JP2004101740A - Photonic crystal waveguide - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photonic crystal waveguide having structure capable of realizing highly efficient connection with a conventional optical waveguide. <P>SOLUTION: In the photonic crystal waveguide having linear defects obtained by linearly removing a part of lattices of slab type two-dimensional photonic crystal in a waveguide direction, the shape or arrangement of a structure on the lattice points arranged on both sides between which linear defects in a prescribed range from the end of the photonic crystal waveguide in the waveguide direction is held is made different from the shape or arrangement of a structure formed on other lattice points so that influence exerted by crystal lattices on propagation light made incident from the end of the photonic crystal waveguide is gradually changed in accordance with the approach of the propagation light. For instance, width between sidewalls on the linear defect center side of the structure on lattice points between both the sides holding the linear defects between them is gradually narrowed in accordance with the increase of distances from the end of the structure. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a photonic crystal waveguide that can be used for a basic structure and an optical component of various optical devices such as a laser and an optical integrated circuit used for optical information processing and optical transmission.
[0002]
[Prior art]
Since current optical devices confine light using a difference in refractive index, the confined region of light cannot be reduced, so that the element cannot be configured small. Furthermore, if a steeply bent waveguide is formed to increase the degree of integration of the element, scattering loss occurs. Therefore, the optical circuit cannot be miniaturized and integrated, and its size is much larger than that of an electronic device. Therefore, a photonic crystal that can confine light with a concept completely different from the conventional one is expected as a new light material that can solve the above-described problems.
[0003]
A photonic crystal is an artificial multidimensional periodic structure in which two or more media having different refractive indices form a periodicity comparable to the wavelength of light, and a light band structure similar to an electron band structure. Having. Therefore, a light forbidden band (photonic band gap) appears in the specific structure.
[0004]
It has been theoretically pointed out that introducing a line defect that disturbs periodicity into a photonic crystal having a photonic band gap can form an optical waveguide in which a waveguide mode is formed within the frequency range of the band gap and completely confine light. (For example, see Non-Patent Document 1). J. D. Joannopoulos et al. Disclose a line defect in which a single column is not arranged in a two-dimensional photonic crystal in which a cylinder having a radius a / 5 having a large refractive index similar to that of a semiconductor is arranged on a square lattice having a lattice constant a of about the wavelength of light. It was theoretically shown that it is possible to construct an optical waveguide in which scattering loss does not occur in principle even when bent at a sharp angle. Such a waveguide can be a very important waveguide in forming a micro optical integrated circuit.
[0005]
Further, Japanese Patent Application No. 2001-394499 describes a slab type two-dimensional photonic crystal waveguide having a single mode with low loss and large group velocity.
[0006]
[Non-patent document 1]
J. D. Joannopoulos, P .; R. Villeneuve, and S.M. Fan, "Photonic Crystal: Putting a New Twist on Light", Nature 386, 143 (1997).
[0007]
[Problems to be solved by the invention]
However, the characteristics of the mode of light propagating through a photonic crystal waveguide having a line defect introduced therein are significantly different from those of a conventional optical waveguide, and therefore, it is very difficult to connect to a conventional optical waveguide. This means that a large light loss occurs when an optical circuit using a photonic crystal is connected to an external network or when a photonic crystal functional device is incorporated into another optical circuit, which is a very serious problem.
[0008]
The present invention has been made in view of the above points, and solves the above-described problems in an optical waveguide using a slab type two-dimensional photonic crystal, and realizes a highly efficient connection with a conventional optical waveguide. It is an object of the present invention to provide a photonic crystal waveguide having:
[0009]
[Means for Solving the Problems]
In order to achieve the above object, a photonic crystal waveguide of the present invention has the following structure.
[0010]
That is, the present invention is a photonic crystal waveguide having a line defect in which a part of a lattice of a slab type two-dimensional photonic crystal is linearly removed in a waveguide direction or a part of the lattice is deformed or moved. In order to gradually change the influence of the crystal lattice on the propagation light incident from the end of the photonic crystal waveguide as the propagation light enters, a line defect in a predetermined range in the waveguide direction from the end is sandwiched. The shape or arrangement of the structures on the grid points on both sides is different from the shape or arrangement of the structures on the other grid points.
[0011]
In the above photonic crystal waveguide, the width between the side walls on the center side of the line defect of the structure on the lattice points on both sides sandwiching the line defect becomes narrower as the distance from the end of the structure increases. be able to.
Further, when connecting the photonic crystal waveguide and a conventional optical waveguide, the width of the conventional optical waveguide is the width of the structure on the lattice points on both sides sandwiching the line defect in the predetermined range. The width is made wider than the width between the side walls on the center side of the line defect.
According to the present invention, the influence of the crystal lattice on the propagating light can be gradually increased as the propagating light enters, so that the conventional optical waveguide and the photonic crystal waveguide having greatly different modes have different properties. It is possible to reduce the optical loss at the connection portion of the optical circuit, to connect an optical circuit using a photonic crystal to an external network, and to incorporate a photonic crystal functional device into another optical circuit.
[0012]
Further, the lattice spacing on both sides of the line defect of the photonic crystal waveguide is set to 0.5 of the lattice spacing on both sides of the defect portion in the ordinary defect structure, except for one row of the lattice of the slab type two-dimensional photonic crystal. The single mode condition can be satisfied by setting any of the values from the double to 1.6 times.
[0013]
The shape of each structure on the grid points on both sides of the line defect in the predetermined range may be a shape obtained by cutting a part of a cylinder by a plane parallel to the center axis thereof.
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 shows a structural diagram of a coupling system of a simple single-line defect type photonic crystal waveguide and a conventional linear waveguide (Si fine-wire waveguide) according to an embodiment of the present invention. In the example shown in FIG. 1, a single-mode Si fine-wire waveguide having a width of 0.74 μm is joined to a photonic crystal waveguide having a lattice constant of 0.39 μm constituted by single-row line defects.
[0014]
Note that the photonic crystal waveguide in the embodiment of the present invention is a slab type two-dimensional photonic crystal waveguide. A slab-type two-dimensional photonic crystal is defined as a cylindrical or polygonal low refractive index column (for example, an air hole having a lower refractive index than a dielectric thin film slab) in a dielectric thin film slab. Are sometimes referred to as a structure) at an appropriate two-dimensional periodic interval, and the upper and lower portions of the dielectric thin film slab are sandwiched between an upper cladding layer and a lower cladding layer having a lower refractive index than the dielectric thin film slab. Nick crystal.
[0015]
The photonic crystal waveguide according to an embodiment of the present invention has a shape of a hole on both sides of a line defect near a connection portion in order to smoothly perform mode conversion from a conventional linear waveguide to a photonic crystal waveguide. Is a shape obtained by cutting a circle by a straight line (a shape obtained by cutting a cylinder by a plane parallel to its central axis), and the area to be cut becomes smaller as it goes deeper into the photonic crystal waveguide.
[0016]
Next, the reason why the structure of the present invention as shown in FIG. 1 can reduce the optical loss in the connection between the conventional optical waveguide and the photonic crystal optical waveguide of the present invention will be described with reference to FIG. .
[0017]
FIG. 2 is a diagram illustrating a dispersion curve of a conventional typical single-line defect type photonic crystal waveguide. In the case of the propagation constant / frequency region shown in FIG. 2, the dispersion curve of the Si wire waveguide is almost straight, whereas the dispersion curve for the photonic crystal waveguide is shown by a complicated curve. The most unique point of the photonic crystal waveguide is that the dispersion curve falls within a normalized frequency (normalized propagation constant) k = 0 to 0.5 as shown in FIG. This is because in the photonic crystal waveguide, the lattice points of the crystal are periodically arranged in the direction in which the light propagates, and the light is subjected to Bragg reflection at k = 0.5. The dispersion curve has a more special shape due to the periodicity of the photonic crystal in a two-dimensional plane.
[0018]
Considering the coupling between such a typical single-row line defect type photonic crystal waveguide and a conventional Si fine line waveguide, the following problems occur.
[0019]
Paying attention to the waveguide mode shown by the solid line in FIG. 2, a linear region asymptotic to the dashed line shown in the figure and a region that bends greatly and moves away from the dashed line (circled portion in FIG. 2) Can be classified. In the linear region, the mode field pattern and the optical characteristics are relatively similar to those of the Si wire waveguide, and if this region is used, the coupling efficiency between the two waveguides is high.
[0020]
However, since the photonic crystal waveguide exhibits leaky characteristics on the higher frequency side than the light line of the cladding, this region cannot be used. On the other hand, in a region on the lower frequency side than the light line that can be used as a photonic crystal waveguide (the portion indicated by a circle in FIG. 2), the dispersion curve of the waveguide mode of the photonic crystal waveguide is largely bent, and Since the field pattern and the optical characteristics are significantly different from those of the Si wire waveguide, a high coupling efficiency cannot be obtained even if the two waveguides are directly coupled. Further, the waveguide mode indicated by the dotted line is not suitable for application to an optical integrated circuit because two modes exist in a frequency region where the waveguide mode exists, that is, a single mode condition is not satisfied.
[0021]
One solution is to shift the light line of the clad to a higher frequency side by making the clad layer in the photonic crystal waveguide air, so that the region where the dispersion curve of the photonic crystal waveguide is close to a straight line is lower than the light line. There is a method used on the frequency side. However, in this method, it is necessary to float a Si thin wire or a photonic crystal having a thickness of a few microns in a comma, and there is a problem that strength cannot be secured.
[0022]
On the other hand, according to the structure of the present invention as shown in FIG. 1, when light is incident on the photonic crystal waveguide from the conventional linear waveguide, the periodicity of the crystal, which is a factor that greatly changes the mode properties, is propagated. By placing a region that gradually increases the effect on light, the sudden mode conversion that occurs when light is incident is reduced, reflection, scattering, and leakage at the coupling part of the waveguide are suppressed, and light loss is reduced. Therefore, the coupling efficiency of the two waveguides can be significantly improved in the region of ○ in FIG. 2 where the photonic crystal functions.
[0023]
Next, FIG. 3 shows light transmission characteristics when a conventional Si fine-wire waveguide and the simple single-row defect photonic crystal waveguide of the present invention are coupled.
[0024]
The solid line in FIG. 3 shows the light transmission characteristics when using the photonic crystal waveguide of the present invention, and the dotted line shows the light transmission characteristics when using the conventional simple single line defect type photonic crystal waveguide. Show. In the example shown in FIG. 3, the photonic crystal waveguide of the present invention has a conversion region for 10 holes in the waveguide direction near the junction with the Si wire waveguide.
[0025]
As described above, it is difficult to obtain a high coupling efficiency in a low frequency region (shaded portion in FIG. 3) where the photonic crystal waveguide functions by the conventional direct junction between the photonic crystal waveguide and the Si wire waveguide. , Low transmittance. On the other hand, the transmittance characteristic of the coupling system according to the embodiment of the present invention in which the conversion region is provided is significantly improved in this region (the hatched portion in FIG. 3). This result indicates that the introduction of the mode conversion mechanism of the present invention makes it possible to efficiently connect a conventional optical waveguide and a photonic crystal waveguide having greatly different mode properties to each other.
[0026]
Next, another embodiment using this mechanism will be described with reference to FIG. FIG. 4 is a diagram showing a structure diagram of a coupling system of the Si fine wire waveguide and the width-variable photonic crystal waveguide in the present embodiment.
[0027]
In this embodiment, the width of a single-mode Si thin-wire waveguide having a width of 0.4 μm and the entire crystal (lattice constant a = 0.39 μm) sandwiching the single-line defect are shifted to reduce the width of the simple single-line waveguide. A photonic crystal waveguide narrowed to 0.7 times the width of the defect structure (referred to as a variable-width photonic crystal waveguide) is joined. In order to smoothly perform mode conversion from a single-mode Si fine-wire type waveguide to a photonic crystal waveguide, the waveguide width is narrowed down from 0.4 μm to 0.35 μm in a 25 μm length in the region of the Si fine wire. In the area of the nick crystal waveguide, the shape of the holes on both sides of the line defect is set to a shape obtained by cutting a circle by a straight line (a shape obtained by cutting a cylinder vertically by a plane). The cutting area is reduced.
[0028]
FIG. 5 shows a dispersion curve of the width-variable photonic crystal waveguide shown in FIG. As shown by the circles in FIG. 5, this photonic crystal waveguide has a wide transmission band (a large slope of the dispersion curve) in a region on the lower frequency side than the light line of the cladding layer, and the dispersion curve has a straight line in that region. It is characterized by being close to.
[0029]
Since the transmission band near the straight line in the region on the lower frequency side than the light line of the cladding layer is wide, if the Si thin wire is directly connected to the width-variable photonic crystal waveguide, the high transmission characteristics will be wider than before. can get. However, the dispersion curve tends to deviate from a straight line in a lower frequency region. Since physical phenomena peculiar to the photonic crystal waveguide appear in this region, it is expected to be a frequency band that will be a key point of a future photonic crystal device (M. Notomi, k. Yamada, A. Shinya, J. M. Takahashi, C.Takahashi, and I.Yokohama, "Extremely Large Group-Velocity Dispersion of Line-Defect waveguides in Photonic Crystal Slabs", Physical Review Letters 87, 253902 (2001).), too features the conventional optical waveguide Due to the difference, high connection efficiency cannot be obtained even if the Si thin wire is directly connected to the width-variable photonic crystal waveguide.
[0030]
FIG. 6 shows light transmission characteristics when the Si fine-wire waveguide and the width-variable photonic crystal waveguide of the present invention are coupled. FIG. 6A shows the case where the structure of the present invention is used, that is, when a conversion region for 50 holes is installed in the input / output section (near the connection portion) in the waveguide direction, and FIG. FIG. 9 is a diagram illustrating an experimental result of a light transmission characteristic when not performed.
[0031]
As described above, in a conventional direct junction between a photonic crystal waveguide and a Si wire waveguide, high coupling efficiency is obtained in a long wavelength region (X portion in FIG. 6B) where a phenomenon peculiar to the photonic crystal waveguide appears. Since it is difficult to obtain, the transmitted light intensity is low. On the other hand, as shown in FIG. 6A, the transmittance characteristic of the coupling system between the photonic crystal waveguide of the present invention and the Si fine wire waveguide in which the mode conversion region is provided is significantly improved in this region (X portion). You can see that it is done.
[0032]
This result indicates that the introduction of the mode conversion mechanism of the present invention makes it possible to efficiently connect a conventional optical waveguide and a photonic crystal waveguide having greatly different mode properties to each other. This indicates that the mechanism can be a mechanism for extracting a specific physical phenomenon to the outside of the crystal with high efficiency.
[0033]
Next, the structural conditions of the photonic crystal waveguide for achieving these highly efficient connections will be described.
[0034]
In the photonic crystal waveguide of the present invention, as light propagated through the conventional optical waveguide (Si fine-wire waveguide) enters the photonic crystal waveguide, both sides sandwiching the line defect of the photonic crystal waveguide gradually. Need to be affected by the lattice points of To this end, as shown in FIGS. 1 and 4, the width (AA ′) of the conventional waveguide (Si fine-wire waveguide) is arranged on lattice points on both sides in the photonic crystal waveguide. It is necessary that the width of the side wall (BB ') on the center side of the waveguide of the structure be wider. The above condition is determined by determining the shape of the structure or the position of the structure such that the side wall of the structure on the center side of the waveguide is along the straight line AB, A′-B ′. Can be satisfied. The side wall is not limited to a plane.
[0035]
Further, it is desirable that a photonic crystal waveguide used as an optical integrated circuit satisfies a single mode condition. Therefore, the distance W between the lattice points on both sides of the line defect of the photonic crystal waveguide is 0.5 times the distance Wo of the lattice points on both sides of the line defect except for one line of the lattice of the two-dimensional photonic crystal. It must be one of 1.6 times. The structure of the photonic crystal waveguide for satisfying the single mode condition is described in, for example, Japanese Patent Application No. 2001-394499.
[0036]
Further, the structure of the present invention in which the distance between the side walls of the structures on both sides of the line defect is gradually narrowed from the connection portion with the straight waveguide toward the back of the photonic crystal waveguide is described in Japanese Patent Application No. 2001-394499. As described in this document, the present invention can be applied to a photonic crystal waveguide in which a structure (dielectric column) on a lattice point in an optical waveguide is shifted or a diameter of the structure is changed. In this specification, the optical waveguide having such a structure is also referred to as a line defect.
[0037]
The present invention is not limited to the above embodiments, but can be variously modified and applied within the scope of the claims.
[0038]
【The invention's effect】
As described above, by using the structure of the photonic crystal waveguide having the mode conversion region as shown in FIGS. 1 and 4, the conventional optical waveguide and the photonic crystal optical waveguide having greatly different modes can be used. Can be connected with high efficiency, an optical circuit using a photonic crystal can be connected to an external network, and a photonic crystal functional device can be incorporated into another optical circuit.
[Brief description of the drawings]
FIG. 1 is a structural diagram of a coupling system of a conventional single-mode linear waveguide (Si fine-wire waveguide) and a simple single-line defect photonic crystal waveguide of the present invention.
FIG. 2 is a view for explaining the waveguide mode dispersion of a conventional typical simple single line defect photonic crystal waveguide.
FIG. 3 is a diagram showing light transmission characteristics when a Si fine wire waveguide and a simple single line defect type photonic crystal waveguide are coupled.
FIG. 4 is a structural diagram of a coupling system of a Si fine-wire waveguide and a width-variable photonic crystal waveguide.
FIG. 5 is a diagram for explaining the waveguide mode dispersion of the width-variable line-defect photonic crystal waveguide.
FIG. 6 is a diagram showing light transmission characteristics when a Si fine wire waveguide and a width-variable line defect type photonic crystal waveguide are coupled.
[Explanation of symbols]
a Lattice constant W Distance between lattice points on both sides of a line defect of photonic crystal waveguide Wo Distance between lattice points on both sides of a line defect except one line of lattice of photonic crystal

Claims (5)

In a photonic crystal waveguide having a line defect in which a part of the lattice of the slab type two-dimensional photonic crystal is linearly removed in the waveguide direction or a part of the lattice is deformed or moved,
Both sides sandwiching a line defect in a predetermined range from the end in the waveguide direction so that the influence of the crystal lattice on the propagation light incident from the end of the photonic crystal waveguide is gradually changed as the propagation light enters. A photonic crystal waveguide characterized in that the shape or arrangement of a structure on a lattice point is different from the shape or arrangement of a structure on another lattice point. In the predetermined range, the width between the sidewalls on the center side of the line defect of the structure on the lattice points on both sides sandwiching the line defect is reduced as the distance from the end of the structure is increased. Item 2. A photonic crystal waveguide according to item 1. In the case where the photonic crystal waveguide is connected to a conventional optical waveguide, the width of the conventional optical waveguide is set at the center of the line defect center of the structure on the lattice points on both sides sandwiching the line defect in the predetermined range. 3. The photonic crystal waveguide according to claim 2, wherein the width is wider than the width between the side walls. The lattice spacing on both sides of the line defect of the photonic crystal waveguide is 0.5 times the lattice spacing on both sides of the defect portion in the normal defect structure except for one row of the lattice of the slab type two-dimensional photonic crystal. 4. The photonic crystal waveguide according to claim 3, wherein the value is any one of 1.6 times. 5. The structure according to claim 1, wherein each of the structures on the lattice points on both sides of the line defect in the predetermined range has a shape obtained by cutting a part of a cylinder by a plane parallel to a central axis thereof. The photonic crystal waveguide according to any one of the above.

Images