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Patent US6643439 - Photonic crystal waveguide - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsA two-dimensional photonic crystal slab waveguide is provided in which a line defect is formed, wherein a first width which is a distance between centers of nearest two lattice points located on both sides of the line defect is different from a second width which is a corresponding distance in a normal...http://www.google.com/patents/US6643439?utm_source=gb-gplus-sharePatent US6643439 - Photonic crystal waveguideAdvanced Patent SearchTry the new Google Patents, with machine-classified Google Scholar results, and Japanese and South Korean patents.Publication numberUS6643439 B2Publication typeGrantApplication numberUS 10/033,766Publication dateNov 4, 2003Filing dateDec 26, 2001Priority dateDec 27, 2000Fee statusPaidAlso published asDE60127729D1, DE60127729T2, DE60127730D1, DE60127730T2, DE60129286D1, DE60129286T2, EP1219984A2, EP1219984A3, EP1219984B1, EP1512994A1, EP1512994B1, EP1512995A1, EP1512995B1, US20020118941Publication number033766, 10033766, US 6643439 B2, US 6643439B2, US-B2-6643439, US6643439 B2, US6643439B2InventorsMasaya Notomi, Koji Yamada, Akihiko Shinya, Junichi Takahashi, Chiharu Takahashi, Itaru YokohamaOriginal AssigneeNippon Telegraph And Telephone CorporationExport CitationBiBTeX, EndNote, RefManPatent Citations (10), Non-Patent Citations (7), Referenced by (73), Classifications (11), Legal Events (4) External Links: USPTO, USPTO Assignment, EspacenetPhotonic crystal waveguide
US 6643439 B2Abstract
A two-dimensional photonic crystal slab waveguide is provided in which a line defect is formed, wherein a first width which is a distance between centers of nearest two lattice points located on both sides of the line defect is different from a second width which is a corresponding distance in a normal two-dimensional photonic crystal slab waveguide. In addition, a two-dimensional photonic crystal slab waveguide is provided in which low refractive index columns in a optical waveguide part are placed at positions shifted from normal positions in a normal two-dimensional photonic crystal slab.
What is claimed is: 1. A two-dimensional photonic crystal slab waveguide in which a part of holes in a lattice structure of a two-dimensional photonic crystal slab do not exist linearly so that a line defect is formed, wherein:
a first width which is a distance between centers of nearest two lattice points located on both sides of said line defect is different from a second width which is a distance between centers of nearest two lattice points located on both sides of a line defect in a normal two-dimensional photonic crystal slab waveguide which simply lacks holes of a single line, wherein said first width is a value from 0.5 times to 0.85 times of said second width. 2. The two-dimensional photonic crystal slab waveguide as claimed in claim 1, said lattice structure is formed by air-hole triangle lattices.
3. The two-dimensional photonic crystal slab waveguide as claimed in claim 1, said two-dimensional photonic crystal slab waveguide includes an oxide cladding or a polymer cladding.
4. The two-dimensional photonic crystal slab waveguide as claimed in claim 3, wherein said two-dimensional photonic crystal slab waveguide is formed by using a Silicon-On-Insulator (SOI) substrate.
5. A two-dimensional photonic crystal slab waveguide in which a part of holes in a lattice structure of a two-dimensional photonic crystal slab do not exist linearly so that a line defect is formed, wherein:
a first width which is a distance between centers of nearest-two lattice points located on both sides of said line defect is different from a second width which is a distance between centers of nearest two lattice points located on both sides of a line defect in a normal two-dimensional photonic crystal slab waveguide which simply lacks holes of a single line; wherein said first width is wider than said second width in which a single-mode appears in a high frequency side of a light line of cladding in a dispersion relation of waveguiding modes of said two-dimensional photonic crystal slab waveguide having said first width, and said first width is a value from 1.3 times to 1.6 times of said second width. 6. The two-dimensional photonic crystal slab waveguide as claimed in claim 5, said lattice structure is formed by air-hole triangle lattices.
7. The two-dimensional photonic crystal slab waveguide as claimed in claim 5, said two-dimensional photonic crystal slab waveguide includes an oxide cladding or a polymer cladding.
8. The two-dimensional photonic crystal slab waveguide as claimed in claim 7, wherein said two-dimensional photonic crystal slab waveguide is formed by using a Silicon-On-Insulator (SOI) substrate.
9. A two-dimensional photonic crystal slab waveguide in which dielectric cylindrical or polygon columns having lower refractive index than that of a dielectric thin-film slab are provided in said dielectric thin-film slab in a two-dimensional lattice pattern, and said dielectric thin-film slab is sandwiched by a top cladding layer and a bottom cladding layer which have lower refractive index than that of said dielectric thin-film slab, wherein:
dielectric columns in an optical waveguide part in said two-dimensional photonic crystal slab waveguide are located at positions which are shifted in an optical propagation direction from positions at which said dielectric columns should be positioned in a normal two-dimensional photonic crystal slab. 10. The two-dimensional photonic crystal slab waveguide as claimed in claim 9, wherein:
a first diameter of said dielectric columns in said optical waveguide part is different from a second diameter of other dielectric columns located in parts other than said optical waveguide part, and said first diameter is a value by which said dielectric columns does not contact with said other dielectric columns. 11. The two-dimensional photonic crystal slab waveguide as claimed in claim 9, wherein each dielectric column of dielectric columns in said optical waveguide part is apart from a position at which said each dielectric column should be positioned in said normal two-dimensional photonic crystal slab by a half of the lattice constant of said normal two-dimensional photonic crystal slab.
12. The two-dimensional photonic crystal slab waveguide as claimed in claim 9, wherein said dielectric cylindrical or polygon columns are arranged in a triangle lattice pattern having a lattice constant “a”, a radius or a half-breadth of said dielectric cylindrical or polygon columns is from 0.2a to 0.45a, and said radius or half-breadth is determined such that said dielectric cylindrical or polygon columns do not contact with dielectric columns in said optical waveguide part.
13. The two-dimensional photonic crystal slab waveguide as claimed in claim 12, wherein:
a refractive index of said dielectric thin-film slab is from 3.0 to 4.5, and each of refractive indices of parts other than said dielectric thin-film slab is from 1.0 to 1.7. 14. The two-dimensional photonic crystal slab waveguide as claimed in claim 9, wherein said dielectric cylindrical or polygon columns are arranged in a square lattice pattern having a lattice constant “a”, a radius or a half-breadth of said dielectric cylindrical or polygon columns is from 0.35a to 0.45a, and said radius or half-breadth is determined such that wherein said dielectric cylindrical or polygon columns do not contact with dielectric columns in said optical waveguide part.
15. The two-dimensional photonic crystal slab waveguide as claimed in claim 14, wherein:
a refractive index of said dielectric thin-film slab is from 3.0 to 4.5, and each of refractive indices of parts other than said dielectric thin-film slab is from 1.0 to 1.7. 16. The two-dimensional photonic crystal slab waveguide as claimed in claim 9, wherein:
silicon, germanium, gallium arsenide base compound, indium phosphide base compound, or indium antimony base compound is used as a material of said dielectric thin-film slab, and silica, polyimide base organic compound, epoxy base organic compound, acrylic base organic compound, air or vacuum is used as a material of parts other than said dielectric thin-film slab. 17. The two-dimensional photonic crystal slab waveguide as claimed in claim 16, wherein said two-dimensional photonic crystal slab waveguide is formed by using a Silicon-On-Insulator (SOI) substrate.
It is theoretically known that, when a line defect which disturbs periodicity of the photonic crystal is included in the photonic crystal, an optical waveguide which completely confines light and has a waveguiding mode in a frequency region of the photonic band-gap can be realized (J. D. Joannopoulos, P. R. Villeneuve, and S. Fan, Photonic Crystal: putting a new twist on light, Nature 386,143 (1997)). J. D. Joannopoulos and others applied a line defect in a two-dimensional photonic crystal in which cylindrical columns having large refractive index almost the same as that of a semiconductor are arranged on a square lattice of lattice constant “a” which is about light wavelength and the radius of each cylindrical column is a/5, and, J. D. Joannopoulos and others theoretically indicated that an optical waveguide having no scattering loss even when it is steeply bent can be realized. This waveguide can be very important for realizing a large scale integrated optical circuit.
FIGS. 1A and 1B are schematic diagrams of a structure of a single missing-hole line defect photonic crystal waveguide of a typical air-hole type according to a conventional technology. FIG. 1A shows a top view and FIG. 1B shows a B-B′ section view. The conventional single missing-hole line defect photonic crystal waveguide can be also called as a normal two-dimensional photonic crystal slab waveguide in this specification. In FIGS. 1A and 1B, 5 indicates an optical waveguide part, 2 indicates an Si layer, 3 indicates an SiO2 layer which is a cladding layer, and 4 indicates an air-hole triangle lattice point, in which the lattice constant is represented as “a”. Each air-hole is a cylindrical column or a polygon column which penetrates the Si layer 2. The diameter of the air-hole is 0.215 μm in this example. In the air-hole triangle lattice, the air-hole is placed in each lattice point of the triangle lattice. The triangle lattice is a regular lattice in which lattice points are placed on vertices of regular triangles which are arranged over the two-dimensional plane.
As representative two-dimensional photonic crystals having the photonic band-gap, there are two structures. One is a structure in which columns of high refractive index are provided in air. Another is a structure in which air-holes are provided in a high refractive index layer like the above-mentioned example. (The air-hole can be also called a low refractive index column or a low refractive index cylindrical column.) The former structure, which was used by J. D. Joannopoulos and others, requires a cladding layer for supporting the columns. Since the refractive index of the cladding layer is larger than that of the air which is a core for the line defect waveguide, very long columns are necessary for preventing light leakage to the upper and lower sides so that manufacturing such structure becomes very difficult. On the other hand, as for the latter structure, since the air-hole can stand by itself, the cladding layer can be freely chosen, and it is easy to determine a core having refractive index larger than that of the cladding layer. Thus, limitation on manufacturing is small so that it is easy to select structural condition that light hardly leak to the upper and lower sides.
FIG. 2 shows a dispersion relation of waveguiding modes of a conventional typical single missing-hole line defect photonic crystal waveguide. When such waveguide is formed by using the two-dimensional photonic crystal slab on oxide cladding, the waveguiding modes become as shown in FIG. 2. In the figure, normalized frequency represented by (lattice constant/wavelength) which is a dimensionless number is used. In addition, normalized propagation constant represented by (wave number×lattice constant/2π) is used. The light line of the cladding (SiO2, refractive index 1.46) is also shown in FIG. 2.
In the conventional structure shown in FIG. 2, the waveguiding mode which satisfies the condition that light does not leak to the cladding layer is only in a region circled by an ellipse which is below the light line. However, inclination of the waveguiding mode in the region is very small so that group velocity (energy propagation velocity) of the waveguiding mode, which is determined depending on the inclination, is very small. There are many problems for using the waveguide having the waveguiding mode of very small group velocity since time for light transmission becomes long. In addition, since heterogeneity exist in an actual structure to some extent, the mode of very small group velocity is affected by the heterogeneity so that light may not propagate. In addition, in the mode above the light line (high frequency region), light can not propagate since diffraction loss in the photonic crystal is too large. That is, light in the photonic crystal waveguide propagates while being perturbed by periodic structure of the photonic crystal, and light leaks to the cladding layer by diffraction loss in the mode above the light line.
In order to use the mode below the light line, it is necessary to move the light line upward or to move the waveguiding mode appropriately in the graph of FIG. 2. However, as long as the oxide cladding structure is used, since the position of the light line is determined by the refractive index of the cladding, the position of the light line can not be changed largely. As for the waveguiding mode, as long as the single-mode within the band-gap should be used, it is difficult to obtain a waveguiding mode having large group velocity below the light line by using the structure shown in FIG. 1. As for crystal structures other than the triangle lattice such as square lattice, it is more difficult to obtain such waveguiding mode. Therefore, it is very difficult to use waveguiding modes below the light line.
FIGS. 3A-3C are figures for explaining the conventional single missing-hole line defect photonic crystal waveguide (optical waveguide). FIG. 3A shows a top view of the optical waveguide, FIG. 3B shows an A-A′ section view, and FIG. 3C shows a B-B′ section view.
In FIG. 3A, the optical waveguide 30 includes dielectric thin-film slab 31 (which corresponds to the above-mentioned high refractive index plate) sandwiched between a top cladding layer 36 and a bottom cladding layer 37. A photonic crystal structure is formed in the dielectric thin-film slab 31 by providing low refractive index cylindrical columns 35 having lower refractive index than that of the dielectric thin-film slab 31 in a triangle lattice pattern. In addition, one line of the low refractive index cylindrical columns 35 is replaced by a dielectric having the same refractive index as the dielectric thin-film slab 31 such that the part of the one line can be used as an optical waveguide part 32. Arrows ←→ in the optical waveguide part 32 indicate optical propagation directions. The waveguide shown in FIG. 1 is an example of a structure shown in FIG. 3 in which the top cladding layer 36 and the low refractive index cylindrical column 35 are air, the bottom cladding layer 37 is SiO2, and the dielectric thin-film slab 31 is Si.
Here, it is assumed that refractive indices of the dielectric thin-film slab 31, the low refractive index cylindrical column 35, the top cladding layer 36 and the bottom cladding layer 37 are n1=3.5, n2=1.0, n3=n4=1.46 respectively, and that radius of the low refractive index cylindrical column 35 is 0.275a and thickness of the dielectric thin-film slab 31 is 0.50a, in which “a” represents the lattice constant (triangle lattice in this example) of the photonic crystal. The low refractive index cylindrical column 35 having the refractive index 1.0 is the same as an air-hole. Characteristics of the optical waveguide 30 will be described in the following.
These refractive indices of the optical waveguide 30 correspond to those of Si, air (vacuum) and SiO2 which are often used for forming waveguides targeted for infrared light for optical communication having a wavelength about 1.55 μm.
Since a relative dielectric constant corresponds to a square of refractive index, “relative dielectric constant” or “dielectric constant” can be used instead of “refractive index” in this specification.
FIGS. 4A-4C are figures for explaining waveguiding modes of the above-mentioned optical waveguide. FIG. 4A shows dispersion curves of waveguiding modes which can propagate through the optical waveguide part. The dispersion curves of waveguiding modes are obtained by using a plane wave expansion method (R. D. Meade et al., Physical Review B 48,8434 (1993)) to which periodic boundary condition is applied. This figure is similar to FIG. 2. FIG. 4B shows magnetic field component perpendicular to the dielectric thin-film slab according to a mode 1 in FIG. 4A, and FIG. 4C shows magnetic field component perpendicular to the dielectric thin-film slab according to a mode 2 in FIG. 4A.
Each amount in FIG. 4A is normalized by the lattice constant or speed of light c. The diagonally shaded regions correspond to the outside of photonic band-gap (J D. JoannoPoulos, R D. Meade, J N. Winn, “Photonic Crystals”, Princeton University Press, Princeton (1995)), that is, the diagonally shaded areas show regions in which light can not be confined in the optical waveguide part 32 (A. Mekis et al., Physical Review B 58,4809 (1998)).
In the vertical line hatching region, power of light confinement caused by difference of refractive indices between the dielectric thin-film slab 31 and the top cladding layer 36/bottom cladding layer 37 is weakened so that light can not be confined in the optical waveguide part 32 (S G. Johnson et al., Physical Review B 60,5751 (1999)). The vertical line hatching region corresponds to the before-mentioned above region of the light line. That is, a region to be considered used for the waveguide is only a white region in FIG. 4A.
As is understood by the figure, two waveguiding modes 1 and 2 exist in the white region of the conventional optical waveguide 30. Further waveguiding modes may exist when the band-gap is wider, however, the two modes 1 and 2 will be considered here for the sake of simplicity. The mode 1 corresponds to the mode circled by the ellipse in FIG. 2, and the mode 2 corresponds to the mode of upper dotted line.
In these two modes 1 and 2, the mode 1 in the low frequency side generally has magnetic field distribution shown in FIG. 4B, and the mode 2 in the high frequency side generally has magnetic field distribution shown in FIG. 4C.
Thus, the mode 1 is used for the conventional waveguide. However, as is known from FIG. 4A, since the frequency hardly change even when the propagation constant change in this mode 1, the mode 1 has a defect that usable frequency band is very small. In this example, the frequency band is about 1%.
The above object can be achieved by a two-dimensional photonic crystal slab waveguide in which a part of holes in a lattice structure of a two-dimensional photonic crystal slab do not exist linearly so that a line defect is formed, wherein:
a first width which is a distance between centers of nearest two lattice points located on both sides of the line defect is different from a second width which is a distance between centers of nearest two lattice points located on both sides of a line defect in a normal two-dimensional photonic crystal slab waveguide which simply lacks holes of a single line.
In the two-dimensional photonic crystal slab waveguide, the first width may a value from 0.5 times to 0.85 times of the second width.
According to the invention, since the first width which is a distance between centers of nearest two lattice points located on both sides of the line defect is different from the second width which is a corresponding distance in a normal two-dimensional photonic crystal slab waveguide, an optical waveguide which can form a single waveguiding mode having large group velocity below the light line can be provided. The two-dimensional photonic crystal slab waveguide may be called a single missing-hole line defect photonic crystal waveguide.
In the two-dimensional photonic crystal slab waveguide, the lattice structure may be formed by air-hole triangle lattices, and the two-dimensional photonic crystal slab waveguide may include an oxide cladding or a polymer cladding. In addition, the two-dimensional photonic crystal slab waveguide may be formed by using a Silicon-On-Insulator (SOI) substrate.
In the two-dimensional photonic crystal slab waveguide, the first width may be wider than the second width in which a single-mode appears in a high frequency side of a light line of cladding in a dispersion relation of waveguiding modes of the two-dimensional photonic crystal slab waveguide having the first width, and the first width may a value from 1.3 times to 1.6 times of the second width.
According to the invention, an optical waveguide can be provided which can form a single waveguiding mode of low loss above the light line can be provided.
The above object can be also achieved by a two-dimensional photonic crystal slab waveguide in which dielectric cylindrical or polygon columns having lower refractive index than that of a dielectric thin-film slab are provided in the dielectric thin-film slab in a two-dimensional lattice pattern, and the dielectric thin-film slab is sandwiched by a top cladding layer and a bottom cladding layer which have lower refractive index than that of the dielectric thin-film slab, wherein:
positions of dielectric columns which form one line of the lattice of two-dimensional photonic crystal slab for an optical waveguide part are shifted in an optical propagation direction.
That is, the dielectric columns in an optical waveguide part in the two-dimensional photonic crystal slab waveguide are located at positions which are shifted in an optical propagation direction from positions at which the dielectric columns should be positioned in a normal two-dimensional photonic crystal slab.
In the two-dimensional photonic crystal slab waveguide, the dielectric cylindrical or polygon columns may be arranged in a triangle lattice pattern having a lattice constant “a”, a radius or a half-breadth of the dielectric cylindrical or polygon columns is from 0.2a to 0.45a, and the radius or half-breadth is determined such that the dielectric cylindrical or polygon columns do not contact with dielectric columns in the optical waveguide part.
In addition, in the two-dimensional photonic crystal slab waveguide, the dielectric cylindrical or polygon columns may be arranged in a square lattice pattern having a lattice constant “a”, a radius or a half-breadth of the dielectric cylindrical or polygon columns is from 0.35a to 0.45a, and the radius or half-breadth is determined such that wherein the dielectric cylindrical or polygon columns do not contact with dielectric columns in the optical waveguide part.
FIGS. 1A and 1B are schematic diagrams of a structure of a single missing-hole line defect photonic crystal waveguide of a typical air-hole type according to a conventional technology, in which FIG. 1A shows a top view and FIG. 1B shows a B-B′ section view;
FIG. 2 shows a dispersion relation of waveguiding modes of a conventional typical single missing-hole line defect photonic crystal waveguide;
FIGS. 3A-3C are figures for explaining the conventional single missing-hole line defect photonic crystal waveguide (optical waveguide), in which FIG. 3A shows a top view of the optical waveguide, FIG. 3B shows an A-A′ section view, and FIG. 3C shows a B-B′ section view;
FIGS. 4A-4C are figures for explaining waveguiding modes of the conventional single missing-hole line defect photonic crystal waveguide;
FIGS. 5A and 5B are figures showing a structure of a single missing-hole line defect photonic crystal waveguide according to a first embodiment of the present invention, in which FIG. 5A shows a top view and FIG. 5B shows a B-B′ section view;
FIG. 6 shows a dispersion relation of waveguiding modes of the single missing-hole line defect photonic crystal waveguide according to the first embodiment of the present invention;
FIG. 7 is a figure for explaining FIG. 6;
FIGS. 8A and 8B show results of theoretical calculation of waveguiding mode dispersion in the cases where the width of the optical waveguide part is 0.85W and 0.50W respectively;
FIGS. 9A and 9B are figures showing a structure of a single missing-hole line defect photonic crystal waveguide according to a second embodiment of the present invention, in which FIG. 9A shows a top view and FIG. 9B shows a B-B′ section view;
FIG. 10 shows a dispersion relation of waveguiding modes of the single missing-hole line defect photonic crystal waveguide according to the second embodiment of the present invention;
FIG. 11 shows dependence of bandwidth of single-mode above the light line (shown by dotted line) and propagation loss (shown by solid line) on the width of the optical waveguide part according to the second embodiment of the present invention;
FIGS. 12A-12C are figures for explaining about mode calculation of the two-dimensional photonic crystal waveguide;
FIGS. 13A-13B are figures for explaining a two-dimensional photonic crystal slab waveguide of an example 3-1, in which FIG. 13A shows a top view of the optical waveguide and FIG. 13B shows an A-A′ section view;
FIGS. 14A-14C are figures for explaining waveguiding modes in the photonic crystal waveguide in the example 3-1;
FIGS. 15A-15B are figures for explaining a two-dimensional photonic crystal slab waveguide of an example 3-2, in which FIG. 15A shows a top view of the optical waveguide and FIG. 15B shows an A-A′ section view.
The optical waveguide in the embodiments is structured such that, in a two-dimensional photonic crystal slab waveguide in which a part of holes in a lattice structure of a two-dimensional photonic crystal slab do not exist linearly so that a line defect is formed, a first width which is a distance between centers of nearest two lattice points located on both sides of the line defect is narrower (first embodiment) or wider (second embodiment) from a second width which is a distance between centers of nearest two lattice points located on both sides of a line defect in a normal two-dimensional photonic crystal slab waveguide which simply lacks holes of a single line. By forming the waveguide in this way, a structure having low loss and large group velocity, and enabling a single waveguiding mode can be realized while light confinement is performed by using photonic band-gap and difference of refractive indices.
In addition, the same effect can be obtained by shifting the position of one line of low refractive index columns (air-holes and the like) in the light propagation direction, or, in addition to that, by changing the radius of the holes (third embodiment).
First, the first embodiment of the present invention will be described in the following.
FIGS. 5A and 5B are figures showing a structure of a single missing-hole line defect photonic crystal waveguide according to a first embodiment of the present invention. The single missing-hole line defect photonic crystal waveguide can also be called as a two-dimensional photonic crystal slab waveguide. FIG. 5A shows a top view and FIG. 5B shows a B-B′ section view. In this embodiment, single line defect is formed in a two-dimensional photonic crystal slab on oxide cladding by removing holes of the single line, and, width of the line defect (in other words, width of the optical waveguide part 5) is tuned by shifting positions of the whole crystal lattices of both sides of the line defect. As a result, single waveguiding mode having a large group velocity below the light line of the cladding can be obtained. In first and second embodiments, the width of the line defect and the width of the optical waveguide part have the same meaning, in which the width is a distance between centers of nearest two lattice points located on both sides of the line defect.
That is, an air-hole triangle lattice photonic crystal of lattice constant a=0.39 μm was manufactured on an SOI substrate consisting of an Si layer 2 of 0.2 μm thickness and an SiO2 layer of 2, 3 μm thickness by electron beam lithography and dry etching, and, various single-line defects of various width were applied to the air-hole triangle lattice photonic crystal. Tuning of the width of a waveguide part is performed by shifting crystal lattices of both sides of the line defect by a predetermined distance in a direction perpendicular to the line defect.
The width of the line defect of a normal single missing-hole line defect photonic crystal waveguide (which may be called a normal width W) is defined as a distance between centers of nearest lattice points 4 of both sides of the line defect. Width of the line defect of this embodiment can be represented by a constant number—times of the normal width W. The photonic crystal itself has the photonic band-gap between a wavelength 1.35 μm and a wavelength 1.57 μm. Thus, transmission of light was not observed in a part having no defect in the crystal within the wavelength range. Next, light transmission spectrum for each line defect was measured. As for a simple (normal) single line defect (the width is 1.0W), light transmission was not observed in the photonic bad-gap frequency range. On the other hand, when using a waveguide having the width of 0.7W, light transmission was clearly observed in a frequency range of the photonic band-gap.
FIG. 6 shows a dispersion relation of waveguiding modes of the single missing-hole line defect photonic crystal waveguide according to the first embodiment of the present invention. FIG. 2 can be represented as FIG. 7 for comparing with FIG. 6. The light line of the cladding (SiO2in this embodiment) is overlaid on FIG. 7 like in FIG. 6. As shown in FIG. 7, a waveguiding mode (which corresponds to the mode 1 in FIG. 4) below the light line of the normal waveguide (the width is 1.0W) has small inclination. On the other hand, according to the present invention shown in FIG. 6, a waveguiding mode which has large inclination, that is, which has large group velocity exists below the light line in the photonic band-gap. In addition, it is clear from the figure that the single-mode condition is satisfied in the region.
A wavelength region in which light transmission was detected in an experiment is the same as the region where this waveguiding mode exists. This shows that the light transmission is realized since the single waveguiding mode having large group velocity is formed by decreasing the width of the line defect (optical waveguide part). As a result of calculating waveguiding mode dispersion by a finite-difference time-domain method and the plane wave expansion method by using various structural parameters, it was found that a waveguiding mode having large group velocity below the light line can be formed when the width of the line defect (optical waveguide part) is set as a value from 0.50W to 0.85W. In addition, light propagation was observed within the width range in experiments.
FIGS. 8A and 8B show results of theoretical calculation of waveguiding mode dispersion in the cases where the width of the optical waveguide part is 0.85W and 0.50W. FIG. 8A shows the case of 0.85W and FIG. 8b shows the case of 0.50W. When the width of the optical waveguide part is a value between 0.85W and 0.50W, the transmission bandwidth of the waveguiding mode is wider than those of the cases shown in FIGS. 8A and 8B as shown in FIG. 6 (0.7W), for example. Therefore, it can be understood that the waveguide having the line defect within the width range practically functions.
The reason that the above-mentioned effect can be obtained by narrowing the width is as follows.
As described in the related art, in the typical (normal) single-line defect state as shown in FIG. 2, the mode (circled mode in FIG. 2) in the low frequency region below the light line within the photonic band-gap has very small group velocity. Thus, this mode is not practical. Here, a waveguiding mode in the outside of the band-gap having large group velocity is considered.
To narrow the width of the line defect is equivalent to decreasing refractive index. Thus, the mode of the lowest mode in FIG. 2 can be shifted to the high frequency side by narrowing the width. Accordingly, this mode can be used within the photonic band-gap and below the light line. Therefore, it becomes possible that a mode having large group velocity can be realized within the band gap and below the light line.
The refractive index of the waveguide can also be lowered by using a medium of low refractive index in the waveguide part, by providing holes in the line defect and the like. A method corresponding to providing holes in the line defect will be described in the third embodiment.
FIGS. 9A and 9B are figures showing a structure of a single missing-hole line defect photonic crystal waveguide 1A according to a second embodiment of the present invention. FIG. 9A shows a top view and FIG. 9B shows a B-B′ section view. In this embodiment, when forming single line defect in an two-dimensional photonic crystal slab on oxide cladding, width of the line defect is widened by shifting the whole crystal lattice of both sides of the line defect. As a result, single waveguiding mode having a large group velocity can be obtained above the light line of the cladding.
That is, an air-hole triangle lattice photonic crystal of lattice constant a=0.39 μm was manufactured on an SOI substrate consisting of an Si layer 2 of 0.2 μm thickness and an SiO2 layer of 2, 3 μm thickness by electron beam lithography and dry etching, and, various single line defects of various widths were applied to the air-hole triangle lattice photonic crystal. Tuning of the width of the optical waveguide part is performed by shifting crystal lattices of both sides of the line defect by a predetermined distance in a direction perpendicular to the line defect such that the width is widened.
According to the embodiment, the width of the optical waveguide part is 1.5 times of the normal width of a normal optical waveguide part.
Also in this case, the normal width of the line defect of a normal single missing-hole line defect photonic crystal waveguide is defined as a distance between centers of nearest lattice points 4 of both sides of the line defect of a normal single missing-hole line defect photonic crystal waveguide. Width of line defect of this embodiment can be represented by a constant number—times of the normal width W. The photonic crystal itself has the photonic band-gap between a wavelength 1.35 μm and a wavelength 1.57 μm. Thus, transmission of light was not observed in parts having no defect in the crystal within the wavelength range.
Next, light transmission spectrum for each line defect was measured. As for a simple (normal) single line defect (the width is 1.0W), light transmission was not observed in the band-gap frequency range. On the other hand, when using a waveguide having the width of 1.5W, light transmission was clearly observed in a frequency band in the band-gap.
FIG. 10 shows a dispersion relation of waveguiding modes of the single missing-hole line defect photonic crystal waveguide according to the second embodiment of the present invention. The light line of the cladding (SiO2in this embodiment) is overlaid on FIG. 10 like FIG. 2. The wavelength region in which light transmission was observed by an experiment is the same as a region within which an even mode exists in the single-mode region in FIG. 10. In this case, the single-mode region exists in the photonic band-gap above the light line. However, in this embodiment, the problem of light leaking is solved by widening the width of the optical waveguide part since diffraction loss decreases. That is, a waveguiding mode of low loss can be obtained although the waveguiding mode exists above the light line.
FIG. 11 shows dependence of bandwidth of single-mode above the light line (shown by dotted line) and propagation loss (shown by solid line) on the width of the optical waveguide part according to the second embodiment of the present invention. The bandwidth is normalized by a central frequency of the single-mode band. As shown in FIG. 11, as the width is widened, the bandwidth of the single-mode decreases after increasing, and, then, the single-mode bandwidth can not be obtained when the width exceeds 1.6W. The reason that the single-mode bandwidth changes in this way is that the mode in the photonic band-gap in FIG. 10 shifts to the low frequency side by widening the waveguide width. The propagation loss decreases as the width is widened.
As a result of calculating waveguiding mode dispersion by the finite-difference time-domain method by using various structural parameters, it was found that a waveguiding mode satisfying the single-mode condition can be formed above the light line when the width is set to a value equal to or smaller than 1.6W, and the propagation loss becomes equal to or smaller than 20 dB/mm which is a practical loss when the width is set to a value equal to or larger than 1.3W. In addition, light propagation was observed within the width of this range in experiments. That is, by setting the waveguide width as a value from 1.3W to 1.6W, the waveguiding mode which satisfies the single-mode condition and suppresses propagation loss below the practical loss can be obtained.
It is difficult to realize a structure having a waveguiding mode of large group velocity in the low frequency side of the light line when the width is widened. However, since perturbation caused by the crystal periodicity becomes small by widening the width of the waveguide part, the group velocity is increased and the diffraction loss is suppressed. As a result, as shown in FIG. 10, propagation loss can be effectively decreased even though the single-mode exists above the light line (the high frequency side). However, since effective refractive index of the waveguide increases as the width is widened, the single-mode band shifts to the low frequency side as mentioned above so that the single-mode bandwidth becomes narrow gradually. Therefore, it is necessary that the width is equal to or smaller than 1.6W for realizing the single-mode.
In the following, a calculation method of waveguiding mode dispersion curves (FIGS. 6, 7, 10 and the like) which are used for explanation of the present invention will be described.
Next, mode calculation of the two-dimensional photonic crystal waveguide will be described. Basically, the same calculation as the above-mentioned one is performed. However, in the photonic crystal used in the present invention, the line defect is incorporated in the inside of the crystal so that the periodicity is disturbed in the direction perpendicular to the line defect. Therefore, a structure shown in FIGS. 12A-12C is used as the unit cell. That is, the periodic boundary condition which satisfies the Bloch condition is applied in the propagation direction of the light. In the direction perpendicular to the light propagation direction, periodic structure is realized by using mirror boundary in which waveguides are placed such that large interference does not occur, and a region for absorbing leaking light which does not become a mode is provided in the thickness direction.
FIG. 12A shows the structure of FIG. 1. FIG. 12B is a magnified view of a part in a dotted line box of FIG. 12A, and FIG. 12C shows stereoscopic view of the structure of FIG. 12B which is the unit cell of the photonic crystal waveguide. The wave number—frequency graph can be obtained by solving the Maxwell equation by the FDTD method for the unit cell. The graph is the dispersion curve described in this specification.
In this calculation, a mode other than the eigenmode can be picked up as a peak of the spectrum if the mode exists for along time in the waveguide. Therefore, analysis for the leaky mode which is above the light line of the cladding layer becomes available. This feature of this method is an advantage point which can not be obtained by other eigenvalue analysis methods. This superiority is applied in this invention. In addition, since lifetime of waveguiding modes in the waveguide can be calculated according to this calculation method, theoretical propagation loss can be presented by using group velocity (energy propagation velocity) obtained by the dispersion curve.
For the waveguide in which the width of the optical waveguide part is narrowed, it was recognized that the single-mode region exists not only in the low frequency side of the light line but also in the high frequency side of the light line.
In the first and second embodiments, the waveguides can be manufactured by using, for example, the Silicon-On-Insulator(SOI) using Si and SiO2as the medium. However, it is clear that the effect of the present invention can also be obtained by using other materials. Generally, when forming a single-line defect waveguide by using the photonic crystal slab in which a dielectric of low refractive index is placed under thin-film medium of high refractive index, it is possible to form a waveguiding mode which satisfies the single-mode condition above or below the light line by adjusting the width like the above-mentioned embodiments.
For example, semiconductor such as gallium arsenide base compound (GaAs, InGaAs, InGaAsP and the like), indium phosphide base compound (InP and the like) and the like can be used instead of Si. In addition, polymer, alumina and the like can be used instead of SiO2. Further, although SiO2 is used as the bottom cladding and air is used as the top cladding in the embodiments, it is clear that same effect can be obtained if dielectric cladding such as SiO2is used for both of the top and bottom claddings.
Next, the third embodiment of the present invention will be described by using examples 3-1-3-2.
FIGS. 13A-13B are figures for explaining a two-dimensional photonic crystal slab waveguide of the example 3-1. FIG. 13A shows a top view of the optical waveguide, FIG. 13B shows an A-A′ section view.
FIGS. 14A-14C are figures for explaining waveguiding modes in the photonic crystal waveguide in the example 3-1. FIG. 14A shows dispersion curves of waveguiding modes. FIG. 14B shows magnetic field component perpendicular to the dielectric thin-film slab in modes 1 and 2 shown in FIG. 14A, and FIG. 14C shows magnetic field component perpendicular to the dielectric thin-film slab in a mode 3 shown in FIG. 14A.
In this case, FIG. 14A shows dispersion curves of waveguiding modes which can pass through the optical waveguide part 12. As shown in FIG. 14A, three waveguiding modes exist. As shown in FIG. 14B, electromagnetic field distribution of the mode 1 and mode 2 which are first and second modes from low frequency side is similar to that of a general single-mode waveguide. Especially, in the second mode (mode 2), change amount of frequency with respect to propagation constant is large in the whole effective propagation constant region. As a result, effective frequency band is enlarged to about 4.7% and the group velocity increases.
When this structure is applied to infrared light for optical communication near 1.55 μm, and, when Si is used for the dielectric thin-film slab 11, air (or vacuum) is used for the low refractive index column 15 and for the low refractive index column A13, and SiO2 is used for the other parts, the period of the triangle lattice becomes about 0.42 μm, the radius of the low refractive index columns 15 becomes about 0.115 μm, the radius of the low refractive index column A13 placed in the optical waveguide part 12 becomes about 0.094 μm. This structure having these values can be manufactured by using a conventional semiconductor processing technology.
In this example, although the radius of the low refractive index column A13 placed in the moved position in the optical waveguide part 12 is 0.225a, it is obvious that frequency band or group velocity can be changed by changing the radius. Considering the structure of the optical waveguide 10, that is, considering a range in which the low refractive index columns A13 placed in the optical waveguide part 12 and other low refractive index columns do not contact with each other, it is practical that the radius of the low refractive index column A13 is about 0.1-0.4 times of the minimum width of the optical waveguide part 12.
As for materials, when infrared light for communication near 1.55 μm wavelength is used as waveguide light, silicon, germanium, gallium arsenide base compound, indium phosphide base compound, indium antimony base compound and the like can be used as the material of the dielectric thin-film slab which has high refractive index, can transmit infrared light and has few problem in manufacturability and stability. The refractive index of the materials is about from 3.0 to 4.5. In addition, silica, polyimide base organic compound, epoxy base organic compound, acrylic base organic compound, air and vacuum and the like can be used as the material of the parts other than the dielectric thin-film slab which has low refractive index, can transmit infrared light and has few problem in manufacturability and stability. The refractive index of these materials is about from 1.0 to 1.7.
These materials can be also used in the first embodiment and the second embodiment in the same way. In addition, the two-dimensional photonic crystal slab waveguide of the third embodiment can be manufactured by using the Silicon-On-Insulator (SOI) substrate.
FIGS. 15A-15B are figures for explaining a two-dimensional photonic crystal slab waveguide of the example 3-2. FIG. 15A shows a top view of the optical waveguide, FIG. 15B shows an A-A′ section view.
In the example 3-1, although low refractive index region of cylindrical columns or polygon columns is provided in the outside of the optical waveguide part 12 in the triangle lattice pattern in order to realize the photonic crystal, same effect can be obtained by using a square lattice pattern as shown in FIG. 15A instead of the triangle lattice pattern. That is, in a structure in which low refractive index columns 25 having lower refractive index than that of the dielectric thin-film slab 21 are provided in the square lattice pattern and the dielectric thin-film slab 21 is sandwiched by a top cladding layer 26 and a bottom cladding layer 27 having lower refractive index than that of the dielectric thin-film slab 21, a single line of the low refractive index columns 25 which exists in a part to become an optical waveguide part 22 is placed in a position moved by a half distance of the lattice constant of the square lattice from the normal lattice position in the optical propagation direction, in addition, the radius of the moved low refractive index columns is changed as necessary according to waveguide characteristics.
The range of the radius of the low refractive index columns for providing the photonic crystal can be a range which allows existence of photonic band-gap. According to theoretical calculation by the plane wave expansion method, when the refractive index of the dielectric thin-film slab 21 is about from 3 to 4.5, and the refractive index of the other parts is about from 1 to 1.7, the radius of the low refractive index columns 25 is about from 0.35a to 0.45a.
As mentioned above, the two-dimensional photonic crystal slab waveguide of present invention is formed such that a part of holes in a lattice structure of a two-dimensional photonic crystal slab do not exist linearly so that a line defect is formed, and that a first width which is a distance between centers of nearest two lattice points located on both sides of the line defect is narrower than a second width which is a distance between centers of nearest two lattice points located on both sides of a line defect in a normal two-dimensional photonic crystal slab waveguide which simply lacks holes of a single line. Therefore, an optical waveguide which can form a single waveguiding mode having large group velocity below the light line can be provided.
In addition, by forming the two-dimensional photonic crystal slab waveguide such that the first width is wider than the second width, an optical waveguide which can form a single waveguiding mode having low loss above the light line can be provided.
Also, by shifting positions of low refractive index columns which form one line of the lattice of the two-dimensional photonic crystal slab in the optical propagation direction, or, in addition to that, by changing the radius of the columns, it becomes possible to provide an optical waveguide which can form a single waveguiding mode having large group velocity below the light line.
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