The present invention relates to a photonic crystal waveguide, and a directional coupler using the same and more particularly to a photonic crystal waveguide in a form of a photonic crystal micro optical circuit for optical communications.
It has been known in the art that the photonic crystal waveguide is used as an optical filter for selecting transmittable optical wavelength, an optical multiplexer/demultiplexer, and an optical dispersion-compensating device in the various fields of optical communication systems, optical switching systems and optical measuring systems. The directional coupler uses the photonic crystal waveguide.
In recent years, a photonic crystal has received a great deal of attention as a three-dimensional periodic structure having a refractive index in the same order as the optical wavelength. This photonic crystal has a potential capability of a remarkable size reduction of the optical circuit by three digits or more. For this reason, it has been on the great expectation to apply the photonic crystal to the micro-optical circuit. Various structures of the optical waveguide in the form of the micro-optical circuit formed in the photonic crystal have been proposed.
FIG. 1 is a fragmentary schematic perspective view illustrative of a first conventional optical waveguide in the photonic crystal. The optical waveguide comprises a line defect introduced in the photonic crystal which has a complete photonic band gap to a wavelength of the optical wave to be propagated through the optical waveguide. Namely, the line defect in the photonic crystal is used as the optical waveguide. This line defect optical waveguide has a high optical confinement function, for which reason the line defect optical waveguide is responsible to a abrupt or tight curve. Thus, the line defect optical waveguide provides a large freedom in pattern of the optical circuit and also allows a remarkable size reduction of the optical circuit.
The photonic crystal has a three dimensional periodical structure which comprises lamination structures of a bottom cladding layer 11 of a first material having a low refractive index, a core layer 12 of a second material having a high refractive index and a top cladding layer 13 of the first material, wherein the core layer 12 is sandwiched between the top and bottom cladding layers 11 and 13. The core layer 12 has a high refractive index, whilst the top and bottom cladding layers 11 and 13 have a low refractive index. The core layer 12 may be made of silicon. Each of the top and bottom cladding layers 11 and 13 may be made of silicon dioxide. The three dimensional periodical structure has a photonic band gap defined by forbidden bands against the propagation of a light having a specific wavelength. If a light is generated in the photonic crystal having the photonic band gap, then the light is confined in the photonic crystal, wherein the propagation of the light is inhibited. The complete photonic band gap inhibits the three dimensional propagation of light. If the line defect is introduced into the photonic crystal having the complete photonic band gap, the line defect permits the propagation of light along the line defect in the photonic crystal. The line defect serves as the waveguide in the photonic crystal. In FIG. 1, hexagons represent lattice structures of the crystal. The photonic crystal has lattice defects aligned in a direction along arrow marks, wherein the lattice defects are represented by the absences of the hexagons. The incident light 10 represented by the arrow mark is propagated through the line defect in the photonic crystal.
Japanese laid-open patent publication No. 11-218627 discloses the following second conventional technique for stabilizing properties of the photonic crystal waveguide and reducing the manufacturing cost. The second conventional photonic crystal waveguide has a dielectric slab waveguide over a surface of a silicon substrate, wherein the dielectric slab waveguide has a matrix array of a lattice array of refractive index varying regions which are different in refractive index from a core layer of the dielectric slab waveguide. The refractive index varying regions are made of the same material as the core layer, and have been subjected to a refractive index varying treatment due to an optical induced effect. The dielectric slab waveguide comprises laminations of a bottom cladding layer, the core layer and a top cladding layer. Those lamination structure may be formed over the substrate by a metal organic chemical vapor deposition method or a liquid phase epitaxy to complete the slab waveguide over the substrate. Subsequently, the core layer is subjected to a selective irradiation through the top cladding layer with any one of an electron beam, a synchrotron orbital radiation light, a ultraviolet ray, and an infrared ray in order to cause a variation in refractive index, due to the optical induced effect, of the irradiated parts of the core layer of the slab waveguide, whereby the refractive index varying regions are formed, which are different in refractive index from the remaining non-irradiated parts of a core layer of the dielectric slab waveguide.
The above first and second conventional photonic crystal waveguides have the following disadvantages. A sectioned area of the line defect waveguide is extremely small and it is difficult to obtain a sufficient optical coupling with any external optical system. The actually available method of forming the line defect waveguide has not yet been established.
In the above circumstances, it had been required to develop a novel photonic crystal waveguide free from the above problem.
Accordingly, it is an object of the present invention to provide a novel photonic crystal waveguide free from the above problems.
It is a further object of the present invention to provide a novel photonic crystal waveguide having a high optical coupling coefficient in coupling the photonic crystal waveguide to an external optical system.
It is a still further object of the present invention to provide a novel photonic crystal waveguide which is suitable for manufacturing the same.
It is yet a further object of the present invention to provide a novel directional coupler utilizing the novel photonic crystal waveguide free from the above problems.
It is a further object of the present invention to provide a novel directional coupler utilizing the novel photonic crystal waveguide having a high optical coupling coefficient in coupling the photonic crystal waveguide to an external optical system.
It is a still further object of the present invention to provide a novel directional coupler utilizing the novel photonic crystal waveguide which is suitable for manufacturing the same.
It is yet a further object of the present invention to provide a novel directional coupler utilizing the novel directional coupler utilizing the novel photonic crystal waveguide.
It is another object of the present invention to provide a novel method of use of a novel photonic crystal waveguide free from the above problems.
It is a further object of the present invention to provide a novel method of use of a novel photonic crystal waveguide having a high optical coupling coefficient in coupling the photonic crystal waveguide to an external optical system.
It is a still further object of the present invention to provide a novel method of use of a novel photonic crystal waveguide which is suitable for manufacturing the same.
It is another object of the present invention to provide a novel method of forming a novel photonic crystal waveguide free from the above problems.
It is a further object of the present invention to provide a novel method of forming a novel photonic crystal waveguide having a high optical coupling coefficient in coupling the photonic crystal waveguide to an external optical system.
It is a still further object of the present invention to provide a novel method of forming a novel photonic crystal waveguide which is suitable for manufacturing the same.
The first present invention provides a photonic crystal waveguide comprising: a substrate; a bottom cladding layer over the substrate; and a core layer over the bottom cladding layer, the core layer having a uniform distribution of holes, wherein the core layer has at least a waveguide region which is thicker than a remaining region of the core layer to cause a refractive index guide effect.
The second present invention provides a photonic crystal waveguide comprising: a substrate; a bottom cladding layer over the substrate; and a core layer with a uniform thickness over the bottom cladding layer, the core layer having a uniform distribution of holes; wherein the core layer has at least a waveguide region, on which a dielectric pattern is provided which has a refractive index higher than a substance in contact with a top surface of the core layer.
The third present invention provides a photonic crystal waveguide comprising: a substrate; a bottom cladding layer over the substrate; and a core layer with a uniform thickness over the bottom cladding layer, the core layer having a uniform distribution of holes wherein the core layer has at least a waveguide region, and the holes except on the waveguide region are filled with an air, whilst the holes on the waveguide regions are filled with a filler material having a refractive index higher than 1.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.