Photonic crystal waveguide

Details Photonic crystal waveguide Photonic crystal waveguide

2001-12-26

2003-11-04

Lee, John D. (Department: 2874)

Optical waveguides

Optical fiber waveguide with cladding

Utilizing nonsolid core or cladding

C385S129000, C385S131000

Reexamination Certificate

active

06643439

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photonic crystal waveguide which can be used as a basic structure which forms photonic devices such as lasers and photonic ICs used for optical information processing, optical transmission and the like.
2. Description of the Related Art
In a conventional photonic device, since light confinement is carried out by using difference of refractive indices, a space for light confinement must be large. Therefore, the device can not be configured very small. In addition, when a steeply bent waveguide is used in order to increase the scale of integration of the device, scattering loss occurs. Thus, it is difficult to integrate photonic circuits and it is difficult to downsize the photonic device. As a result, the size of the photonic device is much larger than that of an electric device. Therefore, the photonic crystal is expected to be a new photonic material which can solve the above-mentioned problem, in which the photonic crystal can perform light confinement by a concept completely different from the conventional one.
The photonic crystal has an artificial multidimensional periodic structure in which periodicity, which is almost the same as light wavelength, is formed by using more than one kinds of mediums having different refractive indices, and the photonic crystal has a band structure of light similar to a band structure of electron. Therefore, forbidden band of light (photonic band-gap) appears in a specific structure so that the photonic crystal having the specific structure functions as a nonconductor for light.
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.
In order to realize the optical waveguide for forming the large scale integrated optical circuit, it is necessary to realize a single waveguiding mode in the photonic band-gap frequency band. When a multi-mode waveguide having a plurality of modes is used as a bent waveguide, there is a problem, for example, in that a part of mode may be converted into a different mode in a bending part. Thus, the multi-mode waveguide can not be used as an effective bent waveguide necessary for realizing the large scale integrated optical circuit. That is the reason for requiring the single-mode. In addition, the multi-mode waveguide is not suitable for high-speed communication.
Some types of waveguides have been manufactured. In the various waveguides, waveguides using the two-dimensional photonic crystal is promising since it is very difficult to fabricate waveguides by a three-dimensional photonic crystal which has full band-gap.
When using the two-dimensional photonic crystal for the waveguide, it is necessary to confine light in the direction perpendicular to the two-dimensional plane. Several methods has been proposed as the method of light confinement. In the methods, using a two-dimensional photonic crystal slab on oxide cladding is preferable since a structure having a large area can be easily manufactured by the two-dimensional photonic crystal slab on oxide cladding and it is easy to add various function elements in the same structure. The two-dimensional photonic crystal slab on oxide cladding is based on a structure in which a thin semiconductor film of high refractive index (from 3 to 3.5) is deposited on a dielectric of low refractive index (oxide or polymer in many cases, the refractive index is about 1.5).
In addition, a substrate called Silicon-On-Insulator (SOI) substrate is being applied to LSIs, and high-quality SOI substrate can be manufactured in recent years. The SOI substrate is formed by providing a silicon (Si) thin-film on silica (SiO
2
). By using the SOI substrate, there is a merit that the two-dimensional photonic crystal slab on oxide cladding having high quality can be easily manufactured. The merit can not be obtained by using other structures (for example, two-dimensional photonic crystal air-bridge slab in which cladding of both sides is air).
As mentioned above, the two-dimensional photonic crystal slab on oxide cladding has the advantage of being easier to manufacture than the two-dimensional photonic crystal air-bridge slab and the like. However, the structure has following problems so that the single waveguiding mode was not realized in the photonic band-gap frequency band according to the conventional structure.
In waveguiding modes generated by the line defect in the optical waveguide of the two-dimensional photonic crystal slab, light is strongly confined in the directions of the two-dimensional plane by the photonic band-gap and scattering loss does not exist in the directions. However, light is generally leaky in a high frequency region above a light line of cladding, that is, the light may be leaked to the cladding. (The light line represents the lowest frequency, with respect to propagation constant, by which light can transmit in the cladding, and, the light line can be represented by a line defined by w=ck
(w: angular frequency, c: light speed, n: refractive index, k: wave number).) Therefore, it is customary to use a low frequency region below the light line such that the waveguide light does not leak to cladding layers of both sides.
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 SiO
2
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 &mgr;m 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

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