JP4146201B2 - Two-dimensional photonic crystal with fine wire waveguide - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a two-dimensional photonic crystal optical multiplexing / demultiplexing device and the like used for wavelength division optical multiplexing communication and the like. In particular, the present invention relates to a waveguide for efficiently exchanging light between these devices and the outside of the device such as an optical fiber.
[0002]
[Prior art]
In recent years, photonic crystals have attracted attention as new optical devices. A photonic crystal is a functional material having a periodic refractive index distribution, and forms a band structure with respect to energy of light or electromagnetic waves. In particular, an energy region (photonic band gap) where light or electromagnetic waves cannot be propagated is formed. One field in which photonic crystal devices are applied is optical multiplex communication.
[0003]
As the photonic crystal, a two-dimensional crystal or a three-dimensional crystal can be used. Both have their respective characteristics. Among them, the two-dimensional crystal is advantageous in that it is relatively easy to produce.
[0004]
By introducing an appropriate defect into the refractive index distribution in the photonic crystal, an energy level (defect level) due to this defect is formed in the photonic band gap. As a result, only light having a wavelength corresponding to the energy of the defect level in the wavelength range corresponding to the energy in the photonic band gap can exist. If the defect in the crystal is made linear, it becomes a waveguide, and if the defect in the crystal is made dotted, it becomes a resonator. In Patent Document 1, in a two-dimensional photonic crystal in which a refractive index distribution is provided by arranging cylindrical holes in a triangular lattice shape, a waveguide is formed by deleting the cylindrical holes linearly ([0025] FIG. 1) describes that a point defect is formed in the vicinity of a waveguide ([0029], FIG. 1). Such a two-dimensional photonic crystal is a demultiplexer that extracts light having a specific wavelength out of light propagating in a waveguide from a point defect, or multiplexing that introduces light having a specific wavelength from the point defect into the waveguide. Can be used as a container.
[0005]
[Patent Document 1]
Japanese Patent Laid-Open No. 2001-272555 ([0025], [0029], FIG. 1)
[0006]
[Problems to be solved by the invention]
In optical multiplex communication, which is a typical field to which an optical device using a photonic crystal is applied, it is necessary to efficiently exchange light between a waveguide of the photonic crystal and an optical fiber. The width of a two-dimensional photonic crystal waveguide (approximately 0.4 μm) that transmits light in the wavelength band (around 1.55 μm) that is generally used for optical multiplex communication is 1 / of the diameter (8 to 10 μm) of the core of the optical fiber. There are only about 20. Therefore, even if the end portion of the optical fiber and the end portion of the two-dimensional photonic crystal waveguide are directly connected, light cannot be efficiently exchanged between them.
[0007]
Therefore, in order to couple the optical fiber and the two-dimensional photonic crystal waveguide, a thin wire waveguide 11 is interposed between the optical fiber core 12 and the two-dimensional photonic crystal waveguide 14 as shown in FIG. Thus, it has been studied to improve the light transmittance between the two. When a thin wire waveguide is introduced, the light transmittance between the core 12 of the optical fiber and the thin wire waveguide 11 and the light transmittance between the thin wire waveguide 11 and the two-dimensional photonic crystal waveguide 14 are determined. Both must be high. In Non-Patent Document 1, the transmittance of light between the latter thin wire waveguide and the two-dimensional photonic crystal waveguide is calculated by two-dimensional numerical analysis, and as a result, a transmittance of 90% or more is obtained. However, in this document, the calculation is performed on the assumption that the thickness of the two-dimensional photonic crystal waveguide and the thin wire waveguide is infinite. Therefore, a high transmittance is not always obtained in an actual system having a finite thickness.
[0008]
[Non-Patent Document 1]
Adibi and 4 others, Physical Review, (USA), Published by American Physical Society, 2001, Vol. B64, 033308 (A. Adibi et al., Physical Review, Vol. B64, 033308)
[0009]
The present invention has been made to solve such a problem, and an object thereof is an interface for transmitting and receiving light between the outside and a two-dimensional photonic crystal waveguide. An object of the present invention is to provide a two-dimensional photonic crystal with a thin wire waveguide having a thin wire waveguide with high light transmittance between the two-dimensional photonic crystal waveguide .
[0010]
[Means for Solving the Problems]
The two-dimensional photonic crystal with a thin wire waveguide according to the present invention, which has been made to solve the above problems,
a) a two-dimensional photonic crystal having a waveguide whose upper and lower surfaces face a space where nothing is provided and is in a single mode at a predetermined wavelength;
b) a fine wire waveguide body to which the waveguide and one end face in the longitudinal direction are connected;
c) A refractive index lower than that of the fine wire waveguide body and higher than that of air, so that the light of the predetermined wavelength propagating in the fine wire waveguide body is in a single mode state . out is provided so as to contact at least a portion, and a cladding member not in contact with the two-dimensional photonic crystal,
It is characterized by providing.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
First, the single-mode thin wire waveguide for a two-dimensional photonic crystal according to the present invention will be described. The main body of the thin wire waveguide has a shape that is long in the waveguide direction and has a small cross section in a direction perpendicular to the waveguide direction. A rectangular shape is typical as the cross-sectional shape, but various other shapes such as a circular shape can be taken. Of the two surfaces at both ends in the longitudinal direction of the thin wire waveguide body, one is a surface that transmits and receives light to and from the two-dimensional photonic crystal waveguide, and the other is between the outside such as an optical fiber. It is a surface that transmits and receives light.
[0013]
It is desirable to use the same material as that of the slab body of the two-dimensional photonic crystal to be connected as the material of the thin wire waveguide body. Thereby, it is possible to avoid the influence of reflection or the like due to the interface between the thin wire waveguide and the two-dimensional photonic crystal. In addition, the fine wire waveguide body and the photonic crystal can be manufactured integrally. Examples of such a material include Si and InGaAsP. These are used for a two-dimensional photonic crystal having a 1.55 μm band generally used for the optical multiplex communication as a transmission band of the waveguide.
[0014]
A clad member made of a material having a refractive index lower than that of air and higher than that of air is provided so as to be in contact with at least a part of the side surface of the main body of the thin wire waveguide. Here, the part of the side surface means not a part in the longitudinal direction but a part in the circumferential direction, and a clad member is provided over the whole in the longitudinal direction. For example, when the material of the thin wire waveguide is Si, SiO ₂ having a lower refractive index can be used as the material of the clad member. When the cross-sectional shape is rectangular, one of the four side surfaces is in contact with the clad member. In order to increase the light transmittance while satisfying the single mode condition between the fine wire waveguide and the photonic crystal waveguide, it is desirable to provide the clad member only on one side surface of the main body of the fine wire waveguide.
[0015]
By providing this clad member, the mode of light propagating in the thin wire waveguide main body can be set to a single mode similarly to the light propagating in the two-dimensional photonic crystal waveguide. Without the clad member, light of two types of modes can exist in the fine wire waveguide body, one of which does not couple to the two-dimensional photonic crystal waveguide, and the efficiency is lowered. The light modes described here will be described in detail later using examples.
[0016]
It is desirable that nothing be provided above and below the surface of the two-dimensional photonic crystal connecting the thin-line waveguides of the present invention, and leave a space. When a clad member is provided on either the upper or lower (or both) surfaces of a two-dimensional photonic crystal in the same manner as the fine wire waveguide, the two-dimensional photonic is removed from the fine wire waveguide main body, rather than when a clad member is provided only on the thin wire waveguide. The light transmittance to the crystal waveguide and from the waveguide of the two-dimensional photonic crystal to the fine wire waveguide body is reduced.
[0017]
【The invention's effect】
By providing a cladding member on at least a part of the side surface of the thin waveguide main body as in the present invention, it is higher while satisfying the single mode condition between the two-dimensional photonic crystal waveguide than in the case of the main body alone. Light transmittance can be obtained. By combining a measure capable of obtaining a high light transmittance between the outside of an optical fiber or the like, which has already been devised separately, and the thin wire waveguide, and the thin wire waveguide of the present invention, A high light transmittance can be obtained as a whole between the two-dimensional photonic crystal waveguides.
[0018]
【Example】
(1) Example of Single Mode Thin Wire Waveguide for Two-dimensional Photonic Crystal An example of a single mode thin wire waveguide for a two-dimensional photonic crystal will be described with reference to FIG. This single mode thin wire waveguide 20 has a thin wire main body 21 having a rectangular cross section made of Si. Of the two surfaces at both ends in the longitudinal direction, one end surface 221 is a surface connected to the two-dimensional photonic crystal waveguide 25, and the other end surface 222 is a surface connected to the outside such as an optical fiber.
[0019]
A clad member 23 made of SiO ₂ is provided on one of the four side surfaces of the thin wire waveguide body 21. As shown in FIG. 2A, the clad member 23 may be provided only in a portion covering one side surface of the fine wire waveguide body 21. Further, as shown in (b), the fine wire waveguide body 21 may be provided on the clad member 23 as a substrate. Here, the thin-line waveguide main body 21 and the clad member 23 are combined to form a single-mode thin-line waveguide 20.
[0020]
FIG. 3 shows an example of a device in which the single mode thin wire waveguide of this embodiment is connected to a two-dimensional photonic crystal. First, as the connected two-dimensional photonic crystal 31, for example, the one described in Patent Document 1 can be used. Here, the periodic arrangement of the different refractive regions provided in the slab is a triangular lattice. The material of the slab is Si, and the different refractive region is a cylindrical hole 32. The photonic crystal waveguide 33 is formed by missing the holes 32 in a line for one row. The photonic crystal waveguide 33 is configured so that at least one end thereof reaches the end of the two-dimensional photonic crystal 31. Although not shown in FIG. 3, a point defect can be formed in the vicinity of the photonic crystal waveguide 33 as described above to form a duplexer or a multiplexer.
[0021]
One end of the thin wire waveguide main body 21 and the end of the photonic crystal waveguide 33 are connected. Here, the thin wire waveguide 20 shown in FIG. 2B is used. Alternatively, the fine wire waveguide body 21 and the two-dimensional photonic crystal 31 may be manufactured integrally, and the portion of the fine wire waveguide body 21 may be placed on the cladding member 23. Here, nothing is provided above and below the two-dimensional photonic crystal 31 to form a space. The two-dimensional photonic crystal 31 can be fixed by sandwiching both side surfaces with a support, for example.
[0022]
(2) Light transmittance in the present embodiment and comparative example Below, in the configuration of the embodiment shown in FIG. 3 and the configuration other than the present invention (comparative example), the thin wire waveguide body 21 and the photonic crystal waveguide 33 The light transmittance between the two is obtained by the three-dimensional time domain difference method (FDTD method). The FDTD method calculates an electromagnetic field by directly solving a time-dependent rotation equation among Maxwell's equations. According to this method, since the thickness of the photonic crystal can be set to a finite value, calculation closer to the actual system than the method described in Non-Patent Document 1 can be performed.
[0023]
In this calculation, the refractive index of the thin wire waveguide body and the photonic crystal is 3.4 (refractive index of Si), the refractive index of the clad member is 1.46 (refractive index of SiO ₂ ), and the photorefractive crystal has a refraction area (hole) ) Period a is 0.42 μm, the radius of the hole is 0.29 a, the thickness of the slab and the thin wire waveguide body is 0.6 a, the width of the photonic crystal waveguide is 1.152 a (a hole with a radius of 0.29 a in the triangular lattice) The width when a single column is missing).
[0024]
(2-1) First Comparative Example First, as a first comparative example, a device in which a thin wire waveguide without a clad member shown in FIG. 4 and a photonic crystal waveguide are joined will be examined. The width of the thin wire waveguide body was 1.152a, the same as the width of the photonic crystal waveguide. FIG. 5 shows the light dispersion relationship in this case. Here, the horizontal axis is a wave vector, and the vertical axis is a normalized frequency that is dimensionless by multiplying the frequency of light by a / c (c is the speed of light). For example, the normalized frequency of light having a wavelength of 1.55 μm is 0.27 because the period a is 0.42 μm. In the figure, a curve 51 is a dispersion relationship of light in a symmetric mode in the photonic crystal waveguide, a curve 52 is a dispersion relationship of light in an antisymmetric mode in the photonic crystal waveguide, and a curve 53 is a symmetry in the thin wire waveguide body. The light dispersion relationship of the mode, the curve 54 represents the light dispersion relationship of the antisymmetric mode in the thin wire waveguide body. Curve 55 is the air light line. The light having the frequency and wave number in the region above the light line in the drawing leaks into the air region in contact with the waveguide, and the light propagation efficiency decreases. For this reason, it is desirable to use light having a frequency and wave number in the region below the light line.
[0025]
We pay attention to the range of normalized frequency 0.268 to 0.280 corresponding to the wavelength 1.55 μm band used for optical multiplex communication. From FIG. 5, this photonic crystal waveguide satisfies the single mode condition in which only the light of the symmetric mode can exist in the range of the normalized frequency. On the other hand, the thin-line waveguide is a multimode state in which both symmetric mode light and antisymmetric mode light can exist in the same range. For this reason, when light is incident from a single-mode optical fiber into a fine-line waveguide, only the symmetric mode is excited in the fine-line waveguide unless the central axis of the optical fiber and the central axis of the fine-line waveguide are completely matched. I can't. If an antisymmetric mode is excited in the thin wire waveguide, it cannot be coupled with the symmetric mode in the photonic crystal waveguide, and reflection occurs. Accordingly, the loss in the entire system including the optical fiber becomes large.
[0026]
(2-2) Second Comparative Example As a second comparative example, in a device in which a thin waveguide having no cladding member shown in FIG. Consider an example in which the width is smaller than that in the comparative example. As the width of the thin wire waveguide becomes narrower, the normalized frequency of the light in the antisymmetric mode in the thin wire waveguide increases, and the cutoff frequency increases accordingly. For this reason, the light in the antisymmetric mode cannot exist within the range of the normalized frequencies of 0.268 to 0.280, and only the light in the symmetric mode can exist. FIG. 6 shows the light dispersion relationship when the width of the thin wire waveguide is 0.65a. The thin wire waveguide satisfies a single mode condition in which only light in a symmetric mode can exist. FIG. 7 shows the transmittance between the fine wire waveguide and the photonic crystal waveguide in this case. In the frequency range to be used, the transmittance is about 0.7.
[0027]
(2-3) Third Comparative Example As a third comparative example, as shown in FIG. 8, a device in which a photonic crystal is provided with the same clad member as that of a thin wire waveguide will be examined. In this case, since the wavelength range of light that can exist in the photonic crystal waveguide due to the presence of the cladding member is different from that in the first and second comparative examples, the width of the photonic crystal waveguide is set to 0.61a. The dispersion relationship in the third comparative example is shown in FIG. Here, a curve 56 is a light line of the clad member. Both the photonic crystal waveguide and the thin wire waveguide main body satisfy the single mode condition in which only the light in the symmetric mode can exist. The transmittance between the fine wire waveguide and the photonic crystal waveguide in this case is shown in FIG. The transmittance is generally in the range of 0.6 to 0.7.
[0028]
(2-4) This Example The device shown in FIG. 2 in which a clad member is provided on a thin wire waveguide and the space above and below the photonic crystal is examined. Here, the width of the thin wire waveguide body was set to 0.97a. The width of the photonic crystal waveguide is 1.152a as described above. The dispersion relationship in this case is shown in FIG. In the range of the normalized frequency of 0.268 to 0.280, both the photonic crystal waveguide and the thin wire waveguide main body are in a single mode state in which only symmetrical mode light exists. The transmittance between the thin wire waveguide and the photonic crystal waveguide is shown in FIG. In the frequency range to be used, a transmittance of 0.82 to 0.88 is obtained. This transmittance is higher than that of the second and third comparative examples.
[0029]
(3) Shape of the junction between the thin wire waveguide body and the photonic crystal waveguide In the device in which the thin wire waveguide of the present invention is provided with a cladding member and the space above and below the photonic crystal, the thin wire waveguide body and the photonic crystal The transmittance between the waveguide and the waveguide also depends on the shape of both at the junction. For example, in the triangular lattice two-dimensional photonic crystal shown in FIG. 13, the holes are arranged in the vicinity of the junction so that the holes closest to the end face of the photonic crystal including the junction are next to the photonic crystal waveguide. Belonging to the hole array (FIG. 13 (a)) and those belonging to the hole array nearest to the photonic crystal waveguide (FIG. 13 (b)). FIG. 14 (a) (in the case of hole arrangement in FIG. 13 (a)) and FIG. 14 (b) (in the case of hole arrangement in FIG. 13 (b)) show the results of calculating the transmittance for both. Here, as shown in FIG. 13, x is defined by defining the distance between the end face of the photonic crystal and the center of the nearest hole as a / 2 + x. From FIG. 14, it can be seen that a higher transmittance can be obtained with the hole arrangement of FIG. 13 (b).
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing connection of an optical fiber, a photonic crystal, and a thin wire waveguide.
FIG. 2 is a perspective view illustrating an embodiment of a single-mode thin wire waveguide for a two-dimensional photonic crystal according to the present invention.
FIG. 3 is a perspective view showing an embodiment of a device in which a single mode thin wire waveguide and a photonic crystal waveguide of the present invention are joined.
FIG. 4 is a perspective view showing a device in which a thin-line waveguide not provided with a cladding member and a photonic crystal waveguide are joined as a first comparative example.
FIG. 5 is a graph showing a dispersion relationship between a thin wire waveguide and a photonic crystal waveguide in a first comparative example.
FIG. 6 is a graph showing a dispersion relationship between a thin wire waveguide and a photonic crystal waveguide in a second comparative example.
FIG. 7 is a graph showing the transmittance between a thin wire waveguide and a photonic crystal waveguide in a second comparative example.
FIG. 8 is a perspective view showing a device in which the same clad member is provided on a photonic crystal and a thin wire waveguide, which is a third comparative example.
FIG. 9 is a graph showing a dispersion relationship between a thin wire waveguide and a photonic crystal waveguide in a third comparative example.
FIG. 10 is a graph showing the transmittance between a thin wire waveguide and a photonic crystal waveguide in a third comparative example.
FIG. 11 is a graph showing a dispersion relationship between a thin wire waveguide and a photonic crystal waveguide in the present example.
FIG. 12 is a graph showing the transmittance between a thin wire waveguide and a photonic crystal waveguide in the present example.
FIGS. 13A and 13B are diagrams showing two modes related to the arrangement of photonic crystal vacancies in the vicinity of a junction between a thin wire waveguide and a photonic crystal waveguide. FIGS.
14 is a graph showing the transmittance between the thin wire waveguide and the photonic crystal waveguide in the two modes of FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11, 20 ... Fine wire waveguide 12 ... Optical fiber core 13, 24, 31 ... Two-dimensional photonic crystal 14, 25, 33 ... Two-dimensional photonic crystal waveguide 21 ... Fine wire waveguide main body 23 ... Clad member 32 ... Empty Hole 51 ... Symmetric mode light dispersion relation in photonic crystal waveguide 52 ... Antisymmetric mode light dispersion relation in photonic crystal waveguide 53 ... Symmetric mode light dispersion relation in fine wire waveguide body
54 ... Light dispersion relationship of antisymmetric mode in the thin wire waveguide body 55 ... Air light line 56 ... Light line of the clad member

Claims (4)

a) a two-dimensional photonic crystal having a waveguide whose upper and lower surfaces face a space where nothing is provided and is in a single mode at a predetermined wavelength;
b) a fine wire waveguide body to which the waveguide and one end face in the longitudinal direction are connected;
c) A refractive index lower than that of the fine wire waveguide body and higher than that of air, so that the light of the predetermined wavelength propagating in the fine wire waveguide body is in a single mode state . out is provided so as to contact at least a portion, and a cladding member not in contact with the two-dimensional photonic crystal,
A two-dimensional photonic crystal with a thin wire waveguide, comprising:   The two-dimensional photonic crystal with a thin wire waveguide according to claim 1, wherein a cross section of the thin wire waveguide is rectangular.   The two-dimensional photonic crystal with a thin wire waveguide according to claim 2, wherein the clad member is in contact with only one side surface of the thin wire waveguide body. 4. The two-dimensional photonic crystal with a thin wire waveguide according to claim 1, wherein a material of the thin wire waveguide body is Si, and a material of the clad member is SiO ₂ .

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