectric layer 13. The antenna 14 concentrates an RF signal applied from the outside on a portion of the ferroelectric layer 13 located immediately above the ring-shaped optical waveguide 11. As the antenna 14, as shown in FIG. 1, a bow tie antenna or the like in which a pair of triangles are arranged to face each other in a plane so that their apex angles face each other can be used.

[Method for Manufacturing RF Optical Converter 1]
Such an RF light converter 1 can be manufactured as follows, for example. First, the line-shaped optical waveguide 12 including a core and a clad is formed on the substrate 16. Specifically, the line-shaped optical waveguide 12 is formed through a thin film forming process, a photolithography process, an etching process, and the like. Here, as an example, silicon can be used as the material for forming the core, and silicon oxide can be used as the material for forming the cladding.

  Next, the ring resonator 10 is formed on the line-shaped optical waveguide 12. Specifically, the ring resonator 10 is formed through a thin film formation process, a photolithography process, an etching process, and the like. In this step, a structure is formed in which the lower surface and side surfaces of the core 11a are surrounded by the clad 11b. As an example, silicon can be used as a material for forming the core 11a, and silicon oxide can be used as a material for forming the clad 11b.

  Next, an adhesion improving film 15 a is formed on the ring resonator 10. The adhesion improving film 15a is for improving the adhesion between the ferroelectric layer 13 disposed on the ring resonator 10 and the ring resonator 10 in the next step. For example, when the core 11 a of the ring resonator 10 is formed of silicon and the ferroelectric layer 13 is formed of lithium niobate, silicon oxide can be used as the adhesion improving film 15. This silicon oxide can be formed on the surfaces of the core 11a and the clad 11b by exposing the ring resonator 10 to the atmosphere, for example. As shown in FIG. 3, the adhesion improving film 15 a may be formed by depositing silicon oxide on the ring resonator 10.

  On the other hand, an adhesion improving film 15b is formed on a ferroelectric thin plate made of a ferroelectric material such as lithium niobate single crystal, and this is cut into a predetermined shape to obtain a dielectric thin plate piece. However, depending on the combination of materials of the ferroelectric layer 13 and the ring resonator 10, the adhesion improving film 15b may not be formed.

  Next, the substrate 16 on which the ring resonator 10 and the line-shaped optical waveguide 12 are formed and the dielectric thin plate piece on which the adhesion improving film 15b is formed are joined. At this time, the substrate 16 and the dielectric thin plate piece are bonded so that the adhesion improving film 15a formed on the ring resonator 10 and the adhesion improving film 15b formed on the dielectric thin plate piece face each other. Here, as described above, when the core 11a of the ring-shaped optical waveguide 11 is made of silicon, the clad 11b is made of silicon oxide, and the adhesion improving film 15 is made of silicon oxide, moisture is contained in the adhesion improving film 15. In addition, after the substrate 16 and the dielectric are stacked, it is preferable to apply pressure and heat to bond the substrate 16 and the dielectric.

  Thereby, the ferroelectric layer 13 is formed on the ring resonator 10. Since the ferroelectric layer 13 and the adhesion improving film 15 cover the core 11 a of the ring-shaped optical waveguide 11, they function as a part of the cladding of the ring-shaped optical waveguide 11. Note that the ferroelectric layer 13 may be directly bonded onto the ring resonator 10 without using the adhesion improving film 15.

  Next, the antenna 14 is formed on the ferroelectric layer 13. The distance between the antenna 14 and the ring-shaped optical waveguide 11 will be described later.

  Thereby, the RF light converter 1 is obtained.

[Distance between antenna 14 and ring-shaped optical waveguide 11]
FIG. 4 shows the relationship between the distance D (FIG. 2) from the ring-shaped optical waveguide 11 to the bow tie antenna 14 and the magnification M at which the bow tie antenna 14 increases the RF intensity. As shown in FIG. 4, as the distance D from the ring-shaped optical waveguide 11 to the antenna 14 becomes shorter, the magnification M at which the antenna 14 increases the RF intensity increases. For example, when the distance D is 0.1 mm, the strength of the electric field can be increased about 80 times compared to the case where no antenna is provided. Therefore, as the distance D between the ring-shaped optical waveguide 11 and the antenna 14 is shortened, a larger electric field concentration can be obtained by the antenna 14.

[Refractive index of ferroelectric layer 13]
FIG. 5 shows the relationship between the refractive index change Δn of the ferroelectric layer 13 and the effective refractive index change ΔNeff of the ring-shaped optical waveguide 11. Here, FIG. 5 shows a ring shape when the refractive index of the ferroelectric layer 13 changes when the width of the core 11a portion of the ring-shaped optical waveguide 11 is 1.0 μm and the height is 0.25 μm. The result of having obtained the change of the effective refractive index of the optical waveguide 11 by calculation is represented.

  When an RF signal is applied to the ferroelectric layer 13 from the outside, the refractive index of the ferroelectric layer 13 is changed by the electric field of the RF signal, and the effective refractive index of the ring-shaped optical waveguide 11 is also changed. The refractive index change Δn of the ferroelectric layer 13 and the effective refractive index change ΔNeff of the ring-shaped optical waveguide 11 are proportional to each other as shown in FIG.

  As a result, in the RF optical converter 1 of the present embodiment, the ring-shaped optical waveguide 11 in which light propagates due to the change in the refractive index of the ferroelectric layer 13 provided in the cladding 11b portion of the ring-shaped optical waveguide 11 is obtained. It can be seen that the effective refractive index changes indirectly. That is, the RF optical converter 1 according to the present embodiment does not directly change the refractive index of the EO crystal that couples with the light as in the prior art.

[Operation of RF optical converter 1]
Next, the operation of the RF optical converter 1 of the present embodiment will be described. FIG. 6 shows the concept of the filter characteristics of the ring resonator 10. FIG. 7 shows the relationship with the wavelength λ0 of the light when the filter characteristics of the ring resonator 10 are shifted. Here, in FIGS. 6 and 7, the horizontal axis represents the wavelength, and the vertical axis represents the attenuation. 6 and 7 show a part of the filter characteristics.

  As shown in FIG. 6, when the wavelength of light incident from the light incident end 12i and propagating through the line-shaped optical waveguide 12 is the wavelength λ1 in the pass band of the filter characteristics of the ring resonator 10, it remains as it is (original It propagates through the line-shaped optical waveguide 12 while maintaining the strength. Further, when the wavelength of the light incident from the incident end 12i and propagating through the line-shaped optical waveguide 12 is the wavelength λ2 within the transition region of the filter characteristics of the ring resonator 10, the light intensity is attenuated and the line-shaped optical waveguide 12 To propagate.

  On the other hand, when an RF signal is applied, the electric field concentrates on the ferroelectric layer 13 on the ring-shaped waveguide 11 by the antenna 14. Then, the refractive index of the ferroelectric layer 13 changes according to the strength of the electric field, and as a result, the effective refractive index of the optical waveguide also changes, so that the filter characteristics of the ring resonator 10 shift. As shown in FIG. 7, if the wavelength λ0 of the light incident on the line-shaped optical waveguide 12 is positioned in the transition region of the filter characteristics, the filter characteristics shift according to the strength of the electric field, so that the light The intensity of changes.

  That is, as shown in FIG. 7A, when the filter characteristic is shifted, the wavelength λ0 of the input light is located at a position where the attenuation is large in the transition region. The attenuation of the intensity of light incident and propagating through the line-shaped optical waveguide 12 is increased. Further, as shown in FIG. 7B, when the wavelength λ0 of light is located at a position where the attenuation is moderate in the transition region, the light enters from the light incident end 12i, and the line-shaped optical waveguide 12 is moderately attenuated. As shown in FIG. 7C, when the wavelength λ0 of the light is located at a position where the attenuation is small in the transition region, the light enters from the light incident end 12i, and the line-shaped optical waveguide 12 is The attenuation of the intensity of propagating light is reduced. As described above, since the light intensity is changed by shifting the filter characteristic, the RF signal is converted into the light intensity signal. In other words, the input light is intensity-modulated by the RF signal. 7A to 7C, the light modulated by the electric field is emitted from the light emitting end 12o of the line-shaped optical waveguide 12.

  As described above, in the present embodiment, the ring resonator 10 and the line-shaped optical waveguide 12 manufactured by performing the photolithography process and the etching process are provided on the substrate 16, and the ferroelectric material is provided on the ring resonator 10. Since the layers 13 are stacked, the layers 13 can be integrated. Further, since the antenna 14 is arranged on the ferroelectric layer 13 by patterning, the antenna 14 can be miniaturized and can be integrated on the substrate 16 together with the ring resonator 10 and the like. For this reason, the RF optical converter 1 can be reduced in size as a whole, and is not limited in terms of size when mounted on an application apparatus.

  In addition, the ring resonator 10 is formed by a thin film process, and the core 11a is formed using, for example, inexpensive silicon. Therefore, the ring resonator 10 can be manufactured at low cost.

  While the present invention has been described with reference to the embodiment, the present invention is not limited to the above-described embodiment, and various modifications can be made.

  For example, in the above embodiment, the case where the ferroelectric layer 13 is made of a lithium niobate single crystal or the like has been described. However, the ferroelectric layer 13 may be formed using a coating-type EO material. When the coating type EO material is used, the ferroelectric layer can be formed by using a spin coater or the like, so that the distance from the antenna 14 to the micro ring resonator 10 is shortened, and the ring-shaped light A large effective refractive index change in the waveguide 11 can be obtained.

  As a specific example of the coating type EO material, when an EO polymer manufactured by NTT-AT is used, after the substrate 16 is disposed in the spin coater, the EO polymer is applied at a rotational speed of 6000 rpm. The coating thickness is 0.2 μm. This thickness is sufficient for the ferroelectric layer 13, and as shown in FIG. 4, an electric field concentration of 200 times or more can be obtained by the bow tie antenna 14.

  In the above embodiment, the case where the ferroelectric layer 13 is provided on the entire upper surface of the ring resonator 10 has been described. However, the ferroelectric layer 13 may be provided only on the ring waveguide. Good. Further, the ferroelectric layer 13 may be provided only between the ring waveguide and the antenna 14. Further, the ferroelectric layer 13 may be provided only on the lower side of the ring waveguide, and may be provided on the upper side and the lower side of the ring waveguide.

  Moreover, although the case where one antenna 14 is provided has been described in the above embodiment, a plurality of antennas 14 may be provided at positions overlapping the ring waveguide. Further, when an RF signal having a sufficient intensity that can change the refractive index of the ferroelectric layer 13 is applied, the antenna 14 may not be provided.

  In the above embodiment, the case where light having a single wavelength is propagated to the line-shaped optical waveguide 12 has been described. However, light having a plurality of wavelengths may be propagated to the line-shaped optical waveguide 12. In this case, application to an optical filter is also possible. For example, light having a plurality of wavelengths is incident from the light incident end of the optical waveguide, light having a specific wavelength is selected from the light having the plurality of wavelengths by an electric field, and light having the selected wavelength is emitted from the light emitting end. It is possible to consider a configuration of When the external electric field is based on an RF signal, the selected wavelength changes according to the strength of the electric field that changes with time. As a result, the optical filter in which the wavelength of the output light changes every moment. It is also possible to configure.

  In the above embodiment, the case where the present invention is applied to an optical waveguide type RF optical converter that converts an input RF signal into a light intensity signal and outputs the signal is described. However, the present invention is used as a light modulation element. Is also possible. Since the configuration in this case is the same as that of the optical waveguide type RF optical converter 1, detailed description thereof is omitted. When used as a light modulation element, carrier light having a constant amplitude may be input, the input light may be modulated according to the RF signal, and output as a light intensity signal.

1 is a perspective view of an optical waveguide type RF optical converter according to an embodiment of the present invention. It is sectional drawing in the AA of FIG. It is sectional drawing in the same position as FIG. 2 when the structure of an adhesive improvement film | membrane is changed. It is a figure showing the relationship between the distance D from a ring-shaped optical waveguide to a bow-tie antenna, and the magnification M which a bow-tie antenna strengthens the intensity | strength of RF. It is a figure showing the relationship between refractive index change (DELTA) n of a ferroelectric layer, and effective refractive index change (DELTA) Neff of a ring-shaped optical waveguide. It is a figure showing the concept of the filter characteristic of a ring resonator. This represents the relationship with the wavelength λ0 of light when the filter characteristics of the ring resonator are shifted.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Optical waveguide type | mold RF optical converter, 10 ... Ring resonator, 11 ... Ring-shaped optical waveguide, 11a ... Core, 11b ... Cladding, 12 ... Line-shaped optical waveguide, 13 ... Ferroelectric layer, 14 ... Antenna, 16 …substrate.

Claims (7)

A ring resonator including a ring-shaped optical waveguide configured to include a core and a clad and to be optically coupled to the optical waveguide;
A dielectric layer that is provided so as to form part of the cladding of the ring-shaped optical waveguide, and whose refractive index changes according to the intensity of an electric field applied from the outside,
An optical waveguide type RF optical converter for converting the intensity change of the electric field into an intensity change of light passing through the optical waveguide. The optical waveguide type RF optical converter according to claim 1, wherein the amount of light passing through the optical waveguide changes according to a change amount of a refractive index of the dielectric layer. The optical waveguide type RF optical converter according to claim 1, further comprising an antenna that concentrates the electric field on the dielectric layer. The optical waveguide type RF optical converter according to claim 1, wherein the dielectric layer is made of a ferroelectric material. The optical waveguide type RF optical converter according to claim 1, wherein the ring resonator and the dielectric layer are integrated on a substrate. An optical waveguide having a light incident end and a light exit end;
A ring resonator including a ring-shaped optical waveguide configured to include a core and a clad and provided to optically couple to the optical waveguide;
A dielectric layer that is provided so as to form part of the cladding of the ring-shaped optical waveguide, and whose refractive index changes according to the intensity of an electric field applied from the outside,
A light modulation element that modulates the intensity of light incident from the light incident end of the optical waveguide by the electric field and emits the light from the light emitting end. An optical waveguide having a light incident end and a light exit end;
A ring resonator including a ring-shaped optical waveguide configured to include a core and a clad and provided to optically couple to the optical waveguide;
A dielectric layer that is provided so as to form part of the cladding of the ring-shaped optical waveguide, and whose refractive index changes according to the intensity of an electric field applied from the outside,
An optical filter that selects light of a part of wavelengths from among a plurality of wavelengths incident from a light incident end of the optical waveguide according to the electric field, and emits the light from the light emitting end.

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